Optical filter and variable wavelength laser element

Abstract

To provide an optical filter and a variable wavelength laser element capable of suppressing crosstalk.SOLUTION: An optical filter includes: a first optical multiplexer / demultiplexer having a first input end, a second input end, a first output end, and a second output end; a first waveguide optical coupled to the first input end and a second waveguide optical coupled to the second input end; a third waveguide optically coupled to the first output end; a fourth waveguide optically coupled to the second output end; and a ring resonator optically coupled to the third waveguide and the fourth waveguide.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to optical filters and tunable laser devices. [Background technology]

[0002] Optical filters using silicon photonics are known. Optical filters that form a resonator with multiple waveguides and excite light of a specific wavelength have been developed (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2022-84531 A Summary of the Invention [Problem to be solved by the invention]

[0004] In an optical filter, light is split into waveguides by a multiplexer / demultiplexer. The port through which light propagates can be controlled by setting the splitting ratio of light in the multiplexer / demultiplexer to a specific value. For example, light is emitted from one port and not emitted from another port. However, the splitting ratio may deviate from the design value. When light passes through the multiplexer / demultiplexer multiple times, the effect of the deviation in the splitting ratio becomes greater. As a result, crosstalk between ports increases. Therefore, an object of the present invention is to provide an optical filter and a wavelength tunable laser element capable of suppressing crosstalk. [Means for solving the problem]

[0005] An optical filter according to the present disclosure includes a first multiplexer / demultiplexer having a first input end, a second input end, a first output end, and a second output end, a first waveguide optically coupled to the first input end, a second waveguide optically coupled to the second input end, a third waveguide optically coupled to the first output end, a fourth waveguide optically coupled to the second output end, and a ring resonator optically coupled to the third waveguide and the fourth waveguide. Effect of the Invention

[0006] According to the present disclosure, it is possible to provide an optical filter and a wavelength tunable laser element capable of suppressing crosstalk. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a plan view illustrating an optical filter according to the first embodiment. [Diagram 2] FIG. 2 is a cross-sectional view illustrating an optical filter. [Diagram 3] FIG. 3 is a diagram illustrating the propagation of light in an optical filter. [Figure 4] FIG. 4 is a diagram illustrating the propagation of light in an optical filter. [Figure 5A] FIG. 5A is a diagram illustrating the spectrum of reflected light. [Figure 5B] FIG. 5B is a diagram illustrating the spectrum of transmitted light. [Figure 6] FIG. 6 is a plan view illustrating an optical filter according to a comparative example. [Figure 7A] FIG. 7A is a diagram illustrating the calculation results of light intensity. [Figure 7B] FIG. 7B is a diagram illustrating the calculation results of the light intensity. [Figure 8] FIG. 8 is a plan view illustrating the optical filter according to the second embodiment. [Figure 9] FIG. 9 is a diagram illustrating the propagation of light in an optical filter. [Figure 10] FIG. 10 is a diagram illustrating the propagation of light in an optical filter. [Figure 11] FIG. 11 is a plan view illustrating the wavelength tunable laser device according to the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0009] One embodiment of the present disclosure is an optical filter including: (1) a first multiplexer / demultiplexer having a first input end, a second input end, a first output end, and a second output end, a first waveguide optically coupled to the first input end, a second waveguide optically coupled to the second input end, a third waveguide optically coupled to the first output end, a fourth waveguide optically coupled to the second output end, and a ring resonator optically coupled to the third waveguide and the fourth waveguide. Reducing the number of times that light passes through the first multiplexer / demultiplexer makes it possible to suppress crosstalk. (2) In the above (1), the optical multiplexer / demultiplexer may include a second optical multiplexer having a third input end, a fourth input end, a third output end, and a fourth output end, a fifth waveguide optically coupled to the third output end, and a sixth waveguide optically coupled to the fourth output end, wherein the third waveguide is optically coupled to the third input end, and the fourth waveguide is optically coupled to the fourth input end. By reducing the number of times that light passes through the first optical multiplexer / demultiplexer and the second optical multiplexer / demultiplexer, crosstalk can be suppressed. (3) In the above (2), an optical path length of the third waveguide from the first output end to a coupling portion between the third waveguide and the ring resonator may be equal to an optical path length of the fourth waveguide from the second output end to a coupling portion between the fourth waveguide and the ring resonator, and an optical path length of the third waveguide from the coupling portion between the third waveguide and the ring resonator to the third input end may be equal to an optical path length of the fourth waveguide from the coupling portion between the fourth waveguide and the ring resonator to the fourth input end. Phase shift of light can be suppressed, and crosstalk can be suppressed. (4) In the above (2) or (3), the ring resonator, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide, and the sixth waveguide may be made of silicon, thereby making it possible to suppress optical loss. (5) In the above (1), the optical multiplexer / demultiplexer may further include a third multiplexer / demultiplexer having a fifth input end, a sixth input end, a fifth output end, and a sixth output end, a seventh waveguide optically coupled to the fifth input end, the ring resonator, and the sixth input end, an eighth waveguide optically coupled to the fifth output end, and a ninth waveguide optically coupled to the sixth output end. The number of times that light passes through the first multiplexer / demultiplexer and the third multiplexer / demultiplexer is reduced, thereby suppressing crosstalk. Resonated light can be output. (6) In the above (5), one end of the seventh waveguide may be optically coupled to the fifth input port, and another end of the seventh waveguide may be optically coupled to the sixth input port, and the seventh waveguide may be coupled to the ring resonator between the end and the other end. Resonated light can be propagated to the eighth waveguide and the ninth waveguide. (7) In the above (5) or (6), an optical path length of the third waveguide from the first output end to a coupling portion between the third waveguide and the ring resonator may be equal to an optical path length of the fourth waveguide from the second output end to a coupling portion between the fourth waveguide and the ring resonator, and an optical path length of the seventh waveguide from a coupling portion between the ring resonator and the seventh waveguide to the fifth input end may be equal to an optical path length of the seventh waveguide from a coupling portion between the ring resonator and the seventh waveguide to the sixth input end. Phase shift of light can be suppressed, and crosstalk can be suppressed. (8) In any one of the above (5) to (7), the ring resonator, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the seventh waveguide, the eighth waveguide, and the ninth waveguide may be made of silicon. Light loss can be suppressed. (9) A light source and two optical filters, each of the two optical filters comprising: a first multiplexer / demultiplexer having a first input end, a second input end, a first output end, and a second output end; a first waveguide optically coupled to the first input end; a second waveguide optically coupled to the second input end; a third waveguide optically coupled to the first output end; a fourth waveguide optically coupled to the second output end; a ring resonator optically coupled to the third waveguide and the fourth waveguide; a third multiplexer / demultiplexer having a fifth input end, a sixth input end, a fifth output end, and a sixth output end; and a seventh waveguide optically coupled to the fifth input end, the ring resonator, and the sixth input end; The optical fiber has an eighth waveguide optically coupled to the fifth output end and a ninth waveguide optically coupled to the sixth output end, and the light source is a tunable laser element optically coupled to the first waveguides of the two optical filters. Crosstalk is suppressed by the two optical filters. Light is reflected by the two optical filters, resulting in laser oscillation. The optical filters can be monitored by the light emitted from the optical filters. (10) In the above (9), the circumferential lengths of the ring resonators of the two optical filters may be different from each other. The resonant wavelengths of the two optical filters are matched to cause laser oscillation of light. (11) In the above (9) or (10), a phase adjustment unit may be provided in the two optical filters to adjust the phase of light propagating to the two optical filters. By adjusting the phase of the light, it is possible to control the laser light.

