JP2019152717A - Wavelength multiplexer - Google Patents

Abstract

To provide a wavelength multiplexer that is easy to design, and which tolerates a large manufacturing error.SOLUTION: The wavelength multiplexer comprises a directional coupler 11 and asymmetric directional couplers 12, 13 instead of a Mach-Zehnder interferometer. These directional couplers do not require as much accurate design as required for Mach-Zehnder interferometers. Wavelength multiplexing is possible by adjusting the length of adjacent parallel waveguides and the effective refractive index of each mode. Especially for the 4-wave case, two waves are multiplexed as TE waves, and another two waves are multiplexed as TM waves, and a heat-insulating mode converter having low wavelength dependency is used to dispense with a Mach-Zehnder interferometer whose FSR is small and which is susceptible to manufacturing tolerances, whereby manufacturing tolerances can be improved.SELECTED DRAWING: Figure 8

Description

  The present disclosure relates to a wavelength multiplexer of a planar optical circuit.

  In recent years, not only long-distance transmission but also the opticalization of LAN is progressing so as to support high-speed transmission of LAN (Local Area Network). The 100 Gbps Ethernet (registered trademark) specification has already been established, and using four optical signals as 100 GBASE-LR4 and 100 GBASE-ER4 respectively transmitting 10 km and 40 km using single mode optical fiber, respectively. A system that realizes data transmission of 25 Gbps × 4 = 100 Gbps by performing transmission of about 25 Gbps at a wavelength of 2 is adopted. One of the most effective light sources is an integrated DFB laser for 4 waves and a modulator (see, for example, Non-Patent Document 1).

  This light source has a configuration in which lasers that output four standard waves are integrated on a single chip, each of which is smaller than a light source for four waves by connecting bulk components. Since the number of adjusting mechanisms and the like can be reduced, it is expected to reduce power consumption.

  However, in the configuration of Non-Patent Document 1, a 4 × 1 MMI coupler is used as a 4-wave multiplexer. When the 4 × 1 MMI coupler is used as a 4-wave multiplexer, there is a loss of 6 dB as a principle loss. That is, in principle, only 25% of the output can be extracted outside, and much light is wasted.

  Therefore, it is considered to replace this multiplexer with a low loss one. As a multiplexer used in such a place, Arrayed waveguiding gratings (AWG), etched planar concavating gratings (PCGs), and the like are known. However, the size of the four-wave multiplexer is large, and therefore the waveguide loss increases.

  Thus, a multiplexer using a Mach-Zehnder interferometer including a delay circuit is conceivable. In the case of four waves, a multiplexer can be realized by a two-stage Mach-Zehnder interferometer, and the size can be reduced as compared with AWG and PCG. A light source in which a four-wave multiplexer using a two-stage Mach-Zehnder interferometer, a four-wave DFB laser, and a modulator are integrated has also been reported (for example, see Non-Patent Document 2).

T.A. Fujisawa et al, “1.3-m 4 25-Gb / s Monolithically Integrated Light Source for Metro Area 100-Gb / s Ethernet”, IEEE PHOTOTONICS TECHNOLOGY TECHNOLOGY TECHNOLOGY 23, NO. 6, MARCH 15, 2011. T.A. Fujisawa, S .; Kanazawa, Y .; Ueda, W .; Kobayashi, K. et al. Takahata, A.A. Ohki, T .; Itoh, M.M. Kotoku, and H.K. Ishii, “Low-loss cascaded Mach-Zehnder multipleplexer integrated 25-Gbit / s × 4-lane EADFB laser array for future CFP4 100 GbE transEmitter. 49, no. 12, pp. 1001-1007, Dec. 2013.

  However, when configuring a multiplexer by combining Mach-Zehnder interferometer filters in multiple stages, there are two difficulties in the controllability of the absolute wavelength position of each peak wavelength. To explain this difficulty, first, the operation of the Mach-Zehnder interferometer filter will be described.

[Operation of Mach-Zehnder Interferometer Filter]
The operation of a four-wave multiplexer using a conventional two-stage Mach-Zehnder interferometer will be described. The numerical values and materials are examples, and it does not matter if there are others. In this example, λ0 = 1295.5 nm, λ1 = 1300.0 nm, λ2 = 1304.5 nm, λ3 = 1309 nm in a 1.3 μm wavelength band made of a Si wire waveguide formed on an SOI (Silicon on Insulator) substrate. A filter for combining four waves will be described. The four waves correspond to the four wavelengths used in 100 Gigabit Ethernet (registered trademark), and the interval between the wavelengths is 800 GHz in frequency.

  FIG. 1 shows one Mach-Zehnder interferometer. The Mach-Zehnder interferometer includes two input waveguides, two 3 dB couplers, an arm waveguide connecting the 3 dB couplers, and two output waveguides. The arm waveguide is composed of two waveguides, and a delay circuit is formed for adding a phase difference to light passing through both waveguides.

  In the Mach-Zehnder interferometer filter 1, the thickness of the Si wire waveguide constituting the delay circuit is 210 nm, the width is 300 nm, and the delay amount (difference between the length of the lower arm and the length of the upper arm) ΔL1 is 21 μm. When white light (broad light) is input from the input 1, FIG. 2 shows a spectrum obtained by calculation. Output 1 is indicated by a solid line and output 2 is indicated by a broken line.

  The output 1 has a characteristic that the light is turned on at the wavelength of 1.309 μm of λ3, that is, the light is transmitted, and the light is blocked around 1.300 μm of λ1. Output 2 will be inverted. The case where white light is input from output 1 has been described. However, when light having a wavelength of 1.300 μm is input from input 1 and light having a wavelength of 1.309 μm is input from input 2 due to reciprocity of the light, both lights are combined with output 1. And output. Note that the FSR (Free Spectral Range) of this filter is 3200 GHz.

  On the other hand, the Mach-Zehnder interferometer filter 2 that combines λ0 and λ2 is the same as the Mach-Zehnder interferometer filter 1 in terms of the thickness and width of the Si wire waveguide, but the phase difference is 90 degrees as a delay amount. Therefore, the delay amount is set to ΔL2 = 21.15 μm. FIG. 3 shows a spectrum obtained by calculation when white light is input from the input 1 with this delay amount. FIG. 3 also shows output 1 as a solid line and output 2 as a broken line. The FSR of this filter is also 3200 GHz.

