JP2017103455A - Wavelength tunable laser device and control method of wavelength tunable laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength tunable laser device and a control method of a wavelength tunable laser by which frequency noises in the wavelength tunable laser can be decreased.SOLUTION: The wavelength tunable laser device includes: a wavelength tunable laser, a control part that controls to fix the oscillation conditions of the wavelength tunable laser; an extraction part that extracts a part of laser light emitted from the wavelength tunable laser; a polarized wave rotation part that rotates polarized waves of the laser light extracted by the extraction part within a range of 90°±5° and allows the rotated laser light to enter the wavelength tunable laser; and an adjusting part that adjusts the power of the laser light entering the wavelength tunable laser so as to decrease frequency noises of the laser light emitted from the wavelength tunable laser.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a wavelength tunable laser device and a wavelength tunable laser control method.

  There has been disclosed a technique for suppressing frequency noise of laser output by reflecting laser output light outside and re-entering the return light into the laser (for example, see Non-Patent Document 1). A technique for reducing the laser line width by monitoring the laser light output through a filter having a wavelength-dependent transmission characteristic and negatively feeding the output back to the phase adjustment current of the SG-DBR laser is disclosed. (For example, refer nonpatent literature 2). A technique for rotating the polarization of output light of a DFB laser and re-entering the DFB laser is disclosed (for example, see Non-Patent Document 3).

“Optical Negative Feedback for Linewidth Reduction of Semiconductor Lasers”, IEEE Photon. Technol. Lett., Vol.27, pp340-343, 2015 “Integrated Linewidth Reduction of a Tunable SG-DBR Laser”, CLEO2013, CTu1L.2 “FM Noise and Spectral Linewidth Reduction by Incoherent Optical Negative Feedback”, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 27, NO. 2, FEBRUARY 1991

  In the technique of Non-Patent Document 1, in order to obtain good characteristics, it is necessary to strictly control the optical path length of negative feedback. With the technique of Non-Patent Document 2, it is difficult to reduce frequency noise over a wide band. In the technique of Non-Patent Document 3, there are cases where frequency noise cannot be reduced when performing negative feedback in a wavelength tunable laser.

  Accordingly, it is an object of the present invention to provide a wavelength tunable laser device and a wavelength tunable laser control method capable of reducing the frequency noise of the wavelength tunable laser with a simple configuration.

  A wavelength tunable laser device according to the present invention includes a wavelength tunable laser, a control unit that performs control for fixing an oscillation condition of the wavelength tunable laser, and an extraction unit that extracts a part of laser light emitted from the wavelength tunable laser. A polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the wavelength tunable laser, and a laser beam emitted from the wavelength tunable laser An adjustment unit that adjusts the power of the laser beam incident on the wavelength tunable laser so that the frequency noise of the laser is reduced.

  According to the above invention, the frequency noise of the wavelength tunable laser can be reduced with a simple configuration.

1 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to a first embodiment. It is a typical sectional view showing the whole wavelength variable laser composition. FIG. 6 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to a second embodiment. FIG. 6 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to a third embodiment. FIG. 10 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to a fourth embodiment. FIG. 10 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to a fifth embodiment. FIG. 10 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to a sixth embodiment. FIG. 10 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to a seventh embodiment. FIG. 10 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to an eighth embodiment. FIG. 10 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to Example 9. It is a figure which illustrates the example of the transmission characteristic of the ring resonator of Si waveguide device. FIG. 10 is a block diagram illustrating another example of the wavelength tunable laser device according to the ninth embodiment. FIG. 10 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to Example 10. FIG. 10 is a block diagram illustrating an overall configuration of a wavelength tunable laser device according to an eleventh embodiment.

[Description of Embodiment of Present Invention]
First, the contents of the embodiments of the present invention will be listed and described.
(1) A wavelength tunable laser device includes a wavelength tunable laser, a control unit that performs control to fix an oscillation condition of the wavelength tunable laser, and an extraction unit that extracts a part of laser light emitted from the wavelength tunable laser. A polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit within a range of 90 ° ± 5 ° and enters the wavelength tunable laser, and a laser beam emitted from the wavelength tunable laser. An adjustment unit that adjusts the power of the laser beam incident on the wavelength tunable laser so that frequency noise is reduced.
(2) The extraction unit is a mirror that reflects a part of output light emitted from the wavelength tunable laser, and the polarization rotation unit is disposed between the wavelength tunable laser and the mirror. It is good also as a / 4 wavelength plate.
(3) The adjustment unit may be an optical amplifier or an attenuator monolithically integrated in the wavelength tunable laser.
(4) The wavelength tunable laser may be a horizontal resonator, and both emission end faces of the wavelength tunable laser may be covered with an AR film.
(5) An optical path from the extraction unit to the wavelength tunable laser may include a loss unit having frequency dependence on optical loss.
(6) A detection unit that detects the intensity of light lost by the loss unit may be provided, and the adjustment unit may adjust the power according to a detection result of the detection unit.
(7) You may provide the wavelength adjustment part which adjusts the output wavelength of the said wavelength variable laser using the detection result of the said detection part.
(8) The loss part may be an etalon.
(9) You may provide the loss adjustment part which adjusts the optical loss of the said loss part.
(10) The adjustment unit may be a ring resonator.
(11) You may provide the temperature control part which controls the temperature of the said ring resonator.
(12) The polarization rotation unit may have a configuration in which a 45 ° polarizer is sandwiched between two λ / 4 wavelength plates.
(13) The control method of the wavelength tunable laser performs control to fix the oscillation condition of the wavelength tunable laser, extracts a part of the laser light emitted from the wavelength tunable laser, and polarizes the extracted laser light Is incident on the tunable laser so that the frequency noise of the laser light emitted from the tunable laser is reduced. Adjust the light power.

[Details of the embodiment of the present invention]
Specific examples of a wavelength tunable laser device and a wavelength tunable laser control method according to an embodiment of the present invention will be described below with reference to the drawings. In addition, this invention is not limited to these illustrations, is shown by the claim, and intends that all the changes within the meaning and range equivalent to a claim are included.

