CN1771446A - Beam shaping and practical methods of reducing loss associated with mating external sources and optics to thin silicon waveguides - Google Patents

Abstract

A practical realization for achieving and maintaining high-efficiency transfer of light from input and output free-space optics to a high-index waveguide of submicron thickness is described. The required optical elements and methods of fabricating, aligning, and assembling these elements are discussed. Maintaining high coupling efficiency reliably over realistic ranges of device operating parameters is discussed in the context of the preferred embodiments.

Description

Practical method for beam shaping and reducing losses caused by connecting external light sources and optical devices to thin silicon waveguides

Cross reference to related applications

The present invention claims the benefit of provisional application No.60/461,697 filed on 10/4/2003.

Technical Field

The present invention relates to coupling devices associated with thin silicon optical waveguides, and more particularly to methods for beam shaping and reducing losses associated with connecting external light sources and optics to such thin waveguides.

Background

In many device applications, the input signal must be pre-processed in the device in order to optimize the device's proprietary technology for performing basic functions; also, the signals emanating from the device core must be post-processed before transmission to the outside in order to produce a signal that is compatible with typical user requirements. For optoelectronic components, the required optical signal processing includes light generation, wavelength control, polarization control, phase control, beam steering, beam shaping, beam splitting or combining, modulation, and detection functions. For ease of use, or to control parameters critical to device performance, a number of pre-or post-processing functions may generally be integrated into the assembly. For example, an important benefit is that the optical insertion loss of the device can generally be reduced by integrating more optical functions into the assembly. This is not only because the choice of components can be more easily optimized for device specific technology, but also because the physical connections between different devices or components are reduced. A low loss opto-electronic component may be used in system applications because it is easier to use in different parts of the system and expands the range of applications of the system. Furthermore, the physical size of the device may be reduced by device integration.

The integration of pre-processing and post-processing optical functions is particularly critical for silicon-based optoelectronic circuits operating at infrared wavelengths. Since silicon lasers have not been widely used in commercial applications, it is not possible to incorporate the light source in the same silicon chip as the signal processing and receiving elements. Therefore, an optical signal must be introduced into the silicon wafer from an external light source. This requires the insertion of optical elements (between the light source and the waveguide) to precondition the signal so that a substantial intensity of light can be transmitted to the waveguide. Furthermore, since silicon-based detectors adapted to infrared wavelengths are just beginning to be developed, optical signals must be transmitted from the silicon waveguide to an external detector or receiving element. The output of the device therefore requires optical components for post-processing of the optical signal. Typical prior art methods of coupling light into a high index contrast waveguide include prism couplers, grating couplers, wedge mode converters, and specially shaped fiber terminations or lensed fibers. While all of these optical components have been used in a laboratory environment to transmit a portion of light from an external source into a high index contrast waveguide, there are still limitations when these components are used in prototypes or end products for low loss devices.

For example, specially shaped fiber terminations, lensed fibers, or wedge mode converters can produce a minimum beam spot size on the order of 1.5 μm, which is not compatible with some sub-micron sized silicon waveguides. In particular, single-mode silicon waveguides having dimensions of about 0.35 μm or less are required in many applications. A mismatch between the mode field diameter of the output beam of a tailored fiber or wedge mode converter and the mode field diameter in the waveguide mode will cause high insertion loss. Even if the waveguide has a diameter of about several microns, the geometry of the device (e.g., the layout and size of the device) has many limitations when the device is coupled to a specially made fiber or wedge mode converter because the input and output ports of the device must be located at the cut plane of the wafer mold containing the waveguide.

The above limitations can be addressed by coupling light into or out of a high index contrast waveguide from an external light source using a grating coupler or a prism coupler. By proper design, light can be successfully coupled into waveguides with thicknesses ranging from tens of nanometers to tens of micrometers. In addition, the grating or prism elements may be placed at any suitable location on the surface of the mold or wafer so that light can enter a substantial portion of the mold or wafer.

Despite their considerable advantages, difficulties in the manufacture of grating and prism couplers have limited their use in some specific applications. The coupling efficiency of a grating coupler is sensitive to grating period, depth and tilt angle. Theoretically, if the design goals of the grating parameters can be met, a coupling efficiency of about 70-80% can be achieved; in practice, the coupling efficiency is found to be around 40% more due to the sensitivity to manufacturing tolerances.

In the prior art, prism couplers require a large bulk optical element (several millimeters in size) to be placed very close to the waveguide and accurately positioned relative to the waveguide. Here, "close proximity" means that the spacing between the optical element and the waveguide allows evanescent coupling of light from the optical element to the waveguide. Typical spacing values for infrared wavelengths used in telecommunications applications fall within the range of 200-500 nm. The control of the motion required to manipulate the prism relative to the waveguide (e.g. using a piezoelectric mount) can be done in a laboratory optical bench or test setup, but this approach cannot be implemented in small electro-optical packages. Therefore, prism coupling applications are limited primarily to waveguide testing and identification.

Because prism couplers have not been used in small optoelectronic packages in the prior art, optical and mechanical components suitable for use with prism couplers in small device structures have not been developed. For example, the prior art does not disclose specific embodiments of typical optical elements for transmitting light to or receiving light from a prism coupler arrangement in a compact optoelectronic package. In laboratory settings, when a signal introduced into a prism coupler changes to some degree (e.g., changes in wavelength, polarization state, beam position, angle of incidence, etc.), the optical elements can generally be adjusted in a number of ways to optimize signal transmission. For small devices, it is appropriate to design a device that is transparent to a wide variety of inputs; that is, only a small portion of the parameters need to be adjusted (or not at all) for the device to function properly when the input state of the signal changes. Thus, the selection of optical parameters associated with the input and output beams, the input and output optical elements, and the prism coupler directly affects the versatility and manufacturability of the device. However, since the prism coupler has not been integrated into a small electro-optical device in the prior art, a detailed design scheme for manufacturing a versatile and manufacturable device has not been developed.

There is therefore a need in the art to design and implement optical systems that can interface with prism couplers in small, low-loss, stable optoelectronic packages.

Disclosure of Invention

The need remaining to be addressed in the prior art will be addressed in the present invention, which relates to the design of an optical system that can be used to process infrared light signals entering and exiting a small prism-coupled optoelectronic device.

In particular, the present invention details several embodiments of optical elements that provide the necessary interface for permanently coupled miniature prism and waveguide components. These interfaces include, but are not limited to: a free space optical element that directs light from an external source into the high index prism structure, an optical element or structure fabricated on the same silicon wafer or die with etched surfaces that serve as the prism input and output surfaces, an evanescent coupling layer that forms the direct physical interface between the high index prism and the waveguide, and a free space optical element that receives the output beam exiting the prism output surface.

The various embodiments described above are particularly suitable for use with thin silicon waveguides in the wavelength range commonly used in telecommunications applications. However, the various interface devices of the present invention are equally applicable to other devices, and may use larger size waveguides and/or other wavelength ranges. Specific embodiments of the emitting optics are detailed and conditions are provided for providing a new and compact packaging solution for prism coupling devices. Designs are disclosed that minimize the end-to-end insertion loss of small optoelectronic devices using prism coupling, and the theoretical coupling efficiency of particular embodiments is calculated. It would be further advantageous to specify a specific and manufacturable embodiment of an evanescent coupling layer that produces a desired output beam intensity profile and reduced insertion loss.

The advantage of reducing the required free-space beam size relative to manufacturing requirements will appear during the course of the following description with reference to the accompanying drawings.

Drawings

Reference is now made to the drawings.

Figure 1 shows a silicon-based prism coupler permanently affixed to a silicon-on-insulator (SOI) wafer containing a silicon waveguide layer.

FIG. 2 shows the geometrical path of a beam of light propagating within a prismatic structure, including the internal and external emission angles of the prism (corresponding to θ, respectively)_airAnd theta_Si) And the physical dimensions of the optical coupling region at the prism surface, the prism surface being directly connected to the evanescent coupling layer;

FIG. 3 shows the external beam emission angle θ of the prism of the embodiment of FIG. 1_SiWithin a certain telecommunications wavelength range, and within ranges under three different silicon waveguide thickness conditions;

FIG. 4 shows the beam emission angle θ in air for the embodiment of FIG. 1_air(outside the prism) in a range of telecommunication wavelengths and ranges at three different silicon waveguide thicknesses;

FIG. 5 shows the prism structure interior (θ)_Si) And a prism surface (theta)_air) The full range of external emission angles, covering the device silicon layer thickness range of 0.1 to 0.21 μm, and the wavelength range of 1290 to 1590 nm;

FIG. 6 shows a coupling efficiency curve covering a certain free-space input beam diameter value and at three different coupling constant values

Full width at half maximum of lower angle (using FWHM (theta)_air) Represents);

FIG. 7 shows the coupling efficiency of the embodiment of FIG. 1 as a function of the thickness of the silicon oxide evanescent coupling layer, simulated results at three different waveguide layer thicknesses in a silicon-on-insulator wafer;

fig. 8 shows the results of simulations for three different materials constituting the evanescent coupling layer as a function of the thickness of the evanescent coupling layer for coupling efficiencies for embodiments similar to the embodiment of fig. 1.

Fig. 9 shows a plot of the maximum shift in flatness (referred to as "wedge angle") as a function of free-space input beam diameter, consistent with a theoretical model of the constant thickness of the evanescent coupling layer of the embodiment of fig. 1.

Fig. 10 shows the optimum wedge angle for an embodiment similar to that of fig. 1 with a wedge-shaped evanescent coupling layer as a function of the free-space input beam diameter.