  The Mach-Zehnder interferometer filter 3 is the same as the Mach-Zehnder interferometer filter 1 in terms of the thickness and width of the Si wire waveguide, but the delay amount is set to ΔL3 = 42 μm, which is twice that of the Mach-Zehnder interferometer filter 1. FIG. 4 shows the characteristics of the Mach-Zehnder interferometer filter 3. FIG. 4 also shows output 1 as a solid line and output 2 as a broken line. The FSR of this filter is 1600 GHz, which is half of the above-described filter. This filter outputs light in which λ0, λ1, λ2, and λ3 are combined by inputting a combined wavelength of λ0 and λ2 to one of the input waveguides and a combined wavelength of λ1 and λ3 to the other.

  FIG. 5 is a diagram for explaining a two-stage Mach-Zehnder interferometer type four-wave multiplexer. That is, if the Mach-Zehnder interferometer filter 1 and the Mach-Zehnder interferometer filter 2 are arranged in parallel as shown in FIG. 5 and their outputs are input to the Mach-Zehnder interferometer filter 3, λ0 input to the input waveguide is obtained. , Λ1, λ2, and λ3 can be combined into one output waveguide.

  However, it is very difficult to manufacture such a Mach-Zehnder interferometer type multiplexer. The characteristics of the Mach-Zehnder interferometer filter are very sensitive to the waveguide width w and the delay amount ΔL, making it difficult to design and manufacture. The difficulty will be described below.

  FIG. 6 is a diagram for explaining the dependency between the 1.300 μm peak wavelength displacement Δλ and the waveguide width change Δw in the Mach-Zehnder interferometer filter of FIG. 1 in which the design value width of the Si wire waveguide is 300 nm. It is. This dependency is almost the same regardless of whether the FSR is 3200 GHz or 1600 GHz. FIG. 6 shows that when the waveguide width is shifted by about 5 nm, the peak wavelength position is shifted by about 6 nm. This dependency is particularly severe for the Mach-Zehnder interferometer 3 (FSR = 1600 GHz) having the characteristics shown in FIG. This is because the output of the Mach-Zehnder interferometer 3 is just reversed when the peak wavelength is shifted by 800 GHz (about 5 nm), and is not suitable as the second-stage filter of the Mach-Zehnder interferometer-type multiplexer shown in FIG. It is.

  FIG. 7 is a diagram for explaining the dependency between the peak wavelength positional deviation Δλ and the delay amount change ΔL of the Mach-Zehnder interferometer filter 1 having an FSR of 3200 GHz. Here, the position of the peak wavelength when the delay amount is 21 μm is set to zero. From FIG. 7, it can be seen that the peak wavelength position changes greatly with a very small change in delay in units of 100 nm. For example, if the delay amount changes by 0.6 μm, it changes by about 18 nm (approximately 3200 GHz) and returns to the original spectrum. FIG. 7 shows that it is difficult to control the delay amount so that the phase is shifted by exactly 90 degrees between the Mach-Zehnder interferometer filter 1 and the Mach-Zehnder interferometer filter 2.

  As described above, the characteristics of the Mach-Zehnder interferometer filter are very sensitive to the waveguide width w and the delay amount ΔL, and even if the waveguide width w is fixed at the design stage, In order to maintain a 90-degree phase difference with the Mach-Zehnder interferometer filter 2, ΔL must be finely adjusted, which complicates the design. If a 90-degree phase difference can be maintained between the Mach-Zehnder interferometer filter 1 and the Mach-Zehnder interferometer filter 2, even if there is an absolute wavelength position, the peak of the Mach-Zehnder interferometer filter 3 can be further increased from there. It is necessary to align the wavelength position. As described above, the FSR of the Mach-Zehnder interferometer filter 3 is half that of the Mach-Zehnder interferometer filters 1 and 2, and ΔL needs to be finely adjusted, which complicates the design.

  At the manufacturing stage, there are manufacturing errors, and it is very difficult to realize the delay amount ΔL and the waveguide width w as design values. If there is a deviation between the delay amount ΔL and the waveguide width w due to a manufacturing error, the position of the peak wavelength of the filter greatly changes (in particular, the Mach-Zehnder interferometer filter 3 has an FSR that is half that of the Mach-Zehnder interferometer filters 1 and 2. Therefore, the influence of the manufacturing error is great.) Even if a filter is combined, it cannot be used as a desired multiplexer.

  As described above, the absolute wavelength position of the peak wavelength of the Mach-Zehnder interferometer filter is controlled by the two parameters of the waveguide width w and the delay amount ΔL. These two parameters are low in sensitivity to the wavelength interval FSR and do not vary greatly, but are sensitive to the absolute wavelength position, and the absolute wavelength position changes greatly if these parameters vary minutely. For example, a four-wave multiplexer is composed of three Mach-Zehnder interferometer filters, and the absolute wavelength positions of the peak wavelengths of all the filters need to be exactly matched to the desired wavelengths. Is very difficult. In other words, when a multiplexer is configured by combining Mach-Zehnder interferometer filters in multiple stages, there are problems that the design of the Mach-Zehnder interferometer is complicated, and that tolerance for manufacturing errors during manufacturing is small.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a wavelength multiplexer that is easy to design and has a high tolerance for manufacturing errors.

  In order to achieve the above object, a wavelength multiplexer according to the present invention includes a directional coupler that converts a wavelength propagation mode and multiplexes them, so that a Mach-Zehnder interferometer with a complicated design and a small manufacturing error tolerance is provided. It was decided not to use.

Specifically, the wavelength multiplexer according to the present invention is a wavelength multiplexer of a planar optical circuit,
An input unit for inputting a plurality of wavelengths to different waveguides in a fundamental mode;
A directional coupler in which two waveguides are close to each other in parallel, and the respective wavelengths of the two waveguides are combined in a fundamental mode;
An asymmetric directional coupler in which two waveguides are close to each other in parallel, one wavelength of the two waveguides is converted to a higher order mode while the other wavelength is converted into a fundamental mode, and combined.
Based on the wavelength of the higher-order mode among the combined wavelengths in which the plurality of wavelengths input to the input unit are all combined via at least one of the directional coupler and the asymmetric directional coupler. A mode converter to convert the mode,
An output unit that outputs the combined wavelength converted into the fundamental mode by the mode converter;
Is provided.

