Light source unit

Abstract

To provide a light source unit capable of suitably resolving the problem of wavelength dependency.SOLUTION: A light source unit 1A includes a light source LS configured to be wavelength tunable and outputting light with a wavelength of 2 μm or more, an optical integrated circuit 20, and a control device 60. The optical integrated circuit 20 includes an optical coupler 41 connected to the upstream end of each of the waveguides P1 and P2 and performing branching and combining of light, a polarization rotating splitter 45 connected to the downstream end of each of the waveguides P1 and P2 and the upstream end of the waveguide Pout and performing branching and combining of light, and a phase shifter 44 provided in the waveguide P1 and adjusting the phase of the light passing through the waveguide P1. The control device 60 controls the amount of phase adjustment by the phase shifter 44.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a light source unit. [Background technology]

[0002] Patent Document 1 discloses a configuration in which an optical integrated circuit having an optical element provided in an optical waveguide on a substrate has a coupler that branches light from an input waveguide into two branch waveguides, and a phase adjustment plate arranged on each branch waveguide. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-155892 Summary of the Invention [Problem to be solved by the invention]

[0004] In the configuration disclosed in Patent Document 1, the amount of phase adjustment in each branching waveguide is set as a fixed value corresponding to the phase adjustment plate disposed on each branching waveguide. Therefore, there is a risk that the characteristics of the optical integrated circuit may change depending on the wavelength of the input light, temperature, manufacturing errors of each component (coupler, phase adjustment plate, etc.) that constitutes the optical integrated circuit, etc. In particular, when measurements are performed at multiple wavelengths using a wavelength-tunable light source, it is necessary to eliminate wavelength dependency (the tendency for the characteristics of the optical integrated circuit to change as the wavelength changes).

[0005] Therefore, an object of one aspect of the present disclosure is to provide a light source unit that can suitably solve the problem of wavelength dependency. [Means for solving the problem]

[0006] The present disclosure includes the following light source units [1] to

[10] .

[0007] [1] A light source configured to be wavelength tunable and outputting light with a wavelength of 2 μm or more; an optical integrated circuit including a substrate and an optical waveguide provided on the substrate; a control unit that controls the operation of at least a part of the optical integrated circuit, The optical waveguide is an input waveguide into which input light from the light source is input; a first waveguide located downstream of the input waveguide; a second waveguide located downstream of the input waveguide; an output waveguide that outputs output light corresponding to the input light, The optical integrated circuit comprises: a first optical coupler connected to the upstream ends of the first waveguide and the second waveguide, and configured to split and combine light; a second optical coupler connected to the downstream ends of the first waveguide and the second waveguide and the upstream end of the output waveguide, for branching and combining light; a first phase shifter provided in the first waveguide and configured to adjust the phase of light passing through the first waveguide; The control unit controls a phase adjustment amount of the first phase shifter.

[0008] Optical couplers and phase shifters generally have wavelength dependence. Therefore, when distance measurement, spectroscopic measurement, etc. are performed using a wavelength-tunable light source, the characteristics of the optical coupler and phase shifter change depending on the wavelength, which may change the characteristics of the optical integrated circuit, making it difficult to perform measurements using the optical integrated circuit. The light source unit described above in [1] is configured so that the phase adjustment amount of the first phase shifter arranged on at least one path (first waveguide) of two paths (first waveguide and second waveguide) between the first optical coupler and the second optical coupler can be controlled by a control unit. Therefore, with the light source unit, the phase adjustment amount of the first phase shifter can be controlled in accordance with changes in the wavelength of the input light, thereby effectively solving the wavelength-dependence problem described above.

[0009] [2] The second optical coupler is configured to combine the light from the first waveguide and the light from the second waveguide to output light of two orthogonal polarization components to the output waveguide, and to separate and output light of two orthogonal polarization components contained in reflected light corresponding to the output light input from the output waveguide to the first waveguide and the second waveguide, respectively; The control unit controls the phase adjustment amount of the first phase shifter so that the light traveling upstream through the first waveguide due to the reflected light and the light traveling upstream through the second waveguide due to the reflected light are in opposite phase in the input waveguide.

[0010] According to the configuration of [2] above, the optical integrated circuit can be used as a circulator.

[0011] [3] The optical integrated circuit is a third optical coupler that receives the input light from the input waveguide and splits the input light into light that is output to a third waveguide and light that is output to a fourth waveguide; a second phase shifter provided in the third waveguide and configured to adjust the phase of light passing through the third waveguide; the first optical coupler is connected to a downstream end of each of the third waveguide and the fourth waveguide; The light source unit according to [1] or [2], wherein the control unit controls a phase adjustment amount of the second phase shifter.

[0012] According to the configuration [3] above, the input light is branched into the third and fourth waveguides by the third optical coupler, and by controlling the phase adjustment amount of the second phase shifter provided in the third waveguide, which is one of the waveguides, it is possible to appropriately set the phase difference between the two light beams (light in the third waveguide and light in the fourth waveguide) input to the first optical coupler. As a result, it is possible to appropriately adjust the intensity ratio of the light beams output to the first and second waveguides downstream of the first optical coupler.

[0013] [4] The optical integrated circuit is a feedback waveguide connected to the first optical coupler, for passing feedback light generated in the first optical coupler based on light traveling upstream through each of the first waveguide and the second waveguide due to reflected light corresponding to the output light; a photodetector connected to an end of the feedback waveguide opposite to the end connected to the first optical coupler, and configured to detect the feedback light; The light source unit according to [1] or [2], wherein the control unit controls a phase adjustment amount of the first phase shifter based on a detection result of the photodetector.

[0014] According to the configuration [4] above, by monitoring the feedback light based on the reflected light, it becomes easy to appropriately control the phase adjustment amount of the first phase shifter so that the optical integrated circuit exhibits suitable characteristics.

[0015] [5] The optical integrated circuit comprises: a feedback waveguide connected to the third optical coupler, for passing feedback light generated in the third optical coupler based on light traveling upstream through each of the third waveguide and the fourth waveguide due to reflected light corresponding to the output light; a photodetector connected to an end of the feedback waveguide opposite to the end connected to the third optical coupler, and configured to detect the feedback light; The light source unit according to [3], wherein the control unit controls a phase adjustment amount of at least one of the first phase shifter and the second phase shifter based on a detection result of the photodetector.

[0016] According to the configuration [5] above, by monitoring the feedback light based on the reflected light, it becomes easy to appropriately control the phase adjustment amount of at least one of the first phase shifter and the second phase shifter so that the optical integrated circuit exhibits suitable characteristics.

