Optical filter and wavelength tunable laser element

Abstract

The invention provides an optical filter and a wavelength tunable laser element capable of suppressing crosstalk. An optical filter is provided with: a first multiplexer / demultiplexer having a first input terminal, a second input terminal, a first output terminal, and a second output terminal; a first waveguide optically coupled to the first input; a second waveguide optically coupled to the second input; a third waveguide optically coupled to the first output; the fourth waveguide is optically coupled with the second output end; and a ring resonator optically coupled to the third waveguide and the fourth waveguide.

Description

Technical Field

[0001] The present disclosure relates to optical filters and wavelength tunable laser elements. Background Art

[0002] Optical filters using silicon photonics are known. An optical filter has been developed that forms a resonator using a plurality of waveguides and excites light of a specific wavelength (for example, Patent Document 1).

[0003] Prior art literature Patent Literature Patent Document 1: Japanese Patent Application Publication No. 2022-84531 Summary of the invention Problems to be solved by the invention In an optical filter, light is branched to a waveguide by a combiner / demultiplexer. By setting the branching ratio of light in the combiner / demultiplexer to a specific value, the port through which the light propagates can be controlled. For example, light is emitted from a certain port, but not from other ports. However, sometimes the branching ratio deviates from the design value. Light passes through the combiner / demultiplexer multiple times, so the influence of the deviation of the branching ratio becomes larger. As a result, the crosstalk between ports increases. Therefore, the purpose of the present disclosure is to provide an optical filter and a wavelength tunable laser element capable of suppressing crosstalk.

[0004] Means used to solve problems The optical filter involved in the present disclosure 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; and a ring resonator optically coupled to the third waveguide and the fourth waveguide.

[0005] Effects of the Invention 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

[0006] Figure 1 It is a plan view showing an example of the optical filter according to the first embodiment.

[0007] Figure 2 is a cross-sectional view showing an example of an optical filter.

[0008] Figure 3 This is a diagram showing an example of the propagation of light in an optical filter.

[0009] Figure 4 This is a diagram showing an example of the propagation of light in an optical filter.

[0010] Figure 5A This is a diagram showing an example of the spectrum of reflected light.

[0011] Figure 5B It is a diagram showing an example of the spectrum of transmitted light.

[0012] Figure 6 It is a top view showing an example of an optical filter according to a comparative example.

[0013] Fig. 7A This is a diagram showing an example of calculation results of light intensity.

[0014] Figure 7B This is a diagram showing an example of calculation results of light intensity.

[0015] Figure 8 It is a plan view showing an example of the optical filter according to the second embodiment.

[0016] Fig. 9 This is a diagram showing an example of the propagation of light in an optical filter.

[0017] Fig.10 This is a diagram showing an example of the propagation of light in an optical filter.

[0018] Fig.11 FIG. 1 is a plan view showing an example of a wavelength tunable laser element according to the third embodiment.

[0019] Description of Reference Numerals 10, 12: substrate; 14: waveguide core; 16: coating 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: combiner / demultiplexer; 32a, 32b, 36a, 36b, 52a, 52b: input terminals; 32c, 32d, 36c, 36d, 52c, 52d: output terminals; 34: Ring resonator; 40a, 40b, 42a, 42b: end; 60: light source; 62, 64, 66: heater; 100, 110, 200, 200a, 200b: optical filters; 300: Wavelength tunable laser element. DETAILED DESCRIPTION

[0020] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0021] One embodiment of the present disclosure is (1) an optical filter, wherein the optical filter 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; and a ring resonator optically coupled to the third waveguide and the fourth waveguide. The number of times light passes through the first multiplexer / demultiplexer is reduced, thereby suppressing crosstalk.

[0022] (2) In the above (1), the optical filter may include: 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 terminal; and a sixth waveguide optically coupled to the fourth output terminal, wherein the third waveguide is optically coupled to the third input terminal, and the fourth waveguide is optically coupled to the fourth input terminal. The number of times light passes through the first multiplexer / demultiplexer and the second multiplexer / demultiplexer is reduced, thereby suppressing crosstalk.

[0023] (3) In the above (2), the optical path length of the third waveguide from the first output end to the coupling portion between the third waveguide and the ring resonator may be equal to the optical path length of the fourth waveguide from the second output end to the coupling portion between the fourth waveguide and the ring resonator, and the 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 the optical path length of the fourth waveguide from the coupling portion between the fourth waveguide and the ring resonator to the fourth input end. This can suppress the shift in the phase of light and suppress crosstalk.

[0024] (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 formed of silicon. This can suppress light loss.

