Control method for laser light source and laser device

Abstract

This control method for a laser light source, which includes an optical resonator, a gain unit disposed inside the optical resonator, and a wavelength selection unit in which a transmission characteristic or a reflection characteristic changes with respect to a wavelength in the optical resonator, comprises: a current adjustment step for adjusting a drive current supplied to the laser light source so that there are a first period in which a current value changes with respect to time and a second period in which the current value is constant with respect to time; a resonator length increase / decrease step for increasing / decreasing a resonator length of the optical resonator with respect to a reference length; a fluctuation detection step for detecting fluctuation of the power of laser light generated in the optical resonator, the fluctuation being caused by the resonator length increase / decrease step; and a wavelength adjustment step for bringing the wavelength of the laser light close to a target wavelength on the basis of the fluctuation detected in the fluctuation detection step. The fluctuation detection step is executed in the second period in the current adjustment step.

Description

Laser Light Source Control Method and Laser Device

[0001] The present invention relates to a laser light source control method and a laser device.

[0002] As methods for bringing the wavelength of laser light output from a laser light source close to a target wavelength, the methods disclosed in Patent Documents 1 and 2 are known. In this method, a dither signal is used to modulate the resonator mode of an optical resonator of a wavelength-tunable laser on the wavelength axis, and the intensity fluctuation of the laser light output from the wavelength-tunable laser is detected. At this time, when the amplitude of the intensity fluctuation is minimum, it is determined that the wavelength of the laser light matches the target wavelength. Further, when the phase of the dither signal matches the phase of the intensity fluctuation, it is determined that the wavelength of the laser light is shorter than the target wavelength; when the phase of the dither signal and the phase of the intensity fluctuation are shifted by 180°, it is determined that the wavelength of the laser light is longer than the target wavelength. Accordingly, the wavelength of the laser light can be brought close to the target wavelength based on the detected intensity fluctuation. Such a method of bringing the wavelength of the laser light close to the target wavelength is also called dither control.

[0003] Further, Patent Document 3 discloses the following technique: prior to starting output of laser light, feedback control is performed such that the temperature of a laser diode reaches a predetermined temperature corresponding to a target emission wavelength; after starting output of the laser light, while the power of the laser light increases to a final target power, after changing the control constant of a temperature adjustment means, the target value of the power of the laser light is increased in a stepwise manner, thereby increasing the power of the laser light to the final target power in a stepwise manner.

[0004] U.S. Patent No. 7209498, International Publication No. WO 2018 / 146749, Japanese Unexamined Patent Application Publication No. 2012-124287

[0005] However, the inventors have found that when increasing the power of the laser beam while performing dither control, the accuracy of the dither control may decrease. Furthermore, the inventors have found that when decreasing the power of the laser beam while performing dither control, the accuracy of the dither control may also decrease. Moreover, the inventors have found that this problem is particularly significant when the power is changed to increase or decrease rapidly with respect to time.

[0006] The present invention has been made in view of the above, and its object is to provide a laser light source control method and a laser device that can change the power of the laser light while performing dither control with high precision.

[0007] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention is a method for controlling a laser light source comprising: an optical resonator; a gain unit disposed inside the optical resonator; and a wavelength selection unit in the optical resonator whose transmission or reflection characteristics change with respect to wavelength, the method comprising: a current adjustment step of adjusting the drive current supplied to the laser light source so that there is a first period in which the current value changes with respect to time and a second period in which the current value is constant with respect to time; a resonator length increase / decrease step of increasing or decreasing the resonator length of the optical resonator with respect to a reference length; a fluctuation detection step of detecting fluctuations in the power of the laser light generated in the optical resonator due to the resonator length increase / decrease step; and a wavelength adjustment step of bringing the wavelength of the laser light closer to a target wavelength based on the fluctuations detected in the fluctuation detection step, wherein the fluctuation detection step is performed during the second period in the current adjustment step.

[0008] In the current adjustment step, the drive current supplied to the gain unit may be adjusted.

[0009] The laser light source includes an optical amplifier that optically amplifies the laser light output from the optical resonator, and in the current adjustment step, the drive current supplied to the optical amplifier may be adjusted.

[0010] In the wavelength adjustment step, the wavelength of the laser light may be brought closer to the target wavelength based on fluctuations in the power of the laser light output from the optical resonator.

[0011] The first period in the current adjustment step and the period in which the wavelength adjustment step is performed may partially overlap.

[0012] The first period in the current adjustment step and the period in which the resonator length increase / decrease step is performed may partially overlap.

[0013] The first period in the current adjustment step and the wavelength adjustment step may be completed before the variation detection step begins.

[0014] The resonator length increase / decrease step may be performed while the current adjustment step, the fluctuation detection step, and the wavelength adjustment step are being executed.

[0015] In the current adjustment step, the drive current supplied to the gain unit may be adjusted based on the monitoring result of the power of the laser light output from the optical resonator, so that the power of the laser light output from the optical resonator approaches the power target value.

[0016] As the monitoring result of the power of the laser light output from the optical resonator, the average value of the monitoring results during the execution of the variation detection step may be used.

[0017] As a result of monitoring the power of the laser light output from the optical resonator, the value used when the resonator length is the reference length during the execution of the variation detection step may be used.

[0018] In the current adjustment step, the drive current supplied to the optical amplifier may be adjusted based on the monitoring result of the power of the laser light output from the optical amplifier so that the power approaches the power target value.

[0019] As the monitoring result of the power of the laser light output from the optical amplifier, the average value of the monitoring results during the execution of the variation detection step may be used.

