JP6643896B2 - Optical pulse train synchronizer, optical microscope, and optical pulse train synchronization method - Google Patents

Description

  The present invention relates to an optical pulse train synchronization device, an optical microscope, and an optical pulse train synchronization method.

  In an optical microscope using a non-linear optical phenomenon such as a stimulated Raman scattering microscope, an optical pulse train emitted by two pulse lasers (light sources) is emitted in a state where the pulse timings are matched or the timing difference is kept constant. It needs to be focused on the sample.

  Patent Document 1 discloses a stimulated Raman scattering (SRS) microscope that detects an output of a photodetector that detects two-photon absorption as a pulse timing difference and adjusts a pulse cycle so that a detected value becomes a set value. Is described. In Patent Document 1, a part of an optical pulse train emitted by a titanium sapphire laser as a light source is separated and used to synchronize with an optical pulse train emitted by another light source (laser).

  Non-Patent Document 1 describes using an erbium-doped fiber laser as a light source using a fiber laser that is more inexpensive and more compact than a titanium sapphire laser. Specifically, Non-Patent Document 1 describes converting an optical pulse train having a wavelength of 1580 nm generated by an erbium-doped fiber laser into an optical pulse train having a wavelength of 790 nm by generating a second harmonic.

Patent No. 5501360

Opt. Express 20, 13958 (2012)

  However, although a titanium sapphire laser can use light with high output and sufficient intensity for synchronizing an optical pulse train, there is an aspect that the size becomes large. In addition, since light having a wavelength of 790 nm obtained by the method described in Non-Patent Document 1 cannot be amplified, the intensity of light used for synchronization is limited to irradiate sufficient light to a sample. As a result, light of sufficient intensity cannot be used for synchronization, and synchronization may be unstable.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide an optical pulse train synchronizer that can perform synchronization of two optical pulses more stably than before.

An optical pulse train synchronizer according to one aspect of the present invention includes a first light source that outputs a first optical pulse train having a first wavelength, and a second light source that outputs a second optical pulse train having a second wavelength. A light source, wavelength changing means for changing the wavelength of the first light pulse train to a third wavelength, and a light pulse train from the wavelength changing means, the third light pulse train having the first wavelength and the third A first separating unit that separates the second optical pulse train into a fourth optical pulse train having a second wavelength, and a fifth optical pulse train having the second wavelength and a sixth optical pulse train having the second wavelength. of a second separating means for separating into the optical pulse train, and the prior SL third optical pulse train before Symbol fifth detection means for detecting an optical pulse train, on the basis of the detection result of said detecting means, said first The pulse period of the light pulse train or the pulse period of the second light pulse train It characterized by having a a period changing means for changing.
An optical pulse train synchronization method according to another aspect of the present invention is an optical pulse train synchronization method for synchronizing a first optical pulse train having a first wavelength and a second optical pulse train having a second wavelength. Changing a part of the wavelength of the first optical pulse train to a third wavelength, and converting the optical pulse train from the wavelength changing step into a third optical pulse train having the first wavelength and a third optical pulse train. A first separation step of separating the second light pulse train into a fourth light pulse train having a wavelength, and a sixth separation having the fifth light pulse train having the second wavelength and the second light pulse having the second wavelength. A second separation step of separating the light into a light pulse train, a detection step of detecting the third light pulse train and the fifth light pulse train, and the first light based on a detection result in the detection step. The pulse period of the pulse train or the And having between cycle changing step of changing the pulse period of the optical pulse train, the.

  According to the optical pulse train synchronizer as one aspect of the present invention, the synchronization of two optical pulses can be performed more stably than before.

FIG. 2 is a diagram illustrating a configuration of an optical microscope according to the first embodiment. FIG. 4 is a diagram illustrating a relationship between an output voltage of a photodetector and timing differences between respective pulses of two optical pulse trains. (A) Time profile of the intensity of the third optical pulse train when the third optical pulse train and the fifth optical pulse train are synchronized. (B) The time profile of the intensity of the fifth optical pulse train when the third optical pulse train and the fifth optical pulse train are synchronized. FIG. 4 is a diagram illustrating a relationship between an output voltage and a pulse timing difference in the photodetector according to the first embodiment. (A) Time profile of the intensity of the fourth light pulse train. (B) Time profile of the intensity of the sixth light pulse train.