[0010] [Details of the embodiment of the present disclosure] Specific examples of optical filters and tunable laser elements according to embodiments of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0011] First Embodiment Fig. 1 is a plan view illustrating an optical filter 100 according to a first embodiment. As shown in Fig. 1, the optical filter 100 is formed on a substrate 10. Two sides of the substrate 10 are parallel to the X-axis. The other two sides are parallel to the Y-axis. The Z-axis is a normal direction to the upper surface of the substrate 10. The X-axis, Y-axis, and Z-axis are perpendicular to each other.

[0012] The optical filter 100 has a waveguide 20 (first waveguide), a waveguide 22 (second waveguide), a multiplexer / demultiplexer 32 (first multiplexer / demultiplexer), a waveguide 24 (third waveguide), a waveguide 26 (fourth waveguide), a ring resonator 34, a multiplexer / demultiplexer 36 (second multiplexer / demultiplexer), a waveguide 28 (fifth waveguide), and a waveguide 30 (sixth waveguide).

[0013] The multiplexer / demultiplexer 32 and the multiplexer / demultiplexer 36 are 3 dB couplers, for example, a two-input, two-output MMI (Multi Mode Interface). The multiplexer / demultiplexer 32 has an input end 32 a (first input end), an input end 32 b (second input end), an output end 32 c (first output end), and an output end 32 d (second output end). The multiplexer / demultiplexer 36 has an input end 36 a (third input end), an input end 36 b (fourth input end), an output end 36 c (third output end), and an output end 36 d (fourth output end).

[0014] One end of the waveguide 20 and one end of the waveguide 22 are located at one end in the X-axis direction of the substrate 10. The other end of the waveguide 20 is optically coupled to an input end 32a of the multiplexer / demultiplexer 32. The other end of the waveguide 22 is optically coupled to an input end 32b of the multiplexer / demultiplexer 32.

[0015] One end of the waveguide 24 is optically coupled to an output end 32c of the multiplexer / demultiplexer 32. The other end of the waveguide 24 is optically coupled to an input end 36a of the multiplexer / demultiplexer 36. One end of the waveguide 26 is optically coupled to an output end 32d of the multiplexer / demultiplexer 32. The other end of the waveguide 26 is optically coupled to an input end 36b of the multiplexer / demultiplexer 36.

[0016] One end of waveguide 28 is optically coupled to output end 36c of multiplexer / demultiplexer 36. One end of waveguide 30 is optically coupled to output end 36d of multiplexer / demultiplexer 36. The other end of waveguide 28 and the other end of waveguide 30 are located at the other end of substrate 10 in the X-axis direction.

[0017] The ring resonator 34 is a loop-type waveguide. The planar shape of the ring resonator 34 may be a circle, an ellipse, a polygon, or the like. The ring resonator 34 is located between the multiplexer / demultiplexer 32 and the multiplexer / demultiplexer 36. In the X-axis direction, the multiplexer / demultiplexer 32, the ring resonator 34, and the multiplexer / demultiplexer 36 are arranged in this order. In the Y-axis direction, the waveguide 24, the ring resonator 34, and the waveguide 26 are arranged in this order.

[0018] A directional coupler 25 is formed at a position where the waveguide 24 is closest to the ring resonator 34. In the directional coupler 25, the waveguide 24 is optically coupled to the ring resonator 34. A directional coupler 27 is formed at a position where the waveguide 26 is closest to the ring resonator 34. In the directional coupler 27, the waveguide 26 is optically coupled to the ring resonator 34.