[0025] (5) In the above (1), the optical filter may include: 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 terminal, the ring resonator, and the sixth input terminal; an eighth waveguide optically coupled to the fifth output terminal; and a ninth waveguide optically coupled to the sixth output terminal. The number of times light passes through the first multiplexer / demultiplexer and the third multiplexer / demultiplexer is reduced, thereby suppressing crosstalk. The resonated light can be output.

[0026] (6) In the above (5), one end of the seventh waveguide may be optically coupled to the fifth input end, and the other end may be optically coupled to the sixth input end, and the seventh waveguide may be coupled to the ring resonator between the end and the other end, so that the resonated light can be propagated to the eighth waveguide and the ninth waveguide.

[0027] (7) In the above (5) or (6), the optical path length of the third waveguide from the first output end to the coupling portion between the third waveguide and the ring resonator may be equal to the optical path length of the fourth waveguide from the second output end to the coupling portion between the fourth waveguide and the ring resonator, and the optical path length of the seventh waveguide from the coupling portion between the ring resonator and the seventh waveguide to the fifth input end may be equal to the optical path length of the seventh waveguide from the coupling portion between the ring resonator and the seventh waveguide to the sixth input end. This can suppress the shift in the phase of light and suppress crosstalk.

[0028] (8) In any one of (5) to (7) above, 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 formed of silicon. This can suppress light loss.

[0029] (9) A wavelength tunable laser element, wherein the wavelength tunable laser element comprises a light source and two optical filters, wherein the two optical filters respectively comprise: a first wavelength combiner / 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 wavelength combiner / 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, wherein the light source is optically coupled to the first waveguide of the two optical filters. Crosstalk is suppressed by the two optical filters. Light is reflected by the two optical filters to perform laser oscillation. The optical filter can be monitored by light emitted from the optical filter.

[0030] (10) In the above (9), the circumferences of the ring resonators of the two optical filters may be different from each other, and the resonance wavelengths of the two optical filters may be made consistent so that the light performs laser oscillation.

[0031] (11) In the above (9) or (10), the wavelength tunable laser element may include a phase adjustment unit, which is provided in the two optical filters and adjusts the phase of the light propagating to the two optical filters. By adjusting the phase of the light, the laser light can be controlled.

[0032] [Details of the embodiments of the present disclosure] Hereinafter, specific examples of optical filters and wavelength tunable laser elements according to embodiments of the present disclosure will be described with reference to the accompanying drawings. In addition, the present disclosure is not limited to these examples, but is indicated by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims.

[0033] <First Embodiment> Figure 1 FIG. 1 is a top view showing an example of the optical filter 100 according to the first embodiment. Figure 1 As shown, 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 the normal direction of the upper surface of the substrate 10. The X axis, the Y axis and the Z axis are orthogonal to each other.

[0034] The optical filter 100 includes a waveguide 20 (first waveguide), a waveguide 22 (second waveguide), a combiner / demultiplexer 32 (first combiner / demultiplexer), a waveguide 24 (third waveguide), a waveguide 26 (fourth waveguide), a ring resonator 34, a combiner / demultiplexer 36 (second combiner / demultiplexer), a waveguide 28 (fifth waveguide), and a waveguide 30 (sixth waveguide).

[0035] The multiplexer / demultiplexer 32 and the multiplexer / demultiplexer 36 are 3dB couplers, such as dual-input dual-output MMI (Multi Mode Interfere). The multiplexer / demultiplexer 32 has an input end 32a (first input end), an input end 32b (second input end), an output end 32c (first output end), and an output end 32d (second output end). The multiplexer / demultiplexer 36 has an input end 36a (third input end), an input end 36b (fourth input end), an output end 36c (third output end), and an output end 36d (fourth output end).

[0036] 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 the input end 32a of the multiplexer / demultiplexer 32. The other end of the waveguide 22 is optically coupled to the input end 32b of the multiplexer / demultiplexer 32.

[0037] 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.

[0038] One end of the waveguide 28 is optically coupled to the output end 36c of the multiplexer / demultiplexer 36. One end of the waveguide 30 is optically coupled to the output end 36d of the multiplexer / demultiplexer 36. The other end of the waveguide 28 and the other end of the waveguide 30 are located at the other end of the substrate 10 in the X-axis direction.

[0039] The ring resonator 34 is a loop-type waveguide. The planar shape of the ring resonator 34 is circular, elliptical, polygonal, etc. The ring resonator 34 is located between the combiner / demultiplexer 32 and the combiner / demultiplexer 36. In the X-axis direction, the combiner / demultiplexer 32, the ring resonator 34, and the combiner / demultiplexer 36 are arranged in sequence. In the Y-axis direction, the waveguide 24, the ring resonator 34, and the waveguide 26 are arranged in sequence.

