JP2010175646A - Optical wavelength multiplexing/demultiplexing circuit and method of adjusting transmitting waveform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical wavelength multiplexing/demultiplexing circuit capable of adjusting a transmitting spectrum waveform, and to provide a method of adjusting it. <P>SOLUTION: The optical wavelength multiplexing/demultiplexing circuit equipped with an array waveguide grating (AWG) includes an optical splitter, two arm waveguides, an optical mode synthesizing coupler, and a taper waveguide. The optical mode synthesizing coupler makes a base mode light inputted from one arm waveguide couple with a primary mode and makes a base mode light inputted from the other arm waveguide couple with the base mode. The taper waveguide is configured to excite a secondary mode light. In such optical wavelength multiplexing/demultiplexing circuit, the effective refractive index of at least one of the two arm waveguides is varied, so that the phase difference of the primary mode light for the base and the secondary mode light at the opening end of the taper waveguide is varied, thereby the transmitting spectrum waveform can be adjusted. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical wavelength multiplexing / demultiplexing circuit and a transmission waveform adjusting method thereof, and more particularly to an optical wavelength multiplexing / demultiplexing device having a flat transmission band and a transmission waveform adjusting method thereof.

  With the spread of broadband communication services, the demand for higher capacity of optical communication networks is increasing, and optical wavelength division multiplexing (WDM) transmission that transmits a large number of optical wavelength signals at once is the transmission capacity of the network. It is important as a technology that dramatically increases On the other hand, a planar lightwave circuit (PLC) composed of a silica-based glass waveguide formed on a substrate such as silicon has been actively researched and developed as a basic technology for various optical devices. An arrayed waveguide diffraction grating (AWG) using such a silica-based PLC technology has a function of multiplexing or demultiplexing a large number of optical wavelengths and plays a very important role as an optical wavelength multiplexer / demultiplexer in WDM transmission. Plays.

  In WDM transmission, it is desirable that the loss does not vary as much as possible even if the signal light wavelength of the light source varies somewhat. In order to transmit a higher-speed modulated signal without deterioration, it is desirable to transmit a modulated component spread in a certain wavelength range without loss. Therefore, a flat AWG having a wide and flat pass band is required for the AWG as an optical wavelength multiplexer / demultiplexer.

  FIG. 22 is a plan view showing a configuration example of a conventional AWG. The AWG 4100 includes an input waveguide 4101, a first slab waveguide 4102, an arrayed waveguide 4103, a second slab waveguide 4104, and an output waveguide 4105. FIG. 23 is a cross-sectional view taken along line BB ′ in FIG. As shown in the figure, a waveguide core 4201 and a clad 4202 are provided on a silicon substrate 4203.

  A light wave incident from a port of the input waveguide 4101 is magnified by the first slab waveguide and enters the arrayed waveguide 4103. Each of the waveguides of the arrayed waveguide 4103 is set so that its optical path length becomes sequentially longer with a constant optical path length difference, and a constant phase difference is given to the light wave propagated through each waveguide, so that the second The light enters the slab waveguide 4104. These incident light waves interfere with the second slab waveguide 4104 and are condensed on the end face connected to the output waveguide 4105. At this time, the phase difference given by the arrayed waveguide 4103 depends on the wavelength. In other words, since the inclination of the equiphase surface varies depending on the wavelength, the condensing position in the second slab waveguide 4104 also depends on the wavelength. Therefore, a light wave having a wavelength corresponding to the connection position with the second slab waveguide 4104 enters the output waveguide 4105 and is demultiplexed to each port.

  In WDM transmission, the wavelength multiplexed signal input to the input waveguide 4101 is demultiplexed into signals of each wavelength and output to each port of the output waveguide 4105. On the contrary, the signal of each wavelength input to each port of the output waveguide 4105 is combined with the wavelength multiplexed signal and output to a port having the input waveguide 4101.

  In such an AWG, the optical field excited at the connection interface between the input waveguide 4101 and the first slab waveguide 4102 and the connection between the output waveguide 4105 and the second slab waveguide 4104 are excited. The power overlap integral of the optical field becomes the transmission spectrum. Normally, only the fundamental mode light is excited in these optical fields, and the transmission spectrum waveform has a Gaussian function shape. However, a parabolic taper waveguide (disclosed in Patent Document 1) is provided at the connection portion of the input waveguide 4101 to the first slab waveguide 4102, and a part of the fundamental mode light is converted into the second-order mode light to generate an optical field. By deforming, flat AWG is realized.

Japanese Patent No. 311246 Y. Hibino, et al., “Optical frequency tuning by laser-irradiation in silica-based March-Zehnder-type multi / demultiplexers,” PHOTONICS TECHNOLOGY LETTERS, Vol.3, pp.640-642, 1991. J. Leuthold, et al., “Multimode Interference Couplers for the Conversion and Combining of Zero- and First-Order Modes,” JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol.16, pp.1228-1238, 1998. M. Abe, et al., “Optical path length trimming technique using thin film heaters for silica-based waveguides on Si,” ELECTRONICS LETTERS, Vol.32, pp.1818-1820, 1996.

  When an AWG is actually manufactured, an error occurs in a certain phase difference given in the arrayed waveguide. This is mainly due to the refractive index in the waveguide core, the nonuniformity of the core width, and the core thickness. Such a phase error affects the transmission spectrum waveform of the flat AWG. In particular, when the phase error for each arrayed waveguide is asymmetric with respect to the central arrayed waveguide, a slope of transmittance occurs at the center of the transmission band, and flatness deteriorates.

  FIG. 24 shows an example of the phase error distribution in each arrayed waveguide. As shown in the figure, the phase error is asymmetric distribution with respect to the central arrayed waveguide, which is well approximated by a cubic function. FIG. 24 shows three types of I, II, and III depending on the magnitude of the error. ing. In the array waveguide number on the horizontal axis, negative indicates an inner array waveguide, positive indicates an outer array waveguide, and zero indicates a central array waveguide. FIG. 25 is a graph showing an example of a transmission spectrum waveform of a flat type AWG, and shows the case of phase error II (solid line) in FIG. 24 together with the case where there is no phase error (dotted line). FIG. 26 is an enlarged view of the front end of the transmission spectrum of FIG.