[0019] The optical path length of the waveguide 20 is equal to the optical path length of the waveguide 22. The optical path length from the output end 32c to the directional coupler 25 is equal to the optical path length from the output end 32d to the directional coupler 27. The optical path length from the directional coupler 25 to the input end 36a is equal to the optical path length from the directional coupler 27 to the input end 36b. The optical path length of the waveguide 28 is equal to the optical path length of the waveguide 30.

[0020] 2 is a cross-sectional view illustrating the optical filter 100, showing a cross section taken along line AA in FIG. 1. The substrate 10 is a silicon on insulator (SOI) substrate, and includes a substrate 12, a cladding layer 16, and a waveguide core 14. The substrate 10 is made of, for example, silicon (Si). The cladding layer 16 is made of, for example, silicon oxide (SiO 2) and covers one surface of the substrate 10. The waveguide core 14 is made of, for example, Si, is spaced apart from the substrate 10, and is embedded inside the cladding layer 16. The waveguide core 14 functions as a waveguide 24. The distance from the upper surface of the substrate 12 to the lower end of the waveguide core 14 is, for example, 2 μm. The width of the waveguide core 14 in the Y-axis direction is, for example, 440 nm. The thickness of the waveguide core 14 is, for example, 220 nm.

[0021] The cross sections of the waveguide 20, the waveguide 22, the waveguide 26, the waveguide 28, the waveguide 30, and the ring resonator 34 are similar to those in FIG.

[0022] The operation of the optical filter 100 will be described. The ends of the waveguides 20 and 22 function as input and output ports. The ends of the waveguides 28 and 30 function as output ports. The optical filter 100 transmits two independent resonance modes.

[0023] 3 and 4 are diagrams illustrating the propagation of light in the optical filter 100. The portions through which light propagates are marked with diagonal lines.

[0024] In the example of FIG. 3, light I1 is input from an external light source to the waveguide 20. No light is input from the outside to the other waveguides. The light propagates through the waveguide 20 and is input to the input end 32a of the multiplexer / demultiplexer 32. The multiplexer / demultiplexer 32 splits the light into output ends 32c and 32d. Ideally, the ratio of the intensities of the split lights is 50:50. Compared to the light output from output end 32c to the waveguide 24, the phase of the light output from output end 32d to the waveguide 26 is delayed by 90°.

[0025] The light propagating through the waveguide 24 transfers to the ring resonator 34 and propagates clockwise through the ring resonator 34. The light propagating through the waveguide 26 transfers to the ring resonator 34 and propagates counterclockwise through the ring resonator 34.

[0026] The light circulates around the ring resonator 34 multiple times and resonates. The resonant wavelength is determined according to the circumferential length of the ring resonator 34. The light that resonates in the ring resonator 34 is reflected as reflected light, transferred from the ring resonator 34 to the waveguide 24 or the waveguide 26, and propagates toward the multiplexer / demultiplexer 32.

[0027] The reflected light propagating through the waveguide 24 is input to output end 32c of the multiplexer / demultiplexer 32. The multiplexer / demultiplexer 32 splits the reflected light input from the waveguide 24 into input ends 32a and 32b. The reflected light propagating through the waveguide 26 is input to output end 32d. The multiplexer / demultiplexer 32 splits the reflected light input from the waveguide 26 into input ends 32a and 32b. The branching ratio of the reflected light is ideally 50:50.

[0028] The phase of the reflected light that propagates through waveguide 26 and inputs to input end 32b differs by 180° from the phase of the reflected light that propagates through waveguide 24 and inputs to input end 32b. Since the phases are inverted, the two reflected lights cancel each other out.

[0029] The phase of the reflected light propagating through the waveguide 24 and inputting to the input end 32a is equal to the phase of the reflected light propagating through the waveguide 26 and inputting to the input end 32a. The two reflected lights constructively interact with each other. The reflected light R1 propagates through the waveguide 20 and is output from the end of the waveguide 20.

[0030] The transmitted light propagating through the waveguide 24 is input to an input end 36a of the multiplexer / demultiplexer 36. The multiplexer / demultiplexer 36 demultiplexes the transmitted light input from the waveguide 24 to output ends 36c and 36d. The transmitted light propagating through the waveguide 26 is input to an input end 36b. The multiplexer / demultiplexer 36 demultiplexes the transmitted light input from the waveguide 26 to output ends 36c and 36d. The branching ratio is ideally 50:50.

[0031] The phase of the transmitted light that propagates through waveguide 26 and inputs to output end 36c differs by 180° from the phase of the transmitted light that propagates through waveguide 24 and inputs to output end 36c. Since the phases are inverted, the two transmitted lights cancel each other out.

[0032] The phase of the transmitted light propagating through the waveguide 24 and inputting to the output end 36d is equal to the phase of the transmitted light propagating through the waveguide 26 and inputting to the output end 36d. The two transmitted lights constructively interfere with each other. The transmitted light T1 propagates through the waveguide 30 and is output from the end of the waveguide 30.

[0033] 3, XT1 represents light leaking into the waveguide 28. XT2 represents light leaking into the waveguide 22. Leaking light, such as light XT1 and XT2, may be referred to as crosstalk. Ideally, the intensities of light XT1 and XT2 are zero.

[0034] In the example of FIG. 4, light I2 is input from an external light source to the waveguide 22. No light is input from the outside to the other waveguides. The light propagates through the waveguide 22 and is input to the input end 32b of the multiplexer / demultiplexer 32. The multiplexer / demultiplexer 32 demultiplexes the light to the output end 32c and the output end 32d. The light that resonates in the ring resonator 34 is reflected as reflected light, and is transferred from the ring resonator 34 to the waveguide 24 or the waveguide 26, and propagates toward the multiplexer / demultiplexer 32. The reflected light R2 is output from the end of the waveguide 22. The transmitted light T2 is output from the end of the waveguide 28. XT3 in FIG. 4 represents the light leaking into the waveguide 30. XT4 represents the light leaking into the waveguide 20. Ideally, the intensities of the lights XT3 and XT4 are zero.

