Nonlinear optical measurement system, endoscope system having the same, light source device, and method for nonlinear optical measurement

Abstract

To realize a method for a nonlinear optical measurement by a simple optical system.SOLUTION: A nonlinear Raman spectroscopic system 100 includes: a pulse laser light source 11; a QPM device 12; an objective lens 2; and an optical detector 4. The pulse laser light source 11 emits a pulse laser beam as an excitation light which induces a parametric lower conversion (PDC) in a QPM device 12 without using seed light. The objective lens 2 collects signal light and idler light generated coaxially by the PDC of the excitation light in the QPM device 12 and radiates the collected light to a sample. The optical detector 4 detects light generated by a nonlinear optical phenomenon of a sample.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a nonlinear optical measurement system, an endoscope system including the same, a light source device, and a nonlinear optical measurement method. [Background technology]

[0002] In recent years, various nonlinear optical measurement methods have been put to practical use. For example, nonlinear Raman spectroscopy has been applied to various technical fields, and has shown remarkable development in the measurement of biological samples in fields such as life science and medical measurement. JP 2009-222531 A (Patent Document 1), JP 2011-191496 A (Patent Document 2), JP 2022-153601 A (Patent Document 3), and JP 2022-29970 A (Patent Document 4) disclose a technique for analyzing a sample by detecting nonlinear Raman scattering light (more specifically, coherent anti-Stokes Raman scattering (CARS: Coherent Anti-Stokes Raman Scattering) light) emitted from a sample when the sample is irradiated with multiple pulsed lights. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-222531 A [Patent Document 2] JP 2011-191496 A [Patent Document 3] JP 2022-153601 A [Patent Document 4] Patent Publication No. 2022-29970 [Patent Document 5] International Publication No. 2007 / 084112 [Patent Document 6] JP 2017-203646 A [Patent Document 7] International Publication No. 2020 / 195674 [Non-patent literature]

[0004] [Non-Patent Document 1] Yujiro Eto, Enhanced two-photon excited fluorescence by ultrafast intensity fluctuations from an optical parametric generator, Applied Physics Express 14, 012011 (2021). [Non-Patent Document 2] Yujiro Eto, Locally controlled two-photon excited fluorescence by correlated ultrafast intensity fluctuations, Applied Physics Express 14, 022003 (2021). [Non-Patent Document 3] Yujiro Eto and Mutsuo Nuriya, Enhanced two-photon excited fluorescence from green fluorescent proteins by ultrafast fluctuations in intense light pulse, Vol. 1, No. 12 / 15 Dec 2022 / Optics Continuum 2539. Summary of the Invention [Problem to be solved by the invention]

[0005] In nonlinear optical measurement methods, more specifically, nonlinear Raman, sum frequency generation (SFG), second harmonic generation (SHG), multiphoton excitation, and the like, complex optical systems are often employed. For example, in nonlinear Raman spectroscopy, two high-intensity laser beams are irradiated onto a sample to induce a nonlinear response in the sample. A complex optical system may be required to prepare and guide such two high-intensity laser beams to the sample. In order to advance social implementation, there is always a demand to realize a nonlinear optical measurement method using as simple an optical system as possible. This disclosure has been made to solve the above problems, and one of the purposes of this disclosure is to realize a nonlinear optical measurement method using a simple optical system. [Means for solving the problem]

[0006] A nonlinear optical measurement system according to an aspect of the present disclosure includes a nonlinear optical crystal, a light source, a lens, and a photodetector. The light source emits pulsed laser light as pump light that induces parametric down-conversion in the nonlinear optical crystal without using seed light. The lens focuses signal light and idler light that are generated coaxially with each other by parametric down-conversion of the pump light in the nonlinear optical crystal, and irradiates the light onto a sample. The photodetector detects light generated by a nonlinear optical phenomenon in the sample.

[0007] An endoscope system according to another aspect of the present disclosure includes a nonlinear optical measurement system and a plurality of optical fibers through which at least one of signal light, idler light, and light generated by the nonlinear optical phenomenon propagates.

[0008] A light source device according to still another aspect of the present disclosure includes a nonlinear optical crystal, and a light source that emits pulsed laser light as pump light that induces parametric down-conversion in the nonlinear optical crystal without using seed light. Signal light and idler light generated by the parametric down-conversion of the pump light in the nonlinear optical crystal are irradiated onto a sample to generate light due to a nonlinear optical phenomenon of the sample.

[0009] A nonlinear optical measurement method according to yet another aspect of the present disclosure includes a step of preparing light for generating light due to a nonlinear optical phenomenon of a sample. The preparing step includes a step of inputting a pulsed laser light as excitation light into a nonlinear optical crystal without using a seed light, a step of generating signal light and idler light coaxially with each other by parametric down-conversion of the excitation light in the nonlinear optical crystal, and a step of focusing the signal light and idler light propagating coaxially from the nonlinear optical crystal using a lens. The nonlinear Raman spectroscopy method further includes a step of irradiating the signal light and idler light focused by the lens onto the sample, and a step of detecting light generated by a nonlinear optical phenomenon of the sample. Effect of the Invention

[0010] According to the present disclosure, a nonlinear optical measurement method can be realized using a simple optical system. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram for explaining a schematic of CARS spectroscopy. [Diagram 2] 1 is a diagram illustrating an example of an overall configuration of a nonlinear Raman spectroscopy system according to a first embodiment of the present disclosure. [Diagram 3] 2A to 2C are diagrams for explaining the configuration and function of a light source device in more detail. [Figure 4] FIG. 1 is a diagram showing an example of the relationship between the wavelength of light emitted from a QPM device and the crystal temperature. [Diagram 5] 1 is a diagram showing an example of the relationship between the frequency difference between the signal light and the idler light and the crystal temperature T. FIG. [Figure 6] 4 is a flowchart showing a first example of a processing procedure of the nonlinear Raman spectroscopy method according to the first embodiment. [Figure 7] FIG. 13 is a diagram showing an example of the measurement results of the relationship between the intensity of excitation light and the intensity of output light from a QPM device. [Figure 8]FIG. 4 is a diagram showing an example of a result of nonlinear Raman spectroscopy measurement in the first embodiment. [Figure 9] FIG. 1 is a diagram illustrating a first example of an overall configuration of a nonlinear Raman imaging system according to a first embodiment of the present disclosure. [Figure 10] FIG. 2 is a diagram illustrating a second example of the overall configuration of the nonlinear Raman imaging system according to the first embodiment of the present disclosure. [Figure 11] 10 is a flowchart showing a second example of the processing procedure of the nonlinear Raman imaging method according to the first embodiment. [Figure 12] FIG. 11 is a diagram illustrating an example of an overall configuration of a nonlinear Raman spectroscopy system according to a second embodiment of the present disclosure. [Figure 13] 4A and 4B are diagrams for explaining pulse waveforms of signal light and idler light. [Figure 14] 11A and 11B are diagrams illustrating an example of a result of enhancing the intensity of anti-Stokes light. [Figure 15] FIG. 2 is a diagram for explaining CARS spectroscopy in the present embodiment. [Figure 16] 10 is a diagram for explaining the frequency of anti-Stokes light when there is no frequency correlation between signal light and idler light. FIG. [Figure 17] 10 is a diagram for explaining the frequency of anti-Stokes light when a frequency correlation exists between signal light and idler light. FIG. [Figure 18] FIG. 13 is a diagram showing an example of an SFG spectrum when the delay time is set to 0. [Figure 19] FIG. 13 is a conceptual diagram for explaining the difference in SFG spectra. [Figure 20] 10 is a flowchart showing an example of a processing procedure of a nonlinear Raman spectroscopy method according to the second embodiment. [Figure 21] 11 is a flowchart showing an example of a processing procedure of a nonlinear Raman imaging method according to the second embodiment. [Figure 22] FIG. 11 is a diagram illustrating an example of an overall configuration of an endoscope system according to a third embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, the present embodiment will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference characters and their description will not be repeated.

[0013] <Terminology explanation> In the present disclosure and its embodiments, the light generated by the nonlinear optical phenomenon of the sample means light generated by the nonlinear Raman scattering, sum frequency generation (SFG), second harmonic generation (SHG) or multiphoton excitation of the sample. The term "nonlinear optical measurement system" means a measurement system capable of performing at least one of nonlinear Raman spectroscopy, nonlinear Raman imaging, SFG spectroscopy, SFG imaging, SHG spectroscopy, SHG imaging, multiphoton excitation spectroscopy and multiphoton excitation imaging. Similarly, the term "nonlinear optical measurement method" means a measurement method using at least one of the above techniques.

