WO2023243052A1 - 光源 - Google Patents

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Publication number
WO2023243052A1
WO2023243052A1 PCT/JP2022/024174 JP2022024174W WO2023243052A1 WO 2023243052 A1 WO2023243052 A1 WO 2023243052A1 JP 2022024174 W JP2022024174 W JP 2022024174W WO 2023243052 A1 WO2023243052 A1 WO 2023243052A1
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Prior art keywords
pulse train
optical pulse
light
light source
laser
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English (en)
French (fr)
Japanese (ja)
Inventor
茂雄 石橋
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NTT Inc
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Nippon Telegraph and Telephone Corp
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Priority to PCT/JP2022/024174 priority Critical patent/WO2023243052A1/ja
Priority to US18/874,053 priority patent/US20250379412A1/en
Priority to JP2024528037A priority patent/JP7755209B2/ja
Publication of WO2023243052A1 publication Critical patent/WO2023243052A1/ja
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/30—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects
    • H01S3/302—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects in an optical fibre
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/10—Arrangements of light sources specially adapted for spectrometry or colorimetry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/44—Raman spectrometry; Scattering spectrometry ; Fluorescence spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/65—Raman scattering
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0092—Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H01S3/06716—Fibre compositions or doping with active elements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106—Mode locking
    • H01S3/1112—Passive mode locking
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/44—Raman spectrometry; Scattering spectrometry ; Fluorescence spectrometry
    • G01J3/4412—Scattering spectrometry

Definitions

  • the present disclosure relates to light sources, and more specifically, for analysis and observation by Raman scattering spectroscopy to detect second harmonic generation, third harmonic generation, coherent anti-Stokes Raman scattering, etc. Regarding light sources.
  • Raman scattering spectroscopy is widely used in many academic fields such as chemistry, biology, medicine, pharmacy, agriculture, and physics as a means of obtaining vibrational information of molecules, crystals, amorphous structures, etc. It has also been widely put into practical use in industry.
  • Classical Raman scattering spectroscopy applies spontaneous Raman scattering.
  • Spontaneous Raman scattering is a phenomenon that generates scattered light whose frequency is shifted by the frequency of molecular vibration or lattice vibration with respect to incident light. Since this scattered light has a very weak power compared to the power of the original incident light, a light source of the incident light with high power is required in order to obtain the scattered light that can be measured by a detector.
  • CARS coherent anti-Stokes Raman scattering
  • CARS measurement along with the development of pulsed lasers as light sources has been remarkable, and the effect is particularly remarkable when acquiring microscopic images.
  • a pulsed laser with high instantaneous power when used as a light source, it generates not only CARS but also second harmonic generation (hereinafter referred to as SHG) and third harmonic generation (hereinafter referred to as THG). can be detected at the same time.
  • SHG second harmonic generation
  • THG third harmonic generation
  • a microscope with such a configuration is called a multimodal nonlinear optical microscope, and many uses have been proposed in life science, medicine, and pharmacy, and further development is desired (for example, non-patent (See Reference 1).
  • Raman scattering spectroscopy When Raman scattering spectroscopy is used to measure living things and biologically related substances, there are two important wavenumber regions: one is the fingerprint region, which is a wavenumber region of 500 cm -1 to 1800 cm -1 , and the other is carbon- This is the region of wavenumbers from 2800 cm ⁇ 1 to 4000 cm ⁇ 1 due to hydrogen (C-H) bonds, nitrogen-hydrogen (N-H) bonds, or oxygen-hydrogen (O-H) bonds (for example, non-patent documents 1 , 13 reference).
  • C-H hydrogen
  • N-H nitrogen-hydrogen
  • O-H oxygen-hydrogen
  • FIG. 1 is a diagram conceptually showing the structure of a CARS light source 10 according to the prior art.
  • a conventional CARS light source 10 electrically connects a pump light/probe light source 11, a Stokes light source 12, and a pump light/probe light source 11 and a Stokes light source 12 to synchronize pulse timing. It includes an electric signal path 13 and a multiplexer 14 that multiplexes lasers output from the pump light/probe light source 11 and the Stokes light source 12 (for example, see Non-Patent Document 2).