FIG. 11 shows a prismatic coupling surface over a range of telecommunications wavelengths for the embodiment of FIG. 1

Input beam size and input free space beam size

The ratio of (a) to (b), in the case of four different waveguide layer thicknesses in the SOI wafer;

FIGS. 12(a) and (b) show in top and side projection views how an initially unpolarized input beam is converted into separate beams of light of two desired polarization directions, thus allowing efficient coupling of light into a waveguide through a prism structure;

fig. 13 shows an example of using an actuated MEM micro-mirror to control the appropriate emission angle of a light beam from one horizontal light source to the outside of the prism.

FIG. 14 shows a physical layout showing side emission of light entering the device from an edge-emitting diode or other fiber input leading from one fiber, with the output on the opposite side of the assembly;

FIG. 15 shows a physical layout showing side emission of light entering the device from a side-emitting diode or other fiber input leading from one fiber, with the output end on the same side of the assembly;

FIG. 16 shows an example of using an array VCSEL light source and a microprism array to direct a beam of light to a prismatic structure;

FIG. 17 is another embodiment of the device of FIG. 16, in which an edge emitting diode array is used in place of the arrayed VCSEL light source banks;

FIG. 18 is another embodiment similar to FIG. 16 and employing a set of lensed optical fibers disposed in association with a beam steering device;

FIG. 19 shows a prism wafer including additional optical elements to collimate and steer the beam before the evanescent coupling layer interface;

FIG. 20(a) shows the preferred embodiment of FIG. 1 with an evanescent coupling layer of constant thickness, FIGS. 20(b) and (c) show the input and output beam amplitudes as a function of z, and FIG. 20(d) shows the superposition of FIGS. 20(b) and (c) (c);

FIG. 21 contains a schematic diagram showing a particular apparatus for forming a half Gaussian wave; and

FIG. 22(a) shows the preferred embodiment of FIG. 1 with an evanescent coupling layer with a linear variation in thickness, FIGS. 22(b) and (c) show the input and output beam amplitudes as a function of z, and FIG. 22(d) shows the superposition of FIGS. 22(b) and (c);

Detailed Description

For a better understanding of the principles of the invention, its understanding and its transportationThe requirements associated with the input beam are important, the input beam being first transmitted to the input face of a typical prism structure 10 shown in fig. 1 and then coupled through the prism structure to a thin silicon waveguide 12. A detailed schematic of the propagation of an input beam in a prismatic structure is shown in fig. 2. The beam enters the prism structure 10 through a hypotenuse (input face) surface 14 which is coated with a radiation reflecting layer 16 to reduce fresnel losses caused by transmission from a low index medium (atmospheric air) to a high index medium (silicon in the embodiment of fig. 1). Referring to FIG. 2, the input beam makes an angle of incidence θ with the normal of the input face surface 14_airAnd then refracted by the prism. To conform to known optics, the angle (θ) within the prism is adjusted_Si) Most conveniently, the angle between the beam and an axis perpendicular to the waveguide. According to the geometric relationship of FIG. 2, θ_SiAnd theta_airThe following relationships exist:

θ_Si＝θ_pr-sin^-1{sinθ_air/n_Si}，

wherein for a wavelength n in the 1.3-1.6 μm band_SiThe refractive index of silicon is approximately equal to 3.5.

Refraction expands the size of the beam inside the prism, along the axis shown in fig. 2, by the expansion factor:

for coupling efficiency, the projection of this beam onto the prism and evanescent coupling layer interface is a critical parameter. From the geometric relationship associated with FIG. 2, it can be seen that the input free space beam and diameter areThe relationship of the projection of the free-space input beam on the prism coupling surface can be expressed as:

FIG. 2 illustrates the geometrical constraints governing the propagation of the internal and external beams of light of prism 10, while FIG. 1 shows a diagramA preferred arrangement, in which the prism coupler is made from a silicon wafer and is permanently affixed to a bonded SOI wafer 20 containing the waveguide 12. As shown in fig. 1, waveguide layer 12 is separated from silicon substrate 22 by a barrier oxide layer 24. The desired prism surface is made on a silicon wafer by a combination of wiring and etching processes rather than using a separate precision prism optical element. The required portions of the vertical walls 30, 32 can be made by a variety of etching processes, while the prism facets 14, 18 are most easily made by an anisotropic wet etching process. The anisotropic process has different etch rates for different crystallographic planes and thus the prism facets 14, 18 are at a specific angle to the wafer plane. For the configuration in FIG. 1, the silicon prism wafer is<100>The crystal orientation, and thus the anisotropic KOH etch, will result in a crystal plane that is 54.74 degrees from the plane of the wafer. By depositing a layer having a refractive index less than the refractive index of silicon (n) on the upper waveguide surface of a silicon prism wafer or of a joined SOI crystal plane_SiApproximately 3.5) to obtain an evanescent coupling layer 26. The prism coupler is then permanently attached to the SOI wafer containing the waveguide, preferably using a semiconductor bonding process, although adhesive and solder bonding methods may also be used. In the resulting prism coupler/SOI wafer assembly, the base of prism coupler 10 (prism coupling surface 15) is in direct contact with waveguide surface 12 of SOI wafer 20, thus resulting in a prism/evanescent coupling layer/waveguide sandwich. To reduce fresnel losses at the input and output prism bevel surfaces (hereinafter "prism faces"), one (or more) layers of additional material are deposited on the surface of the silicon prism coupler that integrates the prism faces. This one or more layers serve as a light reflecting coating 16 that significantly improves transmission as light traverses the prism faces.

By using the theory known in the prior art, the beam angle θ in the silicon prism structure_SiCan be calculated over a range of waveguide thicknesses compatible with single mode propagation and over the wavelength band used for telecommunications applications. A waveguide thickness of 0.1, 0.14 and 0.21 μm and a wavelength range of 1290-1630nm_SiThe calculation results of (a) are shown in fig. 3. These typical waveguide thicknesses are selectedBecause optical and high-speed electronic functions can be integrated in these relatively thin waveguides. It can be seen that the beam angle θ_Si(defined in fig. 2) covers a range of 38 degrees to 58 degrees over the desired wavelength and waveguide thickness range. For determining a suitable emission angle theta outside the prism_airThe previously obtained θ can be used_SiAnd theta_airThe relationship (2) of (c). As previously described, for the embodiment in FIG. 1, for<100>Oriented silicon wafer, using anisotropic etching process to produce planes with input and output angulations_pr54.74 degrees. However, the use of the embodiment in FIG. 1 is not limited to this particular θ_prA value; any other etch process or different method may be used to obtain θ_prThe value is obtained. FIG. 4 shows θ at the wavelength range of 1290-1630nm and waveguide thicknesses of 0.10, 0.14 and 0.21 μm_airThe calculation result of (2). The range of incident angles in air is much larger, varying from-15 to 90 degrees; this is because the refractive indices of air (n ≈ 1.0) and silicon (n ≈ 3.5) differ greatly.

FIG. 5 provides a prism interior (θ)_Si) And prism outer part (theta)_air) The angular range of (a) for a 54.74 degree prism must be reached so that the use of the device can cover the full range of wavelengths and waveguide thicknesses. Except that for wavelengths above 1590nm for a waveguide thickness of 0.10 μm, air launch conditions can be achieved over a wide range of wavelengths and waveguide thicknesses. Thus, the major advantages of the embodiment shown in FIG. 1 include (1) coordinating common semiconductor wiring, etching, and bonding processes to produce a manufacturable prism coupler and waveguide device, and (2) constituting a useful structure for applications covering a wide range of infrared wavelengths and waveguide thicknesses.

The use of the device illustrated in fig. 1 may be further enhanced by selecting optical and spatial characteristics of the input and output light beams that may simplify the optical signal interface connected to the device in fig. 1. Because the wavelength range and power of the input signal are typically determined by the application, the polarization direction, beam shape, beam (or wavefront) quality, and propagation direction can be adjusted within the package. For lens coupling applications, precise control of these parameters is necessary in accordance with the present invention in order to achieve the desired high coupling efficiency of light from the lens coupler into the waveguide. In particular, the following conditions must be satisfied:

(1) the input optical beam must emerge from an angle of incidence determined by the polarization state and wavelength of the input optical beam, the refractive indices and thicknesses of the silicon device waveguide layer 12 (hereinafter denoted W) and evanescent coupling layer 26, and the refractive indices of the prism 10 and its surrounding medium. If the incident beam emerges from the appropriate angle of incidence, the wave field propagation constants in the prism 10 and the waveguide 12 will match, so that a high coupling efficiency can be obtained.

(2) The beam must be highly collimated at the prism coupling surface 15 so that the finest part of the input gaussian beam falls in the vicinity of the prism coupling surface. It is known that if the phase of the wavefront varies greatly over the range of projection of the wave on the prism coupling surface 15, the coupling efficiency will be reduced.

(3) The input beam must intersect the prism coupling surface 15 at a specific location, depending on the form of the evanescent coupling layer and the beam intensity distribution of the input optical signal. For a gaussian input beam and an evanescent coupling layer 26 of constant thickness, it can be seen that the center of projection of the beam onto the prism coupling surface must be located at a distance from the vertical sidewall 34 of the prism shown in fig. 2

To maximize coupling efficiency. A small portion of the beam, truncated by the vertical sidewall 34, is totally internally reflected, first by the vertical sidewall 34 and then by the prism coupling surface 15 before exiting the output face 18. It is emphasized that relative to this positionA small offset of (a) will cause a slight reduction in coupling efficiency (about 10%). Intercepting the projection of the input beam on the prism coupling surface 15 in this particular way prevents light transmitted from the prism structure to the waveguide 12 from coupling back to the prism structure.

