Optical multiplexing / demultiplexing circuit, wavelength filter, and method for manufacturing optical multiplexing / demultiplexing circuit

Abstract

An optical multiplexing / demultiplexing circuit (10) according to the present disclosure branches input light at a prescribed branching ratio, said optical multiplexing / demultiplexing circuit (10) comprising an interference part (11), a pair of input waveguides (12) connected to one end surface of the interference part (11), and a pair of output waveguides (13) connected to the other end surface of the interference part (11), wherein both side surfaces of the interference part (11) have a curved shape, said curved shape in a top view includes a first curved section where the width of the interference part (11) is narrower than the width (W) at the end surface, and the branching ratio is controlled by the width of the interference part (11) at the first curved section.

Description

Photomultiplier branching circuit, wavelength filter, and method for manufacturing a photomultiplier branching circuit

[0001] This disclosure relates to a photomultiplier branching circuit, a wavelength filter, and a method for manufacturing a photomultiplier branching circuit, all based on a multimode interferometer.

[0002] With the increase in communication traffic, optical transceivers are being put into practical use in wavelength division multiplexing (WDM) communication.

[0003] As a wavelength multiplexer / demultiplexer, a component of WDM communication optical transceivers, a wavelength filter (MZI-type wavelength filter) using a small, low-loss MZI (Mach-Zehnder interface meter) is used. As shown in Figure 9, the MZI-type wavelength filter is composed of an optical multiplexer / splitter circuit (optical splitter) 20 and a delay line 4_1, enabling wavelength multiplexing and demultiplexing. For example, a directional coupler is used in the optical multiplexer / splitter circuit 20. The directional coupler splits the light at an arbitrary splitting ratio.

[0004] A DMZI (Delay Mach-Zehender interface) type wavelength filter using a directional coupler in an optical splitter has been disclosed (Non-Patent Literature 1). However, because this wavelength filter has an operating bandwidth limited to a narrow wavelength range, it is difficult to apply it to CWDM (Coarse Wavelength Division Multiplexing) technology, which expands the channel spacing and transmission bandwidth.

[0005] To apply wavelength filters to CWDM technology, it is necessary to suppress the wavelength dependence of the branching ratio of the directional coupler. To suppress the wavelength dependence of the directional coupler, it is necessary to reduce (e.g., to zero) the group refractive index difference between odd and even modes. However, it is difficult to reduce (e.g., to zero) the group refractive index difference by changing (designing) the inter-core distance or core width of the directional coupler. Therefore, it has been difficult to suppress the wavelength dependence of the branching ratio of the directional coupler.

[0006] On the other hand, a configuration has been disclosed in which the wavelength dependence of the branching ratio of a directional coupler is suppressed by curving the waveguide of the directional coupler (Non-Patent Literature 2). By applying this directional coupler to an MZI type wavelength filter, wavelength multiplexing and demultiplexing over a wide wavelength band has been reported (Non-Patent Literature 3).

[0007] Folkert Horst et al., “Cascaded Mach-Zehnder wavelength filters in silicon photonics for low loss and flat pass-band WDM (de-)multiplexing”, OPTICS EXPRESS, Vol.21, No.10 (2013).<https: / / doi.org / 10.1364 / OE.21.011652> Ahmed H. El-Saeed etal., “Low-Loss Silicon Directional Coupler with Arbitrary Coupling Ratios for Broadband Wavelength Operation Based on Bent Waveguides” ,arXiv:2404.06117v1 [physics.optics] 9 Apr 2024.<https: / / doi.org / 10.48550 / arXiv.2404.06117> Hongnan Xu et al., “Flat-Top CWDM (De)Multiplexer Based on MZI With Bent Directional Couplers”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL.30, NO.2 (2018) p.169-172. DOI:10.1109 / LPT.2017.2779489.

[0008] However, the aforementioned directional coupler has the problem of being difficult to manufacture and unstable because it requires a structure (narrow gap structure) in which the cores of the two optical waveguides are placed at a narrow distance (about 100 nm).

[0009] To solve the problems described above, the optical splitting circuit according to the present disclosure is an optical splitting circuit that splits input light at a predetermined splitting ratio, comprising an interference section, a pair of input waveguides connected to one end face of the interference section, and a pair of output waveguides connected to the other end face of the interference section, wherein both sides of the interference section have a curved shape, and in a top view, the curved shape includes a first curved section in which the width of the interference section is narrower than the width of the end face, and the splitting ratio is controlled by the width of the interference section in the first curved section.

[0010] Furthermore, the present disclosure relates to a method for manufacturing an optical splitter circuit, comprising an interference section, a pair of input waveguides connected to one end face of the interference section, and a pair of output waveguides connected to the other end face of the interference section, wherein the input light is split at a predetermined splitting ratio to output light, and the method for manufacturing an optical splitter circuit comprises the steps of: setting the initial values ​​of the length and width of the interference section, the spacing between the pair of input waveguides, and the spacing between the pair of output waveguides so that the splitting ratio is approximately equivalent to 0.5:0.5, with low loss and suppressed wavelength dependence; setting the spectrum of the input light; setting the wavelength dependence of the splitting ratio; and calculating the wavelength dependence of the splitting ratio when the input light is input to one of the input waveguides, wherein in the calculation step, the length of the interference section, the spacing between the pair of input waveguides, and the spacing between the pair of output waveguides are kept constant, and the width is changed while repeating the calculation.

[0011] According to this disclosure, it is possible to provide a photomultiplier branching circuit, a wavelength filter, and a method for manufacturing a photomultiplier branching circuit that can be easily manufactured and suppress the wavelength dependence of the branching ratio.