[0040] 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.

[0041] 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.

[0042] Figure 2 is a cross-sectional view showing an example of an optical filter 100, showing a cross-sectional view along Figure 1 The substrate 10 is a SOI (Silicon on Insulator) 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 ) is formed to cover one surface of the substrate 10. The waveguide core 14 is formed of Si, for example, and is separated from the substrate 10 and buried 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.

[0043] 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 shown in FIG. Figure 2 same.

[0044] The operation of the optical filter 100 will be described. The ends of the waveguide 20 and the waveguide 22 function as an input port and an output port. The ends of the waveguide 28 and the waveguide 30 function as an output port. The optical filter 100 propagates two independent resonance modes.

[0045] Figure 3 as well as Figure 4 1 is a diagram showing an example of the propagation of light in the optical filter 100. The portion where light propagates is shaded.

[0046] exist Figure 3In the example of , light I1 is incident from an external light source into the waveguide 20. For other waveguides, light is not incident from the outside. The light propagates in the waveguide 20 and is input to the input end 32a of the multiplexer / demultiplexer 32. The multiplexer / demultiplexer 32 demultiplexes the light to the output end 32c and the output end 32d. Ideally, the ratio of the intensity of the branched light is 50:50. The phase of the light output from the output end 32d to the waveguide 26 is delayed by 90° compared to the light output from the output end 32c to the waveguide 24.

[0047] The light propagating in the waveguide 24 is transferred to the ring resonator 34, and propagates clockwise in the ring resonator 34. The light propagating in the waveguide 26 is transferred to the ring resonator 34, and propagates counterclockwise in the ring resonator 34.

[0048] The light circulates and resonates multiple times in the ring resonator 34. The resonance wavelength is determined by the circumference of the ring resonator 34. The light resonated 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.

[0049] The reflected light propagating in the waveguide 24 is input to the output end 32c of the multiplexer / demultiplexer 32. The multiplexer / demultiplexer 32 demultiplexes the reflected light input from the waveguide 24 to the input end 32a and the input end 32b. The reflected light propagating in the waveguide 26 is input to the output end 32d. The multiplexer / demultiplexer 32 demultiplexes the reflected light input from the waveguide 26 to the input end 32a and the input end 32b. The branching ratio of the reflected light is ideally 50:50.

[0050] The phase of the reflected light propagating in the waveguide 26 and input to the input end 32b is different by 180° from the phase of the reflected light propagating in the waveguide 24 and input to the input end 32b. Since the phases are reversed to each other, the two reflected lights cancel each other out.

[0051] The phase of the reflected light propagating in the waveguide 24 and input to the input end 32a is equal to the phase of the reflected light propagating in the waveguide 26 and input to the input end 32a. The two reflected lights reinforce each other. The reflected light R1 propagates in the waveguide 20 and is emitted from the end of the waveguide 20.

[0052] The transmitted light propagating in the waveguide 24 is input to the input terminal 36a of the multiplexer / demultiplexer 36. The multiplexer / demultiplexer 36 demultiplexes the transmitted light input from the waveguide 24 to the output terminal 36c and the output terminal 36d. The transmitted light propagating in the waveguide 26 is input to the input terminal 36b. The multiplexer / demultiplexer 36 demultiplexes the transmitted light input from the waveguide 26 to the output terminal 36c and the output terminal 36d. The branching ratio is ideally 50:50.

[0053] The phase of the transmitted light propagating in the waveguide 26 and input to the output end 36c is different by 180° from the phase of the transmitted light propagating in the waveguide 24 and input to the output end 36c. Since the phases are reversed to each other, the two transmitted lights cancel each other out.

[0054] The phase of the transmitted light propagating in the waveguide 24 and input to the output end 36d is equal to the phase of the transmitted light propagating in the waveguide 26 and input to the output end 36d. The two transmitted lights reinforce each other. The transmitted light T1 propagates in the waveguide 30 and is emitted from the end of the waveguide 30.

[0055] Figure 3 XT1 represents light leaking to the waveguide 28. XT2 represents light leaking to the waveguide 22. Light leaking like the light XT1 and the light XT2 is sometimes described as crosstalk. Ideally, the intensity of the light XT1 and the light XT2 is zero.