[0020] One aspect of the present invention is a laser light source comprising: an optical resonator; a gain unit disposed inside the optical resonator; a wavelength selection unit in the optical resonator whose transmission or reflection characteristics change with respect to wavelength; a resonator length adjustment unit for adjusting the resonator length of the optical resonator; a resonator length increase / decrease unit for increasing or decreasing the resonator length of the optical resonator relative to a reference length; and a photodetector for detecting the power of laser light generated in the optical resonator; a control device comprising: a current supply unit for supplying a drive current to the laser light source such that there is a first period in which the current value changes with respect to time and a second period in which the current value is constant with respect to time; a resonator length increase / decrease control unit for controlling the resonator length increase / decrease unit; and a resonator length adjustment control unit for controlling the resonator length adjustment unit, wherein the resonator length adjustment control unit brings the wavelength of the laser light closer to a target wavelength based on the fluctuation in the power of the laser light detected by the photodetector due to the increase or decrease in the resonator length by the resonator length increase / decrease unit, and the fluctuation detected by the photodetector during the second period.

[0021] In the laser apparatus, the resonator length increase / decrease unit and the resonator length adjustment unit may be composed of the same element.

[0022] The wavelength selection unit may include two etalon filters with different periods, and the laser light source may be configured as a vernier-type tunable light source.

[0023] The optical resonator has at least one wavelength-selective mirror whose reflection characteristics change with wavelength, the wavelength selection unit is configured to include the wavelength-selective mirror, and the laser light source may be configured as a vernier-type tunable light source.

[0024] According to the present invention, the power of the laser beam can be changed while performing dither control with high precision.

[0025] Figure 1 is a schematic diagram of the laser device according to Embodiment 1. Figure 2A is a diagram illustrating the problems of the known technology. Figure 2B is a diagram illustrating the problems of the known technology. Figure 3A is a diagram illustrating the control of the laser light source in the laser device according to Embodiment 1. Figure 3B is a diagram illustrating the control of the laser light source in the laser device according to Embodiment 1. Figure 4 is a schematic diagram of the laser device according to Embodiment 2. Figure 5A is a diagram illustrating the control of the laser light source in the laser device according to Embodiment 2. Figure 5B is a diagram illustrating the control of the laser light source in the laser device according to Embodiment 2. Figure 6 is a diagram illustrating the case where the average value of the monitoring results is used.

[0026] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to these embodiments. Furthermore, in each drawing, the same or corresponding elements are denoted by the same reference numerals as appropriate, and redundant explanations are omitted as appropriate.

[0027] (Embodiment 1) Figure 1 is a schematic diagram of the laser device according to Embodiment 1. The laser device 10 comprises a laser light source 1 and a control device 2.

[0028] [Configuration of the laser light source] The laser light source 1 comprises an optical resonator 1a, a gain unit 1b, a wavelength selection unit 1c, a resonator length adjustment unit 1d, a resonator length increase / decrease unit 1e, a photodetector 1f, heaters 1g and 1h, and temperature detectors 1i and 1j. The laser light source 1 outputs laser beams L1 and L2 generated by the optical resonator 1a. The laser beams L1 and L2 have the same wavelength and their powers are approximately proportional.

[0029] The optical resonator 1a is equipped with mirrors 1aa and 1ab. Mirrors 1aa and 1ab are, for example, dielectric multilayer films formed on the end face of a semiconductor, but they may also be mirrors that reflect a laser beam propagating in free space.

[0030] The gain unit 1b is located inside the optical resonator 1a. The gain unit 1b is a semiconductor element having an optical waveguide composed of, for example, an active layer.

[0031] Laser beams L1 and L2 are generated by the combined action of the optical resonator 1a and the gain unit 1b, and are output from the optical resonator 1a. Laser beam L1 is output outside the laser light source 1 and used for predetermined purposes such as optical communication. On the other hand, laser beam L2 is received by the photodetector 1f.

[0032] The wavelength selection unit 1c is located inside the optical resonator 1a and comprises an optical filter 1ca and an optical filter 1cb. The optical filters 1ca and 1cb are etalon filters with different periods. Such a wavelength selection unit 1c has wavelength characteristics such that the transmittance is maximized at the wavelength where one transmission peak in the transmission spectrum of optical filter 1ca and one transmission peak in the transmission spectrum of optical filter 1cb overlap on the wavelength axis. The wavelength selection unit 1c is an example of a wavelength selection unit in which the transmission or reflection characteristics change with respect to wavelength.

[0033] The resonator length adjustment unit 1d comprises a semiconductor element having an optical waveguide and a microheater for heating the semiconductor element. The optical waveguide of the resonator length adjustment unit 1d constitutes a part of the optical path in the optical resonator 1a. The resonator length adjustment unit 1d adjusts the resonator length of the optical resonator 1a by heating the optical waveguide with the microheater. By adjusting the resonator length of the optical resonator 1a, the wavelength of the resonator mode produced by the optical resonator 1a can be adjusted. The resonator length adjustment unit 1d may be a component such as a glass block made of a material that transmits the light emitted by the gain unit 1b, and may include a heater with adjustable temperature.

[0034] The resonator length adjustment unit 1e has the function of increasing or decreasing the resonator length of the optical resonator 1a relative to the reference length. The resonator length adjustment unit 1e may be composed of a piezoelectric material such as lead zirconate titanate (PZT). In this embodiment, the resonator length adjustment unit 1e is provided on the mirror 1ab, and the resonator length of the optical resonator 1a is increased or decreased relative to the reference length by vibrating the mirror 1ab in the direction of light propagation in the optical resonator 1a. Here, the reference length is the resonator length when the resonator length adjustment unit 1e is not operating.

