Tunable light source and optical filter

Abstract

This invention provides a tunable light source and optical filter that cover two frequency bands with a simple structure. [Solution] A tunable light source 100 comprising a first gain unit 12 that emits light in a first band, a second gain unit 14 that emits light in a second band, an optical filter 11 provided on a substrate 10, a first reflector 33 and a second reflector 34 that reflect light in the first band, and a third reflector 35 and a fourth reflector 36 that reflect light in the second band, wherein the optical filter has a first region 16 that propagates light in the first band, a second region 18 that propagates light in the second band, a first ring resonator 30 and a second ring resonator 32, the first gain unit is optically coupled to the first region and the second gain unit is optically coupled to the second region, a first waveguide 20, a first ring resonator and a second ring resonator are provided between the first reflector and the second reflector, and a fourth waveguide 24, a first ring resonator and a second ring resonator are provided between the third reflector and the fourth reflector.

Description

Technical Field

[0001] The present disclosure relates to a wavelength-variable light source and an optical filter.

Background Art

[0002] Wavelength-variable light sources are used in applications such as communication. There is a technology of joining an element having optical gain to an SOI (Silicon on Insulator) substrate or the like to form a hybrid light source (Non-Patent Document 1 and Non-Patent Document 2). A technology of joining a laser element to a filter by butt joint has also been developed (Non-Patent Document 3). Optical elements such as waveguides and ring resonators are provided on the substrate. Light generated from the element propagates through the waveguide or the like.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

[0004] Optical communications utilize multiple wavelength bands, including the C-band (1530nm to 1565nm) and the L-band (1565nm to 1625nm). Covering both wavelength bands with a single tunable light source is difficult. This often requires increasing the number of elements to accommodate both bands, but this results in a more complex structure. Therefore, the objective is to provide a tunable light source and optical filter with a simple structure that can cover both bands. [Means for solving the problem]

[0005] The tunable light source according to this disclosure comprises: a first gain unit having optical gain and emitting light in a first band; a second gain unit having optical gain and emitting light in a second band; an optical filter provided on a substrate; a first reflector and a second reflector that reflect the light in the first band; a third reflector and a fourth reflector that reflect the light in the second band; wherein the optical filter has a first region for propagating the light in the first band; a second region for propagating the light in the second band; a first ring resonator; a second ring resonator; the first gain unit is optically coupled to the first region; the second gain unit is optically coupled to the second region; and the first region has a first waveguide, a second waveguide, and a third waveguide. The second region has a fourth waveguide, a fifth waveguide, and a sixth waveguide, the first waveguide and the fourth waveguide are optically coupled to the first ring resonator and the second ring resonator, the second waveguide and the fifth waveguide are optically coupled to the first ring resonator, and the third waveguide and the sixth waveguide are optically coupled to the second ring resonator. In the direction of light propagation, the first waveguide, the first ring resonator, and the second ring resonator are provided between the first reflecting section and the second reflecting section, and in the direction of light propagation, the fourth waveguide, the first ring resonator, and the second ring resonator are provided between the third reflecting section and the fourth reflecting section. [Effects of the Invention]

[0006] According to this disclosure, it is possible to provide a tunable light source and optical filter having a simple structure and capable of covering two frequency bands. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a plan view illustrating a tunable light source according to the first embodiment. [Figure 2A] Figure 2A is a cross-sectional view illustrating a tunable light source. [Figure 2B] Figure 2B is a cross-sectional view illustrating a tunable light source. [Figure 3] Figure 3 illustrates a reflectance spectrum. [Figure 4A] FIG. 4A is a plan view showing an example of light propagation. [Figure 4B] FIG. 4B is a plan view showing an example of light propagation. [Figure 4C] FIG. 4C is a plan view showing an example of light propagation. [Figure 4D] FIG. 4D is a plan view showing an example of light propagation. [Figure 5A] FIG. 5A is a plan view illustrating a multiplexer. [Figure 5B] FIG. 5B is a cross-sectional view illustrating a multiplexer. [Figure 6] FIG. 6 is a diagram illustrating the characteristics of a multiplexer. [Figure 7] FIG. 7 is a plan view illustrating a wavelength tunable light source according to a comparative example. [Figure 8] FIG. 8 is a plan view illustrating a wavelength tunable light source according to the second embodiment. [Figure 9] FIG. 9 is a plan view illustrating a wavelength tunable light source according to the third embodiment. [Figure 10] FIG. 10 is a plan view illustrating a band-pass filter. [Figure 11] FIG. 11 is a diagram illustrating transmittance. [Figure 12A] FIG. 12A is a diagram illustrating an optical spectrum when only the L-band laser section is driven without driving the C-band laser section. [Figure 12B] FIG. 12B is a diagram illustrating an optical spectrum when only the L-band laser section is driven without driving the C-band laser section. [Figure 13] FIG. 13 is a plan view illustrating a wavelength tunable light source according to the fourth embodiment.

Embodiments for Carrying Out the Invention

[0008] [Description of Embodiments of the Present Disclosure] First, the content of the embodiments of the present disclosure will be listed and described.

