JP6652274B2 - Wavelength switching method of tunable laser - Google Patents

Description

  The present invention relates to a wavelength switching method for a tunable laser.

  A tunable laser capable of selecting an output wavelength is disclosed (for example, see Patent Document 1). In the technique of Patent Document 1, a control condition for obtaining a grid wavelength determined by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is stored in a memory, and any of the grid wavelengths is stored based on the stored control condition. The control for oscillating at the wavelength of is performed.

JP 2009-026996 A

  In order to improve the use efficiency of the band used for optical transmission, control for switching the output wavelength of the wavelength tunable laser is required. During the period in which the wavelength tunable laser switches wavelengths, light output from the wavelength tunable laser to the outside is suppressed. Reducing the period from when the wavelength tunable laser starts switching the wavelength to when the light is output to the outside again contributes to the convenience for the user.

  Therefore, it is an object of the present invention to provide a wavelength switching method for a wavelength tunable laser, which can shorten the time from when the wavelength switching of the wavelength tunable laser is started to when the optical output is performed outside.

  The wavelength switching method of the wavelength tunable laser according to the present invention includes a temperature control device for controlling the temperature of the etalon, and the output of the wavelength tunable laser is determined by the ratio of the light intensity input to the etalon and the light intensity output from the etalon. A method for controlling a wavelength of the wavelength tunable laser to a target wavelength based on a detection result of the wavelength detection unit, the method including driving the wavelength tunable laser at a first wavelength. After the first step, after the first step, in response to a command to output a second wavelength different from the first wavelength, the second step of suppressing the optical output of the tunable laser, and after the second step A third step of starting control of the temperature control device toward a second etalon temperature corresponding to a second wavelength different from the first etalon temperature corresponding to the first wavelength; Before the etalon reaches the second etalon temperature, the etalon detects that the output wavelength of the tunable laser has reached a temperature range corresponding to an allowable range corresponding to the second wavelength, and A fourth step of releasing the suppression of the optical output of the tunable laser.

  According to the above invention, the optical output can be provided to the outside in a short time after the wavelength switching of the wavelength tunable laser is started.

FIG. 2 is a block diagram illustrating an entire configuration of the wavelength tunable laser according to the first embodiment. FIG. 1 is a schematic cross-sectional view illustrating the entire configuration of a semiconductor laser. It is a figure showing an initial set value and a feedback control target value. It is a table showing a temperature correction coefficient. FIG. 4 is a diagram illustrating a relationship between a required wavelength and a fundamental wavelength in gridless control. It is a figure showing the principle of gridless control. FIG. 6 is a diagram illustrating a relationship between a temperature of a second temperature control device and an oscillation wavelength. 5 is a flowchart illustrating steps from when a user inputs a required wavelength to when driving of a semiconductor laser is started. 9 is a flowchart illustrating a driving operation of the semiconductor laser after the flow in FIG. 8. 5 is a flowchart illustrating an example of a wavelength switching operation.

[Description of Embodiment of the Present Invention]
First, the contents of the embodiment of the present invention will be listed and described.
(1) A wavelength switching method for a tunable laser includes a temperature control device for controlling a temperature of an etalon, and an output wavelength of the tunable laser is determined by a ratio of a light intensity input to the etalon and a light intensity output from the etalon. A method of controlling a wavelength of the tunable laser to a target wavelength based on a detection result of the wavelength detecting unit, wherein a first wavelength driving the tunable laser at a first wavelength is provided. Step, after the first step, in response to a command to output a second wavelength different from the first wavelength, a second step of suppressing the optical output of the tunable laser, and after the second step, A third step of starting control of the temperature control device toward a second etalon temperature corresponding to a second wavelength different from the first etalon temperature corresponding to the first wavelength; Before reaching the second etalon temperature, the etalon detects that the output wavelength of the tunable laser has reached a temperature range corresponding to an allowable range corresponding to the second wavelength, and A fourth step of releasing the suppression of the optical output.
(2) The temperature control device may include a fifth step of detecting that the temperature has reached the second etalon temperature.
(3) In the fifth step, when the temperature of the etalon reaches a range including the second etalon temperature and smaller than the temperature range, it may be detected that the etalon has reached the second etalon temperature.
(4) In the fourth step, after confirming that the temperature of the etalon falls within the temperature range for a predetermined period, the suppression of the optical output of the tunable laser may be released.