  FIG. 1 is a block diagram illustrating an overall configuration of a wavelength tunable laser device 100 according to the first embodiment. As shown in FIG. 1, the wavelength tunable laser apparatus 100 includes a wavelength tunable laser 30 (wavelength tunable semiconductor laser) that is a semiconductor laser capable of controlling the wavelength as a laser device. The wavelength tunable laser 30 of this embodiment has a structure in which a SOA (Semiconductor Optical Amplifier) is monolithically integrated in a laser region (resonator). This SOA functions as a light output control unit. The SOA can arbitrarily increase or decrease the intensity of light output. It is also possible to control the light output intensity to be substantially zero. The wavelength tunable laser 30 may be accommodated in a package.

  Furthermore, the wavelength tunable laser device 100 includes a splitter 31, 32, a light receiving element 33, a controller 34, an attenuator (VOA) 35, a splitter 36, a light receiving element 37, a controller 38, a mirror 39, a half-wave plate (HWP). : Half-wave plate) 40, current supply unit 41, and the like.

  FIG. 2 is a schematic cross-sectional view showing the overall configuration of the wavelength tunable laser 30 in the present embodiment. As shown in FIG. 2, the wavelength tunable laser 30 includes a SG-DFB (Sampled Grafted Distributed Feedback) region A, a CSG-DBR (Chired Sampled Distilled Dielectric Reflector) region, an Ac, and an Amplified Bragg Reflector (B) region. Is provided. That is, the wavelength tunable laser 30 is a laser having a wavelength selection mirror in a semiconductor structure.

  As an example, in the wavelength tunable laser 30, the SOA region C, the SG-DFB region A, and the CSG-DBR region B are arranged in this order from the front side to the rear side. The SG-DFB region A has a gain and includes a sampled grating. The CSG-DBR region B has a sampled grating without gain. SG-DFB region A and CSG-DBR region B correspond to the laser region in FIG. 1, and SOA region C corresponds to the SOA region in FIG.

  The SG-DFB region A has a structure in which a lower cladding layer 2, an active layer 3, an upper cladding layer 6, a contact layer 7, and an electrode 8 are stacked on a substrate 1. The CSG-DBR region B has a structure in which a lower cladding layer 2, an optical waveguide layer 4, an upper cladding layer 6, an insulating film 9, and a plurality of heaters 10 are stacked on a substrate 1. Each heater 10 is provided with a power supply electrode 11 and a ground electrode 12. The SOA region C has a structure in which a lower cladding layer 2, an optical amplification layer 19, an upper cladding layer 6, a contact layer 20, and an electrode 21 are stacked on a substrate 1.

  In the SG-DFB region A, CSG-DBR region B, and SOA region C, the substrate 1, the lower cladding layer 2, and the upper cladding layer 6 are integrally formed. The active layer 3, the optical waveguide layer 4, and the optical amplification layer 19 are formed on the same surface. The boundary between the SG-DFB region A and the CSG-DBR region B corresponds to the boundary between the active layer 3 and the optical waveguide layer 4.

  An end face film 16 is formed on end faces of the substrate 1, the lower clad layer 2, the optical amplifying layer 19, and the upper clad layer 6 on the SOA region C side. In this embodiment, the end face film 16 is an anti-reflection (AR) film. The end face film 16 functions as a front side end face of the wavelength tunable laser 30. An end face film 17 is formed on end faces of the substrate 1, the lower clad layer 2, the optical waveguide layer 4, and the upper clad layer 6 on the CSG-DBR region B side. In this embodiment, the end face film 17 is an AR film. The end face film 17 functions as a rear side end face of the wavelength tunable laser 30.

  The substrate 1 is a crystal substrate made of n-type InP, for example. The lower cladding layer 2 is n-type, and the upper cladding layer 6 is p-type, and each is made of, for example, InP. The lower cladding layer 2 and the upper cladding layer 6 confine the active layer 3, the optical waveguide layer 4, and the optical amplification layer 19 in the upper and lower directions.

The active layer 3 is composed of a semiconductor having a gain. The active layer 3 has, for example, a quantum well structure, for example, a well layer made of Ga _0.32 In _0.68 As _0.92 P _0.08 (thickness 5 nm), Ga _0.22 In _0. It has a structure in which barrier layers made of ₇₈ As _0.47 P _0.53 (thickness 10 nm) are alternately stacked. The optical waveguide layer 4 can be composed of, for example, a bulk semiconductor layer, and can be composed of, for example, Ga _0.22 In _0.78 As _0.47 P _0.53 . In this embodiment, the optical waveguide layer 4 has a larger energy gap than the active layer 3.

The optical amplification layer 19 is a region to which gain is given by current injection from the electrode 21, thereby performing optical amplification. The optical amplifying layer 19 can be configured by, for example, a quantum well structure. For example, a well layer of Ga _0.35 In _0.65 As _0.99 P _0.01 (thickness 5 nm) and a Ga _0.15 In _0. A structure in which barrier layers of ₈₅ As _0.32 P _0.68 (thickness 10 nm) are alternately stacked can be employed. Further, as another structure, for example, a bulk semiconductor made of Ga _0.44 In _0.56 As _0.95 P _0.05 can be adopted. Note that the optical amplification layer 19 and the active layer 3 can be made of the same material.

The contact layers 7 and 20 can be composed of, for example, p-type Ga _0.47 In _0.53 As crystal. The insulating film 9 is a protective film made of a silicon nitride film (SiN) or a silicon oxide film (SiO). The heater 10 is a thin film resistor made of titanium tungsten (TiW). Each of the heaters 10 may be formed across a plurality of segments of the CSG-DBR region B.

  The electrodes 8, 21, the power supply electrode 11, and the ground electrode 12 are made of a conductive material such as gold (Au). A back electrode 15 is formed at the bottom of the substrate 1. The back electrode 15 is formed across the SG-DFB region A, the CSG-DBR region B, and the SOA region C.

The end face film 16 and the end face film 17 are AR films having a reflectance of 1.0% or less, and have characteristics that their end faces are substantially non-reflective. The AR film can be composed of a dielectric film made of, for example, MgF ₂ and TiON. In this embodiment, both ends of the laser are AR films, but the end face film 17 may be formed of a reflective film having a significant reflectance. In the case where a structure having a light absorption layer is provided in the semiconductor in contact with the end face film 17 in FIG. 2, the light output leaking to the outside from the end face film 17 is suppressed by giving the end face film 17 a significant reflectance. Can do. A significant reflectance is, for example, a reflectance of 10% or more. Here, the reflectance refers to the reflectance with respect to the inside of the semiconductor laser.