(4) In order to maximize the coupling efficiency, the thickness of the evanescent wave layer must be adapted to the size of the projection of the input beam onto the prism coupling surface,

it is known from the prior art to realize a projection of an input light beam (of the order of

And a coupling strength parameter (hereinafter referred to as "α") determined mainly by the thickness of the evanescent wave layer, the coupling efficiency can be maximized. This is because of a andis an important parameter in the overlap integral that determines the coupling efficiency.

To meet these conditions in a compact optoelectronic package, suitable collimating, shaping, and beam-redirecting micro-elements, as well as additional polarization and phase control optics, are important to the coupling efficiency of coupling light into the structure of fig. 1. Since the typical size of the prism surface in fig. 1 is about 0.5-1.0 mm, the size of the stop of the optical element must be similar to keep the whole assembly compact. The maximum size of the beam must be slightly smaller than the size of the optical element to prevent transmission losses due to beam confinement. As will be discussed below, other manufacturing factors specific to prism coupling applications impose more stringent limits on the maximum size of the beam. For efficient prism coupling, there is an optimum beam size (related to the properties of the evanescent coupling layer, as described above) and a minimum beam size so that the beam remains collimated as it traverses the prism structure and intersects the prism coupling surface.

The manufacturing tolerances of the device as shown in fig. 1 are more easily met if a suitable maximum beam size is chosen. In particular, significant advantages in terms of the emission angle of the input light beam I and the tolerance of the thickness of the evanescent coupling layer 26 may be obtained.

It is known from the prior art to use for constant thicknessWhen the evanescent coupling layer is formed

Then, an optimal coupling efficiency of 80% can be obtained. α is a parameter indicating the coupling intensity, and is also a characteristic constant of the shape of the light beam emitted from the output surface of the prism structure, and is expressed in the inverse of the length, and the form of the shape of the emitted light beam is g (z) · exp (- α z). The parameter α is mainly determined by the evanescent coupling layer thickness, the propagation constant in the evanescent coupling layer and the phase change caused by reflection at the two boundaries of the waveguide.

If set to 0.68 to optimize coupling, then

The value decreases and a must increase, corresponding to stronger coupling or a thinner evanescent coupling layer. The increased coupling strength causes a wider resonance that allows a wider range of wavelengths or equivalently input angles to couple into the waveguide. In fact, the full width at half maximum (FWHM) of the lorentz profile of the resonance in β space (β represents the propagation constant) is directly proportional to α, with the relationship:

FWHM(β)＝FWHM(n_Sisinθ_Si) The numerator and denominator multiplied by α λ/pi

And according to the following relationship:

θ_Si＝θ_pr-sin^-1{sinθ_air/n_Sican derive the full width at half maximum as the input angle theta_airThe function of (d) is:

wherein,

F(θ_air，θ_pr){1-(sinθ_air/n_Si)²}^1/2/[cos(θ_air)×cos{θ_pr-sin^-1(sinθ_air/n_Si)}].

to aSpecific device configurations, e.g. as shown in FIG. 1, theta_prAnd W (waveguide thickness) are each a fixed amount (θ)_pr)₀And W₀. Furthermore, if a specific wavelength λ is selected₀Then theta_airIs also set at a specific value (theta)_air)0 (as shown in fig. 4). In this case, with respect to θ_airThe intensity full width at half maximum of a small deviation (external emission angle into the prismatic structure) can be expressed as:

this means that the intensity distribution over a certain input angle range is dependent on a parameter determining the coupling efficiency

Increases linearly and increases with the inverse of the projection of the beam diameter on the prism coupling surface 15. For a given coupling efficiencyThe intensity distribution over a range of input angles can be enhanced by reducing the projection of the diameter of the input beam onto the prism coupling surface 15. Also, the coupling constant is slightly increased

The intensity distribution over a range of input angles can be improved while the coupling efficiency is only slightly reduced. From a manufacturing point of view, an appropriate coupling constant is selectedAnd beam projectionIt is important that the final device be ready for small theta values over its lifetime_airThe variation is more stable. The following example shows the range of beam sizes and input angles suitable for high coupling efficiency.

FIG. 6 shows FWHM (. theta.) (_air) As four free space lightsBundle diameter

Value and three different coupling efficiencies

A function of the value. These four selected beam sizes correspond to the following cases: (1)63 μm: standard output beam size of the lensed fiber optic component; (2)100 μm: typical beam dimensions of Vertical Cavity Surface Emitting Lasers (VCSELs) with integrated microlenses in the laser package; (3)200 μm: the minimum beam size achievable in a standard fiber optic collimator (fiber/ferrule assembly aligned with a GRIN or aspherical mirror); and (4)360 μm: the most commonly used beam sizes in standard fiber optic collimators (fiber/ferrule components aligned with a GRIN or aspherical mirror). To calculate the coupling efficiency and half-width at varying input emission angles, the projection of the beam onto the prism coupling surface

From free-space beam diameter using the preceding formula

And (4) calculating. Next consider the adjustment

To change the effect of coupling efficiency. If the evanescent coupling layer is thicker than optimal for a given beam size, the system will be under-coupled, i.e. the beam is not at all visible

Less than the optimum. For theFor the embodiment in fig. 1, a coupling efficiency of 72% can still be obtained. This is not very suitable for input angle tolerances, since the resonance is sharper and θ_airThe tolerance for variation is less than the tolerance under optimal coupling conditions. For the device of fig. 1 operating at a wavelength of 1550nm, a coupling efficiency of 72% under-coupled conditions corresponds to an excessively thick evanescent coupling layer of about 40nm (see fig. 7). It can be seen that for any achievable value of couplingConfiguration, FWHM (θ)_air) Typically not exceeding 0.35 degrees. Under the condition of the optimal coupling, the coupling,time FWHM (theta)_air) Has increased to 0.4-0.6 degrees and remains at about 0.1-0.2 degrees for larger beam diameters. Considering now the case where the evanescent coupling layer is too thin, about 40nm, a coupling efficiency of 72% occurs,over-coupling condition of (c). As can be seen from fig. 6, for

The angular tolerance has been significantly increased to 0.7-1.1 degrees and to about 0.2-0.35 degrees for larger beam diameters. Thus, the use of small beam diameters in moderately over-coupled devices after free-space optics calibration can significantly reduce the sensitivity of the device to small variations that occur during operation or as the device ages.

Other benefits of using a relatively small beam diameter arise from the limited physical extent to which the beam interacts with the evanescent coupling layer. To achieve high coupling efficiency, the thickness of the evanescent coupling layer must be precisely controlled. The change in layer thickness is directly converted into a change in alpha, so that

The value of (c) deviates from the optimum value of 0.68. As an example, the coupling efficiency of the preferred embodiment of FIG. 1 is shown in FIG. 7 as a function of the thickness of the silicon oxide evanescent coupling layer 26 at three different thicknesses of the waveguide layer 12. The thickness of the evanescent coupling layer was estimated with reference to a 1550nm application wavelength and an input free-space beam with a diameter of 63 μm. The range of device layer thicknesses shown in the figures represents the actual spread of layer thicknesses in current silicon-on-insulator processing. The target device layer thickness is 0.14 μm, as shown in the preferred embodiment. It can be seen that the thickness of the evanescent coupling layer must fall within the range of the target value 20nm, in this case about 320nm, to prevent a 10% reduction in coupling efficiency (if tolerances on the thickness of the waveguide layer 12 are taken into account, ± 0.01 μm). Nevertheless, a tolerance of 20nm must be achieved when the beam is coupled in the prismThe entire physical extent of the projection on the surface is maintained to ensure high coupling efficiency. This condition is more easily satisfied if (1) the medium constituting the evanescent coupling layer is selected such that the width of the coupling efficiency curve in fig. 7 is appropriate; (2) the process of affixing the prism coupler to the waveguide surface of the SOI wafer allows the thickness tolerance to be maintained within the physical range of the beam projection; and (3) the physical extent of the projection of the light beam on the prism coupling surface is relatively small.

FIG. 8 shows the results of an analysis similar to that of FIG. 1, but showing the coupling efficiency as a function of the thickness of the evanescent coupling layer at three different evanescent coupling layer refractive indices. These three values represent three different typical media: air (n ≈ 1.0), silicon oxide (n ≈ 1.45), and silicon nitride (n ≈ 2.0). The basic form of the coupling efficiency curves for these three cases is the same, but it is clear that the optimum evanescent coupling layer thickness varies, and that the width of the coupling efficiency curve widens slightly as the refractive index of the evanescent coupling layer increases. Referring to fig. 8, when n is 2.0, the thickness of the evanescent coupling layer must fall within the range of the target value ± 20nm, in this case about 385nm, to prevent a 10% reduction in coupling efficiency (if the thickness tolerance of the silicon waveguide layer (referenced with respect to the waveguide layer of fig. 1) ± 0.01 μm). Thus, a small benefit is obtained by using higher index evanescent coupling layers. Interestingly, these three dielectrics (air, silicon dioxide, and silicon nitride) work well within the scope of the current embodiment as long as the correct evanescent coupling layer thickness is obtained. The width of the coupling curve (20 nm for silicon oxide and 25nm for silicon nitride) is indicated to correspond to a tolerance of 6-7% for the evanescent coupling layer thickness, which is compatible with current process fabrication methods.