[0035] The photodetector 1f receives the laser light L2 generated and output by the optical resonator 1a, and detects the power of the laser light L2 by outputting a current corresponding to the power of the received light. The photodetector 1f is composed of, for example, a photodiode (PD).

[0036] A heater 1g is provided to heat the optical filter 1ca. When the optical filter 1ca is heated by the heater 1g, the transmission spectrum shifts along the wavelength axis. A temperature detector 1i is provided to detect the temperature of the optical filter 1ca. A heater 1h is provided to heat the optical filter 1cb. When the optical filter 1cb is heated by the heater 1h, the transmission spectrum shifts along the wavelength axis. A temperature detector 1j is provided to detect the temperature of the optical filter 1cb. The temperature detectors 1i and 1j are configured to include, for example, thermistors.

[0037] [Configuration of the control device] The control device 2 is connected to a higher-level control device (not shown) equipped with a user interface, for example, and controls the operation of the laser light source 1 according to instructions from the user via the higher-level control device.

[0038] The control device 2 includes, as hardware, a processor, memory, peripheral devices such as input / output interfaces, and a power supply. The processor is, for example, a CPU (Central Processing Unit), a DSP (Digital Signal Processor), or a GPU (Graphics Processing Unit), and performs various arithmetic operations for the functions implemented by the control device 2. The memory includes, for example, a part composed of ROM (Read Only Memory) and a part composed of RAM (Random Access Memory). The ROM stores various programs and data used by the processor to perform arithmetic operations. The RAM is used as a workspace for the processor when performing arithmetic operations and to store the results of the processor's arithmetic operations. The control device 2 may also include a recording medium that can be read by a computer. The functions of the control device 2 are realized through the cooperation of hardware and software, for example, by the processor executing a program read from memory. The control device 2 may also include an FPGA (Field Programmable Gate Array).

[0039] The control device 2 includes, as functional units, a current supply unit 2a, an optical filter control unit 2b, and a resonator length control unit 2c.

[0040] The current supply unit 2a supplies current (Gain current) to the gain unit 1b of the laser light source 1. The optical filter control unit 2b controls the wavelength characteristics of the wavelength selection unit 1c by supplying power to the heaters 1g and 1h based on the temperature detected by the temperature detectors 1i and 1j. The resonator length control unit 2c controls the resonator length adjustment unit 1d by supplying power to the microheater of the resonator length adjustment unit 1d based on the current input from the photodetector 1f. The resonator length control unit 2c also controls the resonator length increase / decrease unit 1e by supplying a voltage signal to the resonator length increase / decrease unit 1e. In other words, in this embodiment, the resonator length control unit 2c functions as both a resonator length increase / decrease control unit and a resonator length adjustment control unit.

[0041] [Basic Operation of Laser Device] Next, a description will be given of the operation of each element when the laser device 10 performs a basic operation, that is, outputs a laser beam L1 having a desired and constant power at a desired wavelength. Note that, in a state where the laser device 10 is outputting the laser beam L1 of the desired wavelength, one of the maximum transmittance values of the wavelength selection unit 1c substantially matches one wavelength of the resonator mode of the optical resonator 1a at the desired wavelength. "Substantially match" means that the two wavelengths are within the allowable error range.

[0042] The current supply unit 2a supplies a Gain current to the gain unit 1b of the laser light source 1 such that the power of the laser beam L1 becomes a desired and constant power. For example, the relationship between the power of the laser beam L1 and the Gain current may be measured in advance and stored in a memory of the control device 2. Further, the laser beam L1 may be controlled by adjusting the Gain current based on the power of the laser beam L2 detected by the photodetector 1f. Further, the target power of the laser beam L1 may be set, for example, according to an instruction from a host control device.

[0043] The optical filter control unit 2b controls the wavelength characteristics of the wavelength selection unit 1c by supplying power to the heaters 1g and 1h based on the temperatures detected by the temperature detectors 1i and 1j. Specifically, feedforward control is performed to supply power to the heaters 1g and 1h such that one transmission peak in the transmission spectrum of the optical filter 1ca and one transmission peak in the transmission spectrum of the optical filter 1cb overlap on the wavelength axis at the target wavelength of the desired laser beam L1. As a result, the wavelength at which the transmittance of the wavelength selection unit 1c is maximized matches the target wavelength of the desired laser beam L1. Note that the relationship between the target wavelength of the laser beam L1 and the target temperatures of the heaters 1g and 1h, or the relationship between the target wavelength of the laser beam L1 and the power supplied to the heaters 1g and 1h, may be measured in advance and stored in a memory of the control device 2, for example. Further, the target wavelength of the laser beam L1 may be set, for example, according to an instruction from a host control device.

[0044] The resonator length controller 2c supplies a voltage signal (dither signal) whose amplitude periodically increases and decreases to the resonator length increasing / decreasing unit 1e, causing the resonator length to increase and decrease relative to a reference length, and modulates the resonator mode of the optical resonator 1a on the wavelength axis. Then, the powers of the laser beams L1 and L2 increase and decrease reflecting the transmission characteristics of the wavelength selector 1c.

[0045] The resonator length controller 2c supplies power to the microheater of the resonator length adjuster 1d based on the current input from the photodetector 1f. Specifically, the resonator length controller 2c acquires the intensity fluctuation of the power of the laser beam L2 and the phase of the intensity fluctuation based on the current input from the photodetector 1f, and performs feedback control that supplies power to the microheater of the resonator length adjuster 1d such that the amplitude of the intensity fluctuation is minimized. Through such dither control, the laser device 10 can output a laser beam L1 having a desired wavelength.