[Details of the embodiment of the present invention]
A specific example of the wavelength switching method of the wavelength tunable laser according to the embodiment of the present invention will be described below with reference to the drawings. It should be noted that the present invention is not limited to these exemplifications, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

  FIG. 1 is a block diagram illustrating the entire configuration of the wavelength tunable laser 100 according to the first embodiment. As shown in FIG. 1, the wavelength tunable laser 100 includes, as a laser device, a semiconductor laser 30 (tunable semiconductor laser) whose wavelength can be controlled. The semiconductor laser 30 of the present embodiment is provided with a region that becomes an SOA (Semiconductor Optical Amplifier) connected to the laser region. This SOA functions as a light output control unit. SOA can arbitrarily increase or decrease the intensity of light output. Further, the intensity of the light output can be controlled to substantially zero. Further, the wavelength tunable laser 100 includes a detection unit 50, a memory 60, a controller 70, and the like. The detection unit 50 functions as an output detection unit and a wavelength locker unit. The controller 70 controls the wavelength tunable laser 100, and includes a RAM (Random Access Memory) therein.

  FIG. 2 is a schematic cross-sectional view illustrating the entire configuration of the semiconductor laser 30 in the present embodiment. As shown in FIG. 2, the semiconductor laser 30 includes a sampled distributed feedback (SG-DFB) region A; Prepare. That is, the semiconductor laser 30 is a laser having a wavelength selection mirror in the semiconductor structure.

  As an example, in the semiconductor 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. The SG-DFB region A and the CSG-DBR region B correspond to the laser region in FIG. 1, and the SOA region C corresponds to the SOA region in FIG.

  The SG-DFB region A has a structure in which a lower clad layer 2, an active layer 3, an upper clad 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, the CSG-DBR region B, and the 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 plane. 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 cladding layer 2, the light amplification layer 19, and the upper cladding layer 6 on the SOA region C side. In the present embodiment, the end face film 16 is an AR (Anti Reflection) film. The end face film 16 functions as a front end face of the semiconductor laser 30. An end face film 17 is formed on end faces of the substrate 1, the lower cladding layer 2, the optical waveguide layer 4, and the upper cladding 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 end face of the semiconductor laser 30.

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

The active layer 3 is made of a semiconductor having a gain. The active layer 3 has, for example, a quantum well structure. For example, a well layer made of, for example, Ga _0.32 In _0.68 As _0.92 P _0.08 (thickness: 5 nm) and a 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 light amplification layer 19 is a region where a gain is given by current injection from the electrode 21 and light amplification is thereby performed. Optical amplification layer 19, for example, can be composed of a quantum well structure, for example, _Ga 0.35 _In 0.65 _{As 0.99} P _0.01 well layers and _Ga 0.15 _{an In 0} (thickness: 5 _nm). 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 light amplification layer 19 and the active layer 3 may be made of the same material.

The contact layers 7 and 20 can be made 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 over 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 lower surface electrode 15 is formed below the substrate 1. The back surface electrode 15 is formed over the SG-DFB region A, the CSG-DBR region B, and the SOA region C.

The end face films 16 and 17 are AR films having a reflectance of 1.0% or less, and have a characteristic that the 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. However, the end face film 17 may be formed of a reflective film having a significant reflectance. In the case where a structure provided with a light absorbing layer in the semiconductor in contact with the end face film 17 in FIG. 2 is provided, the light output leaking from the end face film 17 to the outside can be suppressed by giving the end face film 17 a significant reflectance. Can be. A significant reflectance is, for example, a reflectance of 10% or more. Here, the reflectance indicates the reflectance with respect to the inside of the semiconductor laser.