The diffraction grating (corrugation) 18 is formed at a plurality of positions with a predetermined interval in the lower cladding layer 2 of the SG-DFB region A and the CSG-DBR region B. Thereby, sampled gratings are formed in the SG-DFB region A and the CSG-DBR region B. In the SG-DFB region A and the CSG-DBR region B, the lower cladding layer 2 is provided with a plurality of segments. Here, the segment refers to a region in which a diffraction grating portion where the diffraction grating 18 is provided and a space portion where the diffraction grating 18 is not provided are continuous one by one. That is, a segment refers to a region where a space part sandwiched between both ends by a diffraction grating part and the diffraction grating part are connected. The diffraction grating 18 is made of a material having a refractive index different from that of the lower cladding layer 2. When the lower cladding layer 2 is InP, for example, Ga _0.22 In _0.78 As _0.47 P _0.53 can be used as a material constituting the diffraction grating.

  The diffraction grating 18 can be formed by patterning using a two-beam interference exposure method. The space portion located between the diffraction gratings 18 can be realized by exposing the pattern of the diffraction grating 18 to the resist and then exposing the position corresponding to the space portion again. The pitch of the diffraction grating 18 in the SG-DFB region A and the pitch of the diffraction grating 18 in the CSG-DBR region B may be the same or different. In this embodiment, as an example, both pitches are set to be the same. Moreover, in each segment, the diffraction grating 18 may have the same length or may have a different length. Further, each diffraction grating 18 in the SG-DFB region A has the same length, each diffraction grating 18 in the CSG-DBR region B has the same length, and the SG-DFB region A and the CSG-DBR region B The length of the diffraction grating 18 may be different.

  In the SG-DFB region A, the optical length of each segment is substantially the same. In the CSG-DBR region B, the optical lengths of at least two segments are different from each other. Thereby, the intensity | strength of the peak of the wavelength characteristic of CSG-DBR area | region B comes to have wavelength dependence. The average optical length of the segment in the SG-DFB region A and the average optical length of the segment in the CSG-DBR region B are different. As described above, the segment in the SG-DFB region A and the segment in the CSG-DBR region B form a laser region (resonator) in the wavelength tunable laser 30.

  In each of the SG-DFB region A and the CSG-DBR region B, the reflected lights interfere with each other. The active layer 3 is provided in the SG-DFB region A. When carriers are injected, a discrete gain spectrum having a predetermined wavelength interval with substantially uniform peak intensity is generated. In the CSG-DBR region B, a discrete reflection spectrum having a predetermined wavelength interval with a different peak intensity is generated. The intervals between the peak wavelengths of the wavelength characteristics in the SG-DFB region A and the CSG-DBR region B are different. A wavelength satisfying the oscillation condition can be selected using the vernier effect generated by the combination of these wavelength characteristics.

  The current supply unit 41 supplies a current corresponding to a specified oscillation wavelength to the electrode 8 and each power supply electrode 11. Thereby, the wavelength tunable laser 30 oscillates at a designated wavelength. The current supply unit 41 controls the supply current to be constant if the wavelength tunable laser 30 is laser-oscillated at the designated wavelength. Thereby, the oscillation condition of the laser region (resonator) of the wavelength tunable laser 30 is fixed.

  Referring again to FIG. 1, the splitter 31 branches the vertical light emitted from the front side of the wavelength tunable laser 30. The perpendicular light has a polarization direction parallel to the active layer 3, the optical waveguide layer 4, and the optical amplification layer 19, and has a polarization direction perpendicular to the stacking direction of the semiconductor layers of the wavelength tunable laser 30. A part of one of the branched vertical lights passes through the splitter 32 and is received by the light receiving element 33. The light receiving element 33 transmits an electrical signal to the controller 34 in accordance with the received light power. The branching rate of the splitter 31 and the reflectance (transmittance) of the splitter 32 are constant. Thereby, the controller 34 can detect the power of the vertical light output from the wavelength tunable laser 30. The controller 34 adjusts the amplification factor of the SOA region C so that the power of vertical light becomes a desired value. That is, the controller 34 performs APC (Automatic Power Control) control.

  The vertical light reflected by the splitter 32 enters the attenuator 35. The attenuator 35 attenuates the power of the incident vertical light and enters the splitter 36. Part of the vertical light incident on the splitter 36 passes through the splitter 36 and is received by the light receiving element 37. The light receiving element 37 transmits an electrical signal to the controller 38 in accordance with the received light power. The reflectance (transmittance) of the splitter 36 is constant. Thereby, the controller 38 can detect the power of the vertical light output from the attenuator 35. The controller 38 adjusts the attenuation rate of the attenuator 35 so that the power of the vertical light output from the attenuator 35 becomes a desired value. The vertical light reflected by the splitter 36 is reflected by the mirror 39 and enters the half-wave plate 40. The half-wave plate 40 converts the polarized light of the vertical light into horizontal light by rotating it by 90 degrees, and makes it re-enter from the rear side of the wavelength tunable laser 30.

  In this embodiment, a polarization rotation optical feedback (PROF) loop is formed by the splitter 31, the splitter 32, the attenuator 35, the splitter 36, the light receiving element 37, the controller 38, the mirror 39, and the half-wave plate 40. Is formed.

The dominant factor of the frequency fluctuation Δv in the optical output of the wavelength tunable laser 30 is the carrier density fluctuation ΔN in the laser region (resonator). The relationship between the frequency fluctuation Δv and the carrier density fluctuation ΔN is given by, for example, the following formula (1). In formula (1), alpha alpha parameter, gamma optical confinement coefficient, v _g is the group velocity of light, a is a derivative gain.
Δv = (α / 4π) · Γv _g aΔN (1)

  According to the above formula (1), in order to suppress the frequency fluctuation Δv, it is important to suppress the carrier density fluctuation ΔN, or to adjust the carrier density in the direction in which the fluctuation decreases according to the frequency fluctuation Δv. It is. Here, the polarized light of the vertical light emitted from the wavelength tunable laser 30 is rotated by 90 degrees to be horizontal light, and the horizontal light is returned to the laser region (resonator) so that the return light can be expressed by the rate equation related to the carrier density. Depending on the strength, the carrier density can be increased or decreased. This is because the negative feedback control of (increased carrier density → increased photon density → increased light output → increased return light → increased photon density → decreased carrier density) works.

