Optical filters and tunable laser elements

Abstract

To provide an optical filter and a wavelength-variable laser element capable of monitoring features.SOLUTION: The optical filter comprises: a first loop mirror 20; a second loop mirror 25; a first waveguide 12 optically connected to the first loop mirror and the second loop mirror; a second waveguide 14 optically connected to the first loop mirror and the second loop mirror; a first access waveguide 10 optically connected to the first waveguide; a second access waveguide 16 optically connected to the second waveguide; and an output part 30. The first loop mirror includes a first loop waveguide 22 and a first multiplexer / demultiplexer 24. The second loop mirror includes a second loop waveguide 26 and a second multiplexer / demultiplexer 28. The output part includes a third loop waveguide 32, a third multiplexer / demultiplexer 31, a third waveguide 36, and a fourth waveguide 38. The third loop waveguide is optically connected to the second loop waveguide and the third multiplexer / demultiplexer. The third waveguide and the fourth waveguide are optically connected to the third multiplexer / demultiplexer.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] A tunable laser element having a gain section and a filter that reflects light is known. The light emitted from the gain section is reflected by the filter to cause laser oscillation. There is a technique for forming a filter with two ring resonators (for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] “High-Power,Narrow-Linewidth,Miniaturized Silicon Photonic Tunable Laser With Accurate Frequency Control” Y.Gao et. al. Journal of Lightwave Technology,vol.38, No.2, January 15,2020 p265-271 Summary of the Invention [Problem to be solved by the invention]

[0004] If the filter characteristics cannot be monitored, it is difficult to accurately control the filter characteristics, and the oscillation wavelength becomes unstable. Therefore, an object of the present invention is to provide an optical filter and a wavelength tunable laser element whose characteristics can be monitored. [Means for solving the problem]

[0005] The optical filter according to the present disclosure includes a first loop mirror, a second loop mirror, a first waveguide optically coupled to the first loop mirror and the second loop mirror, a second waveguide optically coupled to the first loop mirror and the second loop mirror, a first access waveguide optically coupled to the first waveguide, a second access waveguide optically coupled to the second waveguide, and an output section, wherein the first loop mirror has a first loop waveguide and a first multiplexer / demultiplexer, the second loop mirror has a second loop waveguide and a second multiplexer / demultiplexer, and the first loop the waveguide is optically coupled to the first multiplexer / demultiplexer, the second loop waveguide is optically coupled to the second multiplexer / demultiplexer, the first waveguide is optically coupled to the first multiplexer / demultiplexer and the second multiplexer / demultiplexer, the second waveguide is optically coupled to the first multiplexer / demultiplexer and the second multiplexer / demultiplexer, the output section has a third loop waveguide, a third multiplexer / demultiplexer, a third waveguide, and a fourth waveguide, the third loop waveguide is optically coupled to the second loop waveguide and the third multiplexer / demultiplexer, and the third waveguide and the fourth waveguide are optically coupled to the third multiplexer / demultiplexer.

[0006] The wavelength tunable laser element according to the present disclosure comprises a gain section having an optical gain, the above-mentioned optical filter, and a reflecting section that reflects light, the gain section being located between the optical filter and the reflecting section and optically coupled to a first access waveguide of the optical filter. Effect of the Invention

[0007] According to the present disclosure, it is possible to provide an optical filter and a wavelength tunable laser element whose characteristics can be monitored. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a plan view illustrating an optical filter according to the first embodiment. [Figure 2A] FIG. 2A is a diagram illustrating the characteristics of an optical filter. [Figure 2B] FIG. 2B is a diagram illustrating the characteristics of an optical filter. [Diagram 3] FIG. 3 is a diagram illustrating the relationship between the coupling coefficient and the light extraction efficiency. [Figure 4] FIG. 4 is a plan view illustrating the wavelength tunable laser element according to the second embodiment. [Figure 5A] FIG. 5A is a cross-sectional view taken along line AA in FIG. [Figure 5B] FIG. 5B is a cross-sectional view taken along line BB in FIG. [Figure 5C] FIG. 5C is a cross-sectional view taken along line CC in FIG. [Figure 6A] FIG. 6A is a schematic diagram illustrating the reflection characteristics of an optical filter. [Figure 6B] FIG. 6B is a schematic diagram illustrating the reflection characteristics of the optical filter. [Figure 6C] FIG. 6C is a schematic diagram illustrating the intensity of the laser light. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0010] One aspect of the present disclosure includes (1) a first loop mirror, a second loop mirror, a first waveguide optically coupled to the first loop mirror and the second loop mirror, a second waveguide optically coupled to the first loop mirror and the second loop mirror, a first access waveguide optically coupled to the first waveguide, a second access waveguide optically coupled to the second waveguide, and an output section, wherein the first loop mirror has a first loop waveguide and a first multiplexer / demultiplexer, the second loop mirror has a second loop waveguide and a second multiplexer / demultiplexer, and the first loop waveguide is , optically coupled to the first multiplexer / demultiplexer, the second loop waveguide optically coupled to the second multiplexer / demultiplexer, the first waveguide optically coupled to the first multiplexer / demultiplexer and the second multiplexer / demultiplexer, the second waveguide optically coupled to the first multiplexer / demultiplexer and the second multiplexer / demultiplexer, the output section has a third loop waveguide, a third multiplexer / demultiplexer, a third waveguide, and a fourth waveguide, the third loop waveguide optically coupled to the second loop waveguide and the third multiplexer / demultiplexer, and the third waveguide and the fourth waveguide are an optical filter optically coupled to the third multiplexer / demultiplexer. When light is incident on the first access waveguide and the second access waveguide, a resonance mode is excited in the first loop mirror and the second loop mirror. Light having a resonance wavelength is transferred from the second loop waveguide to the third loop waveguide of the output section. The light propagating through the third loop waveguide is output from the third and fourth waveguides. The light output from the third and fourth waveguides has high intensity at the resonant wavelength. The output light can be used to monitor the characteristics of the optical filter. (2) The output section may have a fourth multiplexer / demultiplexer, the fourth multiplexer / demultiplexer being optically coupled to the second loop waveguide and the third loop waveguide, and an optical path length of the third loop waveguide in a clockwise direction from the third multiplexer / demultiplexer to the fourth multiplexer / demultiplexer may be equal to an optical path length of the third loop waveguide in a counterclockwise direction of the third loop waveguide. The phase of the light propagating through the third loop waveguide is matched. One of the two resonant modes can be output from the third waveguide, and the other can be output from the fourth waveguide. (3) From the second multiplexer / demultiplexer to the fourth multiplexer / demultiplexer, the optical path length of the second loop waveguide in the clockwise direction of the second loop waveguide may be equal to the optical path length of the second loop waveguide in the counterclockwise direction of the second loop waveguide. The phase of the light propagating through the third loop waveguide is matched. One of the two resonance modes can be output from the third waveguide, and the other can be output from the fourth waveguide. (4) The first multiplexer / demultiplexer may have a symmetrical shape, the second multiplexer / demultiplexer may have a symmetrical shape, the third multiplexer / demultiplexer may have a symmetrical shape, and the first waveguide and the second waveguide may have a symmetrical shape. The two resonance modes have equal resonant wavelengths. The two resonance modes have equal FSRs. By measuring the resonant wavelength and FSR of one of the two resonance modes, the characteristics of the optical filter can be monitored. (5) The first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the first loop waveguide, the second loop waveguide, the third loop waveguide, the first access waveguide, and the second access waveguide may be made of silicon. Light loss can be suppressed. (6) The optical fiber may further include a phase adjusting unit provided in at least one of the first loop waveguide and the second loop waveguide, and adjusting a phase of light propagating through the at least one of the first loop waveguide and the second loop waveguide. By adjusting the phase of the light, the wavelength of the light can be changed. (7) A tunable laser element comprising a gain section having an optical gain, the above-mentioned optical filter, and a reflecting section that reflects light, the gain section being located between the optical filter and the reflecting section and optically coupled to a first access waveguide of the optical filter. Light emitted from the gain section is reflected by the optical filter and the reflecting section. The tunable laser element oscillates. A part of the laser light is transferred to the output section and is emitted from the third waveguide of the output section. The characteristics of the optical filter can be monitored using the light emitted from the fourth waveguide of the output section. The oscillation wavelength of the laser light can be controlled.