[0014] In the present disclosure and its embodiments, "femtosecond order" refers to a time range of 1 femtosecond or more and less than 1 picosecond. "Subpicosecond order" refers to a time range of 100 femtoseconds or more and less than 1 picosecond. "Picosecond order" refers to a time range of 1 picosecond or more and less than 1 nanosecond. "Subnanosecond order" refers to a time range of 100 picoseconds or more and less than 1 nanosecond. "Nanosecond order" refers to a time range of 1 nanosecond or more and less than 1 microsecond. "Three-digit watt order" or "sub-kilowatt order" refers to a power of 100W (=0.1kW) or more and less than 1kW. "Four-digit watt order" refers to a power of 1kW or more and less than 10kW. "Five-digit watt order" refers to a power of 10kW or more and less than 100kW.

[0015] In the present disclosure and its embodiments, "PDC" is distinguished from "Spontaneous Parametric Down-Conversion" (SPDC). The average number of photons in each of the signal light and the idler light generated from the excitation light may be used as a theoretical criterion for distinguishing between SPDC and PDC. In the present disclosure and its embodiments, SPDC is a photon pair generation process in which the number of photons per mode is one or less. In contrast, PDC is a photon pair generation process in which the number of photons per mode is more than one. Note that "per mode" may mean per pulse or the same frequency.

[0016] As a more practical criterion for distinguishing between SPDC and PDC, the intensity of the excitation light may be used. The peak intensity of laser light (which may be continuous wave laser light, but is preferably pulsed laser light) for generating PDC is significantly higher than that of laser light for generating SPDC. Specifically, the peak intensity of laser light (typically continuous wave laser light) for generating SPDC is generally several tens of mW to several W. In contrast, the peak intensity for generating PDC required in the present disclosure and its embodiments is preferably on the order of three watts (sub-kilowatt order) to five watts (i.e., 100 W to 100 kW), and more preferably on the order of three watts to four watts (i.e., 100 W to 10 kW). According to this criterion, a photon pair generation process by irradiation with laser light having a peak intensity of at least 100 W is classified as PDC. When the peak intensity is 100 W, the peak intensity is 10 per mode. 12 Photon pairs on the order of 100 are generated.

[0017] In general, in optical parametric amplification, a weak light is amplified by interacting with a strong pump light in a nonlinear optical crystal. In this disclosure and its embodiments, the term "seed light" refers to the weak light that is amplified by the pump light.

[0018] The "frequency" of the laser light can be converted into angular frequency (angular vibration frequency), wavelength, or wave number according to the convention in the technical field of nonlinear optics, and can be appropriately readjusted. The "frequency difference" between two laser lights can also be appropriately readjusted to angular frequency difference (angular vibration frequency difference), wavelength difference, or wave number difference.

[0019] [Embodiment 1] <CARS Spectroscopy> In the following embodiments, a configuration in which the nonlinear optical measurement system according to the present disclosure is a nonlinear Raman spectroscopy system or a nonlinear Raman imaging system, and CARS spectroscopy is realized by the system will be described as an example.

[0020] FIG. 1 is a diagram for schematically explaining CARS spectroscopy. Population of vibration levels is formed by the first pump light and the Stokes light. Then, anti-Stokes light is generated using the second pump light. Hereinafter, when the first pump light and the second pump light are not distinguished, or when the first pump light and the second pump light are comprehensively represented, the notations "first" or "second" are omitted.

[0021] In the present embodiment, signal light and idler light are generated by parametric down-conversion (PDC) described later. Then, the signal light is used as pump light, and the idler light is used as Stokes light. In the description, if the signal light and the pump light are mixed or the idler light and the Stokes light are mixed, confusion may occur. To avoid such confusion, the notations are unified, and both the signal light and the pump light are described as "signal light", and both the idler light and the Stokes light are described as "idler light". In the figure, the signal light is indicated by L S and the idler light is indicated by L I . However, those skilled in the art can appropriately read the signal light as pump light or read the idler light as Stokes light. The anti-Stokes light corresponds to the "light generated by the nonlinear optical phenomenon of the sample" according to the present disclosure. The anti-Stokes light is indicated by L AS .

[0022] However, the nonlinear Raman spectroscopy to which the nonlinear Raman spectroscopy system according to the present disclosure can be applied is not limited to CARS spectroscopy. Specifically, the nonlinear Raman spectroscopy system according to the present disclosure can also be applied to stimulated Raman scattering (SRS) spectroscopy, stimulated Raman gain spectroscopy (SRGS), inverse Raman spectroscopy (IRS), coherent Stokes Raman spectroscopy (CSRS), etc. The same applies to the nonlinear Raman imaging system.

[0023] <Spectroscopic system configuration> 2 is a diagram illustrating an example of an overall configuration of the nonlinear Raman spectroscopy system according to the first embodiment of the present disclosure. The nonlinear Raman spectroscopy system 100 includes a light source device 1, an objective lens 2, a sample holder 3, a photodetector 4, and a controller 10.

[0024] The light source device 1 generates light to be supplied to a downstream objective lens 2 in accordance with a control command from a controller 10. The light source device 1 includes a pulsed laser light source 11, a quasi-phase-matched (QPM) device 12, and a heater 13.

[0025] The pulsed laser light source 11 emits excitation light for exciting a QPM device 12 (nonlinear optical crystal). E The QPM device 12 generates signal light and idler light coaxially by parametric down-conversion (PDC) of pump light from the pulsed laser source 11. No seed light is used in addition to the pump light. The heater 13 heats the QPM device 12. The pulsed laser source 11, the QPM device 12 and the heater 13 will be described in more detail with reference to FIG. 3.

[0026] The objective lens 2 is disposed between the light source device 1 (QPM device 12) and the sample holder 3. The objective lens 2 focuses the signal light and the idler light coaxially generated by the QPM device 12. The objective lens 2 will also be described in detail with reference to FIG. 3. The objective lens 2 corresponds to the "lens" according to the present disclosure. The "lens" may be a reflective lens.

[0027] The phrase "coaxially generating or emitting two laser beams" or "coaxially propagating" two laser beams includes, but is not limited to, the case where the two laser beams are exactly the same in the strict sense, and also includes the case where the two laser beams are approximately the same. The two laser beams only need to be coincident to such an extent that they can be treated as one laser beam in the optical system at the subsequent stage (in other words, to such an extent that separate optical systems do not need to be assembled), and such a degree can be understood by a person skilled in the art in consideration of the description of this specification and common technical knowledge, etc.

[0028] The sample holder 3 is disposed between the objective lens 2 and the photodetector 4. The sample holder 3 holds a sample (denoted by SP). The type of the sample is not particularly limited, but may be, for example, a biological sample such as a cell or tissue.

[0029] When the signal light and the idler light are irradiated onto the sample, anti-Stokes light is generated. In this example, the signal light and the idler light are removed by optical elements (not shown) (such as a short-pass filter or a dichroic mirror) arranged between the sample holder 3 and the photodetector 4, and only the anti-Stokes light reaches the photodetector 4.

[0030] The photodetector 4 detects the anti-Stokes light and outputs a signal indicating the detection intensity to the controller 10. More specifically, in the first embodiment, the photodetector 4 includes an image sensor 41 such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Metal Oxide Semiconductor) sensor. However, the photodetector 4 may include a single-pixel type light receiving element, or may include a spectrometer and a single-pixel type or multi-pixel type light receiving element.

[0031] The controller 10 includes a processor 101, a memory 102, an input device 103, and an output device 104. The processor 101 is an arithmetic processing device such as a central processing unit (CPU) or a micro processing unit (MPU). The memory 102 is a storage device including a read only memory (ROM) and a random access memory (RAM). The memory 102 stores a system program including an operating system (OS), a control program including computer-readable code, and various parameters for performing nonlinear Raman spectroscopy of a sample. The processor 101 reads out the system program, the control program, and the parameters, expands them in the memory 102, and executes them to realize various arithmetic processing for performing nonlinear Raman spectroscopy. The input device 103 is a keyboard, a mouse, a touch panel, an operation button, etc., and accepts operations by a user (a measurer such as a researcher, developer, or student). The output device 104 is a monitor, a printer, etc., and outputs the progress and results of the arithmetic processing performed by the processor 101.