(First embodiment)
The configuration of the optical microscope 100 using the optical pulse train synchronizer 150 (hereinafter, referred to as “synchronizer 150”) of the present embodiment will be described with reference to FIG. FIG. 1 is a diagram illustrating a configuration of an optical microscope 100 according to the embodiment. The optical microscope 100 is a nonlinear optical microscope that simultaneously irradiates a sample with two light pulse trains and measures a nonlinear optical phenomenon generated in the sample. Stimulated Raman scattering and the like are examples of the nonlinear optical phenomenon.

  The synchronizer 150 includes a first light source 1, a second light source 2, a wavelength changing element 3, a first separating unit 5, a second separating unit 4, a mirror 6, a first multiplexing unit 7, and a first multiplexing unit. The optical system includes an optical system 8, a first detecting unit (photodetector) 9, a control circuit 10, and a period changing unit 11. The optical microscope 100 includes a synchronizer 150, a mirror 12, a second multiplexing unit 13, a scanning unit 14, lenses 15, 16, 21, a second optical system 17, a third optical system 19, a filter 20, a second , A conversion circuit 23, an acquisition circuit 24, and a processing means 25.

  The first light source 1 outputs a first optical pulse train 101 having a first pulse period and a first wavelength (λ1). Further, the second light source 2 outputs a second optical pulse train 102 having a second pulse period and a second wavelength (λ2).

  The optical microscope 100 of the present embodiment has a configuration for synchronizing a first optical pulse train 101 from the first light source 1 and a second optical pulse train 102 from the second light source 2. Here, “synchronization” in this specification is defined as maintaining a constant difference between the emission timings of the first optical pulse train 101 and the second optical pulse train 102. The second light source 2 is, for example, a mode-locked laser, and achieves synchronization by changing the resonator length to adjust the second pulse period.

  Note that the optical pulse train includes light of a plurality of different frequencies, and specifically includes light of a wide frequency band. Therefore, the “wavelength” in the optical pulse train indicates a center wavelength or a peak wavelength of light in a wide frequency band included in the optical pulse train. For example, the first optical pulse train 101 has a center wavelength or peak wavelength of the first wavelength (λ1), and the second optical pulse train 102 has a center wavelength or peak wavelength of the second wavelength (λ2).

  By synchronizing the first optical pulse train 101 and the second optical pulse train 102, the fourth optical pulse train 104 from the wavelength changing unit 3 and the sixth optical pulse train 106 from the beam splitter 4 are applied to the sample 18. The irradiation timing can be adjusted. Note that the fourth optical pulse train 104 is an optical pulse train of a third wavelength (λ3) obtained by making the optical pulse train 101 incident on the wavelength changing unit 3. The sixth optical pulse train 106 is an optical pulse train of a second wavelength (λ2) obtained by making the optical pulse train 102 enter the beam splitter 4. In the following description, the fourth light pulse train 104 and the sixth light pulse train 106 may be collectively referred to as “irradiation light”.

  The wavelength changing means 3 changes the wavelength (center wavelength) of the incident first optical pulse train 101 to generate a fourth optical pulse train 104 having a second wavelength (λ3) different from that of the first optical pulse train 101. Let it. As the wavelength changing means 3, a POLN (periodically polled lithium niobate) that generates any of a harmonic, a sum frequency, and a difference frequency, an OPA (optical parametric amplifier), or the like is used.

  The wavelength changing means 3 emits, in addition to the optical pulse train 104, a third optical pulse train 103 having the same wavelength as the incident first optical pulse train 101. This is because the wavelength changing efficiency of the wavelength changing means 3 is generally several tens of percent, only a part of the first optical pulse train is changed in wavelength, and the third optical pulse train 103 is emitted as the light whose wavelength has not been changed. That's because.

  The first separating unit 5 separates the third optical pulse train 103 and the fourth optical pulse train 104. In the present embodiment, the first separating unit 5 includes a dielectric multilayer film designed to transmit the third optical pulse train 103 having the wavelength λ1 and reflect the fourth optical pulse train 104 having the wavelength λ3. A dichroic mirror is used. The third optical pulse train 103 is used for optical pulse train synchronization.

  The second separating means 4 separates the second optical pulse train 102 into a fifth optical pulse train 105 and a sixth optical pulse train 106 having the same wavelength (second wavelength λ2). In the present embodiment, the second separating means 4 reflects a part of the second optical pulse train 102 (the fifth optical pulse train 105) in the right direction on the paper and the rest (the sixth optical pulse train 106) on the paper. Is a beam splitter that transmits downward. Since the second separating means 4 only needs to be able to reflect incident light at a fixed rate, a glass flat plate can be used.

