Wavelength-tunable laser device and method for controlling wavelength-tunable laser device

Abstract

To provide a wavelength variable laser device and a control method of the wavelength variable laser device, in which a stability of a frequency of a laser beam output when continuously changing the frequency in a wide band is high.SOLUTION: In a wavelength variable laser device continuously sweeping a frequency of a laser beam output to an external part, a control part continuously sweeps the frequency of the laser beam while outputting the laser beam to the external part while oscillating a first wavelength variable laser element, sets the number of laser oscillation frequency of a second wavelength variable laser element to a first frequency during the sweeping to oscillate the laser, and an output of the laser beam to the external part is disconnected in order to be a stand-by state. In the case where it is determined that the frequency of the laser beam of the first wavelength variable laser element is appropriately matched to the first frequency, the output to the external part of the laser beam of the first wavelength variable laser element is disconnected, and the stand-by state of the second wavelength variable laser element is cancelled to output the laser beam of the second wavelength variable laser element to the external part as at least one step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] As a light source that continuously changes the wavelength (or frequency corresponding to the wavelength) of laser light over a wide band, a technology is known that includes multiple wavelength-tunable lasers with different wavelength-tunable bands and switches between wavelength-tunable laser elements that output laser light (Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-115411 [Patent Document 2] International Publication No. 2006 / 089802 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, there has been an increasing demand for continuously changing the frequency of laser light over a wide band in light sources for FMCW (Frequency Modulated Continuous Wave)-LiDAR, OCT (Optical Coherence Tomography), OFDR (Optical Frequency Domain Reflectometry), etc. However, with known techniques, the frequency of the output laser light may become unstable when switching between tunable laser elements that output laser light.

[0005] The present invention has been made in view of the above, and aims to provide a wavelength-tunable laser device and a method for controlling a wavelength-tunable laser device that have high stability in the frequency of output laser light when the frequency is changed continuously over a wide band. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a wavelength-tunable laser device that includes a plurality of wavelength-tunable laser elements including a first wavelength-tunable laser element and a second wavelength-tunable laser element that are capable of changing their laser oscillation frequency, and a control unit, and that continuously sweeps the frequency of laser light to be output to the outside, wherein the control unit performs at least once the following steps: oscillating the first wavelength-tunable laser element to output laser light to the outside while continuously sweeping the frequency of the laser light to a first frequency; during the sweeping, setting the laser oscillation frequency of the second wavelength-tunable laser element to the first frequency to cause laser oscillation; and blocking the output of laser light to the outside and putting the second wavelength-tunable laser element into a standby state; and when it is determined that the frequency of the laser light of the first wavelength-tunable laser element approximately matches the first frequency, blocking the output of laser light from the first wavelength-tunable laser element to the outside, canceling the standby state of the second wavelength-tunable laser element, and causing the second wavelength-tunable laser element to output laser light to the outside.

[0007] The control unit may cause the first wavelength tunable laser element and the second wavelength tunable laser element to oscillate at a predetermined frequency based on control parameters stored in a storage unit.

[0008] If the number of wavelength-tunable laser elements included in the plurality of wavelength-tunable laser elements is M, and the number of times that the control unit blocks the output of laser light from the first wavelength-tunable laser element to the outside and releases the standby state of the second wavelength-tunable laser element to output laser light from the second wavelength-tunable laser element to the outside is N, then N≧M may be satisfied.

[0009] The wavelength tunable laser element may be one in which mode hopping, in which the laser oscillation frequency changes discontinuously, occurs.

[0010] The frequency width for sweeping the frequency of the laser light of the first wavelength tunable laser element or the second wavelength tunable laser element may be smaller than three times the frequency interval of the resonator mode of the first wavelength tunable laser element or the second wavelength tunable laser element.

[0011] The laser beam source may further include a monitor unit for monitoring the frequency of laser light, and the case where the control unit determines that the frequency of the laser light of the first wavelength-tunable laser element has approximately matched the first frequency may be a case where the control unit monitors the laser oscillation frequency of the second wavelength-tunable laser element in a state where the laser oscillation frequency is set to the first frequency using the monitor unit, and determines that the monitored laser oscillation frequency has approximately matched the current laser oscillation frequency of the wavelength-tunable laser element being swept.

[0012] The monitor unit may include a filter having a periodic response characteristic with respect to the frequency of light, and the laser light from the first wavelength-tunable laser element may be input from one side of the filter, and the laser light from the second wavelength-tunable laser element may be input from the other side of the filter.

[0013] The monitor unit may include a filter having a periodic response characteristic with respect to the frequency of light, and a changer for changing the frequency characteristic of the filter.

[0014] The monitor section may have a periodic response characteristic with respect to the frequency of light and may include a plurality of filters having different frequency characteristics.

[0015] The monitor unit may be configured to monitor the frequency of the laser light by heterodyne detection.

[0016] The second wavelength-tunable laser element may be oscillated to output laser light to the outside while continuously sweeping the frequency of the laser light, and the frequency width over which the frequency of the laser light of the first wavelength-tunable laser element is continuously swept may be different from the frequency width over which the frequency of the laser light of the second wavelength-tunable laser element is continuously swept.