  Since the return light of the horizontal light does not interfere with the vertical light of the laser oscillation, it does not affect the rate equation regarding the photon density and the optical phase. Therefore, stable laser oscillation can be obtained as compared with the case where vertical light is used as return light. Note that in the case of an edge-emitting laser in which the laser region includes a horizontal resonator, there is a large difference between the threshold gain of vertical light and the threshold gain of horizontal light, so that laser oscillation with horizontal light is suppressed.

  In the present embodiment, the controller 38 controls the power of the returned horizontal light to be constant so that the frequency noise of the vertical light emitted from the front of the wavelength tunable laser 30 is preferably minimized. To do. That is, the controller 38 performs APC (Automatic Power Control) control on the returned horizontal light power. Accordingly, it is possible to suppress the frequency fluctuation of the oscillating laser light while obtaining stable laser oscillation. Note that the power of horizontal light to be returned can be measured in advance. In addition, the oscillation condition can be changed in the wavelength tunable laser. However, since the current supply unit 41 controls to fix the oscillation condition of the wavelength tunable laser 30 as in this embodiment, stable laser oscillation can be obtained by feeding back horizontal light.

  Since the wavelength tunable laser 30 is a wavelength tunable laser, the frequency noise level may vary depending on the wavelength. Therefore, the feedback power is preferably set optimally for each wavelength. The table may store feedback power in association with the output wavelength.

  Since anti-reflection films are formed on both end faces of the wavelength tunable laser 30, resonance other than the laser region generated between the wavelength tunable laser 30 and the PROF loop is suppressed.

  According to the present embodiment, by providing a simple optical feedback circuit and an electric circuit outside the wavelength tunable laser, low frequency noise characteristics can be stably obtained over a wide band. In order to obtain stable operation, it is only necessary to keep the return light level appropriate. To control it, a special electric circuit that must operate at a high frequency, or phase control that adjusts the delay amount of the negative feedback loop No function is required. In addition, since the return light has a polarization orthogonal to that of the laser oscillation light, there is no light interference, and its level is very small compared to the laser oscillation light, affecting the characteristics such as the optical output level and SMSR. Don't give. The tunable laser device according to the present embodiment can be easily downsized and is optimal as a light source for digital coherent communication using multilevel modulation such as 64QAM.

  In this embodiment, the horizontal light is obtained by rotating the polarization of the vertical light emitted from the wavelength tunable laser 30 by 90 degrees, but the present invention is not limited to this. For example, even if vertical light is converted in the range of 90 ° ± 5 °, the effect of reducing frequency noise can be obtained.

  In the present embodiment, the current supply unit 41 functions as an example of a control unit that performs control for fixing the oscillation condition of the laser region (resonator). The splitter 31 functions as an example of an extraction unit that extracts part of the laser light emitted from the wavelength tunable laser. The half-wave plate 40 functions as an example of a polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the laser region. The attenuator 35 and the controller 38 function as an example of an adjustment unit that adjusts the power of the laser light incident on the laser region so that the frequency noise of the laser light emitted from the wavelength tunable laser is reduced.

  FIG. 3 is a block diagram illustrating the overall configuration of the wavelength tunable laser device 100A according to the second embodiment. As illustrated in FIG. 3, the wavelength tunable laser device 100 </ b> A does not include the splitter 32, but includes a demultiplexer / multiplexer 42 and a mirror 43, as compared with the wavelength tunable laser device 100 of FIG. 1. In addition, about the structure similar to Example 1, description is abbreviate | omitted by attaching | subjecting a same sign.

  In the present embodiment, one vertical light branched by the splitter 31 is received by the light receiving element 33. The light receiving element 33 transmits an electrical signal to the controller 34 in accordance with the received light power. The branching rate of the splitter 31 is constant. Thereby, the controller 34 can detect the optical power of the vertical light output from the wavelength tunable laser 30. The controller 34 adjusts the amplification factor of the SOA region C so that the optical power of the vertical light becomes a desired value.

  The vertical light emitted from the rear side (CSG-DBR region B side) of the wavelength tunable laser 30 is reflected to the mirror 43 by the demultiplexer / multiplexer 42. The demultiplexer / multiplexer 42 functions as a PBS (Polarization Beam Splitter) when functioning as a demultiplexer, and functions as a PBC (polarization Beam Combiner) when functioning as a multiplexer.

  The vertical light reflected by the mirror 43 enters the attenuator 35. The attenuator 35 attenuates the power of the incident vertical light and enters the splitter 36. Part of the vertical light incident on the splitter 36 passes through the splitter 36 and is received by the light receiving element 37. The light receiving element 37 transmits an electrical signal to the controller 38 in accordance with the received light power. The reflectance (transmittance) of light incident on the splitter 36 is constant. Thereby, the controller 38 can detect the power of the vertical light emitted from the attenuator 35. The controller 38 adjusts the attenuation rate of the attenuator 35 so that the power of the vertical light emitted from the attenuator 35 becomes a desired value. The vertical light reflected by the splitter 36 is reflected by the mirror 39 and enters the half-wave plate 40. The half-wave plate 40 converts vertical light into horizontal light and makes it re-enter from the rear side of the wavelength tunable laser 30 via the demultiplexer / multiplexer 42.

  According to the present embodiment, the vertical light emitted from the rear side of the wavelength tunable laser 30 is converted into horizontal light and reenters the wavelength tunable laser 30. In this way, a PROF loop may be formed on the rear side of the wavelength tunable laser 30.

  In the present embodiment, the current supply unit 41 functions as an example of a control unit that performs control for fixing the oscillation condition of the laser region (resonator). The demultiplexer / multiplexer 42 functions as an example of an extraction unit that extracts part of the laser light emitted from the wavelength tunable laser. The half-wave plate 40 functions as an example of a polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the laser region. The attenuator 35 and the controller 38 function as an example of an adjustment unit that adjusts the power of the laser light incident on the laser region so that the frequency noise of the laser light emitted from the wavelength tunable laser is reduced.