[0017] [6] The light source unit according to any one of [1] to [5], wherein the optical integrated circuit has a delay circuit formed in at least one of the first waveguide and the second waveguide.

[0018] According to the configuration [6] above, the amount of phase adjustment required by the first phase shifter can be reduced.

[0019] [7] The light source has a laser gain medium and an external resonator that controls the wavelength of the input light; The external resonator is any one of the light source units [1] to [6], which is mounted on the substrate.

[0020] According to the configuration [7] above, it is possible to easily control the wavelength of the input light by the external resonator while suppressing an increase in the size of the light source unit.

[0021] [8] The light source unit of [7], wherein the control unit controls the external resonator so that the wavelength of the input light becomes a predetermined target wavelength, and controls the phase adjustment amount of the first phase shifter according to the target wavelength.

[0022] According to the configuration of [8] above, when the target wavelength is set in the control unit, it is possible to control the wavelength of the input light by the external resonator and control the phase adjustment amount of the first phase shifter according to the target wavelength in parallel, thereby shortening the time from when the target wavelength is determined until the control of the phase adjustment amount is completed and appropriate measurement can be performed.

[0023] [9] The light source unit according to [7] or [8], wherein the laser gain medium is not mounted on the substrate.

[0024] According to the configuration of [9] above, by separating the laser gain medium from the optical integrated circuit, it is possible to improve the degree of freedom in the material and arrangement of the laser gain medium.

[0025]

[10] The light source unit according to [7] or [8], wherein the laser gain medium is mounted on the substrate.

[0026] According to the configuration of

[10] above, by mounting both the laser gain medium and the external resonator that constitute the light source on a substrate, the light source unit can be made smaller. [Effects of the Invention]

[0027] According to one aspect of the present disclosure, it is possible to provide a light source unit that can suitably solve the problem of wavelength dependency. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a block diagram showing the functional configuration of a light source unit 1A according to the first embodiment. [Figure 2] FIG. 2 is a diagram showing an example of the configuration of the light source unit 1A. [Figure 3] FIG. 3 is a diagram for explaining the operation of a mirror formed by an optical waveguide included in an external resonator. [Figure 4] FIG. 4 is a diagram schematically showing a circuit section 40 mounted on the optical integrated circuit 20. As shown in FIG. [Figure 5] FIG. 5 is a flowchart showing an example of the third control by the control device 60. [Figure 6] FIG. 6 is a diagram showing an example of the configuration of a light source unit 1B according to the second embodiment. [Figure 7] FIG. 7 is a diagram showing an example of the configuration of a light source unit 1C according to the third embodiment. [Figure 8] FIG. 8 is a diagram showing a modified example of the circuit section. DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the following description, the same or equivalent elements will be designated by the same reference numerals, and redundant description will be omitted.

[0030] [First embodiment] A light source unit 1A of the first embodiment will be described with reference to FIGS. 1 to 5. As shown in FIG. 1, the light source unit 1A includes a QCL 10 (laser gain medium), an optical integrated circuit 20, a control device 60 (controller), and a laser driver D. As shown in FIGS. 1 and 2, the optical integrated circuit 20 includes a substrate 21 and various optical elements including an optical waveguide provided on the substrate 21. The substrate 21 is, for example, a rectangular plate-shaped silicon substrate. In this embodiment, an external resonator 30, a circuit section 40, and a photodetector 50 are mounted on one main surface 21a of the substrate 21 that constitutes the optical integrated circuit 20.

[0031] (light source) The QCL 10 and the external resonator 30 constitute a light source LS. The light source LS emits light (input light L) that is wavelength-tunable in a wavelength range of 2 μm or more (mainly in the mid-infrared region). in ) to the circuit section 40.

[0032] QCL 10 is a quantum cascade laser device that serves as an example of a laser gain medium constituting light source LS. QCL 10 is used as a semiconductor optical amplifier (SOA). That is, QCL 10 is optically coupled to an external resonator 30 mounted on a principal surface 21a of a substrate 21. As shown in FIG. 2 , QCL 10 is disposed adjacent to substrate 21, and laser light is input from end face 10a of QCL 10 facing substrate 21 to waveguide 31 of external resonator 30. Note that the laser gain medium is not limited to a quantum cascade laser device; however, using a quantum cascade laser device as a laser gain medium enables suitable oscillation of laser light in the mid-infrared range.

[0033] As shown in FIG. 2, in this embodiment, as an example, the external resonator 30 is configured as a ring resonator. The external resonator 30 has five independent optical waveguides (waveguides 31, 33, and 35 and ring waveguides 32 and 34). These waveguides are formed of, for example, the same material as the substrate 21 (e.g., silicon). Alternatively, these waveguides may be formed of a material different from that of the substrate 21 (e.g., silicon nitride, germanium, etc.). The same applies to the optical waveguides included in the circuit unit 40, which will be described later.

[0034] Waveguide 31 is optically coupled to facet 10a of QCL 10. Waveguide 33 is coupled to waveguide 31 via ring waveguide 32. Specifically, when an integer multiple of the wavelength of laser light passing through waveguide 31 in the direction of arrow A in FIG. 2 becomes equal to the circuit length of ring waveguide 32, laser light passing through waveguide 33 in the direction of arrow B in FIG. 2 is generated.

[0035] The waveguide 35 is coupled to the waveguide 33 via the ring waveguide 34. Specifically, when an integral multiple of the wavelength of the laser light passing through the waveguide 33 in the direction of the arrow B in Fig. 2 is equal to the circuit length of the ring waveguide 34, a laser light is generated that passes through the waveguide 35 in the direction of the arrow C in Fig. 2. The end of the waveguide 35 in the direction of the arrow C in Fig. 2 is connected to the waveguide P in is connected to.

[0036] The waveguides 35 are provided with a mirror portion 36 folded back in a ring shape so that the waveguides 35 approach each other to form a directional coupler 36a. The mirror portion 36 separates the reflected light (returned light) reflected toward the QCL 10 and the light reflected toward the waveguide P in The above operation of the mirror section 36 will be described with reference to FIG.

[0037] As shown in Fig. 3A, laser light La passes through waveguide 35 toward mirror section 36. As shown in Fig. 3B, laser light La incident on mirror section 36 is split into laser light Lb and laser light Lc by directional coupler 36a. The ratio between laser light Lb and laser light Lc can be determined appropriately depending on the design of mirror section 36.

