Mode-synchronized laser device and method for controlling same

Abstract

A ring resonator (10) and a NALM (20) configured as a saturable absorption mechanism constitute a laser oscillator. A multiport BPF (12) is provided to a path through which light propagates in the laser oscillator, the multiport BPF (12) passing light in a prescribed band of the propagating light, and branching light outside of the prescribed band as monitoring light (L). A photodetector (1) detects the monitoring light (L). A control unit (2) controls the oscillation state of the light propagating through the laser oscillator on the basis of the intensity of the monitoring light (L), thereby controlling the mode synchronization of the light propagating through the laser oscillator.

Description

Mode-locked laser device and control method thereof

[0001] The present disclosure relates to a mode-locked laser device and a control method thereof, and more particularly to a mode-locked laser device that generates ultrashort pulses that are applied to, for example, laser micromachining, terahertz wave generation, ultrafast spectroscopy, and high-resolution microscopes such as multiphoton microscopes, and a control method thereof.

[0002] The pulsed light generated by an ultrashort pulse laser device has a pulse width in the picosecond to femtosecond range, with a spectral width of 1 nm or more. Ultrashort pulse laser devices are often composed of components such as a mode-locked laser device, multiple optical amplifiers that increase pulse energy and peak power, an optical pulse picker that reduces the repetition frequency of the optical pulse to an integer fraction, an optical pulse stretcher that widens the pulse width of the optical pulse to eliminate element damage and instability due to peak power, an optical pulse compressor that compresses pulse widths of tens of picoseconds to several nanoseconds to picoseconds or femtoseconds, and a wavelength conversion crystal system that reduces the center wavelength of the optical pulse to an integer fraction. Among these components, the mode-locked laser device is an important factor that determines the wavelength, basic repetition frequency, stability, and maintenance cycle of the optical pulse in an ultrashort pulse laser device. Among mode-locked laser devices, a mode-locked fiber laser oscillator, as disclosed in Patent Document 1, for example, is characterized by its easy manufacturability, compact size, and low manufacturing cost. Furthermore, mode-locked fiber laser oscillators have become the mainstream oscillator for ultrashort pulse lasers due to their advantage of high long-term stability.

[0003] Mode-locked lasers can be broadly divided into active mode-locked laser oscillators and passive mode-locked laser oscillators. Active mode-locked laser oscillators are configured to forcibly lock the mode using an optical modulator inserted inside the oscillator. Passive mode-locked laser oscillators are configured to generate mode-locked pulses using a saturable absorption mechanism, such as a semiconductor saturable absorption mirror (SESAM), a nonlinear optical loop mirror (NOLM), or a nonlinear polarization rotation mechanism, inserted inside the oscillator. Active mode-locked laser oscillators require intensity modulators and phase modulators, and high-speed, precise electrical signal control is required to generate mode-locked pulses, resulting in high manufacturing costs. In contrast, passive mode-locked laser oscillators have the advantage of low manufacturing costs because the saturable absorption mechanism automatically generates mode-locked pulses.

[0004] Generally, a passive mode-locked laser device comprises a saturable absorption mechanism, a pumping light source, an optical amplification mechanism consisting of a laser medium, and an optical resonator such as a ring resonator or a linear resonator. Before a passive mode-locked laser device reaches a mode-locked state after starting operation, the intensity of the light circulating in the optical resonator is weak and fluctuates randomly over time. When the intensity of the light circulating in the resonator suddenly increases above a threshold value through control such as increasing the optical output of the pumping light source or the amplification factor in the optical amplification mechanism, some components of the light are selectively amplified by the saturable absorption mechanism and the optical amplification mechanism, and the selectively amplified light forms an optical pulse. As the optical pulse circulates in the optical resonator, its peak intensity increases and its pulse width shortens, eventually transitioning to a steady state. The optical pulse thus formed is a mode-locked pulse, which has a duration ranging from picoseconds to femtoseconds. The mode-locked operation described above, from the start of operation of a passive mode-locked laser device to the generation of mode-locked laser pulse light, is generally referred to as self-starting.

[0005] The mode-locking determination, which determines whether a passive mode-locked laser device has entered a mode-locked state through self-starting, uses the results of observing the output light from the passive mode-locked laser device. One known method for determining mode-locking is to use an optical spectrum analyzer. This method monitors the spectrum of the output light using an optical spectrum analyzer, and can determine whether the passive mode-locked laser device is in a mode-locked state from its bandwidth and intensity.

[0006] Also, as a method for determining mode locking, a method is known in which an electronic circuit including a photodiode and an amplifier is used to monitor the repetition frequency of an optical pulse, as proposed in Patent Document 2. This method makes it possible to determine whether a passive mode-locked laser device is in a mode-locked state based on the monitoring results of whether the observed repetition frequency is within a predetermined range and whether the frequency and power or voltage are stable.

[0007] Another possible method for determining mode locking is to monitor the average intensity of the output light from a passively mode-locked laser device, as described in, for example, Non-Patent Documents 1 and 2. While the above-mentioned method of monitoring the repetition frequency of optical pulses monitors high-frequency electrical signal components of approximately 10 MHz to 100 MHz, this method monitors low-frequency electrical signal components of approximately 100 kHz or less and DC electrical signal components. This method can determine mode locking by utilizing the fact that the average intensity of the output light decreases during self-start operation and increases when the mode-locked state is released.

[0008] JP 2010-238865 A JP 2008-510322 A

[0009] Yi Han, Haochen Tian, ​​Fei Meng, Kai Wang, and Shiying Cao, "Environment-stable sub-100 fs Er: fiber laser with a 3 dB bandwidth of 78 nm," Opt. Express 30, 48021-48029 (2022).Hu, Mengyun, Jiawei Shen, Yuzhi Cao, Shuai Yuan, and Heping Zeng. 2023. "Generation of 48 fs, 1 GHz Fundamentally Mode-Locked Pulses Directly from an Yb-doped “Solid-State Fiber Laser"" Photonics 10, no. 2: 192. Yang Liu, Peng Zhang, Yunlong Fan, Yuzhu Ning, Shuang He, and Shoufeng Tong, "1.7 μm all-fiber figure-9 mode-locked laser based on a fiber Bragg grating," Laser Phys. 33(9), 095103 (2023).

[0010] However, although mode-locking determination using an optical spectrum analyzer can reliably determine the transition to a mode-locked state, it requires a large and expensive optical spectrum analyzer, and therefore this method is limited to applications such as research and development of mode-locked laser oscillators, and is difficult to use generally for controlling mode-locked laser oscillators.

[0011] When a multi-pulse state occurs during self-start, where multiple pulses are present, it is difficult to distinguish between noise and optical pulses because the peak value of each pulse is small. For this reason, in research and development of mode-locked laser oscillators, for example, optical pulses are monitored using an oscilloscope, with the voltage measurement range and trigger voltage value of the oscilloscope being manually changed depending on the mode-locked state. As a result, this method also has limited applications, making it difficult to use in general for controlling mode-locked laser oscillators.

[0012] When a mode-locked laser device is designed to oscillate continuously strong light before self-starting, the method of monitoring the average intensity of the output light of the mode-locked laser device cannot detect the change because the change in the average intensity of the output light is too small when the mode-locked laser device transitions from a continuous light oscillation state to self-starting, as in Non-Patent Document 3. Similarly, it cannot detect the change in the average intensity of the output light when mode-locking is released.

[0013] Therefore, it is desired to realize a passive mode-locked laser oscillator that can reliably transition to a mode-locked state with a simple configuration.