  Alternatively, a flat plate coated with a predetermined reflectance or a polarizing beam splitter may be used as the second separating unit 4. When a polarization beam splitter is used, the second optical pulse train 102 may be configured to enter the polarization beam splitter via a half-wave plate (not shown). The intensity ratio between the fifth optical pulse train 105 and the sixth optical pulse train 106 can be changed by adjusting the angle of the half-wave plate (not shown) around the optical axis.

  After being reflected by the mirror 6, the fifth optical pulse train 105 is multiplexed coaxially with the third optical pulse train 103 by a dichroic mirror as the first multiplexing means 7. The first multiplexing means 7 of the present embodiment is a dielectric multilayer film designed to transmit the third optical pulse train 103 having the wavelength λ1 and reflect the fifth optical pulse train 105 having the wavelength λ2. .

  The first optical system 8 guides the combined third optical pulse train 103 and fifth optical pulse train 105 to the photodetector 9. Although the first optical system 8 of the present embodiment has an objective lens, a mirror, a lens, and the like may be appropriately combined.

  The photodetector 9 is a detection unit that detects the third optical pulse train 103 and the fifth optical pulse train 105. The photodetector 9 includes a light receiving element such as a photodiode, and an electric circuit that converts a current generated on a light receiving surface of the light receiving element into a voltage and outputs the voltage. In order to increase the current generated by two-photon absorption on the light receiving surface of the light receiving element, it is preferable that the numerical aperture of the objective lens of the first optical system 8 be 0.5 or more.

  In order to obtain a two-photon absorption signal, the light receiving element of the photodetector 9 has a photon energy E1 (E1∝1 / λ1) of the third light pulse train 103 and a photon energy E2 (E2∝1) of the fifth light pulse train 105. / Λ2) has sensitivity at a wavelength corresponding to the sum (E1 + E2). That is, the light receiving element of the photodetector 9 has sensitivity at the wavelength λ1 · λ2 / (λ1 + λ2) corresponding to E1 + E2. As an example, when λ1 is 1580 nm and λ2 is 1030 nm, it is sufficient that the light detection sensitivity is 624 nm.

  FIG. 2 shows the output voltage V as a detection result of the photodetector 9 and the pulse of the optical pulse train 105 reaching the light receiving surface from the timing when the pulse of the third optical pulse train 103 arrives at the light receiving surface of the photodetector 9. And the timing difference Δt obtained by subtracting the calculated timing. 2, the vertical axis represents the output voltage V, and the horizontal axis represents the timing difference Δt.

  The output of the photodetector 9 includes two-photon absorption signals corresponding to E1 + E1 and E2 + E2 in addition to the two-photon absorption signals corresponding to E1 + E2. Therefore, a two-photon absorption signal corresponding to each of E1 + E1 and E2 + E2 is added to a two-photon absorption signal corresponding to E1 + E2.

  Two-photon absorption corresponding to E1 + E1 occurs in the first pulse cycle. Two-photon absorption corresponding to E2 + E2 occurs in the second pulse cycle. In the present embodiment, the band frequency of the output voltage of the photodetector 9 is set smaller than the frequencies corresponding to the first pulse period and the second pulse period. Therefore, the two-photon absorption signals corresponding to E1 + E2 and E2 + E2 are DC components that are not modulated in the first pulse cycle and the second pulse cycle.

  The two-photon absorption signal corresponding to E1 + E2 is obtained when the timing of the pulse of the third optical pulse train 103 coincides with the timing of the pulse of the fifth optical pulse train 105, that is, when the pulse reaches the light receiving surface of the photodetector 9 at the same time (Δt = 0). When the peak intensities of the two pulses completely match, a maximum signal is obtained, and the signal is proportional to the product of the intensity of the optical pulse train 103 and the intensity of the optical pulse train 105.

An output voltage as a detection result from the photodetector 9 is input to the control circuit 10. The control circuit 10 outputs a signal for synchronizing the light pulse train to the period changing means 11 based on the output voltage V from the light detector 9. Specifically, the control circuit 10, so that the output voltage of the photodetector 9 is _(any voltage in the range in which the voltage varies by two-photon absorption signal corresponding to the E1 + E2) the target value V _n shown in FIG. 2 Then, a signal for adjusting the pulse timing difference is output to the period changing means 11.

