JP2016018978A - Optical pulse synchronizer, lighting system, optical microscope and synchronization method of optical pulse - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pulse synchronizer which operates stably, even if the optical intensity, wavelength, pulse width, arrangement state of the objective lens and optical detection means, or the like, changes.SOLUTION: An optical pulse synchronizer 50 for synchronizing a first optical pulse generated at a first repetition frequency and a second optical pulse generated at a second repetition frequency has modulation means 10 for changing the optical path length of at least one of the first and second optical pulses by a modulation frequency, optical detection means 16 for outputting a first signal by receiving the first and second optical pulses, synchronization detection means 17 for outputting a second signal based on the signal of modulation frequency and the first signal, and adjustment means 19 for adjusting at least one of the first and second repetition frequencies depending on the second signal.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical pulse synchronization device, an illuminating device, an optical microscope, and an optical pulse synchronization method for matching the timings of pulses of two optical pulse trains emitted by two pulse lasers.

  In recent years, pulse lasers have been used in various fields. As an application of a pulsed laser, a Raman scattering microscope using a nonlinear optical process has been studied. For example, a CARS (coherent anti-Stokes Raman scattering) microscope and a SRS (stimulated Raman scattering) microscope are known. In these microscopes, an optical pulse train emitted by two pulse lasers is focused on a sample in a state where the pulse timings are matched. That is, it is necessary to synchronize the optical pulse trains of the two pulse lasers.

  In the SRS microscope of Patent Document 1, the output of a photodetector that detects two-photon absorption is detected as a pulse timing difference, and the repetition frequency of the optical pulse train is controlled so that the output of the photodetector becomes a set value. Yes. In the CARS microscope of Patent Document 2, two similar photodetectors for detecting the pulse timing difference are used, and the difference between the outputs of the two photodetectors is used for controlling the repetition frequency of the optical pulse train. Therefore, synchronization of the optical pulse train can be realized even if the light intensity, wavelength, or pulse width changes.

International Publication No. 2010/140614 Japanese Patent No. 4862164

  According to the optical pulse train synchronization method disclosed in Patent Document 1, when the light intensity, wavelength, or pulse width emitted by the pulse laser changes, the output circuit of the photodetector or the photodetector is used to realize the synchronization of the optical pulse train. The set value of the output of must be changed. Further, in the CARS microscope of Patent Document 2, in order to stably synchronize the optical pulse train even if the light intensity, wavelength, or pulse width emitted by the pulse laser changes, two photodetectors that detect the timing difference between the pulses are used. Is necessary. These two photodetectors must be configured so that the sensitivity and the wavelength characteristics of the sensitivity are the same, and the light intensity and the pulse width input to each are the same. Otherwise, the optical pulse train cannot be synchronized, and a timing difference occurs between the pulses of the optical pulse train emitted by the two pulse lasers when the wavelength changes. Furthermore, since the output of the photodetector greatly depends on the arrangement state of the objective lens and the light receiving surface of the photodetector, the optical pulse train is not synchronized unless the arrangement state of the objective lens and the photodetector is matched with the two lights.

  In view of such problems, the present invention provides an optical pulse synchronizer that operates stably even when the light intensity, wavelength, pulse width, and arrangement state of the objective lens and the light detection means change. Objective.

  An optical pulse synchronization device according to one aspect of the present invention synchronizes a first optical pulse generated at a first repetition frequency and a second optical pulse generated at a second repetition frequency. A modulation means for changing an optical path length of at least one of the first and second optical pulses at a modulation frequency, and light for receiving the first and second optical pulses and outputting a first signal. Detection means; synchronization detection means for outputting a second signal based on the signal of the modulation frequency and the first signal; and at least one of the first and second repetition frequencies according to the second signal And adjusting means for adjusting.

  An optical pulse train synchronization method according to another aspect of the present invention is an optical pulse for synchronizing a first optical pulse generated at a first repetition frequency and a second optical pulse generated at a second repetition frequency. A modulation step of changing an optical path length of at least one of the first and second optical pulses at a modulation frequency; and receiving the first and second optical pulses and receiving a first signal A photodetection step of outputting, a synchronization detection step of outputting a second signal based on the signal of the modulation frequency and the first signal, and of the first and second frequencies according to the second signal And an adjusting step for adjusting at least one of them.