[0012] Figure 1 is a schematic diagram showing the configuration of a wavelength filter according to the first embodiment of this disclosure. Figure 2A is a schematic diagram showing the configuration of a delay interferometer according to the first embodiment of this disclosure. Figure 2B is a schematic diagram showing the configuration of a delay interferometer according to the first embodiment of this disclosure. Figure 2C is a schematic diagram showing the configuration of a delay interferometer according to the first embodiment of this disclosure. Figure 3A is a schematic top view showing the configuration of an optical splitter circuit according to the first embodiment of this disclosure. Figure 3B is a schematic top view illustrating the configuration of an optical splitter circuit according to the first embodiment of this disclosure. Figure 4A is a diagram illustrating the operation of a conventional optical splitter circuit. Figure 4B is a diagram illustrating the operation of a conventional optical splitter circuit. Figure 4C is a diagram illustrating the operation of a conventional optical splitter circuit. Figure 5A is a diagram illustrating the operation of a conventional wavelength filter. Figure 5B is a diagram illustrating the operation of a conventional wavelength filter. Figure 5C is a diagram illustrating the operation of a conventional wavelength filter. Figure 6A is a diagram illustrating the effects of an optical splitter circuit according to the first embodiment of this disclosure. Figure 6B is a diagram illustrating the operation and effect of the photomultiplier / branching circuit according to the first embodiment of this disclosure. Figure 6C is a diagram illustrating the operation and effect of the photomultiplier / branching circuit according to the first embodiment of this disclosure. Figure 7A is a diagram illustrating the operation and effect of the wavelength filter according to the first embodiment of this disclosure. Figure 7B is a diagram illustrating the operation and effect of an ideal wavelength filter. Figure 8 is a flowchart illustrating the manufacturing method of the photomultiplier / branching circuit according to the first embodiment of this disclosure. Figure 9 is a schematic diagram showing the configuration of a conventional delay interferometer.

[0013] <First Embodiment> A wavelength filter and photomultiplier / branching circuit according to the first embodiment of this disclosure will be described with reference to Figures 1 to 7B.

[0014] <Wavelength Filter Configuration> Figure 1 shows the basic configuration of a DMZI type filter as an example of the configuration of the wavelength filter 1 according to this embodiment. The wavelength filter 1 consists of a 2-input, 2-output x-stage delay interferometer (x stage A) 2_1 and 2-input, s-output x-stage delay interferometers 2_2, 2_3 (x stage B, x stage C) connected to its output.

[0015] For example, the wavelength filter 1 uses Si, SiN, or SiON as the waveguide core through which light propagates on a Si substrate, and uses SiO as the cladding around the waveguide core ２ to be configured.

[0016] <Configuration of Delay Interferometer> In the wavelength filter 1, the delay interferometer 2 includes, for example, a single-stage delay interferometer configured by an optical multiplexing / demultiplexing circuit 10 with a coupling coefficient (branching ratio) κ and a delay line, or a two-stage delay interferometer in which such delay interferometers are cascaded. By increasing the number of stages of the delay interferometer, crosstalk between channels can be suppressed, and the transmission band can be flattened.

[0017] FIGS. 2A to 2C show configuration examples of the delay interferometer 2. For example, as shown in FIG. 2A, the single-stage delay interferometer sequentially includes an optical multiplexing / demultiplexing circuit 10 with a branching ratio κ １ , a first waveguide pair (two waveguides) 3_1, 3_2, and an optical multiplexing / demultiplexing circuit 10 with a branching ratio κ ２ . In the first waveguide pair, one waveguide (delay line) 3_1 is longer than the other waveguide 3_2 by a waveguide length difference ΔL １ .

[0018] As shown in FIG. 2B, the delay interferometer 2 may be a two-stage delay interferometer. The two-stage delay interferometer sequentially includes an optical multiplexing / demultiplexing circuit 10 with a branching ratio κ １ , a first waveguide pair (two waveguides) 3_1, 3_2, an optical multiplexing / demultiplexing circuit 10 with a branching ratio κ ２ , a second waveguide pair 3_3, 3_4, and an optical multiplexing / demultiplexing circuit 10 with a branching ratio κ ３ . In the first waveguide pair, one waveguide (delay line) 3_1 is longer than the other waveguide 3_2 by a waveguide length difference ΔL １ . In the second waveguide pair, the other waveguide (delay line) 3_4 is longer than one waveguide 3_3 by a waveguide length difference ΔL ２ .

[0019] As shown in FIG. 2C, the delay interferometer 2 may be a three-stage delay interferometer. The three-stage delay interferometer sequentially includes an optical multiplexing / demultiplexing circuit 10 with a branching ratio κ １ , a first waveguide pair (two waveguides) 3_1, 3_2, and an optical multiplexing / demultiplexing circuit 10 with a branching ratio κ ２The photomultiplier branching circuit 10, the second waveguide pair 3_3, 3_4, and the branching ratio κ ３ The photomultiplier branching circuit 10, the third waveguide pair 3_5, 3_6, and the branching ratio κ ４ The system includes an optical splitting circuit 10. In the first waveguide pair, one waveguide (delay line) 3_1, 3_2 has a waveguide length difference ΔL from the other waveguide 3_2. １ And it is long. In the second waveguide pair, the other waveguide (delay line) 3_4 has a waveguide length difference ΔL compared to the other waveguide 3_3. ２ And it is long. In the third waveguide pair, the other waveguide (delay line) 3_6 has a waveguide length difference ΔL from the other waveguide 3_5. ３ And it's long.

[0020] <Configuration of the Optical Splitter Circuit> As shown in Figure 3A, the optical splitter circuit 10 according to this embodiment is a multimode interferometer (MMI) and comprises an interfering section 11, two input waveguides 12 connected to one end face of the interfering section 11, and two output waveguides 13 connected to the other end face of the interfering section 11.