For the device configuration shown in FIG. 1, if a bundle diameterA free-space input beam of 63 μm is transmitted to the input prism face at the coupling surfaceThe maximum dimension of the projection on the waveguide is about 110 μm (for a wavelength of 1550nm, a waveguide thickness of 0.14 μm and a silicon oxide evanescent coupling layer thickness of about 320 nm). Furthermore, as can be seen from FIG. 7, the thickness of the evanescent coupling layer can vary by + -20 nm while still maintaining a coupling efficiency in excess of 70% for the same device configuration. The prism coupling surface is generally not absolutely parallel to the waveguide plane during device fabrication. A small shift from the parallel position will cause the amplitude of the evanescent coupling layer thickness to vary slightly along the prism coupling surface. Fig. 9 shows the relative parallel position shift that can be supported over a range of input beam sizes for an embodiment such as that shown in fig. 1, but still in accordance with a model of an equal thickness evanescent coupling layer. As shown in FIG. 7, for the evanescent coupling layer to be a coupling region of substantially constant thickness, a maximum + -20 nm, or 40nm total, thickness variation in the optical coupling region can be supported. Thus, if the projection of the input beam on the prism coupling surface is 110 μm, the maximum allowed wedge angle is about 4 × 10 ═ 0.04 μm/110 μm^-4Radian, or 0.02 degrees. Similar calculations show that the maximum allowable flatness offset for a 62 μm diameter free-space beam can be increased to as much as 0.026 degrees if a silicon nitride evanescent coupling layer is used. If a large size beam is used, the optimum thickness of the evanescent coupling layer will increase, but the variation in thickness allowing high coupling efficiency will remain substantially unchanged, about ± 20 nm. For a free space beam size of 360 μm

For the device configuration in fig. 1, the projection of the corresponding beam on the prism surface is about 610 μm. Similar calculations give that the allowed wedge angle has been reduced to 0.04 μm/610 μm-6.6 × 10^-5Radian, or 0.004 degrees. Most of the improvement in wedge angle tolerance stems from the fact that the critical gap spacing needs to be maintained over a smaller range for smaller beam sizes. Since all the permissible wedge angles mentioned above are very small and the projection of the light beam onto the prism coupling surface decreases in the opposite direction, the production of the device shown in fig. 1, which can couple light efficiently, is apparent from the design using a relatively small beam sizeIs improved.

In a variation of the device configuration shown in fig. 1, small variations in evanescent coupling layer thickness along the input and output optical coupling regions can improve coupling efficiency by over 80%. It is known from the prior art that the gradient thickness of the evanescent coupling layer is such that the intensity of the output free-space light beam is gaussian in nature, the thickness of the light coupling region is greater than the optimal thickness where the light beam is first separated from the waveguide by the output prism, and the thickness of the light coupling region is less than the optimal thickness where the last remaining light intensity in the waveguide is separated by the output prism. This is in contrast to the case of an evanescent coupling layer of constant thickness, where the output beam intensity distribution is exponential. The improved mode matching between the gaussian input beam and the essentially gaussian output beam, brought about by the wedge-shaped evanescent coupling layer, increases the theoretical coupling efficiency from 80% to about 97%. Without a detailed mathematical discussion, the basic information for calculating the appropriate angle for the wedge in FIG. 10 can be obtained from FIG. 7. As mentioned before, the parameter α, which is related to the coupling strength and appears in the functional form of the intensity distribution of the output beam, is mainly determined by the thickness of the evanescent coupling layer. For a wedge-shaped evanescent coupling layer with a thickness (z) that varies along the direction of propagation of light in the waveguide, the coupling strength at a given value of z is directly related to the local value of α, α (z). The thickness of the evanescent coupling layer must change from weakly coupled (small α (z)) to strongly coupled (large α (z)) on a distance scale approximately equal to the projection of the input beam onto the prism coupling surface. To achieve high coupling efficiency, it is necessary to determine a suitable average thickness value (which yields an alpha value close to the optimum value for alpha for optimum coupling for a constant thickness evanescent coupling layer) and a suitable linear variation of the thickness with z, or "wedge angle". For the example of fig. 7, when W is 0.14 μm, it can be seen that the coupling efficiency drops to about 37% (or 1/e) of its maximum value at evanescent wave thicknesses of 250nm and 450 nm. This corresponds to a total variation of 200nm across the projection of the beam on the coupling surface. For the embodiment of FIG. 1 and the configuration shown in FIG. 7, the output beam is in the coupling table when the total projection beam length is 110 μmElectric field amplitude of projection on surface from distance peakDown to about 37% of its maximum value. From broad theory and detailed overlap integration in the prior art, it can be concluded that this match will cause a high degree of overlap between the output and input beams. Thus, the optimum slope of the linear variation gap is 1.8 × 10 at 200nm/100 μm^-3rad is 0.1. It is noted that this condition also applies for a relatively small (but still achievable) free space diameter of 63 μm. If silicon nitride (fig. 8, n ≈ 2.0) is used instead of a silicon oxide evanescent coupling layer, the optimum slope increases well up to 0.13 ° for a free-space beam with a diameter of 63 μm.

Fig. 10 shows the optimal wedge angle as a function of free-space beam size for the exact same calculation as used for the beam size in the calculation shown in fig. 9. It is noted that the optimum wedge angle increases by a factor of 6-7, from 0.02 for a free-space beam size of 360 μm to 0.10 for a free-space beam size of 63 μm. Also, the tolerance improvement is mainly due to the need to maintain precise thickness variation over a smaller distance range for smaller beam sizes.

Since the required wedge angle for both constant and sloped thickness evanescent coupling layers is relatively small, an increase in the wedge angle significantly improves the producibility of the final device. As can be seen from fig. 9 and 10, the tolerance of the required wedge angle starts to improve as the free-space beam diameter decreases below 200 μm. More benefits are obtained when the free-space beam size is reduced below 100 μm.

While the foregoing discussion indicates that reducing the beam size can significantly improve the manufacturability of the device for a number of reasons, the size and layout of the prism coupler (and any input optics before) limits the minimum beam size compatible with this layout. Small diameter beams diverge rapidly over a relatively small propagation distance. A common quality factor for this distance is called the Rayleigh range (denoted herein as z)_R) And by the relationship

Where n is the refractive index of the medium through which the beam propagates, and the other symbols are as defined above. Physically, the rayleigh range roughly corresponds to the distance over which the beam remains collimated. When using a prism structure to transmit light from an external light source to the waveguide, in order to obtain high coupling efficiency, the beam waist must be located in the vicinity of the projection of the input beam on the prism coupling surface. The light beam must travel some distance within the silicon prism coupler (typically within the air or other input optics) before intersecting the prism coupling surface. If the beam size is too small, the path length allowed in air, input optics and silicon will be too small to be practically achievable. An example calculation will be described in detail below in the context of the device configuration in fig. 1.

If the base dimension of the prismatic structure shown in FIG. 1 is 0.45mm (horizontal along the deepest portion of the V-groove to the corner edge created by the etching process), and light having a wavelength of 1550nm is emitted from the silicon prism coupler at a θ of 45.5 °_SiValue emission, the corner beams from the input prism face to the prism structure and prism coupling surface must travel a path length of about 400 μm. The launch distance prior to the prism facet must be included in the calculation of the position of the beam waist. The emission distance includes the path length of the beam in air, and the thickness of the optical element used to pre-condition the beam. The beam path prior to the input face ranges from 1mm (a reasonable manufacturing tolerance for device alignment) to several mm, depending on the number of components required. Because the index of refraction of air (and more generally the index of refraction of the input optics) is much lower than that of silicon, and the path length preceding the prism facets typically exceeds the path length in the prism structure, rayleigh range calculations are largely determined by the launch preceding the input prism facets. Using z as given above_RThe relationship of (a) gives a rayleigh path of 0.2mm in air and 0.7mm in silicon for a beam diameter of 20 μm. For a larger beam diameter of 63 μm, the rayleigh path in air is about 2.1mm and in silicon is 7.3 mm. For a beam diameter of 100 μm, the Rayleigh path in air is about 5.1mm, siliconMedium 17.6 mm. In order to obtain a transmission distance in air of the order of a few millimeters, calculations have shown that it is feasible to use beams with dimensions of the order of 60-100 μm.

Because typical micro-optical elements such as polarizing beam splitters, waveplates, and micro-wedges or prisms can be up to 0.5mm or less thick, some elements can be used to shape, control, and adjust the polarization direction of the beam after the collimating lens. Thus, a beam size of 60-100 μm meets the requirements of a beamlet, a patterning assembly and an input column of micro-optical elements. To simplify packaging and other assembly issues discussed above, it is appropriate to select a design range of input beam diameters on the order of 60-100 μm.

A final consideration regarding the input beam size is that it is the lower limit of the beam size imposed by the prism coupler, evanescent coupling layer and waveguide. The properties of these three elements determine the angle, θ, of the beam within the prism_SiAnd thus directly affect the projection of the beam onto the prism coupling surface,(see FIG. 2). In addition, the prism coupler material and geometry will be in accordance with the relationshipDetermines how the beam is refracted at the input prism face. It is noted that, in general, due to the refraction at the inclined plane and the projection at the surface of the coupling plane,

defined as the beam diameter in free space). Typically, the projection of a light beam on a prism coupling surface

One to three times larger than the free space beam diameter.