[0035] When light is incident on both waveguide 20 and waveguide 22, two independent resonant modes are excited. As shown in the example of Figure 3, reflected light R1 is emitted from waveguide 20, and transmitted light T1 is emitted from waveguide 30. As shown in the example of Figure 4, reflected light R2 is emitted from waveguide 22, and transmitted light T2 is emitted from waveguide 28.

[0036] FIG. 5A is a diagram illustrating the spectrum of reflected light. The horizontal axis represents the wavelength of light. The vertical axis represents the intensity of light. The spectrum of reflected light has peaks at resonance wavelengths λ1, λ2, λ3, λ4, and λ5. At the peaks, the intensity shows a maximum value. The interval between two adjacent resonance wavelengths is the FSR (Free Spectral Range).

[0037] FIG. 5B is a diagram illustrating the spectrum of transmitted light. The horizontal axis represents the wavelength of light. The vertical axis represents the intensity of light. The transmitted light is extinct at resonance wavelengths λ1, λ2, λ3, λ4, and λ5. In other words, the intensity reaches a minimum value.

[0038] The two reflected lights R1 and R2 have the same wavelength characteristics. The FSR of the reflected light R1 is in principle the same as the FSR of the reflected light R2. For example, when one reflected light R1 shows a peak, the other reflected light R2 also shows a peak. By measuring one of the two reflected lights, the characteristics of the optical filter 100, such as the resonant wavelength and the FSR, can be monitored.

[0039] The two transmitted lights T1 and T2 have the same wavelength characteristics. In principle, the FSRs of the two transmitted lights match. For example, when the wavelength of light is the resonant wavelength, when one transmitted light T1 is extinguished, the other transmitted light T2 is also extinguished. The reflected light shows a peak. The characteristics of the optical filter 100 can also be monitored by measuring the transmitted light.

[0040] (Comparative Example) 6 is a plan view illustrating an optical filter 110 according to a comparative example. The optical filter 110 includes a waveguide 40, a waveguide 41, a waveguide 42, a waveguide 43, a waveguide 44, a waveguide 45, a multiplexer / demultiplexer 46, and a multiplexer / demultiplexer 47.

[0041] Waveguide 40 and waveguide 42 are access waveguides for inputting and outputting light, and extend from one end to the other end of substrate 10. Waveguide 40 and waveguide 41 are close to each other, for example, within several hundred nm, and are optically coupled. Waveguide 42 and waveguide 43 are optically coupled.

[0042] The multiplexer / demultiplexer 46 and the multiplexer / demultiplexer 47 are two-input, two-output couplers. One end of each of the waveguide 41 and the waveguide 43 is optically coupled to an input end of the multiplexer / demultiplexer 46. The other end of each of the waveguide 41 and the waveguide 43 is optically coupled to an input end of the multiplexer / demultiplexer 47.

[0043] The waveguide 44 and the waveguide 45 have a loop shape. Both ends of the waveguide 44 are coupled to the output ends of the multiplexer / demultiplexer 46. Both ends of the waveguide 45 are coupled to the output ends of the multiplexer / demultiplexer 47.

[0044] When light I3 is input from end 42a of waveguide 42, the light transfers from waveguide 42 to waveguide 43. The light propagating through waveguide 43 is distributed to both ends of waveguide 44 in multiplexer / demultiplexer 46, and to both ends of waveguide 45 in multiplexer / demultiplexer 47. Waveguide 43, waveguide 44, waveguide 45, multiplexer / demultiplexer 46, and multiplexer / demultiplexer 47 function as resonators. The light propagates repeatedly through waveguide 43, waveguide 44, waveguide 45, multiplexer / demultiplexer 46, and multiplexer / demultiplexer 47, and resonates at a wavelength determined by the optical path length. Resonating light R3 transfers from waveguide 43 to waveguide 42 and is output from end 42a. Transmitted light T3 is output from the other end 42b of waveguide 42.

[0045] Ideally, no light propagates to the waveguide 41 and the waveguide 40. However, leaked light XT5 may propagate toward the end 40a of the waveguide 40, and leaked light XT6 may propagate toward the end 40b of the waveguide 40.

[0046] Crosstalk depends on the branching ratio of light in the multiplexer / demultiplexer. If an error occurs in the branching ratio, crosstalk increases.

[0047] 7A and 7B are diagrams illustrating calculation results of light intensity. The horizontal axis represents the phase of light in the resonator, and θ=2nπ is the resonance condition (n is an integer). The vertical axis represents the light intensity.

[0048] ΔT represents the deviation of the branching ratio in the multiplexer / demultiplexer. The ideal branching ratio is 50:50. The branching ratio in the examples of FIGS. 7A and 7B is (50+ΔT):(50-ΔT). The power coupling coefficient between the resonator and the waveguide is sinφ 2 It is expressed as:

[0049] FIG. 7A shows the calculation results for the comparative example. The thick solid line represents reflected light R3 (ΔT=1%, φ=π / 4). The thin solid line represents crosstalk XT5 (ΔT=1%, φ=π / 4). The dotted line represents reflected light R3 (ΔT=1%, φ=π / 8). The dashed line represents crosstalk XT5 (ΔT=1%, φ=π / 8). The dashed line represents reflected light R3 (ΔT=2%, φ=π / 8). The dashed line represents crosstalk XT5 (ΔT=2%, φ=π / 8). Although crosstalk XT6 is not shown, the results are similar to those of crosstalk XT5.

