JP2016040572A - Optical receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical receiver that sufficiently reduces polarization dependency with a single wavelength branch circuit using no polarization diversity configuration.SOLUTION: An optical receiver 100 includes: an input waveguide 101; an input side slab waveguide 102; an array waveguide 103; an output side slab waveguide 104; an output waveguide 105 that is a multimode waveguide; and a photodetector 106. Between the input waveguide and the input side slab waveguide, a polarization conversion circuit 107 is further provided that outputs the basis mode light of the input TE polarization directly and converts the basis mode light of the input TM polarization into the primary mode light of the TE polarization to output it.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical receiver, and more particularly, to an optical receiver for wavelength division multiplexing signals formed of a silicon integrated circuit.

  Wavelength division multiplexing signal receivers (hereinafter referred to as WDM optical receivers) have recently been put to practical use in devices that transmit and receive high-speed Ethernet (registered trademark) signals of 100 gigabits / second or higher. It has been actively developed as an important device. The WDM optical receiver has a function of demultiplexing an input WDM signal into signals of each wavelength, detecting a signal of each wavelength with a photodetector, converting the signal into an electric signal, and outputting the electric signal.

  As a first conventional example of such a WDM optical receiver, a configuration is proposed in which an optical waveguide demultiplexing circuit of an arrayed waveguide diffraction grating type using a planar lightwave circuit made of quartz material and a photodetector or a photodetector array are combined. (For example, refer to Patent Document 1 and Patent Document 2).

  FIG. 20 is a plan view showing a configuration of a WDM optical receiver according to the first conventional example. Here, the WDM optical receiver includes an input waveguide 11, an input side slab waveguide 12, an arrayed waveguide 13, an output side slab waveguide 14, and an output waveguide 15 that is a multimode waveguide. A lattice chip 10, an optical coupling lens 16, and a photodetector 17 are provided.

  In the WDM optical receiver of the first conventional example, the light input to the input waveguide 11 spreads in the input-side slab waveguide 12, is distributed to each array waveguide of the arrayed waveguide 13, propagates, and then the output-side slab The light is input to the waveguide 14 and interferes therewith, and is condensed at different positions on the output-side slab waveguide 14 according to the wavelength. The condensed light is propagated by being coupled to any mode of the output multimode waveguide 15 installed at the condensing position, and reaches the photodetector 17 through the optical coupling lens 16 and is received. Here, if the output multimode waveguide 15 disposed at the condensing position is designed to be able to propagate a sufficiently large number of modes, any of the light condensed in the range of the waveguide width is output to the output multimode waveguide 15. Therefore, the light receiving characteristic of the photodetector 16 can be flat in a certain wavelength range.

  In addition, the waveguide made of quartz material has very little birefringence and can obtain optical characteristics that are almost independent of the polarization. Therefore, the polarization dependence of the light reception characteristics in the conventional WDM optical receiver is also high. It is sufficiently suppressed.

  As described above, an optical wavelength demultiplexing circuit using a planar lightwave circuit made of quartz material provides excellent optical characteristics, but there is a limit to miniaturization of the chip size, and it is difficult to follow the recent demand for miniaturization of optical communication devices. It has been pointed out that there is.

  On the other hand, an optical circuit using a silicon fine wire optical waveguide has an advantage that the circuit size can be remarkably reduced as compared with a conventional material such as quartz, and research and development are actively performed. In addition, a silicon optical circuit can form a photodetector in an optical thin line waveguide and a series of manufacturing steps, and the optical thin line waveguide and the photodetector can be integrated in the same chip. Therefore, it is attracting attention as a technology that can make WDM optical receivers dramatically smaller and simpler.

  However, in general, a silicon optical wire waveguide has a very large effective birefringence. In other words, when AWG (Arrayed Waveguide Gratings) or ring resonators are used as a multiplexing / demultiplexing circuit, the input light is greatly different in characteristics of demultiplexing depending on the TE polarization and TM polarization, and reception is not dependent on the polarization of the input signal. It cannot be applied as it is to WDM receivers that require characteristics. As a means for solving this problem and realizing a WDM receiver by a silicon optical circuit, a polarization diversity configuration has been proposed.

  FIG. 21 is a plan view showing a configuration of a WDM receiver using a silicon optical circuit to which a polarization diversity configuration is applied according to a second conventional example. Here, the WDM receiver includes two identical arrayed waveguide diffraction gratings on the silicon optical circuit chip 20, and the arrayed waveguide diffraction grating includes the input waveguide (21 a, 21 b) and the input side slab waveguide (22 a). 22b), arrayed waveguides (23, 23b), output-side slab waveguides (24a, 24b), and output waveguides (25a, 25b) which are multimode waveguides. On the chip 20 of the silicon optical circuit, an input waveguide 26 and a polarization separation / rotation circuit 27 are provided. Further, a photodetector 28 is also integrated in the silicon optical chip 20.

  In the WDM optical receiver of the second conventional example, the light input to the input waveguide 26 is separated into a TE polarization component and a TM polarization component by the polarization separation / rotation circuit 27, and the TM polarization component is further converted into a TE polarization component. Rotated by wave and output. The two arrayed waveguide diffraction gratings are designed to operate correctly with respect to the TE polarization component, and wavelength-demultiplex the two TE polarization lights output from the polarization separation / rotation circuit 27. Here, light from two waveguides (25a, 25b) corresponding to the same output port of each arrayed waveguide diffraction grating is inputted to the same photodetector 28 from both directions and is received. That is, the light input to the WDM optical receiver of the second conventional example is separated by the polarization, and is wavelength-demultiplexed by the arrayed waveguide diffraction grating, and the two lights having the same wavelength reach the same photodetector 28. Therefore, the entire circuit can operate without polarization dependency. Further, since the output waveguides (25a, 25b) are multimode waveguides, the light receiving characteristics of the photodetector 28 can obtain flat wavelength characteristics in a certain wavelength range.

International Publication No. 2003/083534 Pamphlet JP 2004-361660 A

W. D. Sacher et al, “Polarization rotator-splitters in standard active silicon photonics platforms”, OPTICS EXPRESS, Vol. 22, pp.3779-3786, (2014)

  However, the WDM optical receiver using the silicon optical circuit described with reference to FIG. 21 has the following problems.