[0009] One embodiment of the present disclosure comprises (1) a first gain unit having optical gain and emitting light in a first band, a second gain unit having optical gain and emitting light in a second band, an optical filter provided on a substrate, a first reflector and a second reflector that reflect the light in the first band, and a third reflector and a fourth reflector that reflect the light in the second band, wherein the optical filter has a first region for propagating the light in the first band, a second region for propagating the light in the second band, a first ring resonator, and a second ring resonator, the first gain unit being optically coupled to the first region, the second gain unit being optically coupled to the second region, the first region having a first waveguide, a second waveguide, and a third waveguide, and the second region being This is a tunable light source having four waveguides, a fifth waveguide, and a sixth waveguide, wherein the first waveguide and the fourth waveguide are optically coupled to the first and second ring resonators, the second waveguide and the fifth waveguide are optically coupled to the first ring resonator, and the third waveguide and the sixth waveguide are optically coupled to the second ring resonator. In the direction of light propagation, the first waveguide, the first ring resonator, and the second ring resonator are provided between the first and second reflectors, and in the direction of light propagation, the fourth waveguide, the first ring resonator, and the second ring resonator are provided between the third and fourth reflectors. The light emitted from the first gain section propagates in the first region. The light emitted from the second gain section propagates in the second region. The tunable light source can cover two bands. The first and second regions share the first and second ring resonators. The structure becomes simpler. (2) In (1) above, the first waveguide and the second waveguide are located opposite each other with respect to the first ring resonator and extend from the first ring resonator to the same side; the fourth waveguide and the fifth waveguide are located opposite each other with respect to the first ring resonator and extend from the first ring resonator to the same side; the first waveguide and the third waveguide are located opposite each other with respect to the second ring resonator and extend from the second ring resonator to the same side; and the fourth waveguide and the sixth waveguide are located opposite each other with respect to the second ring resonator and extend from the second ring resonator to the same side. The light emitted from the first gain unit travels between the first waveguide, the first ring resonator and the second waveguide, and between the first waveguide, the second ring resonator and the third waveguide. The light emitted from the second gain section travels between the fourth waveguide, the first ring resonator, and the fifth waveguide, and also between the fourth waveguide, the second ring resonator, and the sixth waveguide. The light emitted from the first gain section propagates in the first region. The light emitted from the second gain section propagates in the second region. The tunable light source can cover two frequency bands. (3) In (1) or (2) above, the fourth waveguide may extend from the first ring resonator and the second ring resonator to the side opposite to the first waveguide, the fifth waveguide may extend from the first ring resonator to the side opposite to the second waveguide, and the sixth waveguide may extend from the second ring resonator to the side opposite to the third waveguide. Light emitted from the first gain section is less likely to leak into the second region. Light emitted from the second gain section is less likely to leak into the first region. Light loss can be reduced. (4) In any of (1) to (3) above, the first reflector is a loop mirror provided in the second waveguide, the second reflector is a loop mirror provided in the third waveguide, the third reflector is a loop mirror provided in the fifth waveguide, the fourth reflector is a loop mirror provided in the sixth waveguide, the first gain unit and the second gain unit are bonded to one surface of the substrate, the first gain unit is provided in a position overlapping with the first waveguide and optically coupled to the first waveguide, and the second gain unit is provided in a position overlapping with the fourth waveguide and optically coupled to the first waveguide. Laser resonators are configured in the first region and the second region, respectively. Laser oscillation occurs from the emitted light of the first gain unit and the emitted light of the second gain unit. (5) In any of (1) to (4) above, the first reflector is a loop mirror provided in the second waveguide, the third reflector is a loop mirror provided in the fifth waveguide, the first gain section and the second gain section are butt-joint-jointed to the substrate, the first gain section is optically coupled to the third waveguide, the second gain section is optically coupled to the sixth waveguide, the second reflector is a reflective film provided in a position opposite to the optical filter of the first gain section, and the fourth reflector is a reflective film provided in a position opposite to the optical filter of the second gain section. Laser resonators are configured in the first region and the second region, respectively. Laser oscillation occurs from the emitted light of the first gain section and the emitted light of the second gain section. (6) In any of (1) to (5) above, the optical filter may have a multiplexer, a seventh waveguide and an eighth waveguide, wherein the second waveguide is optically coupled to the first input terminal of the multiplexer, the fifth waveguide is optically coupled to the second input terminal of the multiplexer, the seventh waveguide is optically coupled to the first output terminal of the multiplexer, and the eighth waveguide is optically coupled to the second output terminal of the multiplexer. Light can be emitted from the seventh waveguide and the eighth waveguide. (7) In (6) above, the multiplexer may be a mosaic element. Light can be emitted from the 7th waveguide and the 8th waveguide. Light returning to the 2nd waveguide and the 5th waveguide can be reduced. (8) In any of (1) to (7) above, a first heater provided in the first ring resonator and a second heater provided in the second ring resonator may be provided. The wavelength can be easily adjusted by changing the refractive index of the first ring resonator and the second ring resonator. (9) In any of (1) to (8) above, the substrate has a silicon layer or a silicon nitride layer, and the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide and the sixth waveguide, the first ring resonator and the second ring resonator may be provided in the silicon layer or the silicon nitride layer. By confining light in the waveguides and ring resonators, the loss of light can be reduced. (10) In any of (1) to (8) above, the second band is a band with a longer wavelength than the first band, and is equipped with a bandpass filter, the bandpass filter has a first port, a second port and a third port, the first port and the second port are connected to the first waveguide, in the first band the transmittance between the first port and the second port is higher than the transmittance between the first port and the third port, and in the second band the transmittance between the first port and the third port is higher than the transmittance between the first port and the second port. Abnormal oscillation can be prevented. (11) In (10) above, the bandpass filter may be a mosaic element. The bandpass filter can be made smaller. (12) An optical filter comprising a first region, a second region, a first ring resonator, and a second ring resonator, wherein the first region has a first waveguide, a second waveguide, and a third waveguide, the second region has a fourth waveguide, a fifth waveguide, and a sixth waveguide, the first waveguide and the fourth waveguide are optically coupled to the first ring resonator and the second ring resonator, the second waveguide and the fifth waveguide are optically coupled to the first ring resonator, and the third waveguide and the sixth waveguide are optically coupled to the second ring resonator. The optical filter can cover two frequency bands. The structure is simplified.

[0010] [Details of the embodiments of this disclosure] Specific examples of tunable light sources and optical filters according to the embodiments of this disclosure will be described below with reference to the drawings. However, this disclosure is not limited to these examples and is intended to include all modifications within the meaning and scope of the claims as indicated by the claims.

[0011] <First Embodiment> Figure 1 is a plan view illustrating a tunable light source 100 according to the first embodiment. The tunable light source 100 is a hybrid laser element and comprises a substrate 10, a gain unit 12 (first gain unit), and a gain unit 14 (second gain unit). Two sides of the substrate 10 are parallel to the X-axis direction. The other two sides are parallel to the Y-axis direction. The top surface of the substrate 10 is parallel to the XY plane. The Z-axis direction is the normal direction to the top surface and is the thickness direction of the substrate 10. The X-axis, Y-axis, and Z-axis directions are orthogonal to each other. The length L1 of the substrate 10 in the X-axis direction is, for example, 0.6 mm. The length L2 of the substrate 10 in the Y-axis direction is, for example, 2.5 mm.

[0012] An optical filter 11 is formed on the substrate 10. The optical filter 11 has a region 16 (first region) and a region 18 (second region), a multiplexer 27, a waveguide 28 (seventh waveguide), and a waveguide 29 (eighth waveguide). In Figure 1, each region is shown by a dashed line. Gain units 12 and 14 are bonded to the substrate 10. Gain unit 12 is bonded to region 16 of the optical filter 11. Gain unit 14 is bonded to region 18 of the optical filter 11. Tapered sections are provided at both ends of the gain unit in the X-axis direction. The tapered sections are located above the waveguides. The length of the gain unit in the X-axis direction is, for example, 50 μm.

[0013] The tunable light source 100 can emit light in two wavelength bands. The gain section 12 and region 16 correspond to one wavelength band, forming a laser resonator corresponding to, for example, the C band (wavelength 1530 nm to 1565 nm, first band). The gain section 14 and region 18 correspond to a wavelength band other than the C band, forming a laser resonator corresponding to, for example, a part of the L band (second band) (wavelength 1565 nm to 1600 nm). The light from the tunable light source 100 is, for example, single-mode. The laser light is emitted from waveguides 28 and 29. The tunable light source 100 has an anti-reflective coating 90 on the light emission end face. The end faces of waveguide 28 and waveguide 29 are covered with the anti-reflective coating 90.

[0014] Region 16 of the optical filter 11 has waveguides 20 (first waveguide), 22 (second waveguide), and 23 (third waveguide). Region 18 has waveguides 24 (fourth waveguide), 25 (fifth waveguide), and 26 (sixth waveguide). Regions 16 and 18 share ring resonators 30 (first ring resonator) and 32 (second ring resonator). The width of the waveguides is, for example, 500 nm. The ring resonators are formed by ring-shaped waveguides, such as annular ones. The circumference of ring resonator 30 is different from the circumference of ring resonator 32. The circumference of ring resonator 30 is, for example, 140 μm. The circumference of ring resonator 32 is, for example, 150 μm.

[0015] In the example in Figure 1, waveguides 20, 22, 23, 24, 25, 26, 28, and 29 each have a portion parallel to the X-axis and a portion parallel to the Y-axis. The shape of the waveguides may be changed from the example in Figure 1; for example, they do not have to be parallel to the X-axis and Y-axis, and may include portions that are inclined from the X-axis and Y-axis, or may include curves, etc.

[0016] Waveguides 20 and 24 are located between ring resonators 30 and 32. Near ring resonators 30 and 32, waveguides 20 and 24 are connected, forming a loop-shaped waveguide. The portion of waveguide 20 parallel to the X-axis and the portion of waveguide 24 parallel to the X-axis are spaced apart from each other.