[0011] [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.

[0012] First Embodiment 1 is a plan view illustrating an optical filter 100 according to a first embodiment. As shown in FIG. 1, the optical filter 100 includes an access waveguide 10 (first access waveguide), a waveguide 12 (first waveguide), a waveguide 14 (second waveguide), an access waveguide 16 (second access waveguide), a loop mirror 20 (first loop mirror), a loop mirror 25 (second loop mirror), and an output section 30.

[0013] The access waveguides 10 and 16, the waveguides 12 and 14, the loop mirrors 20 and 25, and the output section 30 are formed on a substrate 40. The upper surface of the substrate 40 extends in the XY plane. Two sides of the substrate 40 extend in the X-axis direction. Two sides of the substrate 40 extend in the Y-axis direction. The X-axis direction is perpendicular to the Y-axis direction. The Z-axis direction is the thickness direction of the substrate 40 and is perpendicular to the X-axis and Y-axis directions. One end of the substrate 40 in the X-axis direction is referred to as end 40a, and the other end is referred to as end 40b.

[0014] The loop mirror 20 and the loop mirror 25 are aligned in the X-axis direction. The loop mirror 20 is located on the end 40a side of the substrate 40. The loop mirror 25 is located on the end 40b side.

[0015] The loop mirror 20 has a loop waveguide 22 (first loop waveguide) and a multiplexer / demultiplexer 24 (first multiplexer / demultiplexer). The loop mirror 25 has a loop waveguide 26 (second loop waveguide) and a multiplexer / demultiplexer 28 (second multiplexer / demultiplexer). The loop waveguides 22 and 26 are each a loop-shaped optical waveguide. The loop mirror 20 has a reflective structure that reflects light to the input end of the multiplexer / demultiplexer 24 when light is input to the input end of the multiplexer / demultiplexer 24. The loop mirror 25 has a reflective structure that reflects light to the input end of the multiplexer / demultiplexer 28 when light is input to the input end of the multiplexer / demultiplexer 28.

[0016] The multiplexers / demultiplexers 24 and 28 are, for example, 3 dB couplers. The multiplexers / demultiplexers 24 and 28 in the example of FIG. 1 are directional couplers. In a directional coupler, two optical waveguides are close to each other to a distance of about the wavelength of the propagating light. The power coupling coefficient of each of the multiplexers / demultiplexers 24 and 28 is 0.5. As the multiplexers / demultiplexers, a two-input, two-output (2×2) multimode interference waveguide (MMI) may be used instead of a directional coupler. The loop waveguide 22 is optically coupled to the multiplexer / demultiplexer 24. The loop waveguide 26 is optically coupled to the multiplexer / demultiplexer 28.

[0017] Waveguides 12 and 14 extend in the X-axis direction, are located between loop mirror 20 and loop mirror 25, and are connected to loop mirror 20 and loop mirror 25. Waveguide 12 and waveguide 14 are symmetrical in the Y-axis direction in which waveguide 12 and waveguide 14 are aligned. In other words, when waveguide 14 is folded back with respect to the X-axis, it overlaps with waveguide 12.

[0018] The waveguides 12 and 14 are curved to approach the multiplexer / demultiplexer 24 and 28, respectively, and are optically coupled to the multiplexer / demultiplexer 24 and 28. More specifically, a first end of the waveguide 12 is optically coupled to the multiplexer / demultiplexer 24. A second end of the waveguide 12 is optically coupled to the multiplexer / demultiplexer 28. A first end of the waveguide 14 is optically coupled to the multiplexer / demultiplexer 24. A second end of the waveguide 14 is optically coupled to the multiplexer / demultiplexer 28. The two loop mirrors 20 and 25 and the waveguide 12 form a resonator 11. The two loop mirrors 20 and 25 and the waveguide 14 form a resonator 13. An example of the configuration of the optical waveguides of the optical filter 100 will be described in the second embodiment.