[0014] A mode-locked laser device according to one aspect of the present disclosure includes a laser oscillator connected to a first oscillation means and a second oscillation means configured as a saturable absorption mechanism having an optical amplification means for amplifying propagating light; filtering means provided in a path along which the light propagates in the laser oscillator, the filtering means passing light of a predetermined band among the propagating light and branching off light of bands other than the predetermined band as supervisory light; optical detection means detecting the supervisory light; and control means for controlling mode-locking of the light propagating in the laser oscillator by controlling an oscillation state of the light propagating in the laser oscillator based on the intensity of the supervisory light detected by the optical detection means.

[0015] A control method for a mode-locked laser device according to one aspect of the present disclosure includes a mode-locked laser device having a laser oscillator connected to a first oscillation means and a second oscillation means configured as a saturable absorption mechanism including an optical amplification means for amplifying propagating light, and filtering means provided in a path along which light propagates in the laser oscillator, for passing light of a predetermined band among the propagating light and branching off light outside the predetermined band as supervisory light, the method detecting the supervisory light and controlling the oscillation state of the light propagating in the laser oscillator based on the intensity of the supervisory light, thereby controlling mode-locking of the light propagating in the laser oscillator.

[0016] According to the present disclosure, in a mode-locked laser device, the mode-locked state can be reliably controlled with a simple configuration.

[0017] 1 is a diagram schematically illustrating a general configuration of a mode-locked laser device according to a first embodiment; FIG. 2 is a diagram schematically illustrating a configuration of a mode-locked laser device according to the first embodiment; FIG. 3 is a diagram illustrating a passband of a multi-port BPF; FIG. 4 is a diagram schematically illustrating a spectrum of light emitted from a port of an optical coupler in a mode-locked state; FIG. 5 is a diagram schematically illustrating an example of light intensity measurement when performing self-start in a general mode-locked laser device; FIG. 6 is a diagram illustrating an example of a relationship between the intensity of light detected by a photodetector provided at a monitor port and the optical output of a pumping light source; FIG. 7 is a diagram illustrating an example of a relationship between the intensity of light detected by a photodetector provided at a monitor port and the optical output of a pumping light source; FIG. 8 is a diagram illustrating an example of a relationship between the intensity of light detected by a photodetector provided at a monitor port and the optical output of a pumping light source; FIG. 9 is a diagram illustrating the intensity of monitor light detected by a photodetector in a mode-locked laser device according to the first embodiment; FIG. 10 is a diagram illustrating the spectrum of laser light detected at a monitor port when the optical output of a pumping light source is 250 mW in a comparative example; and FIG. 11 is a diagram illustrating the spectrum of laser light detected at a monitor port when the optical output of a pumping light source is 78 mW in a comparative example. 1 is a diagram showing the spectrum of supervisory light when the optical output of the pumping light source is 250 mW in the mode-locked laser device according to the first embodiment; FIG. 2 is a diagram showing the spectrum of supervisory light when the optical output of the pumping light source is 78 mW in the mode-locked laser device according to the first embodiment; FIG. 3 is a flowchart showing the self-start operation of the mode-locked laser device according to the first embodiment; FIG. 4 is a diagram showing a schematic configuration of a mode-locked laser device according to a second embodiment; FIG. 5 is a diagram showing the intensity of supervisory light detected by a photodetector in the mode-locked laser device according to the second embodiment; FIG. 6 is a diagram showing a schematic configuration of a mode-locked laser device according to a third embodiment; FIG. 7 is a diagram showing a schematic configuration of a mode-locked laser device according to a fourth embodiment; FIG. 8 is a diagram showing a schematic configuration of a mode-locked laser device according to a fifth embodiment;

[0018] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same elements are designated by the same reference numerals, and redundant explanations will be omitted as necessary.

[0019] A mode-locked laser device according to embodiment 1 will be described below. The mode-locked laser device according to embodiment 1 is configured as a figure-8 passive mode-locked laser made of a polarization-maintaining single-clad single-mode optical fiber.

[0020] 1 is a diagram illustrating a schematic configuration of a mode-locked laser device according to a first embodiment. The mode-locked laser device 100 includes a ring resonator 10, a nonlinear amplifying loop mirror 20, a photodetector 1, and a controller 2. Hereinafter, the nonlinear amplifying loop mirror will be abbreviated as NALM.

[0021] The ring resonator 10 and the NALM 20 are optically connected to form a single laser oscillator. In the mode-locked laser device 100, mode-locked single pulse light is generated by controlling the oscillation state of light circulating in the laser oscillator, and output light L from the output port OUT. ＯＵＴ In the following embodiment, the oscillation state of the light circulating in the laser oscillator is controlled by controlling the amplification degree in the NALM 20 for the light circulating in the laser oscillator.

[0022] The ring resonator 10 is provided with a bandpass filter (BPF) that narrows the band of the circulating light. Of the light that passes through the bandpass filter, light in a band outside the passband is branched off as supervisory light L to be monitored by the photodetector 1 and output to the photodetector 1. The photodetector 1 detects the incident supervisory light L and outputs a detection signal DET indicating the detection result to the control unit 2. The control unit 2 observes the intensity of the supervisory light L using the detection signal DET and controls the amplification degree in the NALM 20 for the light circulating in the laser oscillator based on the observation result.

[0023] The configuration of the mode-locked laser device 100 will be specifically described below. Fig. 2 is a diagram schematically showing the configuration of the mode-locked laser device according to the first embodiment. Fig. 2 further shows the pumping light source 3 and the optical isolator 4 included in the NALM 20 provided in the mode-locked laser device 100.

[0024] The ring resonator 10 and the NALM 20 are connected by a polarization-maintaining 3x3 optical coupler 21 provided in the NALM 20 to form a single laser oscillator. Hereinafter, the NALM that constitutes the laser oscillator will also be referred to as the second oscillation means or the second oscillator, and the ring resonator or the resonator connected to the NALM will also be referred to as the first oscillation means or the first oscillator. The optical coupler provided in the NALM and connected to the ring resonator or the resonator will also be referred to as the optical multiplexing / demultiplexing section or the optical multiplexing / demultiplexing means.

[0025] The ring resonator 10 has a configuration in which light propagates along a circular path formed by a polarization-maintaining optical fiber 11, both ends of which are connected to optical couplers 21. The NALM 20 has a configuration in which light propagates along a circular path formed by a polarization-maintaining optical fiber 22, both ends of which are connected to optical couplers 21. As a result, light circulates through a laser oscillator formed by the ring resonator 10 and the NALM 20. Hereinafter, the polarization-maintaining optical fiber 11 will also be referred to as a first optical waveguide. The polarization-maintaining optical fiber 22 will also be referred to as a second optical waveguide.

[0026] The optical coupler 21 may be configured as, for example, an optical fiber tap coupler (also referred to as an optical fiber coupler or an optical tap coupler). Ports P1, P2, and PA are provided on the NALM 20 side of the optical coupler 21, and a transmission port P3, a reflection port P4, and a port PB are provided on the ring resonator 10 side. Hereinafter, ports P1 and P2 will also be referred to as the first and second ports, respectively. The transmission port P3 and reflection port P4 will also be referred to as the third and fourth ports, respectively.

[0027] One end of the polarization-maintaining optical fiber 11 is connected to the transmission port P3, and the other end is connected to the reflection port P4. The port PB is an open end. One end of the polarization-maintaining optical fiber 22 is connected to the port P1, and the other end is connected to the port P2. The port PA is connected to the output light L of the mode-locked laser device 100. ＯＵＴ In this example, the output light L ＯＵＴ In order to prevent the return light from returning to the port PA, an optical isolator 4 is inserted.

[0028] In the ring resonator 10, a multi-port bandpass filter 12 and an optical isolator 13 are inserted in the circular path of the polarization-maintaining optical fiber 11. For convenience of explanation, the light L11 propagating in the clockwise direction through the circular path of the polarization-maintaining optical fiber 11 will also be referred to as clockwise light L11. Furthermore, the light L12 propagating in the counterclockwise direction through the circular path of the polarization-maintaining optical fiber 11 will also be referred to as counterclockwise light L12. Furthermore, the clockwise direction of the circular path of the polarization-maintaining optical fiber 11 will also be referred to as the third direction, and the counterclockwise direction will also be referred to as the fourth direction.