[0032] ≪Light source device≫ 3 is a diagram for explaining the configuration and functions of the light source device 1 in more detail. As shown in FIG. 3, the excitation light from the pulsed laser light source 11 is collected by a lens 141 and enters the QPM device 12. The signal light and idler light from the QPM device 12 are converted into parallel light by a lens 142 and emitted toward the downstream objective lens 2. If the frequency of the excitation light is represented as 2ω, the peak frequency of the signal light is ω+Δω, and the peak frequency of the idler light is ω-Δω. The frequency difference (peak frequency difference) between the signal light and the idler light is 2Δω.

[0033] The pulsed laser light source 11 is a nanosecond laser light source or a subnanosecond laser light source. The pulsed laser light source 11 is preferably a Q-switched laser light source in which the pulse width of the excitation light is on the order of nanoseconds. As a specific example, a Cobolt Tor series pulsed laser device (wavelength 532 nm) may be used. A device that emits a fundamental wave with a wavelength of 1064 nm may be employed, and a second harmonic may be generated from the fundamental wave using a nonlinear optical crystal (e.g., lithium triborate (LBO) crystal).

[0034] Generally, a mode-locked laser is used to generate ultrashort pulsed laser light on the order of femtoseconds to picoseconds. A mode-locked laser is not preferable as the pulsed laser light source 11. The reason for this will be described later (see FIG. 13).

[0035] The QPM device 12 includes a nonlinear optical crystal having a periodically poled structure. In this example, the nonlinear optical crystal has a constant poled period (pitch). Instead of the nonlinear optical crystal of this example, the QPM device 12 may include a nonlinear optical crystal that satisfies a Type I or Type II phase matching condition. The material of the nonlinear optical crystal is, for example, lithium niobate (LiNbO3). Such a QPM device is called PPLN (periodically poled lithium niobate). The length of the PPLN in the optical path direction is, for example, 1 cm to 2.5 cm. When the QPM device 12 is PPLN and the wavelength of the pump light is 532 nm, both the signal light and the idler light are near-infrared light. In this example, the linewidth of each of the signal light and the idler light is about 5 nm.

[0036] There is no particular limitation on the material of the nonlinear optical crystal included in the QPM device 12. The material of the nonlinear optical crystal may be, for example, silver gallium sulfide (AgGaS2), lithium tantalate (LiTaO3), potassium titanyl phosphate (KTiOP4), potassium niobate (KNbO3), or the like.

[0037] In this embodiment, parametric down-conversion (PDC) is used as a process for generating signal light and idler light from the pump light in the QPM device 12. If the intensity of the pump light is too low, only spontaneous parametric down-conversion (SPDC) is induced, and in this case, almost no nonlinear Raman scattering light is generated in the sample. Conversely, if the intensity of the pump light is too high, the PPLN may be damaged (thermal damage, dielectric breakdown, etc.). The intensity of the pump light is determined in consideration of the beam size of the pump light, the size of the PPLN (particularly the length in the optical path direction), etc., so as to avoid damage to the PPLN while generating sufficient nonlinear Raman scattering light. More specifically, the lower limit of the peak intensity of the pump light is on the order of three watts (lower limit value = 100 W), and the upper limit of the peak intensity of the pump light is on the order of five watts (upper limit value = 100 kW).

[0038] As described above, the objective lens 2 collects the signal light and the idler light coaxially generated by the QPM device 12. This increases the light intensity density in the downstream sample.

[0039] In addition, the objective lens 2 has a function related to the phase matching condition. In order to generate anti-Stokes light for nonlinear Raman scattering, it is necessary that the phase matching condition is satisfied even in the sample. In this case, the phase matching condition is determined by the wave vector of the signal light, the wave vector of the idler light, and the wave vector of the anti-Stokes light. In other words, by setting the wave vector of the signal light and the wave vector of the idler light in a certain direction, the anti-Stokes light is generated in a direction that satisfies the phase matching condition. Since the direction that satisfies the phase matching condition can be calculated from the refractive index dispersion of the sample, it is also possible to irradiate the signal light and the idler light from the calculated direction to the sample. However, in this embodiment, instead of such an adjustment of the optical system, the signal light and the idler light focused by the objective lens 2 are irradiated to the sample. Then, the signal light and the idler light each having various wave vector components are generated by the focusing by the objective lens 2, and the wave vector components that satisfy the phase matching condition in the sample are included therein. Therefore, it is not necessary to adjust the optical system to a direction that satisfies the phase matching condition.

[0040] Heater 13 is, for example, an oven configured to heat QPM device 12. More specifically, heater 13 heats QPM device 12 in accordance with a control command from controller 10 so that the temperature of QPM device 12 becomes a target temperature. Hereinafter, the temperature of QPM device 12 is referred to as "crystal temperature T." By changing the target temperature, crystal temperature T can be variably set to various temperatures. Changing crystal temperature T effectively changes the pitch of the nonlinear optical crystal, and therefore changes the wavelengths of the signal light and the idler light that satisfy the phase matching condition of QPM device 12.

[0041] FIG. 4 is a diagram showing an example of the relationship between the wavelength of light emitted from QPM device 12 and crystal temperature T. The horizontal axis represents crystal temperature T. The vertical axis represents the wavelength of the signal light or the idler light. The wavelength of the signal light is a measured value. The wavelength of the idler light is a calculated value based on the measured value of the wavelength of the signal light. As shown in FIG. 4, by changing the crystal temperature T, the combination of the wavelengths of the signal light and the idler light that satisfies the phase matching condition of QPM device 12 changes, and therefore the frequency difference 2Δω between the signal light and the idler light can be changed. In this way, heater 13 is an example of a "frequency difference changer" according to the present disclosure.

[0042] FIG. 5 is a diagram showing an example of the relationship between the frequency difference between the signal light and the idler light and the crystal temperature T. The horizontal axis represents the crystal temperature T. The vertical axis represents the frequency difference 2Δω between the signal light and the idler light. A relationship such as that shown in FIG. 5 is prepared in advance and stored in the memory 102 of the controller 10. This makes it possible to calculate the frequency difference 2Δω from the crystal temperature T, and to calculate the crystal temperature T required to obtain a desired frequency difference 2Δω.

[0043] The QPM device 12 shown in FIG. 3 is an element with a constant pitch of the nonlinear optical crystal. However, the QPM device 12 may be a fan-out type element with a pitch that changes in a fan shape. By changing the incident position of the excitation light on the fan-out type element, the wavelength that satisfies the phase matching condition changes, and the frequency difference 2Δω is changed. Although not shown, the light source device 1 may include a device (typically an electric precision stage) configured to be able to move the fan-out type element, instead of the heater 13. In that case, the device corresponds to the "frequency difference changer" according to the present disclosure. Alternatively, an optical system may be constructed such that the incident position of the excitation light on a fixed fan-out type element can be changed. In that case, the optical system corresponds to the "frequency difference changer" according to the present disclosure. The QPM device 12 may be a chirp type element with a pitch that changes along the optical path direction.

[0044] <Spectroscopic processing flow> 6 is a flowchart showing a first example of a processing procedure of the nonlinear Raman spectroscopy method according to the first embodiment. The processing shown in this flowchart is executed, for example, when the input device 103 such as an operation button accepts a user operation. Each step is basically realized by software processing by the controller 10 (processor 101), but may also be realized by hardware processing by an electronic circuit arranged in the controller 10. The same applies to other flowcharts described later. Hereinafter, steps are abbreviated as "S".

[0045] The process shown in the flowchart of Fig. 6 is related to the measurement for the purpose of spectroscopic analysis (typically identification) of a sample. The following processes of S101 to S104 are processes for setting the measurement conditions in advance. The order of these processes can be changed as appropriate.

[0046] In S101, the controller 10 sets a scanning range of the frequency difference 2Δω between the signal light and the idler light. The controller 10 acquires the scanning range of the frequency difference 2Δω by accepting a user operation on the input device 103, for example.

[0047] In S102, the controller 10 determines the scanning range of the crystal temperature T from the scanning range of the frequency difference 2Δω set in S101. The controller 10 can perform the conversion process based on, for example, a map or table (see FIG. 5) showing the correspondence relationship between the crystal temperature T and the frequency difference 2Δω.