[0019] An access waveguide 10, a waveguide 12, a waveguide 14, and an access waveguide 16 are arranged in this order along the Y-axis direction. The access waveguide 10 extends in the X-axis direction and curves to approach the waveguide 12. The access waveguide 10 is spaced apart from the waveguide 12 by a distance of, for example, several hundred nm, and is optically coupled to the waveguide 12. The access waveguide 16 extends in the X-axis direction and curves to approach the waveguide 14. The access waveguide 16 is spaced apart from the waveguide 14 by a distance of, for example, several hundred nm, and is optically coupled to the waveguide 14.

[0020] An end 10a of the access waveguide 10 and an end 16a of the access waveguide 16 are located at an end 40a of the substrate 40. The other end 10b of the access waveguide 10 and the other end 16b of the access waveguide 16 are located at an end 40b of the substrate 40.

[0021] The output section 30 is located between the loop waveguide 26 of the loop mirror 25 and the end 40b of the substrate 40. The output section 30 has a multiplexer / demultiplexer 31 (third multiplexer / demultiplexer), a loop waveguide 32 (third loop waveguide), a multiplexer / demultiplexer 34 (fourth multiplexer / demultiplexer), a waveguide 36 (third waveguide), and a waveguide 38 (fourth waveguide). The multiplexer / demultiplexer 31 is, for example, a directional coupler, and the power coupling efficiency is determined arbitrarily according to the desired extraction efficiency. The multiplexer / demultiplexer 34 is a 3 dB coupler, for example, a directional coupler. The power coupling coefficient of the multiplexer / demultiplexer 34 is 0.5.

[0022] The loop waveguide 32 is a loop-shaped optical waveguide, which is located next to the loop waveguide 26 and is optically coupled to the loop waveguide 26. More specifically, the loop waveguide 32 and the loop waveguide 26 approach each other to the extent that they are close to each other to the wavelength of light, thereby forming a directional coupler (multiplexer / demultiplexer 31). The loop waveguide 32 is optically coupled to the multiplexer / demultiplexer 34.

[0023] The waveguide 36 and the waveguide 38 are aligned along the Y-axis direction. The waveguides 36 and 38 are located between the loop waveguide 32 and the end 40b of the substrate 40, and are optically coupled to the multiplexer / demultiplexer 34. The waveguides 36 and 38 extend in the X-axis direction from the multiplexer / demultiplexer 34 toward the end 40b. The end 36a of the waveguide 36 and the end 38a of the waveguide 38 are located at the end 40b. The output section 30 may be optically coupled to the loop waveguide 22 of the loop mirror 20.

[0024] One of the ends 10a and 10b of the access waveguide 10 (e.g., end 10a) and one of the ends 16a and 16b of the access waveguide 16 (e.g., end 16a) are defined as input ports of the optical filter 100. The end 36a of the waveguide 36 and the end 38a of the waveguide 38 are defined as output ports of the optical filter 100.

[0025] When light is incident from a light source (not shown) disposed outside the optical filter 100 through the end 10a to the access waveguide 10 as shown by an arrow A1 in Fig. 1, a resonance mode is excited in the resonator 11. Light having a resonance wavelength transfers from the loop waveguide 26 to the loop waveguide 32 of the output unit 30, propagates through the waveguide 36, and is emitted to the outside of the optical filter 100 from the end 36a as shown by an arrow A7. When light is incident on the access waveguide 16 through the end 16a as shown by an arrow A4, a resonance mode is excited in the resonator 13. Light having a resonance wavelength transfers from the loop waveguide 26 to the loop waveguide 32 of the output unit 30, propagates through the waveguide 38, and is emitted to the outside of the optical filter 100 from the end 38a as shown by an arrow A8.

[0026] More specifically, light incident from the end 10a propagates through the access waveguide 10 and transfers from the access waveguide 10 to the waveguide 12. The light propagating through the waveguide 12 is input to the multiplexers / demultiplexers 24 and 28. The light is distributed from the waveguide 12 through the multiplexer / demultiplexer 24 to both ends of the loop waveguide 22. The intensity of the distributed light is 1:1, and the phases are shifted by π / 2 from each other. The phase of the light propagating counterclockwise through the loop waveguide 22 is delayed by π / 2 relative to the light propagating clockwise.

[0027] The light propagating counterclockwise through the loop waveguide 22 is distributed through the multiplexer / demultiplexer 24 to the waveguide 12 without any phase change and to the waveguide 14 with a phase delay of π / 2. The light propagating clockwise through the loop waveguide 22 is distributed through the multiplexer / demultiplexer 24 to the waveguide 12 with a phase delay of π / 2 and to the waveguide 14 without any phase change. The light propagating clockwise through the loop waveguide 22 and input to the waveguide 14 and the light propagating counterclockwise through the loop waveguide 22 and input to the waveguide 14 are in opposite phase to each other and cancel each other out. On the other hand, the light propagating counterclockwise through the loop waveguide 22 and input to the waveguide 12 and the light propagating clockwise through the loop waveguide 22 and input to the waveguide 12 are in phase to each other and are multiplexed without canceling each other out.

[0028] Light is distributed from the waveguide 12 to both ends of the loop waveguide 26 through the multiplexer / demultiplexer 28. The light propagating clockwise through the loop waveguide 26 and input to the waveguide 14 and the light propagating counterclockwise through the loop waveguide 26 and input to the waveguide 14 are in opposite phase to each other and cancel each other out. On the other hand, the light propagating counterclockwise through the loop waveguide 26 and input to the waveguide 12 and the light propagating clockwise through the loop waveguide 26 and input to the waveguide 12 are in phase with each other and are multiplexed.

[0029] When light is transferred from the access waveguide 10 to the waveguide 12, a resonance mode is generated in the resonator 11. The resonant wavelength of the resonator 11 is a wavelength at which the change in phase of the light when it goes around the two loop mirrors 20 and 25 is 2πn (n is an integer). The light having the resonance wavelength is transferred from the waveguide 12 to the access waveguide 10, and is reflected toward the end 10a of the access waveguide 10 as shown by the arrow A2 in FIG. 1. The light having a wavelength other than the resonance wavelength is output from the end 10b of the access waveguide 10 as shown by the arrow A3 in FIG. 1. A part of the light having the resonance wavelength is transferred from the loop waveguide 26 to the loop waveguide 32 of the output unit 30 through the multiplexer / demultiplexer 31.