  • the pump light/probe light source 11 is a Ti-sapphire laser, and outputs a picosecond pulse train having a wavelength of 0.73 ⁇ m, a pulse wavelength band of 0.11 nm, a pulse width of 5 ps, and a repetition frequency of 80 MHz.
  • the Stokes light source 12 is also a Ti-sapphire laser, and outputs a femtosecond optical pulse train with a center wavelength of 0.80 ⁇ m, a pulse wavelength band of 80 nm, a pulse width of 12 fs, and a repetition frequency of 80 MHz.
  • These two systems of optical pulse trains are combined by a multiplexer 14 and input to a microscope 15 along the same optical path. By simultaneously irradiating the sample with these two systems of light pulses, a CARS signal is obtained from the sample.
  • the pulse width of the Stokes light is expanded to 1.54 ps due to optical dispersion in the optical path reaching the sample surface.
  • FIG. 2 is a diagram showing an energy diagram of molecules of a sample when measuring CARS spectroscopy.
  • the molecules of the sample are CARS light (angular frequency ⁇ CARS ) corresponding to the angular frequency ⁇ of the vibration mode is generated.
  • the peak intensity I ⁇ 0 (W/m 2 ), pulse width ⁇ (s), repetition frequency f rep (Hz), average power P ⁇ av (W), and oscillation wavelength ⁇ ( ⁇ m) ( ⁇ 2 ⁇ c ⁇ ⁇ 1 , c: speed of light)
  • the duty ratio D is defined by (Equation 1).
  • the beam focal area A is set to the minimum possible value, it will be proportional to the square of the wavelength ⁇ , so it can be expressed by the following (Equation 2).
  • the power P CARS ⁇ av of the CARS signal is the minimum of the peak intensity of each of the pump light, probe light, and Stokes light, the beam focal area of the CARS light, and the duty ratio of each of the pump light, probe light, and Stokes light. It is proportional to the product of five elements of the duty ratio. If the wavelength of the pump light and the probe light is ⁇ 1 and the wavelength of the Stokes light is ⁇ 3 , then the beam focal area of the CARS light is approximately proportional to the square of ⁇ 1 . From these, the following (Formula 4) holds true.
  • the power of the CARS signal obtained in CARS measurement depends on the average power, wavelength, and duty ratio of each incident light (pump light, Stokes light, etc.).
  • Non-Patent Document 4 For linear response mechanisms, it is necessary to set an upper limit on the total incident power to avoid this damage. In this case, it can be seen from (Equation 4) that reducing the duty ratio D is effective in obtaining a high CARS signal. However, reducing D while keeping the total incident power constant leads to an increase in pulse peak power, and there is a concern that damage to mechanisms exhibiting high-order responses may occur.
  • Non-Patent Document 4 the optimum repetition frequency for a pulse width of 2.5 ps is 1-4 MHz (see, for example, Non-Patent Document 4). Therefore, when using a typical solid-state mode-locked laser with a repetition frequency of 80 MHz, as in the prior art shown in Fig. 1, it is necessary to take some measures, and one such measure is the introduction of a method using line illumination. (For example, see Non-Patent Document 5).
  • FIG. 3 is a diagram conceptually showing the principle of a CARS microscope using line illumination
  • FIG. 3(a) is a diagram showing information allocation on the spectrometer CCD surface in the CARS microscope
  • FIG. ) is a diagram showing an elliptical focus and its scanning direction in a CARS microscope.
  • the optical system is set so that one axis of the two-dimensional CCD 31 attached to the spectrometer of the microscope shown in FIG. 3(a) is the Y-axis position on the line, and the other axis is the spectral spectrum. Set. Then, by scanning the incident laser beam at high speed over one line of the target sample, substantial repetition for one pixel is reduced.
  • the X-axis and Z-axis sweeps are performed by moving the sample using a piezo stage.