[0017] One aspect of the present invention is a control method for a wavelength-tunable laser device that includes a plurality of wavelength-tunable laser elements, including a first wavelength-tunable laser element and a second wavelength-tunable laser element, each capable of changing its laser oscillation frequency, and a control unit, and that continuously sweeps the frequency of laser light output to the outside. The control unit performs the following steps at least once: oscillating the first wavelength-tunable laser element to output laser light to the outside while continuously sweeping the frequency of the laser light to a first frequency; during the sweeping, setting the laser oscillation frequency of the second wavelength-tunable laser element to the first frequency to cause laser oscillation; blocking the output of laser light to the outside and putting the second wavelength-tunable laser element into a standby state; and, when it is determined that the frequency of the laser light of the first wavelength-tunable laser element approximately matches the first frequency, blocking the output of laser light from the first wavelength-tunable laser element to the outside, canceling the standby state of the second wavelength-tunable laser element, and causing the second wavelength-tunable laser element to output laser light to the outside. [Effects of the Invention]

[0018] According to the present invention, when the frequency is changed continuously over a wide band, the frequency of the output laser light is highly stable. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a schematic diagram of a wavelength tunable laser device according to the first embodiment. [Figure 2] FIG. 2 is a diagram illustrating a configuration of an example of a wavelength tunable laser element. [Figure 3] FIG. 3 is an explanatory diagram of the adjustment of the laser oscillation frequency. [Figure 4] FIG. 4 is a diagram of table data showing an example of the relationship between the laser oscillation frequency and the drive power of the first to third heaters. [Figure 5] FIG. 5 is a diagram illustrating an example of the configuration of the monitor unit. [Figure 6] FIG. 6 is an explanatory diagram of an example of a frequency monitor. [Figure 7] FIG. 7 is a diagram illustrating an example of sweeping the frequency of a laser beam. [Figure 8] FIG. 8 is a flow diagram of an example of sweeping the frequency of laser light. [Figure 9] FIG. 9 is a schematic diagram of a wavelength tunable laser device according to the second embodiment. [Figure 10] FIG. 10 is a schematic diagram of a wavelength tunable laser device according to the third embodiment. [Figure 11] FIG. 11 is an explanatory diagram of a plurality of discrimination curves. [Figure 12] FIG. 12 is a diagram showing an example of a configuration for preparing a plurality of discrimination curves. [Figure 13] FIG. 13 is a diagram showing another example of a configuration in which a plurality of discrimination curves are prepared. [Figure 14] FIG. 14 is an explanatory diagram of an example in which the frequency widths of the two sweeps are different. [Figure 15] FIG. 15 is a diagram illustrating a configuration of yet another example of the monitor unit. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the embodiments described below. In addition, in the description of the drawings, the same or corresponding elements are appropriately designated by the same reference numerals.

[0021] (Embodiment 1) Fig. 1 is a configuration diagram of a wavelength tunable laser device according to embodiment 1. In Fig. 1, solid arrows indicate input and output of light, and dashed arrows indicate input and output of electrical signals and power. The wavelength tunable laser device 100 continuously sweeps the frequency of laser light output to the outside.

[0022] The wavelength-tunable laser device 100 includes wavelength-tunable laser elements 11 and 12, which are multiple wavelength-tunable laser elements capable of changing the laser oscillation frequency, optical couplers 21 and 22, an output switching unit 30, a monitor unit 40, and a control unit 50. The wavelength-tunable laser element 11 is an example of a first wavelength-tunable laser element or a second wavelength-tunable laser element, and the wavelength-tunable laser element 12 is an example of a second wavelength-tunable laser element or a first wavelength-tunable laser element. That is, the multiple wavelength-tunable laser elements include a first wavelength-tunable laser element and a second wavelength-tunable laser element.

[0023] FIG. 2 is a configuration diagram of an example of a wavelength-tunable laser element 11. The wavelength-tunable laser element 11 is a vernier-type wavelength-tunable laser element including a first reflecting mirror 11a, a gain section 11b, a phase adjusting section 11c, and a second reflecting mirror 11d. The first reflecting mirror 11a is a sampled grating-distrobuted Bragg reflector (SG-DBR) mirror whose reflection spectrum has periodic peaks with respect to frequency. The second reflecting mirror 11d is an SG-DBR mirror whose reflection spectrum has periodic peaks with respect to frequency at a different period from that of the first reflecting mirror 11a. The first reflecting mirror 11a and the second reflecting mirror 11d form a laser resonator. The second reflecting mirror may be a reflecting mirror using a ring resonator filter, as disclosed in Japanese Patent Application Laid-Open No. 2016-178283.

[0024] The gain section 11b is disposed within the laser resonator, and generates optical gain when supplied with drive power from the control section 50. The phase adjustment section 11c is a waveguide disposed within the laser resonator.

[0025] The first reflecting mirror 11a is provided with a first heater. The first heater receives driving power from the control unit 50 to heat the first reflecting mirror 11a. This heating controls the reflection spectrum of the first reflecting mirror 11a. The second reflecting mirror 11d is provided with a second heater. The second heater receives driving power from the control unit 50 to heat the second reflecting mirror 11d. This heating controls the reflection spectrum of the second reflecting mirror 11d. The phase adjusting unit 11c is provided with a third heater. The third heater receives driving power from the control unit 50 to heat the phase adjusting unit 11c. This heating adjusts the cavity length of the laser cavity. Adjusting the cavity length allows the frequency of the longitudinal mode (cavity mode) of the laser cavity to be controlled.

[0026] By adjusting the driving power supplied to each of the first heater, the second heater, and the third heater, the wavelength tunable laser element 11 oscillates at a frequency at which the reflection peak of the first reflecting mirror 11a, the resonator mode of the laser resonator, and the reflection peak of the second reflecting mirror 11d approximately match, and outputs laser light L1 which is CW (continuous wave) light. That is, the first heater, the second heater, and the third heater constitute a plurality of control elements that control the laser oscillation frequency of the wavelength tunable laser element 11 by supplying driving power thereto.

[0027] 1, the wavelength tunable laser element 12 may have the same configuration as the wavelength tunable laser element 11. The wavelength tunable laser element 12 outputs laser light L2, which is CW light.

[0028] The optical coupler 21 receives the laser light L1 output from the wavelength tunable laser element 11, splits the laser light L1 into laser light L11 and laser light L12, outputs the laser light L11 to the output switching unit 30, and outputs the laser light L12 to the monitor unit 40.