[0056] exist Figure 4 In the example of , light I2 is incident on the waveguide 22 from an external light source. For other waveguides, light is not incident from the outside. Light propagates in the waveguide 22 and is input to the input end 32b of the combiner / demultiplexer 32. The combiner / demultiplexer 32 demultiplexes the light to the output end 32c and the output end 32d. The light resonated 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 combiner / demultiplexer 32. The reflected light R2 is emitted from the end of the waveguide 22. The transmitted light T2 is emitted from the end of the waveguide 28. Figure 4 XT3 represents light leaking into the waveguide 30. XT4 represents light leaking into the waveguide 20. Ideally, the intensity of light XT3 and light XT4 is zero.

[0057] When light is incident on both waveguide 20 and waveguide 22, two independent resonance modes are excited. Figure 3 As in the example of FIG. 2 , reflected light R1 is emitted from the waveguide 20, and transmitted light T1 is emitted from the waveguide 30. Figure 4 As in the example of FIG. 2 , reflected light R2 is emitted from the waveguide 22 , and transmitted light T2 is emitted from the waveguide 28 .

[0058] Figure 5A This is a diagram showing an example of 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 the resonance wavelength λ1, the resonance wavelength λ2, the resonance wavelength λ3, the resonance wavelength λ4, and the resonance wavelength λ5. At the peak, the intensity shows a maximum value. The interval between two adjacent resonance wavelengths is FSR (Free Spectral Range).

[0059] Figure 5BThis is a diagram showing an example of 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 extinguished at the resonance wavelength λ1, the resonance wavelength λ2, the resonance wavelength λ3, the resonance wavelength λ4, and the resonance wavelength λ5. That is, the intensity is a minimum value.

[0060] The two reflected lights R1 and R2 have the same wavelength characteristics. The FSR of the reflected light R1 and the FSR of the reflected light R2 are consistent in principle. 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 FSR can be monitored.

[0061] The two transmitted lights T1 and T2 have the same wavelength characteristics. The FSR of the two transmitted lights is consistent in principle. For example, when the wavelength of the light is the resonance wavelength, when one transmitted light T1 is extinguished, the other transmitted light T2 is also extinguished. The reflected light shows a peak. By measuring the transmitted light, the characteristics of the optical filter 100 can also be monitored.

[0062] (Comparative Example) Figure 6 1 is a plan view showing an example of an optical filter 110 according to a comparative example. The optical filter 110 includes waveguides 40 , 41 , 42 , 43 , 44 , 45 , a multiplexer / demultiplexer 46 , and a multiplexer / demultiplexer 47 .

[0063] The waveguide 40 and the waveguide 42 are access waveguides for inputting and outputting light, and extend from one end to the other end of the substrate 10. The waveguide 40 and the waveguide 41 are close to each other, for example, by several hundred nanometers, and are optically coupled. The waveguide 42 and the waveguide 43 are optically coupled.

[0064] The multiplexer / demultiplexer 46 and the multiplexer / demultiplexer 47 are dual-input dual-output couplers. One end of each of the waveguide 41 and the waveguide 43 is optically coupled to the input end of the multiplexer / demultiplexer 46. The other end of each of the waveguide 41 and the waveguide 43 is optically coupled to the input end of the multiplexer / demultiplexer 47.

[0065] The waveguide 44 and the waveguide 45 have a loop shape. Both ends of the waveguide 44 are coupled to the output end of the multiplexer / demultiplexer 46. Both ends of the waveguide 45 are coupled to the output end of the multiplexer / demultiplexer 47.

[0066] When light I3 is incident from the end 42a of the waveguide 42, the light is transferred from the waveguide 42 to the waveguide 43. The light propagating in the waveguide 43 is distributed to both ends of the waveguide 44 in the multiplexer / demultiplexer 46, and is distributed to both ends of the waveguide 45 in the multiplexer / demultiplexer 47. The waveguide 43, the waveguide 44, the waveguide 45, the multiplexer / demultiplexer 46, and the multiplexer / demultiplexer 47 function as resonators. The light repeatedly propagates in the waveguide 43, the waveguide 44, the waveguide 45, the multiplexer / demultiplexer 46, and the multiplexer / demultiplexer 47, and resonates at a wavelength determined by the optical path length. The resonated light R3 is transferred from the waveguide 43 to the waveguide 42, and is emitted from the end 42a. The transmitted light T3 is emitted from the other end 42b of the waveguide 42.

[0067] Ideally, light does not propagate toward the waveguide 41 and the waveguide 40. However, the leaked light XT5 may propagate toward the end 40a of the waveguide 40, and the leaked light XT6 may propagate toward the end 40b of the waveguide 40.

[0068] Crosstalk depends on the branching ratio of light in the wavelength combiner / demultiplexer. When there is an error in the branching ratio, crosstalk increases.