The diffraction gratings (corrugations) 18 are formed at a plurality of locations in the lower clad layer 2 of the SG-DFB region A and the CSG-DBR region B at predetermined intervals. Thereby, a sampled grating is 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, a plurality of segments are provided in the lower cladding layer 2. Here, the segment refers to a region in which a diffraction grating portion provided with the diffraction grating 18 and a space portion not provided with the diffraction grating 18 are continuous one by one. In other words, a segment refers to a region where a space portion sandwiched between both ends by a diffraction grating portion and the diffraction grating portion are connected. The diffraction grating 18 is made of a material having a different refractive index 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 located between the diffraction gratings 18 can be realized by exposing the pattern of the diffraction grating 18 to a resist and then exposing the resist to a position corresponding to the space 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 the present embodiment, as an example, both pitches are set to be the same. In each segment, the diffraction grating 18 may have the same length or may have different lengths. 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 And 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, at least two segments have optical lengths different from each other. As a result, the intensity of the peaks of the wavelength characteristics of the CSG-DBR region B has wavelength dependence. The average optical length of the segment in the SG-DFB region A is different from the average optical length of the segment in the CSG-DBR region B. Thus, the segment in the SG-DFB region A and the segment in the CSG-DBR region B constitute a resonator in the semiconductor 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, and when carriers are injected, a discrete gain spectrum having a predetermined wavelength interval with almost uniform peak intensity is generated. In the CSG-DBR region B, discrete reflection spectra having different peak intensities and having a predetermined wavelength interval are generated. The peak wavelength intervals of the wavelength characteristics in the SG-DFB region A and the CSG-DBR region B are different. The wavelength satisfying the oscillation condition can be selected by using the Vernier effect generated by the combination of these wavelength characteristics.

  As shown in FIG. 1, the semiconductor laser 30 is disposed on a first temperature control device 31. The first temperature control device 31 includes a Peltier element and functions as a TEC (Thermoelectric cooler). The first thermistor 32 is disposed on the first temperature control device 31. The first thermistor 32 detects the temperature of the first temperature control device 31. The temperature of the semiconductor laser 30 can be specified based on the temperature detected by the first thermistor 32.

  In the wavelength tunable laser 100, the detection unit 50 is disposed on the front side of the semiconductor laser 30. Since the detection unit 50 functions as a wavelength locker, the wavelength tunable laser 100 can be called a front rocker type. The detection unit 50 includes a first light receiving element 42, a beam splitter 51, an etalon 52, a second temperature control device 53, a second light receiving element 54, and a second thermistor 55.

  The beam splitter 41 is arranged at a position where the output light from the front side of the semiconductor laser 30 is branched. The beam splitter 51 is arranged at a position where the light from the beam splitter 41 is branched. The first light receiving element 42 is disposed at a position where one of the two lights split by the beam splitter 51 is received. The etalon 52 is arranged at a position where the other of the two lights split by the beam splitter 51 is transmitted. The second light receiving element 54 is arranged at a position for receiving the transmitted light transmitted through the etalon 52.

  The etalon 52 has a characteristic that the transmittance changes periodically according to the wavelength of the incident light. In the present embodiment, a solid etalon is used as the etalon 52. The periodic wavelength characteristic of the solid etalon changes as the temperature changes. The etalon 52 is arranged at a position where the other of the two lights split by the beam splitter 51 is transmitted. The etalon 52 is disposed on the second temperature control device 53. The second temperature control device 53 includes a Peltier element and functions as a TEC (Thermoelectric cooler).

  The second light receiving element 54 is arranged at a position for receiving the transmitted light transmitted through the etalon 52. The second thermistor 55 is provided to specify the temperature of the etalon 52. The second thermistor 55 is disposed, for example, on the second temperature control device 53. In the present embodiment, the temperature of the etalon 52 is specified by detecting the temperature of the second temperature control device 53 with the second thermistor 55.