[0046] When changing the wavelength of the laser beam L1 in the laser light source 1, the optical filter controller 2b performs feedforward control on the wavelength at which the transmittance of the wavelength selector 1c reaches a maximum to the wavelength after change. Thereafter, the resonator length controller 2c performs dither control, and performs feedback control such that the wavelength of the resonator mode becomes the wavelength after change. The laser light source 1 capable of changing the wavelength in this manner is an example of a laser light source configured as a vernier-type wavelength-tunable light source using two etalon filters having different periods.

[0047] [When increasing the power of laser beam] Next, a case where the laser device 10 operates to change the power of the laser beam L1 while performing dither control will be described with an example of increasing the power. In the following, problems in the case of an operation for increasing the power of a laser beam while performing dither control in a known technique will be described first, and then control of the laser light source 1 in the laser device 10 according to the first embodiment will be described.

[0048] FIGS. 2A and 2B are diagrams for explaining problems in a known technique. As shown in FIG. 2A, it is assumed that when increasing the powers of the laser beams L1 and L2, the Gain current is continuously increased over time.

[0049] The case where dither control is performed while continuously increasing the Gain current over time will be explained using Figure 2B. First, let λ1 be the wavelength of the resonator mode (i.e., the wavelengths of the laser beams L1 and L2) when the resonator length is the reference length. In this case, if the transmission characteristics of the wavelength selection unit 1c are represented by curve C1, the wavelength of the peak C11 of curve C1 is also λ1.

[0050] In contrast, when the resonator length is varied using the resonator length adjustment unit 1e, the wavelength of the laser light L2 varies as shown at points p11, p12, and p13. First, let's assume that when the wavelength is at point p11, the laser light L2 becomes laser light L21. In this case, if the current output by the photodetector 1f is the PD current, then the value of the PD current corresponds to the current P11 at the point on curve C1.

[0051] Next, when the wavelength of the laser beam L2 reaches point p12, curve C1 is fixed, so if the power of the laser beams L1 and L2 is constant, the value of the PD current should be the current P12A corresponding to the point on curve C1. However, in this example, the power of the laser beams L1 and L2 is increased by continuously increasing the Gain current over time, so the laser beam L2 becomes like the laser beam L22. As a result, the value of the PD current becomes the current P12, which is above the current P12A. In this case, the resonator length control unit 2c mistakenly determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C2, and controls the resonator length adjustment unit 1d to bring the wavelength of the resonator mode closer to the peak C21 of curve C2, which has a peak C21 on the longer wavelength side than peak C11.

[0052] Furthermore, when the wavelength of the laser beam L3 reaches point p13, curve C1 is fixed, so if the power of the laser beams L1 and L2 is constant, the value of the PD current should be the current P11 corresponding to a point on curve C1. However, in this example, the Gain current is further increased, further increasing the power of the laser beams L1 and L2, so the value of the PD current becomes an even larger current P13. In this case, the resonator length control unit 2c mistakenly determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C3, and controls the resonator length adjustment unit 1d to bring the wavelength of the resonator mode closer to peak C31 of curve C3, which has peak C31 on the shorter wavelength side than peak C11.

[0053] When dither control is performed as described above, the control adjusts the wavelengths of the resonator modes, i.e., the wavelengths of the laser light L1 and L2, to other wavelengths even though they are the desired wavelengths, thus reducing the accuracy of the dither control.

[0054] In contrast, Figures 3A and 3B illustrate the control of the laser light source 1 by the control device 2 in the laser device 10 according to Embodiment 1.

[0055] In this control, as shown in Figure 3A, the current supply unit 2a of the control device 2 performs a current adjustment step to adjust the Gain current, which is the drive current supplied to the gain unit 1b of the laser light source 1, so that there is a first period in which the current value changes with respect to time and a second period in which the current value is constant with respect to time. In this embodiment, the current value of the Gain current increases with respect to time during the first period.

[0056] Then, the resonator length control unit 2c of the control device 2 executes a resonator length increase / decrease step, which increases or decreases the resonator length of the optical resonator 1a relative to the reference length. As a result, the PD current changes as shown in Figure 3A. In the graph showing the change in PD current, the thick dashed line indicates the fluctuation of the PD current due to the resonator length increase / decrease step.

[0057] Meanwhile, the resonator length control unit 2c performs a variation detection step, which detects variations in the power of the laser light L2 output from the optical resonator 1a (power variations caused by the resonator length increase / decrease step) based on the variation in the PD current obtained from the photodetector 1f.

[0058] Furthermore, the resonator length control unit 2c executes a wavelength adjustment step to bring the wavelength of the laser light L2 closer to the target wavelength based on the fluctuation detected in the fluctuation detection step. In this control, as shown in Figure 3A, the wavelength adjustment step is executed during the "Enable" period, which does not overlap in time with the resonator length increase / decrease step, and the execution of the wavelength adjustment step is stopped during the "Disable" period.

[0059] Here, the resonator length control unit 2c performs the fluctuation detection step during the second period in the current adjustment step when the current value of the Gain current is constant. Based on the fluctuations in the power of the laser light L2 detected by the photodetector 1f due to the increase or decrease in the resonator length by the resonator length increase / decrease unit 1e, and the fluctuations detected by the photodetector 1f during the second period when the current value of the drive current supplied by the current supply unit 2a is constant, the resonator length control unit 2c controls the wavelengths of the laser light L1 and L2 to approach the target wavelength.