[0029] The optical coupler 22 receives the laser light L2 output from the wavelength tunable laser element 12, splits the laser light L2 into laser light L21 and laser light L22, outputs the laser light L21 to the output switching unit 30, and outputs the laser light L22 to the monitor unit 40.

[0030] The output switching unit 30 receives the laser beams L11 and L21 and selectively outputs one of the laser beams L11 and L21. In the first embodiment, the output switching unit 30 includes light blocking elements 31 and 32 and an optical coupler 33. The laser beam L11 is received by the light blocking element 31, and passes or blocks the laser beam L11 under the control of the control unit 50. The laser beam L21 is received by the light blocking element 32, and passes or blocks the laser beam L21 under the control of the control unit 50. The control unit 50 controls the light blocking elements 31 and 32 so that the light blocking element 32 blocks the laser beam L21 while the light blocking element 31 passes the laser beam L11, and blocks the laser beam L11 while the light blocking element 32 passes the laser beam L21.

[0031] The light-blocking elements 31 and 32 can be configured using, for example, semiconductor amplifiers. In this case, when the light-blocking elements 31 and 32 block the laser light, the semiconductor optical amplifiers are controlled to operate in reverse bias, and when the light-blocking elements 31 and 32 allow the laser light to pass, the semiconductor optical amplifiers are controlled to operate in forward bias. The light-blocking elements 31 and 32 can also be configured using Mach-Zehnder optical switches or 2 × 1 optical switches.

[0032] The optical coupler 33 outputs the laser light L11 that has passed through the light blocking element 31 or the laser light L21 that has passed through the light blocking element 32 to the outside of the wavelength tunable laser device 100 as the selected laser light L3.

[0033] The laser beams L21 and L22 are input to the monitor unit 40. The laser beams L21 and L22 are used to monitor the frequencies of the laser beams L11 and L12. The configuration of the monitor unit 40 will be described in detail later.

[0034] The control unit 50 controls the power supplied to the gain units and the first to third heaters of the wavelength tunable laser elements 11 and 12. It also controls the operation of the light blocking elements 31 and 32.

[0035] The control unit 50 includes a calculation unit, a memory unit, an input unit, an output unit, and a power supply unit. The calculation unit includes, for example, a CPU, and performs various calculation processes for control. The memory unit includes a memory unit such as a ROM that stores various programs and data used by the calculation unit to perform the calculation processes, and a memory unit such as a RAM that is used, for example, as a workspace when the calculation unit performs the calculation processes and to store the results of the calculation processes of the calculation unit.

[0036] The input unit receives instruction signals from a higher-level device of the wavelength-tunable laser device 100, current signals from the monitor unit 40, and the like. Information contained in the received signals is stored in the memory unit. The input unit includes, for example, an analog-to-digital converter (ADC). The output unit receives instruction signals generated by the calculation unit through calculation processing, converts them into appropriate instruction signals, and outputs them to the power supply unit, light-blocking elements 31, 32, and the like. The output unit includes, for example, a digital-to-analog converter (DAC). The power supply unit supplies power to the wavelength-tunable laser elements 11, 12 based on the instruction signals.

[0037] (Laser oscillation frequency adjustment) Next, adjustment of the laser oscillation frequency will be explained. Figure 3 is an explanatory diagram of adjustment of the laser oscillation frequency. The upper part shows the reflection spectrum of the first reflecting mirror, the middle part shows the reflection spectrum of the second reflecting mirror, and the lower part shows the spectrum of the resonator mode.

[0038] When the drive power supplied to the first heater is adjusted and controlled, the reflection spectrum of the first reflecting mirror shifts on the frequency axis from the shape shown by the solid line to the shape shown by the dashed line, as indicated by the thick arrow. Similarly, when the drive power supplied to the second heater is adjusted and controlled, the reflection spectrum of the second reflecting mirror shifts on the frequency axis from the shape shown by the solid line to the shape shown by the dashed line. When the drive power supplied to the third heater is adjusted and controlled, the spectrum of the resonator mode shifts on the frequency axis from the shape shown by the solid line to the shape shown by the dashed line.

[0039] In the state indicated by the solid line, laser oscillation occurs at a frequency fy where the reflection peak of the first reflecting mirror 11a, the resonator mode of the laser resonator, and the reflection peak of the second reflecting mirror 11d coincide. To achieve this state, the first and second heaters are each set to the frequency positions where the reflection spectra of the first reflecting mirror 11a and the second reflecting mirror 11d peak based on the power supplied. The third heater is also set to the frequency position where the resonator mode peaks based on the power supplied. By controlling each heater to achieve the state indicated by the dashed line, the wavelength where the reflection peak of the first reflecting mirror 11a, the resonator mode of the laser resonator, and the reflection peak of the second reflecting mirror 11d coincide can be set to frequency fx, thereby changing the laser oscillation frequency to frequency fx. By finely adjusting the drive power when controlling each heater, the laser oscillation frequency can be continuously (swept) finely adjusted while maintaining the coincidence between the resonator mode and the two reflection peaks. The drive power to each heater can be controlled by the current supplied.

[0040] The relationship between the laser oscillation frequency and the driving power of the first to third heaters for each of the wavelength-tunable laser elements 11 and 12 is stored in the memory of the control unit 50 and is referred to when setting the laser oscillation frequency. FIG. 4 is a diagram of table data showing an example of the relationship between the laser oscillation frequency and the driving power of the first to third heaters. In FIG. 4, "A" in "laser element" refers to the setting related to the wavelength-tunable laser element 11, and "B" refers to the setting related to the wavelength-tunable laser element 12. In the example shown in FIG. 4, when the laser oscillation frequency of the wavelength-tunable laser element 11 is set to frequency f0, the driving powers of the first to third heaters are set to W1_0, W2_0, and W3_0, respectively. The driving powers of the first to third heaters are an example of control parameters for causing the wavelength-tunable laser element to oscillate at a predetermined frequency.