  In this example, the refractive index of the cladding was 1.44425, the relative refractive index difference Δ of the waveguide was 1.5%, and the thickness of the waveguide core was 4.5 μm. Also, the design of the AWG is a multiplexing / demultiplexing wavelength interval of 0.8 nm (optical frequency interval of 100 GHz), the path length difference ΔL between adjacent array waveguides is 33.9 μm, the number of arrayed waveguides is 250, the first and second The length of the slab waveguide is 8100 μm, and the output waveguides are arranged at intervals of 16 μm in the portion connected to the second slab waveguide. The parabolic taper waveguide connected to the first slab waveguide has a length of 150 μm and an opening end width of 16 μm, and the width of the linear taper waveguide of the output waveguide connected to the second slab waveguide is 8 μm. It was. When there is no phase error, the transmission spectrum waveform is flat near the transmission center wavelength. However, when there is a phase error, the transmission spectrum waveform is inclined and the flatness of the transmission band is degraded. .

  As described above, when a flat type AWG is actually manufactured, a phase error occurs in the array waveguide, and particularly when the phase error is asymmetric with respect to the central array waveguide, in the vicinity of the transmission center wavelength. An inclination of transmittance occurs, and flatness deteriorates. Therefore, depending on the degree of the phase error, the expected flat transmission waveform cannot be realized, and there is a problem that the optical characteristics required for the flat AWG cannot be obtained with a high yield.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical wavelength multiplexing / demultiplexing circuit capable of adjusting a transmission spectrum waveform and an adjusting method thereof. As a result, in an optical wavelength multiplexing / demultiplexing circuit, even if a phase error occurs in the arrayed waveguide due to a manufacturing error or the like and the phase error fluctuates depending on the circuit, a flat transmission band characteristic is stably provided. Can get to.

  In order to solve the above-mentioned problems, it is considered desirable to change the slope of the transmission spectrum waveform and adjust it to a flat transmission waveform after the circuit is once manufactured. In order to obtain such a change in transmission waveform in the AWG, the present invention focuses on the optical mode excited at the connection portion of the AWG input waveguide to the first slab waveguide.

  In an AWG with a flat transmission spectrum according to the prior art, second-order mode light is excited at a certain intensity ratio by a tapered waveguide having a parabolic shape or the like connected to an input waveguide, and connected to the first slab waveguide. In the part, the field is symmetrical bimodal. On the other hand, only the fundamental mode light exists in the output waveguide, and it is a unimodal field at the connection to the second slab waveguide. The transmission spectrum of the AWG is determined by the power overlap integration of both light fields and has a flat waveform.

  Here, when the first-order mode light is further mixed in the optical field on the input waveguide side at a specific intensity ratio, the field becomes asymmetrical bimodal, and the asymmetry is the fundamental and second-order mode light and the first-order mode. Determined by the phase difference of light. When this field is asymmetric, it appears as a waveform slope in the transmission spectrum of AWG.

  Therefore, if the primary mode light having a predetermined intensity ratio is excited in the input waveguide and the phase difference between the primary mode light and the base and secondary mode light can be changed by an appropriate mechanism, In the flat type AWG based on the technology, it is possible to further change the waveform inclination of the transmission spectrum and compensate for the waveform inclination caused by the phase error of the arrayed waveguide.

  Based on the above considerations, the invention according to claim 1 is directed to a first slab waveguide, an arrayed waveguide composed of a plurality of waveguides connected to the first slab waveguide, and the arrayed waveguide. An arrayed waveguide diffraction grating comprising a second slab waveguide connected to a plurality of waveguides, an optical splitter, first and second arm waveguides connected to the optical splitter, and the first And an optical mode synthesis coupler connected to the second arm waveguide, wherein the fundamental mode light input from the first arm waveguide is coupled to the primary mode and input from the second arm waveguide. An optical mode synthesis coupler that couples the fundamental mode light to the fundamental mode, and a tapered waveguide that is connected to the optical mode synthesis coupler and excites secondary mode light, and is further connected to the first slab waveguide The Characterized by comprising a waveguide.

  The invention according to claim 2 is the optical wavelength multiplexing / demultiplexing circuit according to claim 1, wherein the taper waveguide propagates fundamental and first-order mode light and does not propagate second-order mode light. It is connected to the optical mode synthesis coupler through a multimode waveguide.

  The invention according to claim 3 is the optical wavelength multiplexing / demultiplexing circuit according to claim 1 or 2, further comprising a heater for heating at least one of the first and second arm waveguides. It is characterized by that.

  According to a fourth aspect of the present invention, there is provided the optical wavelength multiplexing / demultiplexing circuit according to any one of the first to third aspects, wherein the optical mode combining coupler comprises two waveguides having different widths. It is characterized by being a directional coupler.

  The invention according to claim 5 is the optical wavelength multiplexing / demultiplexing circuit according to any one of claims 1 to 3, wherein the optical mode synthesis coupler is composed of two waveguides having different widths. The narrower waveguide is characterized by ending with a gradually decreasing width.

  The invention according to claim 6 is the optical wavelength multiplexing / demultiplexing circuit according to any one of claims 1 to 5, wherein the optical splitter is configured by a wavelength independent coupler (WINC). It is characterized by.

  The invention according to claim 7 is the optical wavelength multiplexing / demultiplexing circuit according to any one of claims 1 to 6, wherein the arrayed waveguide diffraction grating and the other waveguides are made of silica-based waveguides. It is configured.

  The invention according to claim 8 is a first slab waveguide, an arrayed waveguide composed of a plurality of waveguides connected to the first slab waveguide, and a plurality of waveguides of the arrayed waveguide. An arrayed waveguide grating comprising a second slab waveguide connected to the optical splitter, an optical splitter, first and second arm waveguides connected to the optical splitter, and the first and second An optical mode synthesis coupler connected to the arm waveguide, wherein the fundamental mode input from the first arm waveguide is coupled to the primary mode and the fundamental mode input from the second arm waveguide. An optical mode combining coupler that couples light to a fundamental mode; and a tapered waveguide that is connected to the optical mode combining coupler and excites secondary mode light, and further connected to the first slab waveguide. With waveguide In the optical wavelength multiplexing / demultiplexing circuit, a transmission spectrum waveform is adjusted by changing a difference in optical path length between the first arm waveguide and the second arm waveguide to change the secondary mode. It is characterized in that the phase difference of the first-order mode light with respect to the base and second-order mode light at the opening end of the tapered waveguide to be excited is changed.

  The invention according to claim 9 is the method according to claim 8, wherein at least one of the first and second arm waveguides is irradiated with ultraviolet light to change its effective refractive index. It is characterized by.

