wavelength-tunable laser device

Abstract

To provide a wavelength variable laser device that produces high quality laser light.SOLUTION: A wavelength variable laser device includes a first mirror, a second mirror, an optical amplifier provided between the first mirror and the second mirror, a wavelength tunable filter provided between the first mirror and the second mirror, and an optical waveguide coupling the optical amplifier and wavelength tunable filter. The optical waveguide includes a first waveguide formed with a first width and a second waveguide formed with a second width greater than the first width.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a wavelength tunable laser device. [Background technology]

[0002] Wavelength-tunable laser devices are becoming increasingly popular as optical devices for achieving high-capacity optical communications. Silicon photonics, which utilizes complementary metal oxide semiconductor (CMOS) technology to realize high-density optical integrated circuits, is also attracting attention.

[0003] A wavelength-tunable laser device includes, for example, an amplifier and an external resonator. When silicon photonics is used, the external resonator is configured by a silicon optical integrated circuit including a silicon waveguide. Here, since the silicon waveguide has a high refractive index, the optical length (or optical path length) can be made long. Therefore, in a wavelength-tunable laser device in which the external resonator is formed by a silicon optical integrated circuit, the spectral width of the output light can be narrowed while realizing miniaturization of the device. Note that a wavelength-tunable laser device including an external resonator is described in, for example, Patent Document 1 or Non-Patent Document 1. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-046144 [Non-patent literature]

[0005] [Non-Patent Document 1] N.Kobayashi, K.Sato et al, Silicon Photonic Hybrid Ring-Filter External Cavity Wavelength Tunable Lasers, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.33, NO.6, P1241-PP1246, MARCH 15, 2015 Summary of the Invention [Problem to be solved by the invention]

[0006] Transmitting high-quality multilevel optical signals requires laser light with a narrow spectral width. To generate laser light with a narrow spectral width, it is preferable to increase the optical length of the cavity. However, within the cavity, multiple weak spectral components called "longitudinal modes" emerge. Longitudinal modes appear at wavelength intervals inversely proportional to the optical length of the cavity. Therefore, increasing the optical length of the cavity to narrow the spectral width of the laser light narrows the wavelength intervals of the longitudinal modes, making it more likely that longitudinal modes will exist near the oscillation wavelength. Furthermore, if light unnecessary for oscillation is generated within the cavity, energy may be transferred to longitudinal modes that appear near the oscillation wavelength due to a phenomenon called mode hopping. In other words, the longitudinal modes may be amplified. This results in unstable laser oscillation and a deterioration in the quality of the laser light.

[0007] An object of one aspect of the present invention is to provide a wavelength tunable laser device that generates high-quality laser light. [Means for solving the problem]

[0008] A wavelength tunable laser device according to one aspect of the present invention includes a first mirror, a second mirror, an optical amplifier unit provided between the first mirror and the second mirror, a wavelength tunable filter provided between the first mirror and the second mirror, and an optical waveguide coupling the optical amplifier unit and the wavelength tunable filter. The optical waveguide includes a first waveguide formed with a first width and a second waveguide formed with a second width wider than the first width. [Effects of the Invention]

[0009] According to the above-described aspect, it is possible to provide a wavelength tunable laser device that generates high-quality laser light. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating an example of a wavelength tunable laser device. [Figure 2] FIG. 10 is a diagram showing an example of the arrangement of a waveguide and a heater. [Figure 3] FIG. 2 is a diagram illustrating an example of the configuration of a wavelength tunable filter. [Figure 4] FIG. 10 is a diagram illustrating the distribution of optical power in an optical waveguide. [Figure 5] 1 is a diagram illustrating a first example of a wavelength tunable laser device according to an embodiment of the present invention. [Figure 6] FIG. 10 is a diagram showing an example of the arrangement of a wide waveguide and a heater. [Figure 7] 1A and 1B are diagrams illustrating an example of the structure of an optical waveguide. [Figure 8] FIG. 10 shows measurement results for reflection from the sidewall of the core. [Figure 9] FIG. 2 is a diagram showing a second example of a wavelength tunable laser device according to an embodiment of the present invention. [Figure 10] 10A and 10B are diagrams illustrating the layout of a wavelength tunable filter and a mirror. [Figure 11] FIG. 10 is a diagram illustrating a third example of a wavelength tunable laser device according to an embodiment of the present invention. [Figure 12]FIG. 10 is a diagram showing a fourth example of a wavelength tunable laser device according to an embodiment of the present invention. [Figure 13] 1 is a diagram showing an example of a connection surface between a waveguide of an optical amplification section and a waveguide of an optical integrated circuit. FIG. [Figure 14] FIG. 10 is a diagram illustrating a fifth example of a wavelength tunable laser device according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Fig. 1 shows an example of a wavelength tunable laser device. The wavelength tunable laser device 100 shown in Fig. 1 is formed on a silicon substrate 10. That is, the wavelength tunable laser device 100 is formed using silicon photonics technology. In this embodiment, the shape of the silicon substrate 10 is rectangular.