  FIG. 4 is a block diagram illustrating an overall configuration of a wavelength tunable laser device 100B according to the third embodiment. As shown in FIG. 4, the tunable laser apparatus 100B is provided with a splitter 32, a splitter 36, a mirror 39, a half-wave plate 40, and a demultiplexer / multiplexer 42, compared to the tunable laser apparatus 100A of FIG. A quarter wave plate (QWP) 44 and a mirror 45 are provided. The mirror 45 has a reflectance of several% or less (for example, 5% or less). In addition, about the structure similar to Example 2, description is abbreviate | omitted by attaching | subjecting a same sign.

  The vertical light emitted from the rear side (CSG-DBR region B side) of the wavelength tunable laser 30 passes through the quarter wavelength plate 44 and enters the attenuator 35. At that time, the quarter-wave plate 44 rotates the polarization by 45 degrees. The attenuator 35 attenuates the power of the incident light and enters the mirror 45. Part of the light incident on the mirror 45 passes through the mirror 45 and is received by the light receiving element 37. The light receiving element 37 transmits an electrical signal to the controller 38 in accordance with the received light power. The light reflected by the mirror 45 enters the attenuator 35 again. The reflectance (transmittance) of light incident on the mirror 45 is constant. Thereby, the controller 38 can detect the power of the light emitted from the attenuator 35. The controller 38 adjusts the attenuation rate of the attenuator 35 so that the power of the light emitted from the attenuator 35 becomes a desired value. The light emitted from the attenuator 35 enters the quarter wavelength plate 44. The quarter wave plate 44 further rotates the polarization by 45 degrees. Thereby, the light emitted from the quarter-wave plate 44 becomes horizontal light. The horizontal light emitted from the quarter wavelength plate 44 is incident again from the rear side of the wavelength tunable laser 30.

  According to the present embodiment, the PROF loop is formed only by sandwiching the quarter wavelength plate 44 and the attenuator 35 between the wavelength tunable laser 30 and the mirror 45. Thereby, the PROF loop length can be further shortened as compared with the first and second embodiments. Thereby, frequency noise can be reduced in a wide band. The attenuator 35 and the mirror 45 may be an integrated product such as a variable reflectance filter. Further, by detecting the power of light transmitted through the mirror 45, the power of the return light of the horizontal light can be adjusted.

  In the present embodiment, the current supply unit 41 functions as an example of a control unit that performs control for fixing the oscillation condition of the laser region (resonator). The mirror 45 functions as an example of an extraction unit that extracts part of the laser light emitted from the wavelength tunable laser. The quarter-wave plate 44 functions as an example of a polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the laser region. The attenuator 35 and the controller 38 function as an example of an adjustment unit that adjusts the power of the laser light incident on the laser region so that the frequency noise of the laser light emitted from the wavelength tunable laser is reduced.

  FIG. 5 is a block diagram illustrating an overall configuration of a wavelength tunable laser device 100C according to the fourth embodiment. As shown in FIG. 5, the wavelength tunable laser device 100C is not equipped with the wavelength tunable laser 30 and the attenuator 35, but is equipped with the wavelength tunable laser 30C, as compared with the wavelength tunable laser device 100B of FIG. The wavelength tunable laser 30C is further monolithically integrated with the SOA / VOA region D on the rear side as compared with the wavelength tunable laser 30 of FIG. The function of the SOA / VOA region D as an amplifier and the function of an attenuator can be changed by a forward bias and a reverse bias to the SOA / VOA region D. The controller 38 adjusts the amplification factor or attenuation factor of the SOA / VOA region D so that the power of the light emitted from the SOA / VOA region D becomes a desired value. In addition, about the structure similar to Example 3, description is abbreviate | omitted by attaching | subjecting a same sign.

  According to this embodiment, the attenuator is monolithically integrated in the wavelength tunable laser. Thereby, the PROF loop length is further shortened. As a result, frequency noise can be reduced in a wide band, and the wavelength tunable laser device can be easily downsized. An AR film may be formed at the rear side end of the SOA / VOA region D.

  In the present embodiment, the current supply unit 41 functions as an example of a control unit that performs control for fixing the oscillation condition of the laser region (resonator). The mirror 45 functions as an example of an extraction unit that extracts part of the laser light emitted from the wavelength tunable laser. The quarter-wave plate 44 functions as an example of a polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the laser region. The SOA / VOA area D and the controller 38 function as an example of an adjustment unit that adjusts the power of the laser light incident on the laser area so that the frequency noise of the laser light emitted from the wavelength tunable laser is reduced.

  FIG. 6 is a block diagram illustrating an overall configuration of a wavelength tunable laser device 100D according to the fifth embodiment. As shown in FIG. 6, the wavelength tunable laser device 100D is different from the wavelength tunable laser device 100 shown in FIG. The quarter wavelength plate 44 and the mirror 45 are arranged in this order between the front side end of the wavelength tunable laser 30 and the splitter 36. In addition, about the structure similar to Example 1, description is abbreviate | omitted by attaching | subjecting a same sign.

  The vertical light emitted from the front side of the wavelength tunable laser 30 enters the quarter wavelength plate 44. The quarter wave plate 44 rotates the polarization by 45 degrees. The light transmitted through the quarter wavelength plate 44 enters the mirror 45. Part of the light incident on the mirror 45 is reflected and reenters the quarter-wave plate 44. The quarter wave plate 44 further rotates the polarization by 45 degrees. Thereby, horizontal light is obtained. Horizontal light is incident again from the front side of the wavelength tunable laser 30.

  The splitter 36 reflects a part of the light transmitted through the mirror 45. The reflected light is received by the light receiving element 33. The light receiving element 33 transmits an electrical signal to the controller 34 in accordance with the received light power. The reflectivity (transmittance) of the mirror 45 and the splitter 36 is constant. Thereby, the controller 34 can detect the power of the light emitted from the wavelength tunable laser 30. The controller 34 adjusts the amplification factor of the SOA region C so that the power of the light output from the wavelength tunable laser 30 becomes a desired value.

  As in this embodiment, the PROF loop may be formed on the front side of the wavelength tunable laser 30. In the present embodiment, the power of the returning horizontal light is controlled by the amplification factor of the SOA region C monolithically integrated in the wavelength tunable laser 30.