  First, since two wavelength demultiplexing circuits (in which the arrayed waveguide diffraction grating is applied in the conventional example) are required and the demultiplexing wavelengths of these two circuits need to coincide with the signal wavelength, the wavelength demultiplexing circuit As compared with the case where there is only one, the manufacturing yield of the circuit is deteriorated.

  Second, a polarization separation / rotation circuit, two wavelength demultiplexing circuits, and a photodetector are provided on the silicon optical circuit chip 20, and light is input to the photodetector from two directions. Therefore, the degree of freedom in circuit layout is low.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a silicon whose polarization dependency is sufficiently reduced with a single wavelength demultiplexing circuit without requiring a polarization diversity configuration. An object of the present invention is to provide a WDM optical receiver using an optical circuit.

  In order to achieve such an object, the optical receiver according to the first aspect of the present invention is an output that is an input waveguide, an input-side slab waveguide, an arrayed waveguide, an output-side slab waveguide, or a multimode waveguide. An optical receiver including a waveguide and a photodetector 106. The optical receiver of the first aspect outputs the input TE-polarized base mode light as the TE-polarized base mode light between the input waveguide and the input-side slab waveguide, and inputs the input TM It further includes a polarization conversion circuit that converts the polarized fundamental mode light into TE polarized primary mode light and outputs the converted light.

  In one embodiment, the polarization conversion circuit is a rib-type silicon waveguide having a rib portion and a fin portion, and the core thickness of the fin portion is thinner than the core thickness of the rib portion, It has a tapered shape in which the core width of the fin portion gradually increases.

  Furthermore, in one embodiment, an intermediate taper waveguide can be further provided between the polarization conversion circuit and the input-side slab waveguide. In one embodiment, the polarization conversion circuit may be a tapered waveguide whose width gradually increases. The tapered waveguide is a waveguide composed of a core and a lower clad and an upper clad. The core is made of silicon, and the lower clad and the upper clad can be made of a material having a refractive index lower than that of silicon. In the region near the upper part of the tapered waveguide, the upper cladding may have a first layer in contact with the upper part of the core and a second layer formed on the upper part of the first layer. The first layer of the upper cladding is made of a material having a lower refractive index than silicon, and the second layer has a higher refractive index than the material forming the lower cladding and the material forming the first layer, and the refractive index is higher than that of silicon. May be formed of a low material. Further, the distance between the upper surface of the core and the lower surface of the second layer of the upper clad can be equal to or smaller than the thickness of the core.

  Furthermore, in one embodiment, the polarization conversion circuit is first and second rib-type silicon waveguides each having a rib portion and a fin portion and connected to each other, and the core thickness of the fin portion is The first rib-type silicon waveguide is thinner than the core of the rib portion, and the first rib-type silicon waveguide has a taper shape in which the core width of the rib portion and the core width of the fin portion gradually increase, and the second rib-type silicon waveguide is The taper shape in which the core width of the rib portion gradually increases and the core width of the fin portion gradually decreases can be obtained.

  Furthermore, in one embodiment, the polarization conversion circuit is a rib-type silicon waveguide having a rib portion and a fin portion, and the rib-type silicon waveguide is directly connected to the input-side slab waveguide, and the fin portion The thickness of the core can be made thinner than the thickness of the core of the rib portion, and the taper shape in which the core width of the rib portion and the core width of the fin portion gradually increase can be obtained.

  As described above, according to the present invention, there is provided a WDM optical receiver using a silicon optical circuit that does not require a polarization diversity configuration and sufficiently reduces the polarization dependency with a single wavelength demultiplexing circuit. be able to. Further, according to the present invention, it is possible to prevent deterioration in circuit manufacturing yield, reduce the circuit size, and increase the degree of freedom of circuit layout.

It is a top view which shows the structure of the WDM optical receiver concerning the 1st Embodiment of this invention. It is the figure which expanded the vicinity of the polarization converting circuit of the WDM optical receiver concerning the 1st Embodiment of this invention. It is sectional drawing in the A-A 'sectional line of FIG. It is sectional drawing in the B-B 'sectional line of FIG. It is a figure which shows the calculation result of the effective refractive index with respect to the change of the core width of the waveguide 113b in the main part 113 of the polarization conversion circuit 107 of FIG. It is a figure which shows the result of having calculated the intensity ratio of each polarization component regarding the 2nd mode of FIG. It is a figure which shows the wavelength dependence of the light reception sensitivity of each photodetector in the 1st Embodiment of this invention. It is the figure which expanded the vicinity of the polarization converting circuit 107 of the WDM optical receiver concerning the 2nd Embodiment of this invention. It is sectional drawing in the C-C 'sectional line of FIG. It is a figure which shows the calculation result of the effective refractive index with respect to the width | variety of the silicon core of the taper waveguide 202 of the polarization converting circuit of the WDM optical receiver concerning the 2nd Embodiment of this invention. It is a figure which shows the result of having calculated the intensity ratio of each polarization component regarding the 2nd mode of FIG. It is a figure which shows the relationship between the efficiency of polarization conversion, and the width | variety of a silicon nitride area | region. It is the figure which expanded the vicinity of the polarization converting circuit 107 of the WDM optical receiver concerning the 2nd Embodiment of this invention. It is the figure which expanded the vicinity of the polarization converting circuit 107 of the WDM optical receiver concerning the 3rd Embodiment of this invention. It is sectional drawing in the D-D 'sectional line of FIG. It is a figure which shows the result of having calculated the effective refractive index in each position of the right end of the 2nd rib type waveguide taper from the left end of the 1st rib type waveguide taper of FIG. It is a figure which shows the result of having calculated the intensity ratio of each polarization component regarding the 2nd mode of FIG. It is the figure which expanded the vicinity of the polarization converting circuit 107 of the WDM optical receiver concerning the 4th Embodiment of this invention. It is a figure which shows the wavelength dependence of the light reception sensitivity of each photodetector in the 4th Embodiment of this invention. It is a top view which shows the structure of the WDM optical receiver in a prior art. It is a top view which shows the structure of the WDM optical receiver in a prior art.

  First, the principle of operation of the optical receiver according to various embodiments of the present invention will be described. The optical receiver according to various embodiments of the present invention has a single arrayed waveguide grating composed of silicon optical waveguides, no polarization separation rotation circuit, and an input waveguide of the arrayed waveguide grating. This is different from the optical receiver of the above-described conventional example 2 in that a polarization conversion circuit 107 is inserted between 101 and the input-side slab waveguide 102.