[0012] The tunable laser device 100 includes an optical amplifier 11, a tunable filter 12, an optical waveguide 13, a mirror 14, and a mirror 15. The tunable laser device 100 may include other elements not shown in FIG. 1. For example, the tunable laser device 100 includes an electric circuit (e.g., electrodes) for supplying a drive current to the optical amplifier 11. The tunable laser device 100 also includes an electric circuit (e.g., heater) for controlling the wavelength of the laser light and an electric circuit (e.g., heater) for adjusting the optical length of the cavity.

[0013] As shown in Fig. 1, a silicon substrate 10 has an optical integrated circuit region and a terrace region. In the optical integrated circuit region, a wavelength tunable filter 12, an optical waveguide 13, and a mirror 14 are formed using silicon photonics technology. The terrace region is formed by removing the surface of the silicon substrate 10 to a predetermined depth. An optical amplifier 11 is provided in the terrace region.

[0014] The optical amplifier 11 is, for example, a semiconductor optical amplifier, and is mounted at a predetermined position within the terrace region so that the electrodes of the optical amplifier 11 are in electrical contact with an electrode pattern (not shown) formed in the terrace region.

[0015] The tunable filter 12 includes a ring waveguide and can select a desired wavelength. In the example shown in FIG. 1, the tunable filter 12 includes two ring waveguides, but the embodiment of the present invention is not limited to this configuration. That is, the tunable filter 12 may include one ring waveguide, or three or more ring waveguides. Note that when the tunable filter 12 includes multiple ring waveguides, the tunable filter 12 functions as a Vernier-type tunable filter as a whole, so it is preferable that the diameters of the multiple ring waveguides differ from each other by only a small amount. However, for convenience, the drawings in this application show the same diameter. The configuration of the tunable filter 12 will be described later with reference to FIG. 3.

[0016] The optical waveguide 13 optically couples the optical amplifier 11 and the wavelength-tunable filter 12. The optical waveguide 13 includes a straight waveguide 13a and an inclined waveguide 13b formed in the longitudinal direction of the silicon substrate 10. Although the straight waveguide 13a and the inclined waveguide 13b are connected at a predetermined angle in FIG. 1, they are actually connected continuously by a curved waveguide with a slight curvature. The inclined waveguide 13b is provided between the optical amplifier 11 and the optical integrated circuit to prevent light propagating through the optical waveguide 13 from being perpendicularly incident on the end faces of both elements. This configuration suppresses reflection of light entering / exiting from the optical waveguide 13 to the optical amplifier 11. The waveguides in the optical amplifier 11 are coupled to the optical waveguide 13 with sufficiently small loss. Furthermore, it is preferable that a spot size converter be formed at the tip of the optical waveguide 13 coupled to the optical amplification section 11 (the inclined waveguide 13b in FIG. 1).

[0017] The optical waveguide 13 is realized by, for example, a silicon waveguide formed on the surface of the silicon substrate 10. The cross-sectional shape of the core of the optical waveguide 13 is, for example, rectangular. The height of the core is, for example, 220 nm. In this case, the width of the core of the optical waveguide 13 is determined so that multimode propagation is sufficiently suppressed. As an example, the width of the core of the optical waveguide 13 is approximately 500 nm. Note that if the width of the core of the optical waveguide is 500 nm or less, multimode propagation is sufficiently suppressed.

