Laser device and its control method

Abstract

It includes a laser unit, a temperature controller for controlling the temperature of the laser unit, and a control unit. The laser unit includes: a laser element unit having a phase adjustment unit for adjusting the optical length of the laser resonator, and the frequency of the output laser is made variable by controlling the phase adjustment unit; and a monitoring unit for obtaining a monitoring value corresponding to the frequency of the laser. The control unit is configured to be able to execute: a first control mode in which, while setting the set temperature of the temperature controller to be fixed, the phase adjustment unit is controlled so that the monitoring value becomes a target monitoring value corresponding to a target frequency set as the frequency of the laser; and a second control mode in which, when continuous fine adjustment control for the frequency of the laser is indicated, the temperature controller is controlled so that the frequency of the laser becomes a target frequency set as the frequency of the laser.

Description

Technical Field

[0001] The present invention relates to a laser device and a control method thereof. Background Art

[0002] In optical communication, a wavelength-variable laser, which is a laser device capable of outputting an arbitrary wavelength, is used for wavelength division multiplexing (WDM). As the wavelength-variable laser, there is a wavelength-variable method using two wavelength-dependent filters. The wavelength-variable laser adopting this wavelength-variable method has a structure in which two filters, a gain section, and a phase adjustment section are arranged between two mirrors constituting a laser resonator. Some of these elements may be integrated. For example, a structure in which a filter and a mirror are realized by an integrated distributed Bragg reflector (DBR) mirror is often used (for example, Patent Document 1). By making the wavelengths of the two filters coincide with a desired oscillation wavelength and further controlling the phase adjustment section, a desired wavelength can be obtained. Here, the phase adjustment section has a function of adjusting the optical length of the laser resonator. By changing the optical length of the laser resonator, the wavelength of the resonator mode is changed. In addition, the wavelength of light is inversely proportional to the frequency of light. Hereinafter, descriptions of wavelength and frequency will be appropriately used for expression.

[0003] In recent years, in systems using wavelength-variable lasers, systems using a digital coherent communication method have become mainstream, and thus a narrow linewidth is demanded for wavelength-variable lasers.

[0004] The wavelength-variable laser is used in the form of a wavelength-variable laser module equipped with a wavelength locker. The wavelength locker is used to fix the oscillation frequency to a desired frequency and uses a mechanism for detecting the transmittance of light that has passed through a wavelength filter having periodic transmittance with respect to the frequency. When it is detected that the oscillation frequency is different from the desired value according to the transmittance, feedback control is performed to correct the difference. In a wavelength-variable laser having a phase adjustment section, this feedback is usually performed on the control amount of the phase adjustment section.

[0005] The wavelength-variable laser module is controlled to output a laser having a desired oscillation frequency. Usually, this desired oscillation frequency does not mean continuous variability. In other words, there is no problem even if the driving conditions of the wavelength-variable laser module are changed discontinuously between a certain frequency and another frequency close thereto.

[0006] On the other hand, as a function of a wavelength-variable laser module, a function such as a fine tuning frequency (FTF) is sometimes sought. The FTF is a function that continuously changes the frequency from an initially determined desired oscillation frequency. A control method for a wavelength-variable laser module seeks to be able to implement TF (for example, Patent Document 2). Hereinafter, the control for implementing the FTF is sometimes described as FTF control, continuous fine adjustment control for the frequency of a laser, or simply described as continuous fine adjustment control.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Patent No. 4918203 Gazette

[0010] Patent Document 2: Japanese Patent No. 6241931 Gazette Summary of the Invention

[0011] -Problems to be Solved by the Invention-

[0012] Generally, in order to output a laser of an arbitrary frequency through a wavelength-variable laser module, a phase adjustment unit needs to change the phase of the laser in the laser resonator within a range of 2π radians. The frequency of the resonator mode corresponding to the frequency of the laser changes according to the amount of phase adjustment performed by the phase adjustment unit. When the adjustment amount becomes 2π radians, the frequency reaches the resonator mode adjacent to that resonator mode. Therefore, when the adjustment amount exceeds 2π radians, the adjustment amount can be temporarily restored to 0 radians.

[0013] However, in the case of performing FTF control, it is necessary to continuously change the frequency over a certain frequency range. In this case, it is sometimes necessary to perform a phase adjustment greater than 2π radians. The phase adjustment amount is usually achieved by the power supplied to the phase adjustment unit. Therefore, when the phase adjustment amount is large, there is a problem that the power consumption to be supplied to the phase adjustment unit increases.

[0014] The present invention has been made in view of the above circumstances, and an object thereof is to provide a laser device and a control method thereof capable of suppressing an increase in power consumption during FTF control.

[0015] -Means for Solving the Problems-

[0016] In order to solve the above problems and achieve the object, one aspect of the present invention is a laser device, comprising: a laser unit including a laser element unit and a monitoring unit, the laser element unit having a phase adjustment unit for adjusting the optical length of a laser resonator, and the output laser frequency being made variable by controlling the phase adjustment unit, the monitoring unit obtaining a monitoring value corresponding to the laser frequency; a temperature controller for controlling the temperature of the laser unit; and a control unit, the control unit being configured to be able to execute a first control mode and a second control mode. In the first control mode, while setting the set temperature for the temperature controller to be fixed, the phase adjustment unit is controlled so that the monitoring value becomes a target monitoring value corresponding to a target frequency set as the laser frequency. In the second control mode, when continuous fine adjustment control of the laser frequency is instructed, the temperature controller is controlled so that the laser frequency becomes the target frequency set as the laser frequency.

[0017] Alternatively, in the second control mode, the control unit controls the temperature controller so that the monitoring value becomes a target monitoring value corresponding to a target frequency set as the laser frequency.