[0038] Next, as shown in Fig. 3C, when each of the laser beams Lb and Lc goes around the ring-shaped portion of the mirror unit 36 ​​and returns to the directional coupler 36a, interference between the laser beams Lb and Lc occurs in the directional coupler 36a. As a result, as shown in Fig. 3D, the laser beams Lb and Lc are separated into reflected light Ld that returns to the QCL 10 side and transmitted light Le that travels toward the circuit unit 40 side. The reflected light Ld is used to amplify the laser beam by laser resonance. The transmitted light Le travels through the waveguide P of the circuit unit 40. in Input light L in is extracted as

[0039] The external resonator 30 has a phase shifter 37 for filter control arranged along the ring waveguide 32, a phase shifter 38 for filter control arranged along the ring waveguide 34, and a phase shifter 39 for longitudinal mode control arranged along the upstream portion of the mirror portion 36 (directional coupler 36a) in the waveguide 35.

[0040] The phase shifter 37 is, for example, a thermal phase shifter, which generates heat when supplied with power. The phase shifter 37 is configured, for example, by a thin-film heater. By generating such heat, the phase shifter 37 changes (increases) the refractive index of the portion of the ring waveguide 32 along the phase shifter 37, thereby shifting the phase of the light circulating in the ring waveguide 32. By controlling the power supplied to the phase shifter 37, the amount of phase adjustment (amount of phase shift) of the ring waveguide 32 can be controlled.

[0041] The phase shifter 38 is, for example, a thermal phase shifter similar to the phase shifter 37. That is, the phase shifter 38 generates heat to change (increase) the refractive index of the portion of the ring waveguide 34 along the phase shifter 38, thereby shifting the phase of the light circulating in the ring waveguide 34. The amount of phase adjustment of the ring waveguide 34 can be controlled by controlling the power supplied to the phase shifter 38.

[0042] By controlling the phase adjustment amount in the ring waveguides 32 and 34 using the phase shifters 37 and 38, the transmitted light Le (input light L in ) can be changed. The circumferential length of the ring waveguide 32 and the circumferential length of the ring waveguide 34 may be made different. In this case, the Vernier effect makes it possible to sweep the wavelength of the transmitted light Le in a wider wavelength range than when a ring resonator consisting of a single ring (ring waveguide 32 or 34) is used.

[0043] The phase shifter 39 is, for example, a thermal phase shifter similar to the phase shifters 37 and 38. The phase shifter 39 generates heat to change the refractive index of the portion of the waveguide 35 along the phase shifter 39, thereby shifting the longitudinal mode of the laser light La passing through the waveguide 35. This makes it possible to change the wavelength of the transmitted light Le. In other words, the phase shifter 39 allows fine tuning of the wavelength of the transmitted light Le.

[0044] The external resonator 30 makes it possible to extract transmitted light Le of a desired wavelength by controlling the phase shifters 37, 38, and 39. For example, by controlling the phase shifters 37 and 38 so as to transmit only light of a single longitudinal mode, it becomes possible to extract transmitted light Le of a single mode.

[0045] (Circuit part) 2 and 4, the configuration of the circuit section 40 will be described. The circuit section 40 includes a waveguide P in(input waveguide), waveguide P1 (first waveguide), waveguide P2 (second waveguide), waveguide P3 (third waveguide), waveguide P4 (fourth waveguide), waveguide P out (output waveguide), and waveguide P FB The circuit unit 40 also has optical elements arranged on or along the optical waveguide, such as an optical coupler 41 (first optical coupler), an optical coupler 42 (third optical coupler), a phase shifter 43 (second phase shifter), a phase shifter 44 (first phase shifter), a polarization rotation splitter 45 (second optical coupler), and delay lines 46 and 47 (delay circuits).

[0046] Hereinafter, each element constituting the circuit section 40 will be described from the upstream side to the downstream side. In this specification, among the optical waveguides constituting the circuit section 40, the waveguide P FB The "upstream" and "downstream" of the optical waveguide other than in (i.e., the input light L in On the other hand, the feedback light L generated due to the reflected light Lr is FB The waveguide P FB The terms "upstream" and "downstream" are based on the traveling direction of the reflected light Lr.

[0047] waveguide P in is the input light L from the light source LS (external resonator 30). in The optical waveguide into which the transmitted light Le is input. in is optically coupled to the downstream end of the waveguide 35. in The downstream end of the optical coupler 42 is connected to the optical coupler 42 .

[0048] The optical coupler 42 is a 2x2 coupler having two upstream ports and two downstream ports. As shown in Fig. 2, the optical coupler 42 has waveguides (parts constituting a directional coupler) separated vertically (in the direction of Fig. 2) and arranged close to each other. The upper upstream port of the optical coupler 42 is connected to the waveguide P inThe upper downstream port of the optical coupler 42 is connected to the upstream end of the waveguide P3. The lower upstream port of the optical coupler 42 is connected to the upstream end of the waveguide P FB The lower downstream port of the optical coupler 42 is connected to the upstream end of the waveguide P4. The upper upstream port of the optical coupler 42 is connected to the upstream end of the waveguide P in Input light L from in Input the input light L in into light output to the waveguide P3 and light output to the waveguide P4.

[0049] As an example, the optical coupler 42 is configured as a 3 dB coupler with a branching ratio of 1:1. That is, when light is input to one (upper or lower) upstream port of the optical coupler 42, the optical coupler 42 branches the light at a branching ratio of 1:1 and outputs each branched light from the upper and lower downstream ports, respectively. Similarly, when light is input to one (upper or lower) downstream port of the optical coupler 42, the optical coupler 42 branches the light at a branching ratio of 1:1 and outputs each branched light from the upper and lower upstream ports, respectively.

[0050] As an example, the optical coupler 42 is configured as a directional coupler. That is, when light is input to an upstream port on one side (upper or lower side) of the optical coupler 42, the phase of light output from a downstream port on the other side (opposite the port to which the light is input; i.e., the lower side if the port to which the light is input is the upper side, and the upper side if the port to which the light is input is the lower side) of the optical coupler 42 is delayed by 90° relative to the light output from the downstream port on one side (the same side as the port to which the light is input; i.e., the upper side if the port to which the light is input is the upper side, and the lower side if the port to which the light is input is the lower side). Similarly, when light is input to a downstream port on one side of the optical coupler 42, the phase of light output from the upstream port on the other side of the optical coupler 42 is delayed by 90° relative to the light output from the upstream port on one side of the optical coupler 42.