  In the present embodiment, the current supply unit 41 functions as an example of a control unit that performs control for fixing the oscillation condition of the laser region (resonator). The mirror 45 functions as an example of an extraction unit that extracts part of the laser light emitted from the wavelength tunable laser. The quarter-wave plate 44 functions as an example of a polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the laser region. The SOA region C and the controller 34 function as an example of an adjustment unit that adjusts the power of the laser light incident on the laser region so that the frequency noise of the laser light emitted from the wavelength tunable laser is reduced.

  FIG. 7 is a block diagram illustrating an overall configuration of a wavelength tunable laser device 100E according to the sixth embodiment. As shown in FIG. 7, the wavelength tunable laser device 100E includes a wavelength tunable laser 30C instead of the wavelength tunable laser 30 as in the fourth embodiment, as compared with the wavelength tunable laser device 100D in FIG. The four-wave plates 44f and 44r and the mirrors 45f and 45r are provided. The quarter wave plates 44f and 44r have the same function as the quarter wave plate 44 of FIG. The mirrors 45f and 45r have the same function as the mirror 45 in FIG. In addition, about the structure similar to Example 5, description is abbreviate | omitted by attaching | subjecting a same sign.

  As in this embodiment, PROF loops may be formed on both the front side and the rear side of the wavelength tunable laser 30C. In the fifth to seventh embodiments, the light transmitted through the quarter wavelength plate is reflected by the reflecting mirror, but may be fed back to the wavelength tunable laser 30C using a power splitter or PBS / PBC.

  In the present embodiment, the current supply unit 41 functions as an example of a control unit that performs control for fixing the oscillation condition of the laser region (resonator). The mirrors 45f and 45r function as an example of an extraction unit that extracts part of the laser light emitted from the wavelength tunable laser. The quarter-wave plates 44f and 44r function as an example of a polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit within a range of 90 ° ± 5 ° and enters the laser region. The SOA area C, the SOA / VOA area D, and the controllers 34 and 38 adjust the power of the laser light incident on the laser area so that the frequency noise of the laser light emitted from the wavelength tunable laser is reduced. Functions as an example.

  FIG. 8 is a block diagram illustrating an overall configuration of a wavelength tunable laser device 100F according to the seventh embodiment. As shown in FIG. 8, the tunable laser apparatus 100F includes an etalon 46 between the quarter-wave plate 44 and the mirror 45, as compared with the tunable laser apparatus 100C of FIG. In addition, about the structure similar to Example 4, description is abbreviate | omitted by attaching | subjecting a same sign.

  Here, as shown in the above equation (1), the laser frequency noise reduction using the PROF loop is realized by suppressing carrier fluctuations by the return light of the horizontal light. If a loss having frequency dependence is inserted in the PROF loop, the effect can be further enhanced. In this embodiment, an etalon 46 having frequency dependency is arranged in the PROF loop. By arranging the etalon 46, it is possible to further increase / decrease the intensity of the return light of the horizontal light when the frequency increases / decreases. Thereby, the carrier density in the laser region can be more strongly reduced / increased. As a result, the feedback for suppressing the frequency fluctuation works more strongly. Further, in the vertical light or horizontal light transmitted through the etalon 46, optical power fluctuations corresponding to the frequency fluctuations occur. Therefore, the optical power fluctuation may be received by the light receiving element, the output amplitude (RMS value≈frequency fluctuation) of the light receiving element may be directly monitored, and the loop gain (loss) may be controlled so that the value is minimized. .

  The level adjustment of the return light may not be realized in the SOA / VOA region D but may be realized by controlling the state of the etalon. You may control the temperature of the etalon 46 using the temperature control apparatus comprised from a Peltier device. Alternatively, the angle of the etalon 46 may be controlled using an angle changing device that changes the angle of the etalon 46.

  In the present embodiment, the current supply unit 41 functions as an example of a control unit that performs control for fixing the oscillation condition of the laser region (resonator). The mirror 45 functions as an example of an extraction unit that extracts part of the laser light emitted from the wavelength tunable laser. The quarter-wave plate 44 functions as an example of a polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the laser region. The SOA / VOA area D and the controller 38 function as an example of an adjustment unit that adjusts the power of the laser light incident on the laser area so that the frequency noise of the laser light emitted from the wavelength tunable laser is reduced. The etalon 46 functions as an example of a loss part having frequency dependence on optical loss.

  FIG. 9 is a block diagram illustrating an overall configuration of a wavelength tunable laser device 100G according to the eighth embodiment. As shown in FIG. 9, the wavelength tunable laser device 100G is further provided with a light receiving element 47, a wavelength controller 48, and a splitter 49, compared to the wavelength tunable laser device 100F of FIG. The splitter 49 is disposed between the wavelength tunable laser 30 </ b> C and the quarter wavelength plate 44. In addition, about the structure similar to Example 7, the description is abbreviate | omitted by attaching | subjecting a same sign.

  The splitter 49 reflects part of the vertical light output from the rear side of the wavelength tunable laser 30 </ b> C and enters the light receiving element 47. The light receiving element 47 inputs an electric signal corresponding to the intensity of the received vertical light to the wavelength controller 48. An electric signal output from the light receiving element 37 is also input to the wavelength controller 48. Thereby, the wavelength controller 48 detects the deviation from the desired wavelength by comparing the power of the light transmitted through the etalon 46 and the power of the light before transmitted through the etalon 46. The wavelength controller 48 controls the oscillation wavelength of the wavelength tunable laser 30C so that the detected deviation approaches zero. As described above, by using the etalon 46 as a part of the wavelength locker, the number of components of the wavelength tunable laser device including the wavelength locker can be reduced.

  In the present embodiment, the current supply unit 41 functions as an example of a control unit that performs control for fixing the oscillation condition of the laser region (resonator). The mirror 45 functions as an example of an extraction unit that extracts part of the laser light emitted from the wavelength tunable laser. The quarter-wave plate 44 functions as an example of a polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the laser region. The SOA / VOA area D and the controller 38 function as an example of an adjustment unit that adjusts the power of the laser light incident on the laser area so that the frequency noise of the laser light emitted from the wavelength tunable laser is reduced. The etalon 46 functions as an example of a loss part having frequency dependence on optical loss. The light receiving element 47 functions as an example of a detection unit that detects the intensity of light lost by the loss unit. The wavelength controller 48 functions as an example of a wavelength adjustment unit that adjusts the output wavelength of the wavelength tunable laser using the detection result of the detection unit.