[0018] A heater 21 is provided near the optical waveguide 13. The heater 21 is realized, for example, by an electric circuit formed near the optical waveguide 13. For example, as shown in FIG. 2, the heater 21 can be realized by forming a thin film of metal such as TiN on top of a 500-nm-wide waveguide core, with a width wider than the waveguide core, along the waveguide core. In this case, the electric circuit includes an electrode pattern formed near the optical waveguide 13 and terminals connected to the electrode pattern. The refractive index of the optical waveguide 13 is changed by controlling the current supplied to the heater 21. That is, by controlling the current supplied to the heater 21, the optical length of the optical waveguide 13 is adjusted, and the resonator length of the wavelength-tunable laser device 100 is adjusted. Therefore, the heater 21 can function as a phase adjuster that adjusts the phase of light propagating within the wavelength-tunable laser device 100.

[0019] The mirror 14 is optically coupled to the output end of the wavelength tunable filter 12. Therefore, the light output from the wavelength tunable filter 12 is reflected by the mirror 14 and input to the wavelength tunable filter 12. The mirror 14 is preferably a total reflection mirror with near-zero loss. Although the mirror 14 is not particularly limited, it is realized by a loop mirror in this embodiment. The loop mirror is realized by an optical waveguide.

[0020] The mirror 15 is provided at the output end of the wavelength tunable laser device 100. That is, the mirror 15 is provided on the output surface of the optical amplifier 11. The mirror 15 is a half mirror (i.e., a semi-transparent mirror). Therefore, the mirror 15 outputs a part of the optical components amplified in the optical amplifier 11 and reflects the remaining optical components. The reflectance (or transmittance) of the mirror 15 is set so that the wavelength tunable laser device 100 generates laser light with high efficiency.

[0021] In the wavelength-tunable laser device 100 configured as described above, the optical amplifier 11 is driven to generate light. The light generated by the optical amplifier 11 then propagates between the mirrors 14 and 15. That is, the light output from the optical amplifier 11 is guided to the wavelength-tunable filter 12 via the optical waveguide 13. The light that passes through the wavelength-tunable filter 12 is reflected by the mirror 14 and returns to the wavelength-tunable filter 12. The light that passes through the wavelength-tunable filter 12 is guided to the optical amplifier 11 via the optical waveguide 13. The light that is input from the optical waveguide 13 to the optical amplifier 11 is reflected by the mirror 15 and output to the optical waveguide 13.

[0022] At this time, the wavelength-tunable filter 12 passes light of the specified wavelength. Therefore, light of the specified wavelength propagates between the mirrors 14 and 15. Furthermore, by controlling the current supplied to the heater 21, the phase of the light propagating through the optical waveguide 13 is adjusted. Therefore, light with a consistent phase is generated. That is, laser light is generated. Furthermore, this laser light is amplified in the optical amplifier 11. Then, part of this laser light passes through the mirror 15. Therefore, the wavelength-tunable laser device 100 can generate laser light of a desired wavelength.

[0023] FIG. 3 shows an example of the configuration of the wavelength tunable filter 12. In this embodiment, the wavelength tunable filter 12 includes ring waveguides 12a-12b and waveguides 12c-12e, as shown in FIG. 3(a). The waveguide 12c is optically coupled to the optical waveguide 13 shown in FIG. 1. The waveguide 12c may be a part of the optical waveguide 13. In this case, the wavelength tunable filter 12 includes ring waveguides 12a-12b and waveguides 12d-12e. The ring waveguide 12a is optically coupled to the waveguide 12c. The waveguide 12d is optically coupled to the ring waveguide 12a. That is, the ring waveguide 12a is coupled to the waveguide 12c and the waveguide 12d, respectively. The ring waveguide 12b is optically coupled to the waveguide 12d. The waveguide 12e is optically coupled to the ring waveguide 12b. That is, the ring waveguide 12b is coupled to the waveguide 12d and the waveguide 12e, respectively, and the waveguide 12e is optically coupled to the mirror 14 shown in FIG.

