CN102749676A - Cross waveguide based on linear tapered multimode interference principle - Google Patents

Abstract

The invention discloses a cross waveguide based on a linear tapered multimode interference principle. The cross waveguide comprises a vertical portion and a horizontal portion, each of the vertical portion and the horizontal portion is composed of a front straight waveguide area, a front tapered waveguide area, a tapered multimode interference area, a rear tapered waveguide area and a rear straight waveguide area which are sequentially connected, the horizontal portion and the vertical portion are perpendicularly crossed, and a crossed area is located in the tapered multimode interference area. The cross waveguide based on the linear tapered multimode interference principle has the advantage of being low in loss and crosstalk and small in size.

Description

A cross waveguide based on the principle of linear tapered multimode interference

technical field

The invention belongs to the technical field of optical communication, and is especially used in optical devices with a cross waveguide structure, such as a micro-ring resonant filter and a micro-ring resonant wavelength division multiplexer with a cross waveguide structure.

technical background

Silicon-based nanowaveguide is one of the important materials for manufacturing integrated optical communication devices. The refractive index difference between the core and cladding of the waveguide is very large, so it is beneficial to confine light in the core. The size of the waveguide can be made very small. It is more suitable for the requirements of high-density integration of large-scale integrated optical circuits. And because of the silicon-based waveguide, it is compatible with the current standard CMOS process. Optical waveguides are fundamental photonic interconnect devices. For sub-micron-scale silicon-based nanowaveguides, the transmission loss is very small, about 1.7dB/cm, but when two waveguides cross each other to form a waveguide with a cross structure, the scattering of light in the cross structure will bring additional loss. For the waveguide of the traditional planar cross structure, we must consider its insertion loss and crosstalk. Especially for high refractive index materials such as SOI (Silicon On Insulator), the insertion loss and crosstalk are very serious, which not only affects the performance of the cross waveguide, but also restricts its application in the field of optical devices. Therefore, the development of a cross waveguide with low insertion loss and low crosstalk is an actual demand in the application field and is of great significance. In recent years, some researchers have conducted research on the cross waveguide, especially the method of reducing the insertion loss and crosstalk of the cross waveguide. For example, a waveguide with a multi-layer vertical structure can well reduce the insertion loss and crosstalk introduced by the cross (Htakeyama, Loss-less multilevelcrossing of busline waveguide in vertically coupled microring resonator filter, IEEE Photonics Technology Letters, 2004, 16(2) :473-475), but the manufacturing process of the multilayer vertical structure waveguide is much more complicated than that of the planar waveguide, which is not only difficult to manufacture, but also high in cost. In addition, in order to reduce the insertion loss and crosstalk introduced by the cross structure, it is proposed to use multimode interference MMI (multimode interference) in the cross waveguide. The existing scheme has a cross waveguide structure based on the principle of rectangular multimode interference ( Chen, H.and A.W.Poon, Low-loss multimode-interference-basedcrossings for silicon wire waveguides, IEEE Photonics Technology Letters, 2006, 18(21):2260-2262), cross waveguide structure based on elliptical multimode interference principle (Fukazawa , T., et al., Low loss intersection of Si photonic wirewaveguides, Japanese Journal of Applied Physics, 2004, 43(2):646-647). Although these solutions can reduce the scattering loss of the cross waveguide structure and suppress crosstalk, the length of the cross structure is greater than 10 microns, which is not conducive to the monolithic integration of large-scale integrated optical circuits.

Contents of the invention

In order to overcome the problems of large insertion loss and serious crosstalk in the cross waveguide structure in the prior art, as well as the shortcomings of large structure size and difficult manufacture, the present invention provides a low loss, low crosstalk, small size based on linear tapered multi Cross waveguide based on mode interference principle.

The technical scheme that provides in order to solve the above-mentioned technical problem is:

A cross waveguide based on the principle of linear tapered multi-mode interference, the cross waveguide includes a vertical part and a transverse part, the vertical part and the transverse part are connected in sequence by the front straight waveguide area, the front tapered waveguide area, The tapered multi-mode interference area, the rear tapered waveguide area and the rear straight waveguide area are composed, the horizontal part and the vertical part are vertically intersected, and the intersection area is in the tapered multi-mode interference area.