[0069] Fig. 7A as well as Figure 7B This is a diagram showing an example of the calculation result of the intensity of light. The horizontal axis represents the phase of light in the resonator, and θ=2nπ is a resonance condition (n is an integer). The vertical axis represents the intensity of light.

[0070] ΔT represents the deviation of the branching ratio in the multiplexer / demultiplexer. The ideal branching ratio is 50:50. Fig. 7A as well as Figure 7B In this example, the branching ratio is (50+ΔT):(50-ΔT). The electric coupling coefficient between the resonator and the waveguide is given by sinφ 2 express.

[0071] Fig. 7A Indicates the calculation results in the comparative example. The thick solid line indicates the reflected light R3 (ΔT=1%, φ=π / 4). The thin solid line indicates the crosstalk XT5 (ΔT=1%, φ=π / 4). The dotted line indicates the reflected light R3 (ΔT=1%, φ=π / 8). The single-dash line indicates the crosstalk XT5 (ΔT=1%, φ=π / 8). The double-dash line indicates the reflected light R3 (ΔT=2%, φ=π / 8). The dotted line indicates the crosstalk XT5 (ΔT=2%, φ=π / 8). Although not shown in the figure, the crosstalk XT6 is the same result as the crosstalk XT5.

[0072] The reflected light R3 shows high intensity under the resonance condition 2nπ. The crosstalk XT5 depends on ΔT and φ. The higher the ΔT, the larger the maximum value of the crosstalk XT5. In the comparative example, the light repeatedly passes through the combiner / demultiplexer 46 and the combiner / demultiplexer 47. Due to the influence of the deviation ΔT of the branching ratio in the accumulation combiner / demultiplexer, the crosstalk XT5 increases. When ΔT is the same, the smaller φ is, the higher the crosstalk XT5 is. If φ is small, the Q value of the resonator becomes higher. For crosstalk, when φ is small, it has the same effect as an increase in the number of times the light surrounds the resonator.

[0073] Figure 7B The calculation results in the first embodiment are shown. The solid line represents the crosstalk XT1 (ΔT=1%, φ=π / 8). The single-point dash line represents the crosstalk XT2 (ΔT=1%, φ=π / 8). The dashed line represents the crosstalk XT1 (ΔT=5%, φ=π / 8). The dotted line represents the crosstalk XT2 (ΔT=5%, φ=π / 8). In the example of ΔT=1% (solid line and single-point dash line), the maximum crosstalk is about -34dB, which is suppressed to be lower than the crosstalk of ΔT=1% in the comparative example. In the example of ΔT=5% (dashed line and dotted line), the maximum crosstalk is about -20dB.

[0074] 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 and is reflected in the ring resonator 34. When light is incident on the waveguide 20, reflected light R1 propagates in the waveguide 20. When light is incident on the waveguide 22, reflected light R2 propagates in the waveguide 22.

[0075] The branching ratio of light in the multiplexer / demultiplexer 32 is designed to be, for example, 50:50. Due to manufacturing errors, the branching ratio may deviate from the designed value. From the time the light is incident on the optical filter 100 until the reflected light is emitted, the light passes through the multiplexer / demultiplexer 32 twice. Since the number of times the light passes through the multiplexer / demultiplexer 32 decreases, the light is less susceptible to the deviation of the branching ratio. Therefore, crosstalk can be suppressed. Figure 7B As in the example of , when the branch offset is ΔT=1% and ΔT=5%, the crosstalk XT1 and the crosstalk XT2 can be reduced compared with the comparative example.

[0076] Since crosstalk is suppressed, two reflected lights can be taken out separately. 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 resonance wavelength and FSR.

[0077] When a resonator is formed by a plurality of waveguides, the resonator length becomes longer, so it is difficult to increase the FSR. According to the first embodiment, the optical filter 100 has a ring resonator 34. By shortening the circumference of the ring resonator 34, the FSR can be increased. For example, the circumference may be greater than 100 μm and less than 300 μm, less than 100 μm, or less than 600 μm. If the circumference is greater than 100 μm and about 600 μm, the FSR is greater than 1 nm and less than 6 nm.

[0078] Waveguide 24 is optically coupled to input end 36a of combiner / demultiplexer 36. Waveguide 26 is optically coupled to input end 36b. Waveguide 28 is optically coupled to output end 36c. Waveguide 30 is optically coupled to output end 36d. When light is incident on waveguide 20, transmitted light T1 propagates in waveguide 30. When light is incident on waveguide 22, transmitted light T2 propagates in waveguide 28. Light is incident on optical filter 100, and until the transmitted light is emitted, the light passes through the combiner / demultiplexer twice. Since the number of times the light passes through the combiner / demultiplexer is limited, the light is not easily affected by the deviation of the branching ratio. Therefore, crosstalk can be suppressed. The two transmitted lights can be taken out separately. The characteristics of optical filter 100 can be monitored using the transmitted light.