[0029] The multiport bandpass filter (hereinafter referred to as multiport BPF) 12 is a bandpass filter that passes light in a predetermined passband. Fig. 3 is a diagram showing the passband of the multiport BPF. As indicated by the solid line in Fig. 3, the multiport BPF 12 passes light in a passband B1. On the other hand, the multiport BPF 12 branches light in a band B2 outside the passband B1 from the incident light, and directs it as monitor light L to the photodetector 1.

[0030] The optical isolator 13 transmits the clockwise light L11 propagating through the polarization-maintaining optical fiber 11 and blocks the counterclockwise light L12. As a result, the light L11 incident on the polarization-maintaining optical fiber 11 from the NALM 20 via the transmission port P3 of the optical coupler 21 circulates around the polarization-maintaining optical fiber 11 in the clockwise direction and is recoupled with the reflection port P4 of the optical coupler 21.

[0031] In the NALM 20, a passive optical fiber section 23 configured as a phase difference imparting means for imparting a phase difference by self-phase modulation is provided between light L21 propagating in the clockwise direction and light L22 propagating in the counterclockwise direction through the circular path of the polarization-maintaining optical fiber 22. Hereinafter, for convenience of explanation, the light L21 propagating in the clockwise direction through the circular path of the polarization-maintaining optical fiber 22 will also be referred to as clockwise direction light L21. Furthermore, the light L22 propagating in the counterclockwise direction through the circular path of the polarization-maintaining optical fiber 22 will also be referred to as counterclockwise direction light L22. Furthermore, the clockwise direction of the circular path of the polarization-maintaining optical fiber 22 will also be referred to as the first direction, and the counterclockwise direction will also be referred to as the second direction.

[0032] In the polarization-maintaining optical fiber 22, a WDM (Wavelength Division Multiplex) coupler 24 is inserted in the path from the passive optical fiber portion 23 to the port P1 of the optical coupler 21 in the clockwise direction.

[0033] The excitation light source 3 emits excitation light L Ｅ The excitation light L from the excitation light source 3 is output. Ｅ is coupled to a polarization-maintaining optical fiber 22 by a WDM coupler 24 .

[0034] In the polarization-maintaining optical fiber 22, an erbium-doped optical fiber (hereinafter referred to as EDF) 25 is inserted as an optical amplifier made of a laser medium in the path from the passive optical fiber portion 23 to the port P2 of the optical coupler 21 in the counterclockwise direction. The EDF 25 amplifies the pumping light L coupled from the pumping light source 3 to the polarization-maintaining optical fiber 22 via the WDM coupler 24. Ｅ and amplifies the light passing through it.

[0035] The photodetector 1 detects the monitor light L, which is light in a band other than the passband of the multiport BPF 12 and which has propagated counterclockwise through the polarization-maintaining optical fiber 11 from the reflection port P4 relative to the port P1, reached the multiport BPF 12, and been branched by the multiport BPF 12 to the photodetector 1. The photodetector 1 then outputs a detection signal DET indicating the intensity of the monitor light L to the control unit 2.

[0036] The control unit 2 adjusts the drive current of the excitation light source 3 by providing a control signal CON to the excitation light source 3 based on the detection signal DET output from the photodetector 1. Ｅ In the following, the intensity of the excitation light L output from the excitation light source 3 can be controlled. Ｅ The intensity of the excitation light L output from the excitation light source 3 is also referred to as the optical output of the excitation light source 3. Ｅ may be modulated in a predetermined manner.

[0037] Next, the propagation of light in the mode-locked laser device 100 will be described. Light L21 propagating clockwise through the NALM 20 enters port P1, and light L22 propagating counterclockwise enters port P2, where they interfere in the optical coupler 21. The resulting interference light exits from a transmission port P3 and a reflection port P4 depending on the phase difference between the light L21 propagating clockwise through the NALM 20 and the light L22 propagating counterclockwise. The light exiting from the transmission port P3 and the reflection port P4 will be described later. In this configuration, the light propagating to port PB is exited to the outside.

[0038] Light L11 emitted from the transmission port P3 of the optical coupler 21 propagates clockwise through the polarization-maintaining optical fiber 11, passes through the optical isolator 13, and then enters the multiport BPF 12. Of the clockwise light L11 that enters the multiport BPF 12, light in the passband of the multiport BPF 12 propagates from the multiport BPF 12 to the reflection port P4 of the optical coupler 21, and is then branched to ports P1 and P2 and enters the polarization-maintaining optical fiber 22. Note that light outside the passband of the multiport BPF 12 is blocked within the multiport BPF 12.

[0039] On the other hand, light L12 emitted from reflection port P4 of optical coupler 21 propagates counterclockwise through polarization-maintaining optical fiber 11 and enters multiport BPF 12. Of the counterclockwise light L12 that enters multiport BPF 12, light in the passband of multiport BPF 12 propagates to but is blocked by optical isolator 13. Of the counterclockwise light L12 that enters multiport BPF 12, light in a band other than the passband of multiport BPF 12 is branched by multiport BPF 12 and emitted to photodetector 1 as monitor light L.

[0040] The light L21 emitted from the port P2 and propagating clockwise through the polarization-maintaining optical fiber 22 is the pumping light L ＥThe clockwise light L21 is amplified by propagating through the EDF 25 pumped by the ion beam. The amplified clockwise light L21 undergoes a large phase shift due to self-phase modulation as it propagates through the passive optical fiber section 23. Thereafter, the clockwise light L21 with a large phase shift undergoes a loss of about 1 dB by the WDM coupler 24, and then enters the port P1 of the optical coupler 21.

[0041] The light L22 emitted from port P1 and propagating counterclockwise through the polarization-maintaining optical fiber 22 is subjected to a loss of about 1 dB by the WDM coupler 24 and then enters the passive optical fiber unit 23. The counterclockwise light L22 that enters the passive optical fiber unit 23 has a weaker optical intensity than the clockwise light L21 because it has not yet been amplified by the EDF 25. Therefore, the amount of phase shift that the counterclockwise light L22 undergoes due to self-phase modulation while propagating through the passive optical fiber unit 23 is smaller than that of the clockwise light L21. The counterclockwise light L22 that has propagated through the passive optical fiber unit 23 is amplified by the EDF 25 and then enters port P2 of the optical coupler 21.

[0042] Next, we will explain the light that is output from the transmission port P3 and the reflection port P4 due to interference between the light that is input from the polarization-maintaining optical fiber 22 to ports P1 and P2 of the optical coupler 21. Figure 4 is a diagram schematically showing the spectrum of the light that is output from the ports of the optical coupler in a mode-locked state. As described above, a phase difference occurs between the light that is input to ports P1 and P2 of the optical coupler 21 after circulating through the polarization-maintaining optical fiber 22 due to the difference in the amount of phase shift received by the passive optical fiber section 23.

[0043] When this phase difference is less than 2π / 3 radians, light that has received a large phase shift by the passive optical fiber section 23 is likely to exit from the transmission port P3 and propagate clockwise through the polarization-maintaining optical fiber 11 of the ring resonator 10. On the other hand, light that has received a small phase shift by the passive optical fiber section 23 is likely to exit from the reflection port P4 and propagate counterclockwise through the polarization-maintaining optical fiber 11 of the ring resonator 10.