[0060] Dither control when the control is performed as described above will be explained using Figure 3B. Similar to the case in Figure 2B, the wavelength of the resonator mode (i.e., the wavelengths of laser light L1 and L2) when the resonator length is the reference length is denoted as λ1. In this case, if the transmission characteristics of the wavelength selection unit 1c are represented by curve C1, the wavelength of the peak C11 of curve C1 is also λ1.

[0061] In contrast, when the resonator length is varied, the wavelength of the laser light L2 varies as shown at points p21, p22, p23, p24, p25, and p26. Then, when the wavelength is at point p21, the laser light L2 becomes like laser light L23. At this time, the detected PD current value corresponds to the current P21 at point C1.

[0062] Next, even when the wavelength of the laser light L2 becomes point p22, the Gain current remains constant, so the laser light L2 becomes like laser light L24. In this case, the detected PD current value becomes the current P22 on curve C1. Therefore, the resonator length control unit 2c determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C1.

[0063] Next, even when the wavelength of the laser light L2 becomes point p23, the Gain current remains constant, so the laser light L2 becomes laser light L23. In this case as well, the resonator length control unit 2c determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C1.

[0064] Subsequently, the resonator length control unit 2c controls the resonator length adjustment unit 1d to bring the wavelength of the resonator mode closer to the peak C11 of curve C1 during the "Enable" period, so that dither control is performed correctly.

[0065] Next, the current supply unit 2a increases the Gain current with respect to time. Subsequently, when the wavelength of the laser light L2 moves to point p24, the detected PD current value becomes the current P24 corresponding to the point on curve C4. Next, even when the wavelength of the laser light L2 becomes point p25, the Gain current remains constant, so the detected PD current value becomes the current P25 on curve C4. In this case, the resonator length control unit 2c determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C4. Note that curve C4 is located above curve C1, has the same shape as curve C1, and the wavelength of its peak C41 is the same as that of peak C11.

[0066] Next, even when the wavelength of the laser light L2 becomes point p26, the Gain current remains constant, so the resonator length control unit 2c determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C4.

[0067] Subsequently, the resonator length control unit 2c controls the resonator length adjustment unit 1d to bring the wavelength of the resonator mode closer to the peak C41 of curve C4 during the "Enable" period, so the dither control is performed correctly. In Figure 3B, there is no shift between points p21 and p23 and points p24 and p26 after the Gain current is increased, but in reality, the optical path length of the resonator changes as the Gain current increases, which may cause a shift in wavelength. However, even in this case, this control can accurately recognize the shape of curve C4, so the dither control works normally and the center wavelength of points p24 and p25 can be immediately pulled back to λ1.

[0068] In the laser device 10, by repeatedly performing the above steps, the power of the laser beam can be increased while performing dither control with high precision.

[0069] (Embodiment 2) Figure 4 is a schematic diagram of the laser device according to Embodiment 2. The laser device 10A comprises a laser light source 1A and a control device 2A.

[0070] [Laser light source configuration] Laser light source 1A has the same configuration as laser light source 1 shown in Figure 1, but with the addition of a semiconductor optical amplifier (SOA) 1k, a beam splitter (BS) 1l, and a photodetector 1m, the removal of the resonator length increase / decrease unit 1e, and the swapping of the positions of the wavelength selection unit 1c, gain unit 1b, and resonator length adjustment unit 1d.

[0071] Control device 2A has a configuration in which the resonator length control unit 2c is replaced with a resonator length control unit 2Ac and a current supply unit 2d is added, as in the control device 2 shown in Figure 1.

[0072] The semiconductor optical amplifier 1k optically amplifies the laser light L1 and outputs it as laser light L3. The beam splitter (BS) 1l splits a portion of the laser light L3 into laser light L4. The laser light L3 is output outside the laser light source 1A and used for a predetermined purpose such as optical communication. The semiconductor optical amplifier 1k is an example of an optical amplifier that optically amplifies the laser light output from an optical resonator.

[0073] The photodetector 1m receives the laser light L4 and outputs a current corresponding to the power of the received light. The photodetector 1m monitors the power of the laser light L3 output from the semiconductor optical amplifier 1k. The photodetector 1m is configured to include, for example, a photodetector (PD).

[0074] The resonator length control unit 2Ac supplies a power signal (dither signal) with periodically increasing and decreasing amplitude to the microheater of the resonator length adjustment unit 1d, thereby increasing or decreasing the resonator length relative to the reference length and modulating the resonator mode of the optical resonator 1a on the wavelength axis.

[0075] Furthermore, the resonator length control unit 2Ac supplies power to the microheater of the resonator length adjustment unit 1d based on the current input from the photodetector 1f. Specifically, the resonator length control unit 2Ac acquires the intensity fluctuation of the laser light L2 power and the phase of said intensity fluctuation based on the current input from the photodetector 1f, and performs feedback control (dither control) to supply power to the microheater of the resonator length adjustment unit 1d so that the amplitude of said intensity fluctuation is minimized.

[0076] In other words, in this embodiment, the resonator length adjustment unit 1d also serves as the resonator length increase / decrease unit. This embodiment is an example in which the resonator length increase / decrease unit and the resonator length adjustment unit are composed of the same element.

[0077] The current supply unit 2d supplies current (SOA current) to the semiconductor optical amplifier 1k of the laser light source 1A. Here, the current supply unit 2d adjusts the SOA current based on the monitoring result of the current input from the photodetector 1m, i.e., the power of the laser light L3, and controls the semiconductor optical amplifier 1k so that the power of the laser light L3 reaches a desired value (power target value). The relationship between the power of the laser light L3 and the SOA current may be measured in advance and stored in the memory of the control device 2A. The target power of the laser light L3 may also be set, for example, by instructions from a higher-level control device.