[0050] The reflected light R3 exhibits high intensity under the resonance condition 2nπ. Crosstalk XT5 depends on ΔT and φ. The higher ΔT, the higher the maximum value of crosstalk XT5. In the comparative example, light repeatedly passes through multiplexer / demultiplexer 46 and multiplexer / demultiplexer 47. Crosstalk XT5 increases because the effect of the deviation ΔT in the branching ratio in the multiplexer / demultiplexer accumulates. For the same ΔT, the smaller φ, the higher the crosstalk XT5. When φ is small, the Q value of the resonator becomes high. A small φ has an effect on crosstalk similar to an increase in the number of times light circulates around the resonator.

[0051] FIG. 7B shows the calculation results in the first embodiment. The solid line represents crosstalk XT1 (ΔT=1%, φ=π / 8). The dashed line represents crosstalk XT2 (ΔT=1%, φ=π / 8). The dashed line represents crosstalk XT1 (ΔT=5%, φ=π / 8). The dotted line represents crosstalk XT2 (ΔT=5%, φ=π / 8). In the example of ΔT=1% (solid line and dashed line), the crosstalk is at most about -34 dB, which is suppressed lower than the crosstalk of ΔT=1% in the comparative example. In the example of ΔT=5% (dashed line and dotted line), the crosstalk is at most about -20 dB.

[0052] According to the first embodiment, the waveguide 20 is optically coupled to the input end 32a of the multiplexer / demultiplexer 32. The waveguide 22 is optically coupled to the input end 32b. The waveguide 24 is optically coupled to the output end 32c. The waveguide 26 is optically coupled to the output end 32d. The ring resonator 34 is optically coupled to the waveguide 24 and the waveguide 26. Light resonates in the ring resonator 34 and is reflected. When light is incident on the waveguide 20, reflected light R1 propagates through the waveguide 20. When light is incident on the waveguide 22, reflected light R2 propagates through the waveguide 22.

[0053] The branching ratio of light in the multiplexer / demultiplexer 32 is designed to be, for example, 50:50. The branching ratio may deviate from the design value due to manufacturing errors and the like. The number of times that light passes through the multiplexer / demultiplexer 32 from when it enters the optical filter 100 until the reflected light is emitted is two. Since the number of times that light passes through the multiplexer / demultiplexer 32 is reduced, the light is less susceptible to deviations in the branching ratio. This makes it possible to suppress crosstalk. As in the example of FIG. 7B, when the branching deviations are ΔT=1% and ΔT=5%, the crosstalks XT1 and XT2 can be reduced compared to the comparative example.

[0054] Since crosstalk is suppressed, the two reflected lights can be separated and extracted. The two reflected lights can be used for different purposes. For example, one reflected light can be used for laser oscillation. The other reflected light can be used to monitor the characteristics of the optical filter 100, such as the resonant wavelength and FSR.

[0055] When a resonator is formed using multiple waveguides, the resonator length becomes long, making it difficult to increase the FSR. According to the first embodiment, the optical filter 100 has one ring resonator 34. The FSR can be increased by shortening the perimeter of the ring resonator 34. For example, the perimeter may be 100 μm or more and 300 μm or less, or 100 μm or less, or 600 μm or less. If the perimeter is 100 μm or more and about 600 μm, the FSR is 1 nm or more and 6 nm or less.

[0056] The waveguide 24 is optically coupled to the input end 36a of the multiplexer / demultiplexer 36. The waveguide 26 is optically coupled to the input end 36b. The waveguide 28 is optically coupled to the output end 36c. The waveguide 30 is optically coupled to the output end 36d. When light is incident on the waveguide 20, transmitted light T1 propagates through the waveguide 30. When light is incident on the waveguide 22, transmitted light T2 propagates through the waveguide 28. When light is incident on the optical filter 100, the light passes through the multiplexer / demultiplexer twice until the transmitted light is emitted. Since the number of times the light passes through the multiplexer / demultiplexer is limited, the light is less susceptible to deviations in the branching ratio. This makes it possible to suppress crosstalk. The two transmitted lights can be separated and extracted. The characteristics of the optical filter 100 can be monitored using the transmitted light.

[0057] The optical path length of the waveguide 24 from the output end 32c to the directional coupler 25 is equal to the optical path length of the waveguide 26 from the output end 32d to the directional coupler 27. The optical path length of the waveguide 24 from the directional coupler 25 to the input end 36a is equal to the optical path length of the waveguide 26 from the directional coupler 27 to the input end 36b. There is little phase shift between the light propagating through the waveguide 24 and the light propagating through the waveguide 26. The optical path lengths may be strictly equal, or may be equal within the range of manufacturing error, for example.

[0058] By suppressing the influence of the branching ratio deviation and suppressing the phase deviation, in principle, as in the example of FIG. 4, the reflected lights cancel each other out and do not propagate to the waveguide 22. The transmitted lights cancel each other out and do not propagate to the waveguide 28. As in the example of FIG. 5, the reflected lights cancel each other out and do not propagate to the waveguide 20. The transmitted lights cancel each other out and do not propagate to the waveguide 30. Crosstalk is suppressed, and two resonance modes are excited independently. The reflected lights R1 and R2 can be extracted separately. The transmitted lights T1 and T2 can be extracted separately. These lights can be used for different purposes.

[0059] As shown in FIG. 2, the waveguide and the ring resonator 34 are formed of a Si waveguide core 14. Light can be confined in the curved waveguide to suppress loss. Even if the bending radius of the ring resonator 34 is made small, light loss can be suppressed. The circumference is shortened, and the FSR can be increased.