[0041] (Monitor configuration) Next, a description will be given of the configuration of the monitor unit 40. Fig. 5 is a diagram showing an example of the configuration of the monitor unit 40. The monitor unit 40 includes an optical switch 41, a filter 42, and a photodetector (PD) 43.

[0042] Under the control of the control unit 50, the optical switch 41 selectively passes the laser light L12 or the laser light L22 as the laser light L4. The filter 42 is an example of a filter having periodic response characteristics (transmission characteristics or reflection characteristics) with respect to the frequency of light, and in this embodiment, it is a filter having periodic transmission characteristics with respect to the frequency of light. Such a filter 42 can be configured using, for example, an etalon filter, a ring resonator filter, a Michelson interferometer, or the like. The filter 42 transmits the laser light L4 with a transmittance according to the frequency of the laser light L4. The laser light L4 after passing through the filter 42 is set as laser light L5.

[0043] The photodetector 43 includes, for example, a photodiode, receives the laser light L5, and outputs to the control unit 50 a current signal having a current value corresponding to the intensity of the received light.

[0044] The control unit 50 monitors the frequency of the laser light L5 based on the correspondence relationship between the value of the current signal (PD current value) from the photodetector 43 and the laser oscillation frequency. Such correspondence relationship is determined in advance by an experiment or the like, and is stored in the storage unit as table data or a relational expression.

[0045] FIG. 6 is an explanatory diagram of an example of a frequency monitor. In FIG. 6, the horizontal axis represents the optical frequency, and the vertical axis represents the PD current value normalized by the maximum value. The curve shown in FIG. 6 has a shape corresponding to the transmission spectrum of the filter 42. Such a curve is also called a discrimination curve. In the case shown in FIG. 6, for example, when the PD current value is Ipd, the control unit 50 determines that the frequency of the laser light L5 is 192.25 THz (approximately 1559 nm in wavelength) based on the discrimination curve.

[0046] The control unit 50 then controls the drive power to the first to third heaters so that the PD current value corresponds to a desired laser oscillation frequency, thereby enabling feedback control of the laser oscillation frequency of the wavelength tunable laser element 12.

[0047] (Continuous sweep of laser light frequency) Next, the continuous sweep of the frequency of the laser light output in the wavelength-tunable laser device 100 will be described. Fig. 7 is an explanatory diagram of an example of sweeping the frequency of the laser light. Fig. 7 shows a case where the frequency of the output laser light is continuously swept from frequency f1 (wavelength λ1) to a frequency smaller than frequency f3 (wavelength λ3). In the graph of Fig. 7, the horizontal axis represents elapsed time, and the vertical axis represents wavelength (frequency).

[0048] In this case, the control unit 50 first performs step A. Specifically, as shown by the solid line l31 in the graph of FIG. 7(a), the control unit 50 continuously sweeps the frequency of the laser light from frequency f1 to frequency f2, which is an example of a first frequency, while oscillating the wavelength-tunable laser element 11 to output the laser light to the outside. This sweep can be performed, for example, by continuously changing the driving power of the first to third heaters of the wavelength-tunable laser element 11 until the frequency reaches f2. The control unit 50 then performs step B. Specifically, during this sweep, the control unit 50 sets the wavelength-tunable laser element 12 to frequency f2 and oscillates the laser light, as shown by the dashed line l32', while blocking the output of the laser light to the outside and putting the device into a standby state. In steps A and B, the control unit 50 controls the light-blocking element 31 to pass the laser light L11 and the light-blocking element 32 to block the laser light L21. As a result, the wavelength-tunable laser device 100 continuously sweeps the frequency of the laser light (laser light L3 (L11) in FIG. 1) output to the outside. In the case of such continuous sweeping, the wavelength-tunable laser element may be feedforward controlled. Also, for example, when the wavelength-tunable laser element 12 is set to oscillate at frequency f2, the wavelength-tunable laser element 12 may be feedback controlled so that the laser oscillation frequency becomes frequency f2.

[0049] Next, the control unit 50 performs step C. That is, when it is determined that the frequency of the laser light from the wavelength-tunable laser element 11 substantially matches the frequency f2, it blocks the output of the laser light from the wavelength-tunable laser element 11 to the outside, and it releases the standby state of the wavelength-tunable laser element 12 to output the laser light from the wavelength-tunable laser element 12 to the outside. Note that the determination that the frequency of the laser light from the wavelength-tunable laser element 11 substantially matches the frequency f2 is made when the frequency of the laser light from the wavelength-tunable laser element 11 completely matches the frequency f2 or when the frequency of the laser light from the wavelength-tunable laser element 11 arrives within a predetermined error range from the frequency f2.

[0050] Next, the control unit 50 performs Steps A to C for the second time. However, this time, in Step A, the control unit 50 continuously sweeps the frequency of the laser light from frequency f2 to frequency f3, which is another example of the first frequency, while oscillating the wavelength-tunable laser element 12 to output the laser light to the outside, as shown by the solid line l32 in the graph of FIG. 7(a). Furthermore, in Step B, during this sweep, the control unit 50 sets the wavelength-tunable laser element 11 to frequency f3 and oscillates the laser light, as shown by the dashed line l31', and blocks the output of the laser light to the outside, putting the laser element 11 into a standby state. When setting the laser oscillation frequency in this way, the laser light output from the wavelength-tunable laser element 11 may temporarily become unstable in frequency, as shown by the spikes in the graph of FIG. 7(a). This phenomenon of frequency instability occurs, for example, due to mode hopping if the wavelength-tunable laser element 11 is a wavelength-tunable laser element that can cause mode hopping, in which the laser oscillation frequency changes discontinuously. However, in step B, the laser light from the wavelength tunable laser element 11 is in a standby state with its output to the outside blocked, so that laser light of an unstable frequency or an unintended frequency is not output to the outside.