[0018] Alternatively, the temperature controller has: a first temperature controller for controlling the temperature of the laser element unit; and a second temperature controller for controlling the temperature of the monitoring unit. In the second control mode, while controlling the second temperature controller to change the temperature of the monitoring unit, the first temperature controller is controlled so that the monitoring value becomes the target monitoring value.

[0019] Alternatively, the temperature controller controls the temperatures of the laser element unit and the monitoring unit together. In the second control mode, based on the temperature dependence of the laser frequency in the laser element unit and the temperature dependence of the monitoring value on the laser frequency in the monitoring unit, the target monitoring value is corrected. While changing the corrected target monitoring value, the temperature controller is controlled so that the monitoring value becomes the corrected target monitoring value.

[0020] One aspect of the present invention is a control method for a laser device, the laser device comprising: a laser unit having a laser element unit and a monitoring unit, the laser element unit having a phase adjustment unit for adjusting the optical length of a laser resonator and capable of varying the frequency of the output laser by controlling the phase adjustment unit, the monitoring unit obtaining a monitoring value corresponding to the frequency of the laser; and a temperature controller for controlling the temperature of the laser unit. The laser device is configured to be able to execute a first control step in which, while fixing the set temperature for the temperature controller, the phase adjustment unit is controlled such that the monitoring value becomes a target monitoring value corresponding to a target frequency set as the frequency of the laser. The control method for the laser device includes a second control step in which, when continuous fine adjustment control is specified for the frequency of the laser, the temperature controller is controlled such that the frequency of the laser becomes the target frequency set as the frequency of the laser.

[0021] Alternatively, in the second control step, the temperature controller is controlled such that the monitoring value becomes a target monitoring value corresponding to a target frequency set as the frequency of the laser.

[0022] Alternatively, in the second control step, while changing the temperature of the monitoring unit, the temperature of the laser element unit is controlled such that the monitoring value becomes the target monitoring value.

[0023] Alternatively, in the second control step, the temperatures of the laser element unit and the monitoring unit are controlled together. The second control step includes: a correction step of correcting the target monitoring value based on the temperature dependence of the frequency of the laser in the laser element unit and the temperature dependence of the monitoring value on the frequency of the laser in the monitoring unit; and a control step of controlling the temperature controller such that the monitoring value becomes the corrected target monitoring value while changing the corrected target monitoring value.

[0024] -Advantages of the Invention-

[0025] According to the present invention, there is an effect that an increase in power consumption in FTF control can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a diagram showing the structure of the laser device according to Embodiment 1.

[0027] Figure 2 It is a diagram showing the structure of the laser element unit.

[0028] Figure 3 It is an explanatory diagram of the adjustment of the frequency of the laser.

[0029] Figure 4 This is a flowchart showing the control method executed by the control unit related to Embodiment 1.

[0030] Figure 5 This is an explanatory diagram showing the change in the frequency characteristics of the discrimination curve based on the change in the TEC temperature.

[0031] Figure 6 This is a diagram showing the structure of the laser device related to Embodiment 2.

[0032] Figure 7 This is a flowchart showing the control method executed by the control unit related to Embodiment 2. Detailed implementation manners

[0033] Hereinafter, the embodiments of implementing the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings. Note that the present invention is not limited by the embodiments described below. Further, in the description of the drawings, the same reference numerals are appropriately assigned to the same parts. In addition, the drawings are schematic, and the dimensional relationships between elements, the ratios of the elements, etc. may be different from the actual ones. Further, between the drawings, there may also be parts where the dimensional relationships and ratios are different from each other.

[0034] (Embodiment 1)

[0035] [Outline structure of the laser device]

[0036] Figure 1 This is a diagram showing the structure of the laser device related to Embodiment 1. The laser device 100 includes a modularized laser unit 1 and a control unit 2 that controls the operation of the laser unit 1. In addition, in Figure 1 , the laser unit 1 and the control unit 2 are separately configured, but they may also be modularized into one body.

[0037] [Structure of the laser unit]

[0038] The laser unit 1 includes a housing 3 and the following components that are housed or inserted into the housing 3: TEC (Thermo-Electric Cooler) elements 4, 5, a heat sink 6, a laser element unit 7, a temperature sensor 8, a lens 9, an optical isolator 10, a beam splitter 11, a lens 12, an optical fiber 13, a beam splitter 14, a photodiode (PD) 15, a temperature sensor 16, an etalon filter 17, and a PD 18. The TEC elements 4, 5 are examples of the first temperature controller and the second temperature controller that constitute the temperature controller. The PD 15, the etalon filter 17, and the PD 18 constitute the monitoring unit 19.

[0039] The TEC elements 4 and 5 are mounted on the bottom plate of the housing 3. The TEC elements 4 and 5 are constituted by, for example, Peltier elements. Hereinafter, the TEC elements 4 and 5 may be respectively referred to as TEC1 and TEC2. The TEC elements 4 and 5 are controlled by being supplied with power from the control unit 2.

[0040] The heat sink 6 is mounted on the TEC element 4. The heat sink 6 is made of a material with high thermal conductivity, such as aluminum nitride (AlN).

[0041] The laser element unit 7 is mounted on the TEC element 4 in a state where the heat sink 6 is interposed therebetween. The temperature of the laser element unit 7 is controlled by the TEC element 4. The laser element unit 7 outputs a laser beam L1 through the drive control of the control unit 2.

[0042] Figure 2 It is a diagram showing the structure of the laser element unit 7. The laser element unit 7 includes: a semiconductor unit 71; an n-side electrode 72 formed on the back surface of the semiconductor unit 71; micro heaters 73, 74, and 76 formed on the surface of the semiconductor unit 71; and p-side electrodes 75 and 77.