[0024] In the tunable filter 12 shown in Fig. 3, light input from the optical port K1 is guided to the optical port K2 via the waveguide 12c, the ring waveguide 12a, the waveguide 12d, the ring waveguide 12b, and the waveguide 12e. Therefore, light input to the tunable filter 12 from the optical waveguide 13 shown in Fig. 1 propagates through the waveguide 12c, the ring waveguide 12a, the waveguide 12d, the ring waveguide 12b, and the waveguide 12e, and is guided to the mirror 14 shown in Fig. 1.

[0025] The mirror 14 reflects the light output from the optical port K2 of the tunable filter 12. This reflected light is input to the optical port K2 of the tunable filter 12. The light input from the optical port K2 is guided to the optical port K1 via the waveguide 12e, the ring waveguide 12b, the waveguide 12d, the ring waveguide 12a, and the waveguide 12c. Therefore, the reflected light from the mirror 14 propagates through the waveguide 12e, the ring waveguide 12b, the waveguide 12d, the ring waveguide 12a, and the waveguide 12c, and is guided to the optical waveguide 13.

[0026] 3 are shown for the purpose of explaining the configuration and operation of the tunable filter 12, and do not mean that the tunable filter 12 has physical "ports." That is, the waveguide 12c and the optical waveguide 13 shown in FIG. 1 are preferably realized by continuous optical waveguides. Also, the waveguide 12e and the mirror 14 shown in FIG. 1 are preferably realized by continuous optical waveguides.

[0027] As shown in FIG. 3(b), the wavelength tunable filter 12 includes heaters 22a and 22b near the ring waveguides 12a and 12b, respectively. The cross-sectional structure of this portion is substantially the same as that shown in FIG. 2. That is, a thin-film metal is formed above each ring waveguide core along the ring waveguide core. The heaters 22a and 22b are realized by electrode patterns formed near the ring waveguides 12a and 12b, respectively. Controlling the current supplied to the heaters 22a and 22b changes the refractive index of the ring waveguides 12a and 12b, thereby adjusting the optical lengths of the ring waveguides 12a and 12b. In other words, the heaters 22a and 22b can be used to adjust the resonance wavelengths of the ring waveguides 12a and 12b. At this time, light of a wavelength corresponding to this resonance wavelength is selected and output. Therefore, the ring waveguides 12a, 12b and the heaters 22a, 22b can act as a wavelength selector that selects light of a desired wavelength.

[0028] As described above, the wavelength-tunable laser device 100 shown in Fig. 1 can generate laser light of a desired wavelength. However, in the configuration shown in Fig. 1, unwanted light is generated in the cavity, and mode hopping may occur due to this unwanted light. When mode hopping occurs, the laser oscillation becomes unstable, and the quality of the laser light deteriorates.

[0029] Regarding this problem, the inventors of the present application have found that one of the causes of the generation of unwanted light is reflection (or scattering) within the optical waveguide. Reflection within the optical waveguide will be described below with reference to FIG. 4.

[0030] The graph shown in Figure 4 shows the distribution of optical power in the optical waveguide. The horizontal axis represents the position on the X-ray line that crosses the core of the optical waveguide. The X-ray line is parallel to the surface of the silicon substrate 10. The vertical axis represents the optical power. Note that the power of the light input to the optical waveguide is the same in the case shown in Figure 4(a) and the case shown in Figure 4(b). In addition, the height of the core of the optical fiber is the same in the case shown in Figure 4(a) and the case shown in Figure 4(b).

[0031] In the case shown in Figure 4(a), the optical waveguide is formed so as to suppress multimode propagation. That is, the core width W1 is designed so as to suppress multimode propagation. As an example, the core width W1 is about 500 nm.

[0032] The optical power is highest near the center of the core. The optical power decreases as the distance from the center of the core increases. However, in the case shown in Figure 4(a), the core width W1 is narrow. Therefore, the optical power P1 at the sidewalls of the core (i.e., positions S1 and S2) is relatively large. On the other hand, it is difficult to form the sidewalls of the core (i.e., the boundary between the core and the cladding) perfectly smooth. If the sidewalls of the core are not smooth, random reflections occur in the light propagating within the core. Here, the higher the optical power in contact with the sidewalls of the core, the stronger the reflected light. In other words, the higher the optical power in contact with the sidewalls of the core, the higher the power of unwanted reflected light. Therefore, when the core width W1 is narrow, as shown in Figure 4(a), the power of unwanted reflected light increases.