  • the laser power is increased compared to point sweep in order to maintain the average power per pixel, but this creates the risk of damage due to higher order responses.
  • Existing reports state that it is necessary to set the pulse peak intensity to 20 GW/cm 2 or less (for example, see Non-Patent Document 2). Therefore, in CARS microscopes using line illumination, as shown in Figure 3(b), a method was adopted in which the shape of the beam 32 at the focal point was set to an ellipse so that this value would not be exceeded at the focal position. .
  • the wavelengths used for the pump light, probe light, and Stokes light are limited to the oscillation wavelength of the Ti-sapphire laser, the measurable Raman scattering band may be limited to 1250 cm ⁇ 1 (for example, see Non-Patent Document 2).
  • a measurement band of about 400 to 4000 cm -1 is practically desired. Therefore, a technique is known that uses supercontinuum light as Stokes light to enable measurement of a wide Raman scattering band (for example, see Non-Patent Documents 1, 3, and 13).
  • These conventional CARS microscopes use supercontinuum light generated by a highly nonlinear fiber with anomalous dispersion.
  • a problem with supercontinuum light obtained using such a PM-AND-HNL fiber is that the wavelength band is relatively narrow (see, for example, Non-Patent Document 11).
  • supercontinuum light with the longest wavelength of 1.39 ⁇ m is generated by pump light with a wavelength of 1.049 ⁇ m (see, for example, Non-Patent Document 10), but even when this is used in a CARS microscope, the wavelength is 2300 cm ⁇ 1 Only Raman scattering up to Therefore, there is a need for a light source configuration that can stably measure up to about 4000 cm -1 using supercontinuum light obtained using a PM-AND-HNL fiber.
  • the present disclosure has been made in view of the above-mentioned problems, and its purpose is to provide a Raman scattering spectroscopy method that is highly robust to the type of sample (particularly biological tissue samples).
  • the object of the present invention is to provide a light source for realizing analysis and observation using a CARS microscope (in particular, observation using a CARS microscope).
  • the present disclosure provides a mode-locked laser that is a light source for Raman scattering spectroscopy and outputs a femtosecond optical pulse train with a center wavelength ⁇ s , and a mode-locked laser that outputs a femtosecond optical pulse train with a center wavelength ⁇ s.
  • a demultiplexer that branches into two systems, a first femtosecond optical pulse train and a second femtosecond optical pulse train, a steady oscillation solid-state laser that outputs continuous light, and a continuous light output from the steady oscillation solid-state laser that transmits it, a first multiplexer that reflects the first femtosecond optical pulse train and outputs the continuous light and the first femtosecond optical pulse train on the same axis; and a difference frequency between the continuous light and the first femtosecond optical pulse train.
  • a second-order nonlinear optical element including at least one wavelength conversion element that generates and outputs a difference frequency generation and conversion optical pulse train consisting of picosecond optical pulses having a center wavelength ⁇ c ; an amplifier that amplifies the difference frequency generation and conversion optical pulse train; a polarization-maintaining fully normal dispersion highly nonlinear fiber that converts the second femtosecond optical pulse train into a supercontinuum optical pulse train; A first dispersion medium that converts the pulse width into substantially the same pulse width, and a second dispersion medium that combines and outputs the difference frequency generation/conversion optical pulse train output from the amplifier and the supercontinuum optical pulse train output from the first dispersion medium.
  • a multiplexer, and a center wavelength ⁇ s and a center wavelength ⁇ c are set so that coherent anti-Stokes Raman scattering measurement can be performed using the difference frequency generation/conversion optical pulse train and the supercontinuum optical pulse train output from the amplifier.
  • FIG. 1 is a diagram conceptually showing the structure of a CARS light source 10 according to the prior art. It is a figure which shows the energy diagram of the molecule of a sample in the case of measuring CARS spectroscopy.
  • FIG. 3(b) is a diagram conceptually showing the principle of a CARS microscope using line illumination
  • FIG. 3(a) is a diagram showing information allocation on the spectrometer CCD surface in the CARS microscope. It is a figure which shows the elliptical focus in a microscope, and its scanning direction.