Fig. 11 shows the magnification of the beam on the prism coupling surface resulting from these effects for different device layer thicknesses and over the full telecommunications wavelength range of fig. 1. It can be seen that in most cases the beam is amplified by a factor of 1.6-2.0 along the propagation axis. Is biggerAnd its more rapid increase (e.g., for theta)_pr54.74 deg., W1.0 μm) corresponds to an increase in the refractive effect at high oblique angles of incidence on the prism facets. For the same reason, these structures are not very suitable from an assembly point of view. Thus, from practical considerations, it will be assumed that the beam size at the prism coupling surface is increased by a factor of 1.4-2.4 relative to the free-space value. Also, note that by selecting a particular waveguide thickness and prism angle (e.g., W ═ 1.7 μm, θ)_pr54.74), the projection of the input beam on the coupling surface may be substantially wavelength independent. In this way, a suitably small beam size can be obtained for any wavelength in the wavelength range, thereby simplifying the design of the device. This allows a given prism wafer/evanescent coupling layer/waveguide configuration to be used with high coupling efficiency over a much wider wavelength range than an arbitrary device configuration.

It should be understood that a variety of different components may be designed and mounted to generate, transmit and condition the optical signal in order to achieve the desired beam characteristics of the present invention. The following description includes exemplary configurations of multiple light sources and optical columns that provide a convenient interface for a device similar to that of fig. 1.

Further, for some applications, the characteristics of the prism coupler, evanescent coupling layer, and waveguide may be selected to simplify interfacing with external light sources or receiving elements. In particular, some of the components used to transmit and condition the input optical signal may be designed inside the prism coupler wafer or chip, thus reducing the total number of individual components and simplifying the assembly process. By selecting appropriate materials, thicknesses and geometries for the evanescent coupling layer and the waveguide, favorable launch geometries and beam shapes can be achieved, as well as simplifying the assembly process.

Laser diodes are typical light sources commonly used in optoelectronic devices using telecommunication wavelengths (1.1-1.65 μm). Many infrared laser diodes typically include a multilayer structure of gallium arsenide-based or indium phosphide-based materials with light emitted from cleaved edge facets of the laser chip (referred to in the art as edge-emitting laser diodes). The laser diode can be used directly in the form of such a chip or, as in many of the packaging techniques established in the prior art, the laser chip can be connected to an output fiber through a series of optical elements. A second type of typical laser diode is known in the art as a vertical cavity surface emitting laser, or VCSEL. An infrared VCSEL comprises a multilayer structure (using gallium arsenide, indium phosphide or indium gallium arsenide based materials) in which light is emitted perpendicular to the layer stack and through the top surface of the device.

For some applications, it is desirable to use free-space optics to pass from the laser chip into the prism structure. Direct coupling with the laser can enable very compact packaging and provide a high degree of polarization control. However, since the emitting surface of the laser is small and the infrared wavelength is long, the output beam may be severely diverged. Edge-emitting laser diodes operating in the 1300-1600nm range typically have a FWHM beam divergence of about 32-50 in the direction perpendicular to the junction and a FWHM beam divergence of about 10-25 in the direction parallel to the junction.

Because of the large and anisotropic beam divergence, at least two lenses are required to achieve efficient free-space beam collimation with high wavefront quality. In one lens component, a pair of crossed cylindrical lenses is used to correct astigmatism and provide collimation in the fast and slow axes. To effectively collimate the highly divergent, or "fast" axis, the first cylindrical lens is typically made of a graded index material (referred to in the art as a "GRIN" lens). Second cylindrical lenses that collimate the less divergent or "slow" axis can be made from a variety of optically transparent materials because lens shaping by itself is insufficient to provide collimation. The diameter of the outgoing input beam in a typical laser diode followed by a micro GRIN rod lens can be selected to fall in the range of 40 μm to several mm. In a second configuration, the first lens is used to reduce the divergence angle perpendicular to the junction direction until its value is equal to the divergence angle parallel to the junction direction, to round the beam and correct astigmatism. Such lenses are sometimes referred to as "laser diode correctors" or "circularizers". The second lens can now be a conventional collimating microlens (to reduce the beam divergence to approximately 0), such as a plano-concave or aspheric mirror, and can be made of a variety of optically transparent materials. The advantage of the arrangement in the second is that only one, rather than two, special lenses are required. The output beam diameter exiting a typical laser diode followed by a correction lens may be selected to fall within the range of 100 μm to 1 mm.

The VCSEL emits a moderately divergent beam and the divergence angle covers a range of 29 ° (for a lens type component) to 18 °. The lenses used may be conventional collimating microlenses (to reduce the beam divergence to approximately 0), such as plano-concave or aspherical mirrors, and may be made of a variety of optically transparent materials. To obtain collimated beams of small beam diameter, integrated microprisms can be incorporated as part of the VCSEL structure itself. Thus, for a VCSEL active area of 3 μm, collimated beams of 100-200 μm diameter can be obtained. Although VCSELs at mid-infrared wavelengths (1270-.

In other applications, a length of optical fiber may be used as a conduit for transferring light from a laser light source to a prism coupler. If the laser source is located in a separate enclosure with a fiber output, the prism-coupled waveguide device must be provided with an input fiber component that can be directly connected to the output of the laser source. (if there are multiple fiber arrangements between the laser source and the prism-coupled waveguide arrangement, the input fiber component of the prism-coupled waveguide arrangement must be connected to the terminal fiber output of the link). If the laser light source is incorporated into the same package as a prism-coupled waveguide device, it may still be advantageous for some applications to use interleaved optical fibers between the laser chip and the prism coupler. For example, a wider range of collimated beam sizes and shapes can be achieved using a specially terminated optical fiber. This special termination can be applied to the end of the fiber closest to the prism coupler and typically involves shaping one end of the fiber or fusing a microprism directly to the end of the fiber. By varying the size and radius of the fiber end or the curved portion of the lens, a collimated beam of minimum spot size (also referred to as a "beam waist") can be obtained at a user-specific working distance. Fiber collimators with beam waist diameters in the range of 15 μm to 100 μm can be manufactured in this way using current process technology. The laser source may be connected to the other end of the fiber using a lens assembly as described in detail in the prior art. Thus, for the configurations of fig. 13, 14 and 15, a beam of diameter 60 can be produced from the fused lens/fiber assembly for both fiber and laser input.

Although the lens component provides the necessary beam collimation, it is still necessary to ensure that the beam is in the proper polarization state before entering the prism. While both Transverse Electric (TE) and Transverse Magnetic (TM) polarization states can be efficiently coupled into the waveguide, at a particular θ_SiAt this value, only one polarization state can be efficiently coupled. Since the edge-emitting laser diode emits a beam whose polarization state is stable and known, a microwave plate can be used to rotate the polarization state to the appropriate state. For some applications, the waveplate may be omitted entirely by selecting an appropriate polarization state consistent with emission from the edge-emitting diode. If the input beam passes through a polarization maintaining fiber, the fiber may be rotated during assembly to ensure that the proper polarization state is achieved, and thus, again, no additional polarizing optics are required.

However, the polarization state of the VCSEL is not exactly known. In particular, the polarization state may not change over time, but the direction is unknown, or conversely, the polarization state may change over time or the laser drive current. Similarly, if a non-polarization-maintaining input fiber is used, the polarization state of the light will be uncertain and will drift over time. An element used in prior art optical circulators may also be used in the present invention to obtain the correct polarization state, as shown in fig. 12. The input beam is transmitted to a birefringent element 50 which can separate a single input beam into two polarized beams: one beam is in the desired polarization state and the other beam is in a polarization state perpendicular to the desired polarization state. Because the refractive indices of the light are different for the two polarization states, the two light beams initially travel in different directions within the device 50. The beam in the desired polarization state continues to propagate within a medium that does not affect its polarization state. However, the beam having a polarization state perpendicular to the desired polarization state passes through a second birefringent element, i.e. its polarization state is rotatedA beam direction control element 52 turned 90 degrees to the desired polarization state. The final output is two separate beams, slightly offset from each other, and all in the desired polarization state. In most applications, the two components 50 and 52 are bonded together to form an optical sub-assembly that is easily aligned and manufactured. Natural birefringent materials (e.g. YvO)₄Quartz, rutile, or lithium niobate) or artificial birefringent elements (such as sub-wavelength diffractive optics). If the polarizing components are oriented such that both beams impinge on the prism face 14 at exactly the same angle of incidence, both beams can be efficiently coupled into the waveguide layer 12. For some applications, the light beams may be recombined after entering the waveguide layer. Recombination is readily accomplished by appropriate guiding structures within SOI waveguide layer 12 itself. Once the input beam is collimated and the desired polarization state is achieved, the optical signal must be launched from the prism face at the appropriate angle of incidence if the desired wavelength of light is to be efficiently coupled into the waveguide. For embodiments such as in FIG. 1, the beam may be directed at θ_airAngular emission into the prismatic structure, or a small optical element may be used to redirect the input beam to an input angle θ on the prismatic structure_air. For packaging reasons, it is often convenient for edge-emitting diode sources, fiber-optic inputs, or Vertical Cavity Surface Emitting Lasers (VCSELS) to emit light parallel to the wafer (θ for direct emission)_air-35.3 °). For external light sources, e.g. VCSELs, the same applies to emission perpendicular to the waveguide (θ in direct emission)_air54.74 °). As can be seen from fig. 4, by choosing a suitable waveguide thickness for a given wavelength, suitable emission conditions can be chosen. However, for some designs, the required waveguide thickness for a particular launch angle may be incompatible with competing device requirements. For these reasons, it is appropriate to package some beam direction control optics in the vicinity of the light source. In addition to angle selection, beam direction control optics may be used with other alignment techniques (e.g., positioning of the light source relative to the prism) to ensure that the beam is properly (translationally) positioned on the prism.