[0033] In the case shown in Figure 4(b), the core width W2 is wider than the width W1 shown in Figure 4(a). As an example, the core width W2 is approximately 2 μm. Therefore, the optical power P2 at the sidewalls of the core (i.e., positions S3 and S4) is sufficiently small. Therefore, even if the sidewalls of the core are not smooth, the reflected light relative to the light propagating within the core is weak. Therefore, when the core width W2 is wide, as shown in Figure 4(b), the power of unwanted reflected light is small.

[0034] Therefore, in an embodiment of the present invention, the core width of at least a portion of the optical waveguide 13 that couples the optical amplifier 11 and the wavelength-tunable filter 12 is increased. Specifically, the core width of at least a portion of the optical waveguide 13 is formed wider than the waveguide width (e.g., 500 nm) at which multimode propagation is suppressed. As a result, as explained with reference to FIG. 4, the generation of unnecessary reflected light in the optical waveguide 13 is suppressed. As a result, mode hopping is less likely to occur, and laser oscillation is stabilized, improving the quality of the laser light. Note that, although increasing the core width of the optical waveguide increases the risk of multimode propagation, an embodiment of the present invention is configured to suppress the generation of multimode, as will be explained later.

[0035] Fig. 5 shows a first example of a wavelength tunable laser device according to an embodiment of the present invention. The configuration of the wavelength tunable laser device 1 according to the first example is almost the same as that of the wavelength tunable laser device 100 shown in Fig. 1. However, the configuration of the optical waveguide 13 is different between the wavelength tunable laser device 100 shown in Fig. 1 and the wavelength tunable laser device 1 shown in Fig. 5.

[0036] In the wavelength tunable laser device 1, the optical waveguide 13 includes a wide waveguide 31. Specifically, the optical waveguide 13 includes a straight waveguide 13a, which includes a first waveguide having a core with a first width and a second waveguide having a core with a second width wider than the first width. The wide waveguide 31 shown in FIG. 5 corresponds to the second waveguide. The waveguide portion of the straight waveguide 13a other than the wide waveguide 31 corresponds to the first waveguide.

[0037] As described above, in the wavelength tunable laser device 1, the optical waveguide 13 that couples the optical amplifier 11 and the wavelength tunable filter 12 includes the wide waveguide 31. Here, as described with reference to Fig. 4, widening the core width of the optical waveguide 13 suppresses the generation of unnecessary reflected light in the optical waveguide 13. Therefore, mode hopping is less likely to occur, and laser oscillation is stabilized, improving the quality of the laser light.

[0038] The heater 21 for adjusting the optical length between the mirrors 14 and 15 is provided near the optical waveguide 13, as in the configuration shown in Fig. 1. In the wavelength tunable laser device 1, the heater 21 may be provided near the normal waveguide or near the wide waveguide 31. In Fig. 5, the heater 21 is provided near the wide waveguide 31. In this case, the proportion of the wide waveguide to the entire waveguide of the optical integrated element increases, so reflection from the waveguide decreases, resulting in more stable laser operation.

[0039] FIG. 6 shows an example of the arrangement of a wide waveguide and a heater. The structure when a heater is provided near a wide waveguide may be the same as the structure when a heater is provided near a normal waveguide shown in FIG. 2. The heater provided near the wide waveguide is formed of, for example, a thin-film metal, similar to the heater provided near the normal waveguide shown in FIG. 2. In the examples shown in FIGS. 2 and 6, the heater provided near the normal waveguide and the heater provided near the wide waveguide have the same width, but the heater provided near the wide waveguide may be wider than the heater provided near the normal waveguide.

[0040] FIG. 7 shows an example of the structure of the optical waveguide 13. As described above, the optical waveguide 13 includes a straight waveguide 13a. In the example shown in FIG. 7(a), the straight waveguide 13a is composed of a wide waveguide 31 and single-mode waveguides 32a and 32b. The single-mode waveguide 32a is connected to one end of the wide waveguide 31, and the single-mode waveguide 32b is connected to the other end of the wide waveguide 31. For example, the single-mode waveguide 32a is coupled to the optical amplifier 11 via the inclined waveguide 13b shown in FIG. 5, and the single-mode waveguide 32b is coupled to the wavelength tunable filter 12 shown in FIG. 5.

