light source

Abstract

A light source according to the present disclosure comprises: a mode-locked laser; a beam splitter that splits a femtosecond optical pulse train having a center wavelength λs outputted from the mode-locked laser; a CW solid-state laser; a first combiner that coaxially outputs one femtosecond optical pulse train; a second-order nonlinear optical element that outputs a difference frequency generation / conversion optical pulse train having a center wavelength λc from continuous light and the one femtosecond optical pulse train; an amplifier that amplifies the difference frequency generation / conversion optical pulse train; a polarization-maintaining, all-normal-dispersion, highly nonlinear fiber that converts the other femtosecond optical pulse train into a supercontinuum optical pulse train; a first dispersion medium that converts the supercontinuum optical pulse train into substantially the same pulse width as that of the difference frequency generation / conversion optical pulse train; and a second combiner that combines and outputs the difference frequency generation / conversion optical pulse train and the supercontinuum optical pulse train. λs and λc are set so that a CARS measurement can be performed using the difference frequency generation / conversion optical pulse train and the supercontinuum optical pulse train.

Description

[Technical Field]

[0001] The present disclosure relates to light sources, and more particularly to light sources for analysis and observation by Raman scattering spectroscopy, such as for detecting second harmonic generation, third harmonic generation, and coherent anti-Stokes Raman scattering. [Background technology]

[0002] Raman scattering spectroscopy is widely used in many academic fields, including chemistry, biology, medicine, pharmacy, agriculture, and physics, as a means of obtaining vibrational information on molecules, crystals, and amorphous structures. It is also widely used in medicine and industry. Classical Raman scattering spectroscopy utilizes spontaneous Raman scattering, a phenomenon in which scattered light is generated at a frequency shifted by the frequency of molecular or lattice vibrations relative to the incident light. Because this scattered light has a much weaker power than the original incident light, a high-power incident light source is required to obtain measurable scattered light. However, most samples have an upper limit to the power per unit area that can be irradiated, and exceeding this limit will result in alteration or destruction. In many cases, even with a light source with a power equivalent to the upper limit, the scattered light is weak, requiring significantly longer measurement times to obtain a signal with a high signal-to-noise ratio. In contrast, Coherent Anti-Stokes Raman Scattering (CARS) is a nonlinear optical process using a light source with high instantaneous power. Therefore, when using a light source of the same power as spontaneous Raman scattering, the power of the Raman scattered light is significantly stronger, and as a result, measurements can be taken in a shorter time.

[0003] The development of pulsed lasers as light sources has led to remarkable advances in CARS measurements, particularly in the acquisition of microscopic images. Furthermore, when a pulsed laser with high instantaneous power is used as the light source, not only CARS but also second harmonic generation (SHG) and third harmonic generation (THG) can be simultaneously detected. Microscopes with this configuration are called multimodal nonlinear optical microscopes, and many applications have been proposed in the fields of life science, medicine, and pharmacy, with further developments anticipated (see, for example, Non-Patent Document 1).

[0004] When measuring living organisms and biologically related substances using Raman scattering spectroscopy, there are two important wavenumber regions. One is the 500 cm wavenumber region, known as the fingerprint region. -1 From 1800cm -1 and the other is the region of 2800 cm due to carbon-hydrogen (CH) bonds, nitrogen-hydrogen (NH) bonds, or oxygen-hydrogen (OH) bonds. -1 From 4000cm -1 (For example, see Non-Patent Documents 1 and 13.) When measuring CARS spectroscopy, light of two wavelengths with a wavenumber difference corresponding to the wavenumber in the above-mentioned region (the difference in the wavelengths of the two lights is a wavenumber corresponding to the wavenumber in the above-mentioned region) is incident on the object to be measured (sample).

[0005] FIG. 1 is a conceptual diagram illustrating the structure of a conventional CARS light source 10. As shown in FIG. 1, the conventional CARS light source 10 includes a pump / probe light source 11, a Stokes light source 12, an electrical signal path 13 that electrically connects the pump / probe light source 11 and the Stokes light source 12 and synchronizes their pulse timing, and a multiplexer 14 that multiplexes the lasers output from the pump / probe light source 11 and the Stokes light source 12 (see, for example, Non-Patent Document 2). Here, as an example, the pump / probe light source 11 is a Ti:sapphire laser that outputs a picosecond pulse train with a wavelength of 0.73 μm, a pulse wavelength band of 0.11 nm, a pulse width of 5 ps, and a repetition rate of 80 MHz. Meanwhile, the Stokes light source 12 is also a Ti:sapphire laser that outputs a femtosecond optical pulse train with a center wavelength of 0.80 μm, a pulse wavelength band of 80 nm, a pulse width of 12 fs, and a repetition rate of 80 MHz. These two optical pulse trains are combined by a combiner 14 and input to a microscope 15 along the same optical path. A CARS signal is obtained from a sample by simultaneously irradiating the sample with these two optical pulses. Here, the pulse width of the Stokes light is broadened to 1.54 ps due to optical dispersion in the optical path leading to the sample surface.