[0021] The interfering section 11, the input waveguide 12, and the output waveguide 13 may be made of, for example, Si, SiN, or SiON. The cladding around the interfering section 11, the input waveguide 12, and the output waveguide 13 is SiO ２ It may consist of these elements.

[0022] The length L of the interference portion 11 is 30 μm. The width W of both end faces of the interference portion 11 is... １ It is 2 μm.

[0023] The width W' of each of the two input waveguides 12 is 1 μm. The distance P between the two input waveguides 12 is 2 μm.

[0024] The two output waveguides 13 are the same as the input waveguide 12, with a width W' of 1 μm and a spacing P of 2 μm.

[0025] Figure 3B shows the detailed configuration of the interference section 11 of the photo-combination branching circuit 10.

[0026] When the branching ratio of the photo-combining branching circuit 10 is 0.5:0.5 (solid line in the figure), both end faces and both sides of the interference portion 11 are flat, and in a top view, the shape of the interference portion 11 is rectangular.

[0027] When the branching ratio of the optical power splitter circuit 10 increases, both side surfaces of the interference section 11 are curved in the length direction of the interference section 11 (the direction perpendicular to the end face of the interference section 11).

[0028] In the curved shape of the side surface of the interference section 11, the width W of the interference section 11 changes (modulates) when viewed from the top surface.

[0029] First, a case where the branching ratio is, for example, 0.72:0.28 in the optical power splitter circuit 10 will be described (dotted line in the figure).

[0030] In this configuration, the width W of the interference section 11 is narrower than the width (3.2 μm) at both end faces of the interference section 11 at the center (x=0) in the length direction, and has a minimum value (about 2.9 μm).

[0031] The width W of the interference section 11 increases from the center (x=0) in the length direction toward both ends, is wider than the width at both end faces at about x=±7 μm, and has a maximum value (about 3.3 μm). Further, the width W decreases toward both ends, is narrower than the width at both end faces at about x=±11 μm, and has a minimum value (about 3 μm), a maximum value at about x=±13 μm, and a minimum value at about x=±14 μm.

[0032] In this way, in the variation of the width W of the interference section 11, the extreme value at x=0 is larger than the extreme value on the end face side from x=0. Here, the "extreme value" refers to the distance between a maximum point or a minimum point (the vertex at the curved portion in the interference section 11) and a reference line connecting both ends of the side surface (corresponding to the side surface when the branching ratio is 0.5:0.5, hereinafter referred to as the "reference line").

[0033] In other words, regarding the configuration in which the branching ratio is 0.72:0.28 in the optical power splitter circuit 10, the outer edge of the side surface of the interference section 11 has a plurality of curved portions when viewed from the top surface.

[0034] The outer edge of the side surface of the interference section 11 has a first curved portion inside the reference line at the center in the length direction. Further, a second curved portion is provided outside the reference line at about x=±7 μm from the center in the length direction. A third curved portion is provided on the inner side at about x=±11 μm, a fourth curved portion on the outer side at about x=±13 μm, and a fifth curved portion on the inner side at about x=±14 μm.

[0035] On the side surface of the interfering portion 11, the distance between the vertex of the first curved portion and the reference line is longer than the distance between the vertices of the second to fifth curved portions and the reference line. The distance between the vertex of the first curved portion and the reference line may be the longest.

[0036] Thus, the shape of the interfering portion 11 is mirror-symmetric with respect to the central axis in the length direction (direction perpendicular to the outer edge of the end face) (x=0 in Figure 3B) and the central axis in the width direction (direction parallel to the outer edge of the end face) (y=0 in Figure 3B) when viewed from above.

[0037] Next, we will explain the case where, for example, the branching ratio in the photo-combining branching circuit 10 is 0.92:0.08 (dashed line in the figure).

[0038] In this configuration, the width W of the interference portion 11 is narrower than the width of both end faces of the interference portion 11 (3.2 μm) at the center in the longitudinal direction (x = 0), and has a minimum value (approximately 2.2 μm).

[0039] The width W of the interference portion 11 increases from the center in the longitudinal direction (x=0) toward both ends, and is greater than the width of the end faces at x = ±6 μm and ±9 μm degrees, having a maximum value (3.4 to 3.6 μm). Furthermore, the width W of the interference portion 11 decreases toward both ends, is narrower than the width of the end faces at x = ±11 μm, having a minimum value (approximately 3 μm), and has a maximum value at x = ±13 μm.

[0040] In the configuration of the photomultiplier branching circuit 10 with a branching ratio of 0.92:0.08, the outer edge of the side surface of the interference portion 11 has a first curved portion inside the reference line at the center in the longitudinal direction. It also has a second curved portion outside the reference line at x = ±6 μm and ±9 μm from the center in the longitudinal direction. It has a third curved portion inside at x = ±11 μm, and a fourth curved portion outside at x = ±13 μm.

[0041] On the side surface of the interfering portion 11, the distance between the vertex of the first curved portion and the reference line is longer than the distance between the vertices of the second to fourth curved portions and the reference line. The distance between the vertex of the first curved portion and the reference line may be the longest.

[0042] As described above, in the photo-combining branching circuit 10, when the branching ratio (absolute value) is greater than 0.5:0.5, the interference portion 11, when viewed from above, has an outer edge on its side that curves in the longitudinal direction. The curved shape of the side of the interference portion 11 has a first curved portion in which the width of the interference portion 11 is narrower than the width of both end faces.

[0043] In the photo-combining branching circuit 10, when viewed from above, the curved shape of the side surface of the interference portion 11 may have a first curved portion at the center in the longitudinal direction where the width of the interference portion 11 is narrower than the width of the end faces.