[0048] In S103, the controller 10 sets the peak intensity of the pump light in an ultra-high gain region (hereinafter also referred to as a "PDC region") where PDC can occur.

[0049] 7 is a diagram showing an example of the measurement results of the relationship between the intensity of the pump light and the intensity of the light emitted from QPM device 12. The horizontal axis represents the time-averaged intensity of the pump light incident on QPM device 12. The vertical axis represents the time-averaged intensity of the light (signal light + idler light) emitted from QPM device 12.

[0050] FIG. 7 shows the measurement results when the diameter of the beam waist of the excitation light is about 160 μm and the length of the QPM device 12 (PPLN) is 1 cm. In this example, the time-average intensity of the light emitted from the QPM device 12 was about 6 mW at maximum. This value is equivalent to about 300 W in peak intensity of pulsed light. Based on such measurement results, it is possible to prepare in advance the relationship between the intensity of the excitation light and the peak intensity required to cause PDC. Therefore, the controller 10 can set the intensity of the excitation light (particularly the peak intensity) to a value within the PDC region that can achieve the peak intensity required for PDC.

[0051] It should be noted that simply irradiating the QPM device 12 with excitation light does not necessarily induce a PDC with the intensity required for nonlinear Raman spectroscopy. Conventionally, it has been technically common knowledge that "laser light generated only by PDC without using seed light does not have sufficient intensity required for nonlinear Raman spectroscopy." The measurement results shown in Figure 7 were obtained by optimizing the parameters of the PDC (beam waist of the excitation light, length of the QPM device 12, etc.) so as to sufficiently improve the intensity of the light emitted from the QPM device 12 while avoiding damage to the nonlinear optical crystal.

[0052] 6, in S104, the controller 10 adjusts the irradiation positions of the signal light and the idler light on the sample by controlling a mirror (not shown) provided in the optical system. The light irradiation positions may be determined in advance.

[0053] In S105, the controller 10 waits until it receives a measurement start operation to the input device 103 (NO in S105). When the operation is received (YES in S105), the controller 10 advances the process to S106. The subsequent processes of S106 to S111 are processes for scanning the frequency difference 2Δω.

[0054] In S106, the controller 10 sets the crystal temperature T to a temperature within the scanning range of the crystal temperature set in S102. The crystal temperature T set in the nth measurement (n is a natural number) is denoted as T(n).

[0055] In S107, the controller 10 controls the light source device 1 to start outputting the excitation light. Then, the controller 10 acquires a signal indicating the detection intensity of the anti-Stokes light from the photodetector 4 (S108). After that, the controller 10 controls the light source device 1 to stop outputting the excitation light (S109).

[0056] In S110, the controller 10 determines whether the current crystal temperature T(n) has reached the end value of the scanning range of the crystal temperature set in S102. If the crystal temperature T(n) has not reached the end value (NO in S110), the controller 10 sets the crystal temperature to the next value T(n+1) (S111). Then, the controller 10 returns the process to S106. As a result, the crystal temperature T is scanned within the scanning range.

[0057] On the other hand, when the crystal temperature T(n) reaches the end value (YES in S110), the controller 10 advances the process to S112 and outputs the measurement result of the intensity of the anti-Stokes light to the output device 104. For example, the controller 10 controls the output device 104 to display the intensity of the anti-Stokes light as a spectrum. In this manner, in this embodiment, a CARS spectrum is acquired. A series of processes ends with the process of S112. Note that the timing of displaying the CARS spectrum is not limited to after the end of the light irradiation. The CARS spectrum during measurement may be displayed during the light irradiation in S106 to S111. In this example, the CARS spectrum (Raman spectrum) corresponds to the "Raman data" according to the present disclosure.

[0058] FIG. 8 is a diagram showing an example of a nonlinear Raman spectroscopy result in the first embodiment. The horizontal axis represents the wave number. The vertical axis represents the normalized intensity of the anti-Stokes light. As samples, polystyrene (see black circle) and glass (see white circle) were used.-1 Looking at the vicinity, it can be seen that polystyrene has a Raman transition, whereas glass does not.

[0059] <Imaging system configuration> 9 is a diagram showing a first example of an overall configuration of a nonlinear Raman imaging system according to the first embodiment of the present disclosure. The nonlinear Raman imaging system 100A is a microscope system to which nonlinear Raman imaging is applied. The nonlinear Raman imaging system 100A differs from the nonlinear Raman spectroscopy system 100 (see FIG. 2) in that the nonlinear Raman imaging system 100A further includes an optical scanner 51 between the light source device 1 (QPM device 12) and the objective lens 2.

[0060] The optical scanner 51 is, for example, a galvano scanner, and is controlled according to a control command from the controller 10. By controlling the optical scanner 51, it is possible to irradiate the signal light and the idler light to any position on the sample via the objective lens 2. It is also possible to scan the signal light and the idler light on the sample.

[0061] 10 is a diagram showing a second example of the overall configuration of the nonlinear Raman imaging system according to the first embodiment of the present disclosure. The nonlinear Raman imaging system 100B is a microscope system to which nonlinear Raman imaging is applied, similar to the nonlinear Raman imaging system 100A. The nonlinear Raman imaging system 100B differs from the nonlinear Raman spectroscopy system 100 (see FIG. 2) in that it further includes a movable stage 52 attached to the sample holder 3.

[0062] The movable stage 52 is, for example, an electric precision stage, and is displaced according to a control command from the controller 10. It is also possible to scan the signal light and the idler light on the sample by controlling the movable stage 52. Although not shown, the optical scanner 51 and the movable stage 52 may be combined.

[0063] <Imaging process flow> 11 is a flowchart showing a second example of the processing procedure of the nonlinear Raman imaging method according to the first embodiment. The processing shown in this flowchart relates to a measurement aimed at real-time imaging (moving image capture) of a sample. It is assumed that the scanning range of the optical scanner 51 or the movable stage 52 for acquiring one image (each frame of the moving image) is determined in advance. In the following, a configuration including the optical scanner 51 (see FIG. 9) will be described as an example, but the optical scanner 51 can be read as the movable stage 52.

[0064] In S121, the controller 10 sets the frequency difference 2Δω between the signal light and the idler light. Typically, the frequency difference Δ2ω is set to a value at which the intensity of the anti-Stokes light is the highest. Then, the controller 10 determines the crystal temperature T corresponding to the set frequency difference 2Δω.

[0065] In S122, the controller 10 sets the peak intensity of the excitation light to an appropriate value within the PDC region (a value within a range of three-digit watt order to five-digit watt order).

[0066] In S123, the controller 10 waits until it receives a measurement start operation on the input device 103 (NO in S123). When the operation is received (YES in S123), the controller 10 sets the crystallization temperature T to the temperature set in S121 (S124).

[0067] In S125, the controller 10 controls the light source device 1 to start outputting the excitation light.

[0068] In S126, the mirror angle θ of the optical scanner 51 is set to an angle θ(k) (k is a natural number) within the scanning range. The mirror angle θ is initially set to an initial value. The controller 10 acquires a signal indicating the detection intensity of the anti-Stokes light from the photodetector 4 (S127). This allows the signal intensity of the anti-Stokes light at the pixel (light irradiation position) corresponding to the mirror angle θ(k) to be acquired. It is estimated that the exposure time per pixel may be about 1 millisecond.

[0069] Until scanning at the mirror angle θ for acquiring one image is completed (NO in S128), the controller 10 sequentially acquires signals indicating the detection intensity of the anti-Stokes light while slightly changing the mirror angle θ (S129). When the mirror angle θ reaches an end value and scanning by the optical scanner 51 is completed (YES in S128), the controller 10 outputs an image indicating the distribution of the detection intensity of the anti-Stokes light in a plurality of pixels to the output device 104 (S130).

[0070] If the measurement end operation to the input device 103 has not been received (NO in S131), the controller 10 returns the process to S126. At this time, the mirror angle θ is reset to the initial value. By repeatedly executing the processes of S126 to S130, a video of the sample is taken, and real-time imaging is realized. Still images may be taken instead of video shooting. If the time change of the sample is gradual, time lapse imaging may be performed. In this example, these videos, still images, or time lapse images correspond to the "Raman data" (Raman image) according to the present disclosure. If the measurement end operation is received (YES in S131), the controller 10 controls the light source device 1 to stop outputting the excitation light (S132). This ends a series of processes.