[0017] The portions of waveguide 20 and waveguide 24 parallel to the Y-axis are optically coupled to ring resonators 30 and 32. For example, the portions of waveguide 20 and waveguide 24 parallel to the Y-axis may approach ring resonators 30 and 32, respectively, to form a directional coupler. Waveguides 20 and waveguide 24 extend from ring resonator 30 in opposite directions. Waveguides 20 and waveguide 24 extend from ring resonator 32 in opposite directions.

[0018] Waveguide 22 is located opposite to waveguide 20 of the ring resonator 30 in the X-axis direction. Waveguide 25 is located opposite to waveguide 24 of the ring resonator 30. The portion of waveguide 22 parallel to the X-axis and the portion of waveguide 25 parallel to the X-axis are spaced apart from each other. The portions of waveguide 22 and waveguide 25 parallel to the Y-axis are optically coupled to the ring resonator 30. Waveguides 22 and 25 extend from the ring resonator 30 to opposite sides. Waveguides 20 and 22 extend to the same side (upward in Figure 1) with respect to the ring resonator 30. Waveguides 24 and 25 extend to the same side (downward in Figure 1) with respect to the ring resonator 30.

[0019] Waveguides 22 and 25 extend from the ring resonator 30 to the multiplexer 27. A loop mirror 33 (first reflector) is provided in the middle of waveguide 22. Waveguides 22 curve, and near the end of the curve, the two waveguides 22 come into close proximity, forming a directional coupler and thus creating the loop mirror 33. A loop mirror 35 (third reflector) is provided in the middle of waveguide 25. The reflectivity of the loop mirror 33 for C-band light is, for example, 90% or more. The reflectivity of the loop mirror 35 for L-band light is, for example, 90% or more.

[0020] The multiplexer 27 is, for example, a 2-input, 2-output element. Waveguide 22 is optically coupled to the first input terminal of the multiplexer 27. Waveguide 25 is optically coupled to the second input terminal. Waveguide 28 is optically coupled to the first output terminal of the multiplexer 27 and extends to the edge of the substrate 10. Waveguide 29 is optically coupled to the second output terminal and extends to the edge of the substrate 10.

[0021] Waveguide 23 is located opposite to waveguide 20 of the ring resonator 32 in the X-axis direction. Waveguide 26 is located opposite to waveguide 24 of the ring resonator 32. The portion of waveguide 23 parallel to the X-axis and the portion of waveguide 26 parallel to the X-axis are spaced apart from each other. The portions of waveguide 23 and waveguide 26 parallel to the Y-axis are optically coupled to the ring resonator 32. Waveguides 23 and 26 extend from the ring resonator 32 in opposite directions. Waveguides 20 and 23 extend from the ring resonator 32 in the same direction. Waveguides 24 and 26 extend from the ring resonator 32 in the same direction.

[0022] Waveguides 23 and 26 do not extend to the edge of the substrate 10, but terminate before the edge. A loop mirror 34 (second reflector) is provided in the middle of waveguide 23. A loop mirror 36 (fourth reflector) is provided in the middle of waveguide 26. The reflectivity of loop mirror 34 for C-band light is, for example, 90% or more. The reflectivity of loop mirror 36 for L-band light is, for example, 90% or more.

[0023] A heater 37 (first heater) is provided in the ring resonator 30. A heater 38 (second heater) is provided in the ring resonator 32. A heater 39 is provided in the waveguide 20. A heater 40 is provided in the waveguide 24. Multiple pads are provided on the substrate 10. Pads 37a and 37b are electrically connected to heater 37. Pads 38a and 38b are electrically connected to heater 38. Pads 39a and 39b are electrically connected to heater 39. Pads 40a and 40b are electrically connected to heater 40. The heaters are made of metal, for example, platinum (Pt). The pads are made of metal, for example, gold (Au).

[0024] Figures 2A and 2B are cross-sectional views illustrating a tunable light source 100. Figure 2A shows a cross-section along line AA in Figure 1. Figure 2B shows a cross-section along line BB in Figure 1. As shown in Figures 2A and 2B, the substrate 10 is an SOI substrate and has a substrate 42, a box layer 44, and a silicon layer 46. The box layer 44 is laminated on one side of the substrate 42. The silicon layer 46 is laminated on the side of the box layer 44 opposite to the substrate 42. The substrate 42 and the silicon layer 46 are made of silicon (Si). The box layer 44 is made of an insulator such as silicon oxide (SiO2). The thickness of the silicon layer 46 is, for example, 220 nm. The refractive index of the silicon layer 46 is 3.45. The refractive index of the box layer 44 and the cladding layer 49 (described later) is lower than that of the silicon layer 46, at 1.45. These refractive indices are values ​​for light with a wavelength of 1.55 μm.

[0025] The silicon layer 46 has a waveguide 20, a recess 47, and a terrace 48. The recess 47 is a groove extending along the waveguide 20, located on both sides of the waveguide 20 and between the waveguide 20 and the terrace 48. The terrace 48 is located outside the recess 47 and is a plate-like portion.

[0026] As shown in Figures 2A and 2B, the upper surfaces of the waveguide 20 and the terrace 48 are at the same height in the Z-axis direction. The recess 47 is recessed below the upper surfaces of the terrace 48 and the waveguide 20. The recess 47 may extend partway through the silicon layer 46 in the Z-axis direction, or it may penetrate the silicon layer 46. In the cross-section of Figure 2A, the inside of the recess 47 is filled with air. In the cross-section of Figure 2B, the inside of the recess 47 is filled with the same material (SiO2) as the cladding layer 49. Other waveguides and ring resonators also have the above configuration.

[0027] As shown in Figure 2A, the gain section 12 is bonded to the silicon layer 46 of the substrate 10. The gain section 12 has a mesa 15. The mesa 15 protrudes from the substrate 10 in the Z-axis direction and is located above the waveguide 20.

[0028] The gain section 12 has a damage mitigation layer 50, a cladding layer 52, a photoconfinement layer 53, an active layer 54, a photoconfinement layer 55, a cladding layer 56, and a contact layer 58. The damage mitigation layer 50 is bonded to the upper surface of the silicon layer 46. The damage mitigation layer 50 may be in contact with the upper surface of the silicon layer 46. An adhesive layer, such as a resin, may be provided between the damage mitigation layer 50 and the silicon layer 46. On the side of the damage mitigation layer 50 opposite to the silicon layer 46, the cladding layer 52, the photoconfinement layer 53, and the active layer 54 are laminated in this order. The damage mitigation layer 50, cladding layer 52, photoconfinement layer 53, and active layer 54 are located below the mesa 15 and extend outside the mesa 15. The mesa 15 has an active layer 54, a photoconfinement layer 55, a cladding layer 56, and a contact layer 58. A portion of the active layer 54 protrudes in the Z-axis direction. The light-trapping layer 55, cladding layer 56, and contact layer 58 are laminated on the protruding portion of the active layer 54.

[0029] The damage mitigation layer 50, the photoconfinement layer 53, and the photoconfinement layer 55 are formed of, for example, undoped gallium indium arsenide phosphide (i-GaInAsP). The thickness of the damage mitigation layer 50 is, for example, 200 nm. The thickness of the photoconfinement layer 53 and the photoconfinement layer 55 is, for example, 100 nm. The band gap wavelength of the damage mitigation layer 50, the photoconfinement layer 53, and the photoconfinement layer 55 is, for example, 1.2 μm, which is shorter than the wavelength of the light emitted from the gain section 12.