  The memory 60 is a rewritable storage device. A typical example of the rewritable storage device is a flash memory. The controller 70 includes a central processing unit (CPU: Central Processing Unit), a RAM (Random Access Memory), a power supply, and the like. The RAM is a memory that temporarily stores a program executed by the central processing unit, data processed by the central processing unit, and the like.

  The memory 60 stores an initial set value and a feedback control target value of each unit of the tunable laser 100 in association with a channel. The channel is a number corresponding to the oscillation wavelength of the semiconductor laser 30. The wavelength of each channel is discretely determined within the wavelength tunable band of the wavelength tunable laser 100. For example, each channel corresponds to a grid wavelength (interval of 50 GHz) of ITU-T (International Telecommunication Union Telecommunication Standardization Sector). Alternatively, the initial setting values may be prepared at intervals smaller than the intervals of the ITU-T grid. In this embodiment, the wavelength of each channel is defined as the fundamental wavelength.

FIG. 3 is a diagram showing the initial setting value and the feedback control target value. As shown in FIG. 3, the initial set values are the initial current value I _LD supplied to the electrode 8 in the SG-DFB region A, the initial current value I _SOA supplied to the electrode 21 in the SOA region C, and the semiconductor laser 30. including the initial temperature value _{T LD,} the initial power value _{P Heater1} ~P _Heater3 supplied to the initial temperature value _{T etalon,} and the heaters 10 of the etalon 52. These initial setting values are determined for each channel. The feedback control target value is a target value when the controller 70 performs the feedback control. The feedback control target value is a target value _{Im1 of} the photocurrent output from the first light receiving element 42 and a ratio of the photocurrent _Im2 output from the second light receiving element 54 to the photocurrent _Im1 output from the first light receiving element 42. Target value I _m2 / I _m1 . The control target value is also determined for each channel. Note that these values are obtained for each individual by tuning using a wavelength meter before shipment of the wavelength tunable laser 100. Further, as shown in FIG. 4, the memory 60 stores a temperature correction coefficient C1. The details of the temperature correction coefficient C1 will be described later. In the present embodiment, the temperature correction coefficient C1 is a value common to each channel.

  The tunable laser 100 according to the present embodiment can output a required wavelength that does not match the fundamental wavelength. Control that enables output at a required wavelength different from the fundamental wavelength is hereinafter referred to as gridless control. FIG. 5 is a diagram illustrating a relationship between a required wavelength and a fundamental wavelength in gridless control. As shown in FIG. 5, in the gridless control, the required wavelength is a wavelength between the fundamental wavelength and another neighboring fundamental wavelength. Note that the required wavelength may be equal to the fundamental wavelength.

FIG. 6 is a diagram illustrating the principle of gridless control. In FIG. 6, the horizontal axis indicates the wavelength, and the vertical axis indicates the normalized value of the ratio _Im2 / _Im1 (the transmittance of the etalon 52). 6, the solid line is a wavelength characteristic corresponding to the initial temperature value _{T Etalon} of the etalon 52. The dotted line shows the wavelength characteristic when the temperature of the etalon 52 is increased by the second temperature control device 53. Here, when the ratio I _m2 / I _m1 of the solid circle on the solid line is adopted as the feedback control target value, if the etalon 52 has the initial temperature value T _Etalon , oscillation will _occur at the fundamental wavelength. On the other hand, when the etalon 52 has a temperature corresponding to the wavelength characteristic indicated by the dotted line, the actual oscillation wavelength is equal to the etalon even if the ratio _Im2 / _Im1 is a value for obtaining the fundamental wavelength (black circle on the dotted line). The shift from the fundamental wavelength is caused by the change in the characteristic. That is, by shifting the etalon characteristics by the wavelength difference between the required wavelength and the fundamental wavelength, the required wavelength can be realized without _{changing the} ratio _Im2 / _Im1, which is the feedback control target value. That is, the required wavelength can be realized by performing an operation for changing the etalon temperature based on the wavelength difference ΔF between the required wavelength and the fundamental wavelength, and applying this as the etalon temperature.