[0079] 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. The phase of the light propagating in the waveguide 24 and the light propagating in the waveguide 26 is not easily shifted. The optical path lengths may be strictly equal, or may be equal within the range of manufacturing error, for example.

[0080] By suppressing the influence of branch ratio deviation and suppressing phase deviation, in principle, Figure 3 As in the example of FIG. 2 , the reflected lights cancel each other out and do not propagate toward the waveguide 22. The transmitted lights cancel each other out and do not propagate toward the waveguide 28. Figure 4 As in the example of FIG. 1 , the reflected lights cancel each other out and do not propagate toward the waveguide 20. The transmitted lights cancel each other out and do not propagate toward the waveguide 30. The crosstalk is suppressed and the two resonance modes are excited independently. The reflected light R1 and the reflected light R2 can be taken out separately. The transmitted light T1 and the transmitted light T2 can be taken out separately. These lights can be used for different purposes.

[0081] like Figure 2 As shown, the waveguide and the ring resonator 34 are formed by the waveguide core 14 of Si. Light can be confined in the curved waveguide to suppress loss. Even if the bending radius of the ring resonator 34 is reduced, the loss of light can be suppressed. The circumference is shortened, and the FSR can be increased.

[0082] The optical filter 100 may be formed on a component other than the SOI substrate. The multiplexer / demultiplexer 32 and the multiplexer / demultiplexer 36 are dual-input dual-output couplers, and may be directional couplers other than MMI.

[0083] <Second Embodiment> Figure 8 1 is a plan view showing an example of an optical filter 200 according to the second embodiment. The description of the same configuration as that of the first embodiment will be omitted.

[0084] The optical filter 200 includes 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).

[0085] Ring resonator 34 and waveguide 50 are provided between combiner / demultiplexer 32 and combiner / demultiplexer 52. Waveguide 54 and waveguide 56 are provided on the opposite side of combiner / demultiplexer 52 from waveguide 50. Waveguide 24 and waveguide 26 are not coupled to combiner / demultiplexer 52.

[0086] The multiplexer / demultiplexer 52 has an input terminal 52 a (fifth input terminal), an input terminal 52 b (sixth input terminal), an output terminal 52 c (fifth output terminal), and an output terminal 52 d (sixth output terminal).

[0087] The waveguide 50 is C-shaped and has two ends. The portion of the waveguide 50 between the two ends is closest to the ring resonator 34, forming a directional coupler 51. In the directional coupler 51, the waveguide 50 is optically coupled to the ring resonator 34. 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 migrates between the ring resonator 34 and the waveguide 50. The larger the distance, the lower the coupling efficiency, and the smaller the intensity of the migrated light.

[0088] One end of the waveguide 50 is optically coupled to the input end 52a of the multiplexer / demultiplexer 52. The other end of the waveguide 50 is optically coupled to the input end 52b. One end of the waveguide 54 is optically coupled to the output end 52c. One end of the waveguide 56 is optically coupled to the output end 52d. The other end of the waveguide 54 and the other end of the waveguide 56 are located at the end of the substrate 10 and function as an output port.

[0089] Fig. 9 as well as Fig.102 is a diagram showing an example of light propagation in the optical filter 200. The portion where light propagates is marked with oblique lines. The optical filter 200 excites two independent resonance modes.

[0090] exist Fig. 9 In the example of FIG. 1 , light I4 is incident from an external light source into the waveguide 20. For other waveguides, light is not incident from the outside. Light propagates in the waveguide 20 and is input to the input end 32a of the multiplexer / demultiplexer 32. The multiplexer / demultiplexer 32 demultiplexes the light to the output end 32c and the output end 32d. Light propagating in the waveguide 24 is transferred to the ring resonator 34. Light propagating in the waveguide 26 is transferred to the ring resonator 34.

[0091] Part of the light resonated in the ring resonator 34 is transferred to the waveguide 50 and input to the input terminal 52a and the input terminal 52b of the multiplexer / demultiplexer 52. The multiplexer / demultiplexer 52 demultiplexes the light to the output terminal 52c and the output terminal 52d. Ideally, the branching ratio is 50:50.

[0092] The phase of the light input from input end 52b and output to output end 52d is 180° different from the phase of the light input from input end 52a and output to output end 52d. Since the phases are reversed, the two lights cancel each other out. Ideally, the light does not propagate toward waveguide 56.