[0041] As described above, the core width W1 of the single-mode waveguides 32a and 32b is designed to suppress multimode propagation, and in this embodiment is approximately 500 nm. The core width W2 of the wide waveguide 31 is wider than the core width W1 of the single-mode waveguides 32a and 32b, and in this embodiment is approximately 2 μm. The heights of the wide waveguide 31 and the single-mode waveguides 32a and 32b are the same.

[0042] As described above, the optical waveguide 13 according to the embodiment of the present invention includes the wide waveguide 31. Therefore, as described with reference to FIG.

[0043] However, in the configuration shown in Figure 7(a), the cross-sectional area of ​​the core of optical waveguide 13 changes discontinuously in the light propagation direction. Specifically, the cross-sectional area of ​​the core of optical waveguide 13 changes discontinuously at the boundary between wide waveguide 31 and single-mode waveguide 32a and at the boundary between wide waveguide 31 and single-mode waveguide 32b. When the cross-sectional area of ​​the core of the optical waveguide changes discontinuously, not only does light loss increase but mode conversion may also occur. Therefore, the configuration shown in Figure 7(a) may generate multimode.

[0044] Considering this problem, it is preferable that the optical waveguide 13 be formed so that the cross-sectional area of ​​the core changes continuously between the wide waveguide 31 and the single-mode waveguides 32a and 32b, as shown in FIG. 7(b). Specifically, a tapered waveguide 33a is provided between the wide waveguide 31 and the single-mode waveguide 32a. The core width of the tapered waveguide 33a changes continuously between W1 and W2. Similarly, a tapered waveguide 33b is provided between the wide waveguide 31 and the single-mode waveguide 32b. The core width of the tapered waveguide 33b changes continuously between W1 and W2. The height of the tapered waveguide 33b is the same as that of the wide waveguide 31 and the single-mode waveguides 32a and 32b.

[0045] 7(b) includes a wide waveguide 31 and is formed so that the cross-sectional area of ​​the core changes continuously, thereby suppressing the generation of unnecessary reflected light in the optical waveguide 13 and also suppressing the generation of multimodes.

[0046] As described with reference to FIG. 4, the wider the wide waveguide 31, the weaker the reflected light from the sidewall of the optical waveguide. However, the wider the wide waveguide 31, the easier it is for multimode light to propagate. While the tapered waveguides 33a and 33b shown in FIG. 7(b) can suppress the occurrence of multimode light, it is difficult to eliminate multimode light. In other words, if the width of the wide waveguide 31 is made too wide, the risk of multimode light propagation increases. Therefore, in an embodiment of the present invention, it is preferable to set the maximum width of the wide waveguide 31 taking into consideration both the reflected light from the sidewall of the optical waveguide core and the risk of multimode light propagation.

[0047] For example, since it is difficult to eliminate multimode propagation, the maximum width of the wide waveguide 31 is set to 2 μm, as an example, to suppress multimode propagation. On the other hand, if the sidewalls of the optical waveguide core can be formed smoothly, the width of the wide waveguide 31 may be narrower than 2 μm because the reflected light from the sidewalls of the core is weak. This design suppresses the generation of unnecessary reflected light in the optical waveguide 13 and also suppresses multimode propagation. As a result, laser oscillation is stabilized, improving the quality of the laser light.

[0048] FIG. 8 shows the measurement results of reflection from the sidewall of the core. The horizontal axis of the graph represents the position on the optical waveguide in the propagation direction of light. Positions R1 and R2 represent the ends of the wide waveguide. That is, the wide waveguide is formed at positions R1 and R2. The length of the wide waveguide (i.e., the distance between positions R1 and R2) is, for example, 1 mm. Furthermore, tapered waveguides are formed at both ends of the wide waveguide, as shown in FIG. 7(b). The vertical axis of the graph represents the amount of reflection from the sidewall of the core. The amount of reflection is measured using a reflectometer.