[0043] The semiconductor unit 71 is made of, for example, an InP-based semiconductor material and has an embedded waveguide structure. The semiconductor unit 71 has a structure in which the following structures are arranged in this order: a first DBR unit 713 having a waveguide 713a including a structure of a sampled grating (SG-DBR) of a distributed Bragg reflection type; a phase adjustment unit 714 having a passive waveguide 714a; a gain unit 715 having a waveguide 715a formed of an active layer; a second DBR unit 716 having a waveguide 716a including a structure of SG-DBR; and a semiconductor optical amplifier (SOA) unit 717 having a waveguide 717a formed of an active layer. The active layer has, for example, a multi-quantum well (MQW) structure made of a GaInAsP-based semiconductor material or an AlGaInAs-based semiconductor material. The passive waveguide is made of, for example, an i-type GaInAsP-based semiconductor material having a bandgap wavelength of 1300 nm. The waveguide of the DBR structure is configured such that diffractive gratings are periodically formed by portions made of, for example, a GaInAsP-based semiconductor material or an AlGaInAs-based semiconductor material and having different refractive indexes from each other.

[0044] The micro heaters 73, 74, and 76 are respectively formed on the surfaces of the first DBR unit 713, the phase adjustment unit 714, and the second DBR unit 716. The p-side electrodes 75 and 77 are respectively formed on the surfaces of the gain unit 715 and the SOA unit 717.

[0045] The first DBR section 713 and the second DBR section 716 form a laser resonator. The first DBR section 713 and the second DBR section 716 have comb-shaped reflection peaks with a periodic frequency interval according to the reciprocal of the period of the diffraction grating. In the first DBR section 713 and the second DBR section 716, the periods are different, and it becomes a structure capable of coarsely adjusting the frequency of the laser L1 by a method called the Vernier method. The micro heater 73 heats the first DBR section 713, thereby changing the refractive index and enabling the comb-shaped reflection peaks to shift in the direction of the frequency axis. Similarly, the micro heater 76 heats the second DBR section 716, thereby changing the refractive index and enabling the comb-shaped reflection peaks to shift in the direction of the frequency axis.

[0046] The gain section 715 is disposed between the first DBR section 713 and the second DBR section 716. By applying a voltage between the n-side electrode 72 and the p-side electrode 75 to allow current to flow, an optical amplification effect is exerted. As a result, laser oscillation is caused.

[0047] The phase adjustment section 714 is disposed between the first DBR section 713 and the second DBR section 716. The micro heater 74 heats the phase adjustment section 714, thereby changing the refractive index and enabling adjustment of the optical length of the laser resonator. By adjusting the optical length of the laser resonator, it is possible to finely adjust the frequency of the resonator mode (cavity mode) while shifting in the direction of the frequency axis. By finely adjusting the resonator mode, it is possible to select which resonator mode to perform laser oscillation, and at the same time, change the frequency within a small range.

[0048] The SOA section 717 is disposed on the opposite side of the first DBR section 713 and the gain section 715 with respect to the second DBR section 716. By applying a voltage between the n-side electrode 72 and the p-side electrode 77 to allow current to flow, an optical amplification effect is exerted. The SOA section 717 optically amplifies the laser output from the second DBR section 716 due to laser oscillation and outputs it as the laser L1 with increased power to the outside.

[0049] On the laser L1 output side of the SOA section 717, a bent waveguide for suppressing reflection at the end face may be provided. A bent waveguide for suppressing reflection at the end face may also be provided on the opposite side of the first DBR section 713 from the phase adjustment section 714.

[0050] Return Figure 1 The temperature sensor 8 is formed using, for example, a thermistor and detects the temperature of the laser element section 7. The temperature sensor 8 outputs an electrical signal including information on the detected temperature to the control section 2.

[0051] The lens 9 makes the laser L1 output from the laser element unit 7 into parallel light. The optical isolator 10 is mounted on the TEC element 5, allows the laser L1 to pass through to the right side of the figure, and blocks the light traveling from the left side of the figure.

[0052] The beam splitter 11 allows the laser L1 to pass through the lens 12 side, and reflects a part of the laser L1 as the laser L2 to the beam splitter 14 side. The lens 12 focuses the laser L1 and couples it with the optical fiber 13. The optical fiber 13 transmits the laser L1.

[0053] The beam splitter 14 allows the laser L2 to pass through to the PD15 side, and reflects a part of the laser L2 as the laser L3 to the etalon filter 17 side. The PD15 receives the laser L2 and outputs an electrical signal corresponding to the received light intensity to the control unit 2.

[0054] The etalon filter 17 has a transmittance characteristic that varies periodically with respect to the frequency of light. The etalon filter 17 transmits the laser L3 with a transmittance corresponding to the frequency of the laser L3. The PD18 receives the laser L3 transmitted by the etalon filter 17 and outputs an electrical signal corresponding to the received light intensity to the control unit 2. This electrical signal includes information on the frequency of the laser L1.

[0055] The electrical signals respectively output from the PD15 and 18 are used for the wavelength locking control (control for setting the wavelength of the laser L1 output from the laser element unit 7 to the target wavelength) performed by the control unit 2. Specifically, although it will be described later, the ratio of the current value of the electrical signal output from the PD18 to the current value of the electrical signal output from the PD15 (monitor PD current ratio) is equivalent to the monitoring value corresponding to the frequency of the laser L1. That is, the monitoring unit 19 functions to obtain the monitoring value corresponding to the frequency of the laser L1.

[0056] The temperature sensor 16 is formed using, for example, a thermistor, and detects the temperature of the etalon filter 17. The temperature sensor 16 outputs an electrical signal including information on the detected temperature to the control unit 2.

[0057] 〔Outline Structure of the Control Unit〕

[0058] The control unit 2 is connected to, for example, an upper control device (not shown) having a user interface, and controls the operation of the laser unit 1 according to an instruction from a user via the upper control device.

[0059] In addition, for the convenience of explanation below, the structure mainly shown as the structure of the control unit 2 is the structure that performs wavelength locking control and FTF control.

[0060] The control unit 2 includes an analog-to-digital converter (ADC), an arithmetic unit, a storage unit, and a current source.