  According to the present invention, it is possible to provide an optical pulse synchronization device that operates stably even if the light intensity, wavelength, pulse width, and the arrangement state of the objective lens and the light detection means change.

It is a conceptual diagram of the optical pulse synchronizer which concerns on embodiment of this invention. It is a time profile regarding the optical pulse train which a photodetector receives. It is a time profile of the output voltage of a photodetector, and the voltage of a modulation frequency. It is a figure which shows the relationship between the output voltage of a synchronous detection circuit, and the timing difference of a pulse. It is a conceptual diagram of the optical pulse synchronizer which has the optical path length modulation means of a structure different from FIG. It is a conceptual diagram of the SRS microscope using an optical pulse synchronizer.

  Hereinafter, an optical pulse synchronizer as an example of an optical apparatus embodying the present invention will be described in detail with reference to the drawings. In each figure, the same members are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a conceptual diagram of an optical pulse synchronizer according to an embodiment of the present invention. The optical pulse synchronizer 50 includes a first pulse generated by a pulse laser (first light generating means) 1 at a first repetition frequency and a pulse laser (second light generation means) 2 having a second repetition frequency. The timings of the second pulses generated in the above are matched.

  The half mirror 3 multiplexes the optical pulse trains irradiated from the pulse lasers 1 and 2 coaxially and demultiplexes them in two directions. One of the demultiplexed optical pulse trains is used in the optical pulse synchronizer 50, and the other optical pulse train is used in a system that requires a synchronized optical pulse train such as a nonlinear optical microscope. The dichroic mirror 4 is a dielectric multilayer film designed to transmit a first optical pulse train having a wavelength λ1 and reflect a second optical pulse train having a wavelength λ2 different from the wavelength λ1. In this embodiment, after being multiplexed by the half mirror 3, it is demultiplexed into optical pulse trains of two wavelengths by the dichroic mirror 4, but before being multiplexed by the half mirror 3, the first and second light beams are separated. Each pulse train may be demultiplexed by a half mirror.

  The first optical pulse train is introduced into the optical fiber 8 by the collimator lens 5. The second optical pulse train is reflected by the mirror 6 and then introduced into the optical fiber 9 by the collimator lens 7.

  The optical path length modulation means (modulation means) 10 periodically changes the optical path length of the light passing through the optical fiber 9 by expanding and contracting the length of the optical fiber 9 at a frequency (modulation frequency) that can be appropriately changed by the user. Let In the optical path length modulation means 10 of this embodiment, an optical fiber 9 is wound around a cylindrical electrostrictive element. The voltage applied to the electrostrictive element is changed at the modulation frequency, and the electrostrictive element is displaced in the radial direction of the cylinder according to the applied voltage. Therefore, the optical fiber 9 expands and contracts at the modulation frequency, and the optical path length of the light passing through the optical fiber 9 changes periodically. As a result, the timing at which the second optical pulse train reaches the photodetector (light detection means) 16 changes.

  The amount of change in the optical path length due to the expansion and contraction of the optical fiber 9 is about the distance that the light travels during the pulse width of the pulse laser 2. If the pulse width is 1 picosecond, the change amount of the optical path length is about 300 μm. Considering the refractive index of the fiber core, the length of the optical fiber 9 may be about 200 μm. When a cylindrical electrostrictive element having a diameter of several tens of millimeters and an optical fiber having a length of about 10 m are used, an optical fiber expansion / contraction amount of 200 μm can be realized at a modulation frequency of about 10 kHz.

  The first optical pulse train is emitted from the optical fiber 8 again into the space through the collimator lens 11 and reflected by the mirror 13. The second optical pulse train is again emitted from the optical fiber 9 into the space through the collimator lens 12. The dichroic mirror 14 transmits the first optical pulse train and reflects the second optical pulse train. Therefore, the first and second optical pulse trains radiated to the space are coaxially combined by the dichroic mirror 14 and then condensed on the light receiving surface of the photodetector 16 by the objective lens 15. The dichroic mirror 14 may have the same configuration as the dichroic mirror 4. In order to increase the two-photon absorption signal detected by the photodetector 16, an objective lens 15 having a numerical aperture of 0.5 or more is suitable.