  • FIG. 2 is a diagram conceptually showing the structure of a light source 40 according to the present disclosure, in which (a) shows a form in which input light propagates through a wavelength conversion element 451b in a secondary nonlinear optical element 45, and (b) shows a form in which input light propagates through a wavelength conversion element 451b.
  • the nonlinear optical element 45 the form in which input light propagates through the wavelength conversion element 451a is shown.
  • the light source according to the present disclosure focuses on a CARS light source using line illumination as shown in FIG. 3, and the use of line illumination itself is a known technique as described above. be.
  • the pulse width of the pump light (or probe light) and Stokes light is adjusted so that the peak power remains below the reference value even when the laser power is increased compared to point sweep. This differs from the prior art in that it is configured to be variable.
  • FIG. 4 is a diagram conceptually showing the structure of the light source 40 according to the present disclosure, in which (a) shows the form in which input light propagates through the wavelength conversion element 451b in the second-order nonlinear optical element 45, ) respectively show the form in which input light propagates through the wavelength conversion element 451a in the second-order nonlinear optical element 45. As shown in FIG.
  • the light source 40 includes a mode-locked laser 41 that outputs a femtosecond pulse laser with a center wavelength ⁇ s , a demultiplexer 42 that branches the femtosecond optical pulse train into two systems in terms of power, and a wavelength ⁇ p
  • CW steady oscillation
  • a multiplexer 44a coaxially outputs a continuous light with a wavelength ⁇ p and a femtosecond optical pulse train with a center wavelength ⁇ s , and a difference between the continuous light with a wavelength ⁇ p and the femtosecond optical pulse train with a center wavelength ⁇ s .
  • a second-order nonlinear optical element 45 that converts a femtosecond optical pulse train into a difference frequency generation/conversion optical pulse train of picosecond pulses with a center wavelength ⁇ c by frequency generation, an amplifier 46 that amplifies the difference frequency generation/conversion optical pulse train, PM-AND- which converts the femtosecond optical pulse train of another system with the center wavelength ⁇ s branched by the wave generator 42 into supercontinuum light (hereinafter referred to as SC light) having the wave number range necessary for the CARS microscope.
  • SC light supercontinuum light
  • HNL fiber 47 a dispersion medium 48 that converts the SC optical pulse train into an SC optical pulse train having a pulse width that is approximately the same as the pulse width of the difference frequency generation/conversion optical pulse train output from the amplifier 46 , the amplifier 46 and the dispersion medium 48 and a multiplexer 44b that multiplexes the optical pulse trains output from each of the multiplexers and inputs the multiplexed optical pulse trains to the microscope 15.
  • the secondary nonlinear optical element 45 includes wavelength conversion elements 451a and 451b. In the figure, it is depicted as including two wavelength conversion elements, but there may be one or more wavelength conversion elements, and if there are more than one, the lengths of each may be different depending on the design. . In addition, when there is a plurality of wavelength conversion elements, the secondary nonlinear optical element 45 combines input light (continuous light with a wavelength ⁇ p and a femtosecond optical pulse train with a center wavelength ⁇ s ) into a specific wavelength conversion element. It may further include a switch mechanism 452 for guiding.
  • the light source 40 is installed between the secondary nonlinear optical element 45 and the amplifier 46, and chirps the difference frequency generation/conversion optical pulse train output from the secondary nonlinear optical element 45. It may further include a second dispersion medium 49 that extends the pulse width by providing .
  • the laser medium of the mode-locked laser 41 can be, for example, Cr 4+ :YAG, Cr forsterite, Ti sapphire, Cr:LiSAF, Cr:LiCAF, Cr:ZnSe, or Cr:ZnS, etc. .
  • the laser medium of the mode-locked laser 41 is YAG, YVO4 , or glass (bulk and fiber) doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho, etc. obtain.
  • the laser medium of the mode-locked laser 41 may be a semiconductor crystal.
  • the shape of the gain medium that constitutes the mode-locked laser 41 may be a rod, a disk, or a fiber.