[0093] The phase of the light input from the input end 52a and output to the output end 52c is equal to the phase of the light input from the input end 52b and output to the output end 52c. The two lights reinforce each other. The transmitted light T4 propagates in the waveguide 54 and is emitted from the end of the waveguide 54.

[0094] 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 .

[0095] exist Fig.10 In the example of , light I5 is incident on the waveguide 22 from an external light source. For other waveguides, no light is incident from the outside. Transmitted light T5 is emitted from the waveguide 56. Reflected light R5 is emitted from the waveguide 22.

[0096] When light is incident on both the waveguide 20 and the waveguide 22, two independent resonance modes are excited. Transmitted light T4 is emitted from the waveguide 54, and reflected light R4 is emitted from the waveguide 20. Transmitted light T5 is emitted from the waveguide 56, and reflected light R5 is emitted from the waveguide 22.

[0097] The transmitted light T4, the transmitted light T5, the reflected light R4 and the reflected light R5 have the same wavelength characteristics, for example, Figure 5AThat 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.

[0098] According to the second embodiment, the waveguide 50 is optically coupled to the ring resonator 34, the input end 52a and the input end 52b of the combiner / demultiplexer 52. The waveguide 54 is optically coupled to the output end 52c of the combiner / 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 emitted from the waveguide 54 and the waveguide 56. The light is injected into the optical filter 200, and the number of times the light passes through the combiner / demultiplexer is 2 times until the transmitted light is emitted. The number of times the light passes through the combiner / demultiplexer is also 2 times until the reflected light is emitted. Since the number of times the light passes through the combiner / demultiplexer is reduced, the light is not easily affected by the deviation of the branching ratio. Therefore, crosstalk can be suppressed.

[0099] Since the crosstalk is suppressed, the waveguide 54 and the waveguide 56 can be used as waveguides for output, and the resonated light can be taken out separately from each other. 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 resonant wavelength and FSR can be monitored. For example, the transmitted light T5 is used for monitoring, and the wavelength is set to the resonant wavelength. The transmitted light T4 having a peak at the resonant wavelength can be output. The reflected light R4 and the reflected light R5 also have the same wavelength characteristics as the transmitted light. The reflected light can also be used for monitoring.

[0100] The FSR can be increased by shortening the circumference of the ring resonator 34. For example, the circumference may be 100 μm or more and 300 μm or less, 100 μm or less, or 600 μm or less.

[0101] One end of the waveguide 50 is coupled to the input end 52a, and the other end is coupled to the input end 52b. The resonated light propagates in the waveguide 50 and is input from both ends to the multiplexer / demultiplexer 52. The resonated light can propagate in the waveguide 54 and the waveguide 56 coupled to the multiplexer / demultiplexer 52.

[0102] 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 light is not easily shifted.

[0103] By suppressing the influence of branch ratio deviation and suppressing phase deviation, in principle, Fig. 9 As in the example of FIG. 5 , the transmitted lights cancel each other and do not propagate toward the waveguide 56. The reflected lights cancel each other and do not propagate toward the waveguide 22. Fig.10 The same is true for the example. Crosstalk is suppressed, and two resonance modes are excited independently. Transmitted light T4 and transmitted light T5 can be taken out separately. Reflected light R4 and reflected light R5 can be taken out separately. These lights can be used for different purposes.

[0104] like Figure 2 As shown, the waveguide and the ring resonator 34 are formed by the waveguide core 14 of Si. Light can be confined in the curved waveguide to suppress loss. Even if the bending radius of the ring resonator 34 is reduced, the loss of light can be suppressed. The circumference is shortened, and the FSR can be increased.

[0105] <Third Embodiment> Fig.11 1 is a plan view showing an example of a wavelength tunable laser element 300 according to the third embodiment. The description of the same structure as that of the first embodiment or the second embodiment will be omitted.

[0106] The wavelength tunable laser element 300 is formed on a substrate 10 and includes a light source 60 and two optical filters 200. The optical filter 200 has the structure of the second embodiment. One of the two optical filters 200 is set as an optical filter 200a, and the other optical filter 200 is set as an optical filter 200b. The optical filter 200a and the optical filter 200b share a waveguide 20. The circumference of the ring resonator 34 of the optical filter 200a is different from the circumference of the ring resonator 34 of the optical filter 200b. The optical filter 200a and the optical filter 200b form a laser resonator.

[0107] The wavelength tunable laser element 300 includes a heater 62, a heater 64, and a heater 66 (phase adjustment unit). The heater is formed of metal and is electrically connected to an external power source 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. By energizing the heater, the heater generates heat, and the silicon waveguide is heated. By increasing the temperature, the optical path length changes, and the phase of the light changes. By adjusting the phase, the resonance wavelength changes.