  The polarization conversion circuit 107 in the optical receiver according to various embodiments of the present invention outputs the input TE-polarized base mode light as it is as the TE-polarized base mode light, and inputs the TM-polarized base mode. It has a function of converting light into TE-polarized primary mode light and outputting it. The arrayed waveguide diffraction grating is designed to operate correctly with respect to TE polarized light, and collects the input TE polarized fundamental mode light at different positions on the output slab waveguide 104 according to the wavelength. The electric field distribution of the collected light at this time reconfigures the TE polarized fundamental mode input to the input-side slab waveguide 102. On the other hand, if there is no polarization conversion circuit 107, the input TM-polarized fundamental mode light enters the arrayed waveguide grating as the TM-polarized fundamental mode light, and is similar to the TE-polarized fundamental mode light. Although wavelength demultiplexing is not possible, the signal is converted into TE-polarized primary mode light by the polarization conversion circuit 107 and condensed at the same position as the TE-polarized base mode light according to the wavelength. However, the electric field distribution of the condensed light at this time reconstructs the primary mode of TE polarization.

  Further, the light condensed at different positions of the output-side slab waveguide 104 according to the wavelength is propagated by being coupled to any mode of the output multimode waveguide 105 installed at the condensing position. However, if the output waveguide (multi-mode waveguide) 105 is designed to propagate a sufficiently large number of modes, the TE-polarized base mode light is also the TE-polarized primary mode light. Since both are coupled to the output waveguide 105 with almost no loss, the wavelength characteristic of the light received by the photodetector 106 hardly depends on the polarization. That is, according to the present invention, it is possible to realize a light receiving circuit having a light receiving characteristic having almost no polarization dependence while using only a single arrayed waveguide diffraction grating.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[First Embodiment]
FIG. 1 is a plan view showing a configuration of an optical receiver 100 according to the first embodiment of the present invention. The optical receiver 100 of this embodiment includes an input waveguide 101, an input-side slab waveguide 102, an arrayed waveguide 103, an output-side slab waveguide 104, an output waveguide 105 that is a multimode waveguide, A photodetector 106. The optical receiver 100 also includes a polarization conversion circuit 107 that optically connects the input waveguide 101 and the input-side slab waveguide 102.

  The polarization conversion circuit 107 emits the TE-polarized base mode light input from the input waveguide 101 to the input-side slab waveguide 102 as TE-polarized base mode light as it is, and the TM input from the input waveguide 101. It has a function of converting polarized fundamental mode light into TE polarized primary mode light and emitting it to the input-side slab waveguide 102.

  The optical receiver 100 has a silicon optical wire waveguide (input waveguide 101, array waveguide 103, output waveguide 105) core thickness of 0.22 μm, and the core width of the input waveguide 101 and array waveguide 103 is 0.5 μm. The core width of the output waveguide 105 is 3 μm. The clad material of the silicon optical wire waveguide is quartz. The number of arrayed waveguides 103 is 75, and the length of the waveguides is designed to increase sequentially by a certain amount ΔL from the inside toward the outside, and ΔL is 11.73 μm. Further, the length of the input side slab waveguide 102 and the output side slab waveguide 104 is 110 μm, the arrangement interval of the output waveguides 105 in the portion connected to the output side slab waveguide 104 is 4.5 μm, the input side slab waveguide 102 and the output The waveguide interval of the arrayed waveguide 103 at the portion connected to the side slab waveguide 104 is 1.2 μm. The number of output waveguides 105, that is, the number of demultiplexing ports, is four, and the optical frequency interval between ports is designed to be 800 GHz.

  FIG. 2 is a plan view showing in detail the polarization conversion circuit 107 that optically connects the input waveguide 101 and the input-side slab waveguide 102 and the vicinity thereof. The polarization conversion circuit 107 converts the TM-polarized base mode light input from the left side (input waveguide 101 side) into the TE-polarized primary mode light at the main portion 113 by converting it into the TE-polarized primary mode light. The TE polarized base mode light output from the input side slab waveguide 102 side and input from the left in the figure is output from the right in the figure as the TE polarized base mode light. The main portion 113 is formed of a rib-type silicon waveguide (see FIG. 4) including a central rib portion 113a and a peripheral fin portion 113b, and the core width of the peripheral fin portion 113b is a tapered waveguide whose width extends from w1 to w2. It has become. The core width of the central rib portion 113a is constant. A rib type-channel type conversion waveguide that smoothly connects the input waveguide 101 to the rib waveguide 113 is provided in the previous stage (left in the figure) of the main portion 113 of the polarization conversion circuit 107. The rib type-channel type conversion waveguide is a part (rib type waveguide) 111 for converting the waveguide structure from the channel type to the rib type, and a part for changing the core width at the center of the rib type waveguide (rib type waveguide). ) 112. The rib type-channel type conversion waveguides (111, 112) do not directly contribute to the function of polarization conversion, but the discontinuity between the structure of the input waveguide 101 connected to the front and the back and the structure of the main portion 113 is eliminated. It has the effect of eliminating the excessive light loss and is preferably installed.

  Here, the rib-type waveguide 113 of the polarization conversion circuit 107 has a length (length in the light propagation direction, hereinafter the same) of 100 μm, the core width of the rib portion is 0.15 μm, and the core width of the fin portion is increased. The core width of the fin portion at the boundary with the rib-type waveguide 112 is w1 = 0.9 μm, and the core width at the boundary with the input-side slab waveguide 102 is w2 = 1.8 μm. In the rib-to-channel conversion waveguides 111 and 112, the rib-type waveguide 111 has a length of 50 μm, a core width of the rib portion is increased to 0.5 μm, and a core width of the fin portion is increased from 0.5 μm to 0.9 μm. The rib-type waveguide 112 has a length of 200 μm, the core width of the rib portion is changed from 0.5 μm to 0.15 μm, and the core width of the fin portion is 0.9 μm.

  FIG. 3 shows a circuit structure of a cross section taken along the line A-A ′ of the input waveguide 101 of FIG. 2. The input waveguide 101 which is the core of the channel-type waveguide is a waveguide embedded in the upper clad 121 and the lower clad 122 on the silicon substrate 123. The input waveguide 101 is made of silicon. Both the upper clad 121 and the lower clad 122 are made of quartz glass. The input waveguide 101 has a core thickness of 0.22 μm, the upper cladding 121 has a thickness of 1.5 μm, and the lower cladding 122 has a thickness of 2 μm (corresponding to the distance between the silicon substrate 123 and the input waveguide 101).