[0051] The phase shifter 43 is provided along the waveguide P3 connected to the upper downstream port of the optical coupler 42. The phase shifter 43 adjusts (shifts) the phase of light passing through the waveguide P3. The phase shifter 43 is, for example, a thermal phase shifter similar to the phase shifters 37, 38, and 39 constituting the external resonator 30 described above. The phase shifter 43 generates heat to heat a portion of the waveguide P3 adjacent to the phase shifter 43, thereby increasing the refractive index of that portion. As a result, the wavelength of the light passing through that portion can be changed, thereby changing the phase of the light passing through the waveguide P3.

[0052] The optical coupler 41 is connected to the downstream ends of the waveguides P3 and P4 and to the upstream ends of the waveguides P1 and P2, and performs branching and combining of light. The upper upstream port of the optical coupler 41 is connected to the downstream end of the waveguide P3. The upper downstream port of the optical coupler 41 is connected to the upstream end of the waveguide P1. The lower upstream port of the optical coupler 41 is connected to the downstream end of the waveguide P4. The lower downstream port of the optical coupler 41 is connected to the upstream end of the waveguide P2. The optical coupler 41 receives light from each of the waveguides P3 and P4 at its upper and lower upstream ports and separates the light into light output to the waveguide P1 and light output to the waveguide P2.

[0053] As an example, the optical coupler 41 is configured as a 2×2 coupler (3 dB coupler) similar to the optical coupler 42 described above. That is, when light is input to an upstream port on one side (upper or lower) of the optical coupler 41, the optical coupler 41 branches the light at a branching ratio of 1:1 and outputs each branched light from the downstream ports on the upper and lower sides, respectively. Similarly, when light is input to a downstream port on one side (upper or lower) of the optical coupler 41, the optical coupler 41 branches the light at a branching ratio of 1:1 and outputs each branched light from the upstream ports on the upper and lower sides, respectively. Furthermore, when light is input to the upstream port on one side (upper or lower) of the optical coupler 41, the phase of the light output from the downstream port on the other side of the optical coupler 41 is configured to be delayed by 90° relative to the light output from the downstream port on one side of the optical coupler 41. Similarly, when light is input to the downstream port on one side of the optical coupler 41, the phase of the light output from the upstream port on the other side of the optical coupler 41 is configured to be delayed by 90° relative to the light output from the upstream port on one side of the optical coupler 41.

[0054] In this embodiment, the optical couplers 41 and 42, the phase shifter 43, and the waveguides P3 and P4 constitute a Mach-Zehnder interferometer MZI.

[0055] The phase shifter 44 is provided along the waveguide P1 connected to the upper downstream port of the optical coupler 41. The phase shifter 44 adjusts (shifts) the phase of light passing through the waveguide P1. The phase shifter 44 is, for example, a thermal phase shifter similar to the above-mentioned phase shifter 43. That is, the phase shifter 44 generates heat to heat a portion of the waveguide P1 adjacent to the phase shifter 44, thereby increasing the refractive index of that portion. As a result, the wavelength of the light passing through that portion can be changed, thereby changing the phase of the light passing through the waveguide P1.

[0056] The delay line 46 is provided in the waveguide P1 to adjust the length of the waveguide P1. As shown in Figure 2, for example, the delay line 46 is a portion of the waveguide P1 that is provided as a detour in order to extend the overall length of the waveguide P1.

[0057] The delay line 47 is provided in the waveguide P2 to adjust the length of the waveguide P2. As shown in Fig. 2, for example, the delay line 47 is a portion of the waveguide P2 that is provided as a detour in order to extend the overall length of the waveguide P2.

[0058] The polarization rotating splitter 45 is connected to the downstream end of each of the waveguides P1 and P2 and the downstream end of the waveguide P out In this embodiment, the polarization rotation splitter 45 combines the light from the waveguide P1 and the light from the waveguide P2, and outputs two orthogonal polarization components of light to the waveguide P2. out The device is configured to output to

[0059] As an example, polarization rotating splitter 45 has two upstream ports (upper and lower in FIGS. 2 and 4) and one downstream port. Polarization rotating splitter 45 outputs light input to the upper (one side) upstream port to the downstream port without changing the polarization direction. On the other hand, polarization rotating splitter 45 rotates the polarization direction of light input to the lower (other side) upstream port by 90° and outputs the light to the downstream port.

[0060] In this embodiment, the input light L in The polarization mode of the waveguides P1 and P2 is set so that the electric field component is in the TM mode along a direction perpendicular to the main surface 21a of the substrate 21 (out-of-plane direction). In this case, the light passing through the waveguides P1 and P2 is both TM mode light (hereinafter referred to as "TM light"). The light from the waveguide P1 input to the upper upstream port of the polarization rotating splitter 45 is output from the downstream port of the polarization rotating splitter 45 as TM mode light. outOn the other hand, the light from the waveguide P2 input to the lower upstream port of the polarization rotating splitter 45 has its polarization direction rotated by 90°, and is converted into TE mode light (hereinafter referred to as "TE light") whose electric field component is parallel to the direction (in-plane direction) horizontal to the main surface 21a of the substrate 21, and is output from the downstream port of the polarization rotating splitter 45 to the waveguide P out is output to

[0061] Here, by adjusting the phase difference between the light passing through the waveguide P1 and input to the polarization rotation splitter 45 and the light passing through the waveguide P2 and input to the polarization rotation splitter 45 to a predetermined angle (for example, 90°), the polarization of the waveguide P out The light passing through the waveguide P can be made into circularly polarized light rotating in a predetermined direction (for example, clockwise direction) (i.e., a combination of TM light and TE light with a phase difference of 90°). out Right-handed circularly polarized output light L out It is possible to output the output light L out When the reflected light Lr hits an object to be measured (for example, an object to be measured for distance), the reflected light Lr travels through the waveguide P out The reflected light Lr is specularly reflected by the object to be measured, and is then converted into output light L out The circularly polarized light has a rotation direction opposite to that of the light reflected from ...

[0062] The reflected light Lr is input to the downstream port of the polarization rotating splitter 45. The polarization rotating splitter 45 is connected to the waveguide P out The reflected light Lr (output light L outThe polarization rotating splitter 45 is configured to separate two mutually orthogonal polarization components of light (TE light and TM light in this embodiment) contained in the reflected light Lr (reflected light according to the polarization component Lr) and output them to each of the waveguides P1 and P2. That is, the polarization rotating splitter 45 performs a process on the light input from the downstream port that is the reverse of the process performed on the light passing from the upstream port to the downstream port described above, and then outputs the light to each of the two upstream ports. More specifically, the polarization rotating splitter 45 outputs the TM light contained in the reflected light Lr from the upper upstream port to the waveguide P1, and converts the TE light contained in the reflected light Lr into TM light and outputs it from the lower upstream port to the waveguide P2.