[0030] The cladding layer 52 is formed of, for example, n-type indium phosphide (n-InP). The thickness of the cladding layer 52 is, for example, 200 nm. For example, Si is doped as an n-type dopant. The dopant concentration of the cladding layer 52 is, for example, 1 × 10⁻⁶. 19 cm -3 The cladding layer 56 is formed of, for example, p-type InP (p-InP). The thickness of the cladding layer 56 is, for example, 1500 nm. The contact layer 58 is formed of, for example, p+-type gallium indium arsenide ((p+)-GaInAs). For example, zinc (Zn) is doped as a p-type dopant. The dopant concentration of the cladding layer 56 is, for example, 1 × 10⁻⁶. 18 cm -3 Therefore, the dopant concentration of the contact layer 58 is, for example, 1 × 10⁻⁶. 19 cm -3 That is the case.

[0031] The active layer 54 has a multi-quantum well (MQW) structure and includes multiple well layers and multiple barrier layers. The well layers and barrier layers are stacked alternately. One well layer is formed of, for example, gallium indium arsenide phosphide (GaInAsP) with a thickness of 6 nm. One barrier layer is formed of, for example, GaInAsP with a thickness of 10 nm.

[0032] A p-type cladding layer 56 and contact layer 58, an i-type active layer 54, and an n-type cladding layer 52 are stacked to form a PIN (positive-intrinsic-negative) junction. The mesa 15 functions as a current-constricting structure.

[0033] The cladding layer 49 is formed of, for example, SiO2. Its thickness is, for example, 1 μm. The cladding layer 49 covers the surface of the gain portion 12 and the silicon layer 46. The cladding layer 49 covers the side and top surfaces of the mesa 15. The cladding layer 49 has an opening on the mesa 15.

[0034] Electrode 59 is provided on the mesa 15 and is electrically connected to the contact layer 58. Electrode 59 is formed of, for example, a titanium, platinum, and gold laminate (Ti / Pt / Au). An electrode (not shown) is electrically connected to the cladding layer 52. This electrode is formed of, for example, a gold, germanium, and Ni alloy (AuGeNi).

[0035] The gain section 14 has the same configuration as the gain section 12. The active layer 54 of the gain section 12 is designed to have optical gain in the C-band, for example. The active layer 54 of the gain section 14 is designed to have optical gain in the L-band, for example. The wavelength tuning range of the gain section 12 includes the C-band. The wavelength tuning range of the gain section 14 includes the L-band.

[0036] As shown in Figure 2B, in the position where the gain section is not bonded, the cladding layer 49 covers the upper surface of the silicon layer 46. The heater 39 is provided on the side of the cladding layer 49 opposite to the substrate 10.

[0037] The tunable light source 100 can emit C-band and L-band light. When emitting C-band light, a voltage is applied to the gain unit 12, and carriers are injected into the active layer 54 of the gain unit 12. Light is emitted from both ends of the gain unit 12. The gain unit 12 is evanescently photocoupled to the waveguide 20, and the light is transferred from the gain unit 12 to the waveguide 20.

[0038] Light emitted from the first end of the gain unit 12 propagates through the waveguide 20, passes around the ring resonator 30 several times, and then transfers to the waveguide 22. A portion of the light is reflected by the loop mirror 33 in the waveguide 22, propagates through the ring resonator 30 and the waveguide 20, and returns to the gain unit 12. Light emitted from the second end of the gain unit 12 propagates through the waveguide 20, passes around the ring resonator 32 several times, and then transfers to the waveguide 23. A portion of the light is reflected by the loop mirror 34 in the waveguide 23, propagates through the ring resonator 32 and the waveguide 20, and returns to the gain unit 12. The light is repeatedly reflected by the loop mirrors 33 and 34, causing laser oscillation. Light emitted from the gain unit 14 is repeatedly reflected by the loop mirrors 35 and 36, causing laser oscillation. The oscillation wavelength of the light is determined by the ring resonator.

[0039] Figure 3 illustrates a reflection spectrum. The horizontal axis represents the wavelength of light. The vertical axis represents the reflectance of light that has traveled around the ring resonator and reflected by the loop mirror. As shown in Figure 3, peaks occur periodically and theoretically exist infinitely at regular intervals. Light resonates through the ring resonator. A peak occurs at the resonant wavelength. The wavelength of the peak is determined by the circumference of the ring resonator.

[0040] The oscillation wavelength is determined by the Vernier effect between ring resonators 30 and 32. In region 16 of the optical filter 11, reflected light is generated from ring resonator 30 and loop mirror 33. Reflected light is also generated from ring resonator 32 and loop mirror 34. The circumference of ring resonator 30 is different from the circumference of ring resonator 32. The peaks of the spectra of the two reflected lights coincide at a certain wavelength. Laser oscillation occurs at that wavelength. The Vernier effect described above is also utilized in region 18.

[0041] As shown in Figure 1, a heater 37 is provided in the ring resonator 30. A heater 38 is provided in the ring resonator 32. When current flows through the heater, the heater generates heat. The heat generated by the heater heats the ring resonator. The refractive index of the ring resonator changes with the temperature change, and the spectrum changes. The wavelength of the laser light can be changed. A heater 39 is provided in the waveguide 20. A heater 40 is provided in the waveguide 24. By heating the waveguide using these heaters, it is also possible to fine-tune the phase of the light. The relationship between the power input to the heater and the amount of change in the resonant wavelength of the light should be investigated through tests, etc.

[0042] Figures 4A to 4D are plan views illustrating examples of light propagation, with magnified views of the area around the ring resonator 30 and the loop mirror 33. The arrows in the figures indicate the direction of light.

[0043] In Figure 4A, light propagates through waveguide 20 and moves onto the ring resonator 30. The light orbits the ring resonator 30 clockwise. After several orbits of the ring resonator 30, the light moves onto waveguide 22. Ideally, the light does not move from the ring resonator 30 to waveguide 25.

[0044] As shown in Figure 4B, light is reflected by a loop mirror 33 installed in waveguide 22. The light then moves from waveguide 22 to the ring resonator 30. The light circulates around the ring resonator 30 counterclockwise. After circling the ring resonator 30 several times, the light moves back to waveguide 22. Ideally, the light does not move from the ring resonator 30 to waveguide 24.

[0045] In Figure 4C, light propagates through waveguide 24 and moves onto the ring resonator 30. The light orbits the ring resonator 30 counterclockwise. After several orbits of the ring resonator 30, the light moves onto waveguide 25. Ideally, the light does not move from the ring resonator 30 to waveguide 22.

[0046] As shown in Figure 4D, light is reflected by a loop mirror 35 installed in waveguide 25. The light moves from waveguide 25 to the ring resonator 30. The light orbits the ring resonator 30 clockwise. After several orbits of the ring resonator 30, the light moves to waveguide 24. Ideally, the light does not move from the ring resonator 30 to waveguide 20.

[0047] Light propagating through waveguide 20 toward the ring resonator 32 moves from waveguide 20 to the ring resonator 32, and then from the ring resonator 32 to waveguide 23. The light is reflected by the loop mirror 34 in waveguide 23 and returns to waveguide 20. Ideally, this light does not propagate through waveguides 24 and 26. Light propagating through waveguide 24 toward the ring resonator 32 moves from waveguide 22 to the ring resonator 32, and then from the ring resonator 32 to waveguide 26. The light is reflected by the loop mirror 36 in waveguide 26 and returns to waveguide 24. Ideally, this light does not propagate through waveguides 20 and 23.

[0048] As described above, in principle, the light emitted from the gain unit 12 propagates through waveguide 20, ring resonators 30 and 32, waveguide 22 and waveguide 23, but does not propagate through waveguides 24, 25 and 26. The light emitted from the gain unit 14 propagates through waveguide 24, ring resonators 30 and 32, waveguide 25 and waveguide 26, but does not propagate through waveguides 20, 22 and 23. That is, C-band light propagates through region 16 of the optical filter 11 and does not propagate through region 18. L-band light propagates through region 18 of the optical filter 11 and does not propagate through region 16.