  The invention according to claim 10 is the method according to claim 8, wherein at least one of the first and second arm waveguides is heated to change its effective refractive index irreversibly. It is characterized by that.

  According to the present invention, a flat transmission band characteristic can be stably obtained in an optical wavelength multiplexing / demultiplexing circuit even when the slope of the transmission spectrum waveform varies due to a manufacturing error or the like.

It is a figure which shows the structural example of the AWG type | mold optical wavelength multiplexing / demultiplexing circuit concerning one Embodiment of this invention. In the AWG type optical wavelength multiplexing / demultiplexing circuit according to the embodiment of the present invention, the field shapes of the base, first-order, and second-order mode light generated at the opening end of the tapered waveguide connected to the first slab waveguide FIG. It is a figure which shows the change of the synthetic | combination field of the basic | primary, primary, and secondary mode light shown in FIG. It is a figure which shows the change of the transmission spectrum waveform of AWG corresponding to the synthetic | combination field shown in FIG. It is a figure which shows the structure of the AWG type | mold optical wavelength multiplexing / demultiplexing circuit concerning the 1st Example of this invention. FIG. 6 is an enlarged view of the vicinity of a tapered waveguide from an optical splitter in the AWG type optical wavelength multiplexing / demultiplexing circuit of FIG. 5. FIG. 5 is a diagram showing changes in the effective refractive index fluctuation amount and optical phase difference of the arm waveguide when the region on the first or second arm waveguide is irradiated with ultraviolet rays in the AWG type optical wavelength multiplexing / demultiplexing circuit of FIG. It is. FIG. 6 is a diagram illustrating an example of a phase error generated in an arrayed waveguide in the AWG type optical wavelength multiplexing / demultiplexing circuit of FIG. 5. The transmittance change rate of the central wavelength with respect to the effective refractive index change amount of the arm waveguide when the region on the second arm waveguide of the AWG type optical wavelength multiplexing / demultiplexing circuit having a negative waveform inclination is irradiated with ultraviolet light. FIG. It is a figure which shows the waveform front-end | tip part before and behind adjustment of the transmission spectrum waveform by irradiation of the ultraviolet light of FIG. FIG. 6 is a diagram showing another example of a phase error generated in the arrayed waveguide in the AWG type optical wavelength multiplexing / demultiplexing circuit of FIG. 5. The transmittance change rate of the center wavelength with respect to the effective refractive index change amount of the arm waveguide when the region on the first arm waveguide of the AWG type optical wavelength multiplexing / demultiplexing circuit having a positive waveform inclination is irradiated with ultraviolet light. FIG. It is a figure which shows the waveform front-end | tip part before and behind adjustment of the transmission spectrum waveform by irradiation of the ultraviolet light of FIG. FIG. 3 is a diagram illustrating a configuration example of an optical mode combining coupler in an AWG type optical wavelength multiplexing / demultiplexing circuit. FIG. 5 is a diagram illustrating another configuration example of an optical mode synthesis coupler in an AWG type optical wavelength multiplexing / demultiplexing circuit. FIG. 10 is a diagram showing still another configuration example of the optical mode combining coupler in the AWG type optical wavelength multiplexing / demultiplexing circuit. In the AWG type | mold optical wavelength multiplexing / demultiplexing circuit, it is a figure which shows one structural example of an optical splitter. It is a figure which shows the structure of the AWG type | mold optical wavelength multiplexing / demultiplexing circuit concerning the 2nd Example of this invention. FIG. 19 is an enlarged view of the vicinity of the tapered waveguide from the optical splitter in the AWG type optical wavelength multiplexing / demultiplexing circuit of FIG. 18. It is sectional drawing in line segment AA 'of FIG. In the AWG type optical wavelength multiplexing / demultiplexing circuit of FIG. 18, it is a figure which shows the change of the effective refractive index variation | change_quantity and optical phase difference of an arm waveguide when the 1st or 2nd arm waveguide is heated with a heater. It is a figure which shows the structural example of the conventional AWG. It is sectional drawing in line segment BB 'of FIG. It is a figure which shows an example of the phase error distribution in each array waveguide of FIG. It is a figure which shows an example of the transmission spectrum waveform of flat type AWG. It is the figure which expanded the front-end | tip of the transmission spectrum of FIG.

  Embodiments of the present invention will be described below. FIG. 1 shows a configuration example of an AWG type optical wavelength multiplexing / demultiplexing circuit according to an embodiment of the present invention. This AWG type optical wavelength multiplexing / demultiplexing circuit 100 includes an input waveguide 101, an optical splitter 106, a first arm waveguide 107, a second arm waveguide 108, an optical mode synthesis coupler 109, a multimode waveguide 110, and A tapered waveguide 111 that excites secondary mode light is provided. The circuit 100 further includes a first slab waveguide 102, an arrayed waveguide 103, a second slab waveguide 104, and an output waveguide 105.

  The optical mode coupler 109 converts the fundamental mode light input from the first arm waveguide 107 into primary mode light, and combines the fundamental mode light input from the second arm waveguide 108 as the fundamental mode light. The combined fundamental mode light and first order mode light propagate in the multimode waveguide 110 according to the effective refractive index of each mode, and in the tapered waveguide 111, part of the fundamental mode light is converted into second order mode light. Is done. At this time, the primary mode light propagates as it is as the primary mode light. Therefore, a combined field of fundamental mode light, first-order mode light, and second-order mode light is generated at the opening end of the tapered waveguide 111 and the connection portion to the first slab waveguide 102. The intensity ratio and the phase difference between the fundamental mode light and the secondary mode light in this combined field are determined by the shape of the tapered waveguide 111. The intensity ratio of the primary mode light to the fundamental mode light and the secondary mode light is determined by the branching ratio in the optical splitter 106 and the coupling ratio of the optical mode combining coupler 109, and the phase difference is the first arm waveguide 107. And the second arm waveguide 108, the length of the multimode waveguide 110, and the length of the tapered waveguide 111.

  FIG. 2 shows field shapes of base, first-order, and second-order mode light generated at the opening end of the tapered waveguide 111 in the embodiment of the present invention. In the figure, the waveguide width at the open end of the tapered waveguide 111 is W, and the center of the waveguide is zero on the horizontal axis. The vertical axis represents the electric field amplitude.