[0030] The light propagating counterclockwise through the loop waveguide 26 transfers to the loop waveguide 32 with a phase delay of π / 2 through the multiplexer / demultiplexer 31, and propagates clockwise through the loop waveguide 32. The light propagating clockwise through the loop waveguide 26 transfers to the loop waveguide 32 with a phase delay of π / 2 through the multiplexer / demultiplexer 31, and propagates counterclockwise through the loop waveguide 32. The distribution ratio of the light intensity in the multiplexer / demultiplexer 31 is, for example, 1:1.

[0031] The light propagating through the loop waveguide 32 is distributed to the waveguides 36 and 38 through the multiplexer / demultiplexer 34. The light propagating clockwise through the loop waveguide 32 is input to the waveguide 38 without any phase change, and is input to the waveguide 36 with a phase change of π / 2. The light propagating counterclockwise through the loop waveguide 32 is input to the waveguide 38 with a phase change of π / 2, and is input to the waveguide 36 without any phase change. The light propagating clockwise through the loop waveguide 32 and input to the waveguide 38 and the light propagating counterclockwise through the loop waveguide 32 and input to the waveguide 38 are in opposite phase to each other, and cancel each other out. On the other hand, the light propagating counterclockwise through the loop waveguide 32 and input to the waveguide 36 and the light propagating clockwise through the loop waveguide 32 and input to the waveguide 36 are in phase with each other, and are multiplexed.

[0032] By inputting light into the access waveguide 10, a resonance mode can be excited in the resonator 11. The resonance mode of the resonator 11 does not propagate through the waveguide 14, but propagates through the waveguide 12, and is reflected by the access waveguide 10. A part of the resonance mode is transferred to the output section 30. The resonance mode is canceled in the waveguide 38, propagates through the waveguide 36, and is output from the end 36a of the waveguide 36 to the outside of the optical filter 100.

[0033] As shown by arrow A4, a resonance mode is excited in the resonator 13 by incident light on the access waveguide 16. The resonance mode of the resonator 13 does not propagate through the waveguide 12, but propagates through the waveguide 14, and is reflected by the access waveguide 16, as shown by arrow A5. Light having a wavelength other than the resonance wavelength is transmitted through the access waveguide 16, as shown by arrow A6. A part of the resonance mode is transferred to the loop waveguide 32 of the output section 30. The resonance mode is canceled in the waveguide 36, propagates through the waveguide 38, and is output to the outside of the optical filter 100 from the end 38a of the waveguide 38, as shown by arrow A8.

[0034] The resonance mode of resonator 11 is cancelled in waveguide 38 and propagates through waveguide 36. The resonance mode of resonator 13 is cancelled in waveguide 36 and propagates through waveguide 38. To enable the above-mentioned propagation of light, it is only necessary that the optical path lengths between the multiplexers / demultiplexers are equal. The optical path length of loop waveguide 26 in the clockwise direction from multiplexer / demultiplexer 28 to multiplexer / demultiplexer 31 is equal to the optical path length of loop waveguide 26 in the counterclockwise direction from multiplexer / demultiplexer 28 to multiplexer / demultiplexer 31. In other words, loop waveguide 26 is divided in half by multiplexer / demultiplexer 28 and multiplexer / demultiplexer 31. The optical path length of the loop waveguide 32 in the clockwise direction from the multiplexer / demultiplexer 31 to the multiplexer / demultiplexer 34 is equal to the optical path length of the loop waveguide 32 in the counterclockwise direction from the multiplexer / demultiplexer 31 to the multiplexer / demultiplexer 34. In other words, the loop waveguide 32 is divided in half by the multiplexer / demultiplexer 31 and the multiplexer / demultiplexer 34.

[0035] With respect to the X-axis, the shape of multiplexer / demultiplexer 24 is symmetrical, the shape of multiplexer / demultiplexer 28 is symmetrical, and waveguide 12 is symmetrical with waveguide 14. In this case, the resonant wavelength of resonator 11 and the resonant wavelength of resonator 13 will in principle match. The FSR (Free Spectral Range, resonant frequency interval) of resonator 11 and the FSR of resonator 13 will in principle match. If the resonant wavelength and FSR of one of the resonant modes of resonator 11 and resonator 13 are known, the resonant wavelength and FSR of the other can also be known. The shape of multiplexer / demultiplexer 24 may be point-symmetrical with respect to the center of multiplexer / demultiplexer 24 itself. The shape of multiplexer / demultiplexer 28 may be point-symmetrical with respect to the center of multiplexer / demultiplexer 28 itself.

[0036] (characteristic) 2A and 2B are diagrams illustrating the characteristics of the optical filter 100. The vertical axis of FIG. 2A represents reflectance. The reflectance is obtained based on the intensity of light incident on the access waveguide 10 and the intensity of the reflected light. The vertical axis of FIG. 2B represents transmittance. The transmittance is obtained based on the intensity of light incident on the access waveguide 10 and the intensity of the transmitted light. The horizontal axis of FIG. 2A and 2B represents the wavelength of light. A number of lines in FIG. 2A and 2B represent examples of different coupling coefficients κ. The thick solid line is an example of κ=0.1. The dotted line is an example of κ=0.3. The dashed line is an example of κ=0.5. The dashed-dotted line is an example of κ=0.7. The thin solid line is an example of κ=0.9.

[0037] As shown in Figure 2A, the reflectance is maximized at the three resonant wavelengths of the resonator 11. As shown in Figure 2B, the transmittance is minimized at the three resonant wavelengths. The width of the reflectance peak and the transmittance peak is determined by the coupling coefficient κ between the access waveguide 10 and the waveguide 12. The lower the coupling coefficient, the narrower and sharper the reflectance and transmittance peaks are. The higher the coupling coefficient, the wider and gentler the peaks are.

[0038] The coupling coefficient between the access waveguide 10 and the waveguide 12 depends on the distance between the access waveguide 10 and the waveguide 12. When the distance between the access waveguide 10 and the waveguide 12 is reduced, the coupling coefficient between the access waveguide 10 and the waveguide 12 is increased and the Q value of the resonator 11 is reduced. The reflectance and transmittance peaks are broadened. When the distance between the access waveguide 10 and the waveguide 12 is increased, the coupling coefficient between the access waveguide 10 and the waveguide 12 is reduced and the Q value is increased. The reflectance and transmittance peaks are narrowed.