[0049] Waveguides 23 and 26 terminate before the edge of the substrate 10. Therefore, no light is emitted from waveguides 23 and 26.

[0050] Light propagating through waveguide 22 is input to multiplexer 27, and from multiplexer 27 is distributed to waveguides 28 and 29. Light propagating through waveguide 25 is input to multiplexer 27, and from multiplexer 27 is distributed to waveguides 28 and 29. Waveguides 28 and 29 extend to the edge of substrate 10. Light is emitted from waveguides 28 and 29.

[0051] For example, an optical fiber (not shown) is coupled to waveguide 28. The light emitted from is input to a modulator or similar device. The light emitted from waveguide 29 is input to a device other than the modulator.

[0052] Figure 5A is a plan view illustrating a multiplexer 27. The multiplexer 27 is a mosaic element provided on the substrate 10. The shape of the multiplexer 27 in the XY plane is rectangular. The length L3 of the multiplexer 27 in the X-axis direction is, for example, 10.5 μm. The length L4 of the multiplexer 27 in the Y-axis direction is, for example, 3 μm. The multiplexer 27 has a plurality of holes 60 provided in the silicon layer 46. When the multiplexer 27 is viewed from above, the shape of each of the multiple holes 60 is, for example, circular. The shape of each of the multiple holes 60 may also be a square, rectangle, ellipse, etc. The multiple holes form a mosaic pattern. In Figure 5A, a large rectangle or square represents a region in which many holes 60 are arranged. In Figure 5A, a small rectangle represents a region in which several holes 60 are arranged in one or two rows. In the example in Figure 5A, the pattern of the multiple holes 60 is symmetric with respect to a straight line passing through the multiplexer 27 and parallel to the X-axis. Furthermore, in the example shown in Figure 5A, the pattern of the multiple holes 60 is symmetric with respect to a straight line passing through the multiplexer 27 and parallel to the Y-axis.

[0053] In Figure 5A, the right end of the multiplexer 27 is the input terminal, and the left end is the output terminal. Waveguides 22 and 25 are connected to the input terminal. The width W1 of the waveguides in the Y-axis direction is, for example, 0.6 μm. Waveguides 22 and 25 are spaced apart in the Y-axis direction. The distance D1 between the centers of waveguide 22 and waveguide 25 in the Y-axis direction is, for example, 1.2 μm. Waveguides 28 and 29 are connected to the output terminal.

[0054] Figure 5B is a cross-sectional view illustrating the multiplexer 27. The holes 60 extend along the Z-axis from the top surface of the silicon layer 46 to the top surface of the box layer 44. The cladding layer 49 covers the top surface of the silicon layer 46 and the holes 60. The inside of the holes 60 may be filled with the cladding layer 49 as in the example in Figure 5B, or it may be filled with air.

[0055] The refractive index of the holes 60 filled with the cladding layer 49 is different from that of the silicon layer 46. The light input to the multiplexer 27 is scattered and branched within the plane of the multiplexer 27. A portion of the light is output to the waveguide 28. Another portion of the light is output to the waveguide 29.

[0056] Figure 6 illustrates the characteristics of the multiplexer 27. The horizontal axis represents the wavelength of light. The vertical axis represents the transmittance of light. The black squares and dashed lines represent the sum of the transmittances to the two waveguides 28 and 29 (Total). The white triangles and dotted lines represent the transmittance (Tbar) to one of the two waveguides 28 and 29. The black triangles and solid lines represent the transmittance (Tcross) to the other waveguide 29. The white circles and dotted lines represent the proportion of light (Rbar) that returns to the waveguide from which the light was input. The black circles and solid lines represent the transmittance to the other waveguide (Rcross).

[0057] In Figure 6, the wavelength of light is in the range of 1500 nm to 1650 nm, including the C-band and L-band. In the C-band (1530 nm and above, and 1565 nm and below), light is input from waveguide 22 and distributed to waveguides 28 and 29. Tbar represents the transmittance to waveguide 28. Tcross represents the transmittance to waveguide 29. Rbar represents the proportion of light reflected by waveguide 22. Rcross represents the proportion of light reflected by waveguide 25. In the L-band (1565 nm and above, and 1625 nm and below), light is input from waveguide 25 and distributed to waveguides 28 and 29. Tbar represents the transmittance to waveguide 29. Tcross represents the transmittance to waveguide 28. Rbar represents the proportion of light reflected by waveguide 25. Rcross represents the proportion of light reflected by waveguide 22.

[0058] As shown in Figure 6, in the C-band and L-band, the transmittance to the two waveguides 28 and 29 is -10 dB or higher. The transmittance to waveguide 28 and waveguide 29 are approximately the same. The multiplexer 27 distributes the input light to the two waveguides 28 and 29 in a ratio of approximately 1:1. The distribution ratio may be other than 1:1.

[0059] Gain section 12 is for the C band, and gain section 14 is for the L band. When one gain section is driven, the other gain section is not driven. Light may return from the multiplexer 27 to the driven gain section. The phase of the returned light may affect the emitted light, causing degradation of characteristics such as a widening of the spectral linewidth. In Figure 6, Rbar is below -40 dB for most wavelengths. For example, when gain section 12 is driven, light is input from waveguide 22 to multiplexer 27. Of this light, only a very small amount returns to waveguide 22.

[0060] Some of the light input from waveguide 22 is reflected by waveguide 25. This light is reflected by loop mirror 35, input to multiplexer 27 again, and may return to waveguide 22. As shown in Figure 6, Rcross is less than -20 dB. Two reflections occur: one to waveguide 25 and another to waveguide 22 after re-input to multiplexer 27. The Rcross (-20 dB) is added twice, resulting in -40 dB. Therefore, the intensity of the light returning to waveguide 22 is small. Degradation of the spectral linewidth is less likely to occur. The linewidth expansion by multiplexer 27 is, for example, about 10% of the linewidth before input to multiplexer 27.

[0061] (Comparative example) Figure 7 is a plan view illustrating a tunable light source 100R according to a comparative example. The tunable light source 100R has one gain unit 12 and one optical filter 11a. The optical filter 11a has waveguides 20, 22, 23, ring resonators 30 and 32. The optical filter 11a has heaters 37, 38, 39, pads 37a, 37b, 38a, 38b, 39a and 39b. The gain unit 12 emits C-band light. C-band light propagates through the optical filter 11a.

[0062] To emit L-band light in addition to C-band light, another tunable light source 100R corresponding to the L-band is used. This can be achieved by adding another optical filter corresponding to the L-band to the substrate 10 and joining another gain section corresponding to the L-band. However, this doubles the number of waveguides, ring resonators, heaters, and pads. The configuration becomes more complex, and the tunable light source becomes larger. Adjusting the wavelength using heater power also becomes more complex.

[0063] According to the first embodiment, the tunable light source 100 has an optical filter 11, two gain units 12 and a gain unit 14. The gain unit 12 and region 16 of the optical filter 11 are optically coupled to form a laser resonator corresponding to, for example, the C-band. The gain unit 14 and region 18 of the optical filter 11 are optically coupled to form a laser resonator corresponding to, for example, the L-band. The tunable light source 100 can cover the C-band and the L-band. Regions 16 and 18 share ring resonators 30 and 32. Therefore, the configuration is simpler compared to the case where two tunable light sources are arranged side by side as shown in Figure 7. The tunable light source 100 can be miniaturized.

[0064] Gain unit 12 emits light having a C-band wavelength. The light propagates through region 16 and generates laser oscillation. Gain unit 14 emits light having an L-band wavelength. The light propagates through region 18 and generates laser oscillation. To reduce light loss, it is important that the C-band light propagates through region 16 and does not leak into region 18. It is also important that the L-band light propagates through region 18 and does not leak into region 16.