  FIG. 10 is a block diagram illustrating an overall configuration of a wavelength tunable laser device 100H according to the ninth embodiment. As illustrated in FIG. 10, the wavelength tunable laser device 100 </ b> H includes a wavelength tunable laser 30 </ b> C instead of the wavelength tunable laser 30 as compared with the wavelength tunable laser device 100 of FIG. 1. In the wavelength tunable laser 30C, as compared with the wavelength tunable laser 30 of FIG. 4, the SOA region D is further monolithically integrated on the rear side. Further, the attenuator 35, the splitter 36, the light receiving element 37, the controller 38, the mirror 39 and the half-wave plate 40 are not provided, and a demultiplexer / multiplexer 71, a mirror 72, a ring resonator 73, a silicon substrate 70 are provided. A heater 74, light receiving elements 75 and 76, controllers 77 and 78, and a polarization rotation element 79 are provided. The ring resonator 73, the heater 74, and the light receiving elements 75 and 76 are monolithically integrated Si waveguide devices, for example, SiPh (silicon photonics) chips. In addition, about the structure similar to Example 1, description is abbreviate | omitted by attaching | subjecting a same sign.

  The vertical light emitted from the rear side of the wavelength tunable laser 30 </ b> C is reflected by the demultiplexer / multiplexer 71, reflected by the mirror 72, and incident on the ring resonator 73. The demultiplexer / multiplexer 71 functions as a PBS (Polarization Beam Splitter) when functioning as a demultiplexer, and functions as a PBC (polarization Beam Combiner) when functioning as a multiplexer. The ring resonator 73 is a resonator including one or more ring waveguides. Part of the vertical light incident on the ring resonator 73 is received by the light receiving element 75. The light receiving element 75 transmits an electrical signal to the controller 77 according to the received light power. If the branching ratio is constant, the controller 77 can detect the power of vertical light incident on the ring resonator 73. The controller 77 adjusts the amplification factor of the SOA region D so that the power of the vertical light incident on the ring resonator 73 becomes a desired value. That is, the controller 77 performs APC control.

  The vertical light emitted from the ring resonator 73 is polarized and rotated by the polarization rotation element 79 to be converted into horizontal light, and is incident from the rear side of the wavelength tunable laser 30C via the demultiplexer / multiplexer 71. A part of the vertical light emitted from the ring resonator 73 is received by the light receiving element 76. The light receiving element 76 transmits an electrical signal to the controller 78 in accordance with the received light power. If the branching rate is constant, the controller 78 can detect the power of vertical light emitted from the ring resonator 73. The controller 78 adjusts the resonance frequency of the ring resonator 73 by adjusting the temperature of the heater 74 so that the power of the vertical light emitted from the ring resonator 73 becomes a desired value. That is, the controller 78 performs APC control.

FIG. 11 is a diagram illustrating a transmission characteristic example of the ring resonator 73 of the Si waveguide device. The ring resonator 73 is by the controller 78 to have the proper wavelength dependence on transmission characteristics in the vicinity of the laser output center wavelength f ₀ (transmission rate = 0.5 in the example of FIG. 11) the temperature of the heater 74 is adjusted Yes. Since the return light of the horizontal light does not interfere with the vertical light of the laser oscillation of the wavelength tunable laser 30C, it does not affect the rate equation regarding the photon density and the optical phase. Therefore, phase noise is reduced and stable laser oscillation can be obtained as compared with the case where vertical light is used as return light.

Thus, in the present embodiment, a PROF loop is formed in which the vertical light emitted from the rear side of the wavelength tunable laser 30C is converted into horizontal light by the Si waveguide device and returned to the wavelength tunable laser 30C. . The noise of the laser output of the wavelength tunable laser 30C is reduced by the return light of the horizontal light. Further, since the ring resonator 73 is provided in the PROF loop, the loop gain has wavelength dependency. In the example of FIG. 11, when the laser oscillation frequency is f ₀ + Δf, the transmittance increases and the loop gain also increases, and when f ₀ −Δf, the transmittance decreases and the loop gain also decreases. If a slope on the opposite side of the filter characteristic is used, the direction of frequency fluctuation and the direction of increase / decrease in loop gain are reversed. By providing the wavelength dependency in a direction that cancels the wavelength variation accompanying the carrier variation in the resonator, it is possible to further enhance the noise reduction effect as compared with the case where there is no wavelength dependency.

  In the present embodiment, the input level to the ring resonator 73 and the output level from the ring resonator 73 are controlled to be constant by APC control by the controller 77 and APC control by the controller 78. Thereby, the loop gain is stabilized.

  The polarization rotator 79 is composed of a λ / 4 wavelength plate, a 45 ° polarizer, and a λ / 4 wavelength plate, so that the horizontal light incident on the rear end face of the wavelength tunable laser 30C is rear side. Even if the light is reflected by the end face, the reflected light can be prevented from entering the Si waveguide device again.

  If the reflection of horizontal light at the rear end face is not a problem, as shown in FIG. 12, the demultiplexer / multiplexer 71 and the polarization rotation element 79 may be monolithically integrated in the Si waveguide device. Good. In this configuration, the input / output ports of the Si waveguide device can be shared, and further miniaturization is possible as compared with the wavelength tunable laser device 100 of FIG.

  In the present embodiment, the current supply unit 41 functions as an example of a control unit that performs control for fixing the oscillation condition of the laser region (resonator). The demultiplexer / multiplexer 71 functions as an example of an extraction unit that extracts part of the laser light emitted from the wavelength tunable laser. The polarization rotation element 79 functions as an example of a polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the laser region. The heater 74 and the controllers 77 and 78 function as an example of an adjustment unit that adjusts the power of the laser light incident on the laser region so that the frequency noise of the laser light emitted from the wavelength tunable laser is reduced.

  FIG. 13 is a block diagram illustrating an overall configuration of a wavelength tunable laser device 100J according to the tenth embodiment. As illustrated in FIG. 13, the wavelength tunable laser device 100J does not include the heater 74 as compared with the wavelength tunable laser device 100H in FIG. 12, and the temperature control devices 80 and 81, the temperature sensor 82, and the controller 83 are included. It is equipped. The temperature control device 80 is equipped with a wavelength tunable laser 30C and controls the temperature of the wavelength tunable laser 30C. The temperature control device 81 is mounted with a Si waveguide device and controls the temperature of the ring resonator 73. The temperature sensor 82 detects the temperature of the ring resonator 73. The temperature control devices 80 and 81 are composed of Peltier elements or the like.