[0044] As described above, the amount of phase shift that light undergoes varies depending on the propagation direction in the polarization-maintaining optical fiber 22, but the amount of phase shift also varies depending on the spectrum of the light. When the light propagating through the passive optical fiber section 23 is an optical pulse, the light near the peak in the spectrum of the optical pulse has high intensity and therefore undergoes a large phase shift in the passive optical fiber section 23. Therefore, the light near the peak in the spectrum of the optical pulse tends to propagate in the clockwise direction through the polarization-maintaining optical fiber 11 of the ring resonator 10 via the transmission port P3. On the other hand, the light in the skirt portion other than the peak in the spectrum of the optical pulse tends to propagate in the counterclockwise direction through the polarization-maintaining optical fiber 11 of the ring resonator 10 via the reflection port P4. The light propagated to port PB has a spectrum intermediate between the light emitted from the transmission port P3 and the reflection port P4, and is emitted to the outside as described above.

[0045] On the other hand, most of the light L12 propagating counterclockwise through the polarization-maintaining optical fiber 11 from the reflection port P4 is light in a band other than the band near the spectral peak of the optical pulse. Therefore, the intensity of light in the pass band of the multiport BPF 12 is low, and broadband light having peaks in bands on both sides of the pass band is incident on the multiport BPF 12. Therefore, most of the light L12 propagating counterclockwise through the polarization-maintaining optical fiber 11 from the reflection port P4 is emitted from the multiport BPF 12 toward the photodetector 1 as monitor light L. Note that although a portion of the light L12 propagating counterclockwise through the polarization-maintaining optical fiber 11 passes through the multiport BPF 12, it is subsequently blocked by the optical isolator 13.

[0046] The photodetector 1 detects the intensity of the monitoring light L in a band other than the passband that is branched by the multiport BPF 12 out of the light L12 that propagates counterclockwise through the polarization-maintaining optical fiber 11 and reaches the multiport BPF 12.

[0047] In the mode-locked laser device 100, as described above, the light repeatedly circulates in the laser oscillator formed by the ring resonator 10 and the NALM 20 via the optical coupler 21, increasing the peak intensity of the light, forming an optical pulse with a pulse width of approximately picoseconds, and then reaching a steady state after a certain period of time, thereby completing the self-start of mode locking.

[0048] If the phase difference between light L21 and light L22 entering optical coupler 21 from NALM 20 is greater than 2π / 3 radians, splitting of the optical pulse occurs in self-start, resulting in a multi-pulse state in which multiple mode-locked pulses exist. Therefore, to achieve a mode-locked state and a single-pulse state in self-start, it is desirable to make the phase difference between light L21 and light L22 entering optical coupler 21 from NALM 20 smaller than 2π / 3 radians.

[0049] Next, the advantages of self-starting in the mode-locked laser device 100 will be described. Generally, in self-starting a passive mode-locked laser device, the optical output of the pump light source is increased continuously or stepwise from the initial state while the output light of the mode-locked laser device and the light circulating in the mode-locked laser device are observed as described above. Then, based on the observation results, it is determined whether the mode-locked laser device is in a mode-locked state.

[0050] Even after transitioning to a mode-locked state, if the optical output of the pump light source is high, a multi-pulse state or a state in which a continuous-wave component exists may occur. In a multi-pulse state, the number of optical pulses, the intensity of each optical pulse, and the interval between optical pulses are poorly reproducible, and the multi-pulse state is undesirable from the viewpoint of optical noise. Furthermore, the continuous-wave component is difficult to separate from the ultrashort pulse component. When the light is amplified externally, it is amplified preferentially over the optical pulse, significantly compromising the advantages of an ultrashort pulse laser. Therefore, it is preferable to remove the continuous-wave component within the oscillator. Therefore, in general, when self-starting a passively mode-locked laser device, it is preferable to achieve a single-pulse state in which only one optical pulse exists while removing the continuous-wave component.

[0051] In order to realize a single-pulse state while eliminating the continuous-wave component in self-starting, a method of reducing the amplification factor of the optical amplifier is generally used to control the oscillation state of light in a mode-locked laser device to a suitable state. Reducing the amplification factor of the optical amplifier when a multi-pulse state and a continuous-wave component are present further reduces the intensity of the continuous-wave component, which is weaker than the optical pulse, so that the weak continuous-wave component is absorbed by the saturable absorption mechanism. Subsequently, by subsequently reducing the amplification factor of the optical amplifier, the number of optical pulses can be gradually reduced. This removes the continuous-wave component and realizes a single-pulse state. For example, a method of reducing the amplification factor of the optical amplifier involves reducing the intensity of the pump light output from the pump light source, i.e., the optical output.

[0052] In other words, the self-start of a mode-locked laser device is an operation that includes a step of increasing the optical output of the pump light source to transition from a non-mode-locked state to a mode-locked state, and a step of reducing the optical output of the pump light source after transition to the mode-locked state to transition to a single-pulse state while removing the continuous wave component.

[0053] In this case, the simplest way to perform self-start is to use a method of monitoring the state of the mode-locked laser based on the results of observing the output light of the mode-locked laser and the light circulating in the mode-locked laser, as described above. Below, as a comparative example, an example of self-start in a general mode-locked laser will be described.

[0054] 5 is a diagram schematically illustrating an example of measuring the intensity of light when performing self-starting in a typical mode-locked laser. In the typical mode-locked laser 900 of FIG. 5, the ring resonator 10 of the mode-locked laser 100 according to the present embodiment is replaced with a ring resonator 90. In the ring resonator 90, the multiport BPF 12 and the optical isolator 13 of the ring resonator 10 are replaced with a multiport BPF 92 and an optical isolator 93, respectively.

[0055] Unlike the multiport BPF 12, the multiport BPF 92 is configured as a general BPF that blocks light in bands other than the passband.

[0056] From the output port OUT, the output light L ＯＵＴ A part of the signal is branched off and output to the monitor port M1.

[0057] The optical isolator 93 branches a part of the light L11 propagating clockwise through the polarization-maintaining optical fiber 11 and outputs it to the monitor port M2.

[0058] The port PB of the optical coupler 21 is connected to the monitor port M3, and the light emitted from the port PB is output to the monitor port M3.

[0059] The light emitted to the monitor ports M1 to M3 is detected by a photodetector (not shown).

[0060] 6 to 8 show examples of the relationship between the intensity of light detected by the photodetectors provided in the monitor ports M1 to M3 and the optical output of the pump light source. Here, a photodetector-type optical power meter was used as the photodetector. The vertical axis in FIGS. 6 to 8 shows the average intensity of light detected by the photodetector, normalized based on the average intensity of light detected by the photodetector when the optical output of the pump light source 3 was 78 mW and in a non-mode-locked state. For ease of explanation, the normalized average intensity of light detected by the photodetector in FIGS. 6 to 8 will be simply referred to as the "light intensity."

[0061] The circular markers indicate the optical intensity detected at each monitor port during the process of increasing the optical output of the pump light source 3 from zero before the start of self-starting. Therefore, the circular markers indicate values ​​in a non-mode-locked state. In contrast, the square markers indicate the optical intensity detected at each monitor port during the process of reducing the optical output of the pump light source 3 to achieve a single-pulse state after the mode-locked state is achieved after the start of self-starting. Therefore, the square markers indicate values ​​in a mode-locked state. In this example, the lower limit of the optical output of the pump light source 3 at which the mode-locked state can be maintained is 73 mW. In this example, the optical spectrum and oscilloscope waveform at a monitor port where no measurement was performed were observed to determine whether the state was mode-locked or non-mode-locked.

[0062] At each of the monitor ports M1 to M3, the difference in average optical intensity between the mode-locked state (circular marker) and the non-mode-locked state (square marker) is small, as shown in Figures 6 to 8. In Figures 6 to 8, when the optical output of the pump light source 3 in the non-mode-locked state is 78 mW, the optical intensities detected at the monitor ports M1 to M3 are approximately 0.004 mW, approximately 0.021 mW, and approximately 1.9 mW, respectively.