[0063] As mentioned above, the output light L out becomes circularly polarized light (for example, right-handed circularly polarized light), and the reflected light Lr becomes the output light L out When the reflected light Lr is a circularly polarized light that rotates in the opposite direction to the reflected light Lr (for example, left-handed circularly polarized light), the reflected light Lr is out From the waveguide P in The light going to the side passes through the waveguide P in The phase is reversed at the waveguide P FB only from the photodetector 50 to the feedback light L FB More specifically, the input light L is input to the output port of the Mach-Zehnder interferometer MZI (the output port of the optical coupler 41). in The light output to the waveguides P1 and P2 due to out Light that has an inverse relationship to the light that forms right-handed circular polarization in the waveguide P out As a result, the light Lr1 (light traveling upstream in the waveguide P1 due to the reflected light Lr) and the light Lr2 (light traveling upstream in the waveguide P2 due to the reflected light Lr) input to the downstream ports of the Mach-Zehnder interferometer MZI (upper and lower downstream ports of the optical coupler 41) are reflected upstream in the waveguide P1 due to the action inside the Mach-Zehnder interferometer MZI. in On the other hand, the light Lr1 and Lr2 input to the Mach-Zehnder interferometer MZI are transmitted through the waveguide P FBSince the feedback light L FB is the waveguide P FB The light is output to the photodetector 50 through the waveguide P FB and the feedback light L FB The photodetector 50 is configured by, for example, a photodiode or the like.

[0064] As described above, by controlling the phase adjustment amount of the phase shifter 44 so that the phase difference between the light passing through the waveguide P1 and input to the polarization rotation splitter 45 and the light passing through the waveguide P2 and input to the polarization rotation splitter 45 becomes a predetermined angle (for example, 90°), the output light L out is circularly polarized light, and the reflected light Lr is the output light L out As a result, the circuit section 40 can receive the light based on the reflected light Lr (the feedback light L FB ) into the waveguide P FB The control for making the circuit section 40 function as a circulator is executed by the control device 60.

[0065] (Control device) The control device 60 is configured to control at least the phase adjustment amount of the phase shifter 44 (the phase shift amount of light passing through the waveguide P1). In this embodiment, the control device 60 is configured to control the phase adjustment amount of the phase shifters 43 and 44, as well as the phase adjustment amount of the external resonator 30 (phase shifters 37 to 39). The control device 60 may be configured by, for example, a computer system including a processor, memory, storage, communication devices, etc. Each function of the control device 60 is executed by these hardware elements operating according to a predetermined program.

[0066] When the phase shifter 44 is a thermal phase shifter (thin film heater) as in this embodiment, the control device 60 can control the phase adjustment amount of the phase shifter 44 by adjusting the amount of power supplied to the phase shifter 44 to control the amount of heat generated in the phase shifter 44. The same applies to the control of the other phase shifters 37, 38, 39, and 43.

[0067] For example, in order to make the circuit unit 40 function as a circulator, the control device 60 controls the light Lr1 traveling upstream in the waveguide P1 due to the reflected light Lr and the light Lr2 traveling upstream in the waveguide P2 due to the reflected light Lr to flow through the waveguide P in The phase adjustment amount of the phase shifter 44 is controlled so that the phases are opposite to each other.

[0068] The control device 60 may also control the phase adjustment amount (the phase shift amount of the light passing through the waveguide P3) of the phase shifter 43. For example, the control device 60 controls the phase adjustment amount of the phase shifter 43 so that the intensity of the light output from the optical coupler 41 to the waveguide P1 and the intensity of the light output from the optical coupler 41 to the waveguide P2 become equal.

[0069] Hereinafter, some examples of the control by the control device 60 will be described.

[0070] (First control) When information on the wavelength dependency (hereinafter referred to as "dependency information") of each circuit element (each optical waveguide, optical couplers 41, 42, phase shifters 43, 44, polarization rotation splitter 45, etc.) constituting the optical integrated circuit 20 is known in advance, the control device 60 may control the phase adjustment amount of the phase shifters 43, 44 based on the dependency information.

[0071] The dependency information used to control the phase shifter 44 is, for example, the input light L in For each wavelength, the light due to the reflected light Lr flows through the waveguide P in (i.e., the output light L outThe dependency information used to control the phase shifter 43 is, for example, information relating to the amount of phase adjustment of the phase shifter 44 for converting the input light L in This information associates, with each wavelength, the amount of phase adjustment by the phase shifter 43 to equalize the intensity of the light output from the optical coupler 41 to the waveguide P1 and the intensity of the light output from the optical coupler 41 to the waveguide P2.

[0072] According to the first control, it is possible to roughly adjust (control) the phase adjustment amounts of the phase shifters 43 and 44 that are necessary for the circuit section 40 to function as a circulator, based on dependency information that has been grasped in advance.

[0073] (Second control) The control device 60 detects the detection result of the photodetector 50 (for example, the feedback light L FB The phase adjustment amount of at least one of the phase shifters 43 and 44 may be controlled based on the intensity of the phase shifter.

[0074] For example, as described above, the output light L out becomes circularly polarized, the waveguide P in The reflected light (light based on the reflected light Lr) returning to the waveguide P FB Only the feedback light L FB Therefore, the input light L in is input to the circuit section 40, the phase adjustment amount of the phase shifter 44 is changed little by little, the feedback light L FB The detected intensity of the output light L out becomes circularly polarized (i.e., the waveguide P in Therefore, the control device 60 adjusts the feedback light L at the photodetector 50 while changing the phase adjustment amount of the phase shifter 44 (i.e., the amount of power supplied to the phase shifter 44), for example. FB The phase adjustment amount of the phase shifter 44 may be determined so that the detected intensity of the signal is maximized.

[0075] Similarly, the closer the ratio of the intensity of the light output from the optical coupler 41 to the waveguide P1 and the intensity of the light output from the optical coupler 41 to the waveguide P2 is to "1:1", the greater the output light L out Therefore, the polarization state of the input light L can be made closer to a circular polarization state. in is input to the circuit section 40, the phase adjustment amount of the phase shifter 43 is changed little by little, the feedback light L FB The detected intensity of the feedback light L in the photodetector 50 is maximized when the above condition of the intensity ratio is satisfied. FB The phase adjustment amount of the phase shifter 43 may be determined so that the detected intensity of the signal is maximized.