  As described above, the wavelength characteristics of the etalon 52 shift according to its temperature. The frequency variation / temperature variation [GHz / ° C.] in the etalon 52 is referred to as a temperature correction coefficient C1 of the etalon 52. Here, the wavelength is represented by a frequency. The temperature correction coefficient C1 corresponds to a rate of change of the drive condition of the wavelength variable laser with respect to a wavelength change.

The set temperature of the etalon 52 for controlling the required wavelength is Tetln_A [° C.]. The initial temperature of the etalon 52, that is, the temperature of the etalon 52 corresponding to the selected fundamental wavelength is Tetln_B [° C.]. Tetln_B corresponds to T _Etalon and is acquired from the memory 60. Further, a wavelength difference (absolute value) between the fundamental wavelength and the required wavelength is defined as ΔF [GHz]. In this case, the relationship between the parameters can be expressed as in the following equation (1). The set temperature Tetln_A required to obtain the required wavelength can be obtained based on the equation (1).
Tetln_A = Tetln_B + ΔF / C1 (1)

By controlling the temperature of the second temperature control device 53 to the set temperature Tetln_A, the ratio _I m2 _{/ I m1} as it is utilized, it is possible to obtain a required wavelength. By performing the above operation, as shown in FIG. 6, the semiconductor laser 30 can oscillate at a wavelength (required wavelength) shifted from the fundamental wavelength by an amount corresponding to the shift of the characteristics of the etalon 52.

  Next, the wavelength switching operation of the wavelength tunable laser 100 will be described. When an instruction to perform wavelength switching is input to the controller 70, the controller 70 performs control for suppressing optical output to the outside. In the example of FIG. 1, by applying a reverse bias to the SOA and making the SOA function as a light absorber, light output to the outside is suppressed. After the light output to the outside is suppressed, the controller 70 calculates the driving condition corresponding to the oscillation wavelength to be output next. This calculation can employ the same procedure as the procedure described above.

  After a new driving condition is obtained, the controller 70 starts controlling each part of the wavelength tunable laser 100. At this time, the control requiring the longest time is the temperature control of the second temperature control device 53. If gridless control is not performed, the temperature of the second temperature control device 53 can be the same at each grid wavelength if the etalon's FSR (Free Spectrum Range) is determined at grid wavelength intervals. However, when performing gridless control, it is necessary to change the temperature of the second temperature control device 53. In the temperature control, since the temperature of a component having a predetermined heat capacity is controlled, the time required for the control is longer than the control of the current and the voltage of the semiconductor laser 30. For this reason, when performing gridless control, the time for wavelength switching becomes longer than when gridless control is not performed.

  FIG. 7 is a diagram illustrating the relationship between the temperature of the second temperature control device 53 and the oscillation wavelength. FIG. 7 shows the oscillation wavelength obtained when the temperature of the second temperature control device 53 is changed in a state where the driving condition corresponding to the oscillation wavelength to be output next is given. In FIG. 7, Ta is a temperature set in the second temperature control device 53 to realize the wavelength λ1 before the wavelength switching. On the other hand, Tb is the temperature of the second temperature control device 53 necessary to realize a new oscillation wavelength λ2 after the wavelength switching.

  In FIG. 7, a characteristic A indicated by a dotted line is a driving condition for oscillating at a wavelength λ1, and a characteristic B indicated by a solid line is a driving condition for oscillating at a wavelength λ2. When the current or voltage of the semiconductor laser 30 is changed by the wavelength switching command, the characteristic of the semiconductor laser 30 changes from the characteristic A to the characteristic B. At this point, it can be said that the inside of the resonator of the semiconductor laser 30 is set to a state corresponding to the wavelength indicated by the wavelength λ2 ′.