  • the demultiplexer 42 may be a half mirror or a beam splitter cube that separates the power of the input light into two, reflecting one and transmitting the other.
  • the CW solid-state laser 43 is a glass fiber laser, a bulk-shaped single-crystal laser, a bulk-shaped ceramic laser, a waveguide-type single-crystal laser, a waveguide-type ceramic laser, or a semiconductor laser. obtain.
  • the multiplexers 44a, b may be dichroic mirrors that reflect light of a predetermined wavelength and transmit light of other wavelengths.
  • the wavelength conversion element included in the secondary nonlinear optical element 45 is a periodically polarized lithium niobate (Periodically Poled Lithium Niobate: hereinafter referred to as PPLN) that satisfies (Formula 5) described below. It may be Periodically Poled Lithium Tantalate (hereinafter referred to as PPLT) or periodically poled KTP (KaTiOPO 4 ) crystal.
  • PPLN Periodically Poled Lithium Tantalate
  • KTP KaTiOPO 4
  • the amplifier 46 is a glass fiber amplifier doped with one rare earth ion selected from Yb, Er, Nd, Tm, Ho, etc., or a part of the amplifier doped with Yb, Er, Nd, It may be a single crystal fiber amplifier doped with one rare earth ion selected from Tm, Ho, and the like.
  • the difference frequency generation and conversion optical pulse train corresponds to pump light (or probe light)
  • the SC light pulse train corresponds to Stokes light.
  • ⁇ s and ⁇ p are selected such that ⁇ c is a pump light (or probe light) and the SC light is a Stokes light at an appropriate wavelength so as to enable the necessary CARS measurement.
  • the pump light (or probe light) is a selection and dispersion medium of a nonlinear optical element
  • the Stokes light is a dispersion medium, each of which has a variable pulse width. Therefore, in CARS measurement, it is possible to prevent damage to the sample and prevent CARS signal intensity from decreasing more than necessary. This is particularly effective when performing highly accurate measurements on samples such as living tissues where it is important to suppress damage during observation.
  • the light source is, for example, a light source for observing a wave number range of 400 cm ⁇ 1 to 4000 cm ⁇ 1 in CARS spectroscopy of a multimodal nonlinear optical microscope.
  • the mode-locked laser is a mode-locked laser oscillator using a Cr 4+ :YAG laser as a laser medium
  • the wavelength conversion element of the secondary nonlinear optical element is PPLN
  • the amplifier is a Yb-doped glass fiber amplifier (hereinafter referred to as YbFA)
  • CW The solid-state laser is a Yb:YLF laser oscillator that oscillates at a single wavelength of 0.607 ⁇ m.
  • the Cr 4+ :YAG laser used as the mode-locked laser is a femtosecond pulse laser with a center wavelength of 1.42 ⁇ m, a pulse width of 100 fs, and a repetition frequency of 80 MHz (see, for example, Non-Patent Document 6).
  • a CW solid-state laser is an oscillator of CW light having a single wavelength of 0.607 ⁇ m.
  • the femtosecond optical pulse train output from the mode-locked laser is split into two by the splitter.
  • One of the two branched femtosecond optical pulse trains is input to the PPLN as a signal light for generating a difference frequency with the picosecond optical pulse train output from the CW solid-state laser.
  • the CW light output from the CW solid-state laser is input to the PPLN as excitation light for generating a difference frequency.
  • the extraordinary ray refractive index of the PPLN used is calculated using (Equation 5), where the wavelength is ⁇ ( ⁇ m) (for example, see Reference 7)
  • the PPLN when generating a picosecond optical pulse train by difference frequency generation using CW light as excitation light and femtosecond optical pulse train as signal light, it can be used because of the difference in group velocity caused by the difference in wavelength of both optical pulse trains in PPLN.
  • the length of the PPLN is limited.
  • the usable length L ⁇ of the PPLN can be determined using (Equation 6) with reference to the method for determining the usable length L ⁇ in the case of SHG (for example, see Non-Patent Document 8).