  The controller 83 controls the temperature of the ring resonator 73 to a desired value by controlling the temperature control device 81 according to the detection result of the temperature sensor 82. The controller 78 controls the temperature control device 80 in accordance with the light receiving power detected by the light receiving element 76, thereby controlling the temperature of the wavelength tunable laser 30C to a desired value. Thereby, the output power of the ring resonator 73 is controlled to a desired value.

In the present embodiment, negative feedback control is performed so that the temperature of the ring resonator 73 becomes a predetermined temperature. Thereby, the wavelength dependence of the ring resonator 73 is ensured. Further, the temperature of the wavelength tunable laser 30C is subjected to negative feedback control so that the output power of the ring resonator 73 is constant. Thereby, Si waveguide device in addition to the function of improving the laser linewidth characteristics of the tunable laser 30C, may be responsible for a wavelength locker stabilize the laser output center wavelength f _0.

  FIG. 14 is a plan view of the wavelength tunable laser device 100K according to the eleventh embodiment. As shown in FIG. 14, the wavelength tunable laser device 100 </ b> K is a modification of the wavelength tunable laser device 100 </ b> A in FIG. 3 and the wavelength tunable laser device 100 </ b> F in FIG. 8, and further includes an isolator 50. The isolator 50 transmits only the vertical light emitted from the end film 17 on the rear side to the etalon 46 and substantially blocks the light emitted from the etalon 46 to the demultiplexer / multiplexer 42. . The vertical light transmitted through the isolator 50 and the etalon 46 is rotated by 90 ° in the half-wave plate 40. That is, the half-wave plate 40 converts vertical light into horizontal light. Accordingly, the horizontal light from the half-wave plate 40 is reflected by the demultiplexer / multiplexer 42 and is incident from the rear end face film 17 of the wavelength tunable laser 30C. Part of the vertical light transmitted through the etalon 46 is received by the light receiving element 37. The controller 38 feedback-controls the amplification factor or attenuation factor of the SOA region of the wavelength tunable laser 30 </ b> C according to the output of the light receiving element 37. Thereby, the frequency noise of the laser beam emitted from the end face film 16 can be reduced.

  1 substrate, 2 lower clad layer, 3 active layer, 4 optical waveguide layer, 6 upper clad layer, 7 contact layer, 8 electrode, 9 insulating film, 10 heater, 11 power supply electrode, 12 ground electrode, 15 back electrode, 16 edge Face film, 17 End face film, 18 Diffraction grating, 19 Optical amplification layer, 20 Contact layer, 21 Electrode, 30 Wavelength variable laser, 31 Splitter, 32 Splitter, 33 Light receiving element, 34 Controller, 35 Attenuator, 36 Splitter, 37 Light receiving element , 38 controller, 39 mirror, 40 half-wave plate, 41 current supply unit, 42 demultiplexer / multiplexer, 43 mirror, 44, 44f, 44r 1/4 wavelength plate, 45, 45f, 45r mirror, 46 etalon, 47 Light receiving element, 48 wavelength controller, 49 splitter, 50 isolator, 71 demultiplexer / multiplexer Second mirror 73 ring resonator 74 heater, 75 and 76 light-receiving element, 77, 78 controller, 79 polarization rotation element, 80 and 81 temperature controller 82 temperature sensor, 83 a controller, 100 tunable laser

Claims (13)

A tunable laser;
A control unit that performs control to fix the oscillation condition of the wavelength tunable laser; and
An extraction unit for extracting a part of laser light emitted from the wavelength tunable laser;
A polarization rotation unit that rotates the polarization of the laser light extracted by the extraction unit in a range of 90 ° ± 5 ° and enters the wavelength tunable laser;
A wavelength tunable laser device comprising: an adjustment unit that adjusts the power of the laser light incident on the wavelength tunable laser so that frequency noise of the laser light emitted from the wavelength tunable laser is reduced. The extraction unit is a mirror that reflects a part of output light emitted from the wavelength tunable laser,
The wavelength tunable laser device according to claim 1, wherein the polarization rotation unit is a ¼ wavelength plate disposed between the wavelength tunable laser and the mirror.   The wavelength tunable laser device according to claim 1, wherein the adjustment unit is an optical amplifier or an attenuator monolithically integrated in the wavelength tunable laser. The wavelength tunable laser is a horizontal resonator,
The wavelength tunable laser device according to claim 1, wherein both emission end faces of the wavelength tunable laser are covered with an AR film.   5. The wavelength tunable laser device according to claim 1, further comprising a loss unit having a frequency dependence on optical loss in an optical path from the extraction unit to the wavelength tunable laser. A detection unit for detecting the intensity of light lost by the loss unit;
The wavelength tunable laser device according to claim 5, wherein the adjustment unit adjusts the power according to a detection result of the detection unit.   The wavelength tunable laser device according to claim 6, further comprising a wavelength adjustment unit that adjusts an output wavelength of the wavelength tunable laser using a detection result of the detection unit.   The wavelength tunable laser device according to claim 5, wherein the loss part is an etalon.   The wavelength tunable laser device according to claim 5, further comprising a loss adjustment unit that adjusts an optical loss of the loss unit.   The wavelength tunable laser device according to claim 5, wherein the loss unit is a ring resonator.   The wavelength tunable laser device according to claim 10, further comprising a temperature control unit that controls a temperature of the ring resonator.   The wavelength tunable laser device according to claim 1, wherein the polarization rotation unit has a configuration in which a 45 ° polarizer is sandwiched between two λ / 4 wavelength plates. Control to fix the oscillation condition of the tunable laser,
Extract a part of the laser light emitted from the wavelength tunable laser,
The extracted polarization of the laser beam is rotated within a range of 90 ° ± 5 ° and incident on the wavelength tunable laser,
A method of controlling a wavelength tunable laser, wherein the power of the laser beam incident on the wavelength tunable laser is adjusted so that frequency noise of the laser beam emitted from the wavelength tunable laser is reduced.

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