[0039] The access waveguide 16 also has the same reflection characteristic as that of Fig. 2A and the same transmission characteristic as that of Fig. 2B. The coupling coefficient between the access waveguide 16 and the waveguide 14 may be equal to the coupling coefficient between the access waveguide 10 and the waveguide 12, or may be larger or smaller than the coupling coefficient between the access waveguide 10 and the waveguide 12.

[0040] As described above, the resonance mode of resonator 11 is transferred to output section 30 and propagates through waveguide 36. The resonance mode of resonator 13 is transferred to output section 30 and propagates through waveguide 38. The intensity of the light propagating through waveguide 36 and the intensity of the light propagating through waveguide 38 show peaks at the resonance wavelength, as in Fig. 2A. That is, light of the resonance wavelength can be extracted with high intensity from waveguides 36 and 38 of output section 30.

[0041] According to a first embodiment, optical filter 100 includes access waveguides 10 and 16, waveguides 12 and 14, loop mirrors 20 and 25, and output 30. Waveguide 12 is optically coupled to loop mirrors 20 and 25 to form resonator 11. Waveguide 14 is optically coupled to loop mirrors 20 and 25 to form resonator 13.

[0042] The light incident on the access waveguide 10 is transferred to the waveguide 12, and the resonance mode of the resonator 11 is excited. A part of the light propagating through the resonator 11 is transferred from the loop waveguide 26 of the loop mirror 25 to the loop waveguide 32 of the output unit 30, and is output from the waveguide 36. The light incident on the access waveguide 16 is transferred to the waveguide 14, and the resonance mode of the resonator 13 is excited. A part of the light propagating through the resonator 13 is transferred from the loop waveguide 26 to the loop waveguide 32 of the output unit 30, and is output from the waveguide 38. It is possible to extract light of a resonance wavelength from the output unit 30. By observing the output light from the waveguides 36 and 38, the characteristics of the optical filter 100, such as the resonance wavelength and FSR, can be directly monitored.

[0043] For example, the wavelength of light incident on the access waveguides 10 and 16 is changed while the light emitted from the waveguides 36 and 38 is received by a light receiving element such as a photodiode. The wavelength at which the light intensity is maximized is the resonant wavelength. The full resonance range (FSR) can also be measured.

[0044] Aging can change the refractive index of the optical waveguides, which can change the characteristics of the optical filter 100. The light emitted from the waveguides 36 and 38 can be used to directly monitor the changes in the characteristics of the optical filter 100 over time, such as a shift in the resonant wavelength.

[0045] Light is also emitted from end 10b of access waveguide 10 and end 16b of access waveguide 16. The emitted light from ends 10b and 16b is extinct at the resonant wavelength and has high intensity at wavelengths other than the resonant wavelength. On the other hand, the emitted light from waveguides 36 and 38 has high intensity at the resonant wavelength compared to wavelengths other than the resonant wavelength. By detecting the emitted light from waveguides 36 and 38, characteristics such as the resonant wavelength can be directly measured.

[0046] The optical path length of the loop waveguide 32 in the clockwise direction from the multiplexer / demultiplexer 31 to the multiplexer / demultiplexer 34 is equal to the optical path length of the loop waveguide 32 in the counterclockwise direction. In other words, the loop waveguide 32 is divided into two equal parts by the multiplexer / demultiplexer 31 and the multiplexer / demultiplexer 34. The phase of the light propagating through the loop waveguide 32 is matched. The resonance mode of the resonator 11 is canceled in the waveguide 38 and propagates through the waveguide 36. The resonance mode of the resonator 13 is canceled in the waveguide 36 and propagates through the waveguide 38. The resonance modes of the two resonators 11 and 13 can be separated and extracted. The resonant wavelength and FSR of the resonator 11 can be monitored using the light emitted from the waveguide 36. The resonant wavelength and FSR of the resonator 13 can be monitored using the light emitted from the waveguide 38.

[0047] The optical path length of the loop waveguide 26 in the clockwise direction from the multiplexer / demultiplexer 28 to the multiplexer / demultiplexer 31 is equal to the optical path length of the loop waveguide 26 in the counterclockwise direction. In other words, the loop waveguide 26 is divided into two equal parts by the multiplexer / demultiplexer 28 and the multiplexer / demultiplexer 31. The phase of the light propagating through the loop waveguide 26 is matched. By making the optical path lengths of the loop waveguides 26 and 32 equal between the two multiplexers / demultiplexers, the phase of the light propagating through the loop waveguides 26 and 32 is matched. The resonance mode of the resonator 11 is canceled in the waveguide 38 and propagates through the waveguide 36. The resonance mode of the resonator 13 is canceled in the waveguide 36 and propagates through the waveguide 38. The resonance modes of the two resonators 11 and 13 can be separated and extracted.

[0048] In the Y-axis direction in which the waveguide 12 and the waveguide 14 are aligned, the shape of the waveguide 12 and the shape of the waveguide 14 are symmetrical. The shape of the multiplexer / demultiplexer 24 is symmetrical. The shape of the multiplexer / demultiplexer 28 is symmetrical. The resonant wavelength of the resonant mode of the resonator 11 is equal to the resonant wavelength of the resonant mode of the resonator 13. The FSR of the resonant mode of the resonator 11 is equal to the FSR of the resonant mode of the resonator 13. For example, the resonant wavelength and FSR of the resonant mode of the resonator 11 can also be monitored by measuring the resonant wavelength and FSR of the resonant mode of the resonator 13 using the light emitted from the waveguide 38. The light emitted from the waveguide 36 can be used for purposes other than monitoring characteristics. The shape of the multiplexer / demultiplexer 24 may be point-symmetrical with respect to the center of the multiplexer / demultiplexer 24 itself. The shape of the multiplexer / demultiplexer 28 may be point-symmetrical with respect to the center of the multiplexer / demultiplexer 28 itself.

