WO2015097972A1

WO2015097972A1 - Control device, control method, and program

Info

Publication number: WO2015097972A1
Application number: PCT/JP2014/005726
Authority: WO
Inventors: Goro Fujita; Kenji Tanaka; Tsutomu Maruyama; Yoshiyuki Urakawa
Original assignee: Sony Corporation
Priority date: 2013-12-26
Filing date: 2014-11-14
Publication date: 2015-07-02
Also published as: JP2015143827A

Abstract

There is provided a control device including a light source control section (301) to cause laser light emitted from a laser light source (10) to be emitted as pulse laser light by intermittent drive, a drive section (311) to move at least one of reflection sections (207) of a resonator (21) in an optical axis direction, and a servocontrol section (313) to servocontrol a light path length of the resonator (21) by causing the drive section (311) to move the at least one of the reflection sections (207) in such a manner to meet a resonance condition, on the basis of an error signal that indicates a difference between the light path length of the resonator according to a synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulse laser light that enters the resonator. The system may comprise an external cavity laser diode (10) mode-locked by an electric modulation signal (301), its output is amplified by a SOA (131) being driven by a pulsed signal (40) in synchronisation (302) with the ECLD (10) resulting in pulse bursts. The pulse bursts may be used to synchronously pump an OPO (20) having a non-linear optical crystal (213). The OPO has a servo loop for length stabilisation based on the difference of the modulation signal of the laser (301) and the OPO output frequency (307). In order to prevent instabilities in the OPO output, the pulse burst frequency is larger than the cut-off frequency of the servo loop.

Description

CONTROL DEVICE, CONTROL METHOD, AND PROGRAM

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Priority Patent Application JP 2013-270050 filed December 26, 2013, the entire contents of which are incorporated herein by reference.

The present disclosure relates to, a control device, a control method, and a program.

There has been proposed a laser light generation device which has a non-linear optical element in the resonator to effectively conduct wavelength conversion by the non-linear optical element, using the high power density inside the resonator.

In the resonator utilized in such a laser light generation device, the non-linear optical element is provided between at least one pair of opposing mirrors that make up the resonator, and a fundamental-wave laser light enters into the resonator and passes through the non-linear element, for example. At this time, the distance between the mirrors (i.e., the light path length in the resonator) is controlled in such a manner to coincide with an integral multiple of the incoming laser light, so that the laser light resonates in the resonator to generate laser oscillation.

For example, PTL 1 discloses one example of the laser light generation device utilizing the resonator as described above. The laser light generation device according to PTL 1 is configured to move the position of the mirror that makes up the resonator in the optical axis direction, and servocontrols the position of the mirror on the basis of the error signal which is in proportion to the difference of the resonator length in relation to the incoming laser light of the resonator. By configuring like this, in the laser light generation device according to PTL 1, the light path length of the resonator is automatically controlled in such a manner to meet the condition under which the incoming laser light resonates in the resonator, and the resonance behavior of the incoming laser light of the resonator is stabilized.

JP H6-53593A

Summary

Some light sources for emitting laser light output pulse laser light by emitting the laser light intermittently. The light source that outputs the pulse laser light in this way can be applied to, for example, a measurement device that irradiates the living body to be measured with the laser light (for example, a fluorescence microscope), in order to reduce the damage to the living body.

On the other hand, there is a demand for the provision of a laser light generation device capable of efficiently conducting wavelength conversion of the incoming pulse laser light by causing the pulse laser light to enter and resonate in the resonator, and servocontrolling the light path length of the resonator to stabilize the resonance behavior.

However, when the light path length of the resonator is servocontrolled, the intermittent light emission of the pulse laser light occasionally acts as a disturbance. For example, when the band of the intermittent light emission of the pulse laser light interferes the band which is the subject of the servocontrol of closed loop (hereinafter, simply referred to as "closed-loop servocontrol" occasionally), the intermittent light emission of the pulse laser light becomes a disturbance, which occasionally makes it difficult to control the light path length of the resonator accurately.

Therefore, the present disclosure proposes a novel and improved control device, a control method, and a program capable of controlling the light path length of the resonator in such a manner to stably output high power pulse laser light.

According to an embodiment of the present disclosure, there is provided a control device including a light source control section configured to cause laser light that is emitted from a laser light source, to be emitted as pulse laser light by intermittent drive, a drive section configured to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light, and a control section configured to servocontrol a light path length of the resonator by causing the drive section to move the at least one of the reflection sections in such a manner to meet a resonance condition, on the basis of an error signal that indicates a difference between the light path length of the resonator according to a synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulse laser light that enters the resonator. The control section decides a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of the reflected light, in such a manner to meet conditions of conditional inequalities 1 to 3,

where f_RS is a cutoff frequency of frequency characteristics in the servocontrol, j is a margin value for stabilizing the servocontrol, k is a proportionality constant that is decided based on a threshold value of buildup, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from the laser light source, and Duty is a duty ratio of the pulse laser light.

According to another embodiment of the present disclosure, there is provided a control method including causing laser light that is emitted from a laser light source, to be emitted as pulse laser light by intermittent drive, causing a drive section to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light, servocontrolling a light path length of the resonator by causing the drive section to move the at least one of the reflection sections in such a manner to meet a resonance condition, on the basis of an error signal that indicates a difference between the light path length of the resonator according to a synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulse laser light that enters the resonator, and deciding a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of the reflected light, in such a manner to meet conditions of conditional inequalities 1 to 3,

According to another embodiment of the present disclosure, there is provided a program for causing a computer to execute causing laser light that is emitted from a laser light source, to be emitted as pulse laser light by intermittent drive, causing a drive section to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light, servocontrolling a light path length of the resonator by causing the drive section to move the at least one of the reflection sections in such a manner to meet a resonance condition, on the basis of an error signal that indicates a difference between the light path length of the resonator according to a synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulse laser light that enters the resonator, and deciding a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of the reflected light, in such a manner to meet conditions of conditional inequalities 1 to 3,

As described above, the present disclosure provides a control device, a control method, and a program capable of controlling the light path length of the resonator in such a manner to stably output high power pulse laser light.

Note that the above effects are not necessarily restrictive, but any effect described in the present specification or another effect that can be grasped from the present specification may be achieved along with the above effects or instead of the above effects.

FIG. 1 is a configuration diagram illustrating one example of a schematic configuration of a laser light generation device according to an embodiment of the present disclosure. FIG. 2 is a characteristic diagram illustrating a state where the peak power of a laser is increased by intermittent light emission. FIG. 3 is an explanatory diagram for describing a relationship between signals in servocontrol. FIG. 4 is an explanatory diagram for describing characteristics of servocontrol. FIG. 5 is an explanatory diagram for describing a calculation method of a buildup period. FIG. 6 is a graph illustrating a relationship between an amplification factor and a pulse number in a resonator. FIG. 7 is a graph illustrating a relationship between a buildup period and a finesse. FIG. 8 is an explanatory diagram for describing a limitation of a frequency of intermittent light emission of pulse laser light, which is based on a buildup period. FIG. 9 is an explanatory diagram for describing a relationship between a frequency in synchronous detection of a reflected light and a frequency of intermittent light emission of pulse laser light. FIG. 10 is an explanatory diagram for describing a scope of a frequency of synchronous detection of a reflected light and a frequency of intermittent drive. FIG. 11 is a diagram illustrating one example of a hardware configuration of a laser light generation device according to an embodiment of the present disclosure.

Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the appended drawings. Note that, in this specification and the drawings, elements that have substantially the same function and configuration are denoted with the same reference signs, and repeated explanation is omitted.

Note that description will be made in the below order.
1. Configuration of Laser Light Generation Device
2. Advantage of Intermittent Light Emission
3. Challenge of Laser Light Generation Device according to Present Embodiment
4. Detail of Control Section
4. 1. Relationship between Signals
4. 2. Limitation based on Band of Servocontrol
4. 3. Limitation based on Buildup Period in Resonator
4. 4. Limitation based on Generation of Error Signal
4. 5. Conclusion
5. Hardware Configuration
6. Conclusion

<1. Configuration of Laser Light Generation Device>
First, with reference to FIG. 1, description will be made of the configuration of a laser light generation device according to an embodiment of the present disclosure. FIG. 1 is a configuration diagram illustrating one example of a schematic configuration of a laser light generation device according to an embodiment of the present disclosure.

As illustrated in FIG. 1, the laser light generation device 1 according to the present embodiment includes a light source unit 10, a wavelength conversion optical system 20, a control section 30, an adder 31, and a semiconductor optical amplifier (SOA) driver 40.

The light source unit 10 utilizes a pulse laser, and is a light source of the master oscillator power amplifier (MOPA) type which includes a laser (mode locked laser diode (hereinafter, referred to as MLLD)) with a resonator and a semiconductor optical amplifier (SOA).