[0051] Therefore, if the laser oscillation frequency that is prone to mode hopping is known in advance for a certain wavelength tunable laser element, it is preferable to not perform continuous sweep within the frequency range that includes that frequency but to put the laser element into a standby state. For example, if the wavelength tunable laser element is a vernier type, mode hopping is likely to occur when the supermode changes.

[0052] Next, the control unit 50 performs step C. That is, when it is determined that the frequency of the laser light from the wavelength-tunable laser element 12 substantially matches the frequency f3, it blocks the output of the laser light from the wavelength-tunable laser element 12 to the outside, and also releases the standby state of the wavelength-tunable laser element 11 to output the laser light from the wavelength-tunable laser element 11 to the outside. These steps A to C are repeatedly performed until the set end frequency of the continuous sweep is reached.

[0053] The above steps A to C are performed at least once depending on the start frequency and end frequency of the continuous sweep.

[0054] In the wavelength tunable laser device 100 configured as above, as shown in the graph of FIG. 7(b), the frequency of the output laser light is highly stable when the frequency is changed continuously over a wide band.

[0055] Furthermore, in the wavelength-tunable laser device 100, a common monitor unit 40 is used to monitor the frequencies of the laser beams of the two wavelength-tunable laser elements 11 and 12, so that the relative difference between the monitored values ​​of the frequencies of the respective laser beams can be reduced.

[0056] In order to suppress mode hopping, the frequency width (the difference between the sweep start frequency and the sweep end frequency) for sweeping the frequency of the laser light of the wavelength-tunable laser element 11 or 12 is preferably smaller than three times the frequency interval of the resonator mode of the wavelength-tunable laser element 11 or 12. The lower limit of this range is not particularly limited, but may be, for example, one time the frequency interval of the resonator mode.

[0057] The operation of the wavelength-tunable laser device 100 will be further described. Fig. 8 is a flow diagram of an example of sweeping the frequency of laser light. The flow diagram shown in Fig. 8 starts when the wavelength-tunable laser element 11 (hereinafter referred to as LD(A) as appropriate) is outputting laser light to the outside while the frequency of the laser light is continuously swept from frequency f1 to frequency f2, the wavelength-tunable laser element 12 (hereinafter referred to as LD(B) as appropriate) is in a standby state while oscillating at frequency f2, and it is determined that the frequency of the laser light from LD(A) has approximately matched frequency f2.

[0058] In this case, in step S101, the control unit 50 blocks the output of the laser light from LD(A) to the outside. Subsequently, in step S102, the control unit 50 releases the blockage of the output of the laser light from LD(B) to the outside, and starts sweeping the laser light from LD(B).

[0059] Subsequently, in step S103, the control unit 50 changes the driving conditions of the LD(A) so that the target frequency value of the laser oscillation frequency of the LD(A) becomes f3, and further performs feedback control.

[0060] Subsequently, in step S104, the control unit 50 determines whether the laser oscillation frequency of the LD(A) has stabilized. Whether the laser oscillation frequency has stabilized is determined, for example, by determining whether the frequency of the laser light L22 (see FIGS. 1 and 5) monitored by the monitor unit 40 for a predetermined period or a predetermined number of monitoring times is within a predetermined error range.

[0061] If it is determined that the laser oscillation frequency is not stable (step S104, No), the control unit 50 executes step S104 again. If it is determined that the laser oscillation frequency is stable (step S104, Yes), the control unit 50 executes step S105.

[0062] In step S105, the control unit 50 stores the frequency of the laser light L22 monitored by the monitor unit 40 in the storage unit as a switching threshold value.

[0063] Next, in step S106, the control unit 50 determines whether the error between the current frequency of the LD(B) being swept and the switching threshold value has fallen below a certain value. Step S106 is an example of a step of determining that the frequency of the laser light of LD(B) has approximately matched the frequency f3.

[0064] If it is determined that the error is not within the certain value (step S106, No), the control unit 50 executes step S106 again. If it is determined that the error is within the certain value, the control unit 50 executes step S107.

[0065] In step S107, the control unit 50 blocks the output of the laser light from LD(B) to the outside. Subsequently, in step S108, the control unit 50 releases the blockage of the output of the laser light from LD(A) to the outside, and starts sweeping the laser light from LD(A).

[0066] 8, the control unit 50 determines that the frequency of the laser light from LD(B) has approximately matched the frequency f3 when the control unit 50 monitors the laser oscillation frequency of LD(A) with the laser oscillation frequency set to frequency f3 using the monitor unit 40 and determines that the monitored laser oscillation frequency (switching threshold) and the current laser oscillation frequency of the wavelength-tunable laser element being swept have approximately matched. This more reliably prevents discontinuous changes in the frequency of the laser light output when the wavelength-tunable laser element being swept is switched.

[0067] 8, if the number of wavelength-tunable laser elements included in the plurality of wavelength-tunable laser elements is M and the number of times the control unit 50 blocks the first wavelength-tunable laser element from outputting laser light to the outside and releases the standby state of the second wavelength-tunable laser element to output laser light from the second wavelength-tunable laser element to the outside is N, then N≧M holds. Specifically, in the flow shown in FIG. 8, M is 2 and N is also 2. Thus, when N≧M holds, the frequency of the laser light can be swept over a relatively wide band compared to the number of wavelength-tunable laser elements.

[0068] (Embodiment 2) 9 is a schematic configuration diagram of a wavelength tunable laser device according to embodiment 2. The wavelength tunable laser device 100A has a configuration in which the optical couplers 21 and 22, the output switching unit 30, and the monitor unit 40 of the wavelength tunable laser device 100 shown in FIG. 1 are replaced with optical couplers 21A and 22A, an output switching unit 30A, and a monitor unit 40A, respectively.

[0069] The optical coupler 21A receives the laser light L1 output from the wavelength tunable laser element 11, splits the laser light L1 into laser light L11 and laser light L12, outputs the laser light L11 to the output switching unit 30A, and outputs the laser light L12 to the monitor unit 40A.