Fig. 13 and 15 detail an exemplary method of directing a light beam from an edge-emitting diode or optical fiber to a prism face. In fig. 14 and 15, collimated free space beams from an edge emitting diode or fiber are directed to a micro-optic prism or wedge. The magnitude of the beam deflection increases with increasing refractive index and wedge angle of the micro-optics. A similar micro-optic may be used at the output end to direct the output beam to a receiving fiber. Alternatively, a diffractive optical element such as a linear phase grating may be used as the beam direction control element. Diffractive optical elements are very effective in beam steering applications because the dispersion of well-designed gratings can be large, allowing large polarization angles (up to 60 °). Another advantage is that more complex diffractive optical elements can perform more than one optical function simultaneously, providing better performance with fewer elements. As an example, in addition to being a beam direction control element, diffractive optical elements may be used for wavefront correction to improve wavefront quality.

In fig. 13, a micro mirror 54 fabricated using micro-electro-mechanical system (MEMS) processing is used to reflect light to the appropriate angle of incidence θ_air. In the example shown in fig. 13, micro-hinges 56 fabricated by silicon micromachining methods hold the mirror at the correct angle and position. One benefit of using this technique is that the position and angle of the micro-mirrors 54 can be manipulated and adjusted so that θ_airAnd the position of the beam relative to the etched corners can be adjusted to maximize the light transmitted through the waveguide. As before, the same structure is used on the output side to direct the output beam to the receiving fiber.

Fig. 14-19 illustrate specific input and output optical configurations that can be connected with prism-coupled waveguide devices with high coupling efficiency. Although a particular optical element (e.g., the lensed fiber 60 of fig. 15) may be depicted in only one embodiment, it is to be appreciated that a given element may be readily implemented in a variety of different embodiments. Thus, the embodiments detailed in FIGS. 14-19 are merely examples in nature and do not present a possible configuration in detail.

Fig. 14 and 15 show two conventional fiber-optic pigtail optoelectronic package configurations that are connected to prism-coupled waveguide devices. Although the prism structure and the SOI device wafer are joined to form one component, the packaging of the input and output optical columns may comprise a single component. In this case, the optical element is placed and aligned in a holder on a separate carrier, which in turn is connected and aligned with the prism/SOI device waveguide assembly. Alternatively, if the prism structures are fabricated on a silicon wafer, additional masking and etching processes can be used to define the recesses that mount the free space elements in the surface of the silicon wafer opposite the attachment surface. In both cases, grooves of a size approximately equal to the outer dimensions of the optical elements are machined in the substrate material. The free-space optical element is then positioned, aligned and fixed in a specified position in the groove. In fig. 14 and 15, optical signals are introduced into and removed from the assembly via a single optical fiber (referred to in the art as a "pigtailed fiber").

In fig. 14, there are two important independent devices in the pigtailed fiber optic package. The input fiber optic interface and the output fiber optic interface are on one side and the other side of the assembly, respectively. The embodiment shown in fig. 14 uses a polarization maintaining fiber 70 to ensure that the correct polarization state can be achieved without other stray polarizations. A tiny optical lens (corresponding to a micro-sphere, a micro-GRIN lens, or a micro-sphere lens) is used to collimate the diverging light from the fiber, and the collimated beam is then directed to a beam steering element 74 which deflects the beam at an angle such that it is incident at an appropriate angle of incidence θ on an entrance face 76 of a prism 78_airAnd (4) incidence. If the beam steering element 74 is further positioned on a separate sub-mount and can be rotated through an angle as shown in fig. 14, the angle of incidence can be adjusted during assembly and is fixed during the lifetime of the device. At the output of the device, the output beams pass through the same sequence of optical elements in reverse order. The use of polarization maintaining fibers allows the configuration of fig. 14 to be used as a bi-directional system, although polarization maintaining fibers are not necessarily required at the output of the device.

The embodiment in FIG. 15 and the class in FIG. 14Similarly, an assembled device with the output port and input port on the same side is shown. This unique configuration is advantageous when the overall size of the assembly needs to be kept small. As shown in fig. 15, the direction of beam propagation is reversed by reflective optical elements located within the waveguide layer of the SOI wafer. The optical signal is introduced through an optical fiber 80 at the bottom end of the module. In this configuration, a micro-lens 82 is fused directly to the fiber 80, thus achieving a well-collimated beam with a single sub-assembly. Since the beam of light is of unknown polarization after exiting the lensed fiber, polarization control element 84 is used to convert the incident beam of light into two beams of light having the desired polarization. The polarization control element 84 is oriented such that the two outgoing beams are horizontally displaced (i.e., the plane containing the two beams is parallel to the wafer plane). Because the separation between the two beams is small, on the order of hundreds of microns, the two beams can be deflected by the same beam direction control element 86. The two beams of light are at the same angle theta_airTo the entrance face 88 of the prism 89 and coupled into the waveguide layer 12 of the SOI wafer. The two light beams, which are phase-shifted with respect to each other, are then recombined into a single light beam by means of optical elements located in the waveguide layer. After passing through the remaining opto-electronic structures in the SOI waveguide layer, the emerging light exits the output prism face and propagates into a similar optical output column. However, the polarization control element at the output end may be omitted unless it is desired to obtain a further unpolarized output beam.

Fig. 16 shows an alternative embodiment. Wherein a set of laser light sources 90 is directly integrated in one assembly. Because VCSELs emit light through the outer surface and can be small in size (about 100-. As shown in fig. 16, beam collimation and beam direction control may be achieved by an array of refractive lenses 92 and diffractive lenses or beam steering elements. The size of the elements in the lens stack can range from a few microns to a few millimeters. Most compact structures can be achieved by etching the control prisms and/or collimating lenses directly into the VCSEL wafer itself. So that all beams are at exactly the same angle theta_airTransmission ofTo a bank of prisms 94 and then coupled into waveguide 12 and exit at output face 95 of prism bank 94. A similar set of lenses and diffractive elements 96 are used to deflect, shape and focus the beam onto the receiving end array of optical fibers 98. Alternatively, side-emitting diodes may be used in a similar configuration, as shown in FIG. 17, provided the pitch of the array is sufficient to accommodate a slightly larger side-emitting device. Referring to fig. 17, an embodiment utilizing an edge-emitting laser diode 91 further utilizes a laser diode collimating lens array 93 positioned at the output of the edge-emitting laser diode array 91 where the collimating lens array is used to provide a suitable signal profile to the beam direction control element 92. Fig. 18 shows another version of the embodiment of fig. 17 with a lensed fiber array 97 positioned at the output of the collimating lens array 93.

If it is desired to reduce the total number of components and the alignment steps, the desired optical components can be machined in a silicon prism wafer or mold, as shown in FIG. 19. In this configuration, the light beam enters the silicon prism wafer 100 through the surface of any user-specific prism wafer 100, rather than directly at the appropriate angle θ_airThe incident beam is transmitted to an etched "hypotenuse" prism input face. In the example of fig. 19, the beam enters through a surface 102 of the prism wafer 100 that is opposite a surface 104 (prism coupling surface) that is connected to an SOI wafer 106. As the beam propagates in the prism wafer 100, it encounters a series of surfaces that change its direction of propagation until the desired launch angle in silicon, θ, is achieved_Si. These surfaces may consist of the top surface 102 and the bottom surface 104 of the wafer 100 or any other surface formed by an etching process. For a beam propagating in a silicon wafer, total internal reflection is obtained at these surfaces over a wide range of angles of incidence due to the high refractive index of silicon. For an air-silicon interface (assuming air index n ≈ 1 and silicon index n ≈ 3.5), the required angle of incidence must be greater than the critical angle for total internal reflection 16.6 °, whereas for a silicon-silicon nitride interface (assuming silicon nitride index n ≈ 2), the required angle of incidence must be greater than 34.8 °. If the angle of incidence is less than the critical angle, through a portion 10 of surface 1028 gold plating as a mirror, still very high reflectivity can be achieved. Because the thickness of the silicon wafer is relatively small, about 500-700 μm, many different reflecting surfaces are still encountered when the light beam travels a relatively short physical distance (about a few millimeters) within the silicon wafer 100. Thus, the silicon prism wafer itself can be used as a compact, low-loss beam-steering element.

In the simplest configuration, a prism wafer is used to (1) direct the beam to an appropriate angle θ_SiAnd (2) coupling the light beam into the waveguide. After being launched into through the top surface of the silicon prism coupler, the beam is refracted in the silicon wafer and incident on the etched surface. If the angle of incidence on the etched surface is large enough, total internal reflection will occur at this surface. Conversely, a sufficiently small angle of incidence results in totally reflected light being emitted towards the top surface. When the angle of incidence on the top surface is sufficiently large, the beam will again be totally internally reflected at the top surface. After total internal reflection at the surface of the top layer, the light beam is emitted at a suitable emission angle theta_SiEmitting towards the optical coupling region. This method of controlling the beam is very effective because it allows a wider emission angle θ to be obtained than if the beam were directly emitted from the top of the silicon prism coupler into the optical coupling region_Si(due to the high refractive index of silicon). Some additional optical functionality may be added by adding optical elements to the surface of the top layer in the direct path of the light beam. In the example of fig. 19, the optical element may be located at the initial entry point of the light beam into the top surface of the silicon prism coupler, or at the point of total internal reflection on the top surface. These optical elements may include, but are not limited to, the following: a refractive or diffractive lens to collimate a diverging input beam, or other diffractive optical element that provides additional beam steering, beam shaping, wavefront correction, or polarization control capabilities.