  The wavelength multiplexer according to the present invention includes not a Mach-Zehnder interferometer but a directional coupler and an asymmetric directional coupler. These directional couplers do not require as accurate a design as a Mach-Zehnder interferometer. Wavelength multiplexing is possible by adjusting the length of adjacent parallel waveguides and the effective refractive index of each mode. Therefore, the present invention can provide a wavelength multiplexer that is easy to design and has a high tolerance for manufacturing errors.

The asymmetric directional coupler of the wavelength multiplexer according to the present invention is:
The effective refractive index of one fundamental mode of the two waveguides and the effective refractive index of the other higher-order mode of the two waveguides match,
The section in which the two waveguides are parallel and close is such a length that the power of one fundamental mode of the two waveguides shifts to the power of the other higher order mode of the two waveguides. Features.

  The mode converter of the wavelength multiplexer according to the present invention is characterized by having a tapered portion in which the waveguide width continuously changes. By setting the output of the wavelength multiplexer to the basic mode, it is possible to cope with a standard optical network using a single mode fiber as a transmission medium. For example, the mode converter may have a tapered silicon rib waveguide that satisfies the heat insulation condition.

Here, the input unit of the wavelength multiplexer according to the present invention inputs the fundamental mode of TE0 to the waveguide,
The asymmetric directional coupler converts the other wavelength into a higher order mode of TE1,
The mode converter converts the higher-order mode of TE1 into the basic mode of TM0.

The configuration of the four-wavelength multiplexer includes one directional coupler, two asymmetric directional couplers, and one mode converter.
The wavelengths input to the waveguide by the input unit are four wavelengths (λ0, λ1, λ2, and λ3) with a frequency interval f,
In the directional coupler, wavelengths (λ1 and λ3) are input to the two waveguides from the input unit, and the two waveguides are parallel so that the wavelengths (λ1 and λ3) are combined. The adjacent waveguide interval and interval are adjusted,
In the first asymmetric directional coupler, the wavelength (λ1) is input to one of the two waveguides from the input unit and the wavelength (λ1) is combined with the other of the two waveguides by the directional coupler. And λ3) are input, the wavelength λ2 is converted into a higher-order mode and combined with the wavelengths (λ1 and λ3),
In the second asymmetric directional coupler, a wavelength λ0 is input from the input unit to one of the two waveguides, and the other asymmetrical directional coupler is combined with the other of the two waveguides. Wavelength (λ1, λ2, and λ3) are input, the wavelength λ0 is converted into a higher-order mode, and multiplexed to the wavelengths (λ1, λ2, and λ3),
The mode converter uses a wavelength (λ0 and λ2), which is a higher-order mode, of the combined wavelengths (λ0, λ1, λ2, and λ3) combined by the second asymmetric directional coupler as a fundamental mode It is characterized by converting into.

Other configurations of the 4-wavelength multiplexer are:
An input unit in which four wavelengths (λ0, λ1, λ2, and λ3 from the short wavelength side) of the frequency interval f are input in the basic mode;
Two Mach-Zehnder interferometer filters having a 2f FSR (Free Spectral Range) and a phase difference of 90 degrees from each other;
An asymmetric directional coupler in which two waveguides are close to each other in parallel, one wavelength of the two waveguides is converted to a higher order mode while the other wavelength is converted into a fundamental mode, and combined.
A mode converter that converts a higher-order mode wavelength into a fundamental mode;
A wavelength multiplexer comprising:
In the first Mach-Zehnder interferometer filter, the wavelengths (λ1 and λ3) are input to the two input waveguides from the input unit, and the wavelengths (λ1 and λ3) are multiplexed in the fundamental mode,
In the second Mach-Zehnder interferometer filter, the wavelengths (λ0 and λ2) are input to the two input waveguides from the input unit, and the wavelengths (λ0 and λ2) are multiplexed in the fundamental mode,
The asymmetric directional coupler receives a wavelength (λ1 and λ3) combined by the first Mach-Zehnder interferometer filter in one of the two waveguides, and a second in the other of the two waveguides. The wavelengths (λ0 and λ2) combined by the Mach-Zehnder interferometer filter are input, the wavelengths (λ0 and λ2) are converted into higher-order modes, and combined with the wavelengths (λ1 and λ3),
The mode converter converts a wavelength (λ0 and λ2), which is a higher-order mode, of the combined wavelengths (λ0, λ1, λ2, and λ3) combined by the asymmetric directional coupler into a fundamental mode. It is characterized by that.

  This wavelength combiner combines two waves as TE waves and the other two as TM waves, uses an adiabatic mode converter with small wavelength dependence, and has a small FSR and weak manufacturing tolerance. The Mach-Zehnder interferometer filter 3 By eliminating the requirement, manufacturing tolerance can be improved.

  For example, the waveguide of the wavelength multiplexer according to the present invention is a silicon fine wire waveguide.

  The 8-wavelength multiplexer further combines the plurality of wavelength multiplexers to which a plurality of wavelengths that do not overlap in wavelength bands are respectively input, and the combined wavelength output from each of the wavelength multiplexers. And a band multiplexer to be configured.

  The present invention can provide a wavelength multiplexer that is easy to design and has a high tolerance for manufacturing errors.