[0061] The ADC converts the analog electrical signals input from temperature sensors 8, 16, and PD15, 18 into digital signals and outputs them to the arithmetic unit.

[0062] The arithmetic unit performs various arithmetic processes for the control executed by the control unit 2 and is constituted by, for example, a CPU (Central Processing Unit) and an FPGA (Field Programmable Gate Array). The storage unit stores various programs, data, etc. used for the arithmetic unit to perform arithmetic processes. For example, it includes: a part constituted by a ROM (Read Only Memory); a working space when the arithmetic unit performs arithmetic processes; and a part, for example, constituted by a RAM (Random Access Memory) that is used to store the results of the arithmetic processes of the arithmetic unit. The control function of the control unit 2 is realized in the form of software through the functions of the arithmetic unit and the storage unit.

[0063] Based on an instruction from the arithmetic unit, the current source supplies power for outputting the laser L1 and controlling the frequency to the laser unit 1. In the present embodiment, the arithmetic unit indicates a current value to the current source as a control quantity. The current source supplies power of the indicated current value to the laser unit 1.

[0064] 〔Adjustment of Frequency〕

[0065] In the laser element unit 7, the frequency of the laser L1 can be changed using the Vernier effect. Figure 3 It is an explanatory diagram of the adjustment of the laser frequency. The upper part shows the reflection spectrum of the first DBR part 713, the middle part shows the reflection spectrum of the second DBR part 716, and the lower part shows the spectrum of the resonator mode. The resonator mode interval is, for example, about 20 GHz. In order to achieve narrow linewidth broadening of the laser L1, it is advantageous for this resonator mode interval to be small. Therefore, in recent wavelength tunable lasers, a design with a value as small as about 20 GHz is sometimes used.

[0066] If the supplied power is adjusted and the micro heater 73 (hereinafter, sometimes referred to as the first DBR heater) is controlled, its reflection spectrum shifts from the shape shown by the solid line to the shape shown by the dashed line on the frequency axis as indicated by the thick arrow line. Similarly, if the micro heater 76 (hereinafter, sometimes referred to as the second DBR heater) is controlled, its reflection spectrum shifts from the shape shown by the solid line to the shape shown by the dashed line on the frequency axis. Similarly, if the micro heater 74 (hereinafter, sometimes referred to as the phase heater) is controlled, its spectrum shifts from the shape shown by the solid line to the shape shown by the dashed line on the frequency axis.

[0067] In the state shown by the solid line, laser oscillation is performed at a frequency f1 at which the reflection peak of the first DBR section 713, the resonator mode, and the reflection peak of the second DBR section 716 coincide. To set this state, the first DBR heater and the second DBR heater determine the frequency positions at which the reflection spectra of the first DBR section 713 and the second DBR section 716 become peaks, respectively, based on the supplied power. In addition, the phase heater determines the frequency position at which the resonator mode becomes a peak based on the supplied power. If the state shown by the dashed line is achieved through the control of each heater, the frequency at which the reflection peak of the first DBR section 713, the resonator mode, and the reflection peak of the second DBR section 716 coincide can be set to the frequency f2, and the frequency of the laser L1 can be adjusted to the frequency f2. In addition, the power supplied to each heater can be controlled using the current as a control quantity. That is, the control unit 2 controls the frequency of the laser L1 by supplying power corresponding to the current as the control quantity to the laser element section 7. The current or the power is an example of the control quantity.

[0068] Thus, in the case where the frequency is changed by changing the refractive index by heating in the micro heater, the larger the required refractive index change amount (frequency change amount), the greater the power consumption of the micro heater.

[0069] When changing the frequency of the laser L1 from the first frequency to the second frequency, for example, first, feedforward control is performed on the first DBR heater and the second DBR so that the reflection peaks of the first DBR section 713 and the second DBR section 716 coincide at the second frequency, and then feedback control is performed on the phase heater so that any one of the resonator modes coincides with the second frequency. However, the control method is not limited to this.

[0070] 〔Startup control and wavelength locking control of laser device〕

[0071] Next, an example of the implementation method of the startup control and wavelength locking control of the laser device 100 will be described. The wavelength locking control is an example of the first control mode or the first control step.

[0072] First, the control unit 2 sets the target frequency of the laser L1, for example, according to an instruction from a higher-level control device.

[0073] Next, the control unit 2 controls the TEC element 4 so that the laser element unit 7 becomes a fixed temperature, and controls the TEC element 5 so that the etalon filter 17 becomes a fixed temperature. In addition, the temperature of the etalon filter 17 is set according to the target frequency. Specifically, since the transmission characteristic of the etalon filter 17 shifts in the frequency axis direction according to the temperature, the temperature is set so that the slope of the frequency with respect to the transmittance of the etalon filter 17 is relatively large at the target frequency. The relationship between the target frequency and the temperature or the current supplied to the TEC element 5 is stored in the storage unit, for example, as tabular data or a relational expression obtained through prior calibration. In addition, for data not included in the tabular data, it is possible to use the data included in the tabular data and calculate by interpolation.

[0074] Next, the control unit 2 supplies currents to the micro heaters 73, 74, and 76 that provide the on-power corresponding to the target frequency. The relationship between the target frequency and the on-power can be referred to, for example, the data stored as tabular data or a relational expression in the storage unit, or calculated by interpolation.

[0075] Next, the control unit 2 supplies a drive current to the gain unit 715 to cause it to perform laser oscillation.

[0076] Next, the control unit 2 gradually supplies a drive current to the SOA unit 717, and drives it so that the power of the laser L1 gradually increases. Once the power of the laser L1 reaches a certain given value, the drive current is fixed.

[0077] Next, wavelength locking control is performed. Specifically, first, the control unit 2 converts the target frequency into a target PD current ratio corresponding to the target frequency. The relationship between the target frequency and the target PD current ratio can be referred to, for example, the data stored as tabular data or a relational expression in the storage unit, or calculated by interpolation.