Further, one end of the front straight waveguide area of the vertical part or the lateral part is an optical input port, and the light sequentially passes through the front tapered waveguide area, the tapered multimode interference area, the rear tapered waveguide area and the rear straight waveguide area, so One end of the rear straight waveguide region is the optical output port.

Furthermore, the front straight waveguide region, the front tapered waveguide region, the tapered multimode interference region, the rear tapered waveguide region and the rear straight waveguide region are all made of SOI material, wherein the refractive indices of silicon and silicon dioxide are respectively 3.48 and 1.46, the refractive index difference between the waveguide core and the cladding is 2.02.

The technical idea of the present invention is: the mode of the input light is changed in a cross waveguide structure based on the linear tapered multi-mode interference principle of the present invention. The single-mode input light is single-mode in the straight waveguide and tapered waveguide regions, but when entering the tapered multi-mode interference region, multi-mode, single-mode and multi-mode will appear in sequence, and the multi-mode is symmetrical, and then enters In the tapered waveguide area, it becomes a single mode again, and finally outputs in a single mode. It is noteworthy that a single-mode case occurs at the center of the cross. The linear tapered cross waveguide is based on the self-image of the waveguide's mode field from the input plane to its center and output plane. The self-image at the center of the cross-point suppresses the spread of the wavefront, thereby reducing cross-point scattering loss and crosstalk. Since the input field is in the center of the waveguide, it only excites the symmetrical even-order mode field, so in the multimode symmetric mode, the self-image is a symmetric interference.

The beneficial effects of the present invention are: 1. Two vertically intersecting tapered multi-mode interference regions are introduced at the intersection of the cross waveguide, which has a small scattering loss compared with other shapes of waveguides perpendicularly intersecting (such as straight waveguide vertically intersecting). , Low crosstalk characteristics. 2. The introduction of two vertically intersecting tapered multi-mode interference regions can reduce the size of the waveguide, which is beneficial to improve the integration degree of the optical path.

Description of drawings

Fig. 1 is a schematic structural diagram of a cross waveguide based on the linear tapered multi-mode interference principle of the present invention.

Fig. 2 is the operating wavelength obtained by finite difference time domain FDTD (finite-different time-domain) simulation between 1500nm and 1600nm, the straight waveguide of the traditional planar cross structure and the present invention based on a linear tapered multimode interference principle The crosstalk comparison diagram of the cross waveguide.

Fig. 3 is a comparison chart of the return loss of a straight waveguide with a traditional planar cross structure and a cross waveguide based on the principle of linear tapered multi-mode interference obtained by FDTD simulation.

Fig. 4 is the insertion loss comparison diagram obtained by FDTD simulation between the operating wavelength between 1500nm and 1600nm, the straight waveguide of the traditional planar cross structure and the crosstalk crosstalk comparison diagram of the cross waveguide based on the linear tapered multimode interference principle of the present invention .

Detailed ways

Further illustrate the present invention below in conjunction with accompanying drawing:

Referring to Figures 1 to 4, a cross waveguide based on the principle of linear tapered multimode interference consists of two identical parts. One of the sections consists of a straight waveguide region, a tapered waveguide region, a tapered multimode interference region, a tapered waveguide region and a straight waveguide region. The two parts intersect vertically, and the intersecting area is in the cone-shaped multimode interference area.

All domain modules are based on SOI materials with silicon and silicon dioxide having refractive indices of 3.48 and 1.46, respectively. The refractive index difference between the waveguide core and the cladding is 2.02. Since the refractive index difference between the waveguide core and the cladding is large, it is beneficial to confine the light in the core. The size of the waveguide can be made very small, which is more suitable for large Scale integration requirements for high-density integration of optical paths.

As shown in Figure 1, the width of the straight waveguide region is W ₁ , the width of the small aperture of the tapered waveguide region is W ₁ , and the width of the large aperture is W ₂ , the width of the large aperture of the tapered multimode interference region is W ₃ , and the length is L ₁ , the width of the small aperture is W ₄ , the width of the large aperture of the other tapered waveguide region is W ₅ , and the length is L ₂ .