[0044] The width of the interference portion 11 in this first curved section can be used to control the branching ratio. For example, in a photomultiplier branching circuit with a large branching ratio, the width of the interference portion 11 in the first curved section may be narrower compared to a photomultiplier branching circuit with a small branching ratio.

[0045] Furthermore, in the photo-combining branching circuit 10, the curved shape of the side surface of the interference portion 11 may include a second curved portion that is wider than the width of the interference portion 11 at both ends, with the width of the interference portion 11 increasing from the center in the longitudinal direction toward each end face.

[0046] Furthermore, in the photomultiplier branching circuit 10, the curved shape of the side surface of the interference portion 11 may include a third curved portion in which the width of the interference portion 11 decreases toward each end surface and is narrower than the width of the end surfaces, and may also include a fourth curved portion in which the width increases toward each end surface and is wider than the width of the end surfaces.

[0047] Furthermore, in the interference section 11 of the photo-combining branching circuit 10, the extreme value of the width at the central curved section in the longitudinal direction may be greater than the extreme values ​​of the width at the other curved sections (for example, the second to fourth curved sections).

[0048] In the photo-combining branching circuit 10, the central axis in the length direction (direction perpendicular to the outer edge of the end face) (x=0 in Figure 3B) and the central axis in the width direction (direction parallel to the outer edge of the end face) (y=0 in Figure 3B) may be mirror-symmetric with respect to both the length direction and the width direction.

[0049] In other words, regarding the configuration of the light-combining branching circuit 10, in a top view, the outer edge of the side surface of the interference portion 11 has at least one curved portion. The first curved portion curves inward from the reference line.

[0050] For example, the interfering portion 11 may have a first curved portion located inside the reference line at its longitudinal center.

[0051] The branching ratio can be controlled by the distance between the apex of the first curved section and the reference line. For example, in a photomultiplier branching circuit with a large branching ratio, the distance between the apex of the first curved section and the reference line may be shorter than in a photomultiplier branching circuit with a small branching ratio.

[0052] Furthermore, the photo-combining branching circuit 10 may have a second curved portion outside the reference line on both end faces from the center in the longitudinal direction.

[0053] Furthermore, in the optical branching circuit 10, a third curved portion may be provided on the inside of the reference line at a portion of each of the end faces, and a fourth curved portion may be provided on the outside of the reference line at a portion of each of the end faces.

[0054] Furthermore, in the interference section 11 of the light-combining branching circuit 10, the distance between the vertex of the first curved section and the reference line may be longer than the distance between the vertices of the other curved sections (for example, the second to fourth curved sections) and the reference line.

[0055] In the photo-combining branching circuit 10, the circuit may be mirror-symmetric with respect to x=0 and y=0 in the length direction.

[0056] <Effects of the Photomultiplier and Wavelength Filter> The effects of the photomultiplier and wavelength filter 1 according to this embodiment will be explained with reference to Figures 4A to 7B.

[0057] First, for comparison, the operation of a conventional directional coupler will be described. Figures 4A to 4C show the calculated wavelength dependence of the branching ratio of a conventional directional coupler.

[0058] The calculations were performed using the FDTD method. White light was used as the input light and input to one of the input waveguides 12, and the intensity (transmission intensity) of the output light in each of the two output waveguides 13 was calculated. In the figure, the solid line shows the intensity of the output light (through) in one of the output waveguides 13, and the dotted line shows the intensity of the output light (cross) in the other output waveguide 13.

[0059] Figure 4A shows the wavelength dependence of the ideal branching ratio. The branching ratio is the same at all wavelengths, and the branching ratio is wavelength-independent.

[0060] Figure 4B shows the wavelength dependence of the branching ratio in a conventional directional coupler. Here, calculations were performed for a configuration designed so that the branching ratio is 0.5:0.5 (1:1) at a wavelength of 1.30 μm.

[0061] Conventional directional couplers exhibit a branching ratio of 0.5:0.5 (1:1) at a wavelength of 1.30 μm, but this ratio changes with wavelength. Thus, in conventional directional couplers, the branching ratio is wavelength-dependent.

[0062] Figure 4C shows the wavelength dependence of the branching ratio in a directional coupler when design errors occur. The waveguide core used in the calculation is 50 nm thinner than the waveguide core in the above design.

[0063] The wavelength corresponding to a branching ratio of 0.5:0.5 (1:1) is 1.36 μm, which differs from the designed wavelength (1.30 μm). Thus, the branching ratio changes due to manufacturing tolerances. Furthermore, even in this configuration, the branching ratio is wavelength-dependent.

[0064] Figures 5A to 5C show the transmission characteristics of a wavelength filter (DMZI type filter) using a directional coupler having the characteristics shown in Figures 4A to 5C. The calculations were performed using a transfer matrix for the DMZI type filter configuration shown in Figure 1, where each delay interferometer uses one stage delay interferometer. In the figures, spectra A to D show the spectra of the four outputs of the DMZI type filter.

[0065] Figure 5A shows the calculated output light intensity of a wavelength filter using a directional coupler with an ideal branching ratio (Figure 4A). Spectra A to D have a large peak (hereinafter referred to as the "primary peak") and multiple smaller peaks (hereinafter referred to as "sub-peaks"). In spectra A to D, the primary peak and sub-peaks have approximately the same intensity. Also, the primary peak and sub-peaks appear periodically. Thus, a directional coupler with an ideal branching ratio exhibits ideal transmission characteristics with no loss and crosstalk of about 10 dB.

[0066] Figure 5B shows the calculated output light intensity for a filter using a conventional directional coupler having the transmission characteristics shown in Figure 4B. Differences in intensity between the primary and secondary peaks occur in spectra A to D. In particular, the intensity differences in some secondary peaks are large.