  FIG. 3 shows a change in the combined field of the base, first-order, and second-order mode light shown in FIG. In the figure, the intensity ratio of the base, first-order, and second-order mode light is set to 0.85: 0.05: 0.10. The phase difference of the secondary mode light with respect to the fundamental mode light is fixed to zero, and the phase difference of the primary mode light with respect to the fundamental mode light is −π, −0.75π, −0.5π, −0.25π, 0 , 0.25π, 0.5π, 0.75π, and π are changed. By changing the phase difference of the first-order mode light with respect to the fundamental mode light, it can be seen that the bimodal field changes from asymmetric → symmetric → reverse asymmetric → symmetric → asymmetric.

  FIG. 4 shows a change in the transmission spectrum waveform of the AWG 100 when each composite field of FIG. 3 is generated. Here, assuming that the optical field of only the fundamental mode light is assumed at the connection portion of the output waveguide 105 to the second slab waveguide 104, the transmission spectrum is obtained by the power overlap integration with the above synthetic field. From the figure, it can be seen that the tilt state of the transmission spectrum waveform is continuously changed by changing the phase difference of the first-order mode light with respect to the base mode light.

  Here, the phase difference of the primary mode light with respect to the fundamental mode light in the composite field is adjusted by changing the optical path length difference between the first arm waveguide 107 and the second arm waveguide 108 by an appropriate means. Is possible. Since the optical path length is a product of the effective refractive index and the length of the waveguide, adjustment of the optical path length difference can be achieved by, for example, changing the effective refractive index of the first arm waveguide 107 and the second arm waveguide 108. It is done by changing. Increasing the effective refractive index of the first arm waveguide 107 shifts the phase difference in the positive direction, and increasing the effective refractive index of the second arm waveguide 108 shifts the phase difference in the negative direction.

  Hereinafter, a method for adjusting a transmission spectrum waveform in the optical wavelength multiplexing / demultiplexing circuit will be described. First, in the design, the phase difference of the first-order mode light with respect to the base mode light in the synthesis field is set so that the transmission waveform becomes flat. Here, the phase difference is assumed to be −0.5π. The transmission spectrum of the manufactured circuit is evaluated and classified into the following three according to the result.

(1) Transmittance does not change with wavelength (flat)
(2) Transmittance has a positive slope with respect to wavelength (3) Transmittance has a negative slope with respect to wavelength In the case of classification (1), the phase error in the arrayed waveguide 103 is not a problem. This means that the transmission waveform is not required to be adjusted. In the cases of classifications (2) and (3), it means that a phase error has occurred in the arrayed waveguide 103, and the transmission waveform needs to be adjusted. In the case of classification (2), the effective refractive index of the first arm waveguide 107 is gradually increased. As a result, the phase difference of the primary mode light with respect to the fundamental mode light in the composite field is shifted in the positive direction, and the slope of the transmittance with respect to the wavelength gradually changes in the negative direction. If the change in the effective refractive index of the first arm waveguide 107 is stopped when the transmission waveform becomes flat, an AWG having a flat transmission waveform can be obtained. On the other hand, in the case of classification (3), the effective refractive index of the second arm waveguide 108 is gradually increased. As a result, the phase difference of the primary mode light with respect to the fundamental mode light in the composite field shifts in the negative direction, and the slope of the transmittance with respect to the wavelength gradually changes in the positive direction. If the effective refractive index change of the second arm waveguide 108 is stopped when the transmission waveform becomes flat, an AWG having a flat transmission waveform can be obtained.

  Here, the width of the multi-mode waveguide 110 is wide enough to propagate at least up to the first-order mode light, so that the second-order mode light cannot propagate, that is, a width that does not have an effective refractive index of the second-order mode. It is preferable that This is because even when the fundamental mode light input from the first arm waveguide 107 is slightly converted into the second-order mode light by the optical mode coupler 110, the second-order mode light is suppressed by the multi-mode waveguide 110. In order to prevent unnecessary fluctuations from occurring in the combined field at the open end of the tapered waveguide 111.

  FIG. 5 shows the configuration of an AWG type optical wavelength multiplexing / demultiplexing circuit according to the first embodiment of the present invention. This AWG type optical wavelength multiplexing / demultiplexing circuit 1100 includes an input waveguide 1101, an optical splitter 1106, a first arm waveguide 1107, a second arm waveguide 1108, an optical mode synthesis coupler 1109, a multimode waveguide 1110, and A tapered waveguide 1111 for exciting the second-order mode light is provided. The circuit 1100 includes a first slab waveguide 1102, an arrayed waveguide 1103, a second slab waveguide 1104, and an output waveguide 1105.

  In FIG. 5, the length of the arrayed waveguide 1103 is designed to become longer by a certain amount ΔL. This AWG 1100 is made of quartz PLC, and has a clad refractive index of 1.44425, a waveguide relative refractive index difference Δ of 1.5%, and a core thickness of 4.5 μm. The core width of the input waveguide 1101, the arrayed waveguide 1103, the output waveguide 1105, the first arm waveguide 1107, and the second arm waveguide 1108 is 4.5 μm. The number of wavelength channels is 40, the transmission wavelength of the central wavelength channel is 15544.53 μm (194.1 THz), and the wavelength channel interval is 0.8 nm (optical frequency interval 100 GHz). At this time, the number of arrayed waveguides is 250. In this case, ΔL is 33.9 μm. The lengths of the first slab waveguide 1102 and the second slab waveguide 1104 are 8100 μm, and the output waveguide 1105 has the number of wavelength channels at intervals of 16 μm in the portion connected to the second slab waveguide 1104. That is, 40 are arranged.

  FIG. 6 is an enlarged view of the vicinity of the tapered waveguide 1111 from the optical splitter 1106 in the optical wavelength multiplexing / demultiplexing circuit 1100 of FIG. The reference numerals of the respective parts are the same as those in FIG. Here, a directional coupler is used as the optical splitter 1106. As the optical mode coupler 1109, a directional coupler having an asymmetric waveguide width is used. The width of the waveguide 1201 connected to the first arm waveguide 1107 is 2.5 μm, and the second arm waveguide 1108 is used. The width of the waveguide 1202 connected to is set to 8 μm, and the length of the waveguides 1201 and 1202 is set to 500 μm. Further, the waveguide width is smoothly converted from the second arm waveguide 1108 to the waveguide 1202 by a linear taper. At this time, the fundamental mode effective refractive index of the waveguide 2101 and the primary mode effective refractive index of the waveguide 1202 are substantially equal, and the fundamental mode light input from the first arm waveguide 1107 to the waveguide 1201 is The first mode of the waveguide 1202 is coupled. Further, since the fundamental mode light input from the second arm waveguide 1108 propagates as it is in the fundamental mode through the waveguide 1202, the fundamental mode and the primary mode are combined and output to the multimode waveguide 1110. The light intensity ratio between the fundamental mode light and the first-order mode light is determined by the branching ratio in the optical splitter 1106 and the coupling ratio of the optical mode combining coupler 1109 from the waveguides 1201 to 1202. 90%, and the light intensity ratio between the fundamental mode light and the first-order mode light is 95: 5.