The two-photon absorption signal changes depending on a change in the intensity of the light output from the first light source 1 or the second light source 2 and a state of focusing on the light receiving surface of the photodetector 9. Therefore, in order to stably synchronize the optical pulse train, the output voltage at the target value V _n of the output voltage of the photodetector 9, that is, the output voltage at the timing difference Δt _n is set to the intermediate value of the voltage changed by the two-photon absorption signal corresponding to E1 + E2. It is preferable to use a value.

  The cycle changing means 11 changes the second pulse cycle. The period changing means 11 has a stage on which a phase modulator or a mirror is attached, and changes the resonator length of the second light source 2 by applying a voltage to the phase modulator or driving the stage. By changing the resonator length, the second pulse period is changed, and the pulse timing can be adjusted. Further, the period changing means 11 may be provided not in the second light source 2 but in the first light source 1 which is a mode-locked laser to adjust the resonator length of the first light source 1.

Here, the function of the control circuit 10 will be described. In the present embodiment, the control circuit 10 controls the second pulse cycle so that the timing difference Δt _{n in} FIG. 2 is obtained. The relationship between the intensity (vertical axis) and the time t (horizontal axis) of the third optical pulse train 103 and the fifth optical pulse train 105 on the light receiving surface of the photodetector 9 when the optical pulse trains are synchronized, respectively This is shown in FIGS. 3 (a) and 3 (b). In a state where the optical pulse trains are synchronized, the timing difference between the intensity peak of the pulse 31 of the third optical pulse train 103 and the intensity peak of the pulse 51 of the fifth optical pulse train 105 is the timing difference of FIG. consistent with the Δt _n. If the synchronization of the optical pulse train is continuously realized, the pulse of the fifth optical pulse train 105 reaches the light receiving section of the photodetector 9 with a timing difference Δt _n even for the pulse adjacent to the pulse 31.

  In FIG. 3, the ratio of the pulse periods of the third optical pulse train 103 and the fifth optical pulse train 105 is set to 1: 2. However, the present invention is not limited to this, and the present invention can be applied even if the pulse period ratio is m: n. m and n are arbitrary positive integers.

  When the output voltage of the photodetector 9 is equal to or higher than the above-described target value, it means that the pulse 31 and the pulse 51 are closer to each other as compared with the state in which the optical pulse train shown in FIG. Therefore, the control circuit 10 outputs to the cycle changing means 11 a signal that delays the pulse of the fifth optical pulse train 105, that is, increases the second pulse cycle (increases the resonator length).

  Conversely, when the output voltage of the photodetector 9 is equal to or less than the target value, the control circuit 10 generates a signal such that the pulse of the fifth optical pulse train 105 advances with respect to the pulse of the third optical pulse train 103. Output to the changing means 11. More specifically, the control circuit 10 outputs a signal for shortening the second pulse cycle (such as shortening the resonator length) to the cycle changing unit 11. The signal output to the cycle changing means 11 is set to have a band and a voltage amplitude such that the control is stably performed in the vicinity of a region indicated by a chain line in FIG.

  By the above control, a constant timing difference is maintained between the pulse of the third optical pulse train 103 and the pulse of the fifth optical pulse train 105 on the light receiving surface of the photodetector 9.

  Here, the behavior when the output intensity of the first light source 1 or the output intensity of the second light source 2 changes will be described. At this time, not only the two-photon absorption signal corresponding to E1 + E2 changes, but also the two-photon absorption signals corresponding to DC components E1 + E1 and E2 + E2. As a result, the pulse timing difference Δt changes according to the output intensity. When the change in the output intensity is large, the target value of the photodetector 9 set in advance falls outside the change range of the two-photon absorption signal corresponding to E1 + E2, and the synchronization itself of the optical pulse train cannot be realized.

  In the present embodiment, in order to alleviate this problem, the band gap energy Eg of the material of the light receiving element of the photodetector 9 is selected so as not to detect the two-photon absorption signal corresponding to either E1 + E1 or E2 + E2. In order to realize synchronization of the pulse train, the material and the wavelength (λ1 and λ2) of the light receiving element are selected so that the band gap energy Eg is equal to or more than E1 + E2 (more than the sum of E1 and E2). Further, the material and wavelength (λ1 and λ2) of the light receiving element are selected so that the two-photon absorption signal corresponding to E1 + E1 or E2 + E2 is not detected, and the band gap energy Eg is E1 + E1 (twice E1) or E2 + E2 (E2 + E2). (2 times).

