JP2003035832A - Polarization-independent directional coupler and optical circuit using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a directional coupler with which a problem of the polarization-dependability of the directional coupler is solved, thus integrality and a scale expansion are excellently preceded. SOLUTION: In the directional coupler having a substrate and two optical waveguides arranged close to each other on the substrate, the section of the core part of the optical waveguide is rectangular, the length of the rectangular side which is parallel to the main plane of the substrate is shorter than the rest of the sides.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a directional coupler for an optical waveguide used in the field of optical communication, and more specifically,
The present invention relates to a directional coupler having an optical coupling rate that does not depend on the polarization state of signal light.

[0002]

2. Description of the Related Art The development of multimedia communication, which requires a large amount of data transmission as seen in moving image distribution using the Internet, causes an increase in communication traffic, and at the same time further increases the capacity and speed of communication networks. The demand for higher performance and higher functionality is increasing day by day.

In the current optical communication network, electronic parts are used for signal switching (path switching) and routing (path setting) by optical-electrical conversion and electric-optical conversion. It is required to develop an optical communication network having a structure in which all communication networks including an access network are connected without converting signals into electric signals.

As an optical component required for constructing a communication system using such an optical communication network, an optical multiplexer / demultiplexer,
There are optical branching couplers, optical switches, optical filters, and the like.

Among these optical components, the waveguide type optical component is expected as an optical component which can meet the demands of mass productivity and large scale. In particular, an optical waveguide made of silica glass formed on a silicon substrate has features such as low optical loss, high stability, and excellent matching with an optical fiber. It is attracting attention as the most promising means for constructing a practical optical circuit.

In order to realize a waveguide type optical component on a practical level, it is necessary to operate as a waveguide type optical element without depending on the polarization characteristic of the signal light propagating in the optical waveguide (polarization independence). It is essential. In particular, in a directional coupler, which is a basic element for realizing optical components such as optical switches and optical filters, the polarization independence reduces loss polarization dependency (PDL) as an optical component and extinguishes light. It is an important element characteristic for achieving the ratio improvement.

10 to 12 are schematic views showing an example of the configuration of an optical circuit including a conventional directional coupler. In the conventional optical circuit, for example, as shown in FIG. 10, two linear optical waveguides 115 arranged in parallel and close to each other are provided.
And the optical coupling section 121 having 116 and the optical coupling section 1
It is composed of input ports 111 and 112 and output ports 113 and 114, which are coupled to 21. Figure 11
10 is a sectional view taken along the line AA 'shown in FIG. 10, and FIG. 12 is a sectional view taken along the line BB' shown in FIG.

The optical waveguides 115 and 116 have a silica-based glass cladding layer 102 provided on the surface of the silicon substrate 101, and a silica-based glass core portion 103 provided inside the cladding layer 102.

In the optical coupling part 121, the guided modes of the signal light are coupled to each other as the signal light propagates in the two optical waveguides 115 and 116 which are close to each other, and the light of one optical waveguide is coupled. The power gradually moves to the other optical waveguide and then gradually returns to the original waveguide again. The movement of such optical power between the optical waveguides is called optical coupling of a directional coupler, and an optical circuit utilizing this phenomenon is used to combine or demultiplex optical signals.

The rate of optical coupling is called the optical coupling rate, and means the rate of optical power that moves from one optical waveguide to the other optical waveguide. The optical coupling rate is determined by the waveguide width in the optical waveguide of the optical coupling section 121, the waveguide spacing, and the length of the optical coupling section (coupling length). Specifically, the narrower the width of the waveguide and the narrower the distance between the waveguides, the stronger the optical coupling, and a high optical coupling rate can be obtained even in the optical coupling section 121 having a short coupling length.

The cross-sectional shape of the core portion 103 is determined in consideration of the propagation loss based on the normalized frequency (V value) satisfying the single mode, and the aspect ratio is usually 1.0: 1.0 to.
It is a square or a horizontally long rectangle with a ratio of 1.0: 1.5.

In the case of an isolated optical waveguide, birefringence can be controlled by changing the width of the waveguide. For example, it has been reported that array waveguides and interference arm waveguides that compose an arrayed waveguide grating and a Mach-Zehnder interferometer control the polarization dependence of interference characteristics by changing the waveguide width (Y. Inoue et al., OFC 2001 Technical Digest WB4
-2).

However, like a directional coupler, two
In the case of an optical circuit in which the optical waveguides of the book are arranged close to each other, the stress distribution and the like in the core portion of the waveguide become extremely complicated, but no design example of the optical circuit considering these influences is found. In addition, although there is a report that the wavelength dependence of the interference characteristics is controlled by changing the width of the optical waveguide of the directional coupler (A. Takagi et al., IEEE J. Quantum Elec
tron, vol.28. no.4, pp.848-855), and there are no reports of design studies of optical circuits from the viewpoint of polarization dependence.