[0049] The loop waveguide 32 in Fig. 1 has a shape obtained by folding back the loop waveguide 26 with respect to the Y-axis. The shape of the loop waveguide 32 may be different from that of the loop waveguide 26. The optical path length of the loop waveguide 32 may be equal to or different from the optical path length of the loop waveguide 22 and the optical path length of the loop waveguide 26. The optical path length of the loop waveguide 22 may be equal to or different from the optical path length of the loop waveguide 26. The optical waveguide of the optical filter 100 may be provided with an electrode 60, which will be described later with reference to Fig. 4. The electrode 60 functions as a heater, and therefore the resonant wavelength can be changed.

[0050] FIG. 3 is a diagram illustrating the relationship between the coupling coefficient and the light extraction efficiency. The horizontal axis is the coupling coefficient between the loop waveguide 26 and the loop waveguide 32. The vertical axis is the light extraction efficiency from the loop waveguide 26 to the loop waveguide 32. As shown in FIG. 3, the extraction efficiency increases in proportion to the square of the coupling coefficient. The coupling coefficient between the loop waveguide 26 and the output section 30 depends on the distance between the loop waveguide 26 and the loop waveguide 32. By adjusting the distance between the loop waveguide 26 and the loop waveguide 32, the intensity of the light transferred to the loop waveguide 32 of the output section 30 can be changed.

[0051] <Second embodiment> 4 is a plan view illustrating a tunable laser device 200 according to the second embodiment. The tunable laser device 200 includes an optical filter 100, an optical filter 110, and a gain section 50.

[0052] The optical filter 100 has the same configuration as that shown in Fig. 1. The optical filter 110 has a configuration obtained by removing the output section 30 from the optical filter 100. The optical filters 100 and 110 are formed on a substrate 40. The gain section 50 is a light-emitting element formed of, for example, a III-V compound semiconductor, and is bonded to the upper surface of the substrate 40.

[0053] The optical filters 100 and 110 share the access waveguide 10. An end 10a of the access waveguide 10 is located at an end 40a of the substrate 40. An end 10b of the access waveguide 10 is located at an end 40b of the substrate 40.

[0054] The end 16a of the access waveguide 16-1 of the optical filter 100 is located at the end 40c in the Y-axis direction of the substrate 40. The end 16b of the access waveguide 16-1 is located at the end 40b of the substrate 40. The end 16c of the access waveguide 16-2 of the optical filter 110 is located at the end 40c. The end 16d of the access waveguide 16-2 is located at the end 40a. The end 10a of the access waveguide 10, the end 16a of the access waveguide 16-1, and the end 16c of the access waveguide 16-2 function as input ports. The end 36a of the waveguide 36 and the end 38a of the waveguide 38 are located at the end 40b of the substrate 40 and function as output ports.

[0055] The optical filters 100 and 110 each have a plurality of electrodes 60. The electrodes 60 are provided on the loop waveguides 22 and 26 and on the access waveguide 10 of the optical filters 100 and 110, respectively. The electrodes 60 are made of metal, function as heaters, and generate heat when a current is input. When the electrodes 60 generate heat, the refractive index of the optical waveguide changes, and the phase of the light propagating through the optical waveguide changes. The change in the phase of the light can change the resonant wavelength.

[0056] 5A is a cross-sectional view taken along line AA in FIG. 4, illustrating a cross-section of the access waveguide 10. FIG. 5B is a cross-sectional view taken along line BB in FIG. 4, illustrating a cross-section of the loop waveguide 22. As shown in FIG. 5A and FIG. 5B, the substrate 40 is a silicon on insulator (SOI) substrate, and includes a substrate 42, a cladding layer 44 (box layer), and a silicon (Si) layer 46. The substrate 42 is made of, for example, Si. The cladding layer 44 is made of, for example, silicon oxide (SiO2).

[0057] The cladding layer 44 is laminated on the upper surface of the substrate 42. The Si layer 46 is embedded inside the cladding layer 44 and is spaced apart from the upper surface of the substrate 42. The distance between the Si layer 46 and the upper surface of the substrate 42 is, for example, 3 μm. The Si layer 46 includes a terrace 45 and a waveguide core 48. The terrace 45 is a surface extending in the XY plane. The waveguide core 48 is spaced apart from the Si layer 46 in the XY plane. The thickness of the waveguide core 48 is, for example, 0.22 μm. The width of the waveguide core 48 is, for example, 0.42 μm.

[0058] A groove 47 is provided between the waveguide core 48 and the terrace 45. The groove 47 is located on both sides of the waveguide core 48 and extends in the same direction as the waveguide core 48. The inside of the groove 47 is filled with SiO2. In Figs. 5A to 5C, the groove 47 is a portion where no Si is provided. The groove 47 may be, for example, a portion of the Si layer 46 that is recessed below the terrace 45. As shown in Fig. 5B, the electrode 60 is provided on the upper surface of the cladding layer 44 and is located above the waveguide core 48 in the Z-axis direction.

[0059] A portion of the optical waveguide of the tunable laser element 200 where the electrode 60 is not provided has the same configuration as that shown in Fig. 5A. A portion of the optical waveguide of the tunable laser element 200 where the electrode 60 is provided has the same configuration as that shown in Fig. 5B.

[0060] 4, in the X-axis direction, the optical filter 100 is located on one side of the gain section 50, and the optical filter 110 is located on the opposite side. The gain section 50 is located above the access waveguide 10 and is optically coupled to the access waveguide 10. The end of the gain section 50 on the optical filter 110 side and the end on the optical filter 100 side have a tapered shape that tapers along the extension direction of the access waveguide 10 (X-axis direction).

[0061] 5C is a cross-sectional view taken along line CC in FIG. 4, illustrating a cross-section of the gain section 50. In the portion of the substrate 40 to which the gain section 50 is bonded, the cladding layer 44 is provided between the substrate 40 and the Si layer 46, and is not provided on the Si layer 46.

[0062] The gain section 50 has cladding layers 52 and 56, an active layer 54, and a contact layer 58. The cladding layer 52 is bonded to the Si layer 46 of the substrate 40. The lower surface of the cladding layer 52 is in contact with the upper surface of the Si layer 46. The active layer 54, the cladding layer 56, and the contact layer 58 are stacked in this order on the cladding layer 52 to form a mesa 51. The mesa 51 is located on the waveguide core 48 of the substrate 40. The cladding layer 52 extends outside the mesa 51.