[0060] The optical filter 100 may be formed on a component other than an SOI substrate. The multiplexer / demultiplexer 32 and the multiplexer / demultiplexer 36 are two-input, two-output couplers, and may be directional couplers other than MMIs.

[0061] <Second embodiment> 8 is a plan view illustrating an optical filter 200 according to the second embodiment. Description of the same configuration as in the first embodiment will be omitted.

[0062] The optical filter 200 has a waveguide 20, a waveguide 22, a multiplexer / demultiplexer 32, a waveguide 24, a waveguide 26, a ring resonator 34, a waveguide 50 (seventh waveguide), a multiplexer / demultiplexer 52 (third multiplexer / demultiplexer), a waveguide 54 (eighth waveguide), and a waveguide 56 (ninth waveguide).

[0063] The ring resonator 34 and the waveguide 50 are provided between the multiplexer / demultiplexer 32 and the multiplexer / demultiplexer 52. The waveguide 54 and the waveguide 56 are provided on the opposite side of the multiplexer / demultiplexer 52 to the waveguide 50. The waveguide 24 and the waveguide 26 are not coupled to the multiplexer / demultiplexer 52.

[0064] The multiplexer / demultiplexer 52 has an input terminal 52a (a fifth input terminal), an input terminal 52b (a sixth input terminal), an output terminal 52c (a fifth output terminal), and an output terminal 52d (a sixth output terminal).

[0065] The waveguide 50 is C-shaped and has two ends. The portion between the two ends of the waveguide 50 is closest to the ring resonator 34, forming a directional coupler 51. In the directional coupler 51, the waveguide 50 and the ring resonator 34 are optically coupled. The coupling efficiency between the waveguide 50 and the ring resonator 34 is determined by the distance between the waveguide 50 and the ring resonator 34. The smaller the distance, the higher the coupling efficiency, and the greater the intensity of the light that transitions between the ring resonator 34 and the waveguide 40. The greater the distance, the lower the coupling efficiency, and the smaller the intensity of the light that transitions.

[0066] One end of the waveguide 50 is optically coupled to an input end 52a of the multiplexer / demultiplexer 52. The other end of the waveguide 50 is optically coupled to an input end 52b. One end of the waveguide 54 is optically coupled to an output end 52c. One end of the waveguide 56 is optically coupled to an output end 52d. The other ends of the waveguides 54 and 56 are located at ends of the substrate 10 and function as output ports.

[0067] 9 and 10 are diagrams illustrating the propagation of light in the optical filter 200. The portions through which light propagates are marked with diagonal lines. The optical filter 200 excites two independent resonance modes.

[0068] In the example of Fig. 9, light I4 is input to the waveguide 20 from an external light source. No light is input from the outside to the other waveguides. The light propagates through the waveguide 20 and enters the input end 32a of the multiplexer / demultiplexer 32. The multiplexer / demultiplexer 32 demultiplexes the light to output ends 32c and 32d. The light propagating through the waveguide 24 transfers to the ring resonator 34. The light propagating through the waveguide 26 transfers to the ring resonator 34.

[0069] A part of the light resonated in the ring resonator 34 is transferred to the waveguide 50 and input to the input terminals 52a and 52b of the multiplexer / demultiplexer 52. The multiplexer / demultiplexer 52 demultiplexes the light to the output terminals 52c and 52d. Ideally, the branching ratio is 50:50.

[0070] The phase of the light input from input end 52a and output from output end 52d is different from the phase of the light input from input end 52b and output from output end 52d by 180°. Since the phases are inverted, the two lights cancel each other out. Ideally, the light does not propagate through the waveguide 56.

[0071] The phase of the light input from the input end 52a and output from the output end 52c is equal to the phase of the light input from the input end 52b and output from the output end 52c. The two lights constructively interact with each other. The transmitted light T4 propagates through the waveguide 54 and is output from the end of the waveguide 54.

[0072] A part of the resonated light is reflected from the ring resonator 34 toward the multiplexer / demultiplexer 32. The reflected light R4 is emitted from the waveguide 20.

[0073] 10, light I5 is incident on the waveguide 22 from an external light source. No light is incident on the other waveguides from the outside. Transmitted light T5 is emitted from the waveguide 56. Reflected light R5 is emitted from the waveguide 22.

[0074] When light is incident on both waveguide 20 and waveguide 22, two independent resonant modes are excited. Transmitted light T4 is emitted from waveguide 54, and reflected light R4 is emitted from waveguide 20. Transmitted light T5 is emitted from waveguide 56, and reflected light R5 is emitted from waveguide 22.

[0075] The transmitted light T4, the transmitted light T5, the reflected light R4, and the reflected light R5 have the same wavelength characteristics, and show the spectrum of FIG. 5A, for example. That is, the transmitted light and the reflected light have peaks at the same resonance wavelength. By measuring one of the two transmitted lights, the characteristics of the optical filter 200, such as the resonance wavelength and FSR, can be monitored.

[0076] According to the second embodiment, the waveguide 50 is optically coupled to the ring resonator 34 and the input end 52a and input end 52b of the multiplexer / demultiplexer 52. The waveguide 54 is optically coupled to the output end 52c of the multiplexer / demultiplexer 52. The waveguide 56 is optically coupled to the output end 52d. The light resonated in the ring resonator 34 is transferred to the waveguide 50 and is output from the waveguide 54 and the waveguide 56. The number of times that the light passes through the multiplexer / demultiplexer from when it enters the optical filter 200 until the transmitted light is output is two. The number of times that the light passes through the multiplexer / demultiplexer is also two until the reflected light is output. Since the number of times that the light passes through the multiplexer / demultiplexer is reduced, the light is less susceptible to the effect of deviation in the branching ratio. This makes it possible to suppress crosstalk.