[0071] <Comparison with prior art> In various nonlinear Raman spectroscopy methods, it is required to irradiate a sample with two high-intensity laser beams in order to induce a nonlinear response (to generate nonlinear Raman scattering) in the sample. To meet this requirement, the following two methods have been mainly used in the past. The first method uses two laser light source devices with different emission wavelengths (see, for example, Patent Document 1). The second method uses a two-color light source device (a light source capable of simultaneously emitting two pulsed laser beams with different wavelengths) whose intensity is increased by a seed beam. However, these methods require mechanisms such as branching, wavelength conversion, and multiplexing of laser beams. Therefore, it may be difficult to construct an optical system, or it may be difficult to stably maintain the constructed optical system.

[0072] In this embodiment, by adjusting (preferably optimizing) parameters related to the PDC in the light source device 1 (peak intensity of the pumping light, beam waist of the pumping light, length of the nonlinear optical crystal, etc.), a signal light and an idler light having high instantaneous intensity and propagating coaxially are generated using the PDC in the ultra-high gain region of the QPM device 12. Then, the signal light and the idler light having high instantaneous intensity are coaxially emitted from the objective lens 2. As a result, from only one pumping light, a signal light and an idler light having an intensity capable of acquiring nonlinear Raman information of the sample and propagating coaxially can be obtained. As a result, as demonstrated by the present inventors, CARS spectroscopy can be realized by only the PDC without using a seed light (see FIG. 8). According to this method, mechanisms such as branching, wavelength conversion, and multiplexing of the laser light can be omitted. Therefore, according to the first embodiment, the optical system for realizing nonlinear Raman spectroscopy becomes simple. Therefore, it is possible to reduce the size and cost of the system.

[0073] International Publication No. 2007 / 084112 (Patent Document 5) describes the generation of quantum entangled photon pairs by SPDC, and the use of one of the photons as a signal light (see FIG. 3). It also describes the use of quantum entanglement to improve the detection efficiency of Raman signals (see pages 22-23). ​​However, SPDC is a process in a weak region where only signal light and idler light with an average photon number of 1 or less per mode are generated. Therefore, even if the detection efficiency is improved, the Raman signal cannot be detected because it is buried in noise, or it takes an extremely long time to detect the Raman signal. In contrast, in this embodiment, PDC is induced in an ultra-high gain region, preferably by using a Q-switched laser light source as the light source device 1. As a result, the number of photons of the signal light and the idler light increases by several digits to several tens of digits, so that the anti-Stokes light can be effectively enhanced.

[0074] The CARS measurement device disclosed in JP 2017-203646 A (Patent Document 6) includes an optical parametric oscillator (see Figs. 1 to 3). Therefore, the system configuration becomes complicated due to the addition of an electric circuit for stabilizing the optical parametric amplification, and the system size increases by the amount of the resonator. Naturally, the material cost of the optical parametric oscillator is also high. In contrast, according to the present embodiment, since it is not necessary to provide an optical parametric oscillator, a simple, compact, and inexpensive system can be constructed.

[0075] The difficulty in combining nonlinear Raman spectroscopy with PDC can be explained as follows. Nonlinear Raman spectroscopy such as CARS spectroscopy is mainly used in the fields of life science, medical measurement, physical chemistry, etc. On the other hand, PDC is researched in the fields of laser science, nonlinear optics, quantum optics, etc. In other words, nonlinear Raman spectroscopy and PDC are in different technical fields. Therefore, for those skilled in the art who started out with nonlinear Raman spectroscopy, it is not easy to conceive of applying PDC to nonlinear Raman spectroscopy. Conversely, for those skilled in the art who started out with PDC, it is not easy to conceive of nonlinear Raman spectroscopy as an application of PDC.

[0076] Here, an example of acquiring a CARS spectrum (Raman spectrum) has been described, but this is not limiting, and an SFG spectrum, SHG spectrum, or multiphoton excitation spectrum can also be acquired in the same way. When the signal light and idler light are irradiated onto a sample, the SFG spectrum, SHG spectrum, or multiphoton excitation spectrum resulting from the nonlinear optical phenomenon of the sample is detected.

[0077] [Embodiment 2] In the first embodiment, a configuration in which the optical paths of the signal light and the idler light are fixed during measurement has been described. In the second embodiment, a configuration in which the optical path length difference between the signal light and the idler light is adjustable will be described.

[0078] <System configuration> 12 is a diagram showing an example of the overall configuration of a nonlinear Raman spectroscopy system according to embodiment 2 of the present disclosure. Nonlinear Raman spectroscopy system 200 differs from nonlinear Raman spectroscopy system 100 according to embodiment 1 (see FIG. 2) in that nonlinear Raman spectroscopy system 200 further includes dichroic mirrors 61 and 62, a movable mirror 71, a driving device 72, and mirrors 81 to 83, and in that nonlinear Raman spectroscopy system 200 includes a photodetector 4A instead of photodetector 4. Other configurations of nonlinear Raman spectroscopy system 200 (light source device 1, objective lens 2, and sample holder 3) are equivalent to the corresponding configurations of nonlinear Raman spectroscopy system 100.

[0079] The dichroic mirror 61 is disposed after the light source device 1 (QPM device 12). The dichroic mirror 61 splits the signal light and the idler light from the QPM device 12. In this example, the dichroic mirror 61 reflects the signal light while transmitting the idler light. The signal light travels toward the dichroic mirror 62 via the mirror 81. The idler light travels toward the movable mirror 71.

[0080] The movable mirror 71 is configured to be movable along the optical path (propagation direction) of the idler light. Specifically, the movable mirror 71 is provided with a driving device 72. The driving device 72 is an electric actuator, for example, a motor driving device (servo motor, stepping motor, etc.) that is mechanically displaced according to a control command from the controller 10. The driving device 72 may be a piezoelectric element that is displaced according to an applied voltage from the controller 10. By changing the position of the movable mirror 71 using the driving device 72, the optical path length difference between the signal light and the idler light can be set. After being reflected by the movable mirror 71, the idler light travels toward the dichroic mirror 62 via the mirror 82. The movable mirror 71 and the driving device 72 correspond to the "delay time changing unit" according to the present disclosure.

[0081] The dichroic mirror 62 is disposed between the mirrors 81 and 83 in the optical path of the signal light, and between the mirrors 82 and 83 in the optical path of the idler light. In this example, the dichroic mirror 62, like the dichroic mirror 61, reflects the signal light while transmitting the idler light. In this way, the dichroic mirror 62 combines the signal light and the idler light branched by the dichroic mirror 61. The combined signal light and idler light reach the objective lens 2 via the mirror 83.

[0082] Photodetector 4A differs from photodetector 4 in that it includes a spectroscope 42 in addition to image sensor 41. Spectroscope 42 separates anti-Stokes light from a sample. By providing spectroscope 42, it is possible to improve the wavenumber resolution of anti-Stokes light by about one to two orders of magnitude, compared to a configuration in which anti-Stokes light is simply detected by image sensor 41 (see FIG. 2).

[0083] In the system configuration of Fig. 12, the optical path length of the signal light is fixed and the optical path length of the idler light is variable. However, conversely, the optical path length of the idler light may be fixed and the optical path length of the signal light may be variable. Alternatively, both the optical path length of the signal light and the optical path length of the idler light may be variable.

[0084] <Intensity fluctuation> FIG. 13 is a diagram for explaining the pulse waveforms of the signal light and the idler light. Each of the signal light and the idler light exhibits intensity variations due to the superposition of femtosecond-order spikes on a base pulse waveform (pulse envelope) of subnanosecond to nanosecond order. This intensity variation is caused by quantum fluctuations and is called "intensity fluctuations." In SPDC, signal photons and idler photons are generated simultaneously. This simultaneity is not lost even in PDC in the ultra-high gain region. In PDC, spikes of the signal light and spikes of the idler light are generated simultaneously. In other words, the intensity fluctuations (spike patterns) are similar between the signal light and the idler light (see Non-Patent Documents 1 to 3 for details on intensity fluctuations).