In the cone-shaped multi-mode interference region, we can theoretically calculate the beat length of the two lowest-order modes as:

${\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}^{\mathrm{<span\; class="notranslate">\; p</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>}_{\mathrm{<span\; class="notranslate">\; p</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; kz</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the formula, β is the propagation constant, W is the aperture size, z is the transmission distance, κ=tan(θ) is the gradient rate of the cone-shaped multimode interference area, where θ is the cone angle, λ is the working wavelength, n _γ is the The refractive index size of the region. The linear tapered cross waveguide is based on the self-image of the waveguide's mode field from the input plane to its center and output plane. The self-image at the center of the cross-point suppresses the spread of the wavefront, thereby reducing cross-point scattering loss and crosstalk. Since the input field is in the center of the waveguide, it only excites the symmetrical even-order mode field, so in the multimode symmetric mode, the self-image is a symmetric interference. The self-image condition that satisfies this situation is

$\exp \left[j{({\mathrm{\&beta;}}_0-{\mathrm{\&beta;}}_1)}_{p}z\right]=\exp \left[j\frac{\mathrm{\&upsi;}(\mathrm{\&upsi;}+2)\mathrm{\&pi;}}{{3L}_{\mathrm{\&pi;}}^p\left(z\right)}z\right],$ v is even (2)

p=1,2,3,…产生自映像。For even moduli, when

p=1,2,3,...generated from the image.

It is precisely because of the self-image of the tapered multimode interference coupler that the wavefront expansion at the center of the cross is counteracted, and the scattering loss and crosstalk at the cross are suppressed.

In the structure size W ₁ is 0.3 μm, W ₂ is 0.8 μm, W ₃ is 1.1 μm, L ₁ is 3.9 μm, W ₄ is 0.8 μm, W ₅ is 0.6 μm, L ₂ is 1 μm, we have a good understanding of the present invention under FDTD The simulation is carried out, and the obtained simulation results are shown in Fig. 2, Fig. 3 and Fig. 4.

As shown in Figure 2, the working wavelength is between 1500nm and 1600nm. A linear cross waveguide based on the principle of tapered multimode interference in the present invention is more conducive to suppressing crosstalk than a straight waveguide with a traditional planar cross structure, reducing the crosstalk by 30dB to 22dB of crosstalk.

As shown in FIG. 3 , the working wavelength is between 1500nm and 1600nm, and the cross waveguide based on the linear tapered multimode interference principle of the present invention improves the return loss by 53dB to 20dB compared with the straight waveguide of the traditional planar cross structure.

As shown in FIG. 4 , the working wavelength is between 1500nm and 1600nm, and the cross waveguide based on the linear tapered multimode interference principle of the present invention improves the insertion loss by 0.9dB compared with the straight waveguide of the traditional planar cross structure.

In addition, the length of the tapered multi-mode interference region is only 3.9 μm, and the overall size of the cross-section of the present invention is 6 μm×6 μm, which greatly improves the optical path integration. Moreover, the bandwidth transmission we simulate with FDTD is between 1500nm and 1600nm, which is also the optical signal transmission band used by the wavelength division multiplexing WDM (Wavelength Division Multiplex) technology.

The content described in the embodiments of this specification is only an enumeration of the implementation forms of the inventive concept. The protection scope of the present invention should not be regarded as limited to the specific forms stated in the embodiments. Equivalent technical means that a person can think of based on the concept of the present invention.

Claims (3)

1. A cross waveguide based on the principle of linear tapered multi-mode interference, characterized in that: the cross waveguide includes a vertical part and a lateral part, and the vertical part and the lateral part are connected successively by the front straight waveguide area, The front tapered waveguide area, the tapered multi-mode interference area, the rear tapered waveguide area and the rear straight waveguide area are composed, the horizontal part and the vertical part are vertically intersected, and the intersection area is in the tapered multi-mode interference area. 2. The cross waveguide based on the linear tapered multi-mode interference principle according to claim 1, characterized in that: one end of the front straight waveguide area of the vertical part or the transverse part is an optical input port, and the light passes through the front A tapered waveguide area, a tapered multi-mode interference area, a rear tapered waveguide area and a rear straight waveguide area, one end of the rear straight waveguide area is an optical output port. 3. The cross waveguide based on the principle of linear tapered multimode interference as claimed in claim 1 or 2, characterized in that: the front straight waveguide area, the front tapered waveguide area, the tapered multimode interference area, and the rear cone Both the shaped waveguide area and the rear straight waveguide area are made of SOI material, in which the refractive indices of silicon and silicon dioxide are 3.48 and 1.46 respectively, and the refractive index difference between the waveguide core and the cladding is 2.02.

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