[0077] Since crosstalk is suppressed, the resonated light can be separated and extracted by using the waveguides 54 and 56 as output waveguides. The transmitted light T4 and the transmitted light T5 have the same wavelength characteristics. By measuring one of the two transmitted lights, the characteristics of the optical filter 200, such as the resonance wavelength and FSR, can be monitored. For example, the transmitted light T5 is used for monitoring, and its wavelength is set as the resonance wavelength. The transmitted light T4 having a peak at the resonance wavelength can be output. The reflected lights R4 and R5 also have the same wavelength characteristics as the transmitted lights. The reflected light may be used for monitoring.

[0078] The FSR can be increased by shortening the circumferential length of the ring resonator 34. For example, the circumferential length may be 100 μm or more and 300 μm or less, or may be 100 μm or less, or may be 600 μm or less.

[0079] One end of the waveguide 50 is coupled to an input end 52a, and the other end is coupled to an input end 52b. The resonated light propagates through the waveguide 50 and enters the multiplexer / demultiplexer 52 from the two ends. The resonated light can be propagated through the waveguides 54 and 56 that are coupled to the multiplexer / demultiplexer 52.

[0080] The optical path length of the waveguide 24 from the output end 32c of the multiplexer / demultiplexer 32 to the directional coupler 25 is equal to the optical path length of the waveguide 26 from the output end 32d to the directional coupler 27. The optical path length of the waveguide 50 from the directional coupler 51 to the input end 52a is equal to the optical path length of the waveguide 50 from the directional coupler 51 to the input end 52b. The phase of the light is less likely to shift.

[0081] By suppressing the influence of the branching ratio deviation and suppressing the phase deviation, in principle, as in the example of FIG. 9, the transmitted light cancels out and does not propagate to the waveguide 56. The reflected light cancels out and does not propagate to the waveguide 22. The same is true for the example of FIG. 10. Crosstalk is suppressed and two resonance modes are excited independently. The transmitted light T4 and the transmitted light T5 can be extracted separately. The reflected light R4 and the reflected light R5 can be extracted separately. These lights can be used for different purposes.

[0082] As shown in FIG. 2, the waveguide and the ring resonator 34 are formed of a Si waveguide core 14. Light can be confined in the curved waveguide to suppress loss. Even if the bending radius of the ring resonator 34 is made small, light loss can be suppressed. The circumference is shortened, and the FSR can be increased.

[0083] <Third embodiment> 11 is a plan view illustrating a wavelength tunable laser device 300 according to the third embodiment. Description of the same configuration as in the first or second embodiment will be omitted.

[0084] The tunable laser element 300 is formed on the substrate 10, and includes a light source 60 and two optical filters 200. The optical filter 200 has the configuration of the second embodiment. One of the two optical filters 200 is optical filter 200a, and the other is optical filter 200b. The optical filters 200a and 200b share a waveguide 20. The circumferential length of the ring resonator 34 of the optical filter 200a is different from the circumferential length of the ring resonator 34 of the optical filter 200b. The optical filters 200a and 200b form a laser resonator.

[0085] The tunable laser element 300 has a heater 62, a heater 64, and a heater 66 (phase adjustment unit). The heaters are made of metal and are electrically connected to an external power supply or the like. The heater 62 is provided in the waveguide 20. The heater 64 is provided in the ring resonator 34 of the optical filter 200a. The heater 66 is provided in the ring resonator 34 of the optical filter 200b. When electricity is applied to the heater, the heater generates heat and the silicon waveguide is heated. When the temperature rises, the optical path length changes and the phase of the light changes. Adjusting the phase changes the resonance wavelength.

[0086] The light source 60 is, for example, a semiconductor light emitting element, is made of a III-V compound semiconductor, and has optical gain. The light source 60 is bonded to one surface of the substrate 10, is located above the waveguide 20, and is optically coupled to the waveguide 20. When power is input to the light source 60, the light source 60 emits light. The light emitted from the light source 60 is transferred to the waveguide 20 and enters the optical filters 200a and 200b.

[0087] When light is incident from the light source 60, the optical filters 200a and 200b generate reflected light and transmitted light. The reflected light from the optical filter 200a propagates through the waveguide 20 toward the light source 60. The reflected light from the optical filter 200b propagates through the waveguide 20 toward the light source 60. The light is repeatedly reflected between the optical filters 200a and 200b.

[0088] The FSR of optical filter 200a is different from that of optical filter 200b because the perimeters of the two ring resonators 34 are different. Due to the Vernier effect, light is lased at wavelengths at which the two optical filters 200 show peaks. The laser light is emitted from the waveguides 54 of optical filters 200a and 200b.

[0089] Light is also incident on the waveguide 22 of the optical filter 200a and the waveguide 22 of the optical filter 200b. When light is incident on the waveguide 22, the optical filters 200a and 200b generate reflected light and transmitted light. The transmitted light propagates through the waveguide 56 of each optical filter and is emitted. By measuring the transmitted light of the optical filter 200a, the characteristics of the optical filter 200a can be monitored. By measuring the transmitted light of the optical filter 200b, the characteristics of the optical filter 200b can be monitored.