[0049] Figure 8(a) shows the reflection amount when the core width of the wide waveguide is slightly wider than that of the single-mode waveguide. Figure 8(b) shows the reflection amount when the core width of the wide waveguide is wider than that shown in Figure 8(a) but narrower than that shown in Figure 8(c). Figure 8(c) shows the reflection amount when the core width of the wide waveguide is sufficiently wide (e.g., 2 μm). As such, widening the core width of the optical waveguide suppresses reflection (or scattering) from the sidewalls of the core.

[0050] 9 shows a second example of a wavelength tunable laser device according to an embodiment of the present invention. The wavelength tunable laser device 2 according to the second example is configured by adding a wide waveguide 34 to the wavelength tunable laser device 1 shown in FIG.

[0051] As described above, widening the core width of the optical waveguide suppresses reflection from the sidewall of the core. Therefore, to reduce reflection from the sidewall of the core, it is preferable to form a wide waveguide as long as possible in the optical path between the mirrors 14 and 15. However, widening the core width of the curved waveguide may excite propagating light in higher modes, resulting in increased light loss. Therefore, for example, it is not preferable to widen the core width of the ring waveguide in the wavelength tunable filter 12. Therefore, in the embodiment of the present invention, the core width of each ring waveguide 12a, 12b is the same as that of a normal waveguide (e.g., a single-mode waveguide).

[0052] Therefore, the wavelength tunable laser device 2 is configured so that the waveguide 12e in the wavelength tunable filter 12 includes a wide waveguide 34. Therefore, in the wavelength tunable laser device 2, the total length of the wide waveguide is larger than that of the wavelength tunable laser device 1 shown in Fig. 5, and it is considered that reflection from the sidewall of the core is reduced.

[0053] However, as the number of wide waveguides increases, the number of conversions between normal waveguides (single-mode waveguides 32a and 32b in FIG. 7) and wide waveguides also increases. To suppress loss and / or mode conversion, a tapered waveguide, as shown in FIG. 7(b), can be provided between the normal waveguides and the wide waveguides. However, even when a tapered waveguide is provided, it is not easy to achieve perfect waveguide width conversion, and loss and / or mode conversion may occur.

[0054] Therefore, it is preferable that the wavelength tunable laser device according to the embodiment of the present invention meets the following two requirements. (1) In the optical path between the mirrors 14 and 15, a wide waveguide is formed as long as possible. (2) Reduce the number of wide waveguides.

[0055] Fig. 10 is a diagram illustrating the layout of the wavelength tunable filter 12 and the mirror 14. In Fig. 10, the optical waveguide 13 and the waveguide 12c constitute a single optical waveguide. Therefore, in the description relating to Fig. 10, this optical waveguide may be referred to as "waveguide 13_12c."

[0056] The layout shown in Fig. 10(a) corresponds to the wavelength tunable laser device 2 shown in Fig. 9. That is, the wide waveguide 34 shown in Fig. 9 is formed in the section F shown in Fig. 10(a). In this case, the wavelength tunable laser device 2 includes two wide waveguides.

[0057] In the layout shown in FIG. 10(b), the positions of the ring waveguides 12a and 12b are moved toward the mirror 14 compared to the layout shown in FIG. 10(a). Specifically, the positions of the ring waveguides 12a and 12b are shifted so that the distance between the ring waveguide 12b and the mirror 14 becomes as small as possible. At this time, the position of the mirror 14 remains unchanged. Therefore, compared to the layout shown in FIG. 10(a), the waveguide 13_12c is longer by a length corresponding to the section F.

[0058] 10(b), compared to the layout shown in Fig. 10(a), in the optical path between point K3 and mirror 14, the length of waveguide 12e is shorter by section F, and the length of waveguide 13_12c is longer by section F. That is, the optical lengths between point K3 and mirror 14 are the same.

[0059] 11 shows a third example of a wavelength tunable laser device according to an embodiment of the present invention. The wavelength tunable laser device 3 according to the third example includes a wide waveguide 35 between the optical amplifier 11 and the wavelength tunable filter 12. Here, the wavelength tunable filter 12 is formed based on the layout shown in FIG. 10(b). Furthermore, in the wavelength tunable laser device 3, the wide waveguide 34 shown in FIG. 9 is not formed, and a wide waveguide is formed in section F shown in FIG. 10(b). Note that the wide waveguide formed in section F shown in FIG. 10(b) is a part of the wide waveguide 35.