[0065] As shown in Figures 4A to 4D, waveguides 20 and 22 are located opposite each other in the X-axis direction, with the ring resonator 30 in between. Waveguides 20 and 22 extend from the ring resonator 30 to the same side in the Y-axis direction. Waveguides 24 and 25 are located opposite each other in the X-axis direction, with the ring resonator 30 in between. Waveguides 24 and 25 extend from the ring resonator 30 to the same side in the Y-axis direction, and also extend to the opposite side from waveguides 20 and 22.

[0066] As shown in Figure 4A, light that has transferred from waveguide 20 to the ring resonator 30 circulates around the ring resonator 30 clockwise. Clockwise light transfers from the ring resonator 30 to waveguide 22, but does not easily transfer to waveguide 25. As shown in Figure 4B, light reflected by the loop mirror 33 of waveguide 22 circulates around the ring resonator 30 counterclockwise. Counterclockwise light transfers from the ring resonator 30 to waveguide 20, but does not easily transfer to waveguide 24. The light emitted from the gain unit 12 propagates into region 16 and does not easily leak into region 18. As shown in Figures 4C and 4D, light propagating between waveguide 24, ring resonator 30, and waveguide 25 in region 18 does not easily leak into region 16. Light loss can be reduced.

[0067] Waveguides 20 and 23 are located opposite each other with respect to the ring resonator 32 and extend from the ring resonator 32 to the same side. Waveguides 24 and 26 are located opposite each other with respect to the ring resonator 32 and extend from the ring resonator 32 to the same side, and further extend to the opposite side from waveguides 20 and 23. Similar to Figures 4A to 4D, light leakage between region 16 and region 18 is minimized. Light loss can be reduced.

[0068] Gain unit 12 is bonded to the substrate 10 at a position overlapping with waveguide 20. Loop mirror 33 is provided on waveguide 22. Loop mirror 34 is provided on waveguide 23. The light emitted from gain unit 12 is reflected by loop mirrors 33 and 34, travels back and forth between these two loop mirrors, and generates laser oscillation. Gain unit 14 is bonded to the substrate 10 at a position overlapping with waveguide 24. Loop mirror 35 is provided on waveguide 25. Loop mirror 36 is provided on waveguide 26. The light emitted from gain unit 14 is reflected by loop mirrors 35 and 36, travels back and forth between these two loop mirrors, and generates laser oscillation. Laser resonators are formed in region 16 and region 18, respectively. C-band and L-band laser light can be generated.

[0069] The optical filter 11 includes a multiplexer 27, a waveguide 28, and a waveguide 29. The multiplexer 27 splits the C-band light input from waveguide 22 into waveguides 28 and 29 for output. The multiplexer 27 also splits the L-band light input from waveguide 25 into waveguides 28 and 29 for output. C-band and L-band laser light can be output from waveguides 28 and 29. This simplifies the configuration for optical output, such as optical fibers.

[0070] As shown in Figures 5A and 5B, the multiplexer 27 is a mosaic element. By changing the position and number of holes 60, the distribution ratio of light to waveguides 28 and 29 can be adjusted. As in the example in Figure 6, the distribution ratio is, for example, 1:1. In the wavelength range including the C-band and L-band, light of approximately equal intensity can be output to waveguides 28 and 29.

[0071] As shown in Figure 6, the intensity of light reflected from the multiplexer 27 to waveguides 22 and 25 is lower than that of light transmitted to waveguides 28 and 29. This reduces the tendency for the spectral linewidth to broaden and improves the monochromaticity of the laser light. The distribution ratio of the multiplexer 27 does not have to be 1:1. The number of output terminals of the multiplexer 27 may be two, one, or three or more. The multiplexer 27 may be composed of multiple waveguides. For example, the multiplexer may be composed of a first waveguide connecting waveguides 22 and 28, and a second waveguide connecting waveguides 29 and 25. The first waveguide and the second waveguide may constitute a directional coupler.

[0072] The wavelength of light can be adjusted by inputting power to the heater 37 provided in the ring resonator 30 and the heater 38 provided in the ring resonator 32. Regions 16 and 18 of the optical filter 11 share the ring resonators 30 and 32, as well as the heaters 37 and 38. For each heater, the relationship between the heater power and the change in wavelength should be investigated. Both the wavelength of light in region 16 and the wavelength of light in region 18 can be controlled by the heaters 37 and 38.

[0073] The substrate 10 is an SOI substrate and has a silicon layer 46. A waveguide, a ring resonator, and a multiplexer 27 are provided in the silicon layer 46. By confining light in the silicon waveguide, etc., light loss can be reduced. By heating the silicon ring resonator with a heater and changing the refractive index, the wavelength of light can be adjusted. The substrate 10 may be a substrate other than an SOI substrate, for example, it may have a silicon nitride (SiN) layer. The waveguide, ring resonator, and multiplexer 27 may be provided in the SiN layer.

[0074] Gain units 12 and 14 are light-emitting elements formed from III-V compound semiconductors. The wavelengths of the light emitted from gain units 12 and 14 are determined by the composition of the active layer 54, etc. The wavelength range of the light emitted from gain unit 12 is different from that of gain unit 14. For example, the wavelength range of gain unit 12 covers the C band, and the wavelength range of gain unit 14 covers the L band. The wavelength range of the tunable light source 100 may be other than the C band and L band.

[0075] <Second Embodiment> Figure 8 is a plan view illustrating a tunable light source 200 according to the second embodiment. The same configuration as in the first embodiment will not be described.

[0076] The tunable light source 200 includes an optical filter 70, a gain section 72 (first gain section), and a gain section 74 (second gain section). The optical filter 70 is formed on the substrate 10. Loop mirrors are not provided in the waveguides 23 and 26 of the optical filter 70. Waveguides 23 and 26 extend to one end of the substrate 10 in the X-axis direction. A heater 38 is provided on the ring resonator 32. Four pads are connected to the heater 38.

[0077] Gain units 72 and 74 are semiconductor chips formed from, for example, a III-V compound semiconductor. Gain units 72 and 74 are located outside the substrate 10, abutted against one end of the substrate 10, and joined by a butt joint.

[0078] The gain section 72 has a waveguide 73. The waveguide 73 is composed of an active layer and a cladding layer, etc. The waveguide 73 is parallel to the X-axis and extends from the first end to the second end of the gain section 72. The gain section 74 has a waveguide 75. The configuration of the waveguide 75 is the same as that of the waveguide 73.

[0079] The waveguide 73 of the gain section 72 is opposite the end of the waveguide 23 of the optical filter 70 and is optically coupled to the waveguide 23. The waveguide 75 of the gain section 74 is opposite the end of the waveguide 24 and is optically coupled to the waveguide 24.

[0080] A high reflection film (HR film) 76 is provided on the end of the gain section 72 opposite to the substrate 10. The high reflection film 76 (second reflection section) covers the end of the waveguide 73 of the gain section 72. A high reflection film 78 (fourth reflection section) is provided on the end of the gain section 74 opposite to the substrate 10. The high reflection film 78 covers the end of the waveguide 75 of the gain section 74. An anti-reflective film (AR film) 90 may be provided between the gain section 72 and the substrate 10, and between the gain section 74 and the substrate 10.

[0081] The light generated by the active layer of the gain unit 72 propagates through the waveguide 73. The light emitted from the gain unit 72 has, for example, a C-band wavelength. The light emitted from the gain unit 74 has, for example, an L-band wavelength. The light emitted from the gain unit 72 is input to the waveguide 23 and propagates through the region 16 of the optical filter 70. The light emitted from the gain unit 72 is reflected by the loop mirror 33 and the high-reflectivity film 76, travels back and forth between them, and generates laser oscillation. The laser light is output from waveguides 28 and 29.