[0070] The optical coupler 22A receives the laser light L2 output from the wavelength tunable laser element 12, splits the laser light L2 into laser light L21 and laser light L22, outputs the laser light L21 to the output switching unit 30, and outputs the laser light L22 to the monitor unit 40.

[0071] The output switching unit 30A has a 2×1 optical switch that receives the laser beams L11 and L21 and selectively outputs one of the laser beams L11 and L21 under the control of the control unit 50. The output switching unit 30A can be configured using, for example, a Mach-Zehnder type optical switch.

[0072] The monitor unit 40A includes a filter 42 and photodetectors 43A1 and 43A2. The filter 42 receives the laser light L12 from the tunable laser element 11 through a first port 42a, and receives the laser light L22 from the tunable laser element 12 through a second port 42b. After passing through the filter 42, the laser light L12 is transmitted as laser light L13, which is output from the second port 42b. After passing through the filter 42, the laser light L22 is transmitted as laser light L23, which is output from the first port 42a.

[0073] The optical coupler 21A receives the laser beam L23 and outputs it to the photodetector 43A2. The optical coupler 22A receives the laser beam 13 and outputs it to the photodetector 43A1.

[0074] The photodetector 43A1 receives the laser beam L13 and outputs a current signal having a current value corresponding to the intensity of the received light to the control unit 50. The photodetector 43A2 receives the laser beam L23 and outputs a current signal having a current value corresponding to the intensity of the received light to the control unit 50.

[0075] The control unit 50 monitors the frequencies of the laser beams L13 and L23 based on the correspondence relationship between the PD current values ​​of the photodetectors 43A1 and 43A2 and the laser oscillation frequencies. Such correspondence relationship is determined in advance by experiments or the like, and is stored in the storage unit as table data or a relational expression.

[0076] The wavelength-tunable laser device 100A configured as described above has high stability of the frequency of the output laser light when continuously changing the frequency over a wide band, similar to the wavelength-tunable laser device 100. Furthermore, the wavelength-tunable laser device 100A can simultaneously monitor the frequency of the laser light from the wavelength-tunable laser element being transmitted and the frequency of the laser light from the wavelength-tunable laser element in a standby state, so that frequency sweeping and standby state frequency setting can be performed separately and simultaneously for each laser light. This allows for more accurate and continuous sweeping.

[0077] (Embodiment 3) Fig. 10 is a schematic configuration diagram of a wavelength tunable laser device according to embodiment 3. The wavelength tunable laser device 100B has a configuration in which the monitor unit 40A of the wavelength tunable laser device 100A shown in Fig. 9 is replaced with a monitor unit 40B, and optical couplers 21B and 22B are added.

[0078] The monitor unit 40B has a configuration in which photodetectors 43B1 and 43B2 are added to the monitor unit 40A.

[0079] Optical coupler 21B transmits laser beams L12 and L23 and extracts laser beam L14, which is a portion of laser beam L12, and outputs it to photodetector 43B1. Optical coupler 22B transmits laser beams L13 and L22 and extracts laser beam L24, which is a portion of laser beam L22, and outputs it to photodetector 43B2.

[0080] The photodetector 43B1 receives the laser beam L14 and outputs a current signal having a current value corresponding to the intensity of the received light to the control unit 50. The photodetector 43B2 receives the laser beam L24 and outputs a current signal having a current value corresponding to the intensity of the received light to the control unit 50.

[0081] The control unit 50 monitors the frequency of the laser beam L13 based on the correspondence relationship between the ratio of the PD current value of the photodetector 43A1 to the PD current value of the photodetector 43B1 (PD current ratio) and the laser oscillation frequency. The control unit 50 also monitors the frequency of the laser beam L23 based on the correspondence relationship between the ratio of the PD current value of the photodetector 43A2 to the PD current value of the photodetector 43B2 (PD current ratio) and the laser oscillation frequency. Such correspondence relationships are determined in advance by experiments or the like and stored in the storage unit as table data or relational expressions.

[0082] In the wavelength-tunable laser device 100B configured as described above, similarly to the wavelength-tunable laser device 100A, when the frequency is changed continuously over a wide band, the frequency of the output laser light is highly stable and more accurate or continuous sweeping can be achieved.

[0083] Furthermore, the wavelength-tunable laser device 100B monitors the frequency of the laser beam based on the PD current ratio. For example, if the intensity of the laser beam from the wavelength-tunable laser element changes over time, the PD current value changes along with the change in the intensity of the laser beam, but the PD current ratio remains approximately constant, so that the accuracy of monitoring the frequency of the laser beam is prevented from decreasing.

[0084] (An example of the monitor filter configuration) In the example described with reference to Fig. 6, the control unit 50 determines the frequency of the laser light based on the PD current value and the discrimination curve. In Fig. 11, in the region indicated by the double-headed arrow Ar between the two dashed lines, the change in the PD current value relative to the change in frequency is large (the gradient of the discrimination curve is steep), so the accuracy of frequency determination is relatively high. However, outside the range indicated by the double-headed arrow Ar, the gradient of the discrimination curve is small, so the accuracy of frequency determination is relatively low. Such a region outside the range is also called a dead zone.

[0085] Therefore, as shown in FIG. 11, for example, by preparing multiple discrimination curves C1 and C2 with mutually different phases, and using the discrimination curve C1 when monitoring the frequency represented by point P1, and using the discrimination curve C2 when monitoring the frequency represented by point P2, it is possible to achieve relatively high determination accuracy regardless of frequency.

[0086] Fig. 12 is a diagram showing an example of a configuration for preparing a plurality of discrimination curves. In the configuration shown in Fig. 12, the monitor unit includes a filter 42 and a change means 60 that changes the frequency characteristics of the filter 42. The change means 60 is configured using a temperature adjustment element such as a Peltier element or a stress application element such as a piezoelectric element. The change means 60 can change the frequency characteristics of the filter 42 to, for example, the discrimination curves C1 and C2 shown in Fig. 11 by changing the temperature of the filter 42 or the stress applied to the filter 42.