[0076] (Third control) The control device 60 may execute a control that combines the first and second controls described above. An example of the third control will be described with reference to FIG. 5. First, the control device 60 controls the input light L in The target wavelength of the input light L is set (step S1). This process may be performed automatically based on a measurement program prepared in advance, or an arbitrary wavelength may be manually set as the target wavelength by an operator or the like. Next, the control device 60 in The control device 60 controls the phase adjustment amounts of the phase shifters 37, 38, and 39 of the external resonator 30 so that the wavelength of the transmitted light Le becomes the target wavelength (step S2). The control device 60 also executes the above-mentioned "first control" (step S3). That is, the control device 60 roughly controls the phase adjustment amounts of the phase shifters 43 and 44 based on the dependency information and the target wavelength that are known in advance. Thereafter, the control device 60 drives the laser driver D to input the input light L from the light source LS to the optical integrated circuit 20. in (Step S4).

[0077] Here, in step S3, the phase adjustment amounts of the phase shifters 43 and 44 are controlled based on the dependency information and the target wavelength that have been previously determined so that the circuit unit 40 functions as a circulator, but control errors and the like may occur. For this reason, the circuit unit 40 is made to function as a circulator (i.e., the waveguide P in It can be difficult to achieve near-zero return light to the target.

[0078] Therefore, the control device 60 may execute the above-mentioned "second control" after starting the laser driving (step S5). For example, the control device 60 may execute the second control based on the feedback light L detected by the photodetector 50. FB The phase adjustment amounts of the phase shifters 43 and 44 may be controlled (finely adjusted) so that the intensity of the phase shifter 43 is maximized. This allows the circuit section 40 to function as a circulator with higher accuracy.

[0079] (Operation and effect of the first embodiment) Optical elements such as optical couplers and phase shifters generally have wavelength dependency. Therefore, when distance measurement, spectroscopic measurement, etc. are performed using a wavelength-tunable light source, the characteristics of optical elements such as optical couplers and phase shifters change depending on the wavelength, which may change the characteristics of the optical integrated circuit, making it difficult to perform measurements using the optical integrated circuit. The light source unit 1A is configured so that the amount of phase adjustment of the phase shifter 44 arranged on at least one path (waveguide P1) of two paths (waveguides P1 and P2) between the optical coupler 41 and the polarization rotation splitter 45 can be controlled by the control device 60. Therefore, according to the light source unit 1A, the input light L in Since the phase adjustment amount of the phase shifter 44 can be controlled in accordance with the change in wavelength, the above-mentioned wavelength dependency problem can be suitably solved.

[0080] The polarization rotation splitter 45 combines the light from the waveguide P1 and the light from the waveguide P2 to generate two orthogonal polarization components (in this embodiment, the TM light from the waveguide P1 and the TE light obtained by converting the TM light from the waveguide P2) and outputs them to the waveguides P out The polarization rotating splitter 45 is configured to output the light from the waveguide P out The control device 60 is configured to separate two mutually orthogonal polarization components contained in the reflected light Lr input from the downstream port of the optical coupler 41 and output them to the waveguides P1 and P2. in The phase adjustment amount of the phase shifter 44 is controlled so that the phases are opposite in phase. According to the above configuration, the optical integrated circuit 20 (circuit unit 40) can be used as a circulator. In particular, it is known that circulators (or isolators) compatible with wavelengths in the mid-infrared range used in spectroscopic analysis, distance measurement (LiDAR, etc.), etc. are difficult to obtain or are very expensive, but the optical integrated circuit (circuit unit 40) of this embodiment can realize a circulator that is inexpensive and operates stably.

[0081] The control device 60 controls the phase adjustment amount of the phase shifter 43 provided in the waveguide P3. in is branched into waveguides P3 and P4 by the optical coupler 42, and by controlling the phase adjustment amount of the phase shifter 43 provided in one of the waveguides, P3, it is possible to appropriately set the phase difference between the two lights (light from waveguide P3 and light from waveguide P4) input to the optical coupler 41. As a result, it is possible to appropriately adjust the intensity ratio of the lights output to each of the waveguides P1 and P2 downstream of the optical coupler 41. The above configuration is particularly effective when the optical integrated circuit 20 (circuit unit 40) is made to function as a circulator, as in this embodiment. That is, in An optical coupler 42, a waveguide P3 provided with a phase shifter 43, and a waveguide P4 are provided between the optical coupler 41 and the optical coupler 41, and by controlling the phase adjustment amount of the phase shifter 43, it is possible to precisely adjust the intensity ratio ("1:1" in this embodiment) of the light output to the waveguides P1 and P2. As a result, the output light Lout This allows the optical integrated circuit 20 to function favorably as a circulator.

[0082] The optical integrated circuit 20 (circuit section 40) has a delay circuit formed in at least one of the waveguides P1 and P2. In this embodiment, delay lines 46 and 47 are provided as delay circuits in the waveguides P1 and P2, respectively. For example, by providing such delay lines 46 and 47 and appropriately adjusting the ratio of the lengths of the waveguides P1 and P2, the phase difference between the light passing through the waveguide P1 and the light passing through the waveguide P2 can be adjusted to be equal to or smaller than the phase difference between the input light L in This reduces the degree to which the phase varies depending on the wavelength of the waveguide P1. As a result, it is possible to reduce the amount of phase adjustment required by the phase shifter 44. Note that, when it is necessary to adjust the length of only one of the waveguides P1 and P2 (for example, when lengthening the shorter one of the waveguides P1 and P2), only one of the delay lines 46 and 47 may be provided.

[0083] The optical integrated circuit 20 (circuit section 40) is connected to an optical coupler 42, and outputs an output light L out The feedback light L generated by the optical coupler 42 is generated based on the light traveling upstream through each of the waveguides P3 and P4 due to the reflected light Lr corresponding to the FB The waveguide P FB and the waveguide P FB The feedback light L is connected to the end opposite to the end connected to the optical coupler 42. FB The control device 60 controls the phase adjustment amount of at least one of the phase shifters 43 and 44 based on the detection result of the photodetector 50. That is, the control device 60 is configured to be able to execute the above-mentioned "second control" based on the detection result of the photodetector 50. According to the above configuration, the feedback light L based on the reflected light Lr FB By monitoring this, it becomes easy to appropriately control the phase adjustment amounts of the phase shifters 43 and 44 so that the optical integrated circuit 20 (circulator section 40) exhibits favorable characteristics (in this embodiment, the characteristics of a circulator).