[0108] The light source 60 is, for example, a semiconductor light emitting element, formed of a III-V compound semiconductor, and having optical gain. The light source 60 is bonded to one surface of the substrate 10, is located on 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 emitted light of the light source 60 is transferred to the waveguide 20 and is incident on the optical filter 200a and the optical filter 200b.

[0109] When light is incident from the light source 60, the optical filter 200a and the optical filter 200b generate reflected light and transmitted light. The reflected light of the optical filter 200a propagates in the waveguide 20 toward the light source 60. The reflected light of the optical filter 200b propagates in the waveguide 20 toward the light source 60. The light is repeatedly reflected between the optical filter 200a and the optical filter 200b.

[0110] Since the circumferences of the two ring resonators 34 are different from each other, the FSR of the optical filter 200a is different from the FSR of the optical filter 200b. Due to the vernier effect, the light oscillates at the wavelength where the two optical filters 200 show peaks. The laser light is emitted from the waveguide 54 of the optical filter 200a and the optical filter 200b.

[0111] 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 filter 200a and the optical filter 200b generate reflected light and transmitted light. The transmitted light propagates in 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.

[0112] According to the third embodiment, the optical filter 200a and the optical filter 200b form a laser resonator. When light is incident from the light source 60 to the waveguide 20, the light is reflected by the two optical filters and performs laser oscillation. The laser is emitted from the waveguide 54 of the optical filter 200a and the optical filter 200b. When the light is incident to the waveguide 22 of the optical filter 200a and the optical filter 200b, the light is emitted from the waveguide 56. The light emitted from the waveguide 56 is used as monitoring light to monitor the optical filter 200a and the optical filter 200b. The wavelength of light is made consistent between the optical filter 200a and the optical filter 200b. The light performs laser oscillation at this wavelength. The oscillation wavelength can be controlled.

[0113] Since crosstalk is suppressed in the optical filters 200a and 200b, the monitoring light is unlikely to be mixed into the laser light, and these lights can be separated from each other. High-precision monitoring can be performed, and desired laser light can be output.

[0114] The circumference of the ring resonator 34 of the optical filter 200a is different from the circumference of the ring resonator 34 of the optical filter 200b. The resonance wavelength is made consistent between the two optical filters, and light is laser-oscillated at the wavelength.

[0115] Heaters are used to control the wavelength. By energizing heaters 62, 64, and 66, the phase of light can be adjusted to change the resonant wavelength. Fig.11 Compared with the example, the number of heaters is increased.

[0116] 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 outgoing light can be controlled.

[0117] As mentioned above, although the embodiment of this disclosure is described in detail, this disclosure is not limited to this specific embodiment, and various deformation|transformation and changes are possible within the scope of the summary of this disclosure described in a claim.

Claims

1. An optical filter, wherein: The optical filter comprises: A first combiner / 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; as well as A ring resonator is optically coupled to the third waveguide and the fourth waveguide.

2. The optical filter according to claim 1, wherein The optical filter comprises: A second combiner / 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; and a sixth waveguide optically coupled to the fourth output end, The third waveguide is optically coupled to the third input end, The fourth waveguide is optically coupled to the fourth input end.

3. The optical filter according to claim 2, wherein The optical path length of the third waveguide from the first output end to the coupling portion of the third waveguide and the ring resonator is equal to the optical path length of the fourth waveguide from the second output end to the coupling portion of the fourth waveguide and the ring resonator. 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. The optical filter according to claim 2 or 3, 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. The optical filter according to claim 1, wherein The optical filter comprises: A third combiner / 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; as well as A ninth waveguide is optically coupled to the sixth output end.

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

7. The optical filter according to claim 5 or 6, wherein: The optical path length of the third waveguide from the first output end to the coupling portion of the third waveguide and the ring resonator is equal to the optical path length of the fourth waveguide from the second output end to the coupling portion of the fourth waveguide and the ring resonator. 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. The optical filter according to claim 5 or 6, 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 wavelength tunable laser element, wherein: The wavelength tunable laser element comprises a light source and two optical filters. The two optical filters respectively include: A first combiner / 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 combiner / 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; as well as a ninth waveguide optically coupled to the sixth output end, The light source is optically coupled to the first waveguides of the two optical filters.

10. The wavelength tunable laser element according to claim 9, wherein: The circumferences of the ring resonators of the two optical filters are different from each other.

11. The wavelength tunable laser element according to claim 9 or 10, wherein: The wavelength tunable laser element includes a phase adjustment unit provided in the two optical filters and adjusting the phase of the light propagating to the two optical filters.

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