  Next, as the temperature of the second temperature controller 53 changes from Ta to Tb, the state inside the resonator of the semiconductor laser 30 changes according to the characteristic B. Wa in FIG. 7 indicates an allowable wavelength range allowed when the user requests the oscillation wavelength λ2. As shown in FIG. 7, when the temperature of the second temperature control device 53 is in the range of Tb1 to Tb2, it can be understood that the oscillation wavelength can be obtained in the range of Wa. That is, if the temperature of the second temperature control device 53 reaches Tb1 during the temperature change from the temperature Ta before the wavelength switching to the temperature Tb after the wavelength switching, the suppression of the optical output of the wavelength tunable laser 100 is released. be able to.

  For example, if the time required for the temperature change from Tb1 to Tb is 1 second, if the output of the SOA is increased (shutter ON) when the temperature Tb1 is reached, the SOA is switched from the SOA one second before the temperature Tb is reached. Light output can be performed. For this reason, the wavelength control by the detection unit 50 and the controller 70 can also be started one second earlier, so that the wavelength switching time can be reduced. The period permitted by the user for wavelength switching is several seconds, and shortening the period contributes to user convenience.

  In FIG. 7, the characteristic A or B can be approximated by a straight line, and its inclination is almost constant. Therefore, if the range of the allowable wavelength allowed by the user is known, the temperature at which the suppression of the optical output can be released from the target temperature of the second temperature control device 53 can be obtained by calculation. Since the temperature at which the suppression of the optical output can be released can be indicated by a temperature difference from the target temperature, if this temperature difference is stored in the memory 60, the start time of the optical output can be reduced at any target wavelength. be able to.

Next, a control flow of the wavelength tunable laser according to the present embodiment will be described. FIG. 8 is a flowchart illustrating steps from when a user inputs a required wavelength to when driving of the semiconductor laser 30 is started. As shown in FIG. 8, the controller 70 receives an input of a required wavelength (step S1). As the required wavelength, there are a case where a real value of a wavelength or a frequency is input, and a case where indirect information including at least two or more types of parameters is input. For example, a reference frequency (start grid wavelength), a channel number, and a grid interval may be input. For example, the required wavelength can be calculated from the reference frequency FCF (First Channel Frequency), the grid interval Grid, and the channel number CH. Specifically, the frequency F is obtained according to the following equation (2), and the required wavelength can be calculated from the frequency F. If the input information is indirect information, the controller 70 calculates the required wavelength according to the following equation (2).
F = FCF + (CH-1) × Grid (2)

  Next, the controller 70 selects a fundamental wavelength based on the required wavelength (Step S2). Next, the controller 70 calculates a difference between the fundamental wavelength and the required wavelength as a wavelength difference ΔF (step S3). Next, the controller 70 reads the initial setting value and the feedback control target value related to the fundamental wavelength selected in step S2 from the memory 60, and calculates the set temperature Tetln_A of the etalon 72 as an updated set value using the wavelength difference ΔF. (Step S4). Next, the controller 70 calculates an update set value of the semiconductor laser 30 using the wavelength difference ΔF (Step S5). The update setting value of the semiconductor laser 30 is a setting value of the semiconductor laser 30 calculated from the initial setting value in FIG. 3 and the wavelength difference ΔF. Next, the controller 70 writes the update setting value of the etalon 72 and the update setting value of the semiconductor laser 30 into its own RAM (step S6). Next, the controller 70 drives the semiconductor laser 30 using the updated set value written in the RAM as a driving condition (step S7).

FIG. 9 is a flowchart illustrating the driving operation of the semiconductor laser 30 after the flow of FIG. As shown in FIG. 9, the controller 70 determines whether or not the detected temperature TH1 of the first thermistor 32 is within the range of _{T LD} (step S21). The range where T _LD, a predetermined range around the temperature value T _LD of the driving conditions. If it is determined as "No" in step S21, the controller 70 changes the current value detected temperature TH1 of the first thermistor 32 can be supplied to the first temperature control device 31 so as to approach the temperature value T _LD.