[0085] Intensity fluctuations also occur when PDC is induced using a mode-locked laser as pump light. However, because ultrashort pulse laser light spreads in nonlinear optical crystals, it is difficult to excite nonlinear optical crystals in the ultrahigh gain region to induce PDC. In addition, when a spike on the femtosecond order is superimposed on a pulse waveform on the femtosecond order, the base intensity of the signal light and the idler light itself fluctuates. In other words, the pulse energy changes for each pulse. Therefore, ultrashort pulse laser light is not suitable for applications to spectroscopic analysis or imaging as in this embodiment. Therefore, it is not preferable to use a mode-locked laser.

[0086] Usually, when a sample is irradiated, one of the signal light and the idler light is delayed in time with respect to the other due to refractive index dispersion caused by the optical element in the upstream stage. In the example shown in FIG. 13, the idler light is delayed with respect to the signal light. The length of this delay is referred to as "delay time τ". By adjusting the optical path length difference between the signal light and the idler light using the movable mirror 71, the delay time τ can be set to 0. In the second embodiment, the delay time τ is controlled in order to improve the measurement accuracy of Raman data. Two actions that contribute to improving the measurement accuracy will be described below.

[0087] <1. Intensity enhancement of anti-Stokes light> In general, the intensity of the anti-Stokes light in CARS spectroscopy is proportional to the product of the square of the instantaneous intensity of the signal light and the instantaneous intensity of the idler light. In such a nonlinear response, when compared under conditions where the total amount of light (sum of light intensities over time) irradiated to the nonlinear optical crystal is equal, the intensity of the anti-Stokes light obtained from the sample that is the subject of nonlinear Raman scattering is higher when light with a high instantaneous intensity is irradiated than when light with a constant intensity is continuously irradiated over time.

[0088] In this embodiment, the delay time τ is set to 0. The signal light and the idler light have similar intensity fluctuation patterns. Therefore, by setting τ=0, the signal light and the idler light are irradiated onto the sample with their intensity fluctuations synchronized. As a result, when the signal light with high instantaneous intensity is irradiated onto the sample, the irradiation intensity of the idler light also becomes high, and as a result, the intensity of the anti-Stokes light is effectively enhanced.

[0089] Fig. 14 shows an example of the result of enhancing the intensity of the anti-Stokes light. The horizontal axis represents the delay time τ of the idler light relative to the signal light. The vertical axis represents the normalized intensity of the anti-Stokes light. In the example shown in Fig. 14, when the delay time τ=0, the intensity of the anti-Stokes light is enhanced by about 1.2 times compared to when τ≠0. Theoretically, it is estimated that an enhancement of more than 2 times is possible if the dispersion of the signal light and the idler light is appropriately compensated for.

[0090] As shown in FIG. 14, while the intensity of the anti-Stokes light peaks at a delay time τ=0, the peak has a time width of about subpicoseconds (corresponding to an optical path length difference of ±several tens of μm). In other words, even if the delay time τ deviates slightly from 0, the intensity of the anti-Stokes light is greater than 1 (baseline). This shows that the delay time τ does not need to be strictly 0 to increase the intensity of the anti-Stokes light. Therefore, in this disclosure, "the delay time is zero" means that the delay time τ is within a predetermined range including 0, and "the delay time is non-zero" means that the delay time τ is outside that range.

[0091] 2. Improved time resolution and wavenumber resolution The second action aims to achieve both a time resolution at the level of an ultrashort pulse wave and a wavenumber resolution at the level of a continuous wave. First, the reason for the improvement in time resolution will be explained. Here, we consider time-resolved nonlinear Raman spectroscopy, in which the Raman spectrum changes transiently. In this case, the time resolution corresponds to how finely the time interval at which the Raman spectrum, which changes from moment to moment, can be divided into.

[0092] In conventional time-resolved nonlinear Raman spectroscopy, two (or more) pulsed laser beams are irradiated onto a sample. The time at which the Raman spectrum is measured is determined by the time difference between the two pulsed laser beams. In addition, the time width (pulse width) of each pulsed laser beam determines the time resolution. For example, to achieve a time resolution on the order of subpicoseconds, it is necessary to use an ultrashort pulsed laser beam with a time width on the order of subpicoseconds.

[0093] In contrast, in this embodiment, attention is focused on the multiple spikes included in each of the signal light and the idler light described in FIG. 13, and instead of two ultrashort pulsed laser lights, the multiple spikes are made to play the role of ultrashort pulsed laser lights. This makes it possible to realize a measurement similar to a time-resolved measurement using ultrashort pulsed laser lights. To explain in more detail, in this embodiment, the amount equivalent to the time width of the ultrashort pulsed laser light is the time width of the spike (spike width). The time width of the spike fluctuation generated in the signal light is about the line width (the reciprocal of the frequency width) of the signal light. The same is true for the idler light. When the line widths of the signal light and the idler light are both on the nanometer order, the time width of the spike is on the subpicosecond order. Therefore, according to this embodiment, even though each of the signal light and the idler light has a pulse envelope on the subnanosecond to nanosecond order, a time resolution on the subpicosecond order can be realized.

[0094] Next, an improvement in wavenumber resolution will be described. Wavenumber resolution is an index showing how closely two peaks can be separated from each other even if the two peaks are adjacent to each other on a Raman spectrum.

[0095] When high wavenumber resolution is required, conventional nonlinear Raman spectroscopy uses laser light with a sufficiently narrow linewidth. In contrast, in this embodiment, the frequency correlation between signal light and idler light (hereinafter also abbreviated as "frequency correlation") is utilized. To facilitate understanding, the following three items will be summarized and explained in order. (1) Why does using frequency correlation improve wavenumber resolution? (2) Does frequency correlation exist in the PDC region? (3) What kind of processing is required to utilize frequency correlation in the PDC domain?

[0096] (1) Reasons for improved wavenumber resolution 15 is a diagram for explaining the CARS spectroscopy in this embodiment. S and the frequency of the idler light is ωI and the frequency of the anti-Stokes light is expressed as ω AS There is a 2ω difference between the signal light and the idler light. S -ω I =ω AS In addition, if the frequency that resonates with the molecular vibration of the sample is expressed as Ω (constant), then ω S -ω I =Ω.

[0097] In this embodiment, since the signal light and the idler light are generated by the PDC, there is a frequency correlation between the signal light and the idler light. The frequency of the anti-Stokes light in this case will be described in comparison with the case where such a frequency correlation does not exist.

[0098] Fig. 16 is a diagram for explaining the frequency of the anti-Astokes light when there is no frequency correlation between the signal light and the idler light. Fig. 17 is a diagram for explaining the frequency of the anti-Astokes light when there is a frequency correlation between the signal light and the idler light. Fig. 16(A) and Fig. 17(A) show ω S and ω I 16(B) and 17(B) show schematic diagrams of Raman spectra (CARS spectra).

[0099] Referring to FIG. 16(A), when there is no frequency correlation between the signal light and the idler light, ω S and ω I Pair with (ω S ,ω I ) is expressed by a circular frequency domain. The frequency ambiguity of this pair (the spread of frequencies due to the uncertainty relationship between time and energy) is expressed by shading. If we assume that the frequency width of the resonant frequency Ω of the sample is sufficiently narrow, the above two equations (2ω S -ω I =ω AS and ω S -ω I=Ω) and the circular frequency domain simultaneously overlap, anti-Stokes light is generated due to molecular resonances in the sample. The anti-Stokes light has a frequency band of δω AS (ω S ,ω I ) is expressed in a circular region, the frequency band δω AS This shows that when there is no frequency correlation, the wavenumber resolution is low (see FIG. 16(B)).

[0100] In contrast, when there is a frequency correlation between the signal light and the idler light, as shown in FIG. S ,ω I The frequency ambiguity of the frequency band δω is represented by a linear region that has a certain width but is sufficiently narrower than the circular region. In this case, the frequency width δω is smaller than when there is no frequency correlation. AS is significantly narrower. This means that the wavenumber resolution of the anti-Stokes light is significantly improved by utilizing the frequency correlation (see Fig. 17(B)).

[0101] (2) Existence of frequency correlation in the PDC region The existence of frequency correlation between signal light and idler light even in the PDC region is not self-evident and requires experimental proof. The inventors have demonstrated this by using SFG. More specifically, they generated an SFG beam of signal light and idler light and measured the SFG spectrum of a sample (specifically, a BBO crystal).