  4 shows a circuit structure of the rib waveguide 113 of the polarization conversion circuit 107 of FIG. A rib core 113 a and a fin core 113 b of a rib-type waveguide formed of silicon are embedded in the lower clad 132 and the upper clad 131 on the silicon substrate 133. The upper clad 131 and the lower clad 132 are made of quartz glass. The silicon substrate 133, the upper clad 131, and the lower clad 132 correspond to the silicon substrate 123, the upper clad 121, and the lower clad 122 in FIG. 2, respectively. The rib portion 113a has a core thickness of 0.22 μm, the fin portion 113b has a core thickness of 0.06 μm, the upper cladding 131 has a thickness of 1.5 μm, and the lower cladding 132 has a thickness of 2 μm. The rib type waveguides 111 and 112 constituting the rib type-channel type conversion waveguide also have a circuit structure with the same cross section as the rib type waveguide 113, and the core thickness of the rib part and the fin part is as follows. The same as the rib-type waveguide 113.

  FIG. 5 is a diagram for explaining the function of the main part 113 of the polarization conversion circuit 107, and showing the result of calculating the effective refractive index with respect to the core width w of the fin portion of the rib-type waveguide. Here, the material and the cross-sectional structure of the waveguide are the same as those described with reference to FIG. 4, and the effective refractive index is calculated up to three by changing only the core width w of the fin portion. Here, the fundamental mode of TE polarization is maintained regardless of w in the first mode, but the second mode and the third mode are combined modes in which the TE polarization component and the TM polarization component are mixed by changing the core width w. It has become. When the core width w is around 0.8 μm, the second mode is almost the TM polarized base mode, and the third mode is almost the TE polarized primary mode. On the other hand, when the core width w is in the vicinity of 2.4 μm, the second mode is switched to the primary mode of approximately TE polarization, and the third mode is switched to the fundamental mode of TM polarization.

  FIG. 6 shows the intensity ratio between the TE polarization component and the TM polarization component, particularly from the calculation result of the second mode in FIG. It can be seen that as the core width w increases, the intensity ratio of the TM polarization component decreases, the intensity ratio of the TE polarization component increases, and the TM polarization component and the TE polarization component are interchanged. The main portion 113 of the polarization conversion circuit 107 includes a core width w1 on the narrower core width side (boundary side with the rib-type waveguide 112) and a wider side (boundary side with the input-side slab waveguide 102). The core width w2 is designed to include as much as possible the region where the polarization component of the second mode is switched. That is, the core width w1 is set to such a width that the second mode is substantially a TM polarized base mode, and the core width w2 is set to be a width such that the second mode is substantially a TE polarized primary mode. The Specifically, in the present embodiment, as described above, w1 = 0.9 μm and w2 = 1.8 μm. In FIG. 2, when TM-polarized fundamental mode light is input from the left of the main portion 113 that is a rib-type waveguide, the propagating light wave of TM-polarized light is gradually increased from w1 to w2. It is converted from the fundamental mode to the primary mode of TE polarization and output from the right side.

  In the optical receiver of the present embodiment, the TE-polarized fundamental mode light input to the input waveguide 101 is directly input to the input-side slab waveguide 102, and the TE-polarized fundamental mode light is transmitted through the arrayed waveguide 103. After being distributed and coupled to each arrayed waveguide and propagating through the arrayed waveguide 103, it is input to and interferes with the output-side slab waveguide 104, and is condensed at different positions on the output-side slab waveguide 104 according to the wavelength. . The electric field distribution of the collected light at this time reconfigures the fundamental mode of the TE polarization input to the input-side slab waveguide 102. The condensed light is propagated by being coupled to any mode of the output waveguide (multimode waveguide) 105 installed at the condensing position of the output-side slab waveguide 104, and reaches the photodetector 106 to be received. Is done. At this time, which wavelength band light is coupled to which waveguide of the output waveguide 105, that is, which wavelength band of light is received by which detector of the photodetector 106 is given by the arrayed waveguide 103. It depends on the amount of light delay.

  Here, when TM-polarized fundamental mode light is input to the input waveguide 101, if there is no polarization conversion circuit 107, the TM-polarized fundamental mode light is input to the input-side slab waveguide 102 as it is, and TM-polarized light is input. The fundamental wave of the wave is distributed and coupled to each arrayed waveguide by the input side slab waveguide 102. Since the TM-polarized base mode light propagates through the arrayed waveguide 103, the amount of delay applied differs from the TE-polarized light, and the wavelength range received by each detector also differs.

  However, by providing the polarization conversion circuit 107, the TM-polarized base mode light is converted into TE-polarized primary mode light and input to the input-side slab waveguide 102. In this case, since the TE-polarized primary mode light is distributed and coupled to each array waveguide by the array waveguide 103 and propagates through the array waveguide 103, the delay amount given in the array waveguide 103 is This is exactly the same as when TE polarization is input through the input waveguide 101. Therefore, the position of interference in the output-side slab waveguide 104 and condensing according to the wavelength is the same. The electric field distribution of the collected light at this time reconstructs the primary mode of the TE polarization input to the input side slab waveguide 102. The condensed light is propagated by being coupled to any mode of the output waveguide (multimode waveguide) 105 installed at the condensing position, and reaches the photodetector 106 to be received.

  That is, according to the optical receiver of this embodiment, when TE-polarized base mode light having a certain wavelength is input, the electric field distribution of the TE-polarized base mode light is re-established at the condensing position according to the wavelength. When the TM polarized base mode light of the same wavelength is input, the electric field distribution of the TE polarized primary mode light is reconstructed at the same condensing position as the TE polarized base mode light. Is done. Here, if the output waveguide 105, which is a multi-mode waveguide disposed at the condensing position, is designed to be able to propagate a sufficiently large number of modes, the TE-polarized base mode light is also the TE-polarized primary mode light. Since both are coupled to the output waveguide 105 almost without loss, it can be said that the wavelength characteristic of the light received by the photodetector 106 has almost no polarization dependence.