  For example, when λ1 is 1580 nm (E1 to 0.8 eV) and λ2 is 1030 nm (E2 to 1.2 eV), a GaAsP photodiode (Eg to 2.0 eV) is used as a light receiving element of the photodetector 9. This condition is satisfied by using.

  FIG. 4 shows the relationship between the output voltage V of the photodetector 9 and the pulse timing difference Δt when the bandgap energy Eg is set to be larger than E1 + E1. In FIG. 4, the vertical axis represents the output voltage V, and the horizontal axis represents the timing difference Δt. When the output intensity of the first light source 1 changes, the pulse timing difference of the two-photon absorption signal corresponding to E1 + E2 changes. However, since the output voltage V corresponding to E1 + E1 is zero and constant, the pulse train of the pulse train is changed. Synchronization can be more stabilized.

  Patent Literature 1 and Non-Patent Literature 1 do not satisfy the above relationship because the wavelength of light used for pulse train synchronization is about 790 nm and about 1030 nm, and the light receiving element is a GaAsP photodiode. Further, a part of the light guided to the sample among the light from the wavelength changing element is branched and detected for synchronization of the optical pulse train. Since the optical microscope 100 of the present embodiment uses light having a sufficient intensity, whose wavelength has not been changed, for synchronizing an optical pulse train, the intensity of light applied to the sample 18 can be improved as compared with the related art. Further, it is possible to reduce the generation of the two-photon absorption signal due to the light whose wavelength has not been changed by the wavelength changing element 3, and to stably synchronize the optical pulse train.

  The photodetector 9 includes a nonlinear crystal and a photomultiplier (photomultiplier) that detects a sum frequency light generated by the third optical pulse train 103 and the fifth optical pulse train 105 incident on the nonlinear crystal. May be provided. As the nonlinear crystal, for example, a barium borate crystal or the like is used. The cut angle and the arrangement angle of the nonlinear crystal are adjusted so that the wavelength λ1 / λ2 / (λ1 + λ2) corresponding to E1 + E2 is generated. The light of λ1 and λ2 is separated from the light of wavelength λ1 · λ2 / (λ1 + λ2) by a filter composed of a dielectric multilayer film or the like, so that only the wavelength λ1 / λ2 / (λ1 + λ2) is detected by photomultiplier. In this case, since the component of E1 + E1 or E2 + E2 hardly occurs, the optical pulse train can be synchronized more stably. Not only the sum frequency light, but also a configuration in which the difference frequency light generated by the third light pulse train 103 and the fifth light pulse train 105 incident on the nonlinear crystal can be detected.

  In the related art, since a part of the irradiation light applied to the sample 18 is used for synchronizing the optical pulse trains, the light intensity of the two optical pulse trains used for synchronization cannot be sufficiently increased, and the synchronization may be unstable. was there. On the other hand, in the synchronizer 150 of the present embodiment, since the light pulse whose wavelength has not been changed by the wavelength changing element 3 is used as the light pulse train used for synchronization, the intensity of the irradiation light irradiating the sample 18 is not limited and is sufficiently high. An optical pulse train with an appropriate amount of light can be used for synchronization.

  The optical microscope 100 of the present embodiment has a configuration of a scanning microscope. The fourth light pulse train 104 reflected by the mirror 12 and the sixth light pulse train 106 transmitted through the second separating means 4 are coaxially multiplexed by a dichroic mirror as the second multiplexing means 13 and then scanned. The light enters the means 14.

  The scanning unit 14 is a beam scanner that scans the sample 18 with the irradiation light by changing the position of the converging point (spot) of the irradiation light within the sample 18 (including the surface of the sample 18). Specifically, the scanning unit 14 includes a light deflecting element that changes (deflects) the direction of the irradiation light emitted from the scanning unit 14 and changes the incident angle of the irradiation light to the second optical system 17. . The scanning unit 14 has, for example, a galvano scanner and a resonant scanner. For simplicity, two mirrors in the scanning means 14 are represented by one mirror in FIG. 1 in FIG. If a resonant scanner (scan frequency of 8 kHz) and a galvano scanner (scan frequency of 15 Hz) are used, an image of 500 lines can be acquired at about 30 frames per second.