[0087] Fig. 13 is a diagram showing another example of a configuration in which a plurality of discrimination curves are prepared. In the configuration shown in Fig. 13, monitor unit 40C has a configuration in which filter 42 is replaced with filters 42C1 and 42C2, photodetector 43 is replaced with photodetectors 43C1 and 43C2, and optical coupler 44C is added to the configuration of monitor unit 40 shown in Fig. 5. Filters 42C1 and 42C2 are an example of a plurality of filters that have periodic response characteristics with respect to the frequency of light and have different frequency characteristics from each other.

[0088] Under the control of the control unit 50, the optical switch 41 selectively passes the laser light L12 or the laser light L22 as the laser light L4. The optical coupler 44C branches the laser light L4 into the laser light L41 and the laser light L42. The filter 42C1 has characteristics such as the discrimination curve C1 in FIG. 11 and transmits the laser light L41 with a transmittance corresponding to the frequency of the laser light L41. The laser light L41 after passing through the filter 42C1 is designated as the laser light L51. The photodetector 43C1 receives the laser light L51 and outputs a current signal having a current value corresponding to the intensity of the received light to the control unit 50. The filter 42C2 has characteristics such as the discrimination curve C2 in FIG. 11 and transmits the laser light L42 with a transmittance corresponding to the frequency of the laser light L42. The laser light L42 after passing through the filter 42C2 is designated as the laser light L52. The photodetector 43C2 receives the laser light L52 and outputs to the control unit 50 a current signal having a current value corresponding to the intensity of the received light.

[0089] The control unit 50 monitors the frequency of the laser beam L51 or L52 based on the correspondence relationship between the PD current value from the photodetector 43C1 or 43C2 and the laser oscillation frequency. Such correspondence relationship is determined in advance by an experiment or the like and stored in the storage unit as table data or a relational expression.

[0090] The control unit 50 may monitor the frequencies of the laser beams L51 and L52 based on the correspondence between a calculated value (e.g., a linearly combined value) obtained by calculating the PD current value from the photodetector 43C1 and the PD current value from the photodetector 43C2 and the laser oscillation frequency. Such a calculated value includes the effect of a discrimination curve of a filter that is not insensitive to the frequencies of the laser beams L51 and L52, and therefore, the determination accuracy can be relatively high regardless of the frequency.

[0091] (Sweep frequency width) In the above embodiment, the frequency width Δf1 for continuously sweeping the frequency of the laser light from the wavelength-tunable laser element 11 and the frequency width Δf2 for continuously sweeping the frequency of the laser light from the wavelength-tunable laser element 12 may be the same as or different from each other.

[0092] 14 is an explanatory diagram of an example in which the frequency widths of the two sweeps are different. As shown in FIG. 14(a), in this example, the laser oscillation frequency of the wavelength-tunable laser element 11 is continuously swept from frequency fa (wavelength λa) to frequency fb (wavelength λb), and Δf1 = |fb - fa|. On the other hand, the laser oscillation frequency of the wavelength-tunable laser element 12 is continuously swept from frequency fb (wavelength λb) to frequency fc (wavelength λc), and Δf2 = |fc - fb|. And, Δf2 > Δf1. In this example, the wavelength-tunable laser element that outputs laser light to the outside is switched between frequencies fa, fb, and fc.

[0093] In this case, as shown by the circle, triangle, and square symbols on the discrimination curve in Figure 14(b), none of the switching frequencies are in the dead zone, but are in the region indicated by the arrow Ar where the wavelength determination accuracy is relatively high. In this way, the frequency accuracy of the timing to switch the wavelength tunable laser element that outputs laser light to the outside can be made relatively high.

[0094] (Another example of the configuration of the monitor unit) In the above embodiment, for example, the monitor unit 40 is configured using a filter 42 having a periodic response characteristic with respect to the frequency of light, but the configuration of the monitor unit is not limited to this. For example, the monitor unit may be configured to monitor the frequency of the laser light by heterodyne detection, as shown in the following Figure 15.

[0095] 15 is a diagram showing another example of the configuration of the monitor unit. The monitor unit 40D includes an optical multiplexer 41D and a photodetector 43. The optical multiplexer 41D multiplexes the laser beams L12 and L22 and outputs the multiplexed laser beam L6 to the photodetector 43. The photodetector 43 receives the laser beam L6 and outputs a current signal having a current value corresponding to the intensity of the received light to the control unit 50.

[0096] Here, the laser beam L6 contains a difference frequency component between the laser beams L12 and L22. The control unit 50 detects the difference frequency component from the photodetector 43 and determines the frequency difference between the laser beams L12 and L22. If the frequency difference falls within a predetermined range, it is determined that the frequency of the laser beam L12 and the frequency of the laser beam L22 substantially match.

[0097] In the monitor unit 40D having such a configuration, there is no need to control an optical switch to select either the laser light L12 or L22, and this control can be omitted.