  The controller 70 determines whether or not the detected temperature TH2 of the second thermistor 55 is within a set range in parallel with step S21 (step S22). The set range in this case is determined based on the set temperature of the etalon 72 included in the driving conditions. For example, the set range can be a predetermined range centered on the set temperature of the etalon 72. If “No” is determined in step S22, the controller 70 changes the current value supplied to the second temperature control device 53 so that the detected temperature TH2 of the second thermistor 55 approaches the set temperature of the etalon 72.

The controller 70 waits until “Yes” is determined in both step S21 and step S22. If “Yes” is determined in both step S21 and step S22, the controller 70 performs a shutter open operation (step S23). Specifically, the current supplied to the electrode 21 in the SOA region C is controlled to the initial current value _ISOA . As a result, laser light having a wavelength based on the driving conditions is output from the semiconductor laser 30.

Next, the controller 70 ends the temperature control by the first temperature control device 31 with the temperature value _TLD as a control target (step S24). Next, the controller 70 starts AFC control by the first temperature control device 31 (step S25). That is, the feedback control is performed so that the temperature of the first temperature control device 31 satisfies the ratio of the feedback control target value _Im2 / _Im1 . The ratio between the input light and the output light of the etalon 52 (the front-to-back ratio) indicates the oscillation wavelength of the semiconductor laser 30. The first temperature controller 31 is a parameter for controlling the wavelength of the semiconductor laser 30. That is, in step S25, the wavelength of the semiconductor laser 30 is controlled by feedback-controlling the temperature of the first temperature control device 31 so that the front-to-back ratio becomes _Im2 / _Iml . Thereby, the required wavelength is realized. When the controller 70 confirms that the ratio _Im2 / _Im1 is within a predetermined range centered on the target value _Im2 / _Im1 at the fundamental wavelength selected in step S2, it outputs an AFC lock flag (step S26). ). Then, the execution of the flowchart ends.

  Next, the operation of wavelength switching will be described. By the operation shown in FIG. 9, when the AFC lock flag is output and the wavelength is output at the requested wavelength and the user issues a wavelength switching instruction, the controller 70 performs the wavelength switching operation shown in FIG. Execute. FIG. 10 is a flowchart illustrating an example of the wavelength switching operation in the present embodiment.

  As shown in FIG. 10, when the user instructs execution of wavelength switching (step 51), an operation of closing a shutter (SOA) is performed to suppress light output (step 52). In order to suppress the light output, a method using SOA as a light absorber is typical. In addition, a method of driving a mechanical shutter using a micromachine or a shutter using liquid crystal is also conceivable. Next, a drive condition corresponding to the target wavelength to be switched input by the user is calculated (step S53). In this calculation, the same operations as those in steps S1 to S6 shown in FIG. 8 are performed.

  Next, the controller 70 drives the tunable laser 100 according to the driving conditions written in the RAM (step S54). After the driving is started, the voltage and current conditions for each part of the tunable laser 100 quickly reach the designated conditions. On the other hand, as for the temperatures of the first temperature control device 31 and the second temperature control device 53, it is necessary to perform the feedback control until the temperatures reach the specified values (steps S55 and S56).

  In the present embodiment, the temperature detection value TH2 of the second temperature control device 53 is managed in two stages. First, in step S56, it is detected that the temperature has reached the range of temperatures Tb1 to Tb2 corresponding to the allowable range Wa described in FIG. Here, the range of the temperatures Tb1 to Tb2 is set as a first set value. The first temperature control device 31 controls the temperature of the semiconductor laser 30. However, since this temperature determines the basic temperature of the semiconductor laser 30, no major change is made. On the other hand, since the temperature of the second temperature control device 53 needs to change the wavelength characteristic of the etalon, the time required for convergence may be long. Therefore, step S56 typically converges after step 55.