[0063] It can be seen that in these comparative examples, the difference in average optical intensity between the mode-locked state and the non-mode-locked state is so small that it is difficult to detect, and therefore, even if the optical intensity is monitored over a long period of time at monitor ports M1 to M3, it is difficult to distinguish between the mode-locked state and the non-mode-locked state based on the fluctuations in optical intensity.

[0064] In contrast, in the mode-locked laser device 100 according to this embodiment, the intensity of the monitor light L detected by the photodetector 1 varies greatly between the mode-locked state and the non-mode-locked state. Fig. 9 is a diagram showing the intensity of the monitor light detected by the photodetector in the mode-locked laser device according to the first embodiment. In Fig. 9, the values ​​on the vertical and horizontal axes and the values ​​indicated by the circular and square markers are the same as those in Figs. 6 to 8, and therefore redundant explanations will be omitted.

[0065] As shown in Fig. 6, the difference in intensity of the monitor light L detected by the photodetector 1 between the mode-locked state and the non-mode-locked state is larger than in the cases of Figs. 6 to 8. The average intensity of the light in the mode-locked state is higher than the intensity of the light in the non-mode-locked state, regardless of the optical output of the pump light source 3, and a gap D is created between the two. Therefore, by determining whether the intensity of the monitor light L detected by the photodetector 1 falls above or below this gap D, it can be easily determined whether the mode-locked laser device 100 is in a mode-locked state or a non-mode-locked state.

[0066] In other words, by determining whether the intensity of the monitoring light L detected by the photodetector 1 is large or small relative to a threshold value set as a single value for the gap D, it is possible to easily determine whether the mode-locked laser device 100 is in a mode-locked state or a non-mode-locked state.

[0067] 9, the gap D between the intensity of the monitor light L in the mode-locked state and the intensity of the monitor light L in the non-mode-locked state is more than twice the width of the distribution of the average light intensity in the non-mode-locked state, because the monitor light L detected by the photodetector 1 is broadband light emitted from the reflection port P1 of the optical coupler 21.

[0068] In FIG. 9, when the optical output of the pumping light source 3 in a non-mode-locked state is 78 mW, the intensity of the monitor light L detected by the photodetector 1 is approximately 0.26 mW, which is approximately 65 times the optical intensity of approximately 0.004 mW detected at the monitor port M1 in the comparative example of FIG. 6.

[0069] Therefore, even in a non-mode-locked state, the mode-locked laser device 100 can suitably monitor the intensity of the monitoring light L using the photodetector 1. Therefore, highly reliable state monitoring is possible regardless of whether the mode-locked laser device 100 is in a mode-locked state or a non-mode-locked state.

[0070] Next, the difference in light intensity between the mode-locked state and the non-mode-locked state will be explained from the viewpoint of the light spectrum. Figures 10 and 11 are diagrams showing the spectrum of the laser light detected at the monitor port M1 when the optical output of the pump light source 3 is 250 mW and 78 mW, respectively, in a comparative example. In Figures 10 and 11, the spectrum in the mode-locked state is shown by a solid line, and the spectrum in the non-mode-locked state is shown by a dashed line. At the monitor port M1, the output light L output to the outside is ＯＵＴ In order to ensure the intensity of the output light L ＯＵＴ Approximately 1% of the light is branched off and monitored, so the measured light intensity is low.

[0071] 12 and 13 are diagrams showing the spectrum of the monitoring light when the optical output power of the pumping light source is 250 mW and 78 mW, respectively, in the mode-locked laser device according to the first embodiment. It can be seen from Fig. 12 and Fig. 13 that the spectrum in the mode-locked state and the spectrum in the non-mode-locked state are significantly different from each other compared to Fig. 10 and Fig. 11 according to the comparative example.

[0072] The light intensity detected by the photodetector is the integral value of the spectrum, so in order to determine whether the mode-locked laser device 100 is in a mode-locked state or a non-mode-locked state based on the light intensity, it is desirable that the light intensity between the two states be as different as possible.

[0073] 10 and 11, however, the difference in the spectrum between the mode-locked state and the non-mode-locked state is small, and as a result, the difference in the measured light intensity is also small, which shows that it is difficult to determine mode-locking by observing the intensity of the output light of the mode-locked laser device at monitor port M1.

[0074] 12 and 13, the difference in the spectrum between the mode-locked state and the non-mode-locked state is large. As a result, the difference in the intensity of the supervisory light between the mode-locked state and the non-mode-locked state is large. Therefore, based on the intensity of the supervisory light L detected by the photodetector 1, it is possible to easily perform robust and reliable mode-locking determination.

[0075] Next, a description will be given of the control operation of the mode-locked state in the mode-locked laser apparatus 100. Fig. 14 is a flowchart showing the self-start operation of the mode-locked laser apparatus according to the first embodiment. In the mode-locked laser apparatus 100, the photodetector 1 outputs a current signal or a voltage signal indicating the intensity of the detected supervisory light L as a detection signal DET. Hereinafter, if the detection signal DET is a current signal, the current value will be referred to as the intensity of the detection signal DET, and if the detection signal DET is a voltage signal, the voltage value will be referred to as the intensity of the detection signal DET.

[0076] Step S1: The control unit 2 sets the driving current I of the excitation light source 3 to the initial value I ＩＮＩ Set to.

[0077] Step S2: Based on the detection signal DET, the control unit 2 determines whether the intensity P of the monitor light L detected by the photodetector 1 is equal to or greater than a preset reference value P ＲＥＦ Determine whether it is higher than

[0078] Step S3: The intensity P of the monitor light L detected by the photodetector 1 is equal to or greater than the preset reference value P ＲＥＦ If the value is equal to or less than the predetermined value ΔI, the control unit 2 determines that the mode-locked laser device 100 is in a non-mode-locked state. ｍ Then, the process returns to step S2.

[0079] Step S4: The intensity P of the monitor light L detected by the photodetector 1 is equal to the preset reference value P ＲＥＦ , the mode-locked laser device 100 is considered to be in a mode-locked state, as described above. In this case, the control unit 2 sets the drive current I of the pump light source 3 to a preset single-pulse current value I ｓ Reduce it to

[0080] Step S5: Based on the detection signal DET, the control unit 2 determines whether the intensity P of the monitor light L detected by the photodetector 1 is equal to or less than the preset reference value P ＲＥＦ Determine whether it is higher than

[0081] Step S6: The intensity P of the monitor light L detected by the photodetector 1 is equal to the preset reference value P ＲＥＦIf the single pulse current value I ｓ is considered to be too low to maintain the mode-locked laser device 100 in a mode-locked state. Therefore, the control unit 2 sets the single-pulse current value I ｓ Here, for example, the single pulse current value I ｓ a predetermined value ΔI ｓ Then, the process returns to step S1.

[0082] Step S7: The intensity P of the monitor light L detected by the photodetector 1 is equal to the preset reference value P ＲＥＦ If it is higher than the single pulse current value I ｓ It is considered that the mode-locked laser device 100 can be maintained in the mode-locked state and the single-pulse state at this point. Therefore, the control unit 2 ends the self-start.

[0083] The driving current required for the single pulse state may increase due to the surrounding environment of the mode-locked laser device 100 or the deterioration of the optical components mounted therein over time. In this case, the single pulse current value I ｓ Even if the pump light source 3 is driven at this value, a situation may occur in which the mode-locked state cannot be maintained in the mode-locked laser device 100. In this case, the self-start is started again and the cycle of steps S4 to S6 is repeated to obtain the single-pulse current value I ｓ can be updated to a suitable value.

[0084] As described above, according to this configuration, it is possible to easily perform self-starting, which transitions the mode-locked laser device 100 to a mode-locked state and a single-pulse state, based on the intensity of the light circulating in the mode-locked laser device 100 .

[0085] Second Embodiment In this embodiment, a modified example of the mode-locked laser apparatus 100 according to the second embodiment will be described. Fig. 15 is a diagram schematically illustrating the configuration of the mode-locked laser apparatus according to the second embodiment. The mode-locked laser apparatus 200 has a configuration in which the ring resonator 10 of the mode-locked laser apparatus 100 is replaced with a ring resonator 30. The ring resonator 30 has a configuration in which the multi-port BPF 12 of the ring resonator 10 is replaced with a multi-port BPF 32.

[0086] The multiport BPF 32 is a bandpass filter that passes laser light in a predetermined passband, similar to the multiport BPF 12. However, the multiport BPF 32 differs from the multiport BPF 12 in that the multiport BPF 32 branches light outside the passband of the clockwise light L11 to the photodetector 1 as monitor light L.

[0087] The other configurations of the mode-locked laser device 200 are similar to those of the mode-locked laser device 100, so redundant explanations will be omitted.

[0088] Next, the operation of the mode-locked laser device 200 will be described. As described in the first embodiment with reference to Fig. 4, the light L11 propagating in the clockwise direction through the polarization-maintaining optical fiber 11 is light that contains a large amount of the band near the peak of the optical pulse spectrum. Therefore, the intensity of the monitor light L in the band outside the passband of the multi-port BPF 32, which is contained in the clockwise light L11, is smaller than that of the counterclockwise light L12.

[0089] 16 is a diagram showing the intensity of the monitor light detected by the photodetector in the mode-locked laser device according to the second embodiment. The values ​​of the vertical and horizontal axes in Fig. 16 and the values ​​indicated by the circular and square markers are the same as those in Fig. 9, so a duplicated explanation will be omitted.

[0090] In Fig. 16, the intensity of the monitor light L detected by the photodetector 1 is slightly lower than in Fig. 9, and the gap D between the intensity of the monitor light L in the mode-locked state and the intensity of the monitor light L in the non-mode-locked state is narrower. However, even in this case, it is possible to distinguish between the mode-locked state and the non-mode-locked state based on the intensity of the monitor light L, just as in Fig. 9.

[0091] Therefore, according to the mode-locked laser device 200, by comparing the intensity of the monitoring light with a threshold value set as a single value for the gap D, it is possible to easily determine whether the mode-locked laser device 200 is in a mode-locked state or a non-mode-locked state.

[0092] Third Embodiment In this embodiment, in the mode-locked laser device 100 according to the first embodiment, the output light L ＯＵＴ 17 is a diagram schematically illustrating a configuration of a mode-locked laser device according to a third embodiment. The mode-locked laser device 300 has a configuration in which the ring resonator 10 and the NALM 20 of the mode-locked laser device 100 are replaced with a ring resonator 40 and an NALM 50, respectively.

[0093] The ring resonator 40 has a configuration in which the optical isolator 13 of the ring resonator 10 is replaced with an optical isolator 43. The optical isolator 43 branches a part of the light L11 propagating in the clockwise direction through the polarization-maintaining optical fiber 11, and outputs the branched light L ＯＵＴ and output to the output port OUT.

[0094] The NALM 50 has a configuration in which the 3×3 optical coupler 21 of the NALM 20 is replaced with a 2×2 optical coupler 51. As described above, in the mode-locked laser device 300, a part of the clockwise light L11 is branched by the optical isolator 43, and the output light L ＯＵＴThe light is output to the output port OUT as a 2×2 optical coupler 51. Therefore, compared to the mode-locked laser device 100, the mode-locked laser device 300 does not require a connection between the optical coupler of the NALM and the output port, and therefore uses a 2×2 optical coupler 51 in which ports PA and PB of the 3×3 optical coupler are removed. The connection relationships regarding ports P1 and P2, transmission port P3, and reflection port P4 of the optical coupler 51 are the same as those in the mode-locked laser device 100, so redundant explanations will be omitted.

[0095] As described above, according to the mode-locked laser device 300, even if the optical coupler 51 of the NALM 50 has a simpler configuration, the output light L ＯＵＴ can be extracted and output.

[0096] In this embodiment, the optical isolator 43 provided in the ring resonator 40 converts the clockwise light L11 into the output light L ＯＵＴ However, this is merely an example. For example, the clockwise light L11 is output at an arbitrary position of the polarization-maintaining optical fiber of the ring resonator. ＯＵＴ By providing an optical branching means such as an optical coupler that branches the output light L ＯＵＴ You may take it out.

[0097] According to the mode-locked laser device 300, extraction and monitoring of the monitoring light L are similar to those of the mode-locked laser device 100, and therefore similar self-starting is possible.

[0098] Fourth Embodiment In this embodiment, in the mode-locked laser device 100 according to the first embodiment, the pumping light L Ｅ 18 is a diagram schematically illustrating a configuration of a mode-locked laser device according to a fourth embodiment. The mode-locked laser device 400 has a configuration in which the NALM 20 of the mode-locked laser device 100 is replaced with an NALM 60.

[0099] In the mode-locked laser device 400, the pumping light source 3 is connected to the port PB of the optical coupler 21, and emits pumping light LE to the port PB.

[0100] The NALM 60 has a configuration in which the WDM coupler 24 is removed from the NALM 20. The pumping light L incident on the port PB Ｅ is branched by the optical coupler 21, propagates counterclockwise from the port P1 to pump the EDF 25, and propagates counterclockwise from the port P2 to pump the EDF 25. That is, in the mode-locked laser device 400, the pumping light L Ｅ The EDF 25 is excited in both directions by the

[0101] As described above, according to the mode-locked laser device 300, even if the WDM coupler 24 for coupling the pump light from the NALM 60 is removed, the pump light L is transmitted to the EDF 25 via the optical coupler 21. Ｅ Therefore, it is possible to provide a mode-locked laser device having the same functions as those of the above-described embodiment with a simpler configuration.

[0102] Fifth Embodiment In the above-mentioned embodiments, a figure-8 shaped mode-locked laser device has been described, but a figure-9 shaped (Figure-9) mode-locked laser device can also perform similar self-starting. Fig. 19 is a diagram schematically showing the configuration of a mode-locked laser device according to the fifth embodiment. The mode-locked laser device 500 is a figure-9 shaped mode-locked laser device, and has a configuration in which the ring resonator 10 and the NALM 20 of the mode-locked laser device 100 are replaced with a resonator 70 and an NALM 80, respectively.

[0103] The resonator 70 is provided with a polarization-maintaining optical fiber 71, one end of which is connected to the reflection port P4 of the NALM 80 and the other end of which is connected to a fiber mirror 73. A multi-port BPF 72 is inserted into the polarization-maintaining optical fiber 71. In this configuration, light propagating through the polarization-maintaining optical fiber 71 from the reflection port P4 toward the fiber mirror 73 is designated as light L12. Light propagating through the polarization-maintaining optical fiber 71 from the fiber mirror 73 toward the reflection port P4 is designated as light L11.

[0104] The multiport BPF 72 has filter characteristics similar to those of the multiport BPF 12, and outputs light in a band other than the passband of the light L12 traveling from the reflection port P4 to the fiber mirror 73 to the photodetector 1 as monitoring light L.

[0105] The fiber mirror 73 reflects the incident light L12 and emits it as light L11. The fiber mirror 73 is an example of a reflecting means, and various reflecting means such as a loop mirror may be used instead of the fiber mirror 73.

[0106] The NLAM 80 has a configuration in which the optical coupler 21 and the passive optical fiber unit 23 of the NALM 20 are replaced with an optical coupler 81 and a nonreciprocal phase shifter 83, respectively, and an optical coupler 86 is added. The optical coupler 21 is configured as a 2×2 optical coupler, similar to the optical coupler 51.

[0107] The nonreciprocal phase shifter 83 is configured as a phase difference imparting means that imparts a larger phase shift to the clockwise light L21 than to the counterclockwise light L22. In this configuration, the nonreciprocal phase shifter 83 is inserted into the polarization-maintaining optical fiber 22 between the port P1 and the WDM coupler 24. The nonreciprocal phase shifter 83 may be configured as a combination of optical elements such as a Faraday rotator, a λ / 2 plate, and a λ / 4 plate. Alternatively, the nonreciprocal phase shifter 83 may be configured as an element that shifts the phase of propagating light by self-phase modulation, similar to the passive optical fiber section 23.

[0108] The optical coupler 86 is inserted between the port P1 and the nonreciprocal phase shifter 83. The optical coupler 86 branches a part of the light L21 propagating in the clockwise direction through the polarization-maintaining optical fiber 22, and outputs the branched light L ＯＵＴ and output to the output port OUT.

[0109] Alternatively, instead of providing the optical coupler 86, for example, the output light L from the transmission port P3 of the optical coupler 81 may be ＯＵＴ may be emitted.

[0110] As described above, according to the figure-9 mode-locked laser device 500, similar to the figure-8 mode-locked laser device according to the above-described embodiment, self-starting can be performed based on the intensity of the monitoring light L in a band other than the passband output from the multi-port BPF.

[0111] Other Embodiments The present disclosure has been described above with reference to the embodiments, but the present disclosure is not limited to the above-described embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0112] The configurations of the mode-locked laser devices according to the second to fourth embodiments may be combined as appropriate. For example, in the mode-locked laser device according to the second embodiment, similarly to the mode-locked laser device according to the third embodiment, an optical isolator 43 or an optical coupler is provided in the polarization-maintaining optical fiber 11 of the ring resonator, so that the clockwise light L11 can be converted into the output light L ＯＵＴ In the mode-locked laser device according to the second embodiment, similarly to the mode-locked laser device according to the fourth embodiment, the pumping light L is supplied from the pumping light source 3 to the EDF 25 via the optical coupler 21 of the NALM. Ｅ Furthermore, in the mode-locked laser device according to the second embodiment, an optical isolator 43 or an optical coupler is provided in the polarization-maintaining optical fiber 11 of the ring resonator, so that the clockwise light L11 can be converted into the output light L ＯＵＴ and the pumping light L is supplied to the EDF 25 via the optical coupler 21 of the NALM. Ｅ may be supplied.

[0113] In the mode-locked laser device according to the third embodiment, as in the mode-pumped laser according to the fourth embodiment, pumping light L is supplied from the pumping light source 3 to the EDF 25 via the optical coupler 21 of the NALM. Ｅ may be supplied.

[0114] In the mode-locked laser device according to the fifth embodiment, the multi-port BPF 32 according to the second embodiment may be provided to branch the monitoring light L from the light L11. As in the third embodiment, an optical coupler may be provided in the polarization-maintaining optical fiber 71 to branch the monitoring light L from the light L11. ＯＵＴ Furthermore, similarly to the fourth embodiment, the pumping light source 3 may be connected to the transmission port P3 of the optical coupler 81 via an optical coupler, and the pumping light L Ｅ may be supplied.

[0115] In the mode-locked laser devices according to the first to fourth embodiments, the ring resonator may be replaced with the resonator 70 according to the fifth embodiment. As in the fifth embodiment, a non-reciprocal phase shifter 83 of any configuration may be used as a phase difference imparting means instead of the passive optical fiber section 23. Furthermore, as in the fifth embodiment, an optical coupler 86 or the like may be provided in the polarization-maintaining optical fiber 22 of the NALM to provide the output light L ＯＵＴ Alternatively, the output light L may be branched from the transmission port P3 of the optical coupler of the NALM. ＯＵＴ Furthermore, if a desired phase difference can be imparted to the light propagating through the NALM, both the passive optical fiber portion 23 and the nonreciprocal phase shifter 83 may be provided in the NALM.

[0116] In the above-described embodiment, the ring resonator and NLAM of the mode-locked laser device are described as being configured with polarization-maintaining optical fibers, but the optical fibers are not limited to this. Non-polarization-maintaining optical fibers may be used for all or part of the optical fibers. Furthermore, various optical fibers, such as double-clad single-mode fibers and double-clad large-mode-area fibers, may be used for all or part of the optical fibers, and accordingly, an optical fiber combiner may be used instead of a WDM coupler.

[0117] In the above-described embodiment, a silica glass-based erbium-doped optical fiber is assumed as the optical amplifier, but the optical amplifier is not limited to this. For example, a silica glass-based optical fiber doped with one or more elements such as ytterbium, thulium, neodymium, holmium, or bismuth, or a fluoride glass-based optical fiber doped with an element such as praseodymium, may also be used as the laser medium.

[0118] In the above-described embodiment, only one optical amplifier made of EDF is provided in the NALM loop, but the arrangement of the optical amplifier is not limited to this. For example, two or more optical amplifiers may be provided in the NALM loop, or one or more optical amplifiers may be provided in each of the NALM loop and the ring resonator.

[0119] In the above-described embodiment, a 1×2 multiport BPF is used as the multiport BPF, but this is merely an example. For example, a multiport BPF having any number of ports, such as 2×2, may be used as needed.

[0120] In the above-described embodiment, a transmission bandpass filter using an optical filter element is used as the multi-port BPF, but this is merely an example. For example, various filters such as a fused-type WDM coupler or an all-fiber Lyot filter that combines polarization control in an optical fiber with a polarization beam splitter may also be used. Furthermore, a fiber Bragg grating or a chirped fiber Bragg grating may also be used in combination with an optical fiber circulator.

[0121] In the above-described embodiment, the multi-port BPF is provided in a ring resonator or a resonator connected to the NALM, but this is merely an example. The multi-port BPF may be inserted in a path along which light propagates in a saturable absorption mechanism such as the NALM.

[0122] In the above-described embodiment, the mode-locked laser device is provided with only a multi-port optical bandpass filter as an optical filter, but this is merely an example. For example, the mode-locked laser device may be provided with multiple optical filters. For example, the oscillator configured in the mode-locked laser device may be a Mamyshev oscillator including optical filters with two or more different center wavelengths.

[0123] In the above-described embodiment, the optical intensity in the resonator included in the mode-locked laser device is controlled by controlling the optical amplification degree, i.e., by controlling the drive current of the pump light source, but this is merely an example. For example, the loss in the resonator may be controlled by a variable attenuator, the amount of dispersion to be added or subtracted may be controlled by a dispersion compensator, or the polarization state of the light may be controlled by a polarization controller.

[0124] In the above-described embodiment, there is one threshold for determining whether or not a mode-locked state is established, but multiple thresholds for determining whether or not a mode-locked state is established may be set according to the optical output of the pump light source or according to the drive current of the pump light source. Furthermore, the drive current of the pump light source may be changed sharply, gradually, or in preset steps.

[0125] In the above-described embodiment, when determining whether two values ​​are larger or smaller as in steps S2 and S5 of Fig. 14, it may be determined whether the first value is larger than the second value, or it may be determined whether the first value is smaller than the second value. In this case, if the first value and the second value are equal, it may be treated as if the first value is larger than the second value, or it may be treated as if the first value is smaller than the second value, as appropriate. In other words, it may be determined whether the first value is greater than or equal to the second value, or it may be determined whether the first value is less than or equal to the second value.

[0126] Each drawing is merely an example for describing one or more embodiments. Each drawing may not relate to only one particular embodiment, but may also relate to one or more other embodiments. As will be understood by those skilled in the art, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings to create, for example, an embodiment not explicitly shown or described. Not all features or steps shown in any one drawing are necessary to describe an exemplary embodiment, and some features or steps may be omitted. The order of steps described in any drawing may be changed as appropriate.

[0127] This application claims priority based on Japanese Patent Application No. 2024-141865, filed on August 23, 2024, the disclosure of which is incorporated herein in its entirety by reference.

[0128] REFERENCE SIGNS LIST 1 Photodetector 2 Control unit 3 Pumping light source 4 Optical isolator 10, 30, 40, 90 Ring resonator 11, 22, 71 Polarization-maintaining optical fiber 12, 32, 72, 92 Multiport BPF 13, 43, 93 Optical isolator 20, 50, 60, 80 NALM 21, 51, 81, 86 Optical coupler 23 Passive optical fiber unit 24 WDM coupler 25 EDF 70 Resonator 73 Fiber mirror 83 Nonreciprocal phase shifter 100, 200, 300, 400, 500, 900 Mode-locked laser device CON Control signal DET Detection signal L Monitoring light L Ｅ Excitation light L ＯＵＴ Output light L11, L21 Clockwise light L12, L22 Counterclockwise light M1 to M3 Monitor port OUT Output port P1, P2, PA, PB port P3 Transmission port P4 Reflection port

Claims

1. A mode-locked laser device comprising: a laser oscillator connected to a first oscillation means and a second oscillation means configured as a saturable absorption mechanism having an optical amplification means for amplifying propagating light; filtering means provided in a path along which the light propagates in the laser oscillator, for passing light of a predetermined band among the propagating light and branching off light of bands other than the predetermined band as supervisory light; optical detection means for detecting the supervisory light; and control means for controlling the oscillation state of the light propagating in the laser oscillator based on the intensity of the supervisory light detected by the optical detection means, thereby controlling mode-locking of the light propagating in the laser oscillator.

2. A mode-locked laser device according to claim 1, wherein said control means determines that the light propagating through said laser oscillator is in a mode-locked state based on the result of comparing the intensity of said monitor light with a predetermined value.

3. The mode-locked laser device according to claim 2, wherein said control means controls the degree of amplification of said optical amplification means, thereby controlling mode-locking of said light propagating through said laser oscillator.

4. A mode-locked laser device according to claim 3, wherein said control means compares the intensity of said supervisory light with said predetermined value while increasing the amplification degree of said optical amplification means, and when the intensity of said supervisory light becomes greater than said predetermined value, determines that said light propagating through said laser oscillator is in a mode-locked state.

5. The mode-locked laser device according to claim 4, wherein the control means, after determining that the light propagating through the laser oscillator is in a mode-locked state, reduces the amplification degree of the optical amplification means to make the light propagating through the laser oscillator a single-pulse state.

6. The second oscillation means is configured as a nonlinear optical amplification loop mirror, and imparts a larger phase shift to light propagating in a first direction through the optical propagation path of the nonlinear optical amplification loop mirror than to light propagating in a second direction opposite to the first direction through the propagation path; the first oscillation means comprises: a first optical waveguide having one end connected to a transmission port of the second oscillation means and the other end connected to a reflection port, in which light incident from the transmission port propagates in a third direction and light incident from the reflection port propagates in a fourth direction opposite to the third direction; and optical blocking means inserted in the first optical waveguide for blocking light propagating in the fourth direction; the filtering means inserted in the first optical waveguide for passing light in the predetermined band out of light propagating in the third direction and light propagating in the fourth direction, and for branching light in a band other than the predetermined band as the supervisory light; 3. The mode-locked laser device according to claim 1, wherein the light propagating in the third direction is incident on the reflection port of the second oscillation means and is split into light propagating in the first direction and light propagating in the second direction along the propagation path.

7. The second oscillation means is configured as a nonlinear optical amplification loop mirror, and imparts a larger phase shift to light propagating in a first direction through the optical propagation path of the nonlinear optical amplification loop mirror than to light propagating in a second direction opposite to the first direction through the propagation path; the first oscillation means comprises: reflection means for reflecting incident light; and a first optical waveguide having one end connected to a reflection port of the second oscillation means and the other end connected to the reflection means, through which light incident from the reflection port propagates in a third direction and light reflected by the reflection means propagates in a fourth direction opposite to the third direction; the filtering means is inserted into the first optical waveguide, and passes light in the specified band out of light propagating in the third direction and light propagating in the fourth direction, and branches light in bands other than the specified band as the supervisory light; 3. The mode-locked laser device according to claim 1, wherein the light propagating through the first optical waveguide in the third direction is incident on the reflection port of the second oscillation means and is branched into light propagating through the propagation path in the first direction and light propagating in the second direction.

8. A mode-locked laser device according to claim 6, wherein said filtering means splits the supervisory light into one of the light propagating in said third direction and the light propagating in said fourth direction, the one having a stronger intensity of the component in said predetermined band.

9. A mode-locked laser device according to claim 6, wherein said filtering means splits the monitor light into one of the light propagating in said third direction and the light propagating in said fourth direction, the one having a weaker intensity of the component in said predetermined band.

10. A mode-locked laser device according to claim 6, wherein the first oscillation means comprises optical branching means inserted into the first optical waveguide for branching the light propagating in the third direction into two beams and outputting one of the branched beams as output light.

11. The mode-locked laser device according to claim 6, wherein the second oscillation means splits the light incident on the reflection port into light propagating in the first direction through the propagation path, light propagating in the second direction, and output light that is output to the outside.

12. The mode-locked laser device according to claim 6, wherein said second oscillation means outputs said light emitted from said transmission port as output light to be output to the outside.

13. The second oscillation means comprises: an optical multiplexing / demultiplexing means having first and second ports, a third port which is the transmission port, and a fourth port which is the reflection port, and which branches light incident on the fourth port from the first oscillation means to the first port and the second port; a second optical waveguide having one end connected to the second port and the other end connected to the first port, which constitutes the propagation path, through which the light branched to the second port propagates in the first direction and the light branched to the second port propagates in the second direction; a phase difference imparting means inserted into the second optical waveguide, which imparts a larger phase difference to the light propagating in the first direction than to the light propagating in the second direction; and an optical amplifying means inserted into the second optical waveguide so as to amplify the light propagating in the first direction before passing through the phase difference imparting means, and to amplify the light propagating in the second direction after passing through the phase difference imparting means.

7. The mode-locked laser device according to claim 6, wherein light propagating through the second optical waveguide in the first direction is incident on the first port, and light propagating through the second optical waveguide in the second direction is incident on the second port, thereby causing interference in the optical multiplexing / demultiplexing means, and the interference light is emitted from the third and fourth ports.

14. A mode-locked laser device according to claim 13, wherein the second oscillation means comprises optical branching means inserted into the second optical waveguide, which branches a portion of the light propagating in the first direction and outputs the branched portion of the light as output light.

15. A mode-locked laser device according to claim 13, wherein a phase difference is created between the light propagating in the first direction and the light propagating in the second direction by self-phase modulation when propagating through the phase difference providing means.

16. A mode-locked laser device according to claim 13, further comprising a light source that outputs pumping light to said second oscillation means, wherein said optical amplification means amplifies said propagating light by being amplified by said pumping light, and said control means controls the amplification degree of said optical amplification means by controlling the intensity of said pumping light output from said light source.

17. A mode-locked laser device according to claim 16, wherein the second oscillation means comprises optical coupling means for coupling the pumping light output from the light source to the second optical waveguide.

18. The mode-locked laser device according to claim 16, wherein the optical multiplexing / demultiplexing means has a port into which the pumping light from the light source is incident, and the pumping light is branched from the port into the first port and the second port.

19. A control method for a mode-locked laser device having a laser oscillator connected to a first oscillation means and a second oscillation means configured as a saturable absorption mechanism including an optical amplification means for amplifying propagating light, and filtering means provided in a path along which light propagates in the laser oscillator, for passing light of a predetermined band among the propagating light and branching off light outside the predetermined band as supervisory light, the method comprising: detecting the supervisory light; and controlling the oscillation state of the light propagating in the laser oscillator based on the intensity of the supervisory light, thereby controlling the mode-locking of the light propagating in the laser oscillator.

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