It is a figure explaining a Mach-Zehnder interferometer. It is a figure explaining the transmission spectrum of a Mach-Zehnder interferometer. It is a figure explaining the transmission spectrum of a Mach-Zehnder interferometer. It is a figure explaining the transmission spectrum of a Mach-Zehnder interferometer. It is a structural diagram explaining a multistage Mach-Zehnder interferometer type wavelength multiplexer. It is a figure explaining the waveguide width dependence of the peak wavelength of a Mach-Zehnder interferometer. It is a figure explaining the delay amount dependence of the peak wavelength of a Mach-Zehnder interferometer. It is a structural diagram explaining the wavelength multiplexer which concerns on this invention. It is a figure explaining the waveguide cross section of the wavelength multiplexer which concerns on this invention. It is a figure explaining the waveguide cross section of the wavelength multiplexer which concerns on this invention. It is a figure explaining the directional coupler of the wavelength multiplexer which concerns on this invention. It is a figure explaining the transmission spectrum of the directional coupler of the wavelength multiplexer which concerns on this invention. It is a figure explaining the transmission spectrum of the directional coupler of the wavelength multiplexer which concerns on this invention. It is a figure explaining the asymmetrical directional coupler of the wavelength multiplexer which concerns on this invention. It is a figure explaining the transmission spectrum of the asymmetrical directional coupler of the wavelength multiplexer which concerns on this invention. It is a figure explaining the transmission spectrum of the asymmetrical directional coupler of the wavelength multiplexer which concerns on this invention. It is a figure explaining the asymmetrical directional coupler of the wavelength multiplexer which concerns on this invention. It is a figure explaining the transmission spectrum of the asymmetrical directional coupler of the wavelength multiplexer which concerns on this invention. It is a figure explaining the transmission spectrum of the asymmetrical directional coupler of the wavelength multiplexer which concerns on this invention. It is a figure explaining the mode converter of the wavelength multiplexer which concerns on this invention. It is a figure explaining the waveguide width dependence of the effective refractive index of a silicon | silicone thin wire | line rib waveguide in the mode converter of the wavelength multiplexer which concerns on this invention. It is a figure explaining the result of the beam propagation simulation of the heat insulation mode conversion part of the mode converter of the wavelength multiplexer which concerns on this invention. It is a structural diagram explaining the wavelength multiplexer which concerns on this invention. It is a structural diagram explaining the wavelength multiplexer which concerns on this invention. It is a figure explaining the wavelength interval of 8 waves.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
FIG. 8 is a diagram for explaining the wavelength multiplexer 301 of the present embodiment. The wavelength multiplexer 301 is a planar optical circuit wavelength multiplexer,
An input unit 10 for inputting a plurality of wavelengths to different waveguides in a fundamental mode;
A directional coupler 11 in which two waveguides are close to each other in parallel, and each wavelength of the two waveguides is multiplexed in a fundamental mode;
An asymmetric directional coupler (12, 13) in which two waveguides are close to each other in parallel, and one wavelength of the two waveguides is converted to a higher order mode while the other wavelength is converted into a fundamental mode. ,
Of the combined wavelengths in which the plurality of wavelengths input to the input unit 10 are all combined via at least one of the directional coupler 11 and the asymmetric directional coupler (12, 13), a higher-order mode is selected. A mode converter 14 for converting the wavelength of
An output unit 15 for outputting the combined wavelength converted by the mode converter 14 to be in the fundamental mode;
Is provided.

  The wavelength multiplexer 301 is, for example, a 4-wavelength multiplexer for 100 Gigabit Ethernet (registered trademark). Light having a wavelength interval of 800 GHz, λ0 = 1295.5 nm, λ1 = 1300.0 nm, λ2 = 1304.5 nm, and λ3 = 1309 nm is multiplexed into one waveguide. The four input waveguides of the input unit 10 or four wavelengths (λ0 to λ3) are input. Here, TE0 indicates a fundamental (0th order) mode with respect to TE polarization of the waveguide. The output of a semiconductor laser used as a light source for optical communication is normally in the TE0 mode.

  The wavelength multiplexer 301 uses a silicon fine wire waveguide created on an SOI substrate as a waveguide. The height of the waveguide is 210 nm, and this is a buried waveguide in which pure quartz is covered as a cladding. 9A is a cross-sectional view of the silicon wire waveguide, FIG. 9B is a TE0 mode field distribution, and FIG. 9C is a TE1 mode field distribution which is the first higher-order mode.

A method for manufacturing the wavelength multiplexer 301 will be described. The core layer is patterned by photolithography on an SOI substrate having a desired silicon layer thickness. Thereafter, the silicon layer is etched by dry etching to form a silicon fine wire waveguide. In that case, when forming the silicon | silicone thin wire | line rib waveguide demonstrated in FIG. 20, patterning is separately performed by photolithography and it etches to the middle of a silicon layer, and forms a rib waveguide. Finally, pure quartz (SiO ₂ ) serving as an upper clad is deposited to form a waveguide having a cross-sectional structure as shown in FIG. FIG. 10 shows a cross-sectional structure of the silicon fine rib waveguide.

  The operation of the wavelength multiplexer 301 will be described. First, the filters 11 combine the wavelengths λ1 and λ3 input from the input unit 10. The filter 11 may be a Mach-Zehnder interferometer as shown in FIG. 1, but the wavelength multiplexer 301 uses a directional coupler as shown in FIG. The directional coupler has a coupling portion in which two waveguides are close to each other as shown in FIG. This directional coupler can switch the wavelength path by appropriately setting the length Lc of the coupling portion according to the input light wavelength, and can be used as a wavelength multiplexer / demultiplexer as shown in FIG. it can.

  In the case of the directional coupler 11, λ 3 is input from the waveguide 1, light of λ 1 is input from the waveguide 2, and Lc is set so that both lights are emitted from the waveguide 2. At this time, the case of emitting from the waveguide 1 to the waveguide 2 is called cross-port transmission, and the case of emitting from the same waveguide to the same waveguide is called bar-port transmission. The directional coupler 11 is designed such that its transmission spectrum is as shown in FIG. 12, and functions as a filter having an FSR of 3200 GHz.

  FIG. 13 is a calculated value of the transmission spectrum of the directional coupler 11. A solid line indicates bar port transmission, and a broken line indicates cross port transmission. As for the structural parameters, the waveguide widths of the waveguides 1 and 2 are 300 nm, the gap between the two waveguides is 200 nm, and the coupling portion length Lc is 240 μm. As shown in FIG. 12, the directional coupler 11 has FSR = 18 nm (3200 GHz), λ1 light is output to the bar port, λ3 light is output to the cross port, and the operation shown in FIG. 11 is performed. I understand that.