[0084] The light source LS is a QCL10 as a laser gain medium and an input light L in The external resonator 30 controls the wavelength of the input light, and the external resonator 30 is mounted on the substrate 21. According to the above configuration, the light source unit 1A can be prevented from becoming large, and the external resonator 30 (a ring resonator in this embodiment) can easily control the wavelength of the input light.

[0085] The control device 60 detects the input light L in The control device 60 controls the external resonator 30 so that the wavelength of the input light L becomes a predetermined target wavelength, and controls the phase adjustment amount of at least one of the phase shifters 43, 44 in accordance with the target wavelength. That is, the control device 60 is configured to be able to execute the "third control" as shown in FIG. 5. According to the above configuration, when the target wavelength is set in the control device 60, the external resonator 30 controls the phase adjustment amount of the input light L in It is possible to perform wavelength control of the transmitted light Le and control of the phase adjustment amount of at least one of the phase shifters 43, 44 according to the target wavelength in parallel. This shortens the time from when the target wavelength is determined until control of the phase adjustment amount is completed and appropriate measurement can be performed. For example, in the example of FIG. 5, in step S3, a rough control of the phase adjustment amount can be performed so that the circuit unit 40 exhibits suitable characteristics (in this embodiment, the characteristics of a circulator). This makes it possible to complete control of the phase adjustment amount in step S5 after the laser drive starts in a short time.

[0086] The QCL 10 as a laser gain medium is not mounted on the substrate 21 of the optical integrated circuit 20. According to the above configuration, by separating the laser gain medium from the optical integrated circuit 20 (substrate 21), it is possible to improve the degree of freedom in the material and arrangement of the laser gain medium.

[0087] [Second embodiment] The configuration of a light source unit 1B of the second embodiment will be described with reference to Fig. 6. Light source unit 1B differs from light source unit 1A in that a light source LS (QCL 10 and external resonator 30) is mounted on a substrate 21 of an optical integrated circuit 20 (i.e., not only the external resonator 30 but also the QCL 10 is mounted on the substrate 21). With light source unit 1B, by mounting both the QCL 10 and the external resonator 30 that constitute the light source LS on the substrate 21, the light source unit can be more suitably miniaturized.

[0088] [Third embodiment] The configuration of a light source unit 1C of the third embodiment will be described with reference to FIG. 7. The light source unit 1C differs from the light source unit 1A in that the light source LS (elements corresponding to the QCL 10 and external resonator 30 of the first embodiment) is separated from the optical integrated circuit 20. That is, the light source unit 1C differs from the light source unit 1A in that the external resonator 30 is not mounted on the substrate 21 (i.e., the light source unit 1C has an optical integrated circuit 20C that does not include the external resonator 30). The light source unit 1C eliminates the need to mount an optical element corresponding to the external resonator 30 on the substrate 21, thereby simplifying the configuration of the optical integrated circuit 20C. Furthermore, the degree of freedom of the light source LS (such as the degree of freedom in combining the laser gain medium and the external resonator) can be improved.

[0089] [Variations] Although several embodiments of the present disclosure have been described above, the present disclosure is not limited to the configurations shown in the above embodiments. The materials and shapes of each configuration are not limited to the specific materials and shapes described above, and various materials and shapes other than those described above can be used. Furthermore, some of the configurations included in the above embodiments may be omitted or modified as appropriate, or may be combined in any manner.

[0090] For example, the layout of the circuit elements included in the circuit section 40 of the light source units 1A, 1B, and 1C described above may be changed as appropriate, or some circuit elements may be omitted. As an example, the circuit section 40 of the light source units 1A, 1B, and 1C may be replaced with a circuit section 40A shown in FIG. 8. The circuit section 40A differs from the circuit section 40 in that it does not include the waveguides P3 and P4, the optical coupler 42, the phase shifter 43, and the delay lines 46 and 47. That is, in the circuit section 40A, the upper upstream port of the optical coupler 41 is connected to the waveguide P in The lower upstream port of the optical coupler 41 is connected to the downstream end of the waveguide P FB is connected to the upstream end of the

[0091] Even when such a configuration of the circuit section 40A is adopted, the control device 60 can detect the detection result of the photodetector 50 (for example, the feedback light L FB 8, the circuit section 40A is connected to the optical coupler 41, and the output light L out The waveguide P1 and the waveguide P2 are caused by reflected light according to the light beams passing through them. The reflected light is generated by the optical coupler 41 based on the light beams going upstream. FB and the waveguide P FB and a photodetector 50 connected to an end opposite to the end connected to the optical coupler 41, for detecting feedback light. Therefore, the control device 60 can execute control similar to the "second control" described in the above embodiment by controlling the phase adjustment amount of the phase shifter 44 based on the detection result of the photodetector 50. That is, by monitoring the feedback light based on the reflected light, it becomes easy to appropriately control the phase adjustment amount of the phase shifter 44 so that the optical integrated circuit 20 (circuit unit 40A) exhibits suitable characteristics (in this embodiment, the properties of a circulator).

[0092] When the optical coupler 41 behaves ideally as a 3 dB coupler (a coupler with a branching ratio of 1:1), the ratio between the intensity of the light output from the optical coupler 41 to the waveguide P1 and the intensity of the light output from the optical coupler 41 to the waveguide P2 can be set to 1:1 in the circuit unit 40A as well. Therefore, by controlling the phase adjustment amount of the phase shifter 44 by the control device 60 in the same manner as in the above embodiment, the circuit unit 40A can be made to function as a circulator. However, as described above, the optical coupler 41 has wavelength dependency, and therefore, the input light L in Depending on the wavelength, the optical coupler 41 may not behave ideally, and the ratio between the intensity of the light output from the optical coupler 41 to the waveguide P1 and the intensity of the light output from the optical coupler 41 to the waveguide P2 may deviate from "1:1". In contrast, according to the Mach-Zehnder interferometer MZI of the circuit unit 40 described in the first embodiment, the amount of phase adjustment by the phase shifter 43 is adjusted based on the input light L in By controlling the output power of the optical coupler 41 in accordance with the wavelength of the waveguide P1, the wavelength dependency as described above can be absorbed, and the ratio between the intensity of the light output from the optical coupler 41 to the waveguide P2 can be made closer to "1:1."