  The optical mode synthesis coupler 1109 is connected to the first slab waveguide 1102 via the multimode waveguide 1110 and the tapered waveguide 1111. In this embodiment, the tapered waveguide 1111 uses a parabolic taper. The multimode waveguide 1110 has a waveguide width of 8 μm, the parabolic tapered waveguide 1111 has a length of 150 μm, and the opening end has a width of 16 μm. In the tapered waveguide 1111, part of the fundamental mode light is converted into secondary mode light. At this time, the primary mode light propagates as it is as the primary mode light. Therefore, a combined field of the fundamental mode light, the first mode light, and the second mode light is generated at the opening end of the tapered waveguide 1111, that is, at the connection portion to the first slab waveguide 1102.

  The intensity ratio and the phase difference between the fundamental mode light and the secondary mode light in this combined field are determined by the shape of the tapered waveguide 1111. In this embodiment, the intensity ratio of the base, first-order, and second-order mode light at the open end of the tapered waveguide 1111 is 85: 5: 10. Further, the phase difference of the secondary mode light with respect to the base mode light is zero. Here, the phase difference of the first-order mode light with respect to the fundamental mode light is the difference in the length of the first arm waveguide 1107 with respect to the second arm waveguide 1108, the length of the multimode waveguide 1110, and the tapered waveguide. It depends on the length of 1111. In this embodiment, the length difference of the first arm waveguide 1107 with respect to the second arm waveguide 1108 is set to zero, and the phase difference of the primary mode light with respect to the fundamental mode light is set to −0.5π. The length of the mode waveguide 1110 is designed.

  A transmission waveform adjustment method of the optical wavelength multiplexing / demultiplexing circuit in this embodiment will be described. In this embodiment, irradiation with ultraviolet light is used as a method for changing the effective refractive index in the first arm waveguide 1107 or the second arm waveguide 1108. Non-Patent Document 1 discloses a technique for changing the effective refractive index of a waveguide by irradiation with ultraviolet light in a quartz-based PLC. When the quartz waveguide is irradiated with ultraviolet light, the effective refractive index of the waveguide can be increased.

  In FIG. 6, regions 1203 and 1204 indicate regions that are irradiated with ultraviolet light. Actually, a metal plate or the like cut out only in the shape of the region 1203 or 1204 is placed on the upper surface of the PLC, and ultraviolet light is irradiated from the upper surface. Thereby, ultraviolet light can be made to reach only the desired region of the PLC. Here, the waveguide lengths of the portions of the first arm waveguide 1107 or the second arm waveguide 1108 overlapping the regions 1203 and 1204 are each 500 μm.

  FIG. 7 is a diagram showing an effective refractive index fluctuation amount of the waveguide when the region 1203 or 1204 is irradiated with ultraviolet rays and a change in the optical phase difference of the first arm waveguide 1107 with respect to the second arm waveguide 1108. It is. Here, the wavelength of light is assumed to be around 1544 nm. When the region 1203, that is, the first arm waveguide 1107 is irradiated, the optical phase difference is shifted in the positive direction with respect to the effective refractive index change, and the region 1204, that is, the second arm waveguide 1108 is irradiated. It can be seen that the optical phase difference shifts in the negative direction with respect to the effective refractive index change.

  Here, in particular, it is assumed that a phase error as shown in FIG. 8 occurs in the arrayed waveguide 1103 of the AWG 1100 of this embodiment. In the array waveguide number on the horizontal axis, negative indicates an inner array waveguide, positive indicates an outer array waveguide, and zero indicates a central array waveguide. When the phase error tends to increase asymmetry with respect to the central arrayed waveguide as shown in FIG. 8, the transmission spectrum waveform of the AWG 1100 has a negative slope with respect to the wavelength of the transmittance. Therefore, in order to eliminate this negative inclination by adjusting, the region 1204 is irradiated with ultraviolet light, and the effective refractive index of a part of the second arm waveguide 1108 is increased.

Here, in order to show the degree of inclination of the transmittance of the AWG with respect to the wavelength, an index of the transmittance change rate is introduced. Now, let the transmission spectrum of AWG be T (λ). Here, λ is a wavelength. The transmittance change rate is defined as dT (λ) / dλ, and is an index indicating the change rate of the transmittance with respect to the wavelength. An ideal flat waveform as a transmission spectrum means dT (λ) / dλ = 0, and if dT (λ) / dλ ≠ 0, the waveform has a slope. FIG. 9 shows the fluctuation of dT (λ _C ) / dλ with respect to the effective refractive index change amount δn of the waveguide when the AWG region 1204 having the negative waveform inclination described above is irradiated with ultraviolet light. Here, the center wavelength of the 3 dB transmission band is λ = λ _C. As can be seen from the figure, dT (λ _C ) / dλ monotonously increases as the effective refractive index increases, and dT (λ _C ) / dλ = 0 when δn = 4.7 × 10 ^−5. That is, the desired flat transmission waveform is reached. In actual waveform adjustment, it is conceivable to measure the transmission spectrum at the same time while irradiating ultraviolet light, and stop the irradiation when dT (λ _C ) / dλ = 0.

FIG. 10 is a diagram comparing the front end of the transmission spectrum waveform before adjustment and after adjustment after irradiation with ultraviolet light up to δn = 4.7 × 10 ⁻⁵ . In the figure, the horizontal axis is the relative wavelengths and lambda = lambda _C standard. It is confirmed that the transmission waveform adjustment improves the negative slope of the transmittance with respect to the wavelength, and an ideal flat waveform can be obtained.

  As another case, it is assumed that a phase error as shown in FIG. 11 has occurred in the arrayed waveguide 1103 of the AWG 1100 of this embodiment. If the array phase is asymmetric with respect to the central array waveguide as shown in FIG. 11 and the array phase tends to decrease, the transmission spectrum waveform of the AWG 1100 has a positive slope with respect to the wavelength of the transmittance. Therefore, in order to eliminate this positive inclination by adjusting, the region 1203 is irradiated with ultraviolet light, and the effective refractive index of a part of the first arm waveguide 1107 is increased.