  Next, λ2 input to the input unit 10 is combined with λ1 and λ3 combined by the directional coupler 11 by the asymmetric directional coupler 12. FIG. 14 shows a structural diagram of the asymmetric directional coupler 12. It consists of a silicon fine wire waveguide 1 having a width w1 and a silicon fine wire waveguide 2 having a width w2. Light of λ2 is incident from the waveguide 1 and coupled to the waveguide 2. In the asymmetric directional coupler, since the widths of the two waveguides are different from each other, no coupling occurs between the same modes, but the fundamental mode of the waveguide 1 and the first higher-order mode of the waveguide 2 are appropriately designed. The TE1 mode (cross-sectional boundary distribution shown in FIG. 9C) can be combined. The appropriate design here is that the effective refractive index of one fundamental mode of two waveguides matches the effective refractive index of the other higher order mode of the two waveguides, and the two waveguides are parallel. The adjacent section may be a length in which the power of one fundamental mode of the two waveguides shifts to the power of the other higher-order mode of the two waveguides.

  The asymmetric directional coupler 12 couples the TE0 mode having the wavelength λ2 incident from the waveguide 1 to the TE1 mode of the waveguide 2, and transmits the light of λ1 and λ3 of the TE0 mode incident to the waveguide 2 to the waveguide 1. The light is emitted from the waveguide 2 as it is. For this reason, the asymmetric directional coupler 12 can multiplex three light beams of λ1, λ2, and λ3. Note that light of λ1 and λ3 is incident on the waveguide 2 as a TE0 mode, and therefore is not coupled to the waveguide 1 in principle.

  At this time, in an appropriate design, the transmission spectrum from the TE0 mode of the waveguide 1 of the asymmetric directional coupler 12 to the TE1 mode of the waveguide 2 is almost 100% in a wide band (FIG. 15). That is, when λ1, λ2, and λ3 are combined by the asymmetric directional coupler 12, it is not necessary to consider the absolute wavelength position and the FSR.

  FIG. 16 is a diagram illustrating a cross-port transmission spectrum from the TE0 mode of the waveguide 1 of the asymmetric directional coupler 12 to the TE1 mode of the waveguide 2. The structural parameters are w1 = 300 nm, w2 = 629 nm, and the gap 200 nm between the two waveguides. It can be seen that a transmittance of almost 100% is obtained in the desired wavelength band. An element having such a flat wavelength characteristic has a large tolerance for a manufacturing error of the waveguide width.

  That is, the multiplexing element (asymmetric directional coupler 12) that multiplexes the light of λ2 with the light of λ1 and λ3 needs to consider the absolute position of the peak wavelength as compared with the Mach-Zehnder interferometer as shown in FIG. It can be seen that the design is facilitated and the tolerance for manufacturing errors is large.

  Subsequently, λ 0 input to the input unit 10 is combined with λ 1, λ 2, and λ 3 combined by the symmetric directional coupler 12 by the asymmetric directional coupler 13. FIG. 17 shows a structural diagram of the asymmetric directional coupler 13. The asymmetric directional coupler 13 can also have a filter characteristic having a certain FSR like the directional coupler 11 by appropriately designing the length Lc of the coupling portion.

  In the case of the asymmetric directional coupler 13, λ 0 is input from the waveguide 1, light of λ 1, λ 2, and λ 3 is input from the waveguide 2 so that all four lights are emitted from the waveguide 2. Here, since the light of λ1 and λ3 has propagated in the TE0 mode, the light is emitted from the waveguide 2 as it is without being coupled to the waveguide 1 by the asymmetric directional coupler 13. As shown in FIG. 17, the light of λ2 is incident on the waveguide 2 as the TE1 mode and is emitted from the waveguide 2 in the TE1 mode so that the light of λ2 is not emitted from the waveguide 1. The light of λ0 is incident as a TE0 mode from the waveguide 1 and is emitted as a TE1 mode from the waveguide 2. In this way, in order to operate the asymmetric directional coupler 13, the bar port transmission spectrum when light is incident from the waveguide 2 is designed as shown in FIG. 18, and functions as a filter having an FSR of 3200 GHz.

  FIG. 19 shows the calculated value of the transmission spectrum of the asymmetric directional coupler 13. A solid line indicates bar port transmission, and a broken line indicates cross port transmission. The structural parameters are w1 = 300 nm, w2 = 629 nm, the gap between the two waveguides is 200 nm, and the length Lc of the coupling portion is 275 μm. As shown in FIG. 18, the asymmetric directional coupler 13 has FSR = 18 nm (3200 GHz), λ2 light is output to the bar port, λ0 light is output to the cross port, and the operation shown in FIG. 17 is performed. I understand that

  In the asymmetric directional coupler 13, four lights of λ0, λ1, λ2, and λ3 are combined into one waveguide, but the lights of λ0 and λ2 are combined as TE1 mode. In 100GBASE-LR4, which is one of the standards of 100 Gigabit Ethernet (registered trademark), the transmission medium is a single mode fiber, and the TE1 mode does not stably propagate through the single mode fiber. Therefore, it is necessary to convert the light of λ0 and λ2 from the TE1 mode to the fundamental mode.

  In the wavelength multiplexer 301, the mode converter 14 converts the light of λ0 and λ2 from the TE1 mode to the TM0 mode. FIG. 20 shows a structural diagram of the mode conversion unit 14. The mode conversion unit 14 includes an input silicon fine wire waveguide 21, an output silicon fine wire waveguide 22, a taper portion 23, and a rib waveguide 24. A cross section of P-P 'is shown in FIG. That is, the rib waveguide 24 is a portion having a thickness t in FIG. 10, and the input silicon fine wire waveguide 21 or the tapered portion 23 is a portion having a thickness h.

  The rib waveguide 24 includes a thin wire rib conversion taper portion 25, a heat insulation mode conversion portion 26, and a wide silicon rib waveguide 27. The light input to the rib waveguide 24 is the silicon fine wire waveguide 21, and is connected to the wide silicon rib waveguide 27 through the fine wire rib conversion taper portion 25. From there, the waveguide width of the wide silicon rib waveguide 27 is adiabatically reduced by the adiabatic mode converter 26. When light passes through the rib waveguide 24, the TE0 mode is not converted, and the TE1 mode is converted to the TM0 mode. Finally, the light passing through the rib waveguide 24 is output to the output silicon fine wire waveguide 22.