[0090] According to the third embodiment, the optical filter 200a and the optical filter 200b form a laser resonator. When light is incident on the waveguide 20 from the light source 60, the light is reflected by the two optical filters and laser oscillation occurs. The laser light is emitted from the waveguide 54 of the optical filter 200a and the optical filter 200b. When light is incident on the waveguide 22 of the optical filter 200a and the optical filter 200b, the light is emitted from the waveguide 56. The optical filter 200a and the optical filter 200b are monitored using the light emitted from the waveguide 56 as monitoring light. The wavelength of light is matched between the optical filter 200a and the optical filter 200b. The light is laser oscillated at that wavelength. The oscillation wavelength can be controlled.

[0091] Since crosstalk is suppressed in the optical filters 200a and 200b, the monitoring light is less likely to mix with the laser light, and the lights can be separated from each other. High-precision monitoring can be performed to output the desired laser light.

[0092] The circumferential length of the ring resonator 34 of the optical filter 200a is different from the circumferential length of the ring resonator 34 of the optical filter 200b. The resonant wavelengths of the two optical filters are matched, and light is oscillated as laser at that wavelength.

[0093] Heaters are used to control the wavelength. The phase of the light can be adjusted and the resonant wavelength can be changed by passing electricity through heaters 62, 64, and 66. The number of heaters may be increased from the example in FIG.

[0094] The coupling coefficient in the directional coupler 51 of the optical filter 200a may be different from the coupling coefficient in the directional coupler 51 of the optical filter 200b. The ratio of the output lights can be controlled.

[0095] Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present disclosure described in the claims. [Explanation of symbols]

[0096] 10, 12 Board 14 Waveguide core 16 Cladding Layer 20, 22, 24, 26, 28, 30, 40, 41, 42, 43, 44, 45, 50, 54, 56 Waveguide 25,27,51 Directional coupler 32, 36, 46, 47, 52 multiplexer / demultiplexer 32a, 32b, 36a, 36b, 52a, 52b input terminal 32c, 32d, 36c, 36d, 52c, 52d output end 34 Ring resonator 40a, 40b, 42a, 42b Ends 60 light source 62, 64, 66 Heater 100, 110, 200, 200a, 200b Optical Filters 300 Tunable Laser Device

Claims

1. a first multiplexer / demultiplexer having a first input end, a second input end, a first output end, and a second output end; a first waveguide optically coupled to the first input end; a second waveguide optically coupled to the second input end; a third waveguide optically coupled to the first output end; a fourth waveguide optically coupled to the second output end; a ring resonator optically coupled to the third waveguide and the fourth waveguide.

2. a second multiplexer / demultiplexer having a third input terminal, a fourth input terminal, a third output terminal, and a fourth output terminal; a fifth waveguide optically coupled to the third output end; a sixth waveguide optically coupled to the fourth output end; the third waveguide is optically coupled to the third input end; 2. The optical filter of claim 1, wherein the fourth waveguide is optically coupled to the fourth input end.

3. an optical path length of the third waveguide from the first output end to a coupling portion between the third waveguide and the ring resonator is equal to an optical path length of the fourth waveguide from the second output end to a coupling portion between the fourth waveguide and the ring resonator, 3. The optical filter according to claim 2, wherein an optical path length of the third waveguide from a coupling portion between the third waveguide and the ring resonator to the third input end is equal to an optical path length of the fourth waveguide from a coupling portion between the fourth waveguide and the ring resonator to the fourth input end.

4. 4. The optical filter according to claim 2, wherein the ring resonator, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide and the sixth waveguide are formed of silicon.

5. a third multiplexer / demultiplexer having a fifth input terminal, a sixth input terminal, a fifth output terminal, and a sixth output terminal; a seventh waveguide optically coupled to the fifth input end, the ring resonator, and the sixth input end; an eighth waveguide optically coupled to the fifth output end; 2. The optical filter of claim 1, further comprising: a ninth waveguide optically coupled to said sixth output end.

6. one end of the seventh waveguide is optically coupled to the fifth input end and another end of the seventh waveguide is optically coupled to the sixth input end; 6. The optical filter according to claim 5, wherein the seventh waveguide is coupled to the ring resonator between the end and the other end.

7. an optical path length of the third waveguide from the first output end to a coupling portion between the third waveguide and the ring resonator is equal to an optical path length of the fourth waveguide from the second output end to a coupling portion between the fourth waveguide and the ring resonator, 7. The optical filter according to claim 5, wherein an optical path length of the seventh waveguide from a coupling portion between the ring resonator and the seventh waveguide to the fifth input end is equal to an optical path length of the seventh waveguide from a coupling portion between the ring resonator and the seventh waveguide to the sixth input end.

8. 7. The optical filter according to claim 5, wherein the ring resonator, the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the seventh waveguide, the eighth waveguide, and the ninth waveguide are formed of silicon.

9. A light source; two optical filters; Each of the two optical filters comprises: a first multiplexer / demultiplexer having a first input end, a second input end, a first output end, and a second output end; a first waveguide optically coupled to the first input end; a second waveguide optically coupled to the second input end; a third waveguide optically coupled to the first output end; a fourth waveguide optically coupled to the second output end; a ring resonator optically coupled to the third waveguide and the fourth waveguide; a third multiplexer / demultiplexer having a fifth input terminal, a sixth input terminal, a fifth output terminal, and a sixth output terminal; a seventh waveguide optically coupled to the fifth input end, the ring resonator, and the sixth input end; an eighth waveguide optically coupled to the fifth output end; a ninth waveguide optically coupled to the sixth output end; The light source is a tunable laser element optically coupled to the first waveguides of the two optical filters.

10. The tunable laser device according to claim 9 , wherein the peripheral lengths of the ring resonators of the two optical filters are different from each other.

11. 11. The tunable laser element according to claim 9, further comprising a phase adjustment section provided in each of the two optical filters, for adjusting a phase of light propagating to the two optical filters.

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