[0102] 18 shows an example of an SFG spectrum when the delay time τ is set to 0. The upper figure shows the SFG spectrum, and the lower figure shows the spectrum of the excitation light for comparison.

[0103] From Fig. 18, it can be confirmed that when the delay time τ is set to 0, a steep peak is generated, and the wavelength and linewidth of the peak are almost equal to those of the pump light. Hereinafter, this spectral component is referred to as the "peak component". While the frequency width of the pump light is 0.1 nm or less, the signal light and idler light have linewidths (e.g., about 5 nm) that are several tens of times thicker than the pump light. Nevertheless, the SFG beam of the signal light and idler light contains a peak component that is almost equal to the wavelength and linewidth of the pump light. This peak component is generated by satisfying the correlation shown in Fig. 17(A). This proves the existence of frequency correlation in the PDC region.

[0104] On the other hand, the SFG spectrum at delay time τ=0 contains not only a peak component but also a broad spectral component centered on the wavelength of the peak component. Hereinafter, the latter spectral component will be referred to as the "broad component." The broad component is produced by the interaction between uncorrelated light beams.

[0105] (3) Processing for utilizing frequency correlation The processing required to utilize frequency correlation in the PDC domain will first be explained using SFG as an example.

[0106] Fig. 19 is a conceptual diagram for explaining the difference in SFG spectra. Fig. 19 shows five SFG spectra obtained by sequentially scanning the delay time τ at a predetermined scanning step Δτ. In this example, the peak component is extracted by subtracting the SFG spectrum at τ=2Δτ (including only broad components) from the SFG spectrum at τ=0 (including both broad components and peak components).

[0107] Like the SFG spectrum, the CARS spectrum when the delay time τ is set to 0 also includes both spectral components (corresponding to peak components in SFG) generated by pairs of signal light modes and idler light modes with frequency correlation, and spectral components (corresponding to broad components in SFG) generated by pairs with no frequency correlation. Therefore, in this embodiment, the difference between the CARS spectrum when the delay time τ=0 and the spectrum when the delay time τ is non-zero is calculated. In this way, the spectral components generated by pairs with no frequency correlation are removed, and the spectral components generated by pairs with frequency correlation remain. This increases the purity of the spectral components with high wavenumber resolution in the CARS spectrum, thereby improving the wavenumber resolution of the CARS spectrum.

[0108] In this way, instead of the conventional method using a narrow linewidth laser beam, the delay time τ is set to 0, thereby creating a situation in which it is possible to specify which mode of signal light and which mode of idler light interact with each other, thereby improving the wavenumber resolution of the CARS spectrum. According to the second embodiment, it is possible to achieve both a time resolution at the level of an ultrashort pulsed light and a frequency resolution at the level of a continuous wave.

[0109] <Processing flow> Fig. 20 is a flowchart showing an example of a processing procedure of the nonlinear Raman spectroscopy method according to the second embodiment. Due to space limitations, Fig. 20 omits the processing for presetting required for measurement (processing of S101 to S104 in Fig. 6) and the processing for accepting a measurement start operation (processing of S105). The presetting processing may further include a processing for setting the scanning range of the delay time τ (scanning range and scanning step of the movable mirror 71).

[0110] In S201, the controller 10 controls the heater 13 to set the crystal temperature to T(n), where T(n) is the temperature in the nth temperature operation within a preset scanning range of the crystal temperature.

[0111] In S202, the controller 10 controls the driver 72 of the moving mirror 71 to set the delay time to τ(j) (j is a natural number). τ(j) is the time in the j-th time operation within the scanning range of the preset delay time.

[0112] In S203, the controller 10 controls the light source device 1 to start outputting excitation light having a peak intensity within the PDC region. Then, the controller 10 acquires a signal indicating the detected intensity of the anti-Stokes light from the photodetector 4 (S204). After that, the controller 10 controls the light source device 1 to stop outputting the excitation light (S205).

[0113] In S206, the controller 10 judges whether the current value τ(j) of the delay time has reached the end value of the scanning range of the delay time τ. If the delay time τ(j) has not reached the end value (NO in S206), the controller 10 sets the delay time to the next value τ(j+1) (S207). Then, the controller 10 returns the process to S202. As a result, the delay time τ is scanned within the scanning range. If the delay time τ(j) has reached the end value (YES in S206), the controller 10 advances the process to S208.

[0114] In S208, the controller 10 judges whether the current value T(n) of the crystal temperature has reached the end value of the scanning range of the crystal temperature T. If the crystal temperature T(n) has not reached the end value (NO in S208), the controller 10 sets the crystal temperature to the next value T(n+1) (S209). Then, the controller 10 returns the process to S201. As a result, the crystal temperature T is scanned within the scanning range. When the crystal temperature T(n) reaches the end value (YES in S208), the controller 10 advances the process to S210. At this point, a CARS spectrum (the relationship between the crystal temperature T and the detected intensity of the anti-Stokes light) is acquired for each delay time τ.

[0115] In S210, the controller 10 identifies a CARS spectrum at a delay time τ=0 from among a plurality of CARS spectra acquired at various delay times τ. If the spectrum has a peak, the peak can be detected by calculating a differential waveform obtained by differentiating the spectrum with respect to wavenumber. For example, the controller 10 calculates differential waveforms of all CARS spectra, and identifies the one in which a peak is detected as the CARS spectrum at a delay time τ=0.

[0116] In S211, the controller 10 calculates a difference spectrum by taking the difference between the CARS spectrum at delay time τ=0 and the CARS spectrum at delay time τ≠0 (for example, the CARS spectrum at a delay time τ that differs from τ=0 by a specified time). As described above, the difference spectrum achieves wavenumber resolution at a continuous wave level. The difference spectrum is an example of the "difference data" according to the present disclosure.

[0117] In S212, the controller 10 controls the output device 104 to display the difference spectrum. In addition to the difference spectrum, a CARS spectrum at a delay time τ=0 and a CARS spectrum at a delay time τ≠0 may be displayed. CARS spectra other than the difference spectrum may be displayed at any timing during the measurement (for example, at the timing when the intensity of the anti-Stokes light is obtained from the photodetector 4).

[0118] Here, an example has been described in which it is unknown before starting measurement which of the multiple CARS spectra at various delay times τ corresponds to the delay time τ=0. However, when the measurement conditions for delay time τ=0 (the position to which movable mirror 71 should be adjusted so that τ=0) are known in advance, such as when repeating similar measurements, scanning of the delay time τ may be omitted. Specifically, it is sufficient to perform measurements under only two conditions: a measurement condition for delay time τ=0 and a measurement condition for delay time τ≠0. This allows the measurement time to be shortened.

[0119] As in the second embodiment, when CARS spectroscopy is performed with high wavenumber resolution using a spectrometer, a dispersion curve can be measured as a CARS spectrum, instead of a curve including a single peak. The dispersion curve is a curve including multiple vibrational resonance wavenumbers due to interference between a component due to the CARS process and a component due to a non-resonant background process included in the anti-Stokes light, and has a shape similar to a differentiated Gaussian distribution. Even in this case, the multiple vibrational resonance wavenumbers appearing in the dispersion curve coincide with the resonance center wavenumber of the component due to the CARS process. Therefore, it is sufficient to specify the measurement condition under which multiple vibrational resonance wavenumbers appear in the dispersion curve as the condition of delay time τ=0. Alternatively, it is also possible to reconstruct the CARS spectrum from the dispersion curve (extract only the vibrational resonance component due to the CARS process) using a known method such as the maximum entropy model (MEM). The measurement condition under which the vibrational resonance wavenumbers are confirmed in the reconstructed CARS spectrum may be specified as the condition of τ=0.

[0120] 20 is explained based on the system configuration (see FIG. 2) and the flow chart (see FIG. 6) for the purpose of spectroscopic analysis such as sample identification. However, the system configuration (see FIG. 9 and FIG. 10) and the flow chart (see FIG. 11) for imaging of a sample may be used as the basis for the second embodiment.

[0121] 21 is a flowchart showing an example of a processing procedure of a nonlinear Raman imaging method according to embodiment 2. In S221, the controller 10 controls the heater 13 to set the crystal temperature T to a temperature corresponding to a desired frequency difference.