  The principle of mode conversion of the mode converter 14 will be described. FIG. 21 is a diagram for explaining the waveguide width dependence of the effective refractive index of the TE0 mode (solid line), the TM0 mode (broken line), and the TE1 mode (one-dot chain line) in the rib waveguide 24. In this description, h = 140 nm and t = 70 nm. From FIG. 21, it can be seen that the TE1 mode and the TM0 mode are coupled in the vicinity of the waveguide width of 0.6 μm, and the mode crossing occurs.

  Here, paying attention to the alternate long and short dash line, when the waveguide width is larger than 0.6 μm, it is a TE1 like mode, and when it is smaller than 0.6 μm, it is a TM0 like mode. For example, when the TE1 mode is input to a rib waveguide having a waveguide width of 0.8 μm and the waveguide width is reduced adiabatically, mode conversion occurs around 0.6 μm, for example, the waveguide width is 0 When reduced to .4 μm, the output is TM0 mode. This adiabatic mode conversion is almost independent of wavelength because of its principle, and is hardly affected by manufacturing errors of waveguide parameters.

  FIG. 22 is a diagram illustrating the magnetic field distribution of the adiabatic mode conversion unit 26 calculated by the beam propagation simulation. Here, with respect to the adiabatic mode converter 26, the rib waveguide width on the input side (input silicon fine wire waveguide 21 side) is 800 nm, the rib waveguide width on the output side (output silicon fine wire waveguide 22 side) is 400 nm, and the length Lconv. Was 100 um. The two TE1 modes in the x direction of the waveguide having the Hy component as the main component are converted into TM0 modes having one peak in the x direction of the waveguide having the main component in Hx by adiabatic conversion. Recognize. The portion where the magnetic field is stronger than the others is called “mountain”. The x direction is the x direction described in FIGS. 10 and 20.

  When the TE0 mode is input to the rib waveguide 24, as is apparent from FIG. 21, no mixing with other modes occurs, so that the TE0 mode is emitted as it is. In other words, the TE0 mode is not affected at all by the rib waveguide 24.

  That is, with respect to the four lights combined by the asymmetric directional coupler 13, the light of λ1 and λ3 is incident on the mode converter 14 as the TE0 mode, so that the light of λ0 and λ2 is emitted in the TE0 mode. Since it enters the mode converter 14 as a mode, it is converted into a TM0 mode and emitted.

  As described above, the wavelength multiplexer 301 has two waves (λ0 and λ2) out of four fundamental mode lights (λ0 (TE0), λ1 (TE0), λ2 (TE0), and λ3 (TE0)). Is converted into a higher-order mode and combined with two waves of other basic modes, and the two higher-order modes are converted back to the basic mode (TM mode), so that λ0 (TM0), λ1 (TE0), The combined light of λ2 (TM0) and λ3 (TE0) is output.

More specifically, the wavelength multiplexer 301 has one directional coupler 11, two asymmetric directional couplers (12, 13), and one mode converter 14.
The wavelengths input to the waveguide by the input unit 10 are four wavelengths (λ0, λ1, λ2, and λ3) with a frequency interval f,
In the directional coupler 11, the wavelengths (λ1 and λ3) are input to the two waveguides from the input unit 10 and the two waveguides are close to each other in parallel so that the wavelengths (λ1 and λ3) are combined. Waveguide spacing and interval are adjusted,
In the asymmetric directional coupler 12, the wavelength λ2 is input from one of the two waveguides from the input unit 10, and the wavelength (λ1 and λ3) combined by the directional coupler 11 is input to the other of the two waveguides. The wavelength λ2 is converted into a higher order mode and combined with the wavelengths (λ1 and λ3),
In the asymmetric directional coupler 13, the wavelength (λ0) is input from one of the two waveguides from the input unit 10 and the other of the two waveguides is combined by the asymmetric directional coupler 12 (λ1, λ2, and λ3). ) Is input, and the wavelength λ0 is converted into a higher-order mode and combined with the wavelengths (λ1, λ2, and λ3),
The mode converter 14 converts a wavelength (λ0 and λ2), which is a higher-order mode, of the combined wavelengths (λ0, λ1, λ2, and λ3) combined by the asymmetric directional coupler 13 into a fundamental mode. It is characterized by.
Here, the input unit 10 inputs the fundamental mode of TE0 to the waveguide, the asymmetric directional couplers (12, 13) convert the wavelengths λ0 and λ2 into higher-order modes of TE1, and the mode converter 14 , The higher order mode of TE1 is converted to the basic mode of TM0.

(effect)
The wavelength multiplexer 301 has the following advantages over the multistage Mach-Zehnder multiplexer of FIG. First, as apparent from the configuration, the wavelength multiplexer 301 eliminates the Mach-Zehnder interferometer 3 having an FSR of 1600 GHz, which is necessary for the multistage Mach-Zehnder multiplexer. In the multi-stage Mach-Zehnder multiplexer, as described in detail with reference to FIG. 4, the Mach-Zehnder interferometer 3 has the smallest manufacturing error tolerance, and is the main cause that makes it difficult to manufacture the wavelength multiplexer.

  On the other hand, the wavelength multiplexer 301 includes an asymmetric directional coupler and an adiabatic mode converter as an alternative to the filter of FSR = 1600 GHz. As described above, since these two elements have a small wavelength dependency, manufacturing tolerance is large, and it is not necessary to design in consideration of the absolute position of the peak wavelength. Thus, the wavelength multiplexer 301 facilitates the design and manufacture of the wavelength multiplexer.

  As for the two filters with FSR = 3200 GHz, the multistage Mach-Zehnder multiplexer needs to be set accurately so that the phase difference between the two filters becomes 90 degrees as described with reference to FIGS. Therefore, it is necessary to control the length of the delay circuit in units of 100 nm (FIG. 7). However, the wavelength multiplexer 301 does not include a delay circuit that is difficult to control, and it is sufficient that the two filters (asymmetric directional couplers) are appropriately designed, and the design and manufacture are easy.