[0063] The upper surface of the cladding layer 52 and the side and upper surface of the mesa 51 are covered with an insulating film 57. The insulating film 57 has openings on the cladding layer 52 and on the mesa 51. The electrode 53 is provided on the cladding layer 52 and contacts the upper surface of the cladding layer 52 through the opening of the insulating film 57. The electrode 55 is provided on the mesa 51 and contacts the upper surface of the contact layer 58 through the opening of the insulating film 57.

[0064] The cladding layer 52 is made of, for example, n-type indium phosphide (InP). The active layer 54 is made of, for example, aluminum gallium indium arsenide (AlGaInAs) and has a multi-quantum well structure (MQW: Multi Quantum Well). The cladding layer 56 is made of, for example, p-type InP. The contact layer 58 is made of, for example, p-type indium gallium arsenide (InGaAs). The electrode 53 is made of a metal such as a laminate of gold, germanium, and nickel (Au / Ge / Ni). The electrode 55 is made of a metal such as a laminate of titanium, platinum, and gold (Ti / Pt / Au).

[0065] The sum of the optical path lengths of the two loop mirrors of the optical filter 100 is different from the sum of the optical path lengths of the two loop mirrors of the optical filter 110. The FSR of the optical filter 100 is different from the FSR of the optical filter 110.

[0066] Fig. 6A is a schematic diagram illustrating the reflection characteristics of the optical filter 110. Fig. 6B is a schematic diagram illustrating the reflection characteristics of the optical filter 100. The horizontal axis represents the wavelength of light, and the vertical axis represents the reflectance.

[0067] As shown in Fig. 6A, the reflectance of the optical filter 110 shows peaks at resonance wavelengths λ1, λ2, and λ3. The FSR between two adjacent resonance wavelengths is Δλ1. As shown in Fig. 6B, the reflectance of the optical filter 100 shows peaks at resonance wavelengths λ4, λ5, and λ6. The FSR between two adjacent resonance wavelengths is Δλ2, which is smaller than Δλ1. Laser oscillation is performed by utilizing the Vernier effect of the two optical filters 100 and 110.

[0068] The gain section 50 has an optical gain. A current is injected into the mesa 51 using electrodes 53 and 55. The active layer 54 generates light by injecting carriers into it. Light is emitted from both ends of the gain section 50. The light emitted from the first end of the gain section 50 propagates through the access waveguide 10 and is input to the optical filter 110. The light emitted from the second end of the gain section 50 propagates through the access waveguide 10 and is input to the optical filter 100.

[0069] When light is input from the access waveguide 10 to the optical filter 110, the resonance mode of the resonator 11 is excited. Light having a resonance wavelength is reflected and propagates through the access waveguide 10 toward the gain section 50. Light having a resonance wavelength is also reflected from the optical filter 100 and propagates through the access waveguide 10 toward the gain section 50.

[0070] The tunable laser element 200 oscillates as light is repeatedly reflected by the optical filters 100 and 110. The wavelength at which the resonant wavelength of the optical filter 100 and the resonant wavelength of the optical filter 110 match is the oscillation wavelength.

[0071] FIG. 6C is a schematic diagram illustrating the intensity of laser light. The horizontal axis represents the wavelength of light. The vertical axis represents the intensity of light. When one of the multiple resonant wavelengths in FIG. 6A (e.g., λ2) coincides with one of the multiple resonant wavelengths in FIG. 6B (e.g., λ5), laser light is generated at that wavelength. As shown in FIG. 6C, the wavelength λa (=λ2=λ5) becomes the oscillation wavelength, and high-intensity laser light is oscillated.

[0072] The laser light propagates through the access waveguide 10, transfers from the access waveguide 10 to the waveguide 12 of the optical filter 100, and propagates through the loop mirrors 20 and 25. A part of the laser light transfers from the loop waveguide 26 of the loop mirror 25 to the loop waveguide 32. The laser light propagating through the loop waveguide 32 is cancelled out in the waveguide 38. On the other hand, the laser light transferring from the loop waveguide 32 to the waveguide 36 is emitted to the outside from the end 36a of the waveguide 36.

[0073] Light is input from a light source outside the wavelength tunable laser element 200 to the access waveguide 16-1 of the optical filter 100 through the end 16a shown in FIG. 4, and light is input to the access waveguide 16-2 of the optical filter 110 through the end 16c. In the optical filters 100 and 110, the resonance mode of the resonator 13 is excited. Light having a resonance wavelength is reflected by the optical filter 100, propagates through the access waveguide 16-1, and is output from the end 16a to the outside. Light having a resonance wavelength is reflected by the optical filter 110, propagates through the access waveguide 16-2, and is output from the end 16c to the outside. Light having a wavelength other than the resonance wavelength (transmitted light) propagates through the access waveguide 16-1 and is output from the end 16b to the outside. The transmitted light propagates through the access waveguide 16-2 and is output from the end 16d to the outside.

[0074] A part of the resonance mode generated in the resonator 13 of the optical filter 100 is transferred to the output section 30, is cancelled in the waveguide 36, propagates through the waveguide 38, and is emitted to the outside from the end 38a of the waveguide 38.

[0075] According to the second embodiment, the gain section 50 is optically coupled to two optical filters 100 and 110. The output light of the gain section 50 is reflected by the optical filters 100 and 110. The FSR of the optical filter 100 is different from the FSR of the optical filter 110. The Vernier effect of the optical filters 100 and 110 oscillates a laser light. A part of the laser light transits to the output section 30 of the optical filter 100 and is output from the waveguide 36. When light is input to the access waveguide 16-1, a resonance mode is excited in the resonator 13 of the optical filter 100. The light having the resonance wavelength transits to the output section 30 and is output from the waveguide 38.

[0076] Light can be extracted from the waveguides 36 and 38 to the outside of the wavelength tunable laser element 200. Laser light is emitted from the waveguide 36. The laser light emitted from the waveguide 36 can be used for optical communication, for example. Light having a high intensity at a resonant wavelength is emitted from the waveguide 38. The light emitted from the waveguide 38 is used to monitor the characteristics of the optical filter 100. The amount of light extracted from the waveguides 36 and 38 depends on the coupling coefficient between the loop waveguide 26 and the loop waveguide 32 as shown in FIG. 3. By reducing the distance between the loop waveguide 26 and the loop waveguide 32, the coupling coefficient can be increased and the intensity of the emitted light can be increased. For example, the distance between the loop waveguides is determined so that the intensity of the light reflected toward the gain section 50 and the intensity of the light emitted from the waveguide 36 to the outside are appropriate.