[0078] Next, the control unit 2 calculates and obtains the PD current ratio (monitor PD current ratio) corresponding to the current frequency of the laser L1 based on the electrical signals respectively output from the PDs 15 and 18.

[0079] Next, the control unit 2 performs feedback control to control the current supplied to the micro heater 74 (phase heater) so that the monitor PD current ratio becomes the target PD current ratio. Specifically exemplifying that the monitor PD current ratio becomes the target PD current ratio means that the absolute value of the difference between the target PD current ratio and the monitor PD current ratio is within the allowable error range. This feedback control is performed by proportional integral derivative (PID) control or PI control. If the absolute value of the difference between the target PD current ratio and the monitor PD current ratio is within the allowable error range, the laser element unit 7 becomes in a wavelength-locked state. In this wavelength locking, the monitor PD current ratio is used for the feedback control of the phase heater.

[0080] Then, increase the drive current supplied to the SOA unit 717 until the received light intensity detected by the PD15 reaches a desired value. Thus, the laser device 100 enters a stable drive state.

[0081] At the time point when the wavelength locking control has ended, the power supplied to the micro heaters 73 and 76 (first and second DBR heaters) and the control temperature of the TEC element 5 become fixed values corresponding to the target frequency of the laser L1, and the drive current of the gain unit 715 and the control temperature of the TEC element 4 become fixed set values regardless of the target frequency. The drive current of the SOA unit 717 is feedback-controlled based on the detection value of the PD15, and the power supplied to the micro heater 74 (phase heater) is feedback-controlled based on the monitor PD current ratio.

[0082] 〔FTF Control〕

[0083] Next, an example of the implementation method of the FTF control executed by the control unit 2 of the laser device 100 according to Embodiment 1 will be described with reference to Figure 4 the flowchart of. The FTF control is an example of the second control mode or the second control step. Figure 4 The process of starts, for example, when an instruction to execute the FTF control from the current frequency to the target frequency is received from the upper control device.

[0084] First, in step S101, the control unit 2 obtains the frequency difference between the target frequency of the indicated FTF control and the current frequency of the laser L1.

[0085] Next, in step S102, the control unit 2 converts the obtained frequency difference into the temperature difference between the control temperature of the TEC element 5 corresponding to the target frequency and the current control temperature of the TEC element 5, that is, the required temperature change amount (TEC2 temperature change amount) to obtain.

[0086] Next, in step S103, the control unit 2 stops the feedback control of the micro heater 74 (phase heater) based on the monitor PD current ratio. As a result, the power supplied to the phase heater (current) is fixed at the value at the time of stopping.

[0087] Next, in step S104, the control unit 2 changes the control temperature (TEC2 temperature) of the TEC element 5. In addition, the change amount is set to a value obtained by dividing the TEC2 temperature change amount into multiple parts. If the TEC2 temperature is changed, the temperature of the etalon filter 17 changes, and the discrimination curve showing the relationship between the transmittance or the PD current ratio with respect to the frequency shifts in the frequency axis direction.

[0088] Figure 5It is an explanatory diagram of the change in the frequency characteristics of the discrimination curve based on the change in the TEC temperature. For example, the discrimination curve before the change in the TEC2 temperature is represented by curve C1. If the TEC temperature is changed, the discrimination curve will shift in frequency by only Δf as shown by curve C2.

[0089] Next, in step S105, the control unit 2 determines whether the absolute value of the difference between the target PD current ratio and the monitor PD current ratio is within a given error range while keeping the fixed target PD current ratio unchanged. If it is not within the error range (step S105, NO), in step S106, the control temperature (TEC1 temperature) of the TEC element 4 is changed, and the process returns to step S105. If it is within the error range (step S105, YES), the process proceeds to step S107.

[0090] If the TEC1 temperature is changed, the temperature of the laser element unit 7 will also change accordingly. Therefore, even if the currents supplied to the first DBR heater, the second DBR heater, and the phase heater are not changed, the reflection peak and the frequency of the resonator mode will shift.

[0091] As Figure 5 shown, before starting the FTF control, feedback control is performed using the discrimination curve of curve C1 so that the difference between the target PD current ratio and the monitor PD ratio is within the error range, and the frequency is locked at f3. On the other hand, through steps S104 to S106, if feedback control is performed using the discrimination curve of curve C2 so that the difference between the target PD current ratio and the monitor PD ratio is within the error range, the frequency is locked at f4. That is, in this state, the control unit 2 performs feedback control of the TEC element 4 based on the monitor PD current ratio.

[0092] Next, in step S107, the control unit 2 determines whether the difference ΔTEC2 between the TEC2 temperature after the change in step S104 and the TEC2 temperature corresponding to the target frequency is zero. If ΔTEC2 is non-zero (step S107, NO), the process returns to step S104, and the control unit 2 further changes the TEC2 temperature. If ΔTEC2 is zero (step S107, YES), the FTF control ends. That is, the control unit 2 restarts the control of the phase heater in step S108 and restarts the normal wavelength locking control. Then the process ends.

[0093] At the time point of ending the process, the power supplied to the first DBR heater and the second DBR heater is the same as before starting the FFT control. The drive current of the SOA unit 717 is feedback-controlled based on the detection value of PD15. The power supplied to the phase heater is feedback-controlled based on the monitor PD current ratio. The TEC2 temperature is the temperature corresponding to the target frequency and is the same temperature as when the target frequency set for FTF control is from the beginning. The TEC1 temperature is the temperature determined by FTF control.

[0094] In the laser device 100 configured as described above, an increase in power consumption during FTF control can be suppressed. The reasons are described below.