The light source unit 10 includes a mode locked laser (MLLD) section 11,

lenses

121, 127 and 129, a mirror 123, an isolator 125, and an optical amplifier section (SOA section) 131. Note that the MLLD section 11 corresponds to the laser (MLLD) with the resonator.

The behavior of the light source unit 10 is controlled by the control section 30. The control section 30 includes an oscillator 301, a synchronizing signal generator section 302, an isolator 303, a photodetector 305, a bandpass filter 307, a mixer 309, a driving signal generator section 311, and a servocontrol driver 313. In the following, the configuration of the light source unit 10 will be described simultaneously with the configuration of the oscillator 301 and the synchronizing signal generator section 302 among the configuration of the control section 30. Note that other configuration of the control section 30 will be described later separately.

The signal of the frequency f_M output from the oscillator 301 is supplied to the adder 31 and the synchronizing signal generator section 302.

The synchronizing signal generator section 302 generates a SOA driving signal for causing the SOA driver 40, which is described later, to intermittently drive the laser light emitted from the MLLD section 11 in such a manner that the laser light emitted from the MLLD section 11 synchronizes with the signal of the frequency f_M supplied from the oscillator 301. Note that, in the following, the frequency for intermittently driving the laser light, i.e., the frequency of the driving signal is occasionally referred to as f_SOA. The synchronizing signal generator section 302 supplies the generated driving signal of the frequency f_SOA to the SOA driver 40. Note that the detail of the SOA driver 40 and the driving signal supplied to the SOA driver 40 will be described later separately in "4.1. Relationship between Signals".

Also, the synchronizing signal generator section 302 generates, by modulating the signal of the frequency f_M, a sample and hold signal (hereinafter, occasionally referred to as "S/H signal") for allowing the driving signal generator section 311, which is described later, to synchronously detect the reflected signal which is based on the reflected light from the resonator 21. At this time, the synchronizing signal generator section 302 generates the S/H signal that is in synchronization with the frequency of the driving signal f_SOA supplied to the SOA driver 40. The synchronizing signal generator section 302 supplies the generated S/H signal to the driving signal generator section 311. Note that the detail of the driving signal generator section 311 and the S/H signal supplied to the driving signal generator section 311 will be described later separately in "4.1. Relationship between Signals".

The adder 31 adds the direct current component (DC Current) of a predetermined output (amplitude) to the signal of the frequency f_M supplied from the oscillator 301, and supplies the signal of the frequency f_M to which the direct current component is added, to the MLLD section 11 as the modulation signal.

The MLLD section 11 includes a laser light source 111, a lens 113, and a diffraction grating 115.

The laser light source 111 outputs the laser light, and may include various types of laser. In the MLLD section 11 according to the present embodiment, the semiconductor laser is utilized as the laser light source 111, for example.

The laser light emitted from the laser light source 111 is directed via the lens 113 to the diffraction grating 115. Then, a resonator (space resonator) is formed between the mirror of the backward end face of the laser light source 111 and the diffraction grating 115, and the frequency f_MLLD of the laser light is decided by the light path length Lc of the resonator. Specifically, the frequency f_MLLD is decided such that f_MLLD = Lc/C, based on the light path length Lc in the MLLD section 11 and the light speed C.

Also, the modulation signal, which is generated by adding the direct current component to the signal of the frequency f_M supplied from the oscillator 301 at the adder 31, is supplied to the laser light source 111. The laser light of the frequency f_MLLD in the MLLD section 11 is phase modulated by the supplied modulation signal of the frequency f_M.

Note that the phase modulator including, for example, an electro-optics (EO) element or an acousto-optics (AO) element may be utilized to modulate the laser light of the frequency f_MLLD. In this case, the phase modulator modulates the laser light of the frequency f_MLLD by the supplied modulation signal of the frequency f_M.

Also, as another example, the laser light source 111 may be configured to emit the laser light modulated at the frequency f_M, from the laser light source 111, by directly driving the laser light source 111 by using the modulation signal of the frequency f_M as the driving signal.

Also, the laser light source 111 may be equipped with a saturable absorber mirror (SAM) as the mirror of the backward end face. The saturable absorber mirror mainly includes a distributed bragg reflector (DBR) and a saturable absorber. As a specific example, a semiconductor saturable absorber mirror (SESAM) can be taken.

The semiconductor saturable absorber mirror conducts Q-switch mode lock on the laser light emitted from the laser light source 111 by the saturable absorber mechanism, and converts the laser light to the laser light having a narrower pulse width and larger energy than the original laser light.

The diffraction grating 115 reflects a part of the light having a predetermined frequency (i.e., the frequency f_MLLD) among the incident laser light in such a manner to send out the part of the light to the outside of the MLLD section 11, and reflects the other part toward the laser light source 111. Also, the diffraction grating 115 reflects the light having other frequencies different from the frequency f_MLLD, toward directions different from the direction toward the outside of the MLLD section 11 or the laser light source 111. By a configuration like this, the light of the frequency f_MLLD resonates in the inside of the MLLD section 11, and only the light of the frequency f_MLLD is sent out to the outside of the MLLD section 11. Note that, given that the configuration described above is realizable, the diffraction grating 115 may be replaced by another component. As a specific example, a combinational component of a bandpass filter (BPF) and a half mirror may be provided instead of the diffraction grating 115. Also, in the following, the laser light emitted from the MLLD section 11, i.e., the laser light generated by modulating the light of the frequency f_MLLD by the modulation signal of the frequency f_M, is occasionally referred to as "laser light L1".

The laser light L1 emitted from the MLLD section 11 is directed via the lens 121 and the mirror 123 to the isolator 125, transmits through the isolator 125, and enters via the lens 127 into the optical amplifier section (SOA section) 131. Note that, given that the laser light L1 emitted from the MLLD section 11 is directed via the isolator 125 to the optical amplifier section (SOA section) 131, it is needless to say that the components of the optical system located in the light path are not limited to the lens 121 and the mirror 123.

The isolator 125 is interposed between the MLLD section 11 and the optical amplifier section (SOA section) 131, and allows the laser light L1 from the MLLD section 11 to transmit through the isolator 125 toward the optical amplifier section (SOA section) 131. Also, the isolator 125 blocks the reflected light (leaked light) from the optical amplifier section (SOA section) 131, to prevent the reflected light from entering into the MLLD section 11.

The optical amplifier section (SOA section) 131 includes, for example, a semiconductor optical amplifier. The optical amplifier section (SOA section) 131 serves as an optical modulator section that amplifies and modulates the incoming laser light (i.e., the laser light L1 emitted from the MLLD section 11) and is located at a subsequent stage from the isolator 125.

The laser output from the MLLD section 11 has a relatively low power, and therefore is amplified by the optical amplifier section 131.

The optical amplifier section 131 is a small and low cost optical amplifier, and can be utilized as a light gate or a light switch that turns on and off the light. In the present embodiment, the laser light L1 emitted from the MLLD section 11 is modulated by turning on and off of the optical amplifier section 131.

The behavior of the optical amplifier section 131 is controlled by the SOA driver 40. Specifically, the optical amplifier section (SOA section) 131 amplifies the laser light L1 according to the magnitude of the control current (direct current) supplied from the SOA driver 40. Further, during the amplification, the optical amplifier section 131 conducts intermittent drive by the control current of a pulse shape, in order to turn on and off the laser light L1 at a predetermined cycle and output the intermittent laser light, i.e., the pulse laser light L2.

At this time, the SOA driver 40 intermittently drives the optical amplifier section (SOA section) 131 in accordance with the SOA driving signal of the frequency f_SOA supplied from the synchronizing signal generator section 302. In other words, the optical amplifier section (SOA section) 131 modulates the laser light L1 by turning on and off the laser light L1 at the frequency f_SOA, and outputs the pulse laser light L2 after modulation.

Note that the control current of the frequency f_SOA is decided in such a manner to avoid the interference with the band within which the servocontrol driver 313, which is described later, servocontrols the light path length of the resonator 21. The detail of the method to decide the frequency f_SOA will be described later separately in "4. Detail of Control Section".

The pulse laser light L2 sent out from the optical amplifier section (SOA section) 131 is directed via the lens 129 to the isolator 303 of the control section 30, which is described later, transmits through the isolator 303, and enters into the wavelength conversion optical system 20.

Next, description will be made of each component of the wavelength conversion optical system 20. The wavelength conversion optical system 20 includes a resonator 21,

relay lenses

221 and 223, and mirrors 225 and 227.

The pulse laser light L2 output from the light source unit 10 enters into the inside of the resonator 21 from the input coupler 201, via the isolator 303 of the control section 30, which is described later, the

relay lenses

221 and 223, and the

mirrors

225 and 227. Note that, given that the pulse laser light L2 emitted from the light source unit 10 is directed via the isolator 303 to the inside of the resonator 21, the components of the optical system located in the light path are not limited to the

relay lenses

221 and 223, and the

mirrors

225 and 227.