[0067] Figure 5C shows the calculated output light intensity for a filter using a directional coupler with manufacturing tolerances and the transmission characteristics shown in Figure 4C. In spectra A to D, the intensity difference increases between the primary and secondary peaks.

[0068] Thus, in wavelength filters using directional couplers whose branching ratio is wavelength-dependent, optical loss increases and crosstalk increases.

[0069] As described above, the wavelength dependence of the branching ratio of the photomultiplier / branching circuit 10 increases optical loss and crosstalk in the wavelength filter. Furthermore, increasing the number of stages in the delay interferometer increases the number of directional couplers, thus increasing the effect of wavelength dependence.

[0070] On the other hand, in wavelength filters, a configuration using a multimode interferometer (MMI) instead of a directional coupler is conceivable. Multimode interferometers do not require a narrow-gap structure, can be easily manufactured, have low wavelength dependence, and have high processing tolerance.

[0071] In a multimode interferometer, multiple waveguide modes of light are excited within a wide waveguide, and each light has a different propagation constant, causing interference as it propagates. As a result, the propagating light forms an image at a specific location, and by positioning the waveguide at the imaging location, it is ideally possible to output light with the same intensity as the input light.

[0072] Furthermore, because the image formation position branches as a result of interference of propagating light, light can be output with any branching ratio by arranging waveguides at each image formation position. The number and proportion of excited waveguide modes, and the propagation constant of each waveguide mode, are determined by the incident position of light in a wide waveguide, the width and thickness of the waveguide, and the refractive index. By combining these components, light can be output with any branching ratio.

[0073] However, even by comprehensively changing the combinations of the above components, it is difficult to form an image at a specific position with an arbitrary branching ratio. As a result, imaging occurs at positions other than the waveguide output position, leading to optical loss. Therefore, it has been difficult to branch light with low loss at an arbitrary branching ratio using a wavelength filter with a multimode interferometer.

[0074] Next, the effects of the photomultiplier branching circuit 10 according to this embodiment will be explained. Figures 6A to 6C show the calculated results of the wavelength dependence of the branching ratio of the photomultiplier branching circuit 10.

[0075] The calculation was performed using the FDTD method. White light was used as the input light and input to one of the input waveguides 12, and the intensity (transmission intensity) of the output light in each of the two output waveguides 13 was calculated. In the figure, the solid line shows the intensity of the output light (through) in one of the two output waveguides 13, and the dotted line shows the intensity of the output light (cross) in the other output waveguide.

[0076] In the photomultiplier / branching circuit 10, with an MMI configuration having a branching ratio of 0.5:0.5 (solid line in Figure 3B), the input light can be branched with a branching ratio of 0.5:0.5, with low loss and suppressed wavelength dependence, as shown in Figure 6A.

[0077] In the photomultiplier / branching circuit 10, with an MMI configuration having a branching ratio of 0.72:0.28 (dotted line in Figure 3B), the light is branched with a branching ratio of 0.72:0.28, as shown in Figure 6B, and wavelength dependence is suppressed.

[0078] In the photomultiplier / branching circuit 10, with an MMI configuration having a branching ratio of 0.92:0.08 (dashed line in Figure 3B), the light is branched with a branching ratio of 0.92:0.08, as shown in Figure 6C, and wavelength dependence is suppressed.

[0079] In the configuration of the optical splitting circuit 10, the modulation width of the modulation structure on the side of the MMI with a branching ratio of 0.92:0.08 is greater than the modulation width of the modulation structure on the side of the MMI with a branching ratio of 0.72:0.28. In other words, the branching ratio can be increased by increasing the modulation width of the MMI.

[0080] In the optical multiplexer (MMI) 10, the propagation constants of each mode excited in the interfering section (waveguide) 11 can be selectively changed by modulating the width of the interfering section 11, and the imaging ratio can be arbitrarily changed. This makes it possible to realize an MMI with low loss and an arbitrary branching ratio, in which the wavelength dependence of the branching ratio is suppressed.

[0081] In the photomultiplier (MMI) 10, the branching ratio can be increased by reducing the width of the interference portion 11 at the first curved portion at the center in the longitudinal direction (the direction perpendicular to the end face of the interference portion 11). In other words, in the MMI 10, the narrower the width of the interference portion 11 at the first curved portion, the larger the branching ratio. Thus, in the MMI 10, the branching ratio can be controlled by the width of the interference portion 11 at the first curved portion.

[0082] Figure 7A shows the calculation results of the transmission characteristics of the wavelength filter 1 according to this embodiment. For the calculation, a wavelength filter with the configuration shown in Figure 1 and using a two-stage delay interferometer was used.

[0083] The delay interferometer of wavelength filter 1 is, in order, branch ratio κ １ The MMI, the first waveguide pair, and the branching ratio κ ２ The MMI, the second waveguide pair, and the branching ratio κ ３ It is equipped with an MMI. In the first waveguide pair, one waveguide (delay line) has a waveguide length difference ΔL from the other waveguide. １ And it is long. In the second waveguide pair, the other waveguide (delay line) has a waveguide length difference ΔL compared to the other waveguide. ２ And it's long. In MMI, the branching ratio κ １ The ratio is 0.5:0.5, and the branching ratio is κ. ２ The ratio is 0.3:0.7, and the branching ratio is κ. ３ The ratio was set to 0.1:0.9. Also, ΔL １ = 11 μm, ΔL ２ = 22 μm.

[0084] For comparison, Figure 7B shows the transmission characteristics of a wavelength filter using a delay interferometer with an ideal branching ratio.

[0085] In a wavelength filter using a delay interferometer with an ideal branching ratio, the main peak appears periodically in the output waveguides 13A to D with equivalent intensity (lossless) in the wavelength range of 1.25 μm to 1.35 μm. In addition, each sub-peak appears periodically with a similar intensity.