FIG. 12 shows the variation of dT (λ _C ) / dλ with respect to the effective refractive index change amount δn of the waveguide when the AWG region 1203 having the positive waveform inclination described above is irradiated with ultraviolet light. As can be seen from the figure, dT (λ _C ) / dλ monotonously decreases as the effective refractive index increases, and dT (λ _C ) / dλ = 0 when δn = 2.4 × 10 ^−5. That is, the desired flat transmission waveform is reached. FIG. 13 is a diagram comparing the front end of the transmission spectrum waveform before adjustment and after adjustment after irradiation with ultraviolet light up to δn = 2.4 × 10 ⁻⁵ . It is confirmed that the transmission waveform adjustment improves the positive slope of the transmittance with respect to the wavelength, and an ideal flat waveform is obtained.

  In this embodiment, as shown in FIG. 6, an asymmetric directional coupler is applied as the optical mode combining coupler 1109. However, the realization of the optical mode combining coupler 1109 is not limited to this configuration. FIG. 14 is an enlarged view of the vicinity of the optical mode combining coupler 1109 in another configuration. In the configuration of FIG. 14, the output waveguide connected to the waveguide 1301 is terminated by a groove 1303 although it is an asymmetric directional coupler including the waveguides 1301 and 1302 as in FIG. 6. Here, a light shielding material that absorbs light waves is inserted into the groove 1303, and the interface between the light shielding material and the output waveguide is not perpendicular to the waveguide, but is inclined by 8 degrees from the vertical plane. With the configuration of FIG. 14, compared to the configuration of FIG. 6, stray light penetrates into the first slab waveguide 1102 by blocking the light wave that remains slightly without being coupled from the waveguide 1301 to the waveguide 1302. It is possible to suppress light wave reflection. Therefore, it is possible to realize the optical characteristics of the AWG 1100 that are more excellent in crosstalk and reflection characteristics.

  FIG. 15 is an enlarged view of the vicinity of the optical mode combining coupler 1109 in still another configuration. 15 is an asymmetric directional coupler composed of waveguides 1401 and 1402 as in FIG. 6, but the waveguide 1401 has a structure in which the width gradually decreases and the width disappears and terminates. ing. At this time, the lengths of the waveguides 1401 and 1402 are designed to be 1100 μm. With the configuration in FIG. 15, compared with the configuration in FIG. 6, the coupling rate of the light wave from the waveguide 1401 to the waveguide 1402 can be almost 100%. Therefore, the optical characteristics of the AWG 1100 can be realized with better loss characteristics.

  FIG. 16 is an enlarged view of the vicinity of the optical mode combining coupler 1109 in still another configuration. In the configuration of FIG. 16, the optical mode combining coupler 1109 is composed of two multi-mode interferometers (MMI). This configuration is disclosed in detail in Non-Patent Document 2. The optical mode synthesis coupler 1109 includes a first MMI 1501, a second MMI 1502, and intermediate waveguides 1503, 1504, and 1505. The first MMI 1501 has a width of 20 μm and a length of 754 μm, and the second MMI 1502 has a width of 20 μm and a length of 377 μm. The intermediate waveguide 1503 has a width of 4.5 μm and a length of 50 μm, the intermediate waveguide 1504 has a width of 4.5 μm and a length of 51.5 μm, and the intermediate waveguide 1505 has a width of 4.5 μm and a length. 53 μm. In general, the MMI has a smaller change in the branching characteristic with respect to the change in the waveguide width than the directional coupler. Accordingly, with the configuration of FIG. 16, the fundamental mode light input from the arm waveguide 1107 is the primary mode of the multimode waveguide 1110 even when a manufacturing error occurs in the width of the waveguide as compared with the configuration of FIG. As a result, the optical properties of the AWG 1100 can be further improved in production tolerance.

  In this embodiment, as shown in FIG. 6, a single directional coupler is applied as the optical splitter 1106. However, the implementation of the optical splitter 1106 is not limited to this configuration. For example, it can be realized by a Y branch circuit or an MMI. More preferably, the optical splitter 1106 is realized by a wavelength independent coupler (WINC). FIG. 17 is an enlarged view of the vicinity of the optical splitter 1106 configured by WINC. The optical splitter 1106 includes two directional couplers 1601 and 1602 and two arm waveguides 1603 and 1604. The coupling ratios of the directional couplers 1601 and 1602 are 85% and 90%, respectively, the optical path length difference of the arm waveguide 1603 with respect to the arm waveguide 1604 is 0.49 μm, and WINC is an optical splitter having a branching ratio of 5.5%. It is functioning. With the configuration of FIG. 17 using WINC, the wavelength dependence of the branching ratio is small compared to the configuration of FIG. 6 using a single directional coupler, so the optical characteristics of the AWG 1100 are uniform over a wider wavelength range. A waveform adjustment operation can be obtained.

  FIG. 18 shows the configuration of an AWG type optical wavelength multiplexing / demultiplexing circuit according to the second embodiment of the present invention. The AWG type optical wavelength multiplexing / demultiplexing circuit 2100 includes an input waveguide 2101, an optical splitter 2106, a first arm waveguide 2107, a second arm waveguide 2108, an optical mode synthesis coupler 2109, a multimode waveguide 2110, and A tapered waveguide 2111 for exciting the second-order mode light is provided. The circuit 2100 includes a first slab waveguide 2102, an arrayed waveguide 2103, a second slab waveguide 2104, and an output waveguide 2105. Further, heaters 2112 and 2113 are provided on the first arm waveguide 2107 and the second arm waveguide 2108 so that a part of them can be heated.

  In FIG. 18, the length of the arrayed waveguide 2103 is designed to increase sequentially by a certain amount ΔL. This AWG 2100 is made of quartz PLC, and has a clad refractive index of 1.44425, a waveguide relative refractive index difference Δ of 1.5%, and a core thickness of 4.5 μm. The core width of the input waveguide 2101, the arrayed waveguide 2103, the output waveguide 2105, the first arm waveguide 2107, and the second arm waveguide 2108 is 4.5 μm. The number of wavelength channels is 40, the transmission wavelength of the central wavelength channel is 15544.53 μm (194.1 THz), and the wavelength channel interval is 0.8 nm (optical frequency interval 100 GHz). At this time, the number of arrayed waveguides is 250. In this case, ΔL is 33.9 μm. The lengths of the first slab waveguide 2102 and the second slab waveguide 2104 are 8100 μm, and the output waveguide 2105 has the number of wavelength channels at intervals of 16 μm in the portion connected to the second slab waveguide 2104. That is, 40 are arranged.