The wavelength multiplexer 301 is designed so that the light of λ1 and λ3 is combined by the directional coupler 11 and the light of λ0 and λ2 is combined by the asymmetric directional coupler 13, but the light of λ0 and λ2 is directed. The optical coupler 11 may be designed to multiplex the light of λ1 and λ3 by the asymmetric directional coupler 13.
In the wavelength multiplexer 301, an example in which a silicon fine wire waveguide is used as a waveguide structure has been described. However, other materials or waveguides may be used. In the wavelength multiplexer 301, an example in which four waves in the 1.3 micron band are combined has been described assuming application to 100 Gigabit Ethernet (registered trademark), but other wavelength bands may be used. In this case, the length Lc of the coupling portion of the directional coupler and the length Lc of the coupling portion of the asymmetric directional coupler are matched with the wavelength band.
In the present embodiment, only the wavelength multiplexer has been described, but in reality, a light source may be mounted on a planar optical circuit in which the wavelength multiplexer is formed.

(Embodiment 2)
In this embodiment, a wavelength multiplexer 302 having a structure different from that of the wavelength multiplexer 301 of the first embodiment will be described. FIG. 23 is a schematic diagram of a wavelength multiplexer 302 that is a 4-wavelength multiplexer for 100 Gigabit Ethernet (registered trademark). The wavelength multiplexer 302 includes an input unit 10 to which four wavelengths (λ0, λ1, λ2, and λ3 from the short wavelength side) of the frequency interval f are input in the basic mode;
Two Mach-Zehnder interferometer filters (1, 2) having a 2f FSR (Free Spectral Range) and a phase difference of 90 degrees from each other;
An asymmetric directional coupler 12 in which two waveguides are close to each other in parallel, one wavelength of the two waveguides is converted into a higher order mode while the other wavelength is converted into a fundamental mode, and combined.
A mode converter 14 for converting a higher-order mode wavelength into a fundamental mode;
A wavelength multiplexer comprising:
The Mach-Zehnder interferometer filter 1 receives wavelengths (λ1 and λ3) from the input unit 10 and inputs the wavelengths (λ1 and λ3) to the two input waveguides while maintaining the fundamental mode.
The Mach-Zehnder interferometer filter 2 receives the wavelengths (λ0 and λ2) from the input unit to each of the two input waveguides, and combines the wavelengths (λ0 and λ2) while maintaining the fundamental mode.
The asymmetric directional coupler 12 receives the wavelength (λ1 and λ3) combined by the Mach-Zehnder interferometer filter 1 in one of the two waveguides, and the Mach-Zehnder interferometer filter in the other of the two waveguides. Wavelength (λ0 and λ2) combined at 2 is input, the wavelengths (λ0 and λ2) are converted into higher-order modes, and combined into wavelengths (λ1 and λ3),
The mode converter 14 converts a wavelength (λ0 and λ2), which is a higher-order mode, of the combined wavelengths (λ0, λ1, λ2, and λ3) combined by the asymmetric directional coupler 12 into a fundamental mode. It is characterized by that.

  Similar to the two-stage Mach-Zehnder interferometer multiplexer described with reference to FIG. 5, the wavelength multiplexer 302 is different from the Mach-Zehnder interferometer filter 1 and the Mach-Zehnder interferometer filter 1 whose FSR is 3200 GHz. 2 is provided. The wavelength multiplexer 302 combines the light of λ1 and λ3 as TE light by the Mach-Zehnder interferometer filter 1 and the light of λ0 and λ2 by the Mach-Zehnder interferometer filter 2, respectively. After that, the wavelength multiplexer 302 uses the asymmetric directional coupler 12 to combine the combined light of λ0 and λ2 into the combined light of λ1 and λ3 using the TE1 mode. As described in detail in the first embodiment, the characteristics of the asymmetric directional coupler 12 have a small wavelength dependency, and it is not necessary to adjust the absolute position of the peak wavelength. Finally, the wavelength multiplexer 302 converts the light of λ0 and λ2 from the TE1 mode to the TM0 mode by the adiabatic mode conversion unit 14, and combines the four waves of λ0 to λ3.

  That is, the wavelength multiplexer 302 replaces the Mach-Zehnder interferometer filter 3 having an FSR of 1600 GHz in FIG. 3 with a combination of an asymmetric directional coupler 12 and an adiabatic mode converter 14 that are easy to design and have a large tolerance for manufacturing errors. ing. For this reason, the wavelength multiplexer 302 has a large manufacturing error tolerance, and does not need to be designed in consideration of the absolute position of the peak wavelength. Thus, the wavelength multiplexer 302 facilitates the design and manufacture of the wavelength multiplexer.

(Embodiment 3)
FIG. 24 is a diagram illustrating the wavelength multiplexer / demultiplexer 303 of the present embodiment. The wavelength multiplexer / demultiplexer 303 includes two wavelength multiplexers (301 or 302) described in the first and second embodiments in which a plurality of wavelengths that do not overlap wavelength bands are respectively input, and the respective wavelength multiplexers. And a band combiner 35 for further combining the output combined wavelengths.

  The wavelength multiplexer / demultiplexer 303 is, for example, an 8-wavelength multiplexer for 400 Gigabit Ethernet (registered trademark). As shown in FIG. 25, 400 Gigabit Ethernet (registered trademark) has four wavelengths (λ0 to λ3) at intervals of 800 GHz used in 100 Gigabit Ethernet (registered trademark), and 4 at intervals of 800 GHz from light at a wavelength shorter than 4000 GHz from λ0. An 8-wave WDM system is being studied that adds waves (λ4 to λ7). In this case, an interval of one wavelength is left between λ0 and λ7.

  First, the wavelength multiplexer 301 or 302 described in the first or second embodiment can be used as it is as a wavelength multiplexer that multiplexes four lights of λ0 to λ3. Also, a wavelength multiplexer that multiplexes light of λ4 to λ7 can adopt the same configuration as the wavelength multiplexer 301 or 302, but it is necessary to set the coupling portion Lc according to the wavelength. The band multiplexing unit 35 is, for example, an edge filter that transmits light having a longer wavelength than λ0 and reflects light having a shorter wavelength than λ0. The combined light emitted from the two wavelength multiplexers (301 or 302) is multiplexed by the band multiplexing unit 35.