[0086] In wavelength filter 1, an optical loss of about 1 to 2 dB is observed at the main peak, but transmission characteristics almost equivalent to those of a wavelength filter using a delay interferometer with an ideal branching ratio are obtained.

[0087] According to the photomultiplier / branching circuit of this embodiment, light can be branched at any branching ratio with low optical loss, and the wavelength dependence of the branching ratio can be suppressed.

[0088] According to the wavelength filter of this embodiment, low optical loss is achieved, crosstalk between channels can be suppressed, and the transmission bandwidth can be flattened.

[0089] <Manufacturing Method of Photocombination Branch Circuit> An example of a manufacturing method of the photocombination branch circuit 10 according to this embodiment will be described with reference to Figure 8. Figure 8 shows a flowchart illustrating the manufacturing method of the photocombination branch circuit 10.

[0090] The width modulation structure (shape) of the photomultiplier branching circuit 10 is designed by inverse design using electromagnetic field analysis by time-domain difference method.

[0091] First, the materials constituting the MMI are determined. For example, Si (refractive index: 3.48) is set as the material constituting the input / output waveguides 12, 13 and the interference section 11. SiO is set as the material surrounding the Si. ２ (Refractive index: 1.44) may be set.

[0092] Furthermore, the initial values ​​(configuration) of the MMI are designed with a branching ratio of 1:1 (0.5:0.5) to achieve low loss and suppress wavelength dependence, and the length L and width W of the interference section 11, and the spacing P between the input and output waveguides 12 and 13 are determined as initial values ​​(step S1).

[0093] In designing the initial values ​​of the MMI, for example, the length L of the interference section 11 and the spacing P between the input and output waveguides 12 and 13 are set to predetermined values, and the width W of the interference section 11 is varied, and the branching ratio is repeatedly calculated. As a result of the calculation, the width W may be determined to have a branching ratio of 1:1, low loss, and suppressed wavelength dependence.

[0094] For example, in designing the initial values ​​of the MMI, the number of calculation iterations may be set to 50. The tolerance for wavelength dependence for a branching ratio of 0.5:0.5 may be around 0.05.

[0095] For example, the MMI is rectangular in shape when viewed from above, and its dimensions are determined to be L = 30 μm, W = 3 μm, and P = 2 μm. The width of the input and output waveguides 12 and 13 may also be determined to be W' = 1 μm.

[0096] When designing the initial values ​​of the MMI, if the branching ratio is set to any value other than 1:1 (0.5:0.5), the tolerance for wavelength dependence for a branching ratio of 0.5:0.5 does not converge to 0.05 or less. For example, if the number of calculation iterations is 50, the tolerance for wavelength dependence for a branching ratio of 0.5:0.5 is greater than 0.05.

[0097] Next, the input and output light are set (step S2). For example, the input light is set to broad light with a peak at a wavelength of 1.3 μm and spreading from 1.2 μm to 1.4 μm.

[0098] The wavelength dependence of the branching ratio for the output light is set. For example, if the branching ratio is 0.3:0.7, the branching ratio between one output light source and the other output light source of the output waveguide 13 is set to be constant at 0.3:0.7 for wavelengths from 1.2 μm to 1.4 μm. If the branching ratio is 0.1:0.9, the branching ratio between one output light source and the other output light source of the output waveguide 13 is set to be constant at 0.1:0.9 for wavelengths from 1.2 μm to 1.4 μm.

[0099] Next, a predetermined MMI (for example, Si and SiO) based on the above settings ２ Assuming that a set broad light is input to the given refractive index distribution, the wavelength dependence of the output light branching ratio is calculated (step S3).

[0100] For example, first, the spectra of one output light and the other output light from the output waveguide 13 are calculated within a predetermined wavelength range (e.g., 1.2 μm to 1.4 μm). Next, the wavelength dependence of the output light branching ratio is obtained from the ratio of the spectral intensities of one output light and the other output light from the output waveguide 13 at each wavelength.

[0101] In calculating the wavelength dependence of the output light branching ratio, the configuration of the MMI is such that the length L of the interference section 11 and the spacing P between the input and output waveguides 12 and 13 are set to predetermined values, and the width W of the interference section 11 is changed (modulated). At this time, the calculation may also be performed under the condition that the interference section 11 is mirror symmetric about the y-axis (x = 0 μm) and mirror symmetric about the x-axis (y = 0 μm).

[0102] Next, when the calculation or result of the wavelength dependence of the output light branching ratio satisfies the predetermined conditions, the calculation is terminated (step S4).

[0103] For example, the calculation may terminate when it reaches a predetermined number of repetitions.

[0104] Alternatively, the wavelength dependence of the output light branching ratio obtained by calculation may be compared with the wavelength dependence of the set output light branching ratio. For example, the difference between the wavelength dependence of the output light branching ratio obtained by calculation and the wavelength dependence of the set output light branching ratio can be calculated.

[0105] Next, the calculation is terminated when the difference between the wavelength dependence of the output light branching ratio and the wavelength dependence of the set output light branching ratio falls below a predetermined value. The predetermined value may be approximately 0.05 for the wavelength dependence of a branching ratio of 0.5:0.5. The configuration at this point is determined as the configuration of the photocombination branching circuit 10.

[0106] The calculation is repeated if the predetermined conditions are not met. For example, the calculation may be repeated if the number of repetitions is less than the predetermined number. Alternatively, the calculation may be repeated if the difference between the calculated value and the set value is greater than the predetermined value.

[0107] Based on the configuration determined above, the MMI is formed using conventional semiconductor processing processes such as photolithography and etching (step S5). This manufactures the photocombination branching circuit 10.