[0078] [Increasing the Power of the Laser Light] Next, we will explain the case where the laser device 10A changes the power of the laser light L3 while performing dither control, using the case of increasing the power as an example. Figures 5A and 5B are diagrams illustrating the control of the laser light source 1A in the laser device 10A.

[0079] In this control, as shown in Figure 5A, the current supply unit 2d of the control device 2A performs a current adjustment step in which it adjusts the SOA current, which is the drive current supplied to the semiconductor optical amplifier 1k of the laser light source 1A, so that there is a first period in which the current value changes with respect to time and a second period in which the current value is constant with respect to time. In this embodiment, the current value of the SOA current increases with respect to time during the first period. However, the Gain current remains constant.

[0080] Then, the resonator length control unit 2Ac of the control device 2A executes a resonator length increase / decrease step, which increases or decreases the resonator length of the optical resonator 1a relative to the reference length. As a result, the PD current changes as shown in Figure 5A. In the graph showing the change in PD current, the thick dashed line indicates the fluctuation of the PD current due to the resonator length increase / decrease step.

[0081] Meanwhile, the resonator length control unit 2Ac performs a variation detection step, which detects variations in the power of the laser light L2 output from the optical resonator 1a (power variations caused by the resonator length increase / decrease step) based on the variation in the PD current obtained from the photodetector 1f.

[0082] Furthermore, the resonator length control unit 2Ac executes a wavelength adjustment step to bring the wavelength of the laser light L2 closer to the target wavelength based on the fluctuation detected in the fluctuation detection step. In this control, as shown in Figure 5A, the wavelength adjustment step is executed during the "Enable" period, which does not overlap in time with the resonator length increase / decrease step, and the execution of the wavelength adjustment step is stopped during the "Disable" period.

[0083] Here, the resonator length control unit 2Ac performs the fluctuation detection step during the second period in the current adjustment step when the current value of the SOA current is constant.

[0084] Dither control when the control is performed as described above will be explained using Figure 5B. Similar to the cases in Figures 2B and 3B, the wavelength of the resonator mode (i.e., the wavelength of the laser light L1 to L4) when the resonator length is the reference length is denoted as λ1. In this case, if the transmission characteristics of the wavelength selection unit 1c are represented by curve C5, the wavelength of the peak C51 of curve C5 is also λ1.

[0085] In contrast, when the resonator length is varied, the wavelength of the laser light L2 varies as shown at points p31, p32, p33, p34, p35, and p36. Then, when the wavelength is at point p31, the detected PD current value corresponds to the current P31 at point C5.

[0086] Next, even when the wavelength of the laser light L2 becomes point p32, the SOA current remains constant, so the laser light L2 becomes laser light L24. In this case, the detected PD current value becomes the current P32 on curve C1. In this case, the resonator length control unit 2Ac determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C5.

[0087] Next, even when the wavelength of the laser light L2 becomes point p33, the SOA current remains constant. Therefore, in this case as well, the resonator length control unit 2Ac determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C5. The value of the PD current becomes current P33.

[0088] Subsequently, the resonator length control unit 2Ac controls the resonator length adjustment unit 1d to bring the wavelength of the resonator mode closer to the peak C51 of curve C5 during the "Enable" period, so that dither control is performed correctly.

[0089] Next, the current supply unit 2d increases the SOA current with respect to time. As a result, the component of light generated from the semiconductor optical amplifier 1k that is unintentionally received by the photodetector 1f (hereinafter referred to as SOA stray light) also increases. Therefore, when the wavelength of the laser light L2 moves to point p34, the detected PD current value becomes the current P34 corresponding to the point on curve C6. Next, even when the wavelength of the laser light L2 becomes point p35, the SOA current remains constant, so the detected PD current value becomes the current P35 on curve C6. In this case, the resonator length control unit 2Ac determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C6. Curve C6 is located above curve C5, has the same shape as curve C5, and the wavelength of its peak C61 is the same as that of peak C51.

[0090] Next, even when the wavelength of the laser light L2 becomes point p36, the SOA current remains constant, so the resonator length control unit 2Ac determines that the transmission characteristic curve of the wavelength selection unit 1c is curve C6.

[0091] Subsequently, the resonator length control unit 2Ac controls the resonator length adjustment unit 1d to bring the wavelength of the resonator mode closer to the peak C61 of curve C6 during the "Enable" period, so dither control is performed correctly.

[0092] In the laser device 10A, by repeatedly performing the above steps, it is possible to increase the power of the laser beam while performing high-precision dithering control, even in the presence of SOA stray light.

[0093] In the control described above, as shown in Figures 3A and 5A, the timing at which the current supply unit 2a or 2d starts increasing the Gain current or SOA current may be before the end of the "Enable" period. That is, the first period of the current adjustment step and the period during which the wavelength adjustment step is performed may partially overlap. Partial overlap includes cases where there is a small overlap and cases where there is a complete overlap. Furthermore, dither control may be performed continuously at a constant period, and the change in laser light power and resonator length control may be performed over a period shorter than half a period of dithering. In this case as well, if the fluctuation detection step is performed during the second period of the current adjustment step, high-precision dither control can be achieved.

[0094] Furthermore, the first period of the current adjustment step and the period during which the resonator length increase / decrease step is performed may partially overlap. For example, referring to Figure 3B, let us describe the case where no fluctuation detection is performed at point p24, and the fluctuation detection step is performed based on the fluctuations of the PD current detected at points p25 and p26, and the time difference between point p23 and point p24 is small. In this case, the first period of the current adjustment step, which was performed and completed between point p23 and point p24 in the case of Figure 3B, and the resonator length increase / decrease step performed between point p24 and point p25 may partially overlap. Similarly, let us describe the case where no fluctuation detection is performed at point p24, and the fluctuation detection step is performed based on the fluctuations of the PD current detected at points p25 and p26, and point p23 and point p24 coincide in time. In this case, the first period of the current adjustment step may be performed and completed during the resonator length increase / decrease step performed between point p24 and point p25.