  When detecting that the steps 55 and 56 have converged, the controller 70 opens the shutter to release the suppression of the light output (step S57). As a result, light is output to the outside at a wavelength within the allowable wavelength range Wa in FIG. During this time, the second temperature control device continues the temperature control from the range of the first set value toward the temperature Tb of FIG. 7 which is the second set value.

  When the suppression of the light output is released in step S57, the temperature control of the first temperature control device 31 is not the control for the target temperature TH1, but the control for the front-to-back ratio of the etalon to reach a predetermined range, that is, Switching to automatic frequency control is performed (steps S58, S59). Thereafter, it is detected that the temperature of the second temperature control device 53 has reached the predetermined range corresponding to the second set value Tb, and that the front-to-back ratio of the etalon has reached the predetermined range (step S60). , The AFC lock flag is output (step S61), and the switching operation ends. In step S60, the predetermined range corresponding to the second set value Tb is a range including Tb and narrower than Tb1 and Tb2.

  According to the present embodiment described above, light output can be performed before the temperature of the second temperature control device 53 reaches the final target Tb. Therefore, the start of wavelength control using the front-to-back ratio of the etalon can be hastened, and the time required for the wavelength switching operation can be reduced.

  In step S56 of the flowchart in FIG. 10, it is detected that the temperature of the second temperature control device 53 has reached the range of the temperatures Tb1 to Tb2, but the invention is not limited thereto. When controlling the temperature of the second temperature control device 53 from the temperature Ta to the temperature Tb, of the temperature range Tb1 to Tb2 corresponding to the allowable wavelength range Wa, the temperature Tb1 on the temperature Ta side of the temperature Tb (the temperature before the temperature Tb1). ) May be detected to open the shutter. However, by confirming that the temperature of the second temperature control device 53 falls within the temperature range Tb1 to Tb2 for the specified period, it is possible to avoid the influence of overshoot and the like.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Lower cladding layer 3 Active layer 4 Optical waveguide layer 6 Upper cladding layer 7 Contact layer 8 Electrode 9 Insulating film 10 Heater 11 Power supply electrode 12 Ground electrode 15 Back electrode 16 End face film 17 End face film 18 Diffraction grating 19 Optical amplification layer 20 Contact layer 21 Electrode 30 Semiconductor laser 31 First temperature controller 32 First thermistor 41 Beam splitter 42 First light receiving element 50 Detector 51 Beam splitter 52 Etalon 53 Second temperature controller 55 Second thermistor 60 Memory 70 Controller 100 Wavelength variable laser

Claims (2)

A temperature control device that controls a temperature of the etalon, a wavelength detection unit that specifies an output wavelength of the tunable laser based on a ratio of a light intensity input to the etalon and a light intensity output from the etalon; A method of controlling the wavelength of the tunable laser to a target wavelength based on a detection result of the unit,
A first step of driving the tunable laser at a first wavelength;
After the first step, in response to a command to output a second wavelength different from the first wavelength, a second step of suppressing the optical output of the tunable laser;
After the second step, from the first set temperature of the etalon corresponding to the first wavelength, to the second set temperature of the etalon corresponding to the second wavelength different from the first set temperature. A third step of starting control of the temperature control device;
Before the temperature of the etalon reaches the second set temperature, it is detected that the temperature of the tunable laser is within a temperature range corresponding to the second wavelength, and the temperature of the etalon is reduced to the second set temperature . A fourth step of detecting that the temperature reaches an allowable range of the set temperature and canceling the suppression of the optical output of the tunable laser.   The switching operation of the tunable laser to the second wavelength is terminated after confirming that the temperature of the temperature control device falls within a temperature range corresponding to the allowable range of the second set temperature for a predetermined period. 2. The wavelength switching method of the wavelength tunable laser according to 1.

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