Generally, in a directional coupler formed of an optical waveguide (square optical waveguide) having a square or horizontally long rectangular cross-sectional shape, the optical coupling characteristic results in dependence on the polarization state of the signal light. The details of the polarization dependence of the optical coupling will be described later, but it is understood that it is mainly caused by two factors.

In the schematic sectional view of the general rectangular optical waveguide shown in FIG. 6, the center of the optical waveguide 31 is taken as the origin, the waveguide width is 2a, the waveguide thickness is 2d, and the center-to-center distance between the optical waveguides 31 and 32. Is D, the refractive index of the core is n ₁ and the refractive index of the clad is n ₀ , the mode coupling constant x between the two optical waveguides 31 and 32 can be expressed by the following equation (“optical waveguide Basics ", Katsuyuki Okamoto, Corona Publishing Co., Ltd.).

[0016]

[Equation 1]

The mode coupling constant x is given by the above equation in both the TE mode and the TM mode. The constants r _x and k _{x in the equation} have different values in the TE mode and the TM mode, respectively. Each is expressed by the following relational expressions.

In the TE mode,

[0019]

[Equation 2]

In the TM mode,

[0021]

[Equation 3]

It is That is, the mode coupling constant x is T
Polarization dependence occurs due to the difference between the E mode and the TM mode.

Further, in such a rectangular optical waveguide, stress is generated due to the difference in the coefficient of thermal expansion between the clad and the core, and the effective n ₁ and n ₀ sensed by the TE-mode and TM-mode signal lights are different. As a result, it has birefringence. In reality, the main factor that causes the polarization dependence of the optical coupling rate is due to this birefringence, and in order to prevent the optical coupling rate from depending on the polarization, the birefringence is canceled or the Measures are needed to counteract the effect. However, in order to elucidate the mechanism of birefringence development in detail, it is difficult to carry out polarization-independent polarization of the optical coupling rate by analytical means, because complicated calculations are required.

[0024]

The following problems occur in an optical circuit composed of a directional coupler whose optical coupling rate has polarization dependency.

For example, in a gain equalizer which is an application example of a lattice filter constituted by connecting Mach-Zehnder interferometers composed of two directional couplers in multiple stages, a polarization dependence of 0.1 dB or less over the entire operating wavelength range. Loss is required. For example, in the case of a gain equalizer with five stages connected,
The polarization dependence of the optical coupling rate in a directional coupler is 0.2%.
The following must be suppressed. Since the allowable range becomes narrower as the number of stages of the lattice filter increases, it becomes a problem to make the optical coupling rate polarization independent.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a directional coupler having no polarization dependence of the optical coupling rate.

[0027]

In order to solve the above problems, a directional coupler according to the present invention has a directional coupler having a substrate and two optical waveguides arranged on the substrate in close proximity to each other. The coupler is characterized in that the core portion of the optical waveguide has a rectangular cross-sectional shape, and the length of a side of the square parallel to the main surface of the substrate is shorter than the length of the remaining side. Preferably, the aspect ratio of the core cross-sectional shape of the optical waveguide is 1.0:
It is 0.4-1.0: 0.8.

With such a structure, in the directional coupler formed of the optical waveguides arranged on the substrate, the birefringence in the waveguide core is eliminated, and the non-polarization dependence of the optical coupling rate is achieved. You can

The optical waveguide of the directional coupler of the present invention can be made of silica glass containing SiO ₂ as a main component.

Preferably, the optical waveguide is arranged on a silicon substrate, and more preferably, the optical waveguide is formed by a combination of a flame deposition method and a reactive etching method.

An optical circuit of the present invention is characterized by including the above-mentioned optical waveguide, and preferably, the cross-sectional shape of the core portion of the optical waveguide other than the optical waveguide forming the directional coupler is a square. The length of the side parallel to the main surface of the substrate is equal to or longer than the length of the remaining side.

With such a structure, it is possible to provide an optical circuit having an optical coupling rate without polarization dependence.

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 1 to 3 are schematic views showing an outline of an optical circuit provided with a directional coupler according to an embodiment of the present invention, and FIGS. 2 and 3 are A- shown in FIG.
It is sectional drawing in an A'line and BB '.

Here, each of the optical waveguides 11 to 16 has a silicon substrate 1, a cladding layer 2 made of silica glass and a core portion 3 arranged on one main surface 1A thereof. Thickness 1
mm on a silicon substrate 1 with a diameter of 4 inches, S in a flame
A silica-based glass film is deposited by a so-called flame hydrolysis reaction in which a raw material gas such as iCl ₄ or GeCl ₄ is reacted with CH ₄ or H ₂ and oxygen gas of a fuel, and then reactive ion etching is performed on the glass film. Optical waveguide 1
1-16 were formed.