  FIG. 7 shows the position of the condensing position of the input light in the output-side slab waveguide 104 and its electric field distribution according to the wavelength, and the output waveguide (multimode waveguide) 105 in the optical receiver of this embodiment. The wavelength dependence of each light receiving sensitivity of the photodetector 106 calculated from optical coupling is shown. Here, the four photodetectors 106 shown in FIG. 1 are called photodetectors 1, 2, 3, and 4 from the top. In FIG. 7, the solid line represents the light receiving sensitivity when the input light is TE-polarized base mode light, and the broken line represents the light receiving sensitivity when the input light is TM-polarized base mode light. The light receiving sensitivity is expressed as a relative sensitivity based on the sensitivity at the wavelength center of each photodetector. From FIG. 7, the wavelength dependence of the photodetection sensitivity of the photodetector is almost the same regardless of whether the input polarization is TE polarization or TM polarization, and is flat as in the conventional WDM optical receiver. A simple waveform can be obtained.

  As described above, the optical receiver of the present embodiment can realize a WDM optical receiver that does not require a polarization diversity configuration and has a sufficiently reduced polarization dependency with a single wavelength demultiplexing circuit. it can.

[Second Embodiment]
An optical receiver according to the second embodiment of the present invention will be described. The overall configuration of the optical receiver in this embodiment is the same as that of the optical receiver of the first embodiment, as shown in FIG. 1, and the design parameters of the arrayed waveguide grating are also the same as those of the first embodiment. It is the same as that illustrated. The optical receiver of this embodiment is different from the case of the first embodiment (see FIG. 2 and the like) in the configuration of the polarization conversion circuit 107.

  FIG. 8 is a plan view showing in detail the polarization conversion circuit 107 and its vicinity of the optical receiver according to the present embodiment. The polarization conversion circuit 107 includes a main portion 202 that is a tapered waveguide whose core width extends from w1 to w2. The main part (tapered waveguide) 202 has an output side connected to the input side of the input-side slab waveguide 102, and the input side smoothly connects the input waveguide 101 and the main part (tapered waveguide) 202 to the second side. The taper waveguide 201 is connected to the output side. The tapered waveguide 202 and the second tapered waveguide 201 are channel waveguides similar to the input waveguide 101.

  The main portion of the tapered waveguide 202 is 50 μm in length, the core width on the input waveguide 101 side (second taper waveguide side) is w1 = 0.64 μm, and the core width on the input side slab waveguide 102 side. Is w2 = 0.74 μm. The width of the core of the input waveguide 101 is 0.5 μm, and the length of the second tapered waveguide 201 is 15 μm.

The optical receiver according to the present embodiment has a silicon nitride Si ₃ N ₄ layer formed on an upper portion (opposite side of the silicon substrate side, the same applies hereinafter) of a main portion (tapered waveguide) 202. In FIG. 8, a region 204 indicates a region where the silicon nitride Si ₃ N ₄ layer is formed. In addition, the optical receiver of this embodiment, the upper portion of the second tapered waveguide 201 and the input-side slab waveguide 102, regions 203 and 205 of the tapered structure formed of silicon nitride, each silicon nitride Si ₃ It is connected to the region 204 of the N ₄ layer. This causes an asymmetry of the refractive index between the upper clad and the lower clad of the silicon waveguide (201, 202, 102), and this becomes a perturbation from the TM polarization fundamental mode to the TE polarization primary mode. This is to cause the polarization conversion.

  Here, the width 203 of the tapered region 203 changes from 0.2 μm to w1 = 0.64 μm from the input waveguide side. The width 205 of the tapered structure region 205 changes from the main portion 202 side from w2 = 0.74 μm to 0.2 μm. The tapered regions 203 and 205 do not directly contribute to the operation of polarization conversion, but at the connection between the second tapered waveguide 201 and the main portion 202 and between the main portion 202 and the input-side slab waveguide 102. This has the effect of reducing mode mismatch and suppressing the occurrence of excess loss. In other words, by mitigating mode mismatch (discontinuity of structure between tapered structures) that causes reflection and emission of light energy, it is possible to suppress the occurrence of excess loss due to reflection and emission of light energy. it can.

  FIG. 9 shows a circuit structure in a cross section taken along the line C-C ′ of the main part of FIG. 8. A core of a main portion (tapered waveguide) 202 made of silicon is embedded in the lower clad 212 and the upper clad 211 on the silicon substrate 213. The upper cladding 211 includes an upper cladding first layer 214 and an upper cladding second layer 204. Both the lower clad 212 and the upper clad first layer 214 are made of quartz. The second layer 204 of the upper cladding is made of silicon nitride. The second layer 204 may be a material having a refractive index higher than that of the material forming the lower cladding 212 and the material forming the first layer 214 and lower than that of silicon.

  The main portion (tapered waveguide) 202 has a core thickness of 0.22 μm, an upper clad 211 having a thickness of 1.5 μm, and a lower clad 212 having a thickness of 2 μm. The thickness of the second layer 204 of the upper cladding is 0.5 μm, and the thickness of the first layer 214 of the upper cladding from the upper surface of the core of the tapered waveguide 202 to the lower surface of the second layer 204 is 0.1 μm. As described above, the distance between the upper surface of the core 202 and the lower surface of the second layer 204 may be equal to or less than the thickness of the core.

  Here, for proper operation of the polarization conversion circuit 107, it is necessary for the light propagating through the main portion (tapered waveguide) 202 to feel the asymmetry of the refractive index above and below the tapered waveguide. Therefore, it is desirable that the width of the second layer (silicon nitride) 204 of the upper cladding is sufficiently wider than the width of the tapered waveguide 202, and is set to 2 μm in this embodiment.

  FIG. 10 is a diagram for explaining the function of the main part 202 of the polarization conversion circuit 107, and shows the result of calculating the effective refractive index with respect to the core width w of the silicon tapered waveguide 202. FIG. Here, the material and the cross-sectional structure of the waveguide are the same as those described with reference to FIG. 9, and the effective refractive index is calculated up to three by changing only the core width w of the tapered waveguide 202. Here, the fundamental mode of TE polarization is maintained regardless of w in the first mode, but the second mode and the third mode are combined modes in which the TE polarization component and the TM polarization component are mixed by changing the core width w. It has become. When the core width w is around 0.64 μm, the second mode is almost the TM polarized base mode, and the third mode is almost the TE polarized primary mode. On the other hand, when the core width w is in the vicinity of 0.74 μm, the second mode is switched to the primary mode of about TE polarization, and the third mode is switched to the base mode of about TM polarization.