[0095] Furthermore, the first period in the current adjustment step and the wavelength adjustment step may be completed before the variation detection step begins. In this case, the first period in the current adjustment step and the wavelength adjustment step may be completed in a shorter time compared to the resonator length increase / decrease step.

[0096] Furthermore, the resonator length adjustment step may be performed while the current adjustment step, the fluctuation detection step, and the wavelength adjustment step are being executed. During the execution of the resonator length adjustment step, there is substantially no time when the resonator length is constant. Moreover, the resonator length adjustment step may be performed even when the current adjustment step, the fluctuation detection step, and the wavelength adjustment step are not being executed; for example, the resonator length adjustment step may be performed at all times.

[0097] Furthermore, in the control according to Embodiment 2, the resonator length control unit 2Ac may perform dither control based on the current from the photodetector 1m, that is, based on the power of the laser beams L3 and L4 output from the semiconductor optical amplifier 1k.

[0098] Furthermore, in the control according to Embodiment 2, in the current adjustment step, the drive current (SOA current) supplied to the semiconductor optical amplifier 1k may be adjusted based on the power monitoring result of the laser light L3 output from the semiconductor optical amplifier 1k so that the power approaches the power target value. In this case, the average value of the monitoring results during the execution of the variation detection step or the value when the resonator length is the reference length during the execution of the variation detection step may be used as the power monitoring result of the laser light L3.

[0099] Figure 6 illustrates the case where the average value of the monitoring results is used. As shown in Figure 6, in the current adjustment step, the SOA current is adjusted so that the power of the laser beam L3 approaches the power target value. At this time, if the fluctuation detection step is performed, the resonator length increase / decrease step is also performed, and dither control is performed using a dither signal of a predetermined amplitude as shown in Figure 6. In this case, the monitored value of the power of the laser beam L3 also fluctuates according to the dither signal, so there is a risk of errors in the monitoring results. Therefore, by using the average value of the monitoring results in the current adjustment step, errors in the monitoring results can be suppressed. As the average value, for example, the average value over the period of the resonator length increase / decrease step (period of the dither signal) (shown by the dotted line in the figure) or the maximum value y over the said period can be used. ＋ and minimum value y － The average value of the above can be used. Also, the value y when the resonator length is the reference length. ０ This may be used. The case where the resonator length is the reference length is when the amplitude of the dither signal is zero and the fluctuation of the monitor value due to dither control is zero.

[0100] Similarly, in the control according to Embodiment 1, the current adjustment step may also adjust the drive current (Gain current) supplied to the laser light source 1 so that the power of the laser light L1 output from the optical resonator 1a approaches the power target value, based on the power monitoring result of the laser light L2 output from the optical resonator 1a. In this case, the average value of the monitoring results during the execution of the variation detection step or the value when the resonator length is the reference length during the execution of the variation detection step may be used as the power monitoring result of the laser light L2.

[0101] Furthermore, while the laser light sources 1 and 1A according to embodiments 1 and 2 are configured as vernier-type tunable light sources by having a wavelength selection unit 1c equipped with two etalon filters having different periods, the configuration of a vernier-type tunable light source is not limited to this. For example, a vernier-type tunable light source may have an optical resonator with one wavelength-selective mirror whose reflection characteristics change with wavelength, and the wavelength selection unit may be configured to include an etalon filter and a wavelength-selective mirror. Alternatively, a vernier-type tunable light source may have an optical resonator with two wavelength-selective mirrors whose reflection characteristics change with wavelength, and the wavelength selection unit may be configured to include two wavelength-selective mirrors. Such wavelength-selective mirrors are, for example, sample-type DBR (Distributed Bragg Reflector) mirrors or ring-resonator type reflective mirrors. Also, such wavelength-selective mirrors may serve as mirrors 1aa and 1ab.

[0102] Furthermore, in the embodiments 1 and 2 described above, if the mirrors 1aa and 1ab are dielectric multilayer films formed on the end faces of semiconductors, they may be integrated with the semiconductor elements of the gain section 1b and the resonator length adjustment section 1d.

[0103] When mirror 1aa or mirror 1ab is a semiconductor end face or is formed as a semiconductor element in the form of a DBR, some of the mirrors 1aa, mirror 1ab, gain unit 1b, and resonator length adjustment unit 1d may be integrally configured as semiconductor elements, allowing for appropriate rearrangement of the arrangement within the resonator. In particular, when mirror 1ab is a mirror such as a DBR, it may also be integrally configured with a semiconductor optical amplifier 1k.

[0104] Furthermore, the laser light sources 1 and 1A in the above embodiments 1 and 2 are configured as external resonator type laser light sources, with bulk elements such as optical filters 1ca and 1cb arranged inside the optical resonator 1a. However, the present invention is not limited to this. For example, the optical resonator and its internal elements may be integrally constructed from semiconductors.

[0105] Furthermore, although the above embodiment describes an example where the current value is increased during the first period of the current adjustment step, the current adjustment step may also be a step of decreasing the current value during the first period.

[0106] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible.

[0107] This invention can be used in laser light sources.