[0077] Light is also emitted from the end 10a of the access waveguide 10 and the end 16d of the access waveguide 16-2. The emitted light from the access waveguide 16-2 can be used to monitor the characteristics of the optical filter 110. The wavelength at which the emitted light is extinct is the resonant wavelength of the optical filter 110.

[0078] By passing a current through the electrode 60, the refractive index of the optical waveguide is changed, and the phase of the light is adjusted. The change in phase changes the resonant wavelength of the optical filters 100 and 110. That is, the spectra in FIG. 6A and FIG. 6B are shifted. The oscillation wavelength can be changed from λa in FIG. 6C. As described above, the tunable laser element 200 can change the oscillation wavelength of the laser light.

[0079] It is effective to provide the electrode 60 on the loop waveguides 22 and 26 of the optical filter. By heating the loop waveguide with the electrode 60, the refractive index of the loop waveguide is changed, and the optical path length is changed. The change in the optical path length makes it possible to control the resonance wavelengths of the resonators 11 and 13. The electrode 60 may be provided on at least one of the loop waveguides 22 and 26 of the optical filter. The electrode 60 may be provided at a position other than that shown in FIG. 4. A configuration other than the electrode 60 may be used to adjust the phase of light.

[0080] The optical waveguide of the optical filters 100 and 110 has a Si waveguide core 48. The waveguide core 48 is surrounded by a SiO2 cladding layer 44. The refractive index of Si is about 3.5. The refractive index of SiO2 is about 1.4. The waveguide core 48, which has a higher refractive index than the cladding layer 44, can strongly confine light. The loss of light in a bent optical waveguide, such as an access waveguide or a loop waveguide, is suppressed.

[0081] The gain section 50 and the optical filters 100 and 110 may be monolithically integrated on a compound semiconductor substrate. The optical waveguide is, for example, a high mesa structure including III-V group compound semiconductors. The optical filter 110 of the wavelength tunable laser element 200 may be configured to reflect light. For example, a filter or a reflecting mirror other than the optical filter 110 may be used as a configuration for reflection (reflection section). The gain section 50 is disposed between the optical filter 100 and the reflection section. Laser oscillation is possible by reflecting the light emitted from one end of the gain section 50 by the optical filter 100 and reflecting the light emitted from the other end by the reflection section. By disposing the optical filter 100 on the side from which the light is extracted, light with high intensity can be emitted.

[0082] 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]

[0083] 10, 16, 16-1, 16-2 Access waveguide 10a, 10b, 16a, 16b, 16c, 16d Ends 11, 13 resonator 12, 14, 36, 38 Waveguide 20, 25 Loop mirror 22, 26, 32 Loop waveguide 24, 28, 31 Multiplexer / demultiplexer 30 Output section 40, 42 Board 44, 52, 56 Cladding layers 45 Terrace 47 Groove 48 Waveguide Core 57 Insulating Film 50 Gain section 51 Mesa 53, 55, 60 electrodes 54 Active layer 58 Contact layer 100, 110 Optical filter 200 Tunable laser element

Claims

1. A first loop mirror; A second loop mirror; and a first waveguide optically coupled to the first loop mirror and the second loop mirror; a second waveguide optically coupled to the first loop mirror and the second loop mirror; a first access waveguide optically coupled to the first waveguide; a second access waveguide optically coupled to the second waveguide; An output unit, the first loop mirror has a first loop waveguide and a first multiplexer / demultiplexer; the second loop mirror has a second loop waveguide and a second multiplexer / demultiplexer; the first loop waveguide is optically coupled to the first multiplexer / demultiplexer; the second loop waveguide is optically coupled to the second multiplexer / demultiplexer; the first waveguide is optically coupled to the first multiplexer / demultiplexer and the second multiplexer / demultiplexer; the second waveguide is optically coupled to the first multiplexer / demultiplexer and the second multiplexer / demultiplexer; the output section includes a third loop waveguide, a third multiplexer / demultiplexer, a third waveguide, and a fourth waveguide; the third loop waveguide is optically coupled to the second loop waveguide and the third multiplexer / demultiplexer; the third waveguide and the fourth waveguide are optically coupled to the third multiplexer / demultiplexer;

2. the output section has a fourth multiplexer / demultiplexer, the fourth multiplexer / demultiplexer is optically coupled to the second loop waveguide and the third loop waveguide; 2. The optical filter according to claim 1, wherein an optical path length of the third loop waveguide in a clockwise direction of the third loop waveguide from the third multiplexer / demultiplexer to the fourth multiplexer / demultiplexer is equal to an optical path length of the third loop waveguide in a counterclockwise direction of the third loop waveguide.

3. 3. The optical filter according to claim 2, wherein an optical path length of the second loop waveguide in a clockwise direction of the second loop waveguide from the second multiplexer / demultiplexer to the fourth multiplexer / demultiplexer is equal to an optical path length of the second loop waveguide in a counterclockwise direction of the second loop waveguide.

4. 4. The optical filter according to claim 1, wherein a shape of the first multiplexer / demultiplexer is symmetrical, a shape of the second multiplexer / demultiplexer is symmetrical, a shape of the third multiplexer / demultiplexer is symmetrical, and a shape of the first waveguide and a shape of the second waveguide are symmetrical.

5. 5. The optical filter according to claim 1, wherein the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the first loop waveguide, the second loop waveguide, the third loop waveguide, the first access waveguide, and the second access waveguide are formed of silicon.

6. 6. The optical filter according to claim 1, further comprising a phase adjustment section provided in at least one of the first loop waveguide and the second loop waveguide, the phase adjustment section adjusting a phase of light propagating through the at least one of the first loop waveguide and the second loop waveguide.

7. a gain section having an optical gain; An optical filter according to any one of claims 1 to 6; A reflecting portion that reflects light, The gain section is located between the optical filter and the reflector, and is a tunable laser element optically coupled to a first access waveguide of the optical filter.

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