[0108] According to the manufacturing method of the photomultiplier branching circuit of this embodiment, the configuration of the photomultiplier branching circuit is simple, so the photomultiplier branching circuit can be easily manufactured.

[0109] In the embodiments of this disclosure, an example is shown in which a two-stage delay interferometer is used in the wavelength filter. However, the invention is not limited to this, and a one-stage or three or more-stage delay interferometer may also be used.

[0110] The wavelength filter according to the embodiments of this disclosure may be used in communication devices. Alternatively, it may be used in devices such as computing devices, computer systems, and signal processing devices. A device equipped with the wavelength filter according to this embodiment may be used in a data center. Multiple devices may be connected to form a network.

[0111] In the embodiments of this disclosure, the "branching ratio" refers to the ratio of the intensity of the output light in one of the two output waveguides 13 to the intensity of the output light in the other output waveguide (or the ratio of the intensity of the output light in the other output waveguide to the intensity of the output light in one output waveguide). For example, for input light with an intensity of 1.0, the branching ratios are 0.5:0.5 (or 0.5 / 0.5 = 1.0), 0.72:0.28 (or approximately 0.72 / 0.28 = 2.6), 0.92:0.08 (or 0.92 / 0.08 = 11.5), etc. In branching ratios, for example, 0.92:0.08 is greater than 0.5:0.5.

[0112] In the embodiments of this disclosure, examples of the structure, dimensions, materials, etc. of each component in the photomultiplier branching circuit, wavelength filter, and method for manufacturing the photomultiplier branching circuit are shown, but the invention is not limited thereto. Any method that performs the function and effect of the photomultiplier branching circuit, wavelength filter, and method for manufacturing the photomultiplier branching circuit is acceptable.

[0113] It should be noted that this disclosure is not limited to the embodiments described above, and it is evident that many modifications and combinations are possible within the technical concept of this disclosure by a person with ordinary skill in the art.

[0114] Some or all of the embodiments described above, or examples thereof, may also be described as follows, but are not limited to these.

[0115] (Note 1) An optical splitting circuit for splitting input light at a predetermined splitting ratio, comprising an interference section, a pair of input waveguides connected to one end face of the interference section, and a pair of output waveguides connected to the other end face of the interference section, wherein both sides of the interference section have a curved shape, and in a top view, the curved shape includes a first curved section in which the width of the interference section is narrower than the width of the end face, and the splitting ratio is controlled by the width of the interference section in the first curved section.

[0116] (Note 2) The photomultiplier branching circuit according to Note 1, wherein when the branching ratio of one of the photomultiplier branching circuits is greater than the branching ratio of the other photomultiplier branching circuits, the width of the interference portion in the first curved portion of the one photomultiplier branching circuit is narrower than the width of the interference portion in the first curved portion of the other photomultiplier branching circuit.

[0117] (Note 3) The optical splitting circuit according to Note 1 or Note 2, wherein, in a top view, the first curved portion is positioned at the center in a direction perpendicular to the end face of the interference portion, the width of the interference portion includes a second curved portion which increases from the center in the perpendicular direction toward each of the end faces and is wider than the width of the interference portion at both ends, and the distance between the vertex of the first curved portion and a reference line connecting both ends of the side surface is longer than the distance between the vertex of the second curved portion and a reference line connecting both ends of the side surface.

[0118] (Note 4) The photo-combining branching circuit according to any one of Notes 1 to 3, wherein, in a top view, the interfering portion is mirror-symmetric with respect to the central axis in a direction parallel to the outer edge of the end face and the central axis in a direction perpendicular to the outer edge of the end face.

[0119] (Note 5) A wavelength filter comprising a plurality of delay interferometers and waveguides connecting the delay interferometers, wherein the delay interferometers comprise a plurality of optical splitting circuits described in any of Notes 1 to 4 and a pair of waveguides connecting the optical splitting circuits, wherein one waveguide of the pair of waveguides is longer than the other waveguide.

[0120] (Note 6) An apparatus equipped with the wavelength filter described in Note 5.

[0121] (Note 7) A data center equipped with the devices described in Note 6.

[0122] (Note 8) A method for manufacturing an optical splitter circuit comprising an interference section, a pair of input waveguides connected to one end face of the interference section, and a pair of output waveguides connected to the other end face of the interference section, wherein input light is split at a predetermined splitting ratio to output light, the method comprising the steps of: setting the initial values ​​of the length and width of the interference section, the spacing between the pair of input waveguides, and the spacing between the pair of output waveguides so that the splitting ratio is approximately equivalent to 0.5:0.5, with low loss and suppressed wavelength dependence; setting the spectrum of the input light; setting the wavelength dependence of the splitting ratio; and calculating the wavelength dependence of the splitting ratio when the input light is input to one of the input waveguides, wherein in the calculation step, the length of the interference section, the spacing between the pair of input waveguides, and the spacing between the pair of output waveguides are kept constant, and the width is changed, and the calculation is repeated.

[0123] (Note 9) The optical splitting circuit according to any one of Notes 1 to 4, further including a third curved portion that decreases toward each of the two aforementioned end faces and is narrower than the width of the end faces, wherein the distance between the vertex of the first curved portion and the reference line connecting both ends of the side surface is longer than the distance between the vertex of the third curved portion and the reference line connecting both ends of the side surface.

[0124] (Note 10) The optical splitting circuit according to any one of Notes 1 to 4 or 9, further including a fourth curved portion that increases toward each of the two aforementioned end faces and is wider than the width of the end faces, wherein the distance between the vertex of the first curved portion and the reference line connecting both ends of the side surface is longer than the distance between the vertex of the fourth curved portion and the reference line connecting both ends of the side surface.