[0095] For example, in FTF control, it is sometimes instructed to typically change the frequency of the laser L1 within the range of ±8 GHz. On the other hand, in the phase adjustment unit 714, although it is necessary to be able to change the phase of light by only 2π radians, if the resonator mode interval is 20 GHz, being able to change the phase of light by only 2π radians is equivalent to being able to shift the resonator mode by only 20 GHz on the frequency axis.

[0096] In FTF control, based on the change in the phase of the phase adjustment unit 714 starting from the state of 0 radians, the frequency of the laser L1 can be further changed to -8 GHz, and in order to achieve the change in phase, starting from the state of +2π radians, the frequency of the laser L1 can be further changed to the state of +8 GHz. It is necessary to be able to change the phase by only 1.8×2π radians using the phase adjustment unit 714. In this case, compared with the case where the change in the phase based on the phase adjustment unit 714 can be only 2π radians, the power supplied to the phase adjustment unit 714 becomes about twice, so the power consumption of the phase adjustment unit 714 increases.

[0097] Therefore, in the laser device 100, in FTF control, the change in the frequency of the laser L1 is achieved by temperature control of the laser element unit 7 by the TEC element 4. When changing the temperature of the entire laser element unit 7 by the TEC element 4, changing the temperature by only 1 Kelvin can change the frequency of the laser L1 by approximately 10 GHz. In this case, the power supplied to the first and second DBR heaters and the phase heater of the laser element unit 7 can be fixed. On the other hand, if it is about 1 Kelvin, the increase in power consumption caused by the temperature control of the TEC element 4 is small enough to be negligible.

[0098] As described above, the laser device 100 has the effect of being able to suppress an increase in power consumption during FTF control.

[0099] (Embodiment 2)

[0100] 〔Outline Structure of Laser Device〕

[0101] Figure 6 This is a diagram showing the structure of the laser device according to Embodiment 2. The laser device 100A includes a modularized laser unit 1A and a control unit 2A that controls the operation of the laser unit 1A. Additionally, the laser unit 1A and the control unit 2A can also be modularized into one unit.

[0102] 〔Structure of Laser Unit〕

[0103] The laser unit 1A has the following structure: In the structure of the laser unit 1, the TEC elements 4 and 5 are replaced with the TEC element 4A, the beam splitters 14, PDs 15, 18, the temperature sensor 16, and the etalon filter 17 are deleted, and the PD 21, the lens 22, the planar lightwave circuit (PLC) 23, and the PD array 24 housed in the housing 3 are added. The PLC 23 and the PD array 24 constitute the monitoring unit 25. Additionally, the following description of the structural elements shared with the laser unit 1 of the laser unit 1A is appropriately omitted.

[0104] The TEC element 4A is mounted on the bottom plate of the housing 3. The TEC element 4A is constituted by using, for example, a Peltier element. The heat sink 6, the laser element unit 7, the temperature sensor 8, the lens 9, the optical isolator 10, the beam splitter 11, the PD 21, the lens 22, the PLC 23, and the PD array 24 are mounted on the TEC element 4A. The TEC element 4A is an example of a temperature controller that controls the temperature of both the laser element unit 7 and the monitoring unit 25.

[0105] The PD 21 is mounted on the TEC element 4A and, similar to the PD 15 of the laser device 100, receives the laser L2 and outputs an electrical signal corresponding to the received light intensity to the control unit 2. The PD 21 functions to obtain a monitoring value corresponding to the intensity of the laser L1.

[0106] The lens 22 is mounted on the TEC element 4A, condenses the laser L4 output from the first DBR section 713 side of the laser element unit 7 and laser-oscillated by the laser element unit 7 in the same manner as the laser L1, and is optically coupled to the PLC 23.

[0107] The PLC 23 includes a transmissive waveguide 23a and filters 23b and 23c. The PLC 23 branches the laser L4 into three lasers L4a, L4b, and L4c, and inputs them into the transmissive waveguide 23a and the filters 23b and 23c respectively. The transmissive waveguide 23a directly transmits the laser L4a and outputs it to the PD array 24. The filters 23b and 23c both change periodically at substantially the same period with respect to the frequency of light, but have transmissive characteristics with a phase deviation of only π / 2 from each other. The filters 23b and 23c transmit the lasers L4b and L4c at a transmittance corresponding to their frequencies (equal to the frequency of the laser L1) and output them to the PD array 24. The filters 23b and 23c can be configured using ring resonator type filters or asymmetric Mach-Zehnder type filters.

[0108] The PD array 24 includes three PDs, and each PD receives the lasers L4a, L4b, and L4c output from the transmissive waveguide 23a and the filters 23b and 23c respectively, and outputs electrical signals corresponding to the received light intensities to the control unit 2A. These electrical signals include information on the intensity or frequency of the laser L1. Each electrical signal output from the PD array 24 is used for the wavelength locking control performed by the control unit 2A. Specifically, as described later, the ratio (monitor PD current ratio) of the current value of the electrical signal corresponding to the laser L4a from the transmissive waveguide 23a to the current value of the electrical signal corresponding to the laser L4b or L4c from the filter 23b or 23c is equivalent to the monitored value corresponding to the frequency of the laser L1. That is, the monitoring unit 25 functions to obtain the monitored value corresponding to the frequency of the laser L1. In addition, which electrical signal of the lasers from the filters 23b and 23c is used is selected according to the frequency of the laser L1 to be controlled. Specifically, the one of the filters 23b and 23c with a larger slope of the transmittance with respect to the frequency to be controlled is selected. Thereby, the change in transmittance with respect to the change in frequency is large, so the detection sensitivity of the frequency is improved.

[0109] 〔Outline Structure of Control Unit〕

[0110] The control unit 2A has the same structure as the control unit 2, so the description thereof is omitted.

[0111] 〔Startup Control and Wavelength Locking Control of Laser Device〕

[0112] The start control and wavelength locking control of the laser device 100A can be implemented in the same manner as that of the laser device 100. Among them, the output from either the filter 23b or 23c is selected according to the target frequency. In addition, the following aspects should be noted: the temperature of the laser element unit 7 and the temperature of the monitoring unit 25 are controlled together by the TEC element 4A. The monitoring unit 25 may also include the filters 23b and 23c, and by selecting either one, the temperature control of the monitoring unit 25 is not performed separately.