[0082] The light generated by the active layer of the gain unit 74 propagates through the waveguide 75. The light emitted from the gain unit 74 is input to the waveguide 26 and propagates through the region 18 of the optical filter 70. The light emitted from the gain unit 74 is reflected by the loop mirror 35 and the high-reflectivity film 78, travels back and forth between them, and generates laser oscillation. The laser light is output from waveguides 28 and 29.

[0083] According to the second embodiment, the tunable light source 200 has an optical filter 70, two gain sections 72 and a gain section 74. Region 16 of the optical filter 70 and the gain section 72 constitute a laser resonator corresponding to, for example, the C-band. Region 18 of the optical filter 70 and the gain section 74 constitute a laser resonator corresponding to, for example, the L-band. The tunable light source 200 can cover the C-band and the L-band. Regions 16 and 18 share a ring resonator 30 and a ring resonator 32. The configuration is simplified and the tunable light source 200 can be miniaturized.

[0084] <Third Embodiment> Region 16 of the tunable light source has a waveguide 20 and a gain section 12, and generates C-band (1530 nm to 1565 nm, first band) laser light. The band gap of the active layer of the gain section 12 is sized to correspond to the C-band. Region 18 has a waveguide 24 and a gain section 14, and generates L-band (1565 nm to 1625 nm, second band) laser light. The band gap of the active layer 54 of the gain section 14 is sized to correspond to the L-band, and is narrower than the band gap of the gain section 12.

[0085] Waveguides 20 and 24 form a loop-type waveguide (loop waveguide). C-band light may leak from region 16 into waveguide 24. L-band light may leak from region 18 into waveguide 20. The active layer 54 of the gain section 14 absorbs C-band light. C-band light does not easily travel around the loop waveguide. The active layer 54 of the gain section 12 does not easily absorb L-band light. If the bandpass filter 80 is not provided, L-band light travels around the loop waveguide. This can cause unintended Fabry-Perot oscillation. In the third embodiment, such unwanted oscillations are prevented.

[0086] Figure 9 is a plan view illustrating a tunable light source 300 according to the third embodiment. Descriptions of components identical to those in the first and second embodiments are omitted. As shown in Figure 9, the tunable light source 300 includes a bandpass filter 80. The bandpass filter 80 is provided in the waveguide 20 and is located, for example, between the heater 39 and the ring resonator 32 of the waveguide 20. Other components are the same as those of the tunable light source 100 in Figure 1.

[0087] Figure 10 is a plan view illustrating a bandpass filter 80. Both the bandpass filter 80 and the multiplexer 27 in Figure 5A are called mosaic elements, but they have different functions. The multiplexer 27 in Figure 5A is a multiplexer designed to transmit both C-band and L-band light equally. On the other hand, the bandpass filter 80 looks similar to the multiplexer 27 with one input and two outputs, but the wavelength dependence of the transmittance differs between the T1 path and the T2 path in Figure 10, and it functions as a wavelength-selective filter. The details will be described later. The bandpass filter 80 has a region 81, a port 82 (first port), a port 83 (second port), and a port 84 (third port). The planar shape of region 81 is, for example, a rectangle. The length L5 of one side of region 81 is, for example, 10 μm. The region 81 is provided with holes, similar to the multiplexer 27.

[0088] The first and second ends of region 81 face each other. Port 82 is provided at the first end. Ports 83 and 84 are provided at the second end. Port 82 is an input port. Ports 83 and 84 are output ports. Ports 82 and 83 are connected to waveguide 20. Port 84 is not connected to the waveguide of the tunable light source 300, and is extended and terminated at a location away from, for example, waveguide 20 and waveguide 24. T1, T2 and T3 are explained in Figure 11.

[0089] Figure 11 illustrates transmittance. The horizontal axis represents the wavelength of light. The vertical axis represents the transmittance of light. T1 is the transmittance of light input from port 82 and transmitted through port 83, and is represented by a solid line. T2 is the transmittance of light input from port 82 and transmitted through port 84, and is represented by a dotted line. T3 is the reflectance of light input from port 82 and reflected by port 82, and is represented by a dashed line. The dashed lines in Figure 11 represent the C band (1530nm to 1565nm) and the L band (1565nm to 1625nm).

[0090] As shown in Figure 11, the transmittance T1 from port 82 to port 83 is high in the C band and low in the L band. The transmittance T2 from port 82 to port 84 is low in the C band and high in the L band. The reflectance T3 returning to port 82 is lower than the transmittances T1 and T2 at most wavelengths.

[0091] In the L-band, the transmittance T2 between port 82 and port 84 is higher than the transmittance T1 between port 82 and port 83. L-band light leaking from region 18 into waveguide 20 is less likely to propagate to port 83 and is output from port 84. Port 84 is not connected to waveguide 20. The L-band light does not travel around waveguide 20 and waveguide 24 completely. Abnormal oscillation is less likely to occur.

[0092] In the C-band, transmittance T1 is higher than transmittance T2. C-band light generated in region 16 does not propagate easily to port 84 but propagates easily to port 83. The C-band light passes through the bandpass filter 80, propagates through the waveguide 20, and causes laser oscillation. The bandpass filter 80 does not significantly affect the laser oscillation in region 16.

[0093] Figures 12A and 12B illustrate the optical spectrum when only the L-band laser section is driven, and the C-band laser section is not driven. The horizontal axis represents the wavelength of light and represents the wavelength band of the L-band. The vertical axis represents the relative light intensity. The gain section 14 in region 18 is operated with a target wavelength of 1590 nm. The gain section 12 in region 16 is not operated.

[0094] In the example shown in Figure 12A, the bandpass filter 80 is not provided. Oscillations Re1 and Re2 occur. Re1 is located around 1590 nm and is represented by a solid line. Re2 is located around 1615 nm and is represented by a dotted line. Oscillation Re1 is a normal oscillation. On the other hand, oscillation Re2 is an unintended Fabry-Perot oscillation (abnormal oscillation) caused by light circulating through waveguides 20 and 24.

[0095] In the example shown in Figure 12B, a bandpass filter 80 is provided in the waveguide 20. Normal oscillation Re1 occurs. Since the bandpass filter 80 blocks the L band, abnormal oscillation Re2 does not occur.

[0096] According to the third embodiment, a bandpass filter 80 is provided in the waveguide 20 of region 16. Ports 82 and 83 are connected to the waveguide 20. Port 84 is not connected to the waveguide 20. In the C band, the transmittance T1 between ports 82 and 83 is higher than the transmittance T2 between ports 82 and 84. The C band light generated in region 16 passes through the bandpass filter 80 and causes laser oscillation. In the L band, the transmittance T2 is higher than the transmittance T1. The L band light propagating through the waveguide 20 does not easily propagate to port 83 of the bandpass filter 80, but propagates to port 84. The L band light is blocked by the bandpass filter 80 and does not easily travel around the waveguide 20 and waveguide 24. Abnormal oscillation can be prevented.

[0097] The L band is a longer wavelength band than the C band. The C band is a shorter wavelength band than the L band. The active layer 54 of the gain section 14 has a high absorption rate for C band light. The gain section 14 absorbs light leaking from region 16 to region 18. Abnormal oscillation is unlikely to occur.

[0098] The bandpass filter 80 may be a mosaic element or an element other than a mosaic element. A mosaic element bandpass filter 80 can be miniaturized to, for example, about 10 μm × 10 μm. The bandpass filter 80 may be placed at any position in the waveguide 20.