[0125] (Note 11) An optical splitting circuit for splitting input light at a predetermined splitting ratio, comprising an interference section, a pair of input waveguides connected to one end face of the interference section, and a pair of output waveguides connected to the other end face of the interference section, wherein, in a top view, the outer edge of the side surface of the interference section has at least one curved portion, and at the center in a direction perpendicular to the outer edge of the end face, there is one curved portion inside a reference line connecting both ends of the side surface, and the splitting ratio is controlled by the distance between the apex of the first curved portion and the reference line.

[0126] (Note 12) The photomultiplier branching circuit described in Note 11, wherein when the branching ratio of one of the photomultiplier branching circuits is greater than the branching ratio of the other photomultiplier branching circuits, the distance between the vertex of the first curved portion of the one of the photomultiplier branching circuits and the reference line is shorter than the distance between the vertex of the first curved portion of the other photomultiplier branching circuit and the reference line.

[0127] (Note 13) The optical splitting circuit according to Note 11 or Note 12, wherein, in a top view, it has other curved portions outside the reference line on both end faces from the center in the vertical direction, and the distance between the vertex of one curved portion and the reference line is longer than the distance between the vertex of the other curved portion and the reference line.

[0128] (Note 14) The optical splitting circuit as described in Note 13, further comprising a curved portion on the inside of the reference line in each of the parts of both end faces, wherein the distance between the vertex of one curved portion and the reference line is longer than the distance between the vertex of the inner curved portion and the reference line.

[0129] (Note 15) The optical splitting circuit as described in Note 14, further comprising a curved portion outside the reference line on each of the ends of the respective end faces, wherein the distance between the vertex of one curved portion and the reference line is longer than the distance between the vertex of the outer curved portion and the reference line.

[0130] (Note 16) The photomultiplier branching circuit described in any of Notes 1 to 4 or 9 to 15, wherein the branching ratio is greater than 0.5:0.5.

[0131] (Note 17) A delay interferometer comprising a plurality of optical combining and branching circuits described in any of Notes 1 to 4 or 9 to 16, and a pair of waveguides connecting the optical combining and branching circuits, wherein one waveguide of the waveguide pair is longer than the other waveguide.

[0132] (Note 18) The method for manufacturing a photomultiplier branch circuit as described in Note 8, wherein, in the initial state of the interference portion, in a top view, the interference portion is mirror-symmetric with respect to the central axis in a direction parallel to the outer edge of the end face and the central axis in a direction perpendicular to the end face of the interference portion.

[0133] (Note 19) A method for manufacturing a photocombination branch circuit as described in Note 8 or Note 18, comprising the step of calculating the difference between the wavelength dependence of the calculated branch ratio and the wavelength dependence of the set branch ratio, and repeating the calculation when the difference is greater than a predetermined value.

[0134] This disclosure can be applied to communication devices, computing devices, computers, signal processing devices, and the like.

[0135] 10 Optical splitting circuit 11 Interfering section 12 Input waveguide 13 Output waveguide

Claims

1. An optical splitting circuit for splitting input light at a predetermined splitting ratio, comprising: an interference section; a pair of input waveguides connected to one end face of the interference section; and a pair of output waveguides connected to the other end face of the interference section, wherein both sides of the interference section have a curved shape, and in a top view, the curved shape includes a first curved section in which the width of the interference section is narrower than the width of the end face, and the splitting ratio is controlled by the width of the interference section in the first curved section.

2. The photomultiplier branching circuit according to claim 1, wherein when the branching ratio of one of the photomultiplier branching circuits is greater than the branching ratio of the other of the photomultiplier branching circuits, the width of the interference portion in the first curved portion of the one of the photomultiplier branching circuits is narrower than the width of the interference portion in the first curved portion of the other of the photomultiplier branching circuits.

3. In a top view, the first curved portion is positioned at the center in a direction perpendicular to the end face of the interference portion, the width of the interference portion includes a second curved portion which increases from the center in the perpendicular direction toward each of the end faces and is wider than the width of the interference portion at both ends, and the distance between the vertex of the first curved portion and a reference line connecting both ends of the side surface is longer than the distance between the vertex of the second curved portion and a reference line connecting both ends of the side surface.

4. The photo-combining branching circuit according to claim 1 or claim 2, wherein, in a top view, the interfering portion is mirror-symmetric with respect to the central axis in a direction parallel to the outer edge of the end face and the central axis in a direction perpendicular to the outer edge of the end face.

5. A wavelength filter comprising a plurality of delay interferometers and a waveguide connecting the delay interferometers, wherein the delay interferometer comprises a plurality of photomultiplier / branching circuits according to claim 1 or claim 2 and a pair of waveguides connecting the photomultiplier / branching circuits, wherein one waveguide of the pair of waveguides is longer than the other waveguide.

6. An apparatus comprising the wavelength filter described in claim 5.

7. A data center comprising the apparatus described in claim 6.

8. A method for manufacturing an optical splitter circuit comprising an interference section, a pair of input waveguides connected to one end face of the interference section, and a pair of output waveguides connected to the other end face of the interference section, wherein input light is split at a predetermined splitting ratio to output light, the method comprising: setting the initial values ​​of the length and width of the interference section, the spacing between the pair of input waveguides, and the spacing between the pair of output waveguides such that the splitting ratio is approximately equivalent to 0.5:0.5, with low loss and suppressed wavelength dependence; setting the spectrum of the input light; setting the wavelength dependence of the splitting ratio; and calculating the wavelength dependence of the splitting ratio when the input light is input to one of the input waveguides, wherein in the calculation step, the length of the interference section, the spacing between the pair of input waveguides, and the spacing between the pair of output waveguides are kept constant, and the width is changed, and the calculation is repeated.

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