The use of these refractive and diffractive elements provides additional optical functions such as collimation and polarization control to be integrated in a silicon prism coupler. Microlenses can be fabricated in silicon using a combination of conventional lithography, photoresist channeling, plasma etching, diffusion, and implantation techniques. Alternatively, grayscale lithography techniques can be used to create more complex aspherical lens shapes. A number of diffractive elements, i.e. grating structures, can be fabricated in a silicon substrate using conventional lithographic techniques. However, higher resolution lithographic techniques (e.g., electronic lithography) may be required to obtain subwavelength grating structures that can be used as polarization control elements.

By carefully considering how the beam shape affects device performance, a significant improvement in the coupling efficiency of the exemplary device of fig. 1 can be obtained. There are three main interfaces to consider: (1) the shape of the free-space input beam from the input optics; (2) the exact form of the evanescent coupling layer; and (3) the shape of the free-space output beam and the output receiving optics.

In general, the coupling efficiency can be determined by overlap integration, which is well known from the prior art. From this integration, it can be concluded that 100% coupling efficiency can only be achieved if the input and output beam shapes match.

For the exemplary embodiment of fig. 1, three correlation overlap integrals need to be considered:

(1)η₁the beam shape of the light source being relative to the desired beam projection on the prism coupling surface

(2)η₂The shape of the beam on the coupling surface of the input prism relative to the beam transmitted from the coupling surface of the output prism

(3)η₃The shape of the beam transmitted from the output prism coupling surface is relative to the desired beam shape of the output receiving optics.

The coupling efficiency is first discussed herein in the context of the preferred embodiment shown in fig. 20. It can be seen from this example that the input and output silicon prisms are separated from the silicon waveguide by an evanescent coupling layer of constant thickness and constant refractive index.

For the laser input and fiber output of a standard pigtailed fiber, the total coupling efficiency of the embodiment in fig. 20 is defined as:

η＝η₁η₂η₃≈64％

coupling efficiency eta₁Determined by the loss caused by generating a well collimated gaussian beam from a light source such as an optical fiber or laser input. Eta if the optical device is integrated in the light source (e.g. using a lensed fibre or laser source incorporating collimating and beam-shaping devices)₁Will be very high, approaching 100%. Coupling efficiency eta₂Determined by the ratio of the power of the free-space output beam of the prism to the power of the free-space input beam. However, for a free-space Gaussian input beam, η₂Not exceeding 80%. As is known in the art, eta is the difference in modal intensity of the input and output beams of this embodiment₂Are subject to limitations. The intensity of the input beam along the direction of propagation is gaussian, while the intensity distribution of the beam from the output prism along the direction of propagation is exponential (see fig. 20(b) and (c) plots of amplitude versus position). Finally, for the same reason, coupling to the output light efficiency η₃About 80%. Again, this is due to the incomplete overlap of the index-enveloping free-space beam exiting the prism with the desired gaussian beam at the output end of the fiber. Thus, the number of the first and second electrodes,

η＝η₁η₂η₃insertion loss of 0.64 or about 2dB ≈ 1 × 0.8.

It is clear that if the coupling efficiency of the embodiment shown in fig. 20 is to be improved, further beam shaping is required to shape η₂Or η₃The yield is improved to more than 80 percent. For a light source, such as a laser, the most common beam shape is a gaussian or square wave type distribution. It can be proved that the coupling efficiency generated by the two waveforms is eta ₂80%. To increase eta₂It is clear that it is necessary to require an input beam with an intensity distribution close to the exponential envelope of the beam exiting the output prism. To achieve the above, one approach is to use a "half-Gaussian" input waveform. As shown in FIG. 21, the initial input Gaussian beam is incident on oneOn the wavesplitting structure 120 and the gaussian beam center is aligned with the intersection of the surfaces of the wave splitters. The two half beams are then transmitted to a prism (not shown) and coupled to a waveguide. One of the half beams may be inverted using suitable optical elements, such as a fold mirror 122. It is important to ensure that the two half beams do not recombine before entering the waveguide, which would otherwise cause strong interference fringes that modulate the intensity distribution of the input beam. In this case, η₂The overlap integral of the half gaussian waveform and the output exponential waveform is 97%. The coupling efficiency eta can be reduced in the process of converting into two half Gaussian beams₁. It will be apparent that if there were any significant advantage in adjusting the incident beam waveform, there would be η₁Is more than 83 percent. Since the standard method for generating a more gaussian distributed beam from an input beam can significantly reduce the intensity, the coupling efficiency η₃It will be more difficult. For the configuration shown in fig. 20, it is expected that the maximum total coupling efficiency η can reach 80% if there is additional shaping of the incident beam; only about 64% can be achieved without additional incident beam shaping.

A higher and more easily achieved overall coupling efficiency η is obtained in the embodiment shown in fig. 22. In this embodiment, the silicon prism is separated from the silicon waveguide by an evanescent coupling layer having a thickness that varies linearly with position. At the input face, the thickness of the evanescent coupling layer when energy is first transferred to the waveguide layer is less than the thickness of the evanescent coupling layer when most of the energy has been transferred to the waveguide layer. At the output face, the thickness of the evanescent coupling layer has a relatively large value when most of the energy is still within the waveguide. And is reduced when the energy of the light beam is coupled out of the waveguide layer and into the prism. This approach can achieve higher coupling efficiency than the embodiment shown in fig. 20, since an approximately gaussian beam waveform can be maintained throughout the optical path from the incident light source to the exit fiber interface.

As mentioned above, a standard incident beam from a laser source or optical fiber has a high coupling efficiency eta₁A collimated gaussian beam. To improve the coupling efficiency eta₂Free-space output light waves exiting from the prismThe shape must be closer to a gaussian beam. Although the output light is not normally a true gaussian beam, the coupling efficiency can still exceed 80% if the overlap integral of the new output light with the input gaussian beam is greater than the overlap integral of the exponential envelope with the input gaussian beam. It is known from the technical literature that one way to make the output beam more gaussian is to let the thickness of the evanescent coupling layer vary gradually with the direction of propagation of the beam. If the evanescent coupling layer thickness is constant, the beam will couple out of the waveguide with the same coupling strength into the prism at all points, with the result that the output beam waveform can be written as g (z) · exp (- α z) (see fig. 20). The coupling strength decreases with increasing evanescent coupling layer thickness and increases with decreasing evanescent coupling layer thickness. If the output beam waveform is closer to the input beam waveform, the coupling of light exiting the prism surface first will be weak, so most of the light will remain in the waveguide layer. To ensure this, the evanescent coupling layer must be sufficiently higher than the optimal coupling value. To achieve this, the coupling strength of the light must be increased so that most of the light can be extracted, forming a peak that outputs a "gaussian" beam. Therefore, this portion of light must act as an interface where the evanescent coupling layer approaches the optimal thickness. In this way, most of the energy is transferred out of the waveguide and exits the system entirely through the output prism. Although the coupling strength continues to increase with decreasing evanescent coupling layer thickness, the amount of light exiting the prism begins to decrease as the energy of the light within the waveguide layer continues to decrease. This allows to obtain an output light waveform that more closely follows a gaussian distribution. Although the output beam is not normally a true Gaussian waveform, the overlap integral η of the new output beam with the input Gaussian beam₂About 97%. It is important that the slope of the evanescent coupling layer must be of a suitable value to produce the desired beam profile. The determination of this slope has been discussed in the previous section of beam size.

Since the free-space output beam from the prism has the Gaussian waveform required for the output fiber interface, the coupling efficiency η₃Can now be very high. As described above, the overlap integral eta of the approximately Gaussian beam emitted from the output prism and the Gaussian mode that is characteristic of the optical fiber₃Can be as high as 97%. If desired, collimating and rounding optics similar to those used to shape the laser diode beam before the light is transmitted to the fiber optic cable may be used to reduce any output beam divergence or ellipticity. Finally, a lens for converging the collimated beam to the optical fiber is always required. This lens may be an integral part of the lensed fiber or collimator assembly, or a separate spherical lens or gradient index lens used for this purpose in conjunction with a conventional fiber termination.

The overall coupling efficiency of the embodiment shown in FIG. 22 can be expressed as:

η＝η₁η₂η₃and ≈ (1) ≈ (0.97) ≈ 0.94, or about 0.3dB loss. This may be the simplest way to achieve efficient end-to-end coupling from a laser or fiber-based input to a fiber output, and this technique may be used for other applications that are more sensitive to insertion loss. However, the improvement in coupling efficiency must be balanced against the additional requirements of the grayscale lithography required to obtain evanescent coupling layers of varying thickness. It should be noted that any evanescent coupling layer structure that produces a similar approximately gaussian or more output beam from the output prism can achieve high coupling efficiencies of 94%. That is, the improvement of the coupling efficiency is not limited to the evanescent coupling layer whose thickness linearly varies. For purposes of the present invention, "more gaussian" may be defined as any output beam waveform that improves the known overlap integral. For example, it can be shown that an evanescent coupling layer with a thickness that varies logarithmically with distance along the waveguide produces a beam that is closer to gaussian than an evanescent coupling layer with a linearly varying thickness. (plotting the coupling efficiency on a logarithmic scale as a function of layer thickness will result in a more symmetrical coupling efficiency peak or curve). Fabricating such thickness structures is generally more complicated, but such structures are still necessary if the total coupling efficiency of 94% is not sufficient to meet the insertion loss requirements of the application.