  FIG. 11 shows the intensity ratio of the TE polarization component and the TM polarization component from the calculation result of the second mode in FIG. 10 in particular. It can be seen that as the core width w increases, the TM polarization component intensity ratio decreases, the TE polarization component intensity ratio increases, and the TM polarization component and the TE polarization component are interchanged. In the present embodiment, the main portion 202 of the polarization conversion circuit 107 is set such that the core width is set to w1 = 0.64 μm and the core width is set to w2 = 0.74 μm, and the narrow side (the boundary side with the second tapered waveguide side). The core width w1 and the core width w2 on the wide side (the boundary side with the input-side slab waveguide 102) are designed to include as much as possible the region where the polarization component of the second mode is switched. That is, the core width w1 is set to such a width that the second mode is substantially a TM polarized base mode, and the core width w2 is set to be a width such that the second mode is substantially a TE polarized primary mode. ing.

  Therefore, the TE-polarized fundamental mode light input from the input waveguide 101 in FIG. 8 passes through the polarization conversion circuit 107 as it is and is input to the input-side slab waveguide 102, and TM polarization input from the input waveguide 101. The fundamental mode light is converted to TE-polarized primary mode light in the process of propagating through the tapered waveguide 202 and is input to the input-side slab waveguide 102.

  Therefore, in the optical receiver according to the present embodiment, when the fundamental mode light of the TE polarized wave having a certain wavelength is input by the same operation as that of the optical receiver according to the first embodiment, the light is condensed according to the wavelength. When the electric field distribution of the TE-polarized base mode light is reconfigured at the position and TM-polarized base mode light of the same wavelength is input, The electric field distribution of the TE-polarized primary mode light is reconstructed. Further, since both the TE-polarized fundamental mode light and the TE-polarized primary mode light are coupled to the output waveguide (multimode waveguide) 105, which is a multimode waveguide, with almost no loss, the light is received by the photodetector 106. The wavelength dependence of the sensitivity is almost the same regardless of the polarization, and a flat waveform can be obtained as in the conventional WDM optical receiver.

  In addition, the polarization conversion circuit 107 in the optical receiver according to the present embodiment has better structure continuity with the input-side slab waveguide 102 than the optical receiver according to the first embodiment, and polarization conversion. There is also an advantage that the generation of reflection and loss is suppressed at the junction between the circuit 107 and the input-side slab waveguide 102.

  From the above, the optical receiver of the present embodiment does not require a polarization diversity configuration, the WDM light that sufficiently reduces the polarization dependence with a single wavelength demultiplexing circuit and has excellent reflection or loss characteristics. A receiver can be realized.

  Here, in the present embodiment, an example is shown in which the width of the second layer 204 of the upper cladding formed of silicon nitride is set to 2 μm, but the present invention is not limited to this numerical example. FIG. 12 shows the efficiency of polarization conversion in the polarization conversion circuit 107 when the width of the second layer 204 of the upper cladding is changed in the optical receiver of this embodiment. The abscissa specifically indicates how many times the width of the second layer 204 of the upper cladding corresponds to the core width w2 = 0.74 μm of the silicon tapered waveguide 202. As can be understood from FIG. 12, the width of the second layer 204 of the upper cladding is preferably 1.5 times or more the core width w 2 of the tapered waveguide 202. The width = 2 μm of the second layer 204 of the upper cladding satisfies this condition, and polarization conversion close to 100% is realized.

  FIG. 13 shows an example in which the configuration of the polarization conversion circuit 107 is slightly changed as a modification of the present embodiment. Specifically, a third tapered waveguide 221 is further provided between the silicon tapered waveguide 202 and the input-side slab waveguide 102. Here, the third taper waveguide 221 further expands the width of the waveguide core from w2, and is connected to the input-side slab waveguide 102. The TE-polarized base mode light and the TE-polarized wave Both light in the first order mode can propagate. The third tapered waveguide 221 is a channel type waveguide similar to the tapered waveguide 202 and the second tapered waveguide 201. The third taper waveguide 221 can be an intermediate taper waveguide that smoothly connects the polarization conversion circuit and the input side slab waveguide. Therefore, although the operation of the optical receiver is not changed by the installation of the third tapered waveguide 221, the opening width of the waveguide connected to the input side slab waveguide 102 is adjusted to some extent by the third tapered waveguide 221. Therefore, it is preferable in that a degree of freedom can be given to the design of the arrayed waveguide diffraction grating.

[Third Embodiment]
An optical receiver according to the third embodiment of the present invention will be described. The overall configuration of the optical receiver in this embodiment is the same as that of the optical receiver of the first embodiment or the second embodiment, as shown in FIG. Is the same as that exemplified in the first embodiment. The optical receiver of this embodiment is different from the case of the first embodiment (see FIG. 2 and the like) in the configuration of the polarization conversion circuit 107. The polarization conversion circuit 107 in this embodiment is reported in Non-Patent Document 1.

  FIG. 14 is a plan view showing in detail the polarization conversion circuit 107 and its vicinity of the optical receiver according to the present embodiment. The polarization conversion circuit 107 inserted between the input waveguide 101 and the input-side slab waveguide 102 includes a first rib-type waveguide taper (301, 302) and a second rib-type waveguide taper (303). 304). The first rib-type waveguide taper has a rib portion 301 and a fin portion 302 around the rib portion 301. The rib portion 301 has a tapered shape in which the left end is connected to the input waveguide 101, the width of the left end is equal to the width of the input waveguide 101, and the width of the right end changes to w1. The fin portion 302 of the first rib-type waveguide taper has a tapered shape in which the left end width is equal to the left end width of the rib portion 301 (that is, the width of the input waveguide 101) and the right end width changes to w3. . The second rib-type waveguide taper has a rib portion 303 and a fin portion 304 around the rib portion 303. The rib portion 303 of the second rib-type waveguide taper has a left end connected to the right end of the rib portion 301 of the first rib-type waveguide taper, the left end width is w1, and the right end width is changed to w2. Tapered shape. The fin portion 304 of the second rib-type waveguide taper has a tapered shape in which the width at the left end is w3 and the width at the right end is reduced to w2.

  In the present embodiment, the width of the input waveguide 101 is 0.5 μm, the length of the first rib-type waveguide taper (301, 302) is 50 μm, the width is w1 = 0.55 μm, w3 = 1.2 μm, The length of the second rib-type waveguide taper (303, 304) is 50 μm and the width is w2 = 0.85 μm.