The optical circuit comprises a directional coupler 21, input waveguides 11 and 12, and output waveguides 13 and 14. The directional coupler 21 is set to have a coupling length corresponding to the width of the optical waveguide so that the optical coupling rate for unpolarized signal light is 3 dB. In this embodiment, the optical waveguide width is set to 4 to 7 μm over the entire optical circuit. A chip on which an optical circuit having such a directional coupler is arranged is cut out by dicing, and the input / output waveguides 11, 12 and 1 are cut out.
A single mode optical fiber was coupled to 3 and 14.

4 to 7 are views for explaining the manufacturing process of the directional coupler using the above silica optical waveguide.
That is, after the silicon substrate 1 is prepared (FIG. 4), the lower clad layer 2A made of silica glass and the core layer 3A are formed on the one main surface 1A of the silicon substrate 1 by the flame deposition method (FIG. 5). ), Only the portion that will later become the waveguide core portion 3 is left, and the remaining portion of the core layer 3A is selectively removed by reactive ion etching (FIG. 6). Finally, the upper clad layer 2B made of quartz glass is deposited on the lower clad layer 2A by the flame deposition method to form the core portion 3
Is covered with the upper clad layer 2B (FIG. 7). As described above, the silica-based optical waveguide of the present invention is manufactured by the combination of the technology for depositing the silica-based glass film utilizing the flame hydrolysis reaction of the raw material gas such as SiCl ₄ or GeCl ₄ and the reactive ion etching. In this embodiment, the thickness of the core portion 3 was set to 4.5 μm, and the difference in relative refractive index between the silica glass used for the core portion 3 and the cladding layer 2 was set to 1.5%.

FIG. 8 shows a TE mode and TM for a directional coupler configured by changing the optical waveguide width, that is, the width (core width) in the direction parallel to the main surface 1A of the silicon substrate 1.
This is the result of measuring the optical coupling rate of modes and defining the difference between both optical coupling rates as the polarization dependency in the vicinity of 3 dB coupling, and determining the optical waveguide width dependency of the polarization dependency.

The polarization dependence decreases as the width of the optical waveguide becomes narrower and the cross-sectional shape of the core becomes longer. In the case of the present embodiment, the polarization state of the signal light is changed when the width of the optical waveguide is about 3.5 μm. A directional coupler having an independent optical coupling rate was obtained. That is, in the directional coupler of the present invention, in the optical waveguide having the aspect ratio (aspect ratio) of 1.0: 0.7 with respect to the core thickness of 4.5 μm, the optical coupling rate becomes the polarization state of the signal light. An independent directional coupler can be realized. From the practical viewpoint of ensuring the mechanical strength of the core part, the aspect ratio (aspect ratio) of the cross-sectional shape of the core part is 1.0: 0.4.
It is preferable to set to 1.0: 0.8. Here, the horizontal direction means a direction parallel to the main surface 1A of the silicon substrate 1, and the vertical direction means a thickness direction of the silicon substrate 1 orthogonal to the main surface 1A.

FIG. 9 is a result of theoretical analysis of the optical waveguide width dependence of the polarization dependence. Here, the thickness of the core 3 was kept constant and the core width was changed to obtain the polarization rate dependence of the coupling rate in the vicinity of 3 dB coupling. Specifically, the stress distribution in the two waveguide cores 3 and 3 arranged close to each other and the gap between the two cores 3 and 3 is obtained by the finite element method, and the refractive index distribution is calculated based on the stress distribution. A directional coupler model was constructed by simplifying the above, and the polarization dependence of the coupling rate was calculated by the beam propagation method.

According to FIG. 9, as the waveguide width, that is, the core width is narrowed and the cross-sectional shape of the core becomes longer, the TM polarization dominance changes to the TE polarization dominance and the polarization dependence of the optical coupling rate decreases. Was confirmed.

Comparing the experimental results shown in FIG. 8 with the theoretical calculation results shown in FIG. 9, it was found that the polarization dependence tends to decrease as the waveguide width becomes narrower.
The calculation results shown in FIG. 9 qualitatively support the effect of the waveguide width on the polarization dependence of the directional coupler of the present invention. It should be noted that there is a difference in the polarization dependence value between the calculation result and the measurement result, but this is because the model used for the calculation is extremely simplified.

In the above embodiment, an example of a directional coupler using an optical waveguide having a core thickness of 4.5 μm and a relative refractive index difference of 1.5% is shown, but the present invention is not limited to this example. However, it has been confirmed that the same effect can be obtained by making the cross-sectional shape of the core vertically long even in optical waveguides having other core thicknesses and relative refractive index differences.