Claims (46)

1. An optical coupling device for providing a signal path into and out of a silicon optical waveguide formed in a surface layer of a silicon-on-insulator (SOI) wafer including a silicon optical waveguide layer formed on an insulating layer on a silicon substrate, the optical coupling device comprising

A silicon-based prism coupler for intercepting an input beam from a light source, the silicon-based prism coupler being permanently affixed to the SOI wafer such that a first surface of the prism coupler is substantially parallel to and contiguous with the flat surface of the SOI wafer, the silicon-based prism coupler having a refractive index equal to or greater than the refractive index of the silicon optical waveguide;

a free-space micro-optical input element interposed between the light source and the silicon-based prism coupler for collimating, shaping, and directing the light beam to a specific entry point and angle of incidence of the silicon-based prism coupler;

an evanescent coupling region disposed between the silicon-based prism coupler and the silicon optical waveguide; and

a free-space micro-optical element disposed in the path of the beam exiting the output surface of the silicon-based prism coupler for shaping, collimating or converging the beam and directing the beam to a receiving element.

2. The optical coupler apparatus of claim 1, wherein the apparatus further comprises a light source coupled to the free-space micro-optical input element.

3. The optical coupling arrangement of claim 2, wherein the wavelength of the light source falls within the range of 1.1-1.65 μm.

4. The optical coupling arrangement of claim 2, wherein the output beam of the light source is substantially single mode.

5. The optical coupling arrangement of claim 2, wherein substantially the entire intensity of the light source falls within ± 5nm of the center wavelength.

6. The optical coupling arrangement of claim 2, wherein the light source is an edge-emitting laser diode.

7. The optical coupling arrangement of claim 5, wherein the micro-optical free-space input element after the edge-emitting laser diode comprises a first micro-optical element that reduces the divergence angle of the output beam perpendicular to the junction to a magnitude of the divergence angle of the output beam parallel to the junction, corrects for astigmatism, and produces a circular beam, and a second micro-optical element that subsequently collimates the beam.

8. The optical coupling arrangement of claim 6, wherein the micro-optic free-space input element behind the edge-emitting laser diode comprises a gradient index micro-cylindrical lens to collimate an output beam perpendicular to the junction, and a second micro-cylindrical lens to subsequently collimate an output beam parallel to the diode junction.

9. The optical coupling arrangement of claim 6, wherein the micro-optic free-space input element after the edge-emitting laser diode comprises a first spherical lens to collimate the beam, followed by a second spherical lens to focus the beam onto a receiving fiber optic component interposed between the diode and the silicon-based prism coupler.

10. The optical coupling arrangement of claim 6, wherein the micro-optic free-space input element after the edge-emitting laser diode comprises a first aspheric mirror to collimate the beam, and a second aspheric mirror to subsequently focus the beam onto a receiving fiber optic component interposed between the diode and the silicon-based prism coupler.

11. The optical coupling arrangement of claim 6, wherein the micro-optical free-space input element behind the edge-emitting laser diode comprises a micro-optical waveplate for rotating the polarization direction.

12. The optical coupling arrangement of claim 2, wherein the light source is a vertical cavity surface emitting laser diode.

13. The optical coupling arrangement of claim 12, wherein the micro-optic free-space input element after the vertical-cavity surface-emitting laser diode comprises a micro-optic collimating lens.

14. The optical coupling arrangement of claim 13, wherein the micro-optic collimating lens is a silicon microlens.

15. The optical coupling arrangement of claim 12, wherein the micro-optic free-space input element after the vertical-cavity surface-emitting laser diode comprises a micro-optic waveplate for rotating the polarization direction.

16. The optical coupling arrangement of claim 12, wherein the micro-optic free-space input element after the vertical-cavity surface-emitting laser diode comprises an optical element that converts an incident beam of light at an unknown polarization state into two separate output beams of light of the same known polarization state, with the second beam being spaced from, but substantially parallel to, the first beam.

17. The optical coupling arrangement of claim 2, wherein the light source is an optical fiber.

18. The optical coupling arrangement of claim 17, wherein the optical fiber is single mode and supports an arbitrary polarization state.

19. The optical coupling arrangement of claim 17, wherein the optical fiber is a single mode polarization maintaining fiber.

20. The optical coupling arrangement of claim 17, wherein the micro-optic free-space input element behind the optical fiber comprises a micro-optic collimating lens.

21. The optical coupling arrangement of claim 20, wherein the micro-optic collimating lens is fused to the optical fiber to form a lensed fiber.

22. The optical coupling arrangement of claim 21, wherein the collimated beam exiting the lensed fiber has a diameter in the range of 10-110 μm.

23. The optical coupling arrangement of claim 17 wherein the micro-optic free-space input element behind the optical fiber comprises an optical element that converts an incident light beam at an unknown polarization state into two separate output light beams having the same known polarization state, and the second light beam is spaced from, but remains substantially parallel to, the first light beam.

24. The optical coupling arrangement of claim 1, wherein the micro-optic free-space input element includes a refractive wedge angle made of a high refractive index material to deflect an incident optical beam.

25. The optical coupling arrangement of claim 1, wherein the micro-optic free-space input element comprises a reflective element that is movable and rotatable by an electronic actuation mechanism to impart movement and angular deflection to the incident light beam.

26. The optical coupling arrangement of claim 1, wherein the micro-optic free-space input element comprises a diffractive optical element that angularly deflects an incident optical beam.

27. The optical coupling arrangement of claim 1, wherein the evanescent coupling region has a substantially constant thickness.

28. The optical coupling arrangement of claim 1, wherein the evanescent coupling region has a wedge-shaped thickness.

29. The optical coupling arrangement of claim 1, wherein the arrangement further comprises an optical receiving element for receiving the output beam from the free-space micro-optical output element.

30. The optical coupling arrangement of claim 29, wherein the receiving element is an optical fiber.

31. The optical coupling arrangement of claim 29, wherein the receiving element is a lensed fiber.

32. The optical coupling arrangement of claim 1, wherein the input and output micro-optical elements, and the input and output surfaces of the silicon-based prism coupler are coated with an anti-reflective coating.

33. An optical coupling device for providing a signal path into and out of a silicon optical waveguide formed in a surface layer of a silicon-on-insulator (SOI) wafer including a silicon optical waveguide layer formed on an insulating layer on a silicon substrate, the optical coupling device comprising

A silicon-based prism coupler permanently affixed to the SOI wafer such that a first surface of the prism coupler is substantially parallel to and contiguous with the flat surface of the SOI wafer, the silicon-based prism coupler having a refractive index equal to or greater than the refractive index of the silicon optical waveguide;

optical elements forming integral components of the silicon-based prism coupler for collimating, shaping, and directing the input beam to a specific entry point and angle of incidence on the coupling surface of the silicon-based prism coupler;

an evanescent coupling region disposed between the silicon-based prism coupler and the silicon-based optical waveguide; and

a free-space micro-optical output element disposed in the path of the light beam exiting the output surface of the silicon-based prism coupler for shaping, collimating or converging the light beam and directing the light beam to a receiving element.

34. The optical coupling arrangement of claim 33, wherein the micro-lenses are formed in a surface of the silicon-based prism wafer other than the joining surface of the SOI wafer to collimate the incident beam.

35. The optical coupling arrangement of claim 33, wherein the diffractive optical element is machined into a surface of the silicon-based prism wafer other than the attachment surface of the SOI wafer to shape the incident beam or to diverge or angularly deflect the incident beam.

36. The optical coupling arrangement of claim 33, wherein the angled surfaces are anisotropically etched in the silicon-based prism coupler to angularly deflect the incident beam throughout internal reflection.

37. The optical coupling arrangement of claim 33, wherein the sub-surfaces in the silicon-based prism coupler are covered with a thin metal layer that acts as a reflective element to deflect the angle of the incident beam.

38. The optical coupling arrangement of claim 33, wherein the evanescent coupling region is tapered at locations where the light beam enters the waveguide from the silicon-based prism coupler at the input prism coupling surface and exits the waveguide into the silicon-based prism coupler at the output prism coupling surface, such that the light beam at all points in the optical coupling arrangement outside the waveguide of the SOI wafer has a substantially gaussian mode intensity profile.

39. The optical coupling arrangement of claim 38, wherein the substantially gaussian mode intensity profile of the output beam obtained using the tapered evanescent coupling layer enables efficient coupling of the beam to a receiving fiber.

40. The optical coupling arrangement of claim 33, wherein a thickness of the waveguide of the SOI wafer is selected such that light emitted from the light source parallel to the wafer surface and incident on the input prism face is refracted by the silicon-based prism coupler at an angle associated with a high coupling efficiency for a particular wavelength.

41. The optical coupling arrangement of claim 33, wherein a thickness of the waveguide of the SOI wafer is selected such that light emitted from the light source normal to the wafer surface and incident on the input prism face is refracted by the silicon-based prism coupler at an angle associated with high coupling efficiency for a particular wavelength.

42. The optical coupling arrangement of claim 33, wherein the arrangement further comprises a light source.

43. The optical coupling arrangement of claim 42, wherein the light source is a vertical cavity surface emitting laser diode having a suitable wavelength.

44. The optical coupling arrangement of claim 33, wherein the thickness of the waveguide of the SOI wafer is selected such that light emitted from the light source and incident on the input prism face is refracted by the silicon-based prism coupler such that the projection of the light beam onto the prism coupling surface remains substantially constant over a wide range of wavelengths.

45. The optical coupling arrangement of claim 1, wherein the thickness of the waveguide of the SOI wafer is selected such that, for the same wavelength, the thickness of the evanescent coupling region that is optimized for coupling efficiency for a given wavelength and input beam size is substantially equal to the quarter wave thickness of the material comprising the evanescent coupling region.

46. The optical coupling arrangement of claim 45, wherein the evanescent coupling layer and the anti-reflective coating of the silicon-based prism wafer are processed simultaneously in a single processing step.

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