  • ⁇ c is the pulse width (full width at half maximum) of the converted light (difference frequency generation converted light pulse train)
  • v gc is the group velocity of the converted light
  • v gs is the group velocity of the signal light.
  • Equation 7 the relationship (Equation 7) is satisfied between the wavelengths of the converted light, signal light, and pump light.
  • ⁇ c , ⁇ s , and ⁇ p are the respective wavelengths of the converted light, signal light, and pump light.
  • the wavelength of the signal light is 1.42 ⁇ m, so the wavelength of the converted light is 1.06 ⁇ m, which is caused by the difference in group velocity of the two lights within the PPLN.
  • L ⁇ is calculated to be 0.03 m.
  • the efficiency ⁇ (%/W) of difference frequency generation can be calculated using (Equation 9), assuming that the powers of the converted light, signal light, and pumping light are P c , P s , and P p (W), respectively. It will be done.
  • C LN is a constant for PPLN
  • L is the length of PPLN (m)
  • a eff is the beam cross-sectional area ( ⁇ m 2 ) of pump light and signal light in PPLN.
  • the length of the first PPLN is 0.030 m, which is calculated as described above.
  • a 2.5 ps pulse is generated due to the difference in group velocity between the signal light and the converted light as described above.
  • the wavelength of the converted light is included in the gain band of YbFA that constitutes the amplifier.
  • YbFA gain band of YbFA that constitutes the amplifier.
  • a second PPLN with L ⁇ 0.060 m
  • the dispersion medium is composed of at least one of an optical fiber, a dispersion compensating mirror, or a prism pair.
  • optical fibers two types with different lengths or types can be selected; for dispersion compensating mirrors, the number of bounces can be changed; and for prism pairs, the position can be changed.
  • Non-Patent Documents 10 and 11 Compared to SC light generation using an anomalous dispersion fiber or a fiber containing zero dispersion, the SC light from a fully normal dispersion fiber improves the signal to noise of the CARS microscope, realizing high resolution and high speed measurements.
  • an existing report introduces a simulation of SC light generated by a PM-AND-HNL fiber (for example, see Non-Patent Document 11), and for a pump light of 1.04 ⁇ m, approximately 0.8 ⁇ m to 1.0 ⁇ m.
  • SC light is generated over a range of 4 ⁇ m. This corresponds to a band of approximately 5000 cm ⁇ 1 .
  • the center wavelength of the SC light wavelength band and the pump light wavelength match, and the wavelength band is 5000 cm -1
  • the wavelength of the pump light (or probe light) is 1.06 ⁇ m, it is the pump light for SC light generation.
  • the center wavelength ⁇ s of the pulse should be set in the wavelength range from 1.26 ⁇ m to 1.53 ⁇ m. In this embodiment, ⁇ s is 1.42 ⁇ m, which is an appropriate value.
  • the SC optical pulse train output from the PM-AND-HNL fiber passes through a dispersion medium and is adjusted to have approximately the same pulse width as the picosecond pulse train.
  • the optical path length of the other branched femtosecond optical pulse train or the optical path length of the picosecond pulse train must be adjusted between the demultiplexer and the microscope. do it.
  • the light source of this embodiment may be used for spectroscopic measurements by coherent Raman scattering (for example, stimulated Raman gain, stimulated Raman loss, etc.) and spectroscopic measurements other than the CARS measurement microscope.
  • coherent Raman scattering for example, stimulated Raman gain, stimulated Raman loss, etc.
  • SHG and THG may be measured together with the CARS signal, and used as a multimodal nonlinear optical microscope.
  • the pulse widths of the pump light (or probe light) and Stokes light are variable, so it is possible to maintain the average power per pixel.
  • the center wavelength of the femtosecond optical pulse and the wavelength of the CW laser for difference frequency generation are appropriately selected so that the SC light output from the PM-AND-HNL fiber is set in the wavelength range necessary for CARS measurement. . Therefore, the present invention is expected to find application in the medical and industrial fields as an analysis and observation technique that applies Raman scattering spectroscopy, which has high robustness to the type of sample (particularly biological tissue samples).

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