  The irradiation light deflected by the scanning unit 14 enters the second optical system 17 via the lenses 15 and 16. By arranging the lenses 15 and 16 so that the scanning unit 14 and the entrance pupil of the second optical system 17 are conjugate, even if the scanning unit 14 deflects the light beam, the light amount does not change due to light shielding. The light is focused on the sample 18.

  The second optical system 17 is an irradiation optical system that guides irradiation light to the sample 16. The second optical system 17 has an objective lens, and may further have a mirror or the like. The magnification of the optical system by the lenses 15 and 16 is selected so that the size of the entrance pupil of the second optical system 17 and the size of the incident light beam become equal. Then, the light spot size condensed by the second optical system 17 is minimized, and the spatial resolution for detecting a signal due to the nonlinear optical phenomenon is improved. In addition, since the signal due to the nonlinear optical phenomenon increases as the intensity of the light spot increases, the signal-to-noise ratio also improves. The second optical system 17 preferably has an objective lens having a large numerical aperture (NA) from the viewpoint of the spatial resolution for detecting a signal and the signal-to-noise ratio.

  The sample 18 is sandwiched between a cover glass and a slide glass (not shown). Due to the deflection of the light beam by the scanning means 14, two-dimensional scanning is performed by the light spot focused on the sample 18. The optical microscope 100 can acquire a two-dimensional image of a sample using a signal based on a nonlinear optical phenomenon. Since the obtained signal is generated in a condensed light spot, a three-dimensional image can be obtained by moving the sample 18 in the optical axis direction by a stage (not shown).

  The third optical system 19 collects light transmitted through the sample 18 and subjected to intensity modulation by a nonlinear optical phenomenon. In order to receive as much light from the sample 18 as possible, it has an NA equal to or greater than the NA of the second optical system 17. The light emitted from the third optical system 19 is applied to the light receiving surface of the light receiving element of the second detecting means 22 via the filter 20 and the lens 21.

  The filter 20 is a dielectric multilayer film designed to transmit only light in a wavelength band necessary for obtaining a nonlinear optical phenomenon. The second detection means 22 has a light receiving element having sensitivity to the wavelength transmitted through the filter 20. The conversion circuit 23 is an electric circuit that outputs a current signal generated by the second detection unit 22 as a voltage. The acquisition circuit 24 is a bandpass filter or a lock-in amplifier, and is an electric circuit for acquiring a signal due to a nonlinear optical phenomenon with a large signal-to-noise ratio.

  The processing unit 25 acquires the data of the two-dimensional image of the material 18 using the control signal of the scanning unit 14 and using the output of the acquisition circuit 24 (the signal due to the nonlinear optical phenomenon). The two-dimensional image data acquired by the processing unit 25 is displayed on a display unit (not shown). When the processing means 25 is a computer, it may be displayed on a computer screen. Further, the processing unit 25 can also acquire data of a three-dimensional image by using a signal acquired by moving the sample 18 in the optical axis direction on a stage (not shown). Further, the processing means 25 can also display a signal obtained by changing at least one wavelength of the optical pulse train output from each of the first light source 1 and the second light source 2 as a spectrum.

  As described above, according to the synchronization device 150 and the optical pulse train synchronization method of the present embodiment, the synchronization of two optical pulses can be performed more stably than before. Further, according to the optical microscope 100 having the synchronizing device 150, the synchronization of the two optical pulse trains is performed more stably than before, and the fourth optical pulse train 104 from the wavelength changing element 3 is separated for synchronization. It can be used as irradiation light without performing.

(Second embodiment)
In the present embodiment, an example will be described in which the optical microscope 100 of the first embodiment is used as a stimulated Raman scattering (SRS) microscope.

  SRS is one of nonlinear optical phenomena, and is induced by simultaneously irradiating a sample with two light pulses having different wavelengths (two-wavelength light pulse). When the difference between the optical frequencies of the two-wavelength light pulses matches the molecular frequency of the sample, SRS occurs at the focal point of those light pulses. Among the two-wavelength light pulses transmitted through the sample, the intensity of the short-wavelength light pulse (pump light) decreases (stimulated Raman loss), and the intensity of the long-wavelength light pulse (Stokes light) increases (stimulated Raman gain). This intensity change is proportional to the product of the respective intensities of the two-wavelength light pulses.

  In the present embodiment, two optical pulse trains having different wavelengths are synchronized to continuously generate the SRS. That is, the wavelength λ3 of the fourth optical pulse train 104 irradiating the sample 18 is different from the wavelength λ2 of the sixth optical pulse train 106, and the fourth optical pulse train 104 and the sixth optical pulse train 106 irradiate the sample 18 simultaneously. Sync to be.