  The length of the optical fiber 8 is adjusted so that the optical path length through which the first optical pulse train passes and the optical path length through which the second optical pulse train pass in the section from the half mirror 3 to the photodetector 16. In the optical pulse synchronizer of the present embodiment, the timings at which the pulses of the first and second optical pulse trains reach the photodetector 16 are matched. Therefore, if the optical fiber 8 is adjusted as described above, the timing of the pulses in the other light beam (used in a non-linear microscope or the like) demultiplexed by the half mirror 3 matches.

  The photodetector 16 includes, for example, a light receiving element such as a photodiode and an electric circuit that converts a current generated in the light receiving element into a voltage and outputs the voltage. In order to obtain a two-photon absorption signal, the light receiving element of the photodetector 16 has a photon energy (E1∝1 / λ1) of the first optical pulse train and a photon energy (E2∝1 / λ2) of the second optical pulse train. Sensitivity is obtained at the wavelength λ1 · λ2 / (λ1 + λ2) corresponding to the sum, ie, E1 + E2. When the wavelength of the first optical pulse train is 800 nm and the wavelength of the second optical pulse train is 1030 nm, the light receiving element needs to have photodetection sensitivity in the vicinity of 450 nm, and a GaAsP photodiode is suitable. When the wavelength of the first or second optical pulse train is larger, it is preferable to use a Si photodiode as the light receiving element.

  FIG. 2 is a time profile regarding the optical pulse train received by the photodetector 16. 2A is a time profile of the intensity of the first optical pulse train on the light receiving surface of the photodetector 16, and FIG. 2B is a time profile of the intensity of the second optical pulse train on the light receiving surface of the photodetector 16. is there. In the present embodiment, the ratio of the first repetition frequency of the first optical pulse train and the second repetition frequency of the second optical pulse train is 2: 1. Therefore, as shown in FIGS. 2A and 2B, the timing of the pulses included in the first optical pulse train coincides with the timing of the pulses included in the second optical pulse train every other pulse. The ratio between the first repetition frequency of the first optical pulse train and the second repetition frequency of the second optical pulse train is not limited to 2: 1 as in this embodiment, and an arbitrary integer ratio may be used. . FIG. 2C corresponds to the sum E1 + E2 of the photon energies of the first and second optical pulse trains when the first and second optical pulse trains are in the state shown in FIG. 2A and FIG. 2B, respectively. 2 is a time profile of a two-photon absorption signal (current generated in a photodiode). FIG. 2D is a time profile of the intensity of the second optical pulse train when the optical path length modulation means 10 arrives at the photodetector 16 with a delay. FIG. 2 (e) corresponds to the sum E1 + E2 of the photon energies of the first and second optical pulse trains when the first and second optical pulse trains are in the state shown in FIG. 2 (a) and FIG. 2 (d), respectively. 2 is a time profile of a two-photon absorption signal. Since the two-photon absorption signal is proportional to the product of the intensities of two pulses, the two-photon absorption signal when there is a timing difference between the pulses of the first and second optical pulse trains shown in FIG. It is smaller than the two-photon absorption signal when there is no timing difference shown in c).

  In addition to the two-photon absorption component corresponding to E1 + E2, the voltage output from the photodetector 16 includes the two-photon absorption component of the first and second optical pulse trains such as E1 + E1 and E2 + E2, and the original photon such as E1 and E2. Contains energy components. However, components other than E1 + E2 are unnecessary for the optical pulse synchronizer of this embodiment. In particular, when a component due to the original photon energy (E1 and E2) is detected, the component is very large, so that it is difficult to detect the component E1 + E2. Therefore, it is desirable to adjust the band gap of the PN junction of the light receiving element of the photodetector 16 to be equal to or higher than the photon energy E1, E2 so that the component due to the original photon energy is not detected. When the wavelength corresponding to the photon energy E1 is 800 nm and the wavelength corresponding to the photon energy E2 is 1030 nm, the band gap is preferably set to be equal to or greater than the photon energy E1 (= 1.55 electron volts). The GaAsP photodiode satisfies this condition.

  The unnecessary two-photon absorption components (E1 + E1 and E2 + E2) cannot be removed using the band gap, but only the component due to E1 + E2 can be obtained by the method described below.

  By changing the timing difference between the pulses of the first and second optical pulse trains that reach the light receiving element of the photodetector 16 by the optical path length modulation means 10, the component caused by E1 + E2 is changed. On the other hand, the components resulting from E1 + E1 and E2 + E2 do not change because they are generated by each optical pulse train itself. Therefore, by extracting the amplitude of the modulation frequency component included in the output voltage of the photodetector 16, only the component resulting from E1 + E2 can be obtained.

  The synchronous detection circuit (synchronization detection means) 17 is an electric circuit such as a lock-in amplifier, and extracts the amplitude of the modulation frequency component contained in the first voltage (first signal) that is the output voltage of the photodetector 16. And output as a second voltage (second signal). Specifically, a voltage output from the photodetector 16 and a voltage signal having a rectangular or sine waveform having a modulation frequency are mixed by a mixer, and then a voltage signal that has passed through a low-pass filter circuit is output. The cutoff frequency of the low-pass filter is set to about 1 kHz as a frequency suitable for feedback control of the frequency of the optical pulse train.

  FIG. 3 is a time profile of the output voltage of the photodetector 16 and the voltage of the modulation frequency used in the synchronous detection circuit 17. In FIG. 3, the band frequency of the photodetector 16 is schematically shown as being smaller than the repetition frequency of each optical pulse train. That is, the time profile for each pulse cannot be detected, and signals for several pulses to several hundred pulses are averaged. The output voltage of the photodetector 16 includes components due to E1 + E1 and E2 + E2 in addition to components due to changing E1 + E2. FIG. 4 shows the relationship between the pulse timing difference and the output voltage of the synchronous detection circuit 17.

  When the first and second optical pulse trains are synchronized, the output voltage of the photodetector 16 changes periodically as shown by the solid line in FIG. 3 due to the change in the optical path length of the second optical pulse train. Here, the fact that the first and second optical pulse trains are synchronized means that the timings of the pulses of the first and second optical pulse trains coincide at the position of the half mirror 3. When the voltage of the modulation frequency indicated by the two-dot chain line in FIG. 3 is zero, the pulse timings of the first and second optical pulse trains coincide with each other, and the output voltage of the photodetector 16 becomes maximum. When the voltage of the modulation frequency becomes positive or negative, a timing difference occurs in the pulse, and the output voltage of the photodetector 16 decreases. The output voltage of the synchronous detection circuit 17 in this case is zero in FIG. 4 because it is the time average of the product of the solid line and the two-dot chain line in FIG.

  The output of the synchronous detection circuit 17 when the resonator length of the pulse laser 1 or the pulse laser 2 changes due to a disturbance and a timing difference occurs between the pulses of the first and second optical pulse trains will be described. The pulse timing difference changes at a frequency smaller than the modulation frequency and the low-pass filter cutoff frequency of the synchronous detection circuit 17.

  When the first optical pulse train is delayed with respect to the second optical pulse train, the output voltage of the photodetector 16 is delayed by the optical path length modulation means 10 as shown by the dotted line in FIG. It becomes the maximum in the time zone (the time zone in which the signal of the modulation frequency is positive). Since the output voltage of the synchronous detection circuit 17 in this case is the time average of the product of the dotted line and the two-dot chain line in FIG. 3, the output voltage is a positive value in FIG. When the delay of the first optical pulse train is larger, the synchronous detection circuit 17 outputs a larger positive value.

  When the first optical pulse train advances with respect to the second optical pulse train, the output voltage of the photodetector 16 advances by the optical path length modulation means 10 through the second optical pulse train as shown by the one-dot chain line in FIG. It becomes the maximum value in the time zone (time zone in which the signal of the modulation frequency is negative). The output voltage of the synchronous detection circuit 17 in this case is a negative value in FIG. 4 because the product of the one-dot chain line and the two-dot chain line in FIG. 3 is time-averaged. When the advance of the first optical pulse train is larger, the synchronous detection circuit 17 outputs a negative value having a larger absolute value.

  As described above, the output voltage of the synchronous detection circuit 17 reflects the timing difference when the pulses of the first and second optical pulse trains reach the half mirror 3.

  The feedback circuit 18 outputs a voltage to be applied to a frequency adjusting means (adjusting means) 19 in the pulse laser 2 in order to correct a pulse timing difference corresponding to the output voltage of the synchronous detection circuit 17.

  The frequency adjusting means 19 is constituted by a stage to which a phase modulator or a mirror is attached, and adjusts the resonator length by applying a voltage to the phase modulator or driving the stage. The frequency adjusting means 19 may be installed in the pulse laser 1 to adjust the resonator length of the pulse laser 1.

  When the output voltage of the synchronous detection circuit 17 is a positive value, the first optical pulse train is delayed with respect to the second optical pulse train, so that the second light is increased by increasing the resonator length of the pulse laser 2. The first and second optical pulse trains are synchronized by delaying the pulse train. For this purpose, the feedback circuit 18 applies a voltage that increases the resonator length of the pulse laser 2 to the frequency adjusting means 19. When the output voltage of the synchronous detection circuit 17 is a negative value, the opposite is true, and the feedback circuit 18 outputs a voltage that reduces the resonator length of the pulse laser 2. As described above, the pulse timings of the first and second optical pulse trains are made to coincide with each other by feedback control that makes the output voltage of the synchronous detection circuit 17 zero. Note that the feedback circuit 18 can be configured so that the output voltage of the synchronous detection circuit 17 is not zero, and a predetermined timing difference can be given to the pulses of the first and second optical pulse trains.

  In the optical pulse synchronizer of this embodiment, it can be determined by the sign of the output voltage of the synchronous detection circuit 17 whether the first optical pulse train is delayed or advanced with respect to the second optical pulse train. For this reason, even if the light intensity or the pulse width changes, the sign does not change only by the change in the absolute value of the output voltage of the synchronous detection circuit 17, so that the synchronization of the optical pulse train can be realized. Similarly, even if the wavelength changes within a range in which the photodetector 16 has sensitivity, synchronization of the optical pulse train can be realized. Further, since only one photodetector is used, it is not necessary to match the arrangement state of the objective lens and the photodetector between the two photodetectors as in Patent Document 2.

  In the present embodiment, the wavelengths of light emitted by the two pulse lasers are different from each other. However, by using a polarization beam splitter instead of the dichroic mirror, the optical pulse trains of the same wavelength emitted by the two pulse lasers are synchronized. Is easily possible.

  In this embodiment, the optical path length of the optical fiber 9 is changed by the optical path length modulation means 10, but the above-described effect can be obtained even if the optical path length of the optical fiber 8 is changed. In the present embodiment, only one optical path length modulation means is provided, but a plurality of optical path length modulation means may be provided. For example, the same optical path length modulation means as the optical path length modulation means 10 is used for expansion and contraction of the optical fiber 8. At this time, the expansion / contraction phase of the optical fiber 8 is reversed with the expansion / contraction phase of the optical fiber 9. By doing so, the amount of modulation of the optical path length by the two optical path length modulation means is doubled, so that the optical pulse train can be stably synchronized even in an optical pulse train having a larger pulse width.

  Further, in the optical path length modulation means 10 of the present embodiment, the wound optical fiber 9 is expanded and contracted by expanding and contracting the electrostrictive element in the radial direction of the cylinder, but the optical fiber can be expanded and contracted linearly. Good. As shown in FIG. 5, the optical path length modulation means 30 may be constituted by an electric stage (drive means) 31 and mirrors (reflection members) 32 to 35 as optical path length modulation means that do not use an optical fiber or a collimator lens.

  The photodetector 16 may be replaced with another photodetector having sensitivity to a wavelength corresponding to the photon energy of E1 + E2 and having a band equal to or higher than the modulation frequency.

  Next, an SRS (stimulated Raman scattering) microscope using the optical pulse synchronizer of this embodiment will be described with reference to FIG. In the SRS microscope, SRS generated by simultaneously irradiating a sample with two lights having different wavelengths is detected by a detection system. SRS is one of nonlinear optical phenomena and is generated in proportion to the product of the intensity of light of each wavelength. Therefore, in order to generate SRS efficiently, the optical pulse train is synchronized so that the light beams of the lasers of two wavelengths are collected at the same point and the optical pulses of the two wavelengths are simultaneously condensed. When SRS occurs, the intensity of the light pulse having the shorter wavelength of the light pulses having the two wavelengths is weakened, and the intensity of the light pulse having the longer wavelength is increased. In order to efficiently generate SRS, it is desirable to use a short pulse laser having a pulse width of 1 to 10 ps.

  In this embodiment, a solid-state laser (titanium sapphire laser) having a center wavelength (λ1) of 800 nm and a repetition frequency of 80 MHz is used as the pulse laser 1. As the pulse laser 2, an ytterbium-doped fiber laser having a center wavelength (λ2) of 1030 nm and a repetition frequency of 40 MHz is used.

  The half mirror 3 coaxially multiplexes the light beams emitted from the pulse lasers 1 and 2 and demultiplexes them in two directions. One of the demultiplexed light beams enters the optical pulse synchronizer, and the other light beam enters the SRS microscope 100. The pulse lasers 1 and 2 and the optical pulse synchronizer 50 of this embodiment constitute an illumination device.

  The light beams of the pulse lasers 1 and 2 are coaxially incident on the beam scanner 101, deflected by the beam scanner 101, and emitted. The beam scanner 101 includes a galvano scanner and a resonant scanner, and changes the direction of the optical axis in two orthogonal directions. For simplification of the drawing, two mirrors in the beam scanner 101 are represented by one mirror in FIG. If a resonant scanner (scan frequency 8 kHz) and a galvano scanner (scan frequency 15 Hz) are used, an image of 500 lines can be acquired at 30 frames per second.

  The light beam deflected by the beam scanner 101 enters the objective lens 104 through the lenses 102 and 103. By arranging the lenses 102 and 103 so that the entrance pupils of the beam scanner 101 and the objective lens 104 are conjugate, even if the light beam is deflected by the beam scanner 101, the amount of light is collected on the sample 105 without being changed by light shielding. Shine. The focal lengths of the lenses 102 and 103 are selected so that the entrance pupil size of the objective lens 104 is equal to the incident light beam size. By doing so, the size of the light spot collected by the objective lens 104 is minimized, and the spatial resolution for detecting the SRS signal is improved. Further, since the SRS signal is increased by increasing the intensity of the light spot, the SN ratio for detecting the SRS signal is also improved. The objective lens 104 is desirably an objective lens having a large numerical aperture (NA) from the viewpoint of the spatial resolution for detecting the SRS signal and the SN ratio.

  The sample 105 is sandwiched between cover glasses (not shown) having a thickness of several tens to 200 μm. The light spot focused on the sample 105 is two-dimensionally scanned by the deflection of the light beam by the beam scanner 101, and the SRS signal is converted into a two-dimensional image. Since the SRS signal is generated locally at the collected light spot, a three-dimensional image can be obtained by moving the sample 105 in the optical axis direction by a stage (not shown).

  The objective lens 106 is an objective lens having an NA equal to or greater than the NA of the objective lens 104 so as to receive all the light that has passed through the sample 105 and has been intensity-modulated by SRS. The light beam emitted from the objective lens 106 passes through the filter 107 and the lens 108 and is then irradiated on the light receiving surface of the photodiode 109. The filter 107 is formed of a dielectric multilayer film, blocks light of wavelength λ2, and transmits light of wavelength λ1. The photodiode 109 is irradiated with an optical pulse train that is emitted from the pulse laser 1 and repeats intensity modulation by SRS for each pulse. As the photodiode 109, a silicon photodiode having sensitivity to an optical pulse train of 800 nm and having a cutoff frequency of 40 MHz or more is used.

  The current-voltage conversion circuit 110 is an electric circuit for outputting a current signal generated by the photodiode 109 as a voltage.

  The synchronous detection circuit 111 extracts the amplitude of a 40 MHz component from the voltage signal output from the current-voltage conversion circuit 110 and outputs it as a voltage, and uses a mixer circuit or a lock-in amplifier. The output voltage of the synchronous detection circuit 111 indicates how much SRS has occurred at the focal point of the sample 105.

  The computer 112 uses the control signal of the beam scanner 101 to convert the output signal of the synchronous detection circuit 111 into a two-dimensional image and display it. The calculator 112 can also display a three-dimensional image of the SRS signal acquired by moving the sample 105 in the optical axis direction on a stage (not shown). The computer 112 can also display the Raman spectrum from the SRS signal acquired by changing the wavelength of at least one of the two pulse lasers.

  When the timings of the pulses of the first and second optical pulse trains coincide as shown in FIGS. 2A and 2B and are condensed at the same point on the sample 105, the sample 105 is transmitted through the SRS. The light intensity of the applied light pulse changes. Specifically, the light intensity of the pulses 1, 3, and 5 in FIG. 2 (a) decreases, and the light intensity of the pulses 2 and 4 does not change. The difference in the detected light intensity corresponds to the SRS signal, and the information on the molecules contained in the spot where the light beam is condensed is reflected. For example, the SRS signal becomes large when the resonance frequency (c / λ1−c / λ2) between the resonance frequencies of the molecular vibrations contained in the focused spot and the optical frequencies of the two lasers coincide. Note that c represents the speed of light. The Raman spectrum can be acquired by acquiring the SRS signal while changing the difference (c / λ1-c / λ2) between the optical frequencies of the two lasers. It can be estimated from the Raman spectrum what kind of molecules are contained in the sample. Therefore, the SRS microscope can acquire a spectrum equivalent to a microscope using spontaneous Raman. Since the scattering efficiency of SRS is much larger than that of spontaneous Raman scattering, the SRS microscope can acquire a Raman spectrum in a shorter time than a microscope using spontaneous Raman scattering. In the SRS microscope of the present embodiment, the optical pulse train can be stably synchronized by the optical pulse synchronizer even if the wavelength of at least one of the two lasers is changed in order to obtain a Raman spectrum.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

10 Optical path length modulation means (modulation means)
16 photodetector (light detection means)
17 Synchronous detection circuit (synchronous detection means)
19 Frequency adjustment means (adjustment means)

Claims (11)

An optical pulse synchronizer that synchronizes a first optical pulse generated at a first repetition frequency and a second optical pulse generated at a second repetition frequency,
Modulation means for changing an optical path length of at least one of the first and second optical pulses at a modulation frequency;
Light detecting means for receiving the first and second light pulses and outputting a first signal;
Synchronization detecting means for outputting a second signal based on the signal of the modulation frequency and the first signal;
And an adjusting means for adjusting at least one of the first and second repetition frequencies according to the second signal.   2. The optical pulse synchronizer according to claim 1, wherein the first signal is a signal corresponding to a timing difference between the first and second optical pulses when the first and second optical pulses are received.   The optical pulse synchronizer according to claim 1, further comprising an optical fiber through which the first and second optical pulses pass.   4. The optical pulse synchronizer according to claim 3, wherein the modulation means includes an electrostrictive element that expands and contracts the optical fiber.   5. The optical pulse synchronizer according to claim 1, further comprising a reflection member that reflects the first and second optical pulses. 6.   6. The optical pulse synchronizer according to claim 5, wherein the modulation means includes drive means for moving the reflecting member. The light detection means includes a photodiode for receiving the first and second optical pulse trains,
7. The optical pulse synchronizer according to claim 1, wherein the photodiode detects a current generated by two-photon absorption. First light generating means for emitting the first light pulse;
Second light generating means for emitting the second light pulse;
An illuminating device comprising: the optical pulse synchronizer according to claim 1. A lighting device according to claim 8;
And an optical system for irradiating the sample with the first and second light pulses.   The optical microscope according to claim 9, further comprising: a detection system that detects light whose intensity is modulated by Raman scattering generated when the first and second light pulses are applied to the sample. An optical pulse synchronization method for synchronizing a first optical pulse generated at a first repetition frequency and a second optical pulse generated at a second repetition frequency,
A modulation step of changing an optical path length of at least one of the first and second optical pulses at a modulation frequency;
A light detection step of receiving the first and second light pulses and outputting a first signal;
A synchronization detection step of outputting a second signal based on the signal of the modulation frequency and the first signal;
And an adjusting step of adjusting at least one of the first and second repetition frequencies in accordance with the second signal.