  FIG. 15 shows a circuit structure in a cross section taken along the line D-D ′ of the polarization conversion circuit 107 in FIG. 14. The core 303 of the rib part and the core 304 of the fin part of the second rib-type waveguide taper formed of silicon are embedded in the lower clad 312 and the upper clad 311 on the silicon substrate 313. Both the upper clad 311 and the lower clad 312 are made of quartz. The rib portion core 303 has a thickness of 0.22 μm, the fin portion core 304 has a thickness of 0.15 μm, the upper clad 311 has a thickness of 1.5 μm, and the lower clad 312 has a thickness of 2 μm. The cross section of the circuit structure of the first rib-type waveguide taper is the same as that of the second rib-type waveguide taper.

  FIG. 16 is a diagram for explaining the function of the polarization conversion circuit 107, from the left end of the first rib-type waveguide taper (connection end to the input waveguide 101) to the right end of the second rib-type waveguide taper. It is a figure which shows the result of having calculated the effective refractive index in each position (0, 5, 10, 15, 20, 35, 50, 75, 100 micrometer position from the left end of a 1st rib type waveguide taper). In this calculation, up to three effective refractive indexes are calculated. Here, the fundamental mode of TE polarization is maintained regardless of the position of the first mode, but the second mode and the third mode are combined modes in which the TE polarization component and the TM polarization component are mixed as they travel through the waveguide. It has become. When the position is 0 μm, the second mode is almost a TM polarized fundamental mode, and the third mode is almost a TE polarized primary mode, while when the position is 50 μm, the second mode is almost TE polarized. The primary mode and the third mode of the wave are switched to the fundamental mode of TM polarization.

  FIG. 17 shows the intensity ratio of the TE polarization component and the TM polarization component from the calculation result of the second mode in FIG. It can be seen that as the light travels through the polarization conversion circuit 107, the intensity ratio of the TM polarization component decreases, the intensity ratio of the TE polarization component increases, and the TM polarization component and the TE polarization component are switched.

  Therefore, in FIG. 14, the TE-polarized fundamental mode light input from the input waveguide 101 passes through the polarization conversion circuit 107 as it is, and the TM-polarized fundamental mode light input from the input waveguide 101 is polarization-converted. In the process of propagating through the circuit 107, the light is converted into TE polarized primary mode light.

  Further, in the present embodiment, a third taper waveguide 305 is provided between the polarization conversion circuit 107 and the input side slab waveguide 102. The third taper waveguide 305 further expands the width of the waveguide from w2 and is connected to the input-side slab waveguide (102). The TE-polarized fundamental mode light and the TE-polarized primary mode Both light can propagate. The third taper waveguide 305 is a channel type silicon waveguide. The configuration in which the second rib-type waveguide taper and the third taper waveguide 305 are connected can be an intermediate tapered waveguide that smoothly connects the polarization conversion circuit and the input-side slab waveguide. Therefore, although the operation of the optical receiver is not changed by the installation of the third tapered waveguide 305, the opening width of the waveguide connected to the input side slab waveguide 102 is adjusted to some extent by the third tapered waveguide 305. Therefore, it is preferable in that a degree of freedom can be given to the design of the arrayed waveguide diffraction grating.

  As described above, also in the optical receiver of the present embodiment, when the fundamental mode light of the TE polarization of a certain wavelength is input by the same operation as the optical receiver of the first embodiment or the second embodiment, When the electric field distribution of TE-polarized base mode light is reconstructed at the condensing position according to the wavelength, and TM-polarized base mode light of the same wavelength is input, the same condensing as in TE polarized light At the position, the electric field distribution of the TE-polarized primary mode light is reconstructed. Furthermore, since both the TE-polarized fundamental mode light and the TE-polarized primary mode light are coupled to the output waveguide (multimode waveguide) 105 with almost no loss, the wavelength dependence of the light receiving sensitivity at the photodetector 106 is as follows. It is almost the same regardless of the polarization, and a flat waveform can be obtained as in the conventional WDM optical receiver.

  In addition, the polarization conversion circuit 107 in the optical receiver of the present embodiment has better structural continuity with the input side slab waveguide 102 than the first embodiment, and the polarization conversion circuit 107 and the input There is also an advantage that generation of reflection and loss is suppressed at the junction with the side slab waveguide 102.

  From the above, the optical receiver of the present embodiment does not require a polarization diversity configuration, a WDM light that sufficiently reduces polarization dependence with a single wavelength demultiplexing circuit and has excellent reflection or loss characteristics. A receiver can be realized.

  FIG. 18 shows an example in which the configuration of the polarization conversion circuit 107 is slightly changed as a modification of the present embodiment. Specifically, in the second rib waveguide taper (303, 304), the width of the fin portion 304 is further expanded to w4 toward the input-side slab waveguide 102, and the third taper waveguide 305 is It differs from the configuration of the polarization conversion circuit 107 in FIG. 14 in that it does not exist and the second rib-type waveguide taper is directly connected to the input-side slab waveguide 102. As an example of the design of the second rib-type waveguide taper of this modification, w2 = 1.0 μm, w3 = 2.0 μm, and length 100 μm.

  The polarization conversion circuit 107 shown in FIG. 18 has better structural continuity with the input-side slab waveguide 102 than the one shown in FIG. 14 of this embodiment, and the polarization conversion circuit 107 and the input-side slab waveguide. There is an advantage that generation of reflection and loss can be further suppressed at the joint portion with 102.

[Fourth Embodiment]
An optical receiver according to the fourth embodiment of the present invention will be described. The configuration of the optical receiver in this embodiment is the same as that in the third embodiment, but an example in which the wavelength interval of the received WDM signal is different will be described. The configuration of the entire circuit of the optical receiver according to the present embodiment is as shown in FIG. 1. The core width of the output waveguide 105 is 3 μm, and the arrangement interval of the output waveguides 105 connected to the output side slab waveguide 104 is It is the same that the waveguide interval of the arrayed waveguide 103 in the portion connected to 4.5 μm, the number of the output waveguides 105 is 4, and the input side slab waveguide 102 and the output side slab waveguide 104 is 1.2 μm. The wavelength division between the ports is designed to be 20nm. Therefore, the number of arrayed waveguides 103 is 40, which is designed to be successively longer than the inner waveguide by a certain amount ΔL. ΔL is 3.88 μm, the input side slab waveguide (102) and the output side slab waveguide (104 ) Is 64 μm long.