  In order to increase the intensity change due to the SRS while reducing the degradation of the resolution of the Raman spectrum described later, a short pulse laser having a light pulse time width of 1 to 10 picoseconds is used as the first light source 1 and the second light source 2. It is desirable to use. In order to detect the SRS signal efficiently, the pulse periods of the first light pulse train 101 and the second light pulse train 102 generated by each of the first light source 1 and the second light source 2 are set to 1: 2. Here, as the first light source 1, an erbium-doped fiber laser that outputs an optical pulse train having a center wavelength of 1580 nanometers and a pulse period of 12.5 nanoseconds is used. As the second light source 2, an ytterbium-doped fiber laser that outputs an optical pulse train having a center wavelength of 1030 nm and a pulse period of 25 nanoseconds is used.

  The wavelength changing means 3 includes a PPLN for generating a second harmonic (wavelength 790 nm) as the fourth optical pulse train 104, a lens for condensing incident light on the PPLN to efficiently generate the second harmonic, and a PPLN. And a lens that converts light transmitted through the lens into parallel light.

  When the timings at which the fourth light pulse train 104 and the sixth light pulse train 106 are irradiated onto the sample 18 coincide (FIG. 5), and the light beams are condensed on the same condensing point (spot) on the sample 18, the sample The light intensity of the pulse transmitted through 18 changes by SRS. In FIG. 5A, the intensities of the pulses 41, 43, and 45 decrease, and the intensities of the pulses 42 and 44 do not change. The filter 20 transmits 790 nm light and blocks 1030 nm light. The light receiving element of the second detection means 22 is a silicon photodiode having sensitivity to light of 790 nanometers and having a cutoff frequency of 40 MHz (corresponding to a pulse period of 25 nanoseconds) or more.

  Since the intensity difference between adjacent pulses to be detected by the second detection means 22 is very small, an electric circuit (mixer circuit or lock-in amplifier) which extracts the amplitude of the 40 MHz component from the output of the conversion circuit 23 and outputs it as a voltage as the acquisition circuit 24 ).

  The difference between the intensities of the adjacent pulses detected by the second detection means 22 corresponds to the SRS signal, and reflects information on molecules contained in spots where the fourth light pulse train 104 and the sixth light pulse train 106 are collected. You. For example, when the resonance frequency of the molecular vibration included in the spot and the difference (c / λ1−c / λ2) between the optical frequency of the first light source 1 and the optical frequency of the second light source 2 match, the SRS signal increases. Become. Note that c is 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 frequency of the first light source 1 and the optical frequency of the second light source 2. In order to obtain a Raman spectrum, the wavelength of at least one of the first light source 1 and the second light source 2 is changed. From the obtained Raman spectrum, what kind of molecules are contained in the sample 18 can be estimated. The SRS microscope can acquire a spectrum equivalent to a microscope using spontaneous Raman scattering. Since the scattering efficiency of SRS is much higher than the scattering efficiency of spontaneous Raman scattering, an SRS microscope can acquire a Raman spectrum in a shorter time than a microscope using spontaneous Raman scattering.

  As described above, according to the synchronization device 150 and the optical pulse train synchronization method of the present embodiment, the synchronization of two optical pulses can be performed more stably than before. When the optical microscope 100 having the synchronizing device 150 is used as an SRS microscope, the synchronization of two light pulses is performed more stably than before, and the fourth light pulse train 104 from the wavelength changing element 3 is synchronized. Can be used as irradiation light without separation.

  In an SRS microscope, an SRS signal changes when a timing difference between two optical pulse trains changes. On the other hand, according to the synchronization device 150 of the present embodiment, a change in the SRS signal due to the timing difference between the two optical pulse trains can be further reduced. Further, according to the present invention, the synchronization of the optical pulse train can be further continued, and the SRS signal can be obtained more continuously.

  As described above, the preferred embodiments of the present invention have been described, but the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the gist.

  For example, the present invention can be used for an optical microscope using a nonlinear optical phenomenon other than SRS, such as CARS (coherent anti-Stokes Raman scattering). Further, the present invention is not limited to the CRS measurement, and an optical device and a microscope for measuring multiphoton absorption and multiphoton fluorescence may be configured.