(effect)
As described above, the present invention can provide a wavelength multiplexer that is easy to design and has a high tolerance for manufacturing errors.

  The wavelength multiplexer according to the present invention is not limited to the four-wavelength multiplexer described in the embodiment, and can be applied to a three-wavelength wavelength multiplexer or a multi-wavelength multiplexer having five or more waves.

10: Input unit 11, 11-1, 11-2: Directional coupler 12, 13: Asymmetric directional coupler 14: Mode converter 15: Output unit 21: Input silicon fine wire waveguide 22: Output silicon fine wire waveguide 23: Tapered portion 24: Rib waveguide 25: Fine wire rib conversion taper portion 26: Adiabatic mode conversion portion 27: Wide silicon rib waveguide 35: Band multiplexer (edge filter)
301, 302, 303: Wavelength multiplexer

Claims (9)

A wavelength multiplexer for a planar optical circuit,
An input unit for inputting a plurality of wavelengths to different waveguides in a fundamental mode;
A directional coupler in which two waveguides are close to each other in parallel, and the respective wavelengths of the two waveguides are combined in a fundamental mode;
An asymmetric directional coupler in which two waveguides are close to each other in parallel, one wavelength of the two waveguides is converted to a higher order mode while the other wavelength is converted into a fundamental mode, and combined.
Based on the wavelength of the higher-order mode among the combined wavelengths in which the plurality of wavelengths input to the input unit are all combined via at least one of the directional coupler and the asymmetric directional coupler. A mode converter to convert the mode,
An output unit that outputs the combined wavelength converted into the fundamental mode by the mode converter;
A wavelength multiplexer comprising: One directional coupler, two asymmetric directional couplers, and one mode converter;
The wavelengths input to the waveguide by the input unit are four wavelengths (λ0, λ1, λ2, and λ3) with a frequency interval f,
In the directional coupler, wavelengths (λ1 and λ3) are input to the two waveguides from the input unit, and the two waveguides are parallel so that the wavelengths (λ1 and λ3) are combined. The adjacent waveguide interval and interval are adjusted,
In the first asymmetric directional coupler, the wavelength (λ1) is input to one of the two waveguides from the input unit and the wavelength (λ1) is combined with the other of the two waveguides by the directional coupler. And λ3) are input, the wavelength λ2 is converted into a higher-order mode and combined with the wavelengths (λ1 and λ3),
In the second asymmetric directional coupler, a wavelength λ0 is input from the input unit to one of the two waveguides, and the other asymmetrical directional coupler is combined with the other of the two waveguides. Wavelength (λ1, λ2, and λ3) are input, the wavelength λ0 is converted into a higher-order mode, and multiplexed to the wavelengths (λ1, λ2, and λ3),
The mode converter uses a wavelength (λ0 and λ2), which is a higher-order mode, of the combined wavelengths (λ0, λ1, λ2, and λ3) combined by the second asymmetric directional coupler as a fundamental mode The wavelength multiplexer according to claim 1, wherein the wavelength multiplexer is converted into a wavelength multiplexer. An input unit in which four wavelengths (λ0, λ1, λ2, and λ3 from the short wavelength side) of the frequency interval f are input in the basic mode;
Two Mach-Zehnder interferometer filters having a 2f FSR (Free Spectral Range) and a phase difference of 90 degrees from each other;
An asymmetric directional coupler in which two waveguides are close to each other in parallel, one wavelength of the two waveguides is converted to a higher order mode while the other wavelength is converted into a fundamental mode, and combined.
A mode converter that converts a higher-order mode wavelength into a fundamental mode;
A wavelength multiplexer comprising:
In the first Mach-Zehnder interferometer filter, the wavelengths (λ1 and λ3) are input to the two input waveguides from the input unit, and the wavelengths (λ1 and λ3) are multiplexed in the fundamental mode,
In the second Mach-Zehnder interferometer filter, the wavelengths (λ0 and λ2) are input to the two input waveguides from the input unit, and the wavelengths (λ0 and λ2) are multiplexed in the fundamental mode,
The asymmetric directional coupler receives a wavelength (λ1 and λ3) combined by the first Mach-Zehnder interferometer filter in one of the two waveguides, and a second in the other of the two waveguides. The wavelengths (λ0 and λ2) combined by the Mach-Zehnder interferometer filter are input, the wavelengths (λ0 and λ2) are converted into higher-order modes, and combined with the wavelengths (λ1 and λ3),
The mode converter converts a wavelength (λ0 and λ2), which is a higher-order mode, of the combined wavelengths (λ0, λ1, λ2, and λ3) combined by the asymmetric directional coupler into a fundamental mode. A wavelength multiplexer characterized by that. The asymmetric directional coupler is
The effective refractive index of one fundamental mode of the two waveguides and the effective refractive index of the other higher-order mode of the two waveguides match,
The section in which the two waveguides are parallel and close is such a length that the power of one fundamental mode of the two waveguides shifts to the power of the other higher order mode of the two waveguides. The wavelength multiplexer according to any one of claims 1 to 3, wherein The mode converter is
The wavelength multiplexer according to any one of claims 1 to 4, further comprising a tapered portion whose waveguide width continuously changes.   6. The wavelength multiplexer according to claim 5, wherein the mode converter has a tapered silicon rib waveguide that satisfies a heat insulation condition. The input unit inputs a fundamental mode of TE0 to the waveguide,
The asymmetric directional coupler converts the other wavelength into a higher order mode of TE1,
7. The wavelength multiplexer according to claim 1, wherein the mode converter converts a higher-order mode of TE1 into a fundamental mode of TM0.   8. The wavelength multiplexer according to claim 1, wherein the waveguide is a silicon fine wire waveguide. A plurality of wavelength multiplexers according to any one of claims 1 to 8, wherein a plurality of wavelengths that do not overlap wavelength bands are respectively input;
A band combiner for further combining the combined wavelengths output by the respective wavelength combiners;
A wavelength multiplexer comprising: