JP2023015990A - Optical filter, wavelength-variable laser element, wavelength-variable laser module, and control method of wavelength-variable laser module - Google Patents

Abstract

To provide an optical filter, a wavelength-variable laser element, a wavelength-variable laser module, and a control method of a wavelength-variable laser module.SOLUTION: An optical filter includes a first loop mirror, a second loop mirror, a first waveguide optically coupled with the first loop mirror and the second loop mirror, and a first access waveguide. The first loop mirror has a first loop waveguide and a first multiplexer/demultiplexer, and the second loop mirror has a second loop waveguide and a second multiplexer/demultiplexer. The first loop waveguide is optically coupled with the first multiplexer/demultiplexer, and the second loop waveguide is optically coupled with the second multiplexer/demultiplexer. The first waveguide is optically coupled with the first multiplexer/demultiplexer and the second multiplexer/demultiplexer, and the first access waveguide is optically coupled with the first waveguide.SELECTED DRAWING: Figure 3

Description

The present disclosure relates to an optical filter, a wavelength tunable laser element, a wavelength tunable laser module, and a wavelength tunable laser module control method.

A wavelength tunable laser device having a gain section and a filter that reflects light is known. There is a technique of forming a filter with a plurality of ring resonators and loop mirrors (for example, Patent Document 1). There is a technique of forming a filter by optically coupling two branched waveguides and a ring resonator (for example, Patent Document 2). By controlling the filter characteristics and changing the phase of the light, the oscillation wavelength of the laser element is adjusted.

JP 2016-102926 A WO2019/159808

However, the characteristics of the filter may change over time due to temperature changes, current injection, and the like. If the filter characteristics cannot be monitored, it is difficult to accurately control the filter characteristics, and the oscillation wavelength becomes unstable. Accordingly, it is an object of the present invention to provide an optical filter, a wavelength tunable laser element, a wavelength tunable laser module, and a method of controlling the wavelength tunable laser module, which are capable of monitoring characteristics.

An 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 first access waveguide, wherein the first loop mirror has a first loop waveguide and a first multiplexer/demultiplexer, and 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, and the first waveguide A wave path is optically coupled to the first multiplexer/demultiplexer and the second multiplexer/demultiplexer, and the first access waveguide is optically coupled to the first waveguide.

A wavelength tunable laser device according to the present disclosure includes a gain section and two optical filters, wherein the two optical filters are the optical filters described above, and the resonance wavelengths of the two optical filters are separated from each other by Differently, the gain section has optical gain and is optically coupled to the first access waveguide of each of the two optical filters.

A wavelength tunable laser module according to the present disclosure includes the above-described wavelength tunable laser element, a light source for inputting light into a second access waveguide of the wavelength tunable laser element, and an optical element in the second access waveguide of the wavelength tunable laser element. and a light receiving element that is positively coupled.

A method for controlling a wavelength tunable laser module according to the present disclosure includes the steps of: injecting light from a light source into a second access waveguide of a wavelength tunable laser element; 2. controlling the wavelength of light propagating in the access waveguide.

According to the present disclosure, it is possible to provide an optical filter, a wavelength tunable laser element, a wavelength tunable laser module, and a method of controlling the wavelength tunable laser module whose characteristics can be monitored.

1A is a plan view illustrating an optical filter according to a first embodiment; FIG. FIG. 1B is a cross-sectional view along line AA of FIG. 1A. FIG. 2A is a diagram illustrating reflection characteristics of an optical filter; FIG. 2B is a diagram illustrating reflection characteristics of an optical filter; FIG. 3 is a plan view illustrating an optical filter according to the second embodiment; FIG. FIG. 4A is a diagram illustrating characteristics of an optical filter; FIG. 4B is a diagram illustrating characteristics of an optical filter; FIG. 4C is a diagram illustrating characteristics of an optical filter; FIG. 5A is a diagram illustrating characteristics of an optical filter; FIG. 5B is a diagram illustrating characteristics of an optical filter; FIG. 5C is a diagram illustrating characteristics of an optical filter; FIG. 6 is a plan view illustrating an optical filter according to the first modified example. FIG. 7 is a plan view illustrating an optical filter according to a second modification. FIG. 8 is a plan view illustrating the wavelength tunable laser device according to the third embodiment. 9A is a cross-sectional view along line BB of FIG. 8. FIG. 9B is a cross-sectional view along line CC of FIG. 8. FIG. FIG. 10 is a diagram illustrating reflection characteristics of an optical filter. FIG. 11 is a plan view illustrating the wavelength tunable laser device according to the fourth embodiment. 12 is a cross-sectional view along line DD of FIG. 11. FIG. 13 is a cross-sectional view along line EE of FIG. 11. FIG. FIG. 14 is a diagram illustrating a wavelength tunable laser module according to the sixth embodiment. FIG. 15 is a block diagram showing the hardware configuration of the controller. FIG. 16A is a flowchart illustrating processing executed by a control unit; FIG. 16B is a flowchart illustrating processing executed by a control unit; FIG. 17 is a diagram illustrating a transmittance spectrum. FIG. 18A is a diagram illustrating a transmittance spectrum. FIG. 18B is a flowchart illustrating a process executed by a control unit; FIG. 19 is a plan view illustrating an optical filter according to the seventh embodiment; FIG. 20A is an enlarged view of the multiplexer/demultiplexer. FIG. 20B is an enlarged view of the area. FIG. 21A is a schematic diagram illustrating an optical filter. FIG. 21B is a schematic diagram illustrating an optical filter. FIG. 22A is a diagram illustrating characteristics of a multiplexer/demultiplexer; FIG. 22B is a diagram illustrating characteristics of a multiplexer/demultiplexer; FIG. 23A is a diagram illustrating frequency characteristics of an optical filter; FIG. 23B is a diagram illustrating frequency characteristics of an optical filter; FIG. 24A is a diagram illustrating frequency characteristics of an optical filter; FIG. 24B is a diagram illustrating frequency characteristics of an optical filter;

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

One aspect of the present disclosure is (1) a first loop mirror, a second loop mirror, a first waveguide optically coupled to the first loop mirror and the second loop mirror, and a first access waveguide and, wherein the first loop mirror has a first loop waveguide and a first multiplexer/demultiplexer, and the second loop mirror has a second loop waveguide and a second multiplexer/demultiplexer wherein the first loop waveguide is optically coupled to the first multiplexer/demultiplexer, the second loop waveguide is optically coupled to the second multiplexer/demultiplexer, and the second 1 waveguide is optically coupled with the first multiplexer/demultiplexer and the second multiplexer/demultiplexer, and the first access waveguide is an optical filter optically coupled with the first waveguide. . When light is launched into the first access waveguide, light having a resonant wavelength is reflected back into the first access waveguide. Light having wavelengths other than the resonant wavelength is transmitted. Reflected and transmitted light propagating through the first access waveguide can be used to monitor the properties of the optical filter.
(2) A second waveguide optically coupled to the first loop mirror and the second loop mirror, and a second access waveguide, wherein the second waveguide is the first multiplexing/demultiplexing and the second multiplexer/demultiplexer, and the second access waveguide may be optically coupled with the second waveguide. Reflected and transmitted light propagating through one of the first and second access waveguides can be used to monitor the properties of the optical filter.
(3) The shape of the first multiplexer/demultiplexer is symmetrical, the shape of the second multiplexer/demultiplexer is symmetrical, and the shape of the first waveguide and the shape of the second waveguide may be symmetrical. good. The resonance wavelength of the resonance mode excited in the first access waveguide is equal to the resonance wavelength of the resonance mode excited in the second access waveguide. The FSR of the resonant mode excited in the first access waveguide is equal to the FSR of the resonant mode excited in the second access waveguide. By measuring the resonant wavelength and FSR of light propagating through one of the first access waveguide and the second access waveguide, the characteristics of the optical filter can be monitored.
(4) The first multiplexer/demultiplexer and the second multiplexer/demultiplexer may be 2×2 multimode interference waveguides or directional couplers. Light entering the first loop mirror and the second loop mirror from the first waveguide is reflected to the first waveguide. A resonant mode is excited in the first access waveguide that couples to the first waveguide. Light incident on the first loop mirror and the second loop mirror from the second waveguide is reflected to the second waveguide. A resonant mode is excited in the second access waveguide that couples to the second waveguide. One of the two resonant modes can be extracted from the first access waveguide and the other from the second access waveguide.
(5) The first multiplexer/demultiplexer and the second multiplexer/demultiplexer are directional couplers each having two waveguides, and the first loop waveguide side of the first multiplexer/demultiplexer The distance between the two waveguides in and the distance on the side of the first waveguide and the second waveguide are greater than the distance in the central portion of the first multiplexer/demultiplexer, and the second The distance between the two waveguides on the second loop waveguide side of the multiplexer/demultiplexer and the distance between the first waveguide and the second waveguide side of the second multiplexer/demultiplexer It may be greater than the distance in the central portion. In the first demultiplexer and the second demultiplexer, light distribution becomes nearly uniform over a wide wavelength band. The wavelength dependence of crosstalk can be improved, and crosstalk can be suppressed to a low level in a wide wavelength band.
(6) The two waveguides of the first multiplexer/demultiplexer have bends on the first loop waveguide side and bends on the first waveguide and second waveguide sides. and the two waveguides of the second multiplexer/demultiplexer have bends on the second loop waveguide side and bends on the first waveguide and second waveguide sides. may In the first branching filter and the second branching filter, the distance between the first waveguide and the second waveguide at the bend is larger than the distance between the first waveguide and the second waveguide in the center. In the first demultiplexer and the second demultiplexer, light distribution becomes nearly uniform over a wide wavelength band. The wavelength dependence of crosstalk can be improved, and crosstalk can be suppressed to a low level in a wide wavelength band.
(7) The central portion of the two waveguides of the first multiplexer/demultiplexer may be curved. The distance between the waveguides in the curved portion of the central portion is larger than that in the portion away from the central portion. In the first multiplexer/demultiplexer, light distribution becomes nearly uniform over a wide wavelength band. The wavelength dependence of crosstalk can be improved, and crosstalk can be suppressed to a low level in a wide wavelength band.
(8) provided in at least one of the first loop waveguide and the second loop waveguide to adjust the phase of light propagating in at least one of the first loop waveguide and the second loop waveguide; You may comprise a phase adjustment part. By adjusting the phase of light, the wavelength of light can be changed.
(9) A gain section and two optical filters, wherein the two optical filters are the above optical filters, the two optical filters have different resonance wavelength intervals, and the gain section is optical A tunable laser element having gain and optically coupled to the first access waveguide of each of the two optical filters. Light emitted from the gain section propagates through the first access waveguide and is reflected by the optical filter. The vernier effect of the two optical filters causes the wavelength tunable laser element to laser-oscillate. Light propagating in the second access waveguide can be used to monitor the properties of the optical filter.
(10) The two optical filters may be formed on substrates, the gain section and the substrate may be butt-connected, and a reflecting mirror may be provided on the opposite side of the gain section to the substrate. Light emitted from the gain section propagates through the first access waveguide and is reflected by the optical filter and the reflecting mirror. The wavelength tunable laser element laser-oscillates. Light propagating in the second access waveguide can be used to monitor the properties of the optical filter.
(11) A substrate formed of a III-V group compound semiconductor is provided, the gain section and the two optical filters are monolithically integrated on the substrate, and one of the two optical filters is the gain may be positioned at one end of the gain section, and another one of the two optical filters may be positioned at the other end of the gain section. Light emitted from the gain section propagates through the first access waveguide and is reflected by two optical filters. The wavelength tunable laser element laser-oscillates. Light propagating in the second access waveguide can be used to monitor the properties of the optical filter.
(12) The two optical filters are formed on a substrate and include the first waveguide, the second waveguide, the first loop waveguide, the second loop waveguide, the first access waveguide, and the A second access waveguide is a waveguide of silicon formed on the substrate, one of the two optical filters is located on one end side of the gain section, Another one of them may be located on the other end side of the gain section, and the gain section may be bonded to the surface of the substrate. Light emitted from the gain section propagates through the first access waveguide and is reflected by two optical filters. The wavelength tunable laser element laser-oscillates. Light propagating in the second access waveguide can be used to monitor the properties of the optical filter.
(13) The wavelength tunable laser element, the light source for inputting light into the second access waveguide of the wavelength tunable laser element, and the light receiving element optically coupled to the second access waveguide of the wavelength tunable laser element. is a tunable laser module comprising: Light emitted from the gain section propagates through the first access waveguide and is reflected by the optical filter. The vernier effect of the two optical filters causes the wavelength tunable laser element to laser-oscillate. Light propagating in the second access waveguide can be used to monitor the properties of the optical filter.
(14) In the above control method for the wavelength tunable laser module, the steps of: injecting light from a light source into the second access waveguide of the wavelength tunable laser element; and controlling the wavelength of light propagating through the second access waveguide. Light emitted from the gain section propagates through the first access waveguide and is reflected by the optical filter. The vernier effect of the two optical filters causes the wavelength tunable laser element to laser-oscillate. Light propagating in the second access waveguide can be used to monitor the properties of the optical filter.
(15) In the step of controlling the wavelength of light, the wavelength of light propagating through the second access waveguide is controlled using the phase adjuster based on the intensity of light transmitted through the second access waveguide. It may be a process. Light propagating in the second access waveguide can be used to monitor the properties of the optical filter.
(16) The step of controlling the wavelength of the light includes controlling the wavelength of light emitted from a light source based on the intensity of the light transmitted through the second access waveguide, so that the light propagates through the second access waveguide. may be a step of controlling the wavelength of Light propagating in the second access waveguide can be used to monitor the properties of the optical filter.

[Details of the embodiment of the present disclosure]
Specific examples of an optical filter, a wavelength tunable laser element, a wavelength tunable laser module, and a wavelength tunable laser module control method according to embodiments of the present disclosure will be described below with reference to the drawings. The present disclosure is not limited to these examples, but is indicated by the scope of the claims, and is intended to include all modifications within the scope and meaning equivalent to the scope of the claims.

<First embodiment>
FIG. 1A is a plan view illustrating the optical filter 100 according to the first embodiment. As shown in FIG. 1A, the optical filter 100 is a silicon (Si)-based filter element, and includes an access waveguide 10 (first access waveguide), a waveguide 12 (first waveguide), and two loop mirrors 20. and 25.

The top surface of the substrate 30 extends in the XY plane. Two sides of the substrate 30 extend in the X-axis direction. Two sides of the substrate 30 extend in the Y-axis direction. The X-axis direction is orthogonal to the Y-axis direction. The Z-axis direction is the thickness direction of the substrate 30 and is orthogonal to the X-axis direction and the Y-axis direction. One end of the substrate 30 in the X-axis direction is defined as an end 30a, and the other end is defined as an end 30b.

Access waveguide 10 , waveguide 12 and two loop mirrors 20 and 25 are formed in substrate 30 . The loop mirror 20 (first loop mirror) is located on the edge 30a side of the substrate 30 . A loop mirror 25 (second loop mirror) is located on the end 30 b side and faces the loop mirror 20 . The waveguide 12 extends in the X-axis direction, is located between the loop mirrors 20 and 25 , and is connected to the loop mirrors 20 and 25 .

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). Two loop mirrors 20 and 25 and waveguide 12 form a resonator 11 .

The multiplexers 24 and 28 in the example of FIG. 1A are, for example, 3 dB one-input two-output (1×2) multimode interference waveguides (MMI). The multiplexers/demultiplexers 24 and 28 may be, for example, Y-branch waveguides other than 1×2 MMI. Multiplexers 24 and 28 each have one input end and two output ends. A first end of waveguide 12 is optically coupled to an input end of multiplexer/demultiplexer 24 . A second end of waveguide 12 is optically coupled to the input end of multiplexer/demultiplexer 28 .

A loop waveguide 22 of the loop mirror 20 is a loop-shaped optical waveguide. A first end of the loop waveguide 22 is optically coupled to a first output end 24 c of the multiplexer/demultiplexer 24 . A second end of the loop waveguide 22 is optically coupled to a second output end 24 d of the multiplexer/demultiplexer 24 . The loop mirror 20 has a reflecting structure that reflects the light to the input end of the multiplexer/demultiplexer 24 when the light is input from the input end of the multiplexer/demultiplexer 24 .

A loop waveguide 26 of the loop mirror 25 is a loop-shaped optical waveguide. A first end of the loop waveguide 26 is optically coupled to a first output end 28 c of the multiplexer/demultiplexer 28 . A second end of the loop waveguide 26 is optically coupled to a second output end 28 d of the multiplexer/demultiplexer 28 . The loop mirror 25 has a reflecting structure that reflects light to the input end of the multiplexer/demultiplexer 28 when light is input from the input end of the multiplexer/demultiplexer 28 .

Access waveguide 10 and waveguide 12 are aligned in the Y-axis direction. The end 10 a of the access waveguide 10 is located at the end 30 a of the substrate 30 . Another end 10 b of access waveguide 10 is located at end 30 b of substrate 30 . Access waveguide 10 extends in the X-axis direction and curves toward waveguide 12 . Access waveguide 10 is spaced a distance g from waveguide 12 and is optically coupled to waveguide 12 . The distance g is several hundred nm, for example.

1B is a cross-sectional view along line AA of FIG. 1A illustrating a cross-section of access waveguide 10. FIG. Cross sections of waveguide 12 and loop waveguides 22 and 26 are also similar to FIG. 1B. The substrate 30 is an SOI (Silicon on Insulator) substrate and has a substrate 32 , a clad layer 33 and a waveguide core 34 . The substrate 30 is made of Si, for example. The clad layer 33 is made of silicon oxide (SiO ₂ ), for example, and covers one surface of the substrate 30 . The waveguide core 34 is made of Si, for example, is separated from the substrate 30 and is buried inside the clad layer 33 . The distance from the upper surface of the substrate 32 to the lower end of the waveguide core 34 is, for example, 2 μm. The width of the waveguide core 34 in the Y-axis direction is, for example, 0.44 μm. The thickness of the waveguide core 34 is, for example, 0.22 μm.

One of the ends 10 a and 10 b of the access waveguide 10 (for example, the end 10 a ) is used as the input port of the optical filter 100 . Light from a light source (not shown) arranged outside the optical filter 100 enters the optical filter 100 through the end portion 10a as indicated by an arrow A1 in FIG. 1A. A resonance mode is excited in the resonator 11 by the incidence of light on the access waveguide 10 . As indicated by an arrow A2 in FIG. 1A, light having a resonance wavelength (reflected light) is reflected toward the end 10a of the access waveguide 10 and exits the optical filter 100 from the end 10a. As indicated by an arrow A3, light (transmitted light) having a wavelength other than the resonant wavelength is transmitted through the optical filter 100 and emitted from the end portion 10b of the access waveguide 10 to the outside of the optical filter 100. FIG.

More specifically, light propagates through access waveguide 10 and hops from access waveguide 10 to waveguide 12 . Light propagating through the waveguide 12 is distributed to the two output ends 24 c and 24 d of the multiplexer/demultiplexer 24 and propagates through the loop waveguide 22 . The intensity of the light distributed is 1:1 and the phases are equal. The light propagating clockwise through the loop waveguide 22 and the light propagating counterclockwise through the loop waveguide 22 join in the same phase and enter the waveguide 12 from the multiplexer/demultiplexer 24 .

Light propagating through the waveguide 12 is distributed to the two output ends 28 c and 28 d of the multiplexer/demultiplexer 28 and propagates through the loop waveguide 26 . The light propagating clockwise through the loop waveguide 26 and the light propagating counterclockwise through the loop waveguide 26 join in the same phase and enter the waveguide 12 from the multiplexer/demultiplexer 28 .

The resonance wavelength of the resonator 11 is the wavelength at which the phase change of light when going around the two loop mirrors 20 and 25 is 2πn (n is an integer). Light having a resonant wavelength hops onto access waveguide 10 from waveguide 12 and is reflected toward end 10a of access waveguide 10 as indicated by arrow A2 in FIG. 1A. Light having a wavelength other than the resonant wavelength is emitted from the end 10b of the access waveguide 10, as indicated by arrow A3 in FIG. 1A.

That is, the optical filter 100 is a filter that reflects light of the resonance wavelength and transmits light having a wavelength other than the resonance wavelength. The characteristics of the optical filter 100 can be monitored by receiving reflected light or transmitted light from the optical filter 100 with a light receiving element such as a photodiode.

2A and 2B are diagrams illustrating reflection characteristics of the optical filter 100. FIG. The horizontal axis represents the wavelength of light. The vertical axis represents the reflectance of the optical filter 100 . As shown in FIGS. 2A and 2B, the reflectance peaks at the resonance wavelength of the resonator 11. FIG. The interval between two adjacent resonant wavelengths (Free Spectral Range, FSR) is determined by the optical path length of the resonator 11 (waveguide 12, loop mirrors 20 and 25). The resonant wavelength in FIG. 2A is equal to the resonant wavelength in FIG. 2B. The FSR in FIG. 2A is equal to the FSR in FIG. 2B. Let the value of FSR in FIGS. 2A and 2B be FSR1.

On the other hand, the width of the peak in FIG. 2A is wider than the width of the peak in FIG. 2B. The width of the peak is determined by the Q value of the resonator 11. FIG. The Q value is determined by the coupling ratio between access waveguide 10 and resonator 11 . When the distance g between the access waveguide 10 and the waveguide 12 is reduced, the coupling rate between the access waveguide 10 and the waveguide 12 is increased and the Q value is decreased. The lower the Q value, the broader and more gradual the peak, as in FIG. 2A. Increasing the distance g decreases the coupling rate and increases the Q value. The peak becomes narrower and sharper as in FIG. 2B.

According to the first embodiment, optical filter 100 comprises access waveguide 10 , waveguide 12 , loop mirrors 20 and 25 . Waveguide 12 and loop mirrors 20 and 25 are optically coupled to form resonator 11 . When light enters the end portion 10 a of the access waveguide 10 , the light having the resonance wavelength is reflected by the resonator 11 and enters the end portion 10 a of the access waveguide 10 . Light having a wavelength other than the resonant wavelength is transmitted through the optical filter 100 and emitted from the end 10b of the access waveguide 10. FIG. By detecting reflected light or transmitted light of the optical filter 100, the characteristics of the optical filter 100 can be monitored.

The reflectance or transmittance of the optical filter 100 is measured while changing the wavelength of light incident on the access waveguide 10 . As in FIGS. 2A and 2B, the wavelength at which the reflectance is maximum is the resonant wavelength. By measuring the reflectance spectrum, the resonance wavelength and the reflectance peak spacing (FSR) can be directly measured. By changing the distance g between the access waveguide 10 and the waveguide 12, the Q value and the coupling ratio can be adjusted. The shape of the peaks can be varied as in FIGS. 2A and 2B. Transmitted light may be used to monitor the properties of the optical filter 100 . The wavelength at which the transmittance is lowest is the resonant wavelength.

The refractive index of the optical filter 100 may change over time. A change in the refractive index may change the refractive index of the optical waveguide. The change in refractive index changes the characteristics of the optical filter 100 . Light propagating through access waveguide 10 can be used to directly monitor the properties of optical filter 100 . By measuring the reflectance, changes in properties such as shifts in resonant wavelength can be accurately detected.

As shown in FIG. 1B, the optical waveguide of optical filter 100 has a waveguide core 34 of Si. A waveguide core 34 is surrounded by a clad layer 33 . The refractive index of Si is approximately 3.5. The refractive index of _SiO2 is about 1.4. Light can be strongly confined in the waveguide core 34 having a higher refractive index than the cladding layer 33 . Light loss in bent optical waveguides, such as access waveguide 10 and loop waveguides 22 and 26, is suppressed. The optical waveguide of the optical filter 100 may be a waveguide made of Si as shown in FIG. 1B, or a waveguide made of a compound semiconductor as described in FIG. As shown in FIG. 8 and the like, an electrode 35 may be provided on the optical waveguide. The shape of the loop waveguide 22 may be the same as that of the loop waveguide 26, or may be different.

<Second embodiment>
FIG. 3 is a plan view illustrating an optical filter 200 according to the second embodiment. Description of the same configuration as in the first embodiment is omitted. As shown in FIG. 3, optical filter 200 comprises two access waveguides 10 and 16, two waveguides 12 and 14, and two loop mirrors 20 and 25. FIG. Waveguide 12 , loop mirrors 20 and 25 form resonator 11 . Waveguide 14 , loop mirrors 20 and 25 form resonator 13 . The optical waveguide of the optical filter 200 has the same configuration as in FIG. 1B.

From one end (lower end in FIG. 3) to the other end (upper end in FIG. 3) of substrate 30 in the Y-axis direction, access waveguide 10, waveguide 12, guide Waveguide 14 and access waveguide 16 are in series. Access waveguide 10 and waveguide 12 curve toward each other. Access waveguide 10 is optically coupled to waveguide 12 and is spaced apart by a distance g1. Access waveguide 16 and waveguide 14 curve toward each other. Access waveguide 16 is optically coupled to waveguide 14 and is spaced apart by a distance g2.

End 10 a of access waveguide 10 and end 16 a of access waveguide 16 are located at end 30 a of substrate 30 . End 10 b of access waveguide 10 and end 16 b of access waveguide 16 are located at end 30 b of substrate 30 .

Waveguides 12 and 14 are located between loop mirror 20 and loop mirror 25 in the X-axis direction and are connected to loop mirror 20 and loop mirror 25 . The waveguides 12 and 14 are symmetrical in the Y-axis direction (vertical direction in FIG. 3) in which the waveguides 12 and 14 are aligned. In other words, waveguides 12 and 14 are symmetrical about the X-axis. That is, when the waveguide 14 is folded back about the X-axis, it overlaps the waveguide 12 .

Multiplexers 24 and 28 are each 3 dB 2×2 MMI couplers. A first end of waveguide 12 is optically coupled to a first input end 24 a of multiplexer/demultiplexer 24 . A second end of waveguide 12 is optically coupled to a first input end 28 a of multiplexer/demultiplexer 28 . A first end of waveguide 14 is optically coupled to a second input end 24 b of multiplexer/demultiplexer 24 . A second end of waveguide 14 is optically coupled to a second input end 28 b of multiplexer/demultiplexer 28 .

The shape of the multiplexer/demultiplexer 24 is symmetrical in the vertical direction of FIG. That is, the shape of the multiplexer/demultiplexer 24 is symmetrical with respect to the X axis. Similar to the multiplexer/demultiplexer 24, the shape of the multiplexer/demultiplexer 28 is symmetrical in the vertical direction.

Light from a light source (not shown) arranged outside the optical filter 200 enters one end (for example, the end 10a) of the access waveguide 10 as indicated by an arrow A1 in FIG. Light enters one end of access waveguide 16 (eg, end 16b), as indicated by arrow A4.

Light incident on access waveguide 10 transfers to waveguide 12 . The light propagating through waveguide 12 enters multiplexers/demultiplexers 24 and 28 . The light entering the multiplexer/demultiplexer 24 from the waveguide 12 is distributed to the two output ends 24 c and 24 d of the multiplexer/demultiplexer 24 and propagates through the loop waveguide 22 . The ratio of the intensity of the distributed light is 1:1. The phase of the light output from the output end 24d is delayed by 90° with respect to the phase of the light output from the output end 24c. The light propagating clockwise through the loop waveguide 22 from the output end 24 c returns to the input end 24 a of the multiplexer/demultiplexer 24 with a phase delay of 90° and enters the waveguide 12 . Light propagating counterclockwise through the loop waveguide 22 from the output end 24 d returns to the input end 24 a of the multiplexer/demultiplexer 24 and enters the waveguide 12 . That is, in principle, the light entering the loop waveguide 22 from the waveguide 12 is reflected to the waveguide 12 through the input end 24 a without entering the waveguide 14 .

The light incident on the multiplexer/demultiplexer 28 from the waveguide 12 is distributed to the two output ends 28c and 28d of the multiplexer/demultiplexer 28 with an intensity ratio of 1:1 and a phase difference of 90°. propagates In principle, the light entering the loop waveguide 26 from the waveguide 12 is reflected by the waveguide 12 without entering the waveguide 14 . Light in waveguide 12 hops onto access waveguide 10 and propagates toward end 10a.

Light entering the access waveguide 16 is transferred to the waveguide 14 and enters the multiplexers/demultiplexers 24 and 28 . The light entering the multiplexer/demultiplexer 24 from the waveguide 14 is distributed to the two output ends 24c and 24d of the multiplexer/demultiplexer 24 with an intensity ratio of 1:1 and a phase difference of 90°. propagates In principle, the light entering the loop waveguide 22 from the waveguide 14 is reflected by the waveguide 14 without entering the waveguide 12 . In principle, light entering the loop waveguide 26 from the waveguide 14 through the multiplexer/demultiplexer 28 is reflected by the waveguide 14 without entering the waveguide 12 . Light in waveguide 14 hops onto access waveguide 16 and propagates toward end 16b.

That is, the resonance mode of the resonator 11 can be excited by entering light from the end 10a of the access waveguide 10 as indicated by the arrow A1 in FIG. The resonant mode of resonator 11 propagates through loop waveguides 22 and 26 and waveguide 12 and is reflected toward end 10a of access waveguide 10 as indicated by arrow A2. The resonant mode of resonator 11 does not propagate to waveguide 14 and access waveguide 16 . Light having a wavelength other than the resonance wavelength among the light propagating through the access waveguide 10 is transmitted through the optical filter 200 as indicated by arrow A3, and emitted to the outside of the optical filter 200 from the end portion 10b.

By entering light from the end 16b of the access waveguide 16 as indicated by an arrow A4, the resonance mode of the resonator 13 can be excited. The resonant mode of resonator 13 propagates through loop waveguides 22 and 26 and waveguide 14 and is reflected toward end 16b of access waveguide 16 as indicated by arrow A5. The resonant mode of resonator 13 does not propagate to waveguide 12 and access waveguide 10 . Of the light propagating through the access waveguide 16, the light having a wavelength other than the resonant wavelength is transmitted through the optical filter 200 as indicated by arrow A6 and emitted from the end 16a to the outside.

When the multiplexer/demultiplexer 24 is symmetrical with respect to the X axis, the multiplexer/demultiplexer 28 is symmetrical, and the waveguides 12 and 14 are symmetrical, the resonance wavelength of the resonance mode of the resonator 11 and the resonator The resonant wavelengths of the 13 resonant modes coincide in principle. The FSR of the resonance mode of the resonator 11 and the FSR of the resonance mode of the resonator 13 match in principle. If the resonance wavelength and FSR of one of the resonance mode of the resonator 11 and the resonance mode of the resonator 13 are known, the resonance wavelength and FSR of the other are also known.

Reflected light or transmitted light from the optical filter 200 propagating through the access waveguide 16 is detected by a light receiving element, and the resonance wavelength and FSR of the resonance mode of the resonator 13 are measured. From the results of this measurement, it is also possible to monitor the resonance wavelength and FSR of the resonance mode of the resonator 11 without detecting the reflected light and transmitted light of the access waveguide 10 . Access waveguide 16 can be used to monitor the properties of optical filter 200, and access waveguide 10 can be used for applications other than monitoring, such as lasing.

4A to 5C are diagrams illustrating characteristics of the optical filter 200, and are simulation results of the characteristics. The horizontal axis represents the wavelength of light. The vertical axis represents reflectance and transmittance (reflectance/transmittance (a.u.)). A solid line represents the transmittance. The dashed line represents reflectance. The dotted line represents crosstalk between access waveguide 10 and access waveguide 16 . Crosstalk may occur when the distribution ratios of the multiplexers/demultiplexers 24 and 28 change according to the wavelength of light. 4A-4C are reflectance and transmittance spectra obtained using the access waveguide 10. FIG. 4A-4C, crosstalk is the ratio of the intensity of light emitted from access waveguide 16 (arrow A5) to the intensity of light incident on access waveguide 10 (arrow A3 in FIG. 3). 5A-5C are reflectance and transmittance spectra obtained using access waveguide 16. FIG. 5A-5C, crosstalk is the ratio of light exiting access waveguide 10 (arrow A3) to light entering access waveguide 16 (arrow A4 in FIG. 3).

In the examples of FIGS. 4A and 5A, the distance g1 between the access waveguide 10 and the waveguide 12 is 300 nm. In the examples of FIGS. 4B and 5B, the distance g1 is 250 nm. In the examples of FIGS. 4C and 5C, the distance g1 is 350 nm. In both examples, the distance g2 between the access waveguide 16 and the waveguide 14 is 300 nm.

As shown in FIGS. 4A to 5C, at four wavelengths ranging from 1548 nm to 1552 nm, reflectance has maxima and transmittance has minima. The wavelength is the resonant wavelength. In six examples, resonant wavelength and FSR are equal to each other.

The peaks of the reflectance spectrum in FIG. 4C are sharper than those in FIGS. 4A and 4B. The peaks in FIG. 4A are steeper than those in FIG. 4B. The peaks in FIG. 4B are milder than those in FIGS. 4A and 4C. When the distance g1 between the access waveguide 10 and the waveguide 12 is reduced, the shape of the spectral peak becomes gentle. This is because when the distance g1 is reduced, the coupling rate increases and the Q value decreases. Increasing the distance g1 makes the shape of the peak steeper. This is because when the distance g1 is increased, the coupling rate is decreased and the Q value is increased. 5A-5C, the distance g2 between access waveguide 16 and waveguide 14 is constant. Since the coupling rate and Q value are constant, the peak shape of the spectrum is also almost constant. Moreover, while the reflectance is 1 at maximum, the crosstalk is smaller than the reflectance by two orders of magnitude or more.

According to the second embodiment, optical filter 200 has access waveguides 10 and 16 , waveguides 12 and 14 , and loop mirrors 20 and 25 . Loop mirrors 20 and 25 and waveguide 12 form resonator 11 . Loop mirrors 20 and 25 and waveguide 14 form resonator 13 . When light is incident from the end portion 10 a of the access waveguide 10 , the light of the resonance wavelength is reflected at the end portion 10 a of the access waveguide 10 . When light is incident from the end 16 b of the access waveguide 16 , the light of the resonant wavelength is reflected at the end 16 b of the access waveguide 16 . Light having wavelengths other than the resonant wavelength is transmitted through optical filter 200 and propagates through access waveguides 10 and 16 .

By sensing the reflected or transmitted light of optical filter 200 propagating in one of access waveguides 10 and 16, the properties of optical filter 200 can be monitored. For example, access waveguide 16 can be used for monitoring and access waveguide 10 can be used for a different application than monitoring.

The waveguides 12 and 14 are symmetrical in the Y-axis direction in which the waveguides 12 and 14 are aligned. The shape of the multiplexer/demultiplexer 24 is symmetrical. The shape of the multiplexer/demultiplexer 28 is symmetrical. The resonance wavelength of the resonance mode of the resonator 11 matches the resonance wavelength of the resonance mode of the resonator 13 . The resonance mode FSR of the resonator 11 matches the resonance mode FSR of the resonator 13 . By measuring the resonance wavelength and FSR of the resonance mode of the resonator 13 using the access waveguide 16, the resonance wavelength and FSR of the resonance mode of the resonator 11 can also be monitored. Access waveguide 10 can be used for applications other than monitoring. 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 symmetrical with respect to the center of the multiplexer/demultiplexer 28 itself.

A change in the optical filter 200 over time may change the refractive index of the optical waveguide. The characteristics of the optical filter 200 such as the peak wavelength (resonance wavelength) change. Since the light propagating through the access waveguide 16 can be used to monitor the properties of the optical filter 200, changes in properties can be accurately detected.

The multiplexers/demultiplexers 24 and 28 may be 2×2 MMI or directional couplers. A resonance mode excited by light entering access waveguide 10 propagates through loop mirrors 20 and 25 , waveguide 12 and access waveguide 10 , but does not propagate through waveguide 14 and access waveguide 12 . A resonant mode excited by light entering access waveguide 16 propagates through loop mirrors 20 and 25, waveguide 14 and access waveguide 16, but does not propagate through waveguide 12 and access waveguide 10. FIG. One of the two resonant modes can be extracted from one access waveguide.

The coupling ratio between the access waveguide 10 and the waveguide 12 is determined by the distance g1 between the access waveguide 10 and the waveguide 12 . By changing the distance g1, the shape of the spectrum can be changed, as shown in FIGS. 4A to 4C. The coupling ratio between the access waveguide 16 and the waveguide 14 is determined by the distance g2 between the access waveguide 16 and the waveguide 14 . The distance g2 can be determined independently of the distance g1. Therefore, the spectral shape of resonator 11 and the spectral shape of resonator 13 can be adjusted independently of each other. The coupling ratio between access waveguide 10 and waveguide 12 may be equal to the coupling ratio between access waveguide 16 and waveguide 14, or may be of different magnitudes. Light having high intensity propagating through the resonators 11 and 13 may cause nonlinear optical effects and destabilize the characteristics. Distances g1 and g2 can be varied to adjust the coupling ratio so that the intensity of light in resonators 11 and 13 is appropriate. If the distribution ratio of the multiplexer/demultiplexer deviates from the design value, there is a risk that mixing (crosstalk) will occur between the monitor light and the oscillation light. The coupling ratio can also be adjusted according to the actual size of the distribution ratio of the multiplexer/demultiplexer.

(Modification)
FIG. 6 is a plan view illustrating an optical filter 200a according to the first modification. Waveguides 12 and 14 are not curved and extend parallel to the X-axis direction. Other configurations are the same as in FIG.

FIG. 7 is a plan view illustrating an optical filter 200b according to a second modification. Directional couplers are used as multiplexers/demultiplexers 24 and 28 instead of MMI. In a directional coupler, two optical waveguides are brought close to each other by a distance of about the wavelength of light. A directional coupler distributes the light similar to a 2×2 MMI. Other configurations are the same as in FIG.

<Third Embodiment>
FIG. 8 is a plan view illustrating a wavelength tunable laser device 300 according to the third embodiment. Description of the same configuration as in the first embodiment or the second embodiment is omitted. As shown in FIG. 8, wavelength tunable laser device 300 includes gain section 40 and filter device 310 . Filter element 310 is a passive element made of Si or the like, and has substrate 30 .

The gain section 40 is a light-emitting element made of a III-V compound semiconductor, butt-connected to the end portion 30 a of the substrate 30 of the filter element 310 and optically coupled with the filter element 310 . A reflecting mirror 59 is provided on the opposite side of the gain section 40 from the filter element 310 . The reflector 59 is, for example, a distributed Bragg reflector (DBR). Gain section 40 and reflector 59 form a monolithically integrated integrated device.

(filter element)
Filter element 310 has waveguide 15, multiplexer/demultiplexer 17, and two optical filters 200-1 and 200-2. Optical filters 200-1 and 200-2 each have the same configuration as optical filter 200 in FIG. Optical filter 200-1 has access waveguides 10-1 and 16-1. Optical filter 200-2 has access waveguides 10-2 and 16-2. The optical filters 200-1 and 200-2 are arranged in the Y-axis direction.

The waveguide 15 extends in the X-axis direction. A first end of waveguide 15 is located at end 30 a of substrate 30 . A second end of waveguide 15 is optically coupled to an input end of multiplexer/demultiplexer 17 . The multiplexer/demultiplexer 17 is, for example, a 3 dB 1×2 MMI coupler. A first output end of the multiplexer/demultiplexer 17 is optically coupled to one end of the access waveguide 10-1 of the optical filter 200-1. A second output end of the multiplexer/demultiplexer 17 is optically coupled to one end of the access waveguide 10-2 of the optical filter 200-2.

End 10c of access waveguide 10-1, end 10d of access waveguide 10-2, end 16b of access waveguide 16-1, and end 16d of access waveguide 16-2 are the ends of substrate 30. located in section 30b. End 16 a of access waveguide 16 - 1 and end 16 c of access waveguide 16 - 2 are located at end 30 a of substrate 30 .

Since the optical path length of the loop waveguide of optical filter 200-1 differs from the optical path length of the loop waveguide of optical filter 200-2, the FSR of optical filter 200-1 differs from the FSR of optical filter 200-2. .

The two loop waveguides of the optical filter 200-1, the access waveguide 10-1, and the two loop waveguides of the optical filter 200-2 are each provided with an electrode 35 (phase adjuster). Of the optical waveguides of optical filters 200-1 and 200-2, portions where electrodes 35 are not provided have the same configuration as in FIG. 1B.

FIG. 9A is a cross-sectional view along line BB of FIG. 8, illustrating a cross-section of access waveguide 10-1 of optical filter 200-1. The description of the same configuration as in FIG. 1B of the first embodiment is omitted. As shown in FIG. 9A, a waveguide core 34 and an electrode 35 are embedded in the cladding layer 33 in order from the substrate 32 side. The electrode 35 is spaced apart from the waveguide core 34 and positioned above the waveguide core 34 . The electrode 35 is made of metal such as an alloy (nichrome) of nickel (Ni) and chromium (Cr), and functions as a heater. By applying a current to the electrode 35, the electrode 35 generates heat. By heating the waveguide core 34 by the electrode 35 , the refractive index of the waveguide core 34 changes, and the phase of light propagating through the waveguide core 34 can be adjusted. A portion of the optical waveguide of the filter element 310 where the electrode 35 is provided has the same configuration as in FIG. 9A.

(gain section)
FIG. 9B is a cross-sectional view along line CC of FIG. 8 and illustrates a cross-section of the gain section 40. FIG. As shown in FIG. 9B, the gain section 40 has a substrate 42, clad layers 43, 45 and 46, an active layer 44, a contact layer 47, a buried layer 48 and a current blocking layer 49. As shown in FIG. The active layer 44 of the gain section 40 is located at the same height as the waveguide core 34 of the waveguide 15 and faces the waveguide core 34 .

A cladding layer 43 , an active layer 44 and a cladding layer 45 are sequentially stacked on the upper surface of the substrate 42 , and these semiconductor layers form the mesa 41 . The mesa 41 protrudes from the substrate 42 in the Z-axis direction and extends in the X-axis direction. The width of the upper end of the mesa 41 in the Y-axis direction is, for example, 1.5 μm. Embedded layers 48 are provided on both sides of the mesa 41 in the Y-axis direction. A current blocking layer 49 is provided on the buried layer 48 . Two buried layers 48 and two current blocking layers 49 sandwich the mesa 41 from both sides in the Y-axis direction. The width from the side surface of one embedded layer 48 to the side surface of the other embedded layer 48 is, for example, 3 μm. The cladding layer 46 and the contact layer 47 are laminated on the cladding layer 45 and the current blocking layer 49 in this order. The height from the top surface of the substrate 42 to the top surface of the contact layer 47 is, for example, 3 μm.

The insulating film 38 covers the top surface of the substrate 42 , the side surfaces of the mesa 41 and the top surface of the mesa 41 . The insulating film 38 has an opening on the upper surface of the mesa 41 . The electrode 37 is provided on the mesa 41 and contacts the upper surface of the contact layer 47 exposed through the opening of the insulating film 38 . An electrode 36 is provided on the bottom surface of the substrate 42 opposite to the mesa 41 .

The substrate 42, clad layer 43, and current blocking layer 49 are made of, for example, n-type indium phosphide (InP). The clad layers 45 and 46 are made of p-type InP, for example. The contact layer 47 is made of, for example, p-type indium gallium arsenide (InGaAs). The active layer 44 includes, for example, a plurality of alternately stacked well layers and barrier layers, and has a multiple quantum well structure (MQW: Multi Quantum Well). The well layers and barrier layers are made of undoped indium gallium arsenide phosphide (i-InGaAsP), for example. The semiconductor layer may be formed of III-V compound semiconductors other than those described above.

The insulating film 38 is made of an insulator such as silicon nitride (SiN). The electrode 36 is an n-type electrode formed of a laminate (Au/Ge/Ni) in which gold, germanium, and nickel are laminated in order from the substrate 42, for example. The electrode 37 is a p-type electrode formed of a laminate (Ti/Pt/Au) in which titanium, platinum, and gold are laminated in order from the contact layer 47 side, for example.

A current is injected into gain section 40 by applying a voltage to electrodes 36 and 37 . Since the n-type substrate 42 , p-type buried layer 48 , n-type current blocking layer 49 and p-type clad layer 46 are laminated on both sides of the mesa 41 , current does not flow outside the mesa 41 . It is difficult and selectively flows to the mesa 41 . Injection of current into the active layer 44 causes the gain section 40 to emit light.

As indicated by an arrow A7 in FIG. 8, the light incident on the filter element 310 from the gain section 40 propagates through the waveguide 15, and passes through the access waveguides 10-1 and 10-2 at the multiplexer/demultiplexer 17. distributed to When light propagates through the access waveguide 10-1, a resonance mode is excited in the optical filter 200-1, and the light is reflected to the access waveguide 10-1. When light propagates through the access waveguide 10-2, a resonance mode is excited in the optical filter 200-2, and the light is reflected to the access waveguide 10-2. The reflected light from optical filter 200-1 and the reflected light from optical filter 200-2 are multiplexed by multiplexer/demultiplexer 17 and propagate through waveguide 15 as indicated by arrow A8. As the light is repeatedly reflected by the filter element 310 and the reflecting mirror 59, the wavelength tunable laser element 300 performs laser oscillation.

The FSR of the reflection spectrum of optical filter 200-1 is different from the FSR of the reflection spectrum of optical filter 200-2. The FSR of optical filter 200-1 is, for example, FSR1 shown in FIGS. 2A and 2B. FIG. 10 is a diagram illustrating reflection characteristics of the optical filter 200-2. FSR2 of optical filter 200-2 is greater than FSR1 of optical filter 200-1. Using the vernier effect of the two optical filters 200-1 and 200-2, laser oscillation is performed. The wavelength tunable laser device 300 laser-oscillates at a wavelength at which the resonant wavelength of the optical filter 200-1 and the resonant wavelength of the optical filter 200-2 match. As indicated by arrows A9 and A10 in FIG. 8, laser light propagates through access waveguides 10-1 and 10-2 and is emitted outside wavelength tunable laser element 300 from ends 10c and 10d. Laser light does not propagate through access waveguides 16-1 and 16-2.

Light is emitted from the gain section 40, and light is incident on the access waveguides 16-1 and 16-2 from a light source outside the wavelength tunable laser element 300. FIG. Resonant modes are excited in the optical filters 200-1 and 200-2, and reflected light is reflected in the access waveguides 16-1 and 16-2. For example, as indicated by an arrow A11, light enters from the end 16a of the access waveguide 16-1. As indicated by arrow A12, reflected light from optical filter 200-1 is emitted from end portion 16a. As indicated by arrow A13, light transmitted through optical filter 200-1 is emitted from end 16b. As indicated by arrows A14 and A15, when light enters from end 16c of access waveguide 16-2, reflected light from optical filter 200-2 is emitted from end 16c. As indicated by arrow A16, light transmitted through optical filter 200-2 is emitted from end portion 16d.

The resonance wavelength and FSR of the resonance mode occurring in the access waveguide 16-1 match the resonance wavelength and FSR of the resonance mode occurring in the access waveguide 10-1. The resonance wavelength and FSR of the resonance mode occurring in the access waveguide 16-2 match the resonance wavelength and FSR of the resonance mode occurring in the access waveguide 10-2. For example, by measuring the spectrum of the transmitted light of the optical filter 200-1 propagating through the access waveguide 16-1 and the transmitted light of the optical filter 200-2 propagating through the access waveguide 16-2, the optical filter 200- 1 and 200-2 characteristics can be monitored. The reflected light propagating through the access waveguide 10-1 and the reflected light propagating through the access waveguide 10-2 are not used for characteristic monitoring, but are used for laser oscillation.

According to the third embodiment, gain section 40 is butt-connected to filter element 310 and optically connected to access waveguide 10-1 of optical filter 200-1 and access waveguide 10-2 of optical filter 200-2. Join. Light enters optical filters 200-1 and 200-2 from gain section 40 through access waveguides 10-1 and 10-2, and is reflected from optical filters 200-1 and 200-2 to gain section 40. FIG. By reflecting light between the optical filters 200-1 and 200-2 and the reflecting mirror 59, laser oscillation is possible.

The light propagating through access waveguide 16-1 can be used to monitor the characteristics of optical filter 200-1, and the light propagating through access waveguide 16-2 can be used to monitor the characteristics of optical filter 200-2. . Light emitted from the gain section 40 does not propagate through the access waveguides 16-1 and 16-2, but reflected light and transmitted light from the optical filter propagate. The characteristics of optical filters 200-1 and 200-2 can be directly monitored independently of laser oscillation. By adjusting the characteristics of optical filters 200-1 and 200-2, the oscillation wavelength of wavelength tunable laser device 300 can be controlled accurately.

By applying a current to the electrode 35 and heating the first access waveguide 10-1 and the loop waveguide, the refractive index of the waveguide is changed. Since the optical path length also changes due to the change in the refractive index, the phase of light can be adjusted. The wavelength of light reflected by optical filters 200-1 and 200-1 can be adjusted. Since the relationship between the current flowing through the electrode 35 and the refractive index is linear, the refractive index can be controlled with high accuracy.

For example, heating by the electrodes 35 may cause the waveguide core 34 to deteriorate. Such changes over time change the characteristics of the optical filters 200-1 and 200-2. According to the third embodiment, it is possible to monitor the characteristics of the optical filters 200-1 and 200-2 and detect changes in the characteristics, such as shifts in resonant wavelength. The resonant wavelength is controlled by, for example, adjusting the voltage applied to the electrode 35 according to the change in characteristics. The wavelength tunable laser element 300 can laser-oscillate at the wavelength at which the reflectance of the optical filters 200-1 and 200-2 peaks. It is possible to control the oscillation wavelength accurately and stably.

Light may enter the access waveguide 16-1 from one of the ends 16a and 16b. Light may enter the access waveguide 16-2 from one of the ends 16c and 16d. Electrode 35 may be provided, for example, on access waveguide 10-2. Although the filter element 310 is made of an SOI substrate and has an Si optical waveguide, it may be made of, for example, a compound semiconductor other than Si.

<Fourth Embodiment>
FIG. 11 is a plan view illustrating a wavelength tunable laser device 400 according to the fourth embodiment. A description of the same configuration as that of any one of the first to third embodiments will be omitted. In wavelength tunable laser element 400, gain section 40 and two optical filters 200-1 and 200-2 are monolithically integrated on one substrate . Gain section 40 has the same configuration as in FIG. 9B. One end of the substrate 42 in the X-axis direction is defined as an end 42a, and the other end is defined as an end 42b.

As shown in FIG. 11, optical filter 200-1 is positioned on one side of gain section 40 in the X-axis direction, and optical filter 200-2 is positioned on the other side. Access waveguide 10-1 of optical filter 200-1 and access waveguide 10-2 of optical filter 200-2 face each other in the X-axis direction. One end of access waveguide 10 - 1 is optically coupled to gain section 40 . The other end of access waveguide 10-1 does not reach end 42a of substrate 42; One end 10 e of the access waveguide 10 - 2 is located at the end 42 b of the substrate 42 and functions as an output port of the tunable laser device 400 . The other end of access waveguide 10 - 2 is optically coupled to gain section 40 .

The end 16a of the access waveguide 16-1 of the optical filter 200-1 and the end 16c of the access waveguide 16-2 of the optical filter 200-2 are located at the end 42a of the substrate . End 16b of access waveguide 16-1 and end 16d of access waveguide 16-2 are located at end 42b of substrate . The FSR of optical filter 200-1 is different from the FSR of optical filter 200-2.

FIG. 12 is a cross-sectional view taken along line DD in FIG. 11, showing a cross-section of a portion of the loop waveguide of the optical filter 200-1 where the electrode 35 is provided. As shown in FIG. 12, a clad layer 50, a core layer 51, and a clad layer 52 are laminated in this order on the upper surface of the substrate . The clad layer 50, core layer 51, and clad layer 52 form a high-mesa optical waveguide. The core layer 51 is positioned at the same height as the active layer 44 of the gain section 40 . The width of the clad layer 50, the core layer 51, and the clad layer 52 in the Y-axis direction is equal to the width of the mesa 41 of the gain section 40, eg, 1.5 μm. The height from the top surface of the substrate 42 to the top surface of the clad layer 52 is, for example, 3 μm.

The insulating film 38 covers the upper surface of the substrate 42 , the side surfaces of the clad layer 50 , the core layer 51 and the clad layer 52 , and the upper surface of the clad layer 52 . An electrode 35 is provided on the upper surface of the insulating film 38 on the cladding layer 52 . An electrode 36 is provided on the back surface of the substrate 42 opposite to the clad layer 50 .

The cladding layer 50 is made of, for example, n-type InP. The core layer 51 is made of InGaAsP, for example. The cladding layer 52 is made of, for example, p-type InP. The insulating film 38 is made of SiN, for example. The electrode 35 is made of metal such as nichrome. The portions of the optical waveguides of the optical filters 200-1 and 200-2 where the electrodes 35 are provided have the same configuration as in FIG. A portion of the optical waveguide where the electrode 35 is not provided has a configuration obtained by removing the electrode 35 from FIG.

By applying a voltage to electrodes 36 and 37 and injecting a current into gain section 40, gain section 40 emits light. As indicated by arrows A17 and A19 in FIG. 11, light is emitted from both ends of gain section 40 in the X-axis direction and propagates through access waveguides 10-1 and 10-2. Optical filters 200-1 and 200-2 reflect light toward gain section 40, as indicated by arrows A18 and A20. Light is repeatedly reflected by the two optical filters 200-1 and 200-2, causing the wavelength tunable laser element 400 to oscillate. The oscillation wavelength is determined by the vernier effect of the two optical filters 200-1 and 200-2. As indicated by arrow A21, the laser light is emitted to the outside of wavelength tunable laser device 400 from end 10e of access waveguide 10-2. By adjusting the phase of the light in the optical filters 200-1 and 200-2 using the electrode 35, the oscillation wavelength can be changed.

As indicated by arrow A22 in FIG. 11, light from an external light source enters access waveguide 16-1 of optical filter 200-1 through end 16a. Light enters the access waveguide 16-2 of the optical filter 200-2 through the end 16d, as indicated by an arrow A23. Light having a resonant wavelength is reflected by optical filters 200-1 and 200-2. As indicated by arrows A24 and A25, reflected light exits from end 16a or 16d. Light having a wavelength other than the resonant wavelength is transmitted through optical filters 200-1 and 200-2. As indicated by arrows A26 and A27, transmitted light exits from ends 16b or 16c. Transmitted or reflected light propagating through access waveguides 16-1 and 16-2 can be used to monitor the properties of optical filters 200-1 and 200-2.

According to the fourth embodiment, the gain section 40 and the two optical filters 200-1 and 200-2 are monolithically integrated. Light emitted from gain section 40 propagates through access waveguides 10-1 and 10-2 and is reflected by optical filters 200-1 and 200-2. Light is reflected by optical filters 200-1 and 200-2, so that wavelength tunable laser device 400 performs laser oscillation. The light propagating through access waveguide 16-1 can be used to monitor the characteristics of optical filter 200-1, and the light propagating through access waveguide 16-2 can be used to monitor the characteristics of optical filter 200-2. . By monitoring the characteristics of the optical filters 200-1 and 200-2 independently of laser oscillation, it is possible to accurately and stably control the oscillation wavelength.

As shown in FIG. 12, the loss of light can be suppressed by covering the high-mesa structure optical waveguide with an insulating film 38 of SiN. In particular, light loss can be suppressed by using a loop waveguide with a high mesa structure.

<Fifth Embodiment>
A plan view of the wavelength tunable laser device according to the fifth embodiment is the same as FIG. A description of the same configuration as that of any one of the first to fourth embodiments will be omitted. In the fifth embodiment, gain section 40, optical filter 200-1, and optical filter 200-2 are monolithically integrated on substrate 30 instead of substrate 42 in the fourth embodiment. The configurations of optical filters 200-1 and 200-2 are the same as in FIG. A portion of the optical waveguide provided with the electrode 35 has the same configuration as in FIG. 9A. A portion of the optical waveguide where the electrode 35 is not provided has the same configuration as in FIG. 1B.

Access waveguide 10-1 of optical filter 200-1 and access waveguide 10-2 of optical filter 200-2 form one optical waveguide. An access waveguide shared by optical filters 200-1 and 200-2 may be referred to as access waveguide 10-1. A gain section 40 is provided on the access waveguide 10-1.

FIG. 13 is a cross-sectional view along line EE of FIG. 11 illustrating a cross-section of gain section 40 and access waveguide 10-1. As shown in FIG. 13, substrate 30 is an SOI substrate and has substrate 32 , clad layer 33 (box layer) and Si layer 39 . The substrate 32, the clad layer 33 and the Si layer 39 are laminated in this order in the Z-axis direction. The thickness of the Si layer 39 is, for example, 0.22 μm. A waveguide core 34 is provided in the Si layer 39 . The width of the waveguide core 34 is, for example, 0.5 μm. In the Si layer 39, concave portions 39a are provided on both sides of the waveguide core 34 in the Y-axis direction. The Si layer 39 is formed on the bottom surface of the recess 39a. A Si layer 39 having a thickness similar to that of the waveguide core 34 extends outside the recess 39a. A gain portion 40 is joined to the surface of the Si layer 39 opposite to the clad layer 33 . The wavelength tunable laser device in the fifth embodiment is a hybrid laser device.

Gain section 40 has clad layers 53 and 55 , active layer 54 , and contact layer 56 . The clad layer 53 is bonded to the top surface of the Si layer 39 of the substrate 30 . An active layer 54 , a cladding layer 55 and a contact layer 56 are laminated in this order on the cladding layer 53 . The active layer 54, the clad layer 55 and the contact layer 56 protrude in the Z-axis direction and form a mesa 41a with a height of 2 μm, for example. The cladding layer 53 extends outside the mesa 41a in the XY plane. The gain section 40 overlaps the waveguide core 34 and the recess 39a in the Z-axis direction. The cladding layer 53 of the gain section 40 and the Si layer 39 of the substrate 30 may be in direct contact. An insulating film may be provided between the cladding layer 53 and the Si layer 39 . The gain section 40 may have a tapered section on the access waveguide and along the access waveguide.

The insulating film 38 covers the upper surface of the substrate 30, the upper surface of the clad layer 53 and the mesa 41a. The insulating film 38 has an opening on the contact layer 56 and an opening on the cladding layer 53 . The two electrodes 57 are located on both sides of the mesa 41 a and provided on the upper surface of the clad layer 53 exposed from the opening of the insulating film 38 . The electrode 58 is provided on the upper surface of the contact layer 56 exposed from the opening of the insulating film 38 .

The clad layer 53 is made of, for example, n-type InP. The active layer 54 is made of, for example, aluminum gallium indium arsenide (AlGaInAs) and has a multiple quantum well structure. The cladding layer 55 is made of, for example, p-type InP. The contact layer 56 is made of, for example, p-type InGaAs. Electrode 57 is made of metal such as a laminate of gold, germanium and nickel (Au/Ge/Ni). Electrode 58 is made of metal such as a laminate of titanium, platinum and gold (Ti/Pt/Au).

According to the fifth embodiment, light emitted from gain section 40 propagates through access waveguides 10-1 and 10-2 and is reflected by optical filters 200-1 and 200-2. The wavelength tunable laser device 400 performs laser oscillation. The light propagating through access waveguide 16-1 can be used to monitor the characteristics of optical filter 200-1, and the light propagating through access waveguide 16-2 can be used to monitor the characteristics of optical filter 200-2. . By monitoring the characteristics of the optical filters 200-1 and 200-2 independently of laser oscillation, it is possible to accurately and stably control the oscillation wavelength.

<Sixth Embodiment>
FIG. 14 is a diagram illustrating a wavelength tunable laser module 600 according to the sixth embodiment. The wavelength tunable laser module 600 includes a wavelength tunable laser element 610 , a controller 60 , a power supply 61 and a light source 62 .

A wavelength tunable laser element 610 has the same configuration as the wavelength tunable laser element according to the fourth or fifth embodiment, and includes light receiving elements 75 and 76 . Optical filters 200 - 1 and 200 - 2 , gain section 40 , and light receiving elements 75 and 76 are monolithically integrated on substrate 42 or substrate 30 . The light receiving element 75 is provided in the middle of the access waveguide 16-1 of the optical filter 200-1 and is optically coupled with the access waveguide 16-1. The light receiving element 76 is provided in the middle of the access waveguide 16-2 of the optical filter 200-2 and is optically coupled with the access waveguide 16-2. Light receiving elements 75 and 76 may be provided outside wavelength tunable laser element 610 .

The light source 62 is, for example, a tunable laser element. A dotted line in FIG. 14 is the path of the light emitted from the light source 62 . The lens 63, isolator 64, beam splitter 65, half mirror 66, and mirror 67 are arranged in this order from the side closer to the light source 62 along the Y-axis direction. The lens 63 faces the light exit port of the light source 62 .

The light receiving element 68 faces the beam splitter 65 in the X-axis direction. Light receiving elements 68, 75 and 76 each comprise a photodiode. Each photodiode outputs an electrical signal (current) corresponding to the intensity of incident light.

A mirror 69 faces the half mirror 66 in the X-axis direction. Mirror 70 faces half mirror 69 in the Y-axis direction, and faces lens 71 and end 16a of access waveguide 16-1 in the X-axis direction. The mirror 70, the lens 71, and the end portion 16a are arranged in this order.

Mirror 72 faces mirror 67 in the X-axis direction. The mirror 73 faces the half mirror 72 in the Y-axis direction, and faces the lens 74 and the end 16d of the access waveguide 16-2 in the X-axis direction. The end 16d, lens 74, and mirror 73 are arranged in this order.

Light emitted from the light source 62 is incident on the lens 63, the isolator 64, the beam splitter 65, the half mirror 66, and the mirror 67 in order. The beam splitter 65 reflects part of the light toward the light receiving element 68 . The light receiving element 68 outputs a current corresponding to the intensity of light.

Half mirror 66 reflects part of the light toward mirror 69 . The light is reflected by mirrors 69 and 70, passes through lens 71, and enters access waveguide 16-1 from end 16a. The light receiving element 75 outputs a current corresponding to the intensity of light transmitted through the optical filter 200-1.

Light passing through beam splitter 65 and half mirror 66 is reflected by mirrors 67, 72 and 73, passes through lens 74, and enters access waveguide 16-2 through end 16d. The light receiving element 76 outputs a current corresponding to the intensity of light transmitted through the optical filter 200-2.

The control unit 60 is a control device including a computer or the like. Control unit 60 is electrically connected to power source 61 , light source 62 , and light receiving elements 68 , 75 and 76 . The controller 60 functions as a light source controller 80 , a laser element controller 81 , a phase controller 82 and a memory controller 83 .

The light source controller 80 switches the light from the light source 62 on and off and controls the wavelength of the light. The phase control unit 82 controls the phase and wavelength of light propagating through the wavelength tunable laser element 610 by controlling the voltage applied from the power supply 61 to the electrode 35 of the wavelength tunable laser element 610 . The laser device control section 81 controls the voltage applied from the power supply 61 to the gain section 40 of the wavelength tunable laser device 610 . The storage control unit 83 controls the storage device 86 to write and read data.

FIG. 15 is a block diagram showing the hardware configuration of the controller 60. As shown in FIG. As shown in FIG. 15 , the control section 60 includes a CPU (Central Processing Unit) 84 , a RAM (Random Access Memory) 85 , a storage device 86 and an interface 87 . The CPU 84, RAM 85, storage device 86 and interface 87 are connected to each other via a bus or the like. RAM 85 is a volatile memory that temporarily stores programs and data. The storage device 86 is a ROM (Read Only Memory), a solid state drive (SSD) such as a flash memory, a hard disk drive (HDD), or the like. The storage device 86 stores a data table, which will be described later, programs for processing shown in FIGS. 16A, 16B, and 18, and the like.

When the CPU 80 executes the programs stored in the RAM 86, the control unit 60 implements the light source control unit 80, the laser element control unit 81, the phase control unit 82, the storage control unit 83, and the like shown in FIG. Each part of the control unit 60 may be hardware such as a circuit.

16A and 16B are flowcharts illustrating processes executed by the control unit 60. FIG. The processing in FIG. 16A is performed, for example, as an inspection during manufacturing of the wavelength tunable laser device 610 .

As shown in FIG. 16A, the laser device control unit 81 uses the power supply 61 to inject current into the gain section 40 of the wavelength tunable laser device 610, thereby causing the gain section 40 to emit light (step S10). The wavelength of light emitted from the end 10e of the access waveguide 10-2 is measured by a wavemeter (not shown). The phase controller 82 applies voltage to the electrode 35 using the power supply 61 . The current flowing through the electrode 35 changes the wavelength of the light propagating through the wavelength tunable laser element 610 . The phase control unit 82 acquires the wavelength of light and controls the voltage so that the wavelength has a desired magnitude. Repeat the above operation for multiple wavelengths. The storage controller 83 stores the relationship between the wavelength and the voltage in the storage device 86 (step S11). Table 1 is an example of the relationship between wavelength and voltage. Storage device 86 stores a table such as Table 1.

The light source control unit 80 causes the light source 62 to emit light (step S12). Let the wavelengths of light be λm1, λm2, λm3, and so on. These wavelengths are, for example, wavelengths in the vicinity of the wavelengths listed in Table 1, and are preferably wavelengths in a range in which the transmittance changes rapidly with respect to changes in wavelength. Light emitted from the light source 62 is incident on the light receiving element 68 and the access waveguides 16 - 1 and 16 - 2 of the wavelength tunable laser element 610 . Light transmitted through the optical filter 200 - 1 enters the light receiving element 75 . Light transmitted through the optical filter 200 - 2 enters the light receiving element 76 . The light receiving element 68 outputs a current I0 corresponding to the intensity of light immediately after being emitted from the light source 62 . The light receiving element 75 outputs a current I1 corresponding to the intensity of light transmitted through the optical filter 200-1. The light receiving element 75 outputs a current I2 corresponding to the intensity of light transmitted through the optical filter 200-2.

The phase controller 82 applies voltages corresponding to the respective wavelengths in Table 1 to the electrodes 35 . Storage control unit 83 acquires values of currents I0, I1 and I2 for each wavelength, and calculates transmittance I1/I0 of optical filter 200-1 and transmittance I2/I0 of optical filter 200-2. The storage controller 83 stores the transmittance for each wavelength in the storage device 86 (step S13). Table 2 is an example of transmittance for each wavelength. The processing in FIG. 16A ends here.

FIG. 17 is a diagram illustrating a transmittance spectrum. The horizontal axis represents wavelength. The vertical axis represents the transmittance of the optical filter. The transmittance may shift from the solid line to the dashed line due to changes over time. In the example of the solid line in FIG. 17, the transmittance is minimal at the wavelength λ1. At wavelength λm1, the transmittance is I1a/I0a. In the example of the dashed line, the transmittance is minimal at the wavelength λp1. At wavelength λn1, the transmittance is I1a/I0a. A transmission shift changes the transmission value corresponding to the wavelength, but does not change the shape of the spectrum. In the solid line example, the interval between the wavelength λ1 at which the transmittance is minimal and the wavelength λm1 at which the transmittance is I1a/I0a is Δλ1. In the broken line example, the interval between the wavelength λp1 at which the transmittance is minimal and the wavelength λn1 at which the transmittance is I1a/I0a is also Δλ1.

The processing in FIG. 16B is performed, for example, when operating the wavelength tunable laser device 610 . 16A is performed with the transmittance indicated by the solid line in FIG. 17, and then the transmittance shifts to the broken line in FIG. 17 by the wavelength of Δλ2 over time.

As shown in FIG. 16B, the phase control unit 82 determines the voltage to be applied to the electrode 35 based on the relationship between the wavelength and the voltage shown in Table 1, and applies it to the electrode 35 (step S14). The light source control unit 80 causes the light source 62 to emit light (step S15). The phase control unit 82 acquires the transmittances I1/I0 and I2/I0 and determines the voltage based on the transmittances (step S16). The phase control unit 82 controls the voltage applied to the electrode 35 so that the transmittance becomes I1a/I0a shown in Table 2.

The transmittance spectrum shifts from the solid line in FIG. 17 to the dashed line. The wavelength corresponding to the transmittance I1a/I0a is λn1. Let Δλ2 be the interval between the wavelength λm1 and the wavelength λn1. The voltage applied to the electrode 35 is changed by the amount corresponding to the interval Δλ2 between λn1 and λm1 (step S17). The resonance wavelength is adjusted to the wavelength λm1 which is Δλ2 apart from the wavelength λn1, and the transmittance becomes the minimum value at the wavelength λ1. The characteristics of the optical filter 200-1 are adjusted so that the transmittance is minimized. The optical filter 200-2 is also adjusted by similar processing. By this processing, as shown in FIG. 17, the transmittance of the optical filters 200-1 and 200-2 is minimized and the reflectance is maximized. The oscillation wavelength of the wavelength tunable laser element 610 can be adjusted to λ1 accurately and stably.

According to the sixth embodiment, the light receiving element 75 measures the intensity of light transmitted through the access waveguide 16-1 of the optical filter 200-1. The light receiving element 76 measures the intensity of light transmitted through the access waveguide 16-2 of the optical filter 200-2. Based on the light transmittance of the optical filter 200-1 and the light transmittance of the optical filter 200-2, the voltage applied to the electrode 35 is adjusted to control the light wavelength. The wavelength is controlled to λ1, which is the resonant wavelength of the optical filters 200-1 and 200-2, for example. The wavelength tunable laser element 610 can laser-oscillate at the wavelength at which the transmittance peaks. In other words, the oscillation wavelength can be controlled based on the intensity of transmitted light.

In the example of FIGS. 16A and 16B, the characteristics of both optical filters 200-1 and 200-2 are controlled. The characteristics of optical filters 200-1 and 200-2 may be independently controlled. The wavelength tunable laser element 610 may have the configuration shown in FIG.

(Modification)
FIG. 18A is a diagram illustrating a transmittance spectrum. The solid line in FIG. 18A is the spectrum of the optical filter immediately after the tunable laser element 610 is manufactured. Both the solid and dashed line examples have local minima at wavelength λ1. The dashed line is the spectrum after the change over time, and the transmittance has changed compared to the solid line. Even if the transmittance is set to the value (for example, I1a/I0a) obtained in the process of FIG. 16A immediately after manufacturing, the wavelength may not be the desired value λ1.

FIG. 18B is a flowchart illustrating processing executed by the control unit 60, which is performed during operation of the wavelength tunable laser module 600 instead of the processing in FIG. 16B. The process of FIG. 16A is also implemented in the modified example.

The phase control unit 82 determines the voltage to be applied to the electrode 35 based on the relationship between the wavelength and voltage shown in Table 1, and applies, for example, the voltage V1 corresponding to the wavelength λ1 to the electrode 35 (step S14). The light source controller 80 causes the light source 62 to emit light, and changes the wavelength of the light, for example, within a range from λa to λb as shown in FIG. 18B (wavelength dithering, step S15a). The wavelength λa is smaller than the wavelength λ1. Wavelength λb is greater than wavelength λ1. The light source controller 80 obtains the transmittances I1/I0 and I2/I0, and determines the wavelength of the light emitted from the light source 62 based on the transmittances (step S17). That is, the light source control section 80 controls the voltage applied to the light source 62 so that the transmittance becomes a minimum value. The processing ends here. By this process, the wavelength tunable laser element 610 can laser-oscillate at the wavelength λ1 at which the transmittance is minimized and the reflectance is maximized as shown in FIG. 18A.

According to a variant, the wavelength of the light emitted by the light source 62 is varied to find the wavelength at which the transmission is minimal. Even if the resonance wavelength shifts or the magnitude of the transmittance changes, it is possible to adjust the characteristics of the optical filters 200-1 and 200-2 to control the oscillation wavelength.

<Seventh embodiment>
In the seventh embodiment, by using a directional coupler as a multiplexer/demultiplexer, the wavelength dependence of crosstalk in the optical filter is suppressed, and crosstalk is suppressed to a low level. FIG. 19 is a plan view illustrating an optical filter 700 according to the seventh embodiment. Descriptions of the same configurations as those of the first to sixth embodiments are omitted.

The optical filter 700 is symmetrical about the point P in the XY plane. Loop mirror 20 and multiplexer/demultiplexer 24 extend toward one vertex of substrate 30 . The loop mirror 25 and the multiplexer/demultiplexer 28 extend toward a vertex diagonally opposed to the vertex of the substrate 30 .

Loop waveguides 22 and 26 are arc-shaped, for example. A curvature radius R1 of the loop waveguides 22 and 26 is, for example, 15 μm. The curvature radius R2 of the connecting portion between the loop waveguide and the multiplexer/demultiplexer is, for example, 13.675 μm. The radius of curvature R3 of the bent portions of access waveguides 10 and 16 is, for example, 15 μm. The length of the parallel portion of the access waveguides 10 and 12 and the length of the parallel portion of the access waveguides 16 and 14 (length L1) are, for example, 2 μm.

Dotted lines in FIG. 19 are imaginary line segments indicating boundaries between the multiplexers/demultiplexers 24 and 28 and other components. As shown in FIG. 19, the multiplexer/demultiplexer 24 and the multiplexer/demultiplexer 28 are directional couplers each having a degree of coupling of 3 dB. The multiplexer/demultiplexer 24 is convex downward in FIG. The multiplexer/demultiplexer 28 is convex upward.

Multiplexer/demultiplexer 24 has two waveguides 19 and 21 . Waveguide 19 is connected to loop waveguide 22 and waveguide 12 . Waveguide 21 is connected to loop waveguide 22 and waveguide 14 . Multiplexer/demultiplexer 28 has two waveguides 27 and 29 . Waveguide 27 is connected to loop waveguide 26 and waveguide 12 . Waveguide 29 is connected to loop waveguide 26 and waveguide 14 .

FIG. 20A is an enlarged view of the multiplexer/demultiplexer 28. FIG. In the waveguide 27, the side closer to the waveguide 12 (left side in FIG. 20A) is defined as a region 27a, the side closer to the loop waveguide 26 (right side in FIG. 20A) is defined as a region 27b, and the central portion is defined as a region 27c. Waveguide 29, like waveguide 27, has regions 29a, 29b and 29c. The width of waveguides 27 and 29 is, for example, 0.4 μm. Width is constant in all regions.

Regions 27c and 29c have the shape of a curve, such as an arc. A dotted line in FIG. 20A represents the midpoint between the regions 27c and 29c. The radius of curvature of the dotted line is, for example, 30.5 μm. The length of the dotted line (arc length) L2 is, for example, several micrometers or several tens of micrometers.

Regions 27a, 27c, 29a and 29c have a different shape than regions 27c and 29c and are bend waveguides. As an example, the regions 27a, 27c, 29a and 29c are S-bends in an S-shape.

Waveguide 27 is provided away from waveguide 29 in regions 27a and 27c. The distance (gap g3) between the waveguides 27 and 29 in the central portion (regions 27c and 29c) is, for example, 0.25 μm. The distance (gap g4) between waveguides 27 and 29 at the S-bends (regions 27a, 29a, 27b and 29c) is greater than gap g3. The gap g4 increases with increasing distance from the central portion, and is, for example, twice the gap g3 (0.5 μm) or more. The waveguide 19 of the multiplexer/demultiplexer 24 has the same shape as the waveguide 29 . The waveguide 21 of the multiplexer/demultiplexer 24 has the same shape as the waveguide 27 .

FIG. 20B is an enlarged view of the region 27b. Let the left half of the area 27b be an area 27b1 and the right half be an area 27b2. The shape of the region 27b1 and the shape of the region 27b2 are point symmetrical, and are arcs, for example. The length of region 27b1 is equal to the length of region 27b2. The radius of curvature of the region 27b1 and the radius of curvature of the region 27b2 are, for example, 15 μm. A distance L3 between the centers of the regions 27b1 and 27b2 is, for example, 1 μm. The shape of the region 27b1 and the shape of the region 27b2 may be different. In other words, one waveguide of the multiplexer/demultiplexer may be symmetrical or asymmetrical.

21A and 21B are schematic diagrams illustrating the optical filter 700, and are examples in which light is incident from the access waveguide. The portion where the electromagnetic field is distributed is illustrated by shading. In FIG. 21A, light is incident from the left end (end portion 10a) of the access waveguide 10. In FIG. A resonance mode of the resonator 11 is excited. Reflected light R is reflected toward end 10 a of access waveguide 10 . The transmitted light T propagates toward the end 10b of the access waveguide 10. FIG.

Ideally, the light that propagates counterclockwise through the loop waveguide and enters the waveguide 12 and the light that propagates clockwise through the loop waveguide and enters the waveguide 12 are in phase with each other, superimposed. On the other hand, the light that propagates counterclockwise through the loop waveguide and is incident on the waveguide 14 and the light that propagates clockwise through the loop waveguide and is incident on the waveguide 14 have opposite phases and cancel each other out. That is, the resonant mode of resonator 11 propagates through waveguide 12 and access waveguide 10 but does not propagate through waveguide 14 and access waveguide 16 . However, light may leak into waveguide 14 and access waveguide 16 . The light that leaks into the waveguide 14 and propagates toward the end 16a of the access waveguide 16 is crosstalk XT1. Light that leaks into the waveguide 14 and propagates toward the end 16b of the access waveguide 16 is crosstalk XT2.

In FIG. 21B, light enters from the right end (end 16b) of the access waveguide 16. In FIG. A resonance mode of the resonator 13 is excited. Reflected light R is reflected toward end 16 a of access waveguide 16 . Transmitted light T propagates toward end 16b of access waveguide 16 . Light may leak into waveguide 12 and access waveguide 10 . The light propagating toward the end portion 10b of the access waveguide 10 is assumed to be crosstalk XT1. The light propagating toward the end portion 10a of the access waveguide 10 is assumed to be crosstalk XT2.

As described in the second embodiment, the characteristics of the optical filter 700 are monitored by extracting light in the resonance mode from one of the access waveguides 10 and 16 and propagating the other light. It is important to keep crosstalk low between the two access waveguides. Crosstalk occurs due to the imbalance of light distribution in the multiplexer/demultiplexer.

22A and 22B are diagrams illustrating characteristics of the multiplexer/demultiplexer. 22A and 22B show the transmittance Tbar of light to one of the two waveguides of the multiplexer/demultiplexer, and the transmittance Tbar of light to the other waveguide. This is the result of simulating the transmittance Tcross and the distribution ratio. The horizontal axis represents the wavelength of light, and the wavelength ranges from 1530 nm to 1570 nm. The vertical axis on the left represents the transmittance of light to each waveguide. The right vertical axis represents the optical imbalance between the two waveguides. The solid line in the figure represents Tbar. Dotted line represents Tcross. Dashed lines represent imbalances.

Imbalance is the absolute value of the common logarithm of the partition ratio (10*log ₁₀ (Tbar/Tcross)). If the light is distributed in the same proportion in the two waveguides (Tbar=Tcross), the imbalance becomes zero. Imbalance increases when one of Tbar and Tcross is greater than the other. The waveguide core has a refractive index of 2.76, and the clad layer has a refractive index of 1.44. The gap g1 between access waveguide 10 and waveguide 12 and the gap g2 between access waveguide 16 and waveguide 14 are 200 nm (see FIG. 3).

FIG. 22A is an example where two straight waveguides act as a directional coupler. By setting the radius of curvature of the dotted line in FIG. 20A to infinity, a simulation assuming a straight waveguide is performed. The waveguide length is 3.88 μm. As shown in FIG. 22A, Tbar=Tcross=50% at a wavelength of 1547.5 nm. At a wavelength of 1547.5 nm the imbalance becomes zero. The shorter the wavelength, the smaller Tcross and the larger Tbar. The longer the wavelength, the smaller Tbar and the larger Tcross. The imbalance increases with wavelengths away from 1547.5 nm and exceeds 0.1 dB in the wavelength bands below 1545 nm and above 1550 nm.

As shown in FIG. 22A, in the wavelength band shorter than 1547.5 nm, Tbar is high and Tcross is low. In such a wavelength band, for example, when light is incident from the access waveguide 10, light propagating counterclockwise through the loop waveguide 26 (corresponding to Tbar) is strong, and light propagating clockwise through the loop waveguide 26 is strong. (corresponding to Tcross) is weak. Therefore, these lights do not cancel each other out, propagate through the waveguide 14 , and transit to the access waveguide 16 . The light propagating through the loop waveguide 22 also does not cancel each other out. In the wavelength band longer than 1547.5 nm, Tbar is low and Tcross is high. The light propagating clockwise through the loop waveguide 26 is strong, and the light propagating counterclockwise through the loop waveguide 26 is weak. Therefore, the light propagates through the waveguide 14 without canceling each other. When light enters through access waveguide 16 , the light also propagates through waveguide 12 and leaks into access waveguide 10 .

FIG. 22B is an example in which two waveguides have bends at both ends like the multiplexer/demultiplexer 28 of FIG. 20A. The dotted line in FIG. 20A has a radius of curvature of 30.5 μm and a length L2 of 10.8 μm. Gaps g1 and g2 are 200 nm. In the example of FIG. 21B, Tbar and Tcross are within 50%±1% in the wavelength band from 1530 nm to 1570 nm. The imbalance between waveguides is less than 0.1 dB over the above wavelength band.

For example, when light is incident from the access waveguide 10 , the intensity of light propagating counterclockwise through the loop waveguide 26 is approximately the same as the intensity of light propagating clockwise through the loop waveguide 26 . Therefore, these lights cancel each other out and are difficult to propagate to the waveguide 14 . The two lights propagating through the loop waveguide 22 also cancel each other out and are difficult to propagate to the waveguide 14 . Light transitioning from waveguide 14 to access waveguide 16 is also suppressed. When light enters from the access waveguide 16 , the light is less likely to propagate to the waveguide 12 and less likely to leak to the access waveguide 10 .

23A to 24B are diagrams illustrating frequency characteristics of optical filters. The horizontal axis represents the wavelength of light. The vertical axis represents the intensity of light. A solid line in the drawing represents the intensity of the transmitted light T. FIG. A dotted line represents the intensity of the reflected light R. FIG. The dashed line represents the intensity of crosstalk XT1. The dashed-dotted line represents the intensity of crosstalk XT2.

23A and 23B are examples in which the multiplexer/demultiplexer is formed of two linear waveguides, corresponding to the example of FIG. 22A. In FIG. 23A, light is incident from access waveguide 10 . In FIG. 23B, light is injected from access waveguide 16 .

As shown in FIGS. 23A and 23B, the transmitted intensity is minimized and the reflected intensity is maximized at the resonant frequency (and resonant wavelength). Crosstalk XT1 and XT2 are maximal at the resonant frequency. Crosstalk XT1 and XT2 is less than -30 dB in the wavelength range from approximately 1545 nm to 1553 nm. However, for wavelengths below 1545 nm or above 1553 nm, the crosstalk XT1 and XT2 is -30 dB or more.

The light is distributed unbalanced in the multiplexer/demultiplexer as shown in FIG. 22A. When light enters through the access waveguide 10 , the light also propagates through the access waveguide 16 . When light enters from the access waveguide 16 , the light also propagates through the access waveguide 10 . Crosstalk XT1 and XT2 increases as in FIGS. 23A and 23B.

24A and 24B are examples in which the multiplexer/demultiplexer has bends as shown in FIG. 20A, corresponding to the example in FIG. 22B. In FIG. 24A, light is injected from access waveguide 10 . In FIG. 24B, light is injected from access waveguide 16 . As shown in FIGS. 24A and 24B, crosstalk XT1 and XT2 are suppressed to -30 dB or less over the band from 1530 nm to 1570 nm.

As shown in FIG. 22B, light is distributed in a nearly uniform ratio in the multiplexer/demultiplexer. When light is incident from the access waveguide 10 , it is difficult for the light to propagate to the access waveguide 16 . When light is incident from the access waveguide 16 , it is difficult for the light to propagate to the access waveguide 10 . As shown in FIGS. 24A and 24B, the wavelength dependence of crosstalk can be suppressed, and crosstalk can be kept low in a wide wavelength band.

According to the seventh embodiment, the multiplexer/demultiplexer 24 is a directional coupler with two waveguides 19 and 21 . Multiplexer/demultiplexer 28 is a directional coupler having two waveguides 27 and 21 . In each of the multiplexers 24 and 28, the distance between waveguides changes. As shown in FIG. 20A, gap g4 in regions 29a and 29b at both ends of multiplexer/demultiplexer 28 is larger than gap g3 in central region 29c of multiplexer/demultiplexer 28. As shown in FIG. In the multiplexer/demultiplexer 24 as well, the gaps at both ends are larger than the central gap.

When the waveguides 19 and 21 have curved shapes, a phase mismatch occurs between the light propagating through the waveguide 19 of the multiplexer/demultiplexer 24 and the light propagating through the waveguide 21 . When the waveguides 19 and 21 have curved shapes, a phase mismatch occurs between the light propagating through the waveguide 27 of the multiplexer/demultiplexer 28 and the light propagating through the waveguide 29 . As shown in FIG. 22B, the transmittances Tbar and Tcross of light from the multiplexer/demultiplexer to the loop waveguide are around 50% over the wavelength band from 1530 nm to 1570 nm. The wavelength dependence of light distribution is improved. Crosstalk is suppressed to a low level because the lights that have traveled around the loop waveguide cancel each other out. More specifically, as shown in FIGS. 24A and 24B, the wavelength dependence of crosstalk can be reduced, and crosstalk can be kept low over a wide wavelength band. Two resonance modes can be independently generated in a wide band, and for example, laser light oscillation and characteristic monitoring can be performed simultaneously.

As shown in FIG. 20A, multiplexer/demultiplexer 28 has arcuate regions 27c and 29c in the central portion. Multiplexer/demultiplexer 28 has regions 27a and 29a at one end and regions 27b and 29b at the opposite end. Regions 27a, 27c, 29a and 29c are S-bends. Like the multiplexer/demultiplexer 28, the multiplexer/demultiplexer 24 has an arcuate region and an S-bend. The gap g4 at the S-bend is larger than the gap g3 between the two waveguides in the central region. Light can be equally distributed over a wide wavelength band. It is possible to suppress crosstalk to a low level.

As shown in FIG. 20A, the gap g4 at each of the bends at both ends is larger than the gap g3 between the two waveguides at the central portion of the multiplexer/demultiplexer. The gap g4 increases with increasing distance from the central portion, and may be two or more times, or three or more times, the gap g3.

The shape of the multiplexer/demultiplexer is not limited to the example in FIG. 20A. The central region may be a circular arc or a curved line other than a circular arc. An S-bend is shaped like two arcs connected. The S-bend may be, for example, a sine curve. The regions at both ends of the multiplexer/demultiplexer may be S bends or bends other than S bends. The waveguides of the multiplexer/demultiplexer may have bends of different shapes. For example, in the waveguide 27 of the multiplexer/demultiplexer 28, the regions 27a and 27b may have different shapes.

The optical filter 700 is symmetrical with respect to the point P shown in FIG. The multiplexer/demultiplexer 24 is downwardly convex, and the multiplexer/demultiplexer 28 is upwardly convex. The light input from the waveguide 12 to the multiplexer/demultiplexer 24 propagates through the waveguide 19 that expands on the outer peripheral side of the multiplexer/demultiplexer 24 . The light input from the waveguide 12 to the multiplexer/demultiplexer 28 propagates through a waveguide 27 that expands toward the inner peripheral side of the multiplexer/demultiplexer 28 . The light input from the waveguide 14 to the multiplexer/demultiplexer 24 propagates along the inner peripheral side of the waveguide 21 of the multiplexer/demultiplexer 24 . The light input from the waveguide 14 to the multiplexer/demultiplexer 28 propagates along the outer peripheral side of the waveguide 29 of the multiplexer/demultiplexer 28 . The phase change amount of the light returning to the waveguide 12 through the loop mirrors 20 and 25 matches the phase change amount of the light returning to the waveguide 14 through the loop mirrors 20 and 25 . The resonant wavelength of the resonator 11 and the resonant wavelength of the resonator 13 match. The FSR of resonator 11 and the FSR of resonator 13 match.

By adjusting the gap between the access waveguide 10 and the waveguide 12 (corresponding to the distance g1 in FIG. 3) and the gap between the access waveguide 16 and the waveguide 14 (corresponding to the distance g2 in FIG. 3) , can control the finesse of the resonator. Finesse can also be controlled by adjusting the length of the parallel portion of access waveguide 10 and waveguide 12 and the length L1 of the parallel portion of access waveguide 16 and waveguide 14 .

As shown in FIG. 22B, light is nearly equally distributed in the band including the entire C band (wavelength 1530 nm to 1565 nm), and crosstalk is suppressed as shown in FIGS. 24A and 24B. By changing the length of the waveguide, etc., it is also possible to improve the characteristics in other wavelength bands. The waveguide of the optical filter 700 may be an optical waveguide having a Si waveguide core as shown in FIG. 1B, or an optical waveguide having a high mesa structure formed of III-V group compound semiconductors as shown in FIG. An electrode for phase adjustment may be provided on the waveguide. Optical filter 700 may be incorporated into the tunable laser device.

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 can be made within the scope of the gist of the present disclosure described in the claims. Change is possible.

<Appendix 1>
The first waveguide, the second waveguide, the first loop waveguide, the second loop waveguide, the first access waveguide and the second access waveguide may be made of silicon. Light loss can be suppressed.

<Appendix 2>
A first clad layer, a core layer, and a second clad layer, wherein the first clad layer, the core layer, and the second clad layer are formed of a III-V group compound semiconductor, and the first clad layer , the core layer and the second clad layer are laminated in this order to form a mesa, the first waveguide, the second waveguide, the first loop waveguide, the second loop waveguide, The first access waveguide and the second access waveguide may comprise the mesa. Light loss can be suppressed.
<Appendix 3>
The phase adjustment unit may be a heater that generates heat when an electrical signal is input. By changing the refractive index, the phase of light can be adjusted and the wavelength of light can be changed.

<Appendix 4>
A control program for a wavelength tunable laser module, wherein a computer causes a light source to enter a second access waveguide of a wavelength tunable laser element, and based on the intensity of light transmitted through the second access waveguide, A control program for a wavelength tunable laser module that executes a process of controlling the wavelength of light propagating through a second access waveguide. Light emitted from the gain section propagates through the first access waveguide and is reflected by the optical filter. The vernier effect of the two optical filters causes the wavelength tunable laser element to laser-oscillate. Light propagating in the second access waveguide can be used to monitor the properties of the optical filter.

10, 10-1, 10-2, 16, 16-1, 16-2 access waveguides 10a, 10b, 10c, 10d, 10e, 16a, 16b, 16c, 16d ends 11, 13 resonators 12, 14, 15, 19, 21, 29 waveguides 20, 25 loop mirrors 22, 26 loop waveguides 17, 24, 28 multiplexers/demultiplexers 27a, 27b, 27b1, 27b2, 27c, 29a, 29b, 29c regions 24a, 24b, 28a , 28b input end 24c, 24d, 28c, 28d output end 30, 32, 42 substrate 33, 43, 45, 46, 50, 52, 53, 55 clad layer 34 waveguide core 35, 36, 37, 57, 58 electrode 38 insulating film 39a recessed portion 40 gain portion 41, 41a mesa 44, 54 active layer 48 buried layer 49 current blocking layer 47, 56 contact layer 51 core layer 59 reflector 60 control portion 62 light source 63, 71, 74 lens 64 isolator 65 Beam splitter 66 Half mirror 67, 69, 70, 72, 73 Mirror 68, 75, 76 Light receiving element 80 Light source controller 81 Laser element controller 82 Phase controller 83 Storage controller 84 CPU
85 RAM
86 storage device 87 interface 100, 200, 200a, 200b, 200-1, 200-2, 700 optical filter 300, 400, 610 wavelength tunable laser element 310 filter element 600 wavelength tunable laser module

Claims (16)

a first loop mirror;
a second loop mirror;
a first waveguide optically coupled to the first loop mirror and the second loop mirror;
a first access waveguide;
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 first access waveguide is an optical filter optically coupled to the first waveguide. a second waveguide optically coupled to the first loop mirror and the second loop mirror;
a second access waveguide;
the second waveguide is optically coupled to the first multiplexer/demultiplexer and the second multiplexer/demultiplexer;
2. The optical filter of claim 1, wherein said second access waveguide optically couples with said second waveguide. The shape of the first multiplexer/demultiplexer is symmetrical, the shape of the second multiplexer/demultiplexer is symmetrical, and the shape of the first waveguide and the shape of the second waveguide are symmetrical. 3. The optical filter according to 2. 4. The optical filter according to claim 2, wherein said first multiplexer/demultiplexer and said second multiplexer/demultiplexer are 2*2 multimode interference waveguides or directional couplers. The first multiplexer/demultiplexer and the second multiplexer/demultiplexer are directional couplers each having two waveguides,
In the first multiplexer/demultiplexer, the distance between the two waveguides on the first loop waveguide side and the distance on the first waveguide and second waveguide sides are larger than the distance at the center of the branching filter,
The distance between the two waveguides on the second loop waveguide side of the second multiplexer/demultiplexer and the distance on the first waveguide and second waveguide sides are the second multiplexer/demultiplexer 4. An optical filter according to claim 2 or 3, wherein the distance is greater than the distance at the center of the wave. The two waveguides of the first multiplexer/demultiplexer have bends on the first loop waveguide side and bends on the first waveguide and second waveguide sides,
6. Said two waveguides of said second multiplexer/demultiplexer have bends on said second loop waveguide side and bends on said first waveguide and said second waveguide sides. The optical filter described in . 6. The optical filter according to claim 5, wherein the central portion of the two waveguides of the first multiplexer/demultiplexer is curved. A phase provided in at least one of the first loop waveguide and the second loop waveguide and adjusting a phase of light propagating in the at least one of the first loop waveguide and the second loop waveguide 4. The optical filter according to claim 2, further comprising an adjustment section. a gain section;
two optical filters;
The two optical filters are the optical filters according to claim 8,
intervals of resonance wavelengths of the two optical filters are different from each other,
A tunable laser element, wherein the gain section has an optical gain and is optically coupled to the first access waveguides of each of the two optical filters. The two optical filters are formed on a substrate,
the gain section and the substrate are butt-connected;
10. The wavelength tunable laser device according to claim 9, further comprising a reflecting mirror provided on the side of the gain section opposite to the substrate. comprising a substrate formed of a III-V compound semiconductor;
the gain section and the two optical filters are monolithically integrated on the substrate;
one of the two optical filters is located on one end side of the gain section;
10. The wavelength tunable laser device according to claim 9, wherein another one of said two optical filters is positioned on the other end side of said gain section. The two optical filters are formed on a substrate,
The first waveguide, the second waveguide, the first loop waveguide, the second loop waveguide, the first access waveguide, and the second access waveguide are made of silicon formed on the substrate. is a waveguide,
one of the two optical filters is located on one end side of the gain section;
another one of the two optical filters is located on the other end side of the gain section;
10. The wavelength tunable laser device according to claim 9, wherein said gain section is bonded to the surface of said substrate. a wavelength tunable laser device according to claim 9;
a light source for injecting light into the second access waveguide of the wavelength tunable laser element;
a light receiving element optically coupled to the second access waveguide of the wavelength tunable laser element. A control method for a wavelength tunable laser module according to claim 13,
injecting light from a light source into a second access waveguide of the tunable laser device;
and controlling the wavelength of the light propagating through the second access waveguide based on the intensity of the light transmitted through the second access waveguide. The step of controlling the wavelength of the light is a step of controlling the wavelength of the light propagating through the second access waveguide using the phase adjuster based on the intensity of the light transmitted through the second access waveguide. 15. The method of controlling a wavelength tunable laser module according to claim 14. In the step of controlling the wavelength of light, the wavelength of light propagating through the second access waveguide is controlled by controlling the wavelength of light emitted from a light source based on the intensity of light transmitted through the second access waveguide. 15. The method of controlling a wavelength tunable laser module according to claim 14, which is a step of controlling.

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