[0006] Figure 2 shows the energy diagram of the sample molecules when measuring CARS spectroscopy. As shown in Figure 2, when measuring CARS spectroscopy, the pump light (angular frequency ω1), the Stokes light (angular frequency ω2), and the probe light (angular frequency ω3) are incident, and the CARS light (angular frequency ω) corresponding to the angular frequency Ω of the vibration mode of the sample molecules is generated. CARS ) occurs. In the example of the prior art shown in Figure 1, the same picosecond pulse train is used as the pump light and the probe light (i.e., ω1 = ω3), and a broadband femtosecond optical pulse train is used as the Stokes light. This excites many vibrational modes, making it possible to measure broadband CARS light; this type of spectroscopy is called a multiplex CARS process (see, for example, Non-Patent Document 3).

[0007] Here, we will discuss the relationship between the average power of the incident light from the light source and the signal intensity of the CARS signal. i (i=1, 2 or 3) is the light intensity per unit area, ω (t,x i )(W / m 2 ) and the peak intensity I ω·0 (W / m 2 ), pulse width τ(s), repetition frequency f rep (Hz), average power P ω·av (W), and oscillation wavelength λ (μm) (λ=2πcω -1 , c: speed of light), and the duty ratio D is defined by (Equation 1).

[0008]

number

[0009] Furthermore, if the beam focal area A is set to the smallest possible value, it will be proportional to the square of the wavelength λ, and can be expressed by the following equation (2).

[0010]

number

[0011] From these, the average power of the incident light P ω·av The proportional relationship shown in (Equation 3) holds true for these two.

[0012]

number

[0013] Also, the power of the CARS signal P CARS·avis proportional to the product of five elements: the peak intensity of the pump light, probe light, and Stokes light, the beam focal area of ​​the CARS light, and the smallest duty ratio among the duty ratios of the pump light, probe light, and Stokes light. If the wavelengths of the pump light and probe light are λ1 and the wavelength of the Stokes light is λ3, the beam focal area of ​​the CARS light is roughly proportional to the square of λ1. From these, the following (Equation 4) holds.

[0014]

number

[0015] That is, the power of the CARS signal obtained by CARS measurement depends on the average power, wavelength, and duty ratio of each of the incident light (pump light, Stokes light, etc.).

[0016] When observing biological tissue using CARS measurements, it is important to select conditions that avoid damaging the sample tissue. Previous studies have shown that incident light can damage biological tissue through two mechanisms: a linear response to the incident light power and a higher-order response (see, for example, Non-Patent Document 4). For the linear response mechanism, an upper limit on the total incident power must be set to avoid damage. In this case, Equation 4 shows that reducing the duty ratio D is effective for obtaining a high CARS signal. However, reducing D while maintaining a constant total incident power leads to an increase in the pulse peak power, raising concerns about damage caused by mechanisms exhibiting higher-order responses. Previous studies have shown that the optimal repetition rate 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 rate of 80 MHz, as in the prior art shown in Figure 1, ingenuity is required. One such measure is the use of line illumination (see, for example, Non-Patent Document 5).

[0017] Figure 3 conceptually illustrates the principle of a CARS microscope using line illumination. Figure 3(a) illustrates the information allocation on the spectrometer CCD surface in a CARS microscope, and Figure 3(b) illustrates the elliptical focus and its scanning direction in a CARS microscope. In the line illumination method described above, the optical system is configured so that one axis of the two-dimensional CCD 31 attached to the spectrometer of the microscope shown in Figure 3(a) corresponds to the Y-axis position on the line, and the other axis corresponds to the optical spectrum. Then, by rapidly scanning the incident laser light across a line on the target sample, the effective repetition rate per pixel is reduced. Note that the X- and Z-axis sweeps are performed by moving the sample using a piezo stage. In this case, the laser power is increased compared to point sweeps to maintain the average power per pixel, but this increases the risk of damage due to higher-order responses. Previous reports suggest that the pulse peak intensity should be set to 20 GW / cm. 2 It is stated that it is necessary to set the value below (see, for example, Non-Patent Document 2). Therefore, in CARS microscopes using line illumination, a method has been adopted in which the shape of the beam 32 at the focus is set to an ellipse so that this value is not exceeded at the focus position, as shown in FIG. 3(b).

[0018] However, the damage threshold due to linear absorption and the damage threshold due to higher-order response differ depending on the type of biological tissue, which can lead to a problem that in some cases the peak intensity may drop more than necessary, resulting in insufficient CARS generation efficiency.

[0019] Furthermore, the wavelengths used for the pump light, probe light, and Stokes light are limited to the oscillation wavelength of the Ti:sapphire laser, so the measurable Raman scattering band is limited to 1250 cm -1 (For example, see Non-Patent Document 2.) When measuring living organisms and biologically related substances, the practical range is 400-4000 cm -1A measurement bandwidth of approximately 100 fs is desired. Therefore, a technique has been