[0122] In S222, the controller 10 sets the delay time τ=0 by controlling the driving device 72 of the movable mirror 71. Then, the controller 10 executes the imaging process of S223 to S228 under this condition. The imaging process is substantially the same as the process of S125 to S129 in Fig. 11, and therefore the description will not be repeated. As a result, an image showing the distribution of the detected intensity of the anti-Stokes light at the delay time τ=0 is obtained.

[0123] In S229, the controller 10 sets the delay time τ to a non-zero value by controlling the driver 72 of the movable mirror 71. Then, the controller 10 executes an imaging process similar to the processes in S223 to S228 (S230). As a result, an image showing the distribution of the detected intensity of the anti-Stokes light when the delay time τ is non-zero is obtained.

[0124] In S231, the controller 10 creates a difference image between an image when the delay time τ=0 and an image when the delay time τ is non-zero, and displays the difference image on the output device 104. The difference image is another example of the "difference data" according to the present disclosure.

[0125] Here, an example of creating a still image as a difference image has been described. However, by using an optical scanner 51 with a sufficiently fast scanning speed of the mirror angle θ, it is also possible to observe the time change of the sample to be measured in real time (i.e., to capture a video).

[0126] According to the second embodiment, as in the first embodiment, a small-sized and inexpensive nonlinear Raman spectroscopy system can be realized by using a simple optical system. In addition, according to the second embodiment, a time resolution on the level of an ultrashort pulsed light and a wavenumber resolution on the level of a continuous wave can be simultaneously achieved by controlling the delay time τ between the signal light and the idler light. According to estimates by the present inventors, a time resolution of several tens to 100 femtoseconds, a spatial resolution of about 0.5 μm, and a wavenumber resolution of 10 cm can be achieved by optimizing various parameters. -1 and a wavenumber resolution of 600 to 3600 cm -1 It is possible to simultaneously achieve the above wavenumber bands.

[0127] Here, an example of performing CARS spectrum (Raman spectrum) or CARS imaging (Raman imaging) has been described. However, the present invention is not limited to this, and SFG spectrum, SFG imaging, SHG spectrum, SHG imaging, multiphoton excitation spectrum, or multiphoton excitation imaging can also be performed in the same manner.

[0128] [Embodiment 3] In the third embodiment, a configuration in which the nonlinear Raman imaging system according to the first or second embodiment is applied to an endoscope system will be described.

[0129] 22 is a diagram showing an example of an overall configuration of an endoscope system according to embodiment 3 of the present disclosure. Endoscope system 300 differs from nonlinear Raman imaging system 100A (see FIG. 9) according to embodiment 1 in that it does not include a sample holder 3 and includes a dichroic mirror 63 and optical fibers 91 to 94.

[0130] The optical fiber 91 optically couples the light source device 1 (QPM device 12) and the optical scanner 51. The optical fiber 92 optically couples the optical scanner 51 and the dichroic mirror 63. The optical fiber 93 optically couples the dichroic mirror 63 and the objective lens 2. The optical fiber 94 optically couples the dichroic mirror 63 and the photodetector 4 (image sensor 41). Each of the optical fibers 91 to 94 may be provided separately for each propagating light. All of the optical fibers 91 to 94 are not essential, and it is sufficient that at least one of the optical fibers 91 to 94 is provided.

[0131] The dichroic mirror 63 transmits the signal light and idler light from the light source device 1. The objective lens 2 collects the signal light and idler light from the dichroic mirror 63 and irradiates the sample (such as biological tissue) with the light, and also captures the anti-Stokes light from the sample. The dichroic mirror 63 reflects the anti-Stokes light captured by the objective lens 2. As a result, the anti-Stokes light is detected by the image sensor 41.

[0132] Other configurations of the endoscope system 300 are equivalent to the corresponding configurations of the nonlinear Raman imaging system 100A according to the first embodiment. In addition, the processing procedure in the endoscope system 300 is also equivalent to the processing procedure in the first embodiment (see FIG. 11). The endoscope system according to the third embodiment may be based on the nonlinear Raman imaging system 200 according to the second embodiment (see FIG. 12). The processing procedure in that case is also equivalent to the processing procedure in the second embodiment (see FIG. 20 or FIG. 21). Therefore, detailed description will not be repeated. Here, nonlinear Raman imaging has been described as an example, but the present invention is not limited to this, and SFG imaging, SHG imaging, or multiphoton excitation imaging can also be applied to the endoscope system.

[0133] According to the third embodiment, a small and inexpensive endoscope system can be realized by using a simple optical system. Since it is not practical to modify or fine-tune the optical system in the medical field where the endoscope system is used, the high stability of the optical system is a great advantage.

[0134] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is defined by the claims, not by the description of the embodiments described above, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0135] 1 light source device, 11 pulsed laser light source, 12 QPM device, 13 heater, 141, 142 lens, 2 objective lens, 3 sample holder, 4 photodetector, 41 image sensor, 42 spectrometer, 51 optical scanner, 52 movable stage, 61-63 dichroic mirror, 71 moving mirror, 72 drive device, 81-83 mirror, 91-93 optical fiber, 10 controller, 101 processor, 102 memory, 103 input device, 104 output device, 100, 200 nonlinear Raman spectroscopy system, 100A, 100B nonlinear Raman imaging system, 300 endoscope system.

Claims

1. A nonlinear optical crystal; a light source that emits a pulsed laser beam as pumping light that induces parametric down-conversion in the nonlinear optical crystal without using a seed beam; a lens for collecting signal light and idler light, which are generated coaxially with each other by parametric down-conversion of the excitation light in the nonlinear optical crystal, and irradiating the collected light onto a sample; and a photodetector for detecting light generated by a nonlinear optical phenomenon in the sample.

2. the pulse width of the excitation light is on the order of subnanoseconds or nanoseconds; The nonlinear optical measurement system of claim 1 , wherein the peak intensity of the excitation light is in the range of three to five orders of watts.

3. a frequency difference changing unit that changes a frequency difference between the signal light and the idler light; A processor for performing calculations on the detection results by the photodetector, The processor calculates data of the sample based on a detection result by the photodetector when the frequency difference is scanned within a predetermined range or set to a predetermined value; the data being at least one of Raman data, Sum Frequency Generation (SFG) data, Second Harmonic Generation (SHG) data, and multi-photon excitation data; the Raman data being a Raman spectrum or a Raman image; The SFG data is a SFG spectrum or a SFG image; the SHG data is an SHG spectrum or an SHG image; The nonlinear optical measurement system according to claim 1 or 2, wherein the multiphoton excitation data is a multiphoton excitation spectrum or a multiphoton excitation image.

4. a delay time changing unit configured to change a delay time of one of the signal light and the idler light with respect to the other, The nonlinear optical measurement system of claim 3 , wherein the processor calculates the data based on the detection result by the photodetector under a condition in which the delay time is set to zero and intensity fluctuations between the signal light and the idler light are synchronized.

5. the data is the Raman data, The light generated by the nonlinear optical phenomenon of the sample is coherent anti-Stokes Raman scattering (CARS) light, The processor, Calculating first CARS data based on a detection result by the photodetector under a condition where the delay time is zero, and second CARS data based on a detection result by the photodetector under a condition where the delay time is non-zero; The nonlinear optical measurement system according to claim 4 , further comprising: calculating difference data obtained by subtracting the first CARS data from the second CARS data.

6. The nonlinear optical measurement system according to claim 3 ; and one or more optical fibers through which at least one of the signal light, the idler light, and the light generated by the nonlinear optical phenomenon propagates.

7. A nonlinear optical crystal; a light source that emits a pulsed laser beam as pumping light that induces parametric down-conversion in the nonlinear optical crystal without using a seed beam; A light source device, wherein the signal light and the idler light generated by the parametric down-conversion of the pump light in the nonlinear optical crystal are irradiated onto a sample to generate light due to a nonlinear optical phenomenon of the sample.

8. The method includes the step of preparing light for generating light due to a nonlinear optical phenomenon of a sample, The preparing step includes: A step of irradiating a pulsed laser beam as an excitation beam into a nonlinear optical crystal without using a seed beam; generating signal light and idler light coaxially with each other by parametric down-conversion of the pump light in the nonlinear optical crystal; and collecting the signal light and the idler light coaxially propagating from the nonlinear optical crystal using a lens. irradiating the sample with the signal light and the idler light collected by the lens; and detecting light generated by a nonlinear optical phenomenon in the sample.

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