  In each of the above-described embodiments, the case where the two-dimensional scanning of the focal point is performed on the sample 108 using the galvanomirror has been described. However, the configuration of the scanning unit is not limited to this, and two-dimensional scanning may be performed by combining scanning with a one-axis galvanometer mirror and driving of the stage 109 that supports a sample that is driven in a direction orthogonal to the scanning direction. . Further, only the stage 109 may be driven in a two-dimensional plane. If imaging is not necessary, Raman spectrum measurement at one point on the sample 108 may be performed without scanning the light deflection element 105.

  Further, in the measurement using the microscope 100, one repetition frequency of the irradiation light may be configured to be twice the other repetition frequency, or the repetition frequency may be configured to be the same. When the same repetition frequency is used, the second harmonic of Er fiber laser light or Yb fiber laser light is modulated at an arbitrary repetition frequency using a method of modulating any one of intensity, wavelength, and polarization state by an external input signal. I do. Then, it is preferable to lock-in the unmodulated light at the modulation frequency.

  In each of the above-described embodiments, the configuration for measuring the light transmitted through the sample 18 has been described. However, in order to measure the thick sample 18, the measurement for the scattered light backscattered by the sample 18 may be performed. Good. In that case, the second optical system 17 also functions as the third optical system 19. That is, the scattered light backscattered by the sample 18 is collected by the second optical system 17 and then separated from the optical path of the irradiation light by the branching unit arranged at a predetermined position. Is detected.

  In addition, an endoscope, which is one form of the Raman scattering observation apparatus, may be configured by forming a probe for part or all of each component for guiding light to the sample 18.

DESCRIPTION OF SYMBOLS 1 1st light source 2 2nd light source 3 wavelength change means 4 2nd separation means 5 1st separation means 9 detection means 11 period change means

Claims (7)

A first light source that outputs a first optical pulse train having a first wavelength;
A second light source that outputs a second optical pulse train having a second wavelength;
Wavelength changing means for changing the wavelength of the first optical pulse train to a third wavelength;
First separating means for separating the light pulse train from the wavelength changing means into a third light pulse train having the first wavelength and a fourth light pulse train having the third wavelength;
Second separating means for separating the second optical pulse train into a fifth optical pulse train having the second wavelength and a sixth optical pulse train having the second wavelength;
Detecting means for detecting a pre-SL and the third optical pulse train before Symbol fifth optical pulse train,
An optical pulse train synchronizer, comprising: a period changing unit that changes a pulse period of the first optical pulse train or a pulse period of the second optical pulse train based on a detection result of the detecting unit. The detection means outputs a signal corresponding to a difference between a timing at which the third light pulse train is incident on the detection means and a timing at which the fifth light pulse train is incident on the detection means. Item 2. An optical pulse train synchronizer according to Item 1. The detecting means includes a light receiving element that generates a current by two-photon absorption on a light receiving surface that receives the third light pulse train and the fifth light pulse train, and outputs a signal corresponding to the current. The optical pulse train synchronizer according to claim 1 or 2, wherein: The light receiving element is characterized by having at least the light receiving sensitivity to the wavelength corresponding to the sum energy of the second photon energy corresponding to the first photon energy and the second wavelength corresponding to said first wavelength The optical pulse train synchronizer according to claim 3, wherein The period changing unit changes the pulse period of the first light pulse train by changing the resonator length of the first light source, or changes the resonator length of the second light source. The optical pulse train synchronizer according to any one of claims 1 to 4, wherein a pulse cycle of the second optical pulse train is changed. An optical pulse train synchronizer according to any one of claims 1 to 5,
An irradiation optical system that irradiates the sample with the fourth light pulse train from the pulse train synchronizer and the sixth light pulse train from the second separation unit;
An optical microscope, comprising: detection means for detecting light from the sample. An optical pulse train synchronization method for synchronizing a first optical pulse train having a first wavelength and a second optical pulse train having a second wavelength,
Changing a part of the wavelength of the first optical pulse train to a third wavelength;
A first separating step of separating the optical pulse train from the wavelength changing step into a third optical pulse train having the first wavelength and a fourth optical pulse train having a third wavelength;
A second separation step of separating the second light pulse train into a fifth light pulse train having the second wavelength and a sixth light pulse train having the second wavelength;
A detection step of detecting a pre-SL and the third optical pulse train before Symbol fifth optical pulse train,
An optical pulse train synchronization method, comprising: changing a pulse period of the first optical pulse train or a pulse period of the second optical pulse train based on a detection result in the detecting step.

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