[0060] The length of the wide waveguide 35 is equal to or greater than the sum of the lengths of the wide waveguides 31 and 34 shown in FIG. 9. Therefore, the requirement (1) is satisfied. Furthermore, the wavelength tunable laser device 3 shown in FIG. 11 has one wide waveguide. Therefore, the requirement (2) is also satisfied. As a result, it is possible to suppress the occurrence of unnecessary reflected light in the optical waveguide 13 while suppressing loss and / or mode conversion caused by providing a wide waveguide.

[0061] FIG. 12 shows a fourth example of a wavelength tunable laser device according to an embodiment of the present invention. In the wavelength tunable laser device 4 according to the fourth example, the optical amplifier 11 is mounted at an angle to the longitudinal direction of the silicon substrate 10. In addition, as shown in FIG. 13, the connection surface between the waveguide in the optical amplifier 11 and the optical waveguide 13 is formed at an angle to the light propagation direction. In this case, the path from the connection with the optical amplifier 11 to the wide waveguide 35 is entirely made up of straight waveguides, and does not include any curved waveguides of normal width. Therefore, with this configuration, the proportion of the wide waveguides in the entire waveguide of the optical integrated device is increased, which reduces reflection from the waveguide and results in more stable laser operation.

[0062] 14 shows a fifth example of a wavelength tunable laser device according to an embodiment of the present invention. In the first to fourth examples, a wide waveguide is formed between the optical amplifier 11 and the wavelength tunable filter 12. In contrast, in the wavelength tunable laser device 5 according to the fifth example, the optical waveguide 13 formed between the wavelength tunable filter 12 and the mirror 14 includes a wide waveguide 31. Even with this configuration, as in the first to fourth examples, unnecessary reflected light generated in the optical waveguide is suppressed, improving the quality of the laser light. [Explanation of symbols]

[0063] 1~5 Wavelength tunable laser device 10 Silicon substrate 11 Optical amplifier 12 Tunable wavelength filter 12a, 12b Ring waveguide 13 Optical waveguide 13a straight waveguide 14 Mirror (half mirror) 15 Mirror (Loop Mirror) 31, 34, 35 Wide waveguide 32a, 32b Single-mode waveguide 33a, 33b Tapered waveguide

Claims

1. a first mirror; and a second mirror; and an optical amplification unit provided between the first mirror and the second mirror; a tunable filter provided between the first mirror and the second mirror; an optical waveguide that couples the optical amplifier unit and the wavelength tunable filter, The optical waveguide includes a first waveguide formed with a first width and a second waveguide that is a wide waveguide formed with a second width wider than the first width. A wavelength tunable laser device characterized by:

2. The second waveguide is a straight waveguide.

2. The wavelength tunable laser device according to claim 1.

3. a tapered waveguide between the first waveguide and the second waveguide, the width of a first end connected to the first waveguide being the first width, the width of a second end connected to the second waveguide being the second width, and the width continuously changing between the first end and the second end; 2. The wavelength tunable laser device according to claim 1.

4. an electrical circuit that changes the refractive index of the optical waveguide to adjust the optical length between the first mirror and the second mirror; 2. The wavelength tunable laser device according to claim 1.

5. The electrical circuit changes the refractive index of the second waveguide.

5. The wavelength tunable laser device according to claim 4.

6. The tunable filter is a ring waveguide; a third waveguide coupling the ring waveguide and the second mirror; At least a portion of the third waveguide is formed with the second width.

2. The wavelength tunable laser device according to claim 1.

7. The second waveguide is the only waveguide formed with the second width.

2. The wavelength tunable laser device according to claim 1.

8. The connection surface between the waveguide in the optical amplification section and the optical waveguide is formed obliquely with respect to the direction of light propagating through the second waveguide.

2. The wavelength tunable laser device according to claim 1.

9. a first mirror; and a second mirror; and an optical amplification unit provided between the first mirror and the second mirror; a tunable filter provided between the first mirror and the second mirror and coupled to the optical amplifier; an optical waveguide that couples the tunable filter and the second mirror, The optical waveguide includes a first waveguide formed with a first width and a second waveguide that is a wide waveguide formed with a second width wider than the first width. A wavelength tunable laser device characterized by:

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