[0099] <Fourth Embodiment> Figure 13 is a plan view illustrating a tunable light source 400 according to the fourth embodiment. Descriptions of configurations identical to those in any of the first to third embodiments are omitted.

[0100] A bandpass filter 80 is provided in the waveguide 20 of region 16. A bandpass filter 85 is provided in the waveguide 24 of region 18. Bandpass filters 80 and 85 are, for example, mosaic elements, and both are 1-input, 2-output filters.

[0101] The input port of the bandpass filter 85 is connected to the waveguide 24. The first output port of the bandpass filter 85 is connected to the waveguide 24. The second output port is not connected to the waveguide 24. The transmittance between the input port and the first output port of the bandpass filter 85 is low in the C band and high in the L band, as shown at T2 in Figure 11. The transmittance between the input port and the second output port is high in the C band and low in the L band, as shown at T1 in Figure 11.

[0102] According to the fourth embodiment, L-band light passes through the bandpass filter 85 and causes laser oscillation. C-band light may leak from region 16 into waveguide 24. This light is absorbed by the gain unit 14 and propagates to the second output port of the bandpass filter 85, without circling waveguide 24 and waveguide 20. Abnormal oscillation can be prevented.

[0103] Although embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to these specific embodiments, and various modifications and changes are possible within the scope of the gist of the present disclosure as described in the claims. For example, the multiplexer 27 may be a directional coupler. In that case, the multiplexer 27 is composed of a plurality of waveguides. For example, the directional coupler may consist of a first waveguide connecting waveguide 22 and waveguide 28, and a second waveguide connecting waveguide 29 and waveguide 25. [Explanation of symbols]

[0104] 10, 42 boards 11, 11a Optical filters 12, 14, 72, 74 Gain section 15 Mesa 16, 18 areas Waveguides 20, 22, 23, 24, 25, 26, 28, 29, 73, 75 27 Multiplexer 30, 32 Ring resonators 33, 34, 35, 36 Loop Mirror 37, 38, 39, 40 Heater 37a, 37b, 38a, 38b, 39a, 39b, 40a, 40b pad 46 Silicon layer 47 Recess 48 Terrace 49, 52, 56 Clad layers 50 Damage Mitigation Layer 53, 55 Light confinement layer 54 Active layer 58 Contact Layer 59 Electrode 60 holes 76, 78 Highly reflective coating 80, 85 bandpass filter 81 areas Ports 82, 83, and 84 90 Anti-reflection coating 100, 100R, 200 wavelength tunable light source

Claims

1. A first gain unit having optical gain and emitting light in a first band, A second gain unit having optical gain and emitting light in the second band, An optical filter provided on the substrate, A first reflecting section and a second reflecting section that reflect light in the first band, It comprises a third reflecting part and a fourth reflecting part that reflect light in the second band, The optical filter comprises a first region for propagating light in the first band, a second region for propagating light in the second band, a first ring resonator, and a second ring resonator. The first gain unit is optically coupled to the first region. The second gain unit is optically coupled to the second region. The first region has a first waveguide, a second waveguide, and a third waveguide. The second region has a fourth waveguide, a fifth waveguide, and a sixth waveguide. The first waveguide and the fourth waveguide are optically coupled to the first ring resonator and the second ring resonator, The second waveguide and the fifth waveguide are optically coupled to the first ring resonator. The third waveguide and the sixth waveguide are optically coupled to the second ring resonator. In the direction of light propagation, the first waveguide, the first ring resonator, and the second ring resonator are provided between the first reflecting portion and the second reflecting portion. A tunable light source having the fourth waveguide, the first ring resonator, and the second ring resonator positioned between the third reflector and the fourth reflector in the direction of light propagation.

2. The first waveguide and the second waveguide are located opposite each other with respect to the first ring resonator and extend from the first ring resonator to the same side. The fourth waveguide and the fifth waveguide are located opposite each other with respect to the first ring resonator and extend from the first ring resonator to the same side. The first waveguide and the third waveguide are located opposite each other with respect to the second ring resonator and extend from the second ring resonator to the same side. The tunable light source according to claim 1, wherein the fourth waveguide and the sixth waveguide are located opposite each other with respect to the second ring resonator and extend from the second ring resonator to the same side.

3. The fourth waveguide extends from the first ring resonator and the second ring resonator to the side opposite to the first waveguide, The fifth waveguide extends from the first ring resonator to the side opposite to the second waveguide, The tunable light source according to claim 1 or claim 2, wherein the sixth waveguide extends from the second ring resonator to the side opposite to the third waveguide.

4. The first reflecting section is a loop mirror provided in the second waveguide, The second reflecting section is a loop mirror provided in the third waveguide. The third reflecting section is a loop mirror provided in the fifth waveguide, The fourth reflecting section is a loop mirror provided in the sixth waveguide, The first gain unit and the second gain unit are bonded to one surface of the substrate. The first gain unit is provided at a position overlapping with the first waveguide and is optically coupled to the first waveguide. The tunable light source according to claim 1 or 2, wherein the second gain section is provided at a position overlapping with the fourth waveguide and is optically coupled with the fourth waveguide.

5. The first reflecting section is a loop mirror provided in the second waveguide, The third reflecting section is a loop mirror provided in the fifth waveguide, The first gain section and the second gain section are butt-jointed to the substrate. The first gain unit is optically coupled to the third waveguide. The second gain unit is optically coupled to the sixth waveguide. The second reflective portion is a reflective film provided at a position opposite to the optical filter of the first gain portion, The tunable light source according to claim 1 or claim 2, wherein the fourth reflective portion is a reflective film provided at a position opposite to the optical filter of the second gain portion.

6. The optical filter has a multiplexer, a seventh waveguide, and an eighth waveguide. The second waveguide is optically coupled to the first input terminal of the multiplexer. The fifth waveguide is optically coupled to the second input terminal of the multiplexer. The seventh waveguide is optically coupled to the first output terminal of the multiplexer. The tunable light source according to claim 1 or claim 2, wherein the eighth waveguide is optically coupled to the second output terminal of the multiplexer.

7. The wavelength-tunable light source according to claim 6, wherein the multiplexer is a mosaic element.

8. A first heater provided in the first ring resonator, A tunable light source according to claim 1 or claim 2, comprising a second heater provided in the second ring resonator.

9. The substrate has a silicon layer or a silicon nitride layer. The tunable light source according to claim 1 or claim 2, wherein the first waveguide, the second waveguide, the third waveguide, the fourth waveguide, the fifth waveguide, and the sixth waveguide, the first ring resonator, and the second ring resonator are provided in the silicon layer or the silicon nitride layer.

10. The second band is a band with longer wavelengths than the first band. Equipped with a bandpass filter, The bandpass filter has a first port, a second port, and a third port. The first port and the second port are connected to the first waveguide, In the first bandwidth, the transmittance between the first port and the second port is higher than the transmittance between the first port and the third port. The tunable light source according to claim 1 or claim 2, wherein in the second band, the transmittance between the first port and the third port is higher than the transmittance between the first port and the second port.

11. The tunable light source according to claim 10, wherein the bandpass filter is a mosaic element.

12. The first area and, The second area and, The first ring resonator and, It comprises a second ring resonator, The first region has a first waveguide, a second waveguide, and a third waveguide. The second region has a fourth waveguide, a fifth waveguide, and a sixth waveguide. The first waveguide and the fourth waveguide are optically coupled to the first ring resonator and the second ring resonator, The second waveguide and the fifth waveguide are optically coupled to the first ring resonator. The third waveguide and the sixth waveguide are optical filters optically coupled to the second ring resonator.

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