Further, in the above-mentioned embodiment, an example of the optical circuit in which the entire optical circuit is constituted by the optical waveguide having a constant width has been described. However, depending on the optical circuit element, the aspect of the core portion in the optical waveguide other than the optical coupling portion 21 is described. It goes without saying that the ratio may be another ratio, for example, a square structure or a horizontally long structure. In such a case, only the cross-sectional shape of the optical waveguide of the directional coupler of the present invention may be vertically elongated, and the aspect ratio of the core portion of the remaining optical waveguide of the optical circuit may be set to a desired value. In that case, from the viewpoint of reducing the optical loss, it is preferable to use an optical waveguide having a tapered structure in the core-shaped conversion portion.

The present invention has been described above with respect to an optical circuit that uses a planar optical waveguide and is realized by a single mode optical waveguide made of silica glass formed on a silicon substrate as an optical waveguide. Is excellent in integration property, is excellent in large-scale circuitization and cost reduction of manufacturing cost, is low in loss and stable, and is excellent in compatibility with the silica-based optical fiber. However, the invention is not limited to only these combinations. In this case, the polarization dependence of the optical coupling rate is caused by the birefringence resulting from the difference in the coefficient of thermal expansion between the cladding and the core. The present invention can obtain the same effect even in the case of a combination of other materials that exhibit birefringence due to the difference in thermal expansion coefficient between the clad and the core.

[0045]

According to the present invention, in a directional coupler in which two optical waveguides are arranged close to each other, by making the cross-sectional shape of the cores of these optical waveguides longitudinal, the birefringence of the cores can be improved. Therefore, the non-polarization dependence of the optical coupling rate can be realized.

The present invention is extremely effective in putting into practical use an optical circuit which requires polarization independence of the optical coupling rate in a directional coupler.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating an outline of an optical circuit including a directional coupler according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along the line AA ′ of FIG.

3 is a cross-sectional view taken along the line BB ′ of FIG.

FIG. 4 is a schematic cross-sectional view showing a manufacturing process of the optical waveguide of the present invention.

FIG. 5 is a schematic cross-sectional view showing a manufacturing process of the optical waveguide of the present invention.

FIG. 6 is a schematic cross-sectional view showing a manufacturing process of the optical waveguide of the present invention.

FIG. 7 is a schematic cross-sectional view showing a manufacturing process of the optical waveguide of the present invention.

FIG. 8 is a graph illustrating the polarization dependence of the optical coupling rate of the directional coupler provided in the optical circuit of the present invention.

FIG. 9 is a result of analyzing the polarization dependence of the optical coupling rate of the directional coupler included in the optical circuit of the present invention based on a simplified model.

FIG. 10 is a schematic diagram illustrating an outline of an optical circuit including a conventional directional coupler.

11 is a cross-sectional view taken along the line AA ′ of FIG.

12 is a cross-sectional view taken along the line BB ′ of FIG.

FIG. 13 is a schematic diagram of a calculation model for calculating a mode coupling constant of a directional coupler having a general rectangular optical waveguide.

[Explanation of symbols]

1 Silicon substrate 1A Silicon substrate main surface 2 Cladding layer 2A Lower clad layer 2B Upper clad layer 3 core part 3A Waveguide core layer 11 Input waveguide 12 Input waveguide 13 Output waveguide 14 Output waveguide 15 Directional coupler waveguide 16 Directional coupler waveguide 21 Directional coupler 31 Waveguide 32 Waveguide 101 Silicon substrate 102 clad layer 103 core 111 Input waveguide 112 Input waveguide 113 Output waveguide 114 output waveguide 115 Directional coupler waveguide 116 Directional coupler waveguide 121 Directional coupler

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masayuki Okuno             2-3-1, Otemachi, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation F term (reference) 2H047 KA04 KA12 KB04 LA11 RA08                       TA21

Claims (7)

[Claims] 1. A directional coupler having a substrate and two optical waveguides arranged close to each other on the substrate, wherein the core portion of the optical waveguide has a rectangular cross-sectional shape. A directional coupler, wherein a length of a side parallel to the main surface of the substrate is shorter than a length of the remaining side. 2. The aspect ratio of the square is 1.0: 0.4.
The directional coupler according to claim 1, wherein the directional coupler is 1.0: 0.8. 3. The directional coupler according to claim 1, wherein the optical waveguide is a silica-based glass containing SiO ₂ as a main component. 4. The directional coupler according to claim 1, wherein the substrate is a silicon substrate. 5. The directional coupler according to claim 1, wherein the optical waveguide is formed by a combination of a flame deposition method and a reactive etching method. 6. An optical circuit comprising the directional coupler according to claim 1. 7. A cross-sectional shape of a core portion of an optical waveguide other than the optical waveguide forming the directional coupler is a square, and a length of a side of the square parallel to the main surface of the substrate is a remaining side. The optical circuit according to claim 6, wherein the optical circuit is equal to or longer than the length.