The resonator 21 is what is referred to as the optical parametric oscillator (OPO: Optical Parametric Oscillation), which resonates the pulse laser light L2 from the light source unit 10 in the inside, converts the wavelength of the laser light L2, and outputs the pulse laser light L4 of the converted wavelength. In the following, description will be made of the detailed configuration of the resonator 21. Note that, in the following, the pulse laser light which enters into the resonator 21 is occasionally referred to as "excitation laser light", and the pulse laser light which is subjected to wavelength conversion and output from the resonator 21 is occasionally referred to as "OPO laser light". Also, the excitation laser light that resonates in the resonator 21 is occasionally referred to as "excitation laser light L3" or "pulse laser light L3", to distinguish the excitation laser light from the pulse laser light L2 before entering into the resonator 21.

The resonator 21 includes an input coupler 201, mirrors 203, 205, and 207, a dichroic mirror 209, an output coupler 211, and a non-linear optical element 213. The input coupler 201 and the output coupler 211 is generally a partial reflector (partial reflector mirror) having a transmissivity of several percent.

Also, the non-linear optical element 213 is provided between the mirror 203 and the mirror 205.

As the non-linear optical element 213, KTP (KTiOPO₄), LN (LiNbO₃), QPMLN (quasi phase matching LN), BBO (beta-BaB₂O₄), LBO (LiB₃O₄), KN (KNbO₃) and the like are utilized for example.

As one example, the non-linear optical element 213 converts the input laser light (i.e., the excitation laser light L3) into two wavelengths. Then, the laser light of at least one wavelength (for example, the longer wavelength) among the converted two wavelength resonates in the resonator 21 as the OPO laser light L4, and is output from the output coupler 211 to the outside of the resonator 21.

Also, the dichroic mirror 209 is provided between the input coupler 201 and the mirror 203. The dichroic mirror 209 allows the excitation laser light L3, among the light reflected toward the input coupler 201 by the mirror 203, to transmit through the dichroic mirror 209 toward the input coupler 201, and reflects the OPO laser light L4 toward the output coupler 211. By the configuration like this, the resonator 21 according to the present embodiment is made such that the excitation laser light L3 and the OPO laser light L4 are directed via the different light paths in the resonator 21. In the following, description will be made of the detail of the respective light paths of the excitation laser light L3 and the OPO laser light L4 in the resonator 21.

First, an attention is given to the light path of the excitation laser light L3. The excitation laser light L3 that has entered from the input coupler 201 into the inside of the resonator transmits through the dichroic mirror 209, reaches the mirror 207 via the mirror 203, the non-linear optical element 213, and the mirror 205, and is reflected at the mirror 207.

Also, the excitation laser light L3 reflected at the mirror 207 is directed via the mirror 205, the non-linear optical element 213, and the mirror 203 to the dichroic mirror 209, transmits through the dichroic mirror 209, and is directed to the input coupler 201.

The input coupler 201 reflects a part of the directed excitation laser light L3, and sends out the other part to the outside of the resonator 21. In this way, the excitation laser light L3 that has entered into the resonator 21 repeats the reflection between the input coupler 201 and the mirror 207. That is, the light path between the input coupler 201 and the mirror 207 corresponds to the light path length (in other words, the resonator length) of the excitation laser light L3 in the resonator 21, and the light path length is adjusted to the resonance condition of the excitation laser light L3, so that the excitation laser light L3 resonates in the resonator 21.

Also, the excitation laser light sent out from the input coupler 201 to the outside of the resonator 21 is directed, as the reflected light from the resonator 21, toward the photodetector 305 by the isolator 303, and is detected at the photodetector 305.

Next, an attention is given to the light path of the OPO laser light L4. The excitation laser light L3 subjected to wavelength conversion at the non-linear optical element 213, i.e., the OPO laser light L4, reaches the mirror 207 via the mirror 205, and is reflected at the mirror 207.

Also, the OPO laser light L4 reflected at the mirror 207 is directed via the mirror 205, the non-linear optical element 213, and the mirror 203, to the dichroic mirror 209, and is reflected at the dichroic mirror 209 and directed to the output coupler 211.

The output coupler 211 reflects a part of the directed OPO laser light L4, and sends out the other part to the outside of the resonator 21. In this way, the OPO laser light L4 that has entered into the resonator 21 repeats reflection between the output coupler 211 and the mirror 207. That is, the light path between the output coupler 211 and the mirror 207 corresponds to the light path length of the OPO laser light L4 in the resonator 21 (in other words, the resonator length), and the light path length is adjusted to the resonance condition of the OPO laser light L4, so that the OPO laser light L4 resonates in the resonator 21.

Next, description will be made of the behavior related to the adjustment of the respective light path lengths of the excitation laser light L3 and the OPO laser light L4 in the resonator 21. In the resonator 21 according to the present embodiment, the mirror 207 is configured such that the position is adjustable along the optical axis of the excitation laser light L3 and the OPO laser light L4 incident on the mirror 207. Likewise, the output coupler 211 is configured such that the position is adjustable along the optical axis of the OPO laser light L4 incident on the output coupler 211.

In other words, by adjusting the position of the mirror 207, the respective light path lengths of the excitation laser light L3 and the OPO laser light L4 are adjusted. By adjusting the position of the output coupler 211, the light path length of the OPO laser light L4 is adjusted. For that reason, for example, the resonator 21 may be configured such that the position of the mirror 207 is adjusted to meet the resonance condition of the excitation laser light L3, and thereafter the position of the output coupler 211 is adjusted to meet the resonance condition of the OPO laser light L4. By adjusting the positions of the mirror 207 and the output coupler 211 in this order, the light path length can be controlled to meet the respective resonance conditions for the excitation laser light L3 and the OPO laser light L4.

The positions of the mirror 207 and the output coupler 211 are adjusted by the actuator device such as, for example, an electromagnetic actuator (VCM: Voice Coil Motor) and a piezoelectric element component. Note that the behavior of the actuator device for adjusting the positions of the mirror 207 and the output coupler 211 is controlled by the servocontrol driver 313, which is described later. Also, the actuator device that controls the positions of the mirror 207 and the output coupler 211 corresponds to one example of "drive section".

Next, description will be made of each component of the control section 30.

The isolator 303 is interposed between the optical amplifier section (SOA section) 131 and the resonator 21, and allows the pulse laser light L2 output from the optical amplifier section (SOA section) 131 to transmit through the isolator 303 toward the resonator 21.

Also, a part of the excitation laser light L3 that is transmitted through the input coupler 201 of the resonator 21 and sent out to the outside of the resonator 21, i.e., the reflected light from the resonator 21, is directed via the

mirrors

227 and 225 and the

relay lenses

223 and 221 to the isolator 303. The isolator 303 reflects the reflected light from the resonator 21, toward the photodetector 305 located in the direction different from the direction to the optical amplifier section (SOA section) 131, so that the reflected light is prevented from entering into the optical amplifier section (SOA section) 131.

The photodetector 305 includes a photo detector (PD), for example. The photodetector 305 detects the reflected light from the resonator 21, which is directed via the isolator 303. The photodetector 305 outputs the detection result of the reflected light from the resonator 21, as a reflected signal, to the bandpass filter 307.

The bandpass filter 307 allows the signal of the band corresponding to the excitation laser light L3 to pass through the bandpass filter 307 toward the mixer 309, and blocks the signal other than the band. Thereby, among the signals based on the detection result of the photodetector 305, the signal based on the reflected light from the resonator 21 (i.e., the excitation laser light L3 that has leaked from the resonator 21) passes through the bandpass filter 307 to be input into the mixer 309, and the signal based on other light that accompanies disturbance and the like is blocked at the bandpass filter 307.

The mixer 309 generates the error signal by integrating the signal of the frequency f_M supplied from the oscillator 301 by the reflected signal based on the reflected light from the resonator 21, and outputs the generated error signal to the driving signal generator section 311. The error signal generated at this time corresponds to the error signal in what is called Pound-Drever-Hall (PDH) method, and indicates the difference between the light path length of the excitation laser light L3 in the resonator 21 and the light path length that meets the resonance condition of the excitation laser light L3.

The driving signal generator section 311 includes a low-pass filter, a synchronous detection circuit, and a phase compensation section. The error signal output from the mixer 309 is supplied to the synchronous detection circuit, after the high-frequency component (i.e., noise) of the error signal output from the mixer 309 is removed by the low-pass filter of the driving signal generator section 311.

To the synchronous detection circuit of the driving signal generator section 311, the S/H signal is supplied from the synchronizing signal generator section 302. The synchronous detection circuit synchronously detects the error signal from which the noise is removed by the low-pass filter, on the basis of the S/H signal supplied from the synchronizing signal generator section 302. Then, the synchronous detection circuit outputs the error signal synchronously detected on the basis of the S/H signal (hereinafter, occasionally referred to as "the S/H output ") to the phase compensation section.

The phase compensation section of the driving signal generator section 311 compensates the phase of the S/H output supplied from the synchronous detection circuit, and supplies the S/H output of which the phase is compensated, as the driving signal, to the servocontrol driver 313.

The servocontrol driver 313 adjusts the position of the mirror 207, by driving the actuator device in accordance with the driving signal supplied from the driving signal generator section 311. At this time, the driving signal is generated based on the error signal indicating the difference between the light path length of the excitation laser light L3 in the resonator 21 and the light path length that meets the resonance condition of the excitation laser light L3. For that reason, the servocontrol driver 313 controls the position of the mirror 207 in accordance with the driving signal supplied from the driving signal generator section 311, so that the light path length of the excitation laser light L3 in the resonator 21 is servocontrolled.

Note that, when the light path length of the excitation laser light L3 in the resonator 21 is controlled, i.e., the position of the mirror 207 is changed, the light path length of the OPO laser light L4 in the resonator 21 is also changed. For that reason, it is needless to say that, when the servocontrol driver 313 controls the position of the mirror 207, the servocontrol driver 313 may control the light path length of the OPO laser light L4 as well, by adjusting the position of the output coupler 211 in response to the control amount of the position of the mirror 207.

In the above, the configuration of the laser light generation device 1 according to the present embodiment is described with reference to FIG. 1. Note that the control section 30 and the SOA driver 40 described above correspond to one example of "control section". Also, the configuration including the control section 30, the SOA driver 40, and the actuator device for controlling the positions of the mirror 207 and the output coupler 211 corresponds to one example of "control device".

<2. Advantage of Intermittent Light Emission>
Next, the advantage of outputting in a state where the peak power of the laser is increased by intermittent light emission is described with reference to FIG. 2. FIG. 2 is a characteristic diagram illustrating a state where the peak power of a laser is increased by intermittent light emission.

The laser light generation device 1 according to the present embodiment is applicable to the laser scanning microscope which scans a biological sample with the laser light and observes the light from the biological sample, for example. In the observation in which the biological sample is irradiated with the laser light as in the laser scanning microscope, reducing the average power of the laser and increasing the peak power is effective to reduce the damage of the object.

Also, the laser chip that makes up a MOPA light source utilizing a semiconductor laser as the laser light source 111 has an operation limit set by the heat generation of the high electrical power load, due to the smallness of the laser chip.

In contrast, since the light source unit 10 of the laser light generation device 1 according to the present embodiment outputs the intermittent laser light (pulse laser light L2) by the intermittent drive of the optical amplifier section (SOA section) 131, the peak during the light emission is heightened despite the same average electrical power, as compared to the case not conducting the intermittent drive. Also, by conducting the intermittent drive, the heat generation of the laser chip by the high electrical power load is also suppressed.

Here, an attention is given to the case in which the laser light generation device 1 according to the present embodiment is utilized as the light source in the fluorescence microscope, which is particularly referred to as the two-photon excitation microscope among the laser scanning microscope. And, description will be made of the characteristics when the peak power of the laser is increased by the intermittent light emission. Here, the fluorescence microscope means the microscope which scans the biological sample with the laser light emitted from the light source and observes the fluorescence from the biological sample. Also, among the aforementioned fluorescence microscopes, the two-photon excitation microscope uses the fluorescence generation by the two-photon excitation, i.e., the mechanism in which a molecule is shifted to the excited state by the interaction of two photons and a molecule in order to generate the fluorescence.

In the microscope utilizing the light source of the two-photon excitation, FOM (= (peak power)² x pulse width x frequency = peak power x average power) is known as the figure of merit. According to this figure of merit, output is increased in proportion to the product of the peak power by the average power. Accordingly, in the laser microscope observation of the living body, reducing the average power and increasing the peak power is effective to minimize the damage of the object and intensify the output. For this reason, in the present embodiment the intermittent drive is conducted to reduce the duty (duty = pulse width x frequency) ratio and increase the peak power.

For example, the top of FIG. 2 illustrates the characteristics of the laser light output from the laser light source. The characteristics at the left side of the top illustrates the peak power of the continuous light emission. The characteristics at the right side illustrates the peak power of the intermittent light emission when the duty ratio is set at 50 %. In this way, when the duty ratio of the intermittent light emission is set at 50 %, the output signal intensity (2 x I₀) of the intermittent light emission is two times as strong as the signal intensity (I₀) of the continuous light emission.

Also, the middle of FIG. 2 illustrates the characteristics of the fluorescence signal generated by the two-photon excitation. The characteristics of the left side illustrates the peak power of the fluorescence in the continuous light emission. The characteristics of the right side illustrates the peak power of the fluorescence in the intermittent light emission when the duty ratio is set at 50 %. Regarding the figure of merit FOM, in the case of the two-photon excitation, the figure of merit is increased at the square of the peak power. Accordingly, in the case of the intermittent light emission, the signal intensity (= 4 x I₀ ²) of the fluorescence in the two-photon excitation is four times the signal intensity (= I₀ ²) of the fluorescence in the continuous light emission. Also, at the average signal intensity of the pulse light emission point and the pulse non-light emission point, the average signal intensity (= 2 x I₀ ²) of the fluorescence in the two-photon excitation is two times the signal intensity (= I₀ ²) of the fluorescence in the continuous light emission. Accordingly, according to the present embodiment, the peak power and the average signal intensity are increased by conducting intermittent drive in the light source unit 10.

The characteristics at the bottom of FIG. 2 illustrates the signal generated by passing the characteristics of the middle through the low-pass filter for band limitation (the low-pass filter included in the driving signal generator section 311 illustrated in FIG. 1). When the duty ratio of turning on and off is 50 % (1/2), the signal amplitude is 1/2, and as a result the intermittent light emission of the two-photon excitation can obtain two times the signal amplitude in the continuous light emission. Further, when the aforementioned driving signal generator section 311 samples only the signals in the ON state, the intermittent light emission of the two-photon excitation can obtain four times the signal amplitude in the continuous light emission.

<3. Challenge of Laser Light Generation Device according to Present Embodiment>
On the other hand, when the resonance behavior is stabilized by servocontrolling the light path length of the resonator 21, for example, by PDH method as in the laser light generation device 1 according to the present embodiment, the accurate detection of the error signal occasionally becomes difficult when the disturbance signal is included in the band which is subjected to the closed-loop servocontrol.

Particularly, when the band of the intermittent light emission of the pulse laser light interferes the band which is subjected to the closed-loop servocontrol, the intermittent light emission of the pulse laser light becomes the disturbance, and as a result the laser light generation device occasionally makes the accurate detection of the light path length of the resonator difficult.

For that reason, the laser light generation device 1 according to the present embodiment aims to control the light path length of the resonator 21 in such a manner that the pulse laser light of high output is stably output, in the configuration that resonates the pulse laser light emitted from the light source unit 10 in the resonator 21. Specifically, the laser light generation device 1 decides the frequency f_M supplied from the oscillator 301, and the frequency f_SOA of the intermittent light emission of the pulse laser light, in such a manner that the band of the intermittent light emission of the pulse laser light avoids the interference with the band which is subjected to the closed-loop servocontrol.

Therefore, in the following, the detail of the control section 30 in the laser light generation device 1 in the present embodiment will be described with a particular attention to the method to decide the frequency f_M and the frequency f_SOA.

<4. Detail of Control Section>
4. 1. Relationship between Signals
In describing the method to decide the frequency f_M and the frequency f_SOA as the detail of the control section 30, the relationship between the signals of the servocontrol in the laser light generation device 1 according to the present embodiment will be first described with reference to FIG. 3. FIG. 3 is an explanatory diagram for describing a relationship between the signals in the servocontrol, and is a schematic timing chart illustrating the relationship between respective signals.

In FIG. 3, the MLLD output L1 represents the laser light L1 output from the MLLD section 11 (i.e., the light generated by modulating the light of the frequency f_MLLD by the modulation signal of the frequency f_M), which has been described with reference to FIG. 1. The MLLD output L1 has a modulation cycle of 0.1 [microsecond], when the modulation signal supplied from the oscillator 301 has a frequency f_M = 10 [MHz], for example.

Also, the SOA driving signal g11 represents the SOA driving signal for the SOA driver 40 to intermittently drive the optical amplifier section (SOA section) 131. As described above, the SOA driving signal g11 is generated in such a manner to synchronize with the signal of the frequency f_M supplied from the oscillator 301 by the synchronizing signal generator section 302. Note that, in the example illustrated in FIG. 3, the frequency f_SOA of the SOA driving signal g11 is set at f_SOA = 100 [kHz], and in this case the cycle of the SOA driving signal g11 is 5 [microsecond].

The MOPA output L2 represents the modulation signal generated by modulating the aforementioned MLLD output L1 by intermittently driving the optical amplifier section (SOA section) 131 in accordance with the SOA driving signal g11 of the frequency f_SOA. Note that the MOPA output L2 corresponds to the pulse laser light L2 output from the light source unit 10, which has been described with reference to FIG. 1. Note that the wave number of the MLLD output L1 component in the ON state of the MOPA output L2 is 50 [wobble], which is calculated from the cycle 0.1 [microsecond] of the MLLD output L1 and the cycle 5 [microsecond] of the SOA driving signal g11.

The reference sign L3 represents the excitation laser light that resonates in the resonator 21 on the basis of the MOPA output L2 that enters into the resonator 21, and represents the excitation laser light L3 which has been described with reference to FIG. 1.

When the MOPA output L2 enters into the resonator 21, the resonance occurs at the timing when the MOPA output L2 is in the ON state, and the resonance does not occur at the timing when the MOPA output L2 is in the OFF state. Also, when the MOPA output L2 shifts from the OFF state to the ON state, the MOPA output L2 does not immediately shift to the state in which the excitation laser light L3 resonates, but takes a time T_build to shift to the resonant state (hereinafter, occasionally referred to as "buildup period").

For that reason, for example, when the light path length in the resonator 21 is servocontrolled by PDH method or the like, the MOPA output L2, which is represented by the reference sign g21, is in the ON state, and the excitation laser light L3 during the period after the buildup completion (after the lapse of the buildup period T_build) is effective as an error signal.

On the other hand, during the period represented by the reference sign g23 (i.e., the period during which the MOPA output L2 is in the OFF state and the buildup period T_build), particularly, the period during which the MOPA output L2 is in the OFF state, the signal of the frequency f_M is not included sufficiently to conduct the synchronous detection. For that reason, it is difficult to estimate the difference between the light path length of the excitation laser light L3 in the resonator 21, and the light path length that meets the resonance condition of the excitation laser light L3, from the error signal obtained during the period represented by the reference sign g23.

The reference signs g31 and g33 represent the error signal generated by the mixer 309 in FIG. 1, in other words, the error signal generated by integrating the signal of the frequency f_M supplied from the oscillator 301 by the reflected signal based on the reflected light from the resonator 21. The error signal g31 corresponds to the period g21, and the error signal g33 corresponds to the period g23.

Therefore, it is desirable to detect the error signal g31 among the error signals g31 and g33 by the synchronous detection. In other words, the synchronizing signal generator section 302 modulates the signal of the frequency f_M supplied from the oscillator 301 in such a manner to synchronize the signal of the frequency f_M with the timing g21 in order to generate the S/H signal g41, and supplies the generated S/H signal g41 to the driving signal generator section 311. Note that the buildup period T_build is in proportion to the finesse which indicates the sharpness of the resonance in the resonator, and can be calculated in advance based on the respective reflectances of the input coupler 201 and the mirror 207 of the resonator 21, and the frequency of the excitation laser light L3. Note that the detail of the buildup period T_build will be described later separately in "4. 3. Limitation based on Buildup Period in Resonator".

The S/H output g51 represents the signal generated by synchronously detecting the error signal g31 and g33 by the driving signal generator section 311, which has been described with reference to FIG. 1, on the basis of the S/H signal g41 supplied from the synchronizing signal generator section 302.

The driving signal generator section 311 compensates the phase of the S/H output g51, and supplies the S/H output g51 of which the phase is compensated, as the driving signal to the servocontrol driver 313. Thereby, the servocontrol driver 313 can servocontrol the light path length of the resonator 21, synchronizing with the turning on and off of the excitation laser light L3 in the resonator 21, in accordance with the driving signal supplied from the driving signal generator section 311 (i.e., the S/H output g51 of which the phase is compensated).

4. 2. Limitation based on Band of Servocontrol
Next, description will be made of the limitation of the frequency f_M and the frequency f_SOA, for setting the band of the intermittent light emission of the pulse laser light to avoid the interference with the band which is subjected to the closed-loop servocontrol, with reference to FIG. 4. FIG. 4 is a diagram for describing the characteristics of the servocontrol.

In FIG. 4, the reference sign g61 illustrates one example of the gain characteristics of the actuator that servocontrols the position of the mirror 207 in the laser light generation device 1 according to the present embodiment. The vertical axis of the gain characteristics g61 represents the gain [dB], and the horizontal axis represents the frequency f [Hz].

Also, the reference sign g63 represents the phase characteristics of the actuator that servocontrols the position of the mirror 207 in the laser light generation device 1 according to the present embodiment. The vertical axis of the phase characteristics g63 represents the phase [degree], and the horizontal axis represents the frequency f [Hz]. Note that the horizontal axis of the gain characteristics g61 and the horizontal axis of the phase characteristics g63 are same. Also, the reference sign g65 represents the characteristics of the phase compensation section of the driving signal generator section 311, and the fact that the gain characteristics g63 is compensated by the characteristics of the phase compensation section can be understood.

The reference sign m1 represents the gain margin, i.e., how much margin there is until the gain becomes 0 [dB] (i.e., until oscillation) at the frequency f_max at which the phase is -180 [degree] in the phase characteristics g63. Likewise, the reference sign m2 represents the phase margin, i.e., how much margin there is until the phase becomes -180 [degree] (i.e., until oscillation), at the cutoff frequency f_RS at which the gain is 0 [dB] in the gain characteristics g61.

Note that the characteristics of the band f_width at or below the frequency at which the gain margin is determined, i.e., at which the phase characteristics g63 is -180 [degree], affect the servocontrol performance of the light path length of the resonator 21. Further, since in general the actuator follows until the cutoff frequency oscillates, the cutoff frequency f_RS is used as a numerical value representing the servocontrol performance.

The intermittent light emission of the pulse laser light can be considered as a kind of sampling action, and it is known that the sampling frequency needs to be greater than twice the signal component in order to restore the sampled signal correctly. Here, on the basis of the same condition, the limitations to the frequency f_M and the frequency f_SOA are described in detail below.

First, since the sampling frequency needs to be greater than twice the signal component, it is obvious that the frequency f_M and the frequency f_SOA need to be greater than twice the cutoff frequency f_RS. Further, it is known that the phase lead frequency of the phase compensation section is set at one third of the cutoff frequency f_RS and the phase lag frequency is set at three times or more the cutoff frequency f_RS, to achieve the characteristics of the phase compensation section as shown with reference sign g65 illustrated in FIG. 4. Accordingly, it is desired that the characteristics of the phase compensation section be restored correctly until the phase lag frequency in order to restore the characteristics of the phase compensation section correctly, and therefore it is desired that the frequency f_M and the frequency f_SOA be twice or more the phase lag frequency.

From the above, the relationship among the frequency f_M, the frequency f_SOA, and the cutoff frequency f_RS is in below formula 1. From this result, it is desired that the relationship among the frequency f_M, the frequency f_SOA, and the cutoff frequency f_RS be as in formula 2 shown below. Note that formula 2 shown below corresponds to "conditional inequality 1". Also, the constant j corresponds to one example of "margin value j" for stabilizing the servocontrol.

f_SOA > (f_RS x 3) x 2 = j x f_RS (j = 6) (formula 1)
f_SOA > j x f_RS (formula 2)

In the above, description has been made of the limitation of the frequency f_M and the frequency f_SOA for setting the band of the intermittent light emission of the pulse laser light to avoid the interference with the band which is subjected to the closed-loop servocontrol, with reference to FIG. 4.

4. 3. Limitation based on Buildup Period in Resonator
Next, description will be made of the detail of the calculation method of the buildup period T_build of the excitation laser light L3 in the resonator 21 which is described above, and the limitation of the frequency f_SOA of the intermittent light emission of the pulse laser light, which is based on the buildup period T_build.

First, the calculation method of the buildup period T_build will be described with reference to FIG. 5. FIG. 5 is an explanatory diagram for describing the calculation method of the buildup period, and is a diagram that schematically illustrates the configuration of the resonator. In FIG. 5, the reference sign L21 represents the laser light that is emitted from the laser light source and enters into the resonator. The reference sign L31 represents the excitation laser light that resonates in the resonator on the basis of the laser light L21. Also, the reference sign L41 represents the OPO laser light that is sent out from the resonator.

Also, the

reference sign

215 and 217 represent the mirrors which make up the resonator. In the example illustrated in FIG. 5, the reflectance of the mirror 215 is R1, and the reflection amplitude based on the reflectance R1 is r1. Likewise, the reflectance of the mirror 217 is R2, and the reflection amplitude based on the reflectance R2 is r2.

At this time, the finesse F, which indicates the sharpness of the resonance in the resonator is decided based on formula 3 below, by means of the coefficient R' = r1 x r2, which is decided based on the reflection amplitudes r1 and r2.

F = pi x sqrt(R') / (1 - R') (formula 3)

Note that the respective reflectances R1 and R2 of the

mirrors

215 and 217 are occasionally designed to be different reflectances for the excitation laser light L31 and the OPO laser light L41, respectively. For example, the respective reflectances for the excitation laser light L31 and the OPO laser light L41 of the

mirrors

215 and 217 are designed to meet the conditions of the finesse F which are decided in advance for the excitation laser light L31 and the OPO laser light L41 respectively.

Also, the time (photon life time) T_L until the strength of the incoming laser light L21 that has entered into the resonator becomes 1/e is calculated by multiplying the number of reflection that occurs until the strength becomes 1/e, by the time it takes for the laser light L2 to make a round trip in the resonator. In other words, the time T_L is decided based on formula 4 below, from the coefficient R', and the frequency f_MLLD of the laser light L21 that enters into the resonator.

T_L = 1/2 ln(1/R') x (1/f_MLLD) (formula 4)

Here, where the proportionality constant decided dependent on the threshold value of the buildup is k, the buildup period T_build is decided based on formula 5 below, from the above formula 4 and the proportionality constant k.

T_build = k x 1/2 ln(1/R') x (1/f_MLLD) (formula 5)

Note that the proportionality constant k is, for example, 4.5, when the threshold value of the buildup is 80 [%].

Here, description will be made of the relationship between the pulse number and the buildup in the resonator, with a specific example. For example, the graph g71 illustrated in FIG. 6 illustrates the relationship between the amplification factor and the pulse number in the resonator, in the case of the coefficient R' = 0.93. In the graph g71 illustrated in FIG. 6, the vertical axis represents the amplification factor, and the horizontal axis represents the number of the pulses that enter the resonator.

For example, in the example illustrated in FIG. 6, while the number of the pulses that have entered into the resonator is 0 to 50, the amplification factor increases with the increase of the pulse number. When the pulse number is at or greater than 50, the amplification factor converges to a certain value.

Next, the relationship between the finesse F and the buildup will be described with a specific example. For example, the graph g73 illustrated in FIG. 7 illustrates one example of the measurement result showing the relationship between the buildup period T_build and the finesse F. In the example illustrated in FIG. 7, the proportionality constant k is set at 4.5, with the frequency f_MLLD of the excitation laser light L3 at 850 [MHz], and the threshold value of the buildup at 80 [%].

As illustrated in FIG. 7, the buildup period T_build shown in the above formula 5 and the finesse F shown in formula 3 are in a proportional relationship. Therefore, it is known that the buildup period T_build is in a proportional relationship with the coefficient F/f_MLLD, which is based on the finesse F and the frequency f_MLLD.

In the example illustrated in FIG. 7, in the case of the finesse F = 43 with respect to the excitation laser light L3 and the coefficient R' = 0.93 with respect to the excitation laser light L3, the buildup time T_L3 of the excitation laser light L3 is T_L3 = 36 [ns]. Also, in the case of the finesse F = 150 with respect to the OPO laser light L4 and the coefficient R' = 0.98 with respect to the OPO laser light L4, the buildup time T_L4 of the OPO laser light L4 is T_L4 = 127 [ns].

From the above result, the buildup period T_build in the example illustrated in FIG. 7 is T_build = T_L3 + T_L4 = 163 [ns].

In the above, the calculation method of the buildup period T_build has been described with reference to FIG. 7.

Next, description will be made of the limitation of the frequency f_SOA at which the pulse laser light is emitted intermittently, which is based on the buildup period T_build, with reference to FIG. 8. FIG. 8 is an explanatory diagram for describing the limitation of the frequency f_SOA at which the pulse laser light is emitted intermittently, which is based on the buildup period T_build.

In FIG. 8, the reference sign g75 represents the SOA drive pulse that causes the laser light emitted from the laser light source to flash intermittently for the purpose of emitting the pulse laser light. The SOA drive pulse g75 corresponds to the drive pulse of the frequency f_SOA supplied from the synchronizing signal generator section 302 to the SOA driver 40 for driving the optical amplifier section (SOA section) 131 in FIG. 1, for example.

In the SOA drive pulse g75, the period during which each pulse is in the ON state is represented by 1/f_SOA*Duty, where Duty is the duty ratio of the SOA drive pulse g75.

Also, the reference sign g77 schematically illustrates the light density of the OPO laser light in the resonator. Note that the vertical direction of the OPO light density g77 represents the strength of the OPO laser light. Also, the reference sign T_build represents the aforementioned buildup period, and the reference sign T_eff represents the time effective for the OPO conversion in the resonator.

For example, the time T_eff effective for the OPO conversion is T_eff = T50 - T_build, where the period during which each pulse of the SOA drive pulse g75 in the ON state is T50, when the duty ratio of the SOA drive pulse g75 is 50 %. At this time, the condition for the time T_eff > 0 is decided based on formula 6 below, by the above formula 5 for calculating T_build.

1/f_SOA * Duty > T_build = k x 1/2 ln(1/R') x (1/f_MLLD) (formula 6)

Note that, as described above, the proportionality constant k is the proportionality constant decided dependent on the threshold value of the buildup, and the proportionality constant k is equal to 4.5 when the threshold value is 80 [%].

In other words, on the basis of formula 6 above, the frequency f_SOA is set based on the condition shown below by formula 7, from the limitation by the buildup period T_build. Note that formula 7 below corresponds to "conditional inequality 2".

f_SOA < Duty / [k x 1/ln(1/R') x (1/f_MLLD)] (formula 7)

In the above, description has been made of the limitation of the frequency f_SOA of the intermittent light emission of the pulse laser light, which is based on the buildup period T_build, with reference to FIG. 8.

4. 4. Limitation based on Generation of Error Signal
Next, description will be made of the limitation in detecting the difference between the light path length of the excitation laser light L3 in the resonator 21 and the light path length that meets the resonance condition of the excitation laser light L3, as an error signal, on the basis of the reflected light from the resonator 21 as in the PDH method, with reference to FIG. 9. FIG. 9 is an explanatory diagram for describing the relationship between the frequency f_M in the synchronous detection of the reflected light and the frequency f_SOA of the intermittent light emission of pulse laser light.

In FIG. 9, the SOA driving signal g11 shows the SOA driving signal g11 in FIG. 3, and represents the driving signal for the SOA driver 40 to intermittently drive the optical amplifier section (SOA section) 131. Since the frequency of the SOA driving signal g11 is f_SOA as described above, the cycle of the SOA driving signal g11 is 1/f_SOA. At this time, the time during which the SOA driving signal g11 is in the ON state is 1/f_SOA*Duty, where Duty is the duty ratio of the SOA driving signal g11.

Also, in FIG. 9, the MOPA output L2 shows the MOPA output L2 in FIG. 3, and represents the modulation signal generated by modulating the aforementioned MLLD output L1 by intermittently driving the optical amplifier section (SOA section) 131 in accordance with the SOA driving signal g11 of the frequency f_SOA.

When this MOPA output L2 is synchronously detected on the basis of the signal of the frequency f_SOA, at least one wave of the signal of the frequency f_SOA is expected to be included during the light emission time of the MOPA output L2 (i.e., in the ON state), as illustrated in the right side of FIG. 9. Therefore, the light emission time of the MOPA output L2, i.e., the time 1/f_SOA*Duty during which the SOA driving signal g11 is in the ON state is expected to meet the condition shown by formula 8 below.

In other words, in order to synchronously detect the reflected light from the resonator 21 on the basis of the condition shown by formula 8 above, the frequency f_M is expected to meet the condition shown by formula 9 below. Note that formula 9 below corresponds to "conditional inequality 3".

In the above, description has been made of the limitation of the frequency f_M in detecting the difference between the light path length of the excitation laser light L3 in the resonator 21 and the light path length that meets the resonance condition of the excitation laser light L3, as an error signal, on the basis of the reflected light from the resonator 21, with reference to FIG. 9.

4. 5. Conclusion
As described above, the control section 30 of the laser light generation device 1 according to the present embodiment decides the frequency f_M and the frequency f_SOA in such a manner to meet conditional inequalities 1 to 3 which are shown below.

For example, FIG. 10 illustrates an exemplary setting of the frequency f_M and the frequency f_SOA, based on the condition that has been described with reference to FIG. 7. In other words, the example illustrated in FIG. 10 illustrates the case in which the proportionality constant k is set at 4.5, where the frequency f_MLLD of the excitation laser light L3 is at 850 [MHz], and the threshold value of the buildup is 80 [%]. Also, the finesse F with respect to the excitation laser light L3 is 43, and the coefficient R' with respect to the excitation laser light L3 is 0.93, and the finesse F with respect to the OPO laser light L4 is 150, and the coefficient R' with respect to the OPO laser light L4 is 0.98. Note that the buildup period T_build at this time is T_build = 163 [ns].

When the duty ratio Duty of the SOA drive pulse is 0.5, f_SOA < 0.5/0.163 [ns] = 3.06 [MHz] is derived from the duty ratio Duty and the buildup period T_build = 163 [ns], on the basis of conditional inequality 2.

Also, since the frequency f_SOA is j = 6 in conditional inequality 1, the frequency f_SOA is greater than f_RS x 6 = 60 [kHz] on the basis of conditional inequality 1.

Also, when f_SOA is set at 3 [MHz] on the basis of the scope of the frequency f_SOA which is calculated above, the duty ratio Duty is 0.5, and thus the frequency f_M is greater than or equal to f_SOA/Duty = 3 [MHz]/0.5 = 6 [MHz] on the basis of conditional inequality 3. Note that the upper limit value of the frequency f_M is not prescribed, but is virtually decided by the detection performance of the photodetector for detecting the reflected light from the resonator, and the arithmetic performance of the mixer for generating the error signal.

<5. Hardware Configuration>
Next, description will be made of one example of the hardware configuration of the laser light generation device 1 according to the present embodiment, with reference to FIG. 11. FIG. 11 is a diagram illustrating one example of the hardware configuration of the laser light generation device 1 according to the present embodiment.

As illustrated in FIG. 11, the laser light generation device 1 according to the present embodiment includes a processor 901, a memory 903, a storage 905, a light source unit 907, an optical system unit 909, a manipulation device 911, a display device 913, a communication device 915, and a bus 917.

The processor 901 may be, for example, a central processing unit (CPU), a graphics processing unit (GPU), a digital signal processor (DSP) or a system on chip (SoC), and executes various processings of the laser light generation device 1. The processor 901 may be configured by an electronic circuit for executing various types of arithmetic processings, for example. Note that the aforementioned control section 30 may be configured by the processor 901.

The memory 903 includes a random access memory (RAM) and a read only memory (ROM), and stores programs and data executed by the processor 901. The storage 905 may include a memory medium such as a semiconductor memory and a hard disk.

The light source unit 907 is a unit for radiating the pulse laser light L2, and corresponds to the aforementioned light source unit 10. The light source unit 907 is controlled with respect to the strength and the wavelength of the emitted excitation light, by the processor 901.

The optical system unit 909 is the unit which resonates the pulse laser light L2 emitted from the light source unit 907, converts the wavelength of the pulse laser light L2, and outputs the OPO laser light L4 into which the wavelength has been converted. The optical system unit 909 corresponds to the wavelength conversion optical system 20 including the resonator 21.

The manipulation device 911 has the function to generate the input signal for the user to perform a desired manipulation. The manipulation device 911 may be configured by an input section, such as for example a button, a switch, and the like, for the user to input information, an input control circuit for generating the input signal on the basis of the input by the user and supplying the input signal to the processor 901, and the like.

The display device 913 is one example of the output device, and may be a display device such as a liquid crystal display (LCD) device, and an organic light emitting diode (OLED) display device. The display device 913 can provide information by displaying frames to the user.

The communication device 915 is communication means which is included in the laser light generation device 1, and communicates with external devices via a network. The communication device 915 is an interface for wireless communication, and may include a communication antenna, a radio frequency (RF) circuit, a baseband processor, and others.

The communication device 915 has the function to execute various types of signal processings to the signal received from the external device, and is capable of supplying the digital signal generated from the received analog signal to the processor 901.

The bus 917 connects the processor 901, the memory 903, the storage 905, the light source unit 907, the optical system unit 909, the manipulation device 911, the display device 913, and the communication device 915 with each other. The bus 917 may include a plurality of types of buses.

Also, a program for causing the hardware such as the CPU, the ROM and the RAM built in a computer to perform the function equivalent to the configuration of the laser light generation device 1 described above is also producible. Also, a computer-readable memory medium recording the program can also be provided.

<6. Conclusion>
As described above, the control section 30 of the laser light generation device 1 according to the present embodiment decides the frequency f_M and the frequency f_SOA in such a manner to meet conditional inequalities 1 to 3 which are shown below.

By setting the lower limit value of the frequency f_SOA that meets the condition shown by conditional inequality 1 above, the frequency f_SOA, i.e., the band of the intermittent light emission of the pulse laser light is set in such a manner to avoid the interference with the band which is subjected to the closed-loop servocontrol (i.e., the servocontrol of the light path length in the resonator). For that reason, the laser light generation device 1 according to the present embodiment can conduct the control stably without allowing the servocontrol of the light path length of the resonator to oscillate, even in the configuration in which the pulse laser light is the excitation laser light.

Also, by setting the upper limit value of the frequency f_SOA that meets the condition shown by conditional inequality 2 above, the buildup period becomes shorter than the time during which the SOA driving signal is in the ON state, even when the pulse laser light enters into the resonator 21 as the excitation laser light. For that reason, the laser light generation device 1 according to the present embodiment can ensure the time effective for the OPO conversion and emit the OPO laser light, even in the configuration in which the pulse laser light is the excitation laser light.

Also, by setting the frequency f_M that meets the condition shown by conditional inequality 3 above, at least one wave of the signal of the frequency f_SOA is included during the light emission time of the MOPA output (i.e., in the ON state). For that reason, the laser light generation device 1 according to the present embodiment can synchronously detect the reflected light from the resonator to generate the error signal, and servocontrol the light path length of the resonator on the basis of the error signal, even in the configuration in which the pulse laser light is the excitation laser light.

The preferred embodiment of the present disclosure have been described above in detail with reference to the accompanying drawings, whilst the technical scope of the present disclosure is not limited to such an example. A person having ordinary knowledge in the technical field of the present disclosure obviously can conceive of various alterations and modifications within the scope of the technical concept recited in the appended claims, and it should be understood that they will naturally come under the technical scope of the present disclosure.

Also, the effects described in the present specification are only explanatory and exemplary, and are not restrictive. That is, the technology according to the present disclosure can achieve other effects which are obvious for a person skilled in the art from the description of the present specification, along with the above effects or instead of the above effects.

Additionally, the present technology may also be configured as below.
(1)
A control device including:
a light source control section configured to cause laser light that is emitted from a laser light source, to be emitted as pulse laser light by intermittent drive;
a drive section configured to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light; and
a servocontrol section configured to servocontrol a light path length of the resonator by causing the drive section to move the at least one of the reflection sections in such a manner to meet a resonance condition, on the basis of an error signal that indicates a difference between the light path length of the resonator according to a synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulse laser light that enters the resonator,
wherein the servocontrol section decides a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of the reflected light, in such a manner to meet conditions of conditional inequalities 1 to 3,

where
f_RS is a cutoff frequency of frequency characteristics in the servocontrol,
j is a margin value for stabilizing the servocontrol,
k is a proportionality constant that is decided based on a threshold value of buildup,
R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections,
f_MLLD is a frequency of laser light that is emitted from the laser light source, and
Duty is a duty ratio of the pulse laser light.
(2)
The control device according to (1), wherein
the cutoff frequency f_RS is a cutoff frequency in the servocontrol in which a gain is 0 dB.
(3)
The control device according to (1) or (2), wherein
the laser light source is a semiconductor laser, and
the light source control section causes the laser light to be emitted as the pulse laser light, by intermittently driving an optical modulator section that is configured to amplify and modulate laser light that is emitted from the semiconductor laser.
(4)
A control method including:
causing laser light that is emitted from a laser light source, to be emitted as pulse laser light by intermittent drive;
causing a drive section to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light;
servocontrolling a light path length of the resonator by causing the drive section to move the at least one of the reflection sections in such a manner to meet a resonance condition, on the basis of an error signal that indicates a difference between the light path length of the resonator according to a synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulse laser light that enters the resonator; and
deciding a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of the reflected light, in such a manner to meet conditions of conditional inequalities 1 to 3,

where
f_RS is a cutoff frequency of frequency characteristics in the servocontrol,
j is a margin value for stabilizing the servocontrol,
k is a proportionality constant that is decided based on a threshold value of buildup,
R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections,
f_MLLD is a frequency of laser light that is emitted from the laser light source, and
Duty is a duty ratio of the pulse laser light.
(5)
A program for causing a computer to execute:
causing laser light that is emitted from a laser light source, to be emitted as pulse laser light by intermittent drive;
causing a drive section to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light;
servocontrolling a light path length of the resonator by causing the drive section to move the at least one of the reflection sections in such a manner to meet a resonance condition, on the basis of an error signal that indicates a difference between the light path length of the resonator according to a synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulse laser light that enters the resonator; and
deciding a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of the reflected light, in such a manner to meet conditions of conditional inequalities 1 to 3,

where
f_RS is a cutoff frequency of frequency characteristics in the servocontrol,
j is a margin value for stabilizing the servocontrol,
k is a proportionality constant that is decided based on a threshold value of buildup,
R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections,
f_MLLD is a frequency of laser light that is emitted from the laser light source, and
Duty is a duty ratio of the pulse laser light.
(18)
A control device comprising: a light source section configured to cause pulsed laser light by intermittent drive; a drive section configured to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulsed laser light; and a servocontrol section configured to servocontrol a light path length of the resonator by moving the at least one of the reflection sections, wherein the servocontrol section decides a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of a reflected light from the resonator, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from a laser light source, and Duty is a duty ratio of the pulsed laser light.
(19)
The control device according to (18), wherein the cutoff frequency f_RS is a cutoff frequency in the servocontrol section in which a gain is approximately 0 dB.
(20)
The control device according to (18), wherein the margin value j is approximately 6.
(21)
The control device according to (18), wherein the laser light source is a semiconductor laser, and the light source control section causes the laser light to be emitted as the pulsed laser light, by intermittently driving an optical modulator section that is configured to amplify and modulate laser light that is emitted from the semiconductor laser.
(22)
The control device according to (18), wherein the servocontrol section is further configured to servocontrol the light path length on the basis of an error signal that indicates a difference between the light path length of the resonator according to the synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulsed laser light that enters the resonator.
(23)
A control method comprising:
causing laser light that is emitted from a laser light source, to be emitted as pulsed laser light by intermittent drive;
causing a drive section to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light;
servocontrolling a light path length of the resonator by causing the drive section to move the at least one of the reflection sections; and
deciding a frequency f_SOA of the intermittent drive and a frequency fM for synchronous detection of a reflected light from the resonator, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from the laser light source, and Duty is a duty ratio of the pulsed laser light.
(24)
The control method of (23), further comprising servocontrolling the light path length in such a manner to meet a resonance condition in the resonator, on the basis of an error signal that indicates a difference between the light path length of the resonator according to the synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulsed laser light that enters the resonator.
(25)
The control method of (23), wherein the margin value j is approximately 6.
(26)
A storage device having machine-readable instructions that, when executed by a processor: cause laser light that is emitted from a laser light source to be emitted as pulse laser light by intermittent drive;
cause a drive section to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light;
cause a servocontrol section to servocontrol a light path length of the resonator by causing the drive section to move the at least one of the reflection sections; and
determine a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of the reflected light, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from the laser light source, and Duty is a duty ratio of the pulse laser light.
(27)
A laser generation apparatus comprising a mode locked laser section coupled to a resonator having at least one pair of reflection sections and a non-linear optical crystal and a control section, the control section comprising:
a light source section configured to cause pulsed laser light by intermittent drive;
a drive section configured to move at least one of reflection sections of the resonator in an optical axis direction, and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulsed laser light; and
a servocontrol section configured to servocontrol a light path length of the resonator by moving the at least one of the reflection sections, wherein the servocontrol section decides a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of a reflected light from the resonator, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from the mode locked laser section, and Duty is a duty ratio of the pulsed laser light.
(28)
The laser generation apparatus of (27), wherein the cutoff frequency f_RS is a cutoff frequency in the servocontrol section in which a gain is approximately 0 dB.
(29)
The laser generation apparatus of (27), wherein the mode locked laser section includes a semiconductor laser, and the light source section causes the laser light to be emitted as the pulsed laser light, by intermittently driving an optical modulator section that is configured to amplify and modulate laser light that is emitted from the semiconductor laser.
(30)
A microscope comprising a laser light source for illuminating a sample area of the microscope, the laser light source comprising a mode locked laser section coupled to a resonator having at least one pair of reflection sections and a non-linear optical crystal and a control section, the control section comprising:
a light source section configured to cause pulsed laser light by intermittent drive;
a drive section configured to move at least one of reflection sections of the resonator in an optical axis direction, and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulsed laser light; and
a servocontrol section configured to servocontrol a light path length of the resonator by moving the at least one of a reflected light from the resonator, wherein the servocontrol section decides a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of the reflected light, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from the mode locked laser section, and Duty is a duty ratio of the pulsed laser light.

1 laser light generation device
10 light source unit
11 MLLD section
111 laser light source
113 lens
115 diffraction grating
121, 127, 129 lens
123 mirror
125 isolator
131 optical amplifier section (SOA section)
20 wavelength conversion optical system
21 resonator
201 input coupler
203, 205, 207 mirror
209 dichroic mirror
211 output coupler
213 non-linear optical element
215, 217 mirror
221, 223 relay lens
225, 227 mirror
30 control section
31 adder
301 oscillator
302 synchronizing signal generator section
303 isolator
305 photodetector
307 bandpass filter
309 mixer
311 driving signal generator section
313 servocontrol driver
40 SOC driver

Claims

A control device comprising:
a light source section configured to cause pulsed laser light by intermittent drive;
a drive section configured to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulsed laser light; and
a servocontrol section configured to servocontrol a light path length of the resonator by moving the at least one of the reflection sections, wherein the servocontrol section decides a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of a reflected light from the resonator, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from a laser light source, and Duty is a duty ratio of the pulsed laser light.
The control device according to claim 1, wherein the cutoff frequency fRS is a cutoff frequency in the servocontrol section in which a gain is approximately 0 dB.
The control device according to claim 1, wherein the margin value j is approximately 6.
The control device according to claim 1, wherein the laser light source is a semiconductor laser, and the light source control section causes the laser light to be emitted as the pulsed laser light, by intermittently driving an optical modulator section that is configured to amplify and modulate laser light that is emitted from the semiconductor laser.
The control device according to claim 1, wherein the servocontrol section is further configured to servocontrol the light path length on the basis of an error signal that indicates a difference between the light path length of the resonator according to the synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulsed laser light that enters the resonator.
A control method comprising:
causing laser light that is emitted from a laser light source, to be emitted as pulsed laser light by intermittent drive;
causing a drive section to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light;
servocontrolling a light path length of the resonator by causing the drive section to move the at least one of the reflection sections; and
deciding a frequency f_SOA of the intermittent drive and a frequency fM for synchronous detection of a reflected light from the resonator, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from the laser light source, and Duty is a duty ratio of the pulsed laser light.
The control method of claim 6, further comprising servocontrolling the light path length in such a manner to meet a resonance condition in the resonator, on the basis of an error signal that indicates a difference between the light path length of the resonator according to a synchronously detected reflected light from the resonator and a light path length that meets the resonance condition of the pulsed laser light that enters the resonator.
The control method of claim 6, wherein the margin value j is approximately 6.
A storage device having machine-readable instructions that, when executed by a processor: cause laser light that is emitted from a laser light source to be emitted as pulse laser light by intermittent drive;
cause a drive section to move at least one of reflection sections of a resonator in an optical axis direction, the resonator including at least one pair of reflection sections and a non-linear optical crystal and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulse laser light;
cause a servocontrol section to servocontrol a light path length of the resonator by causing the drive section to move the at least one of the reflection sections; and
determine a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of a reflected light from the resonator, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from the laser light source, and Duty is a duty ratio of the pulse laser light.
A laser generation apparatus comprising a mode locked laser section coupled to a resonator having at least one pair of reflection sections and a non-linear optical crystal and a control section, the control section comprising:
a light source section configured to cause pulsed laser light by intermittent drive;
a drive section configured to move at least one of reflection sections of the resonator in an optical axis direction, and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulsed laser light; and
a servocontrol section configured to servocontrol a light path length of the resonator by moving the at least one of the reflection sections, wherein the servocontrol section decides a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of a reflected light from the resonator, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from the mode locked laser section, and Duty is a duty ratio of the pulsed laser light.
The laser generation apparatus of claim 10, wherein the cutoff frequency fRS is a cutoff frequency in the servocontrol section in which a gain is approximately 0 dB.
The laser generation apparatus of claim 10, wherein the mode locked laser section includes a semiconductor laser, and the light source section causes the laser light to be emitted as the pulsed laser light, by intermittently driving an optical modulator section that is configured to amplify and modulate laser light that is emitted from the semiconductor laser.
A microscope comprising a laser light source for illuminating a sample area of the microscope, the laser light source comprising a mode locked laser section coupled to a resonator having at least one pair of reflection sections and a non-linear optical crystal and a control section, the control section comprising:
a light source section configured to cause pulsed laser light by intermittent drive;
a drive section configured to move at least one of reflection sections of the resonator in an optical axis direction, and being configured to resonate the pulse laser light that enters the resonator to convert a wavelength of the pulsed laser light; and
a servocontrol section configured to servocontrol a light path length of the resonator by moving the at least one of the reflection sections, wherein the servocontrol section decides a frequency f_SOA of the intermittent drive and a frequency f_M for synchronous detection of a reflected light from the resonator, in such a manner to satisfy the equations

where f_RS is a cutoff frequency of frequency characteristics associated with the servocontrol section, j is a margin value for stabilizing the servocontrol section, k is a proportionality constant that is decided based on a threshold value of light buildup in the resonator, R' is a coefficient that is decided based on a reflectance of the pair of the reflection sections, f_MLLD is a frequency of laser light that is emitted from the mode locked laser section, and Duty is a duty ratio of the pulsed laser light.