  FIG. 19 is a diagram illustrating a result of calculating the wavelength dependence of each light receiving sensitivity of the photodetector 106 in the optical receiver of the present embodiment. Here, the four photodetectors 106 shown in FIG. 1 are called photodetectors 1, 2, 3, and 4 from the top. The solid line represents the light receiving sensitivity when the input light is TE-polarized base mode light, and the broken line represents the light receiving sensitivity when the input light is TM-polarized base mode light. The light receiving sensitivity is expressed as a relative sensitivity based on the sensitivity at the wavelength center of each photodetector. As in the other embodiments, the wavelength dependence of the light reception sensitivity is substantially the same regardless of whether the input polarization is TE polarization or TM polarization, and a flat waveform can be obtained. It can also be seen that a WDM receiver with a wide wavelength spacing such as a wavelength spacing of 20 nm can be realized by simply changing the design parameters of the arrayed waveguide grating.

  As described above, from various embodiments, according to the present invention, the wavelength dependence of the light receiving sensitivity is substantially the same regardless of the polarization, and a flat waveform can be obtained like the conventional WDM optical receiver. It was shown that an optical receiver can be realized.

  In the above embodiment, quartz glass is used as the material for the upper cladding and the lower cladding, but the scope of application of the present invention is not limited to this material. It may be formed of a material having a refractive index lower than that of silicon.

  In the above embodiment, specific numerical values are used as the thicknesses of the upper clad and the lower clad. However, the scope of the present invention is not limited to this value, and the thickness is equal to or greater than that of the core. I just need it.

  In the polarization conversion circuit portion of the optical receiver of the above embodiment, the design parameter of the silicon waveguide is limited to a specific value. However, the application range of the present invention is not limited to this parameter. The structure of the core width of the silicon waveguide where conversion occurs between the TM-polarized fundamental mode light and the TE-polarized primary mode light depends on the thickness and width of the core, and the refractive index of the upper and lower cladding materials. Determined.

  The thickness of the core of the fin portion of the rib-type waveguide in the first, third, and fourth embodiments is not limited to a specific value, but when the thickness is comparable to the thickness of the core of the rib portion. There is a possibility that an unnecessary higher-order mode higher than the first-order mode may occur, resulting in deterioration of characteristics. Therefore, from the viewpoint of suppressing higher-order modes, the core thickness of the fin portion is about 1/3 or less of the core thickness of the rib portion in the first embodiment, and in the third and fourth embodiments. It is preferable to set it to about 3/4 or less of the thickness of the core of the rib portion.

In the above embodiment, the input waveguide is preferably a single mode waveguide or optically connected to the single mode waveguide from the viewpoint of connectivity with other circuits. The single-mode condition of the silicon waveguide varies depending on the refractive index of each material of the upper cladding and the lower cladding. However, when the most common quartz is used for the cladding, if it is a channel type, the cross-sectional area of the waveguide core is approximately Must be 0.2 μm ² or less.

DESCRIPTION OF SYMBOLS 10 Array waveguide diffraction grating chip 11, 21, 26, 101 Input waveguide 12, 22, 102 Input side slab waveguide 13, 23, 103 Array waveguide 14, 24, 104 Output side slab waveguide 15, 25, 105 Output waveguide (multi-mode waveguide)
16 Optical coupling lens 17, 28, 106 Photo detector 20 Silicon optical circuit chip 27 Polarization separation rotation circuit 100 Optical receiver 107 Polarization conversion circuit 111, 112 Channel type-rib type waveguide conversion circuit 113 Polarization conversion circuit Main portion 113a Rib portion of rib type waveguide 113b Fin portion of rib type waveguide 121, 131, 211, 311 Upper clad 122, 132, 212, 312 Lower clad 123, 133, 213, 313 Silicon substrate 201 Second taper Waveguide 202 Tapered waveguide 203, 205 Tapered structure 204 Layer of silicon nitride (second layer of upper cladding)
214 First layer of upper cladding 221, 305 Third tapered waveguide 301 Rib portion of first rib-type waveguide taper 302 Fin portion of first rib-type waveguide taper 303 303 of second rib-type waveguide taper Rib portion 304 Fin portion of second rib-type waveguide taper

Claims (6)

An optical receiver comprising an input waveguide, an input slab waveguide, an arrayed waveguide, an output slab waveguide, an output waveguide that is a multimode waveguide, and a photodetector,
Between the input waveguide and the input-side slab waveguide, the input TE-polarized base mode light is output as TE-polarized base mode light, and the input TM-polarized base mode light is output as TE. An optical receiver characterized by further comprising a polarization conversion circuit for converting into polarized first-order mode light for output. The polarization conversion circuit is a rib-type silicon waveguide having a rib portion and a fin portion,
The rib-type silicon waveguide has a tapered shape in which the core of the fin portion is thinner than the core of the rib portion, and the core width of the fin portion gradually increases. Item 4. The optical receiver according to Item 1. The polarization conversion circuit is a tapered waveguide whose width gradually increases. The tapered waveguide is a waveguide including a core, a lower clad, and an upper clad. The core is made of silicon, and the lower clad And the upper cladding is formed of a material having a lower refractive index than silicon,
In the region near the upper part of the tapered waveguide, the upper clad has a first layer in contact with the upper part of the core and a second layer formed on the upper part of the first layer, The second layer is formed of a material having a refractive index higher than that of the material forming the lower cladding and the material of forming the first layer, and having a refractive index lower than that of silicon. The optical receiver according to claim 1, wherein a distance between an upper surface of the core and a lower surface of the second layer is equal to or less than a thickness of the core. The polarization conversion circuit includes first and second rib-type silicon waveguides each having a rib portion and a fin portion and connected to each other, and the thickness of the core of the fin portion is equal to that of the core of the rib portion. Thinner than the thickness,
The first rib-type silicon waveguide has a taper shape in which the core width of the rib portion and the core width of the fin portion gradually increase,
The said 2nd rib type | mold silicon waveguide is a taper shape which the core width of the said rib part expands gradually, and the core width of the said fin part reduces gradually, The said 1st characterized by the above-mentioned. Optical receiver.   The optical receiver according to claim 3, further comprising an intermediate taper waveguide between the polarization conversion circuit and the input-side slab waveguide. The polarization conversion circuit is a rib-type silicon waveguide having a rib portion and a fin portion,
The rib-type silicon waveguide has a tapered shape in which the core of the fin portion is thinner than the core of the rib portion, and the core width of the rib portion and the core width of the fin portion gradually increase. The optical receiver according to claim 1, wherein the optical receiver is directly connected to the input-side slab waveguide.

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