[0093] In the above embodiment, directional couplers have been described as an example of the optical couplers 41 and 42, but the optical couplers 41 and 42 may be other optical devices. For example, the optical couplers 41 and 42 may be configured as multi-mode interference (MMI) couplers. Furthermore, the waveguides P3 and P4 connecting the optical couplers 41 and 42 in the Mach-Zehnder interferometer MZI may be set to have the same length or different lengths.

[0094] In the above embodiment, the downstream ends of the waveguides P1 and P2 and the out Although a polarization rotating splitter (PRS) has been described as an example of the optical coupler (second optical coupler) that is connected to the upstream end of the optical coupler and splits and combines light, the optical coupler (second optical coupler) may be an optical device other than the PRS. For example, the optical coupler (second optical coupler) may be configured by a polarization beam splitter (PBS), a polarization rotator, etc.

[0095] Furthermore, the application of the circuit section 40 is not limited to a circulator. The behavior of the optical couplers 41 and 42 and the polarization rotation splitter 45 may be changed as appropriate depending on the application of the circuit section 40.

[0096] The light source LS also emits input light L having a wavelength of less than 2 μm. in That is, the light source LS may be configured to output an input light L that is wavelength tunable in a wavelength range including wavelengths shorter than 2 μm. in may be configured to be able to output.

[0097] In the above embodiment, the input light L in The polarization mode of the input light L was set to be the TM mode. in The polarization mode of the TM light may be set to be the TE mode. In this case, the "TM mode" and "TM light" in the above embodiment are replaced with the "TE mode" and "TE light", and the "TE mode" and "TE light" in the above embodiment are replaced with the "TM mode" and "TM light".

[0098] Furthermore, in the above-described circuit units 40 and 40A, phase shifters may be provided in both the waveguides P1 and P2. With such a configuration, for example, the amount of phase adjustment of the waveguide P1 relative to the waveguide P2 can be more flexibly controlled by an operation (push-pull operation) in which the power supplied to the phase shifter 44 provided in the waveguide P1 is increased (to increase the refractive index) while the power supplied to the phase shifter provided in the waveguide P2 is decreased (to decrease the refractive index). Furthermore, in the above-described circuit unit 40, phase shifters may be provided in both the waveguides P3 and P4. In this case, the amount of phase adjustment of the waveguides P3 and P4 can also be controlled by the same push-pull operation as that of the above-described waveguides P1 and P2. [Explanation of symbols]

[0099] 1A, 1B, 1C...light source unit, 10...QCL (laser gain medium), 20, 20C...optical integrated circuit, 30...external resonator, 41...optical coupler (first optical coupler), 42...optical coupler (third optical coupler), 43...phase shifter (second phase shifter), 44...phase shifter (first phase shifter), 45...polarization rotation splitter (second optical coupler), 50...photodetector, 60...control device (control unit), LS...light source, P FB ...waveguide (feedback waveguide), P in ...waveguide (input waveguide), P out ...waveguide (output waveguide), P1...waveguide (first waveguide), P2...waveguide (second waveguide), P3...waveguide (third waveguide), P4...waveguide (fourth waveguide).

Claims

1. a light source configured to be wavelength tunable and outputting light with a wavelength of 2 μm or more; an optical integrated circuit including a substrate and an optical waveguide provided on the substrate; a control unit that controls the operation of at least a part of the optical integrated circuit, The optical waveguide is an input waveguide into which input light from the light source is input; a first waveguide located downstream of the input waveguide; a second waveguide located downstream of the input waveguide; an output waveguide that outputs output light corresponding to the input light, The optical integrated circuit comprises: a first optical coupler connected to an upstream end of each of the first waveguide and the second waveguide, and configured to split and combine light; a second optical coupler connected to the downstream ends of the first waveguide and the second waveguide and the upstream end of the output waveguide, for branching and combining light; a first phase shifter provided in the first waveguide and configured to adjust the phase of light passing through the first waveguide; The control unit controls a phase adjustment amount of the first phase shifter.

2. the second optical coupler is configured to combine the light from the first waveguide and the light from the second waveguide to output light of two polarized components orthogonal to each other to the output waveguide, and to separate and output light of two polarized components orthogonal to each other, which are included in reflected light corresponding to the output light input from the output waveguide, to each of the first waveguide and the second waveguide, 2. The light source unit according to claim 1, wherein the control unit controls a phase adjustment amount of the first phase shifter so that light traveling upstream through the first waveguide due to the reflected light and light traveling upstream through the second waveguide due to the reflected light are in opposite phase in the input waveguide.

3. The optical integrated circuit comprises: a third optical coupler that receives the input light from the input waveguide and splits the input light into light that is output to a third waveguide and light that is output to a fourth waveguide; a second phase shifter provided in the third waveguide and configured to adjust the phase of light passing through the third waveguide; the first optical coupler is connected to a downstream end of each of the third waveguide and the fourth waveguide; The light source unit according to claim 1 , wherein the control unit controls a phase adjustment amount of the second phase shifter.

4. The optical integrated circuit comprises: a feedback waveguide connected to the first optical coupler, for passing feedback light generated in the first optical coupler based on light traveling upstream through each of the first waveguide and the second waveguide due to reflected light corresponding to the output light; a photodetector connected to an end of the feedback waveguide opposite to the end connected to the first optical coupler, and configured to detect the feedback light; The light source unit according to claim 1 , wherein the control unit controls a phase adjustment amount of the first phase shifter based on a detection result of the photodetector.

5. The optical integrated circuit comprises: a feedback waveguide connected to the third optical coupler, for passing feedback light generated in the third optical coupler based on light traveling upstream through each of the third waveguide and the fourth waveguide due to reflected light corresponding to the output light; a photodetector connected to an end of the feedback waveguide opposite to the end connected to the third optical coupler, and configured to detect the feedback light; The light source unit according to claim 3 , wherein the control unit controls a phase adjustment amount of at least one of the first phase shifter and the second phase shifter based on a detection result of the photodetector.

6. The light source unit according to claim 1 , wherein the optical integrated circuit has a delay circuit formed in at least one of the first waveguide and the second waveguide.

7. the light source has a laser gain medium and an external cavity that controls the wavelength of the input light; The light source unit according to claim 1 , wherein the external resonator is mounted on the substrate.

8. The light source unit according to claim 7 , wherein the control unit controls the external resonator so that the wavelength of the input light becomes a predetermined target wavelength, and controls a phase adjustment amount of the first phase shifter according to the target wavelength.

9. 9. The light source unit according to claim 7, wherein the laser gain medium is not mounted on the substrate.

10. 9. The light source unit according to claim 7, wherein the laser gain medium is mounted on the substrate.

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