[0108] 1, 1A: Laser light source 1a: Optical resonator 1aa, 1ab: Mirror 1b: Gain unit 1c: Wavelength selection unit 1ca, 1cb: Optical filter 1d: Resonator length adjustment unit 1e: Resonator length increase / decrease unit 1f, 1m: Photodetector 1g, 1h: Heater 1i, 1j: Temperature detector 1k: Semiconductor optical amplifier (SOA) 1l: Beam splitter (BS) 2, 2A: Control unit 2a, 2d: Current supply unit 2Ac, 2c: Resonator length control unit 2b: Optical filter control unit 10, 10A: Laser device C1, C2, C3, C4, C5, C6: Curve C11, C21, C31, C41, C51, C61: Peak L1, L2, L3, L4, L21, L22, L23, L24: Laser light p11, p12, p13, p21, p22, p23, p24, p25, p26, p31, p32, p33, p34, p35, p36: Point P11, P12, P12A, P13, P21, P22, P24, P25, P31, P32, P33, P34, P35: Current

Claims

1. A method for controlling a laser light source comprising: an optical resonator; a gain unit disposed inside the optical resonator; and a wavelength selection unit in the optical resonator that changes the transmission or reflection characteristics with respect to wavelength, the method comprising: a current adjustment step of adjusting the drive current supplied to the laser light source such that there is a first period in which the current value changes with respect to time and a second period in which the current value is constant with respect to time; a resonator length increase / decrease step of increasing or decreasing the resonator length of the optical resonator with respect to a reference length; a variation detection step of detecting a variation in the power of the laser light generated in the optical resonator due to the resonator length increase / decrease step; and a wavelength adjustment step of bringing the wavelength of the laser light closer to a target wavelength based on the variation detected in the variation detection step, wherein the variation detection step is performed during the second period in the current adjustment step.

2. The method for controlling a laser light source according to claim 1, wherein the current adjustment step involves adjusting the drive current supplied to the gain unit.

3. The method for controlling a laser light source according to claim 1, wherein the laser light source includes an optical amplifier that optically amplifies the laser light output from the optical resonator, and the current adjustment step involves adjusting the drive current supplied to the optical amplifier.

4. A method for controlling a laser light source according to claim 1 or 3, wherein in the wavelength adjustment step, the wavelength of the laser light is brought closer to a target wavelength based on fluctuations in the power of the laser light output from the optical resonator.

5. The method for controlling a laser light source according to claim 1, wherein the first period in the current adjustment step and the period in which the wavelength adjustment step is performed partially overlap.

6. The method for controlling a laser light source according to claim 1, wherein the first period in the current adjustment step and the period in which the resonator length increase / decrease step is performed partially overlap.

7. The method for controlling a laser light source according to claim 1, wherein the first period in the current adjustment step and the wavelength adjustment step are completed before the start of the variation detection step.

8. A method for controlling a laser light source according to claim 1, wherein the resonator length increase / decrease step is performed while the current adjustment step, the fluctuation detection step, and the wavelength adjustment step are being executed.

9. The method for controlling a laser light source according to claim 2, wherein in the current adjustment step, the drive current supplied to the gain unit is adjusted based on the monitoring result of the power of the laser light output from the optical resonator so that the power of the laser light output from the optical resonator approaches the power target value.

10. A method for controlling a laser light source according to claim 9, wherein the average value of the monitoring results during the execution of the variation detection step is used as the monitoring result of the power of the laser light output from the optical resonator.

11. A method for controlling a laser light source according to claim 9, wherein the value obtained when the resonator length is the reference length during the execution of the variation detection step is used as the power of the laser light output from the optical resonator.

12. The method for controlling a laser light source according to claim 3, wherein in the current adjustment step, the drive current supplied to the optical amplifier is adjusted based on the monitoring result of the power of the laser light output from the optical amplifier so that the power approaches a power target value.

13. A method for controlling a laser light source according to claim 12, wherein the average value of the monitoring results during the execution of the variation detection step is used as the monitoring result of the power of the laser light output from the optical amplifier.

14. A method for controlling a laser light source according to claim 12, wherein the value obtained when the resonator length is the reference length during the execution of the variation detection step is used as the result of monitoring the power of the laser light output from the optical amplifier.

15. A laser light source comprising: an optical resonator; a gain unit disposed inside the optical resonator; a wavelength selection unit in the optical resonator whose transmission or reflection characteristics change with respect to wavelength; a resonator length adjustment unit for adjusting the resonator length of the optical resonator; a resonator length increase / decrease unit for increasing or decreasing the resonator length of the optical resonator relative to a reference length; and a photodetector for detecting the power of laser light generated in the optical resonator; a control device comprising: a current supply unit for supplying a drive current to the laser light source such that there is a first period in which the current value changes with respect to time and a second period in which the current value is constant with respect to time; a resonator length increase / decrease control unit for controlling the resonator length increase / decrease unit; and a resonator length adjustment control unit for controlling the resonator length adjustment unit, wherein the resonator length adjustment control unit brings the wavelength of the laser light closer to a target wavelength based on the fluctuation in the power of the laser light detected by the photodetector due to the increase or decrease in the resonator length by the resonator length increase / decrease unit, and the fluctuation detected by the photodetector during the second period.

16. The laser apparatus according to claim 15, wherein the resonator length increasing / decreasing unit and the resonator length adjustment unit are composed of the same element.

17. The laser apparatus according to claim 15, wherein the wavelength selection unit includes two etalon filters having different periods, and the laser light source is configured as a vernier-type tunable light source.

18. The laser apparatus according to claim 15, wherein the optical resonator has at least one wavelength-selective mirror whose reflection characteristics change with respect to wavelength, the wavelength selection unit is configured to include the wavelength-selective mirror, and the laser light source is configured as a vernier-type tunable light source.

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