[0113] At the time point when the wavelength locking control ends, the energization power to the micro heaters 73 and 76 (the first and second DBR heaters) becomes a fixed value corresponding to the target frequency of the laser L1, and the drive current of the gain unit 715 and the control temperature of the TEC element 4A become fixed set values regardless of the target frequency. The drive current of the SOA unit 717 is feedback-controlled based on the detection value of the PD21, and the power supply to the micro heater 74 (the phase heater) is feedback-controlled based on the monitor PD current ratio.

[0114] 〔FTF control〕

[0115] Next, with reference to Figure 7 the flowchart, an example of the implementation method of the FTF control executed by the control unit 2A of the laser device 100A according to Embodiment 2 will be described. The FTF control is an example of the second control mode or the second control step. For example, it starts when an instruction to execute the FTF control from the current frequency to the target frequency is received from the upper control device Figure 7 of the process.

[0116] First, in step S201, the control unit 2A obtains the frequency difference between the target frequency of the indicated FTF control and the current frequency of the laser L1.

[0117] Next, in step S202, the control unit 2A converts the obtained frequency difference into the difference between the target PD current ratio corresponding to the target frequency and the target PD current ratio corresponding to the current frequency of the laser L1, that is, the target PD current ratio change amount is obtained. The relationship between the frequency and the target PD current ratio can be referred to, for example, the data stored in the storage unit as tabular data or relational expressions, or calculated by interpolation.

[0118] Here, in the laser device 100A, the temperature of the laser element unit 7 and the temperature of the monitoring unit 25 are controlled together by the TEC element 4A. Therefore, if the temperature of the laser element unit 7 is changed by the TEC element 4A, the temperature of the monitoring unit 25 also changes. If the temperature of the monitoring unit 25 changes, the discrimination curve indicating the relationship between the transmittance (or PD current ratio) of the frequency for the filters 23b and 23c shifts in the direction of the frequency axis.

[0119] Therefore, in this control method, in consideration of the temperature dependencies of the laser element unit 7 and the filters 23b and 23c, a correction step of correcting the target PD current ratio stored in the storage unit is performed. For example, if df / dT represents the offset amount f of the transmission characteristics of the filters 23b and 23c in the frequency axis direction with respect to the change in temperature T, and dF / dT represents the change in the frequency F of the laser L1 of the laser element unit 7 with respect to the change in temperature T, the stored target PD current ratio is multiplied by {1 - (df / dT) / (dF / dT)} and corrected.

[0120] In addition, in the FTF control, when the filters used for the wavelength locking control are different at the target frequency and the current frequency of the laser L1, the switching can be performed in advance before the start of the FTF control, or the switching can be performed during the FTF control.

[0121] Next, in step S203, the control unit 2A stops the feedback control of the micro heater 74 (phase heater) based on the monitor PD current ratio. As a result, the power supply (current) to the phase heater is fixed at the value at the time of stopping.

[0122] Next, in step S204, the control unit 2A changes the target PD current ratio from the current value. In addition, the change amount is set to an amount obtained by dividing the target PD current ratio change amount into a plurality of parts.

[0123] Next, in step S205, the control unit 2A determines whether the absolute value of the difference between the changed target PD current ratio and the monitor PD current ratio is within a given error range. If it is not within the error range (step S205, NO), the control temperature (TEC temperature) of the TEC element 4A is changed, and the process returns to step S205. The frequency of the laser L1 changes according to the change in the TEC temperature. If it is within the error range (step S205, YES), the process proceeds to step S207.

[0124] If the TEC temperature is changed, the temperature of the laser element unit 7 also changes accordingly. Therefore, even if the currents supplied to the first DBR heater, the second DBR heater, and the phase heater are not controlled, the reflection peak and the frequency of the resonator mode shift.

[0125] In this state, the control unit 2A performs feedback control of the TEC element 4A based on the changed target PD current ratio and the monitor PD current ratio.

[0126] Next, in step S207, the control unit 2A determines whether the difference Δ(target PD current ratio) between the target PD current ratio changed in step 204 and the target PD current ratio corresponding to the target frequency is zero. When Δ(target PD current ratio) is non-zero (step S207, NO), the process returns to step S204, and the control unit 2A further changes the target PD current ratio. When Δ(target PD current ratio) is zero (step S207, YES), the FTF control ends. That is, the control unit 2A restarts the control of the phase heater in step S208 and restarts the normal wavelength locking control. Then the process ends.

[0127] At the time point when the process ends, the power supplied to the first DBR heater and the second DBR heater is the same as before the start of the FTF control. The drive current of the SOA unit 717 is feedback-controlled based on the detection value of PD21. The power supplied to the phase heater is feedback-controlled based on the monitored PD current ratio. The target PD ratio is a value corresponding to the target frequency and is the same value as when the target frequency set for the FTF control is the same from the beginning. The TEC temperature is the temperature determined by the FTF control.

[0128] In the laser device 100A configured as described above, similar to the laser device 100, an increase in power consumption during the FTF control can be suppressed.

[0129] In addition, in the above-described second embodiment, the two filters 23b and 23c are configured by a PLC, and they may also be configured by an etalon filter and a spatial optical system. Further, in the second embodiment, the monitoring unit 25 is arranged behind the laser element unit 7, but the monitoring unit 25 may also be arranged on the side of the output laser L1 of the laser element unit 7. In this case, a structure using the PD array 24 to monitor the intensity of the laser L1 may also be adopted.