[0098] It should be noted that the present invention is not limited to the above-described embodiments. The present invention also includes configurations in which the components of the above-described embodiments are appropriately combined. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible. [Explanation of symbols]

[0099] 11, 12: Tunable laser element 11a: First reflecting mirror 11b: Gain section 11c: Phase adjustment section 11d: Second reflecting mirror 13: Laser light 21, 21A, 21B, 22, 22A, 22B, 33, 44C: Optical couplers 30, 30A: Output switching section 31, 32: Light blocking element 40, 40A, 40B, 40C, 40D: Monitor section 41: Optical switch 41D: Optical multiplexer 42, 42C1, 42C2: Filter 42a: First port 42b: Second port 43, 43A1, 43A2, 43B1, 43B2, 43C1, 43C2: photodetectors 50: Control unit 60: Change method 100, 100A, 100B: Tunable wavelength laser device Ar: Arrow C1, C2: Discrimination curves L1, L11, L12, L13, L14, L2, L21, L22, L23, L24, L3, L4, L41, L42, L5, L51, L52, L6: Laser light l31, l32: solid lines l31´, l32´: dashed lines

Claims

1. a plurality of wavelength-tunable laser elements including a first wavelength-tunable laser element and a second wavelength-tunable laser element of a Vernier type capable of changing a laser oscillation frequency; A control unit; A wavelength tunable laser device that continuously sweeps the frequency of a laser beam output to the outside, comprising: The control unit oscillating the first wavelength tunable laser element to output laser light to the outside, while continuously sweeping the frequency of the laser light up to a first frequency; During the sweep, the laser oscillation frequency of the second wavelength tunable laser element is set to the first frequency to cause laser oscillation, and the output of the laser light to the outside is blocked to enter a standby state; when it is determined that the frequency of the laser light of the first wavelength tunable laser element substantially matches the first frequency, blocking the output of the laser light of the first wavelength tunable laser element to the outside, and releasing the standby state of the second wavelength tunable laser element to output the laser light of the second wavelength tunable laser element to the outside; Steps are performed at least once, When the number of wavelength tunable laser elements included in the plurality of wavelength tunable laser elements is M, and the number of times that the control unit cuts off the output of laser light from the first wavelength tunable laser element to the outside and releases the standby state of the second wavelength tunable laser element to output laser light from the second wavelength tunable laser element to the outside is N, N≧M is established. Tunable wavelength laser device.

2. The control unit controls the first wavelength tunable laser element and the second wavelength tunable laser element to oscillate at a predetermined frequency based on control parameters stored in a storage unit.

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

3. The control unit In the first wavelength tunable laser element or the second wavelength tunable laser element, when the laser oscillation frequency is in a frequency range including a frequency at which the supermode changes, continuous sweeping is not performed and the laser element is placed in a standby state.

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

4. The control unit oscillating the second wavelength tunable laser element to output laser light to the outside, while continuously sweeping the frequency of the laser light up to a second frequency; During the sweep, the laser oscillation frequency of the first wavelength tunable laser element is set to the second frequency to cause laser oscillation, and the output of the laser light to the outside is blocked to enter a standby state; When it is determined that the frequency of the laser light from the second wavelength tunable laser element substantially matches the second frequency, the output of the laser light from the second wavelength tunable laser element to the outside is blocked, and the standby state of the first wavelength tunable laser element is released, and the frequency of the laser light from the first wavelength tunable laser element is continuously swept up to a third frequency while the laser light is output to the outside.

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

5. The frequency width for sweeping the frequency of the laser light of the first wavelength tunable laser element or the second wavelength tunable laser element is smaller than three times the frequency interval of the resonator mode of the first wavelength tunable laser element or the second wavelength tunable laser element, and is a frequency width in which mode hopping is suppressed.

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

6. a monitor unit for monitoring the frequency of the laser light; The case where the control unit determines that the frequency of the laser light of the first wavelength-tunable laser element has approximately matched the first frequency is the case where the control unit monitors the laser oscillation frequency of the second wavelength-tunable laser element in a state where the laser oscillation frequency is set to the first frequency using the monitor unit, and determines that the monitored laser oscillation frequency and the current laser oscillation frequency of the wavelength-tunable laser element being swept approximately match.

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

7. The monitor unit includes a filter having a periodic response characteristic with respect to the frequency of light, and the laser light from the first wavelength tunable laser element is input to one side of the filter, and the laser light from the second wavelength tunable laser element is input to the other side of the filter.

7. The wavelength tunable laser device according to claim 6.

8. The monitor unit includes a filter having a periodic response characteristic with respect to the frequency of light, and a change unit for changing the frequency characteristic of the filter.

7. The wavelength tunable laser device according to claim 6.

9. The monitor unit has a plurality of filters that have periodic response characteristics to the frequency of light and have different frequency characteristics.

7. The wavelength tunable laser device according to claim 6.

10. The monitor unit is configured to monitor the frequency of the laser light by heterodyne detection.

7. The wavelength tunable laser device according to claim 6.

11. oscillating the second wavelength tunable laser element to output laser light to the outside while continuously sweeping the frequency of the laser light; a frequency width for continuously sweeping the frequency of the laser light from the first wavelength tunable laser element and a frequency width for continuously sweeping the frequency of the laser light from the second wavelength tunable laser element are different from each other; The wavelength tunable laser device according to any one of claims 1 to 10.

12. a plurality of wavelength-tunable laser elements including a first wavelength-tunable laser element and a second wavelength-tunable laser element of a Vernier type capable of changing a laser oscillation frequency; A control unit; A method for controlling a wavelength tunable laser device that continuously sweeps the frequency of laser light output to the outside, comprising: The control unit oscillating the first wavelength tunable laser element to output laser light to the outside, while continuously sweeping the frequency of the laser light up to a first frequency; During the sweep, the laser oscillation frequency of the second wavelength tunable laser element is set to the first frequency to cause laser oscillation, and the output of the laser light to the outside is blocked to enter a standby state; when it is determined that the frequency of the laser light of the first wavelength tunable laser element substantially matches the first frequency, blocking the output of the laser light of the first wavelength tunable laser element to the outside, and releasing the standby state of the second wavelength tunable laser element to output the laser light of the second wavelength tunable laser element to the outside; Steps are performed at least once, When the number of wavelength tunable laser elements included in the plurality of wavelength tunable laser elements is M, and the number of times that the control unit cuts off the output of laser light from the first wavelength tunable laser element to the outside and releases the standby state of the second wavelength tunable laser element to output laser light from the second wavelength tunable laser element to the outside is N, N≧M is established. A method for controlling a wavelength tunable laser device.

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