  FIG. 19 is an enlarged view of the vicinity of the tapered waveguide 2111 from the optical splitter 2106 in the optical wavelength multiplexing / demultiplexing circuit 2100 of FIG. The reference numerals of the respective parts are the same as those in FIG. Here, WINC is used as the optical splitter 2106, and this WINC is composed of two directional couplers 2201 and 2202 and two arm waveguides 2203 and 2204. The coupling ratios of the directional couplers 2201 and 2202 are 85% and 90%, respectively, the optical path length difference of 2203 with respect to the arm waveguide 2204 is 0.49 μm, and WINC functions as an optical splitter with a branching ratio of 5.5%. is doing.

  FIG. 20 is a view showing a cross-sectional structure taken along line AA 'in FIG. On the silicon substrate 2304, cores 2301, 2302 and a clad 2304 are formed. The cores 2301 and 2302 are waveguide cores of the first arm waveguide 2107 and the second arm waveguide 2108, respectively. The heaters 2112 and 2113 are mounted on the surface of the clad 2304 above the waveguide cores 2301 and 2302, and can heat the respective waveguides.

  Further, as shown in FIG. 19, a directional coupler having an asymmetric waveguide width is used as the optical mode coupler 2109. The waveguide 2205 connected to the first arm waveguide 2107 is gradually increased from a width of 2.5 μm. It has a structure that becomes narrower and ends with no width. The width of the waveguide 2206 connected to the second arm waveguide 2108 is 8 μm, and the length of the waveguides 2205 and 2206 is 1100 μm. Further, the waveguide width is smoothly converted from the second arm waveguide 2108 to the waveguide 2206 by a linear taper. The light intensity ratio between the fundamental mode light and the primary mode light output from the optical mode coupler 2109 is determined by the branching ratio in the optical splitter 2106 and the coupling ratio of the optical mode combining coupler 2109 from the waveguides 2205 to 2206. In the embodiment, the light intensity ratio is designed to be 5.5% and 90%, respectively, and the light intensity ratio between the fundamental mode light and the primary mode light is 95: 5.

  The optical mode synthesis coupler 2109 is connected to the first slab waveguide 2102 via the multimode waveguide 2110 and the tapered waveguide 2111. In this embodiment, the tapered waveguide 2111 uses a parabolic taper. The waveguide width of the multimode waveguide 2110 is 8 μm, the length of the parabolic tapered waveguide 2111 is 150 μm, and the width of the open end is 16 μm. In this tapered waveguide 2111, part of the fundamental mode light is converted into secondary mode light. At this time, the primary mode light propagates as it is as the primary mode light. Therefore, a combined field of the fundamental mode light, the first mode light, and the second mode light is generated at the opening end of the tapered waveguide 2111, that is, at the connection portion to the first slab waveguide 2102.

  The intensity ratio and the phase difference between the fundamental mode light and the secondary mode light in this combined field are determined by the shape of the tapered waveguide 2111. In this embodiment, the intensity ratio of the base, first-order, and second-order mode light at the open end of the tapered waveguide 2111 is 85: 5: 10. Further, the phase difference of the secondary mode light with respect to the base mode light is zero. Here, the phase difference of the first-order mode light with respect to the fundamental mode light is the difference in the length of the first arm waveguide 2107 with respect to the second arm waveguide 2108, the length of the multimode waveguide 2110, and the tapered waveguide. 2111 length. In this embodiment, the length difference of the first arm waveguide 2107 with respect to the second arm waveguide 2108 is set to zero, and the first mode light phase difference with respect to the fundamental mode light is set to −0.5π. The length of the mode waveguide 2110 is designed.

  The transmission wavelength adjustment method of the optical wavelength multiplexing / demultiplexing circuit according to the present embodiment changes the effective refractive index in the first arm waveguide 2107 or the second arm waveguide 2108 in the same manner as in the first embodiment, thereby This is performed by changing the phase difference of the primary mode light with respect to the fundamental mode light at the opening end of the tapered waveguide 2111. However, in this embodiment, the first arm waveguide 2107 is provided with a heater 2112, and the second arm waveguide 2108 is provided with a heater 2113. As a method for changing the effective refractive index of each arm waveguide, A method is used in which the heater 2112 or 2113 is heated for a certain period of time to cause an irreversible change in the waveguide material. Non-Patent Document 3 discloses a technique for irreversibly changing the effective refractive index of a waveguide by heating with a heater in a quartz PLC. When the quartz-based waveguide is heated for a certain time, the effective refractive index of the waveguide can be increased.

  In FIG. 19, the length and width of the first arm waveguide 2107 and the second arm waveguide 2108 on which the heaters 2112 and 2113 are mounted are 4000 μm and 50 μm, respectively. FIG. 21 is a diagram showing the effective refractive index fluctuation amount of the waveguide when heated by the heater 2112 or 2113 and the change in the optical phase difference of the first arm waveguide 2107 with respect to the second arm waveguide 2108. . Here, the wavelength of light is assumed to be around 1545 nm. When the heater 2112, that is, the first arm waveguide 2107 is heated, the optical phase difference shifts in the positive direction with respect to the effective refractive index change, and the heater 2113, that is, the second arm waveguide 2108 is heated. It can be seen that the optical phase difference shifts in the negative direction with respect to the effective refractive index change.

  As shown in FIG. 21, by changing the optical phase difference, the phase difference of the primary mode light with respect to the fundamental mode light at the opening end of the tapered waveguide 2111 changes. Thus, the transmission light waveform of the AWG 2100 can be adjusted by changing the combined light field at the opening end of the tapered waveguide 2111. In the present embodiment, the waveform adjustment for the phase error in the arrayed waveguide 2103 can be performed in the same manner as the procedure described in the first embodiment.

  As described above, from the description of the embodiment of the present invention and the two examples, in the optical wavelength multiplexing / demultiplexing circuit according to the present invention, in the optical wavelength multiplexing / demultiplexing circuit based on the conventional flat type AWG, the slope of the transmission spectrum waveform is a manufacturing error. It was shown that an optical wavelength multiplexing / demultiplexing circuit having a flat transmission band characteristic can be stably obtained by an appropriate transmission waveform adjustment method.