[0130] In addition, in the above-described embodiment, in order to change the refractive indices of the first DBR unit 713, the phase adjustment unit 714, and the second DBR unit 716, temperature changes caused by the micro heaters 73, 74, and 76 are used. However, as another method for changing the refractive index, a method of injecting carriers into the waveguide by current injection may also be used. In the case of this method, the phenomenon that the refractive index decreases due to the carrier plasma effect is used. Even when this method is used, an increase in power consumption during the FTF control can be suppressed in the same manner as in the above-described embodiment. Among them, the method using temperature change is advantageous in that the laser L1 has a narrow line width.

[0131] In addition, in the above-described embodiment, the frequency of the laser L1 is roughly adjusted by a method called the cursor type. However, even in a structure using a method other than the cursor type such as a digital supermodel, as long as it has a phase adjustment unit, it is possible to suppress an increase in power consumption in the FTF control in the same manner as the above-described embodiment.

[0132] Furthermore, the present invention is not limited by the above-described embodiment. A solution formed by appropriately combining the above-described respective structural elements is also included in the present invention. In addition, further effects and modification examples can be easily derived by those skilled in the art. Thus, a broader aspect of the present invention is not limited to the above-described embodiment, and various changes can be made.

[0133] -Industrial Applicability-

[0134] The present invention can be used in a laser device and a control method thereof.

[0135] -Symbol Explanation-

[0136] 1, 1A Laser unit

[0137] 2, 2A Control unit

[0138] 3 Housing

[0139] 4, 4A, 5 TEC element

[0140] 6 Heat sink

[0141] 7 Laser element unit

[0142] 8, 16 Temperature sensor

[0143] 9, 12 Lens

[0144] 10 Optical isolator

[0145] 11, 14 Beam splitter

[0146] 13 Optical fiber

[0147] 15, 18, 21 PD

[0148] 17 Etalon filter

[0149] 19, 25 Monitoring unit

[0150] 23 PLC

[0151] 23a Transmission waveguide

[0152] 23b, 23c Filter

[0153] 24 PD array

[0154] 71 Semiconductor section

[0155] 72 n-side electrode

[0156] 73, 74, 76 Micro heaters

[0157] 75, 77 p-side electrodes

[0158] 100, 100A Laser devices

[0159] 713 First DBR section

[0160] 713a, 714a, 715a, 716a Waveguides

[0161] 714 Phase adjustment section

[0162] 715 Gain section

[0163] 716 Second DBR section

[0164] 717 SOA section

[0165] 717a Waveguide

[0166] L1, L2, L3, L4, L4a, L4b, L4c Lasers

Claims

1. A laser device, comprising: A laser unit, comprising a laser element unit and a monitoring unit, wherein the laser element unit has a phase adjustment unit for adjusting the optical length of a laser resonator, and the frequency of the output laser is made variable by controlling the phase adjustment unit, and the monitoring unit obtains a monitoring value corresponding to the frequency of the laser; A temperature controller for controlling the temperature of the laser unit; and A control unit, The control unit is configured to be able to execute a first control mode and a second control mode. In the first control mode, while keeping the set temperature for the temperature controller fixed, the phase adjustment unit is controlled so that the monitoring value becomes a target monitoring value corresponding to a target frequency set as the frequency of the laser. In the second control mode, when continuous fine adjustment control of the frequency of the laser is instructed, the temperature controller is controlled so that the frequency of the laser becomes the target frequency set as the frequency of the laser.

2. The laser device according to claim 1, wherein In the second control mode, the control unit controls the temperature controller so that the monitored value becomes a target monitored value corresponding to a target frequency set as the frequency of the laser.

3. The laser device according to claim 2, wherein The temperature controller includes: a first temperature controller that controls the temperature of the laser element unit; and a second temperature controller that controls the temperature of the monitoring unit. In the second control mode, the control unit controls the second temperature controller to change the temperature of the monitoring unit while controlling the first temperature controller so that the monitored value becomes the target monitored value.

4. The laser device according to claim 2, wherein The temperature controller controls the temperatures of the laser element unit and the monitoring unit together. In the second control mode, the control unit corrects the target monitored value based on the temperature dependence of the frequency of the laser in the laser element unit and the temperature dependence of the monitored value in the monitoring unit on the frequency of the laser. While changing the corrected target monitored value, the control unit controls the temperature controller so that the monitored value becomes the corrected target monitored value.

5. A control method for a laser device, The laser device comprises: A laser unit, comprising a laser element unit and a monitoring unit, wherein the laser element unit has a phase adjustment unit for adjusting the optical length of a laser resonator, and the frequency of the output laser is made variable by controlling the phase adjustment unit, and the monitoring unit obtains a monitoring value corresponding to the frequency of the laser; and A temperature controller for controlling the temperature of the laser unit. The laser device is configured to be able to execute a first control step, in which while keeping the set temperature for the temperature controller fixed, the phase adjustment unit is controlled so that the monitoring value becomes a target monitoring value corresponding to a target frequency set as the frequency of the laser. The control method of the laser device includes a second control step, in which, when continuous fine adjustment control is specified for the frequency of the laser, the temperature controller is controlled so that the frequency of the laser becomes a target frequency set as the frequency of the laser.

6. The control method of the laser device according to claim 5, wherein, In the second control step, the temperature controller is controlled so that the monitored value becomes a target monitored value corresponding to a target frequency set as the frequency of the laser.

7. The control method of the laser device according to claim 6, wherein, In the second control step, while changing the temperature of the monitoring unit, the temperature of the laser element unit is controlled so that the monitored value becomes the target monitored value.

8. The control method of the laser device according to claim 6, wherein, In the second control step, the temperatures of the laser element unit and the monitoring unit are controlled together. The second control step includes: a correction step of correcting the target monitored value based on the temperature dependence of the frequency of the laser in the laser element unit and the temperature dependence of the monitored value in the monitoring unit on the frequency of the laser; and a control step of changing the corrected target monitored value while controlling the temperature controller so that the monitored value becomes the corrected target monitored value.

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