  In all the embodiments, the number of channels, the channel interval, and the transmission wavelength of each channel of the optical wavelength multiplexing / demultiplexing circuit are limited to specific numerical values, but the scope of application of the present invention is not limited to these numerical values.

  In all the examples, the relative refractive index difference, the core width, and the core thickness of the waveguide are limited to specific values, but the scope of application of the present invention is not limited to these values.

  In all the examples, the AWG design parameter is limited to a specific value, but the scope of the present invention is not limited to this parameter.

  In all the embodiments, the parabolic tapered waveguide is applied as the tapered waveguide for exciting the second-order mode light. However, the shape of the taper is not limited to this, and the multimode interferometer, Y-branch, hyperbolic shape, ellipse is used. Any tapered shape that converts a part of the fundamental mode light into the second-order mode light, such as a shape or an exponential function shape, is applicable.

  In all of the embodiments, the power ratio of the base, first-order, and second-order mode light generated at the opening end of the tapered waveguide that excites the second-order mode light is limited to a specific value. It is not limited to this value.

  In the embodiment, as a method of adjusting the optical phase difference between the first arm waveguide and the second arm waveguide, the first or second arm waveguide is irradiated with ultraviolet light to A method of changing the effective refractive index and a method of irreversibly changing the effective refractive index of the waveguide by heating a heater loaded on the first or second arm waveguide are applied. Any method that can adjust the optical phase difference is applicable. For example, a part of the first or second arm waveguide may be divided and a material having a refractive index different from the effective refractive index of the waveguide may be inserted. In this case, the optical phase difference can be adjusted by preparing a plurality of materials having different refractive indexes and selecting the material to be inserted, or by fixing the material to be inserted and changing the width for dividing the waveguide. Is also possible.

100, 1100, 2100 AWG type optical wavelength multiplexing / demultiplexing circuit 101, 1101, 2101 Input waveguide 102, 1102, 2102 First slab waveguide 103, 1103, 2103 Array waveguide 104, 1104, 2104 Second slab guide Waveguides 105, 1105, 2105 Output waveguides 106, 1106, 2106 Optical splitters 107, 1107, 2107 First arm waveguides 108, 1108, 2108 Second arm waveguides 109, 1109, 2109 Optical mode synthesis couplers 110, 1110 , 2110 Multimode waveguide 111, 1111, 2111 Tapered waveguide 1201, 1202, 1301, 1302, 1401, 1402, 2205, 2206 Waveguide 1203, 1204 Region 1303 Groove 1501 First multimode interference Circuit 1502 Second multimode interference circuit 1503, 1504, 1505 Intermediate waveguide 1601, 1602, 2201, 2202 Directional coupler 1603, 1604, 2203, 2204 Arm waveguide 2112, 2113 Heater 2301, 2302 Core 2304 Clad 4100 Array Waveguide diffraction grating 4101 Input waveguide 4102 First slab waveguide 4103 Array waveguide 4104 Second slab waveguide 4105 Output waveguide 4201 Core 4202 Cladding 4203 Silicon substrate

Claims (10)

A first slab waveguide, an array waveguide composed of a plurality of waveguides connected to the first slab waveguide, and a second slab waveguide connected to the plurality of waveguides of the array waveguide; An arrayed waveguide grating comprising:
An optical splitter;
First and second arm waveguides connected to the optical splitter;
An optical mode synthesis coupler connected to the first and second arm waveguides, wherein the fundamental mode light input from the first arm waveguide is coupled to a primary mode, and the second arm guide is coupled. An optical mode synthesis coupler for coupling the fundamental mode light input from the waveguide to the fundamental mode;
A tapered waveguide connected to the optical mode combining coupler for exciting secondary mode light, the taper waveguide further connected to the first slab waveguide. Demultiplexer circuit.   2. The optical wavelength multiplexing / demultiplexing circuit according to claim 1, wherein the tapered waveguide is configured to combine the optical mode via a multi-mode waveguide in which a fundamental mode light and a secondary mode light do not propagate. An optical wavelength multiplexing / demultiplexing circuit connected to a coupler.   3. The optical wavelength multiplexing / demultiplexing circuit according to claim 1, further comprising a heater that heats at least one of the first and second arm waveguides. .   4. The optical wavelength multiplexing / demultiplexing circuit according to claim 1, wherein the optical mode synthesis coupler is a directional coupler composed of two waveguides having different widths. 5. Optical wavelength multiplexing / demultiplexing circuit.   4. The optical wavelength multiplexing / demultiplexing circuit according to claim 1, wherein the optical mode synthesis coupler includes two waveguides having different widths, and the narrower waveguide has a width of Is an optical wavelength multiplexing / demultiplexing circuit characterized by gradually decreasing and terminating.   6. The optical wavelength multiplexing / demultiplexing circuit according to claim 1, wherein the optical splitter is constituted by a wavelength independent coupler (WINC).   7. The optical wavelength multiplexing / demultiplexing circuit according to claim 1, wherein the arrayed waveguide diffraction grating and the other waveguides are constituted by silica-based waveguides. 8. Combined / demultiplexed circuit. A first slab waveguide, an array waveguide composed of a plurality of waveguides connected to the first slab waveguide, and a second slab waveguide connected to the plurality of waveguides of the array waveguide; An arrayed waveguide grating comprising:
An optical splitter;
First and second arm waveguides connected to the optical splitter;
An optical mode synthesis coupler connected to the first and second arm waveguides, wherein the fundamental mode light input from the first arm waveguide is coupled to a primary mode, and the second arm guide is coupled. An optical mode synthesis coupler for coupling the fundamental mode light input from the waveguide to the fundamental mode;
In the optical wavelength multiplexing / demultiplexing circuit, comprising: a tapered waveguide connected to the optical mode combining coupler and exciting a second-order mode light, and further connected to the first slab waveguide. A method for adjusting a transmission spectrum waveform, comprising:
The fundamental mode at the open end of the tapered waveguide that excites the secondary mode by changing the optical path length difference between the first arm waveguide and the second arm waveguide, and the primary mode for the secondary mode light A method characterized by changing the phase difference of light.   9. The method according to claim 8, wherein at least one of the first and second arm waveguides is irradiated with ultraviolet light to change its effective refractive index.   9. The method of claim 8, wherein at least one of the first and second arm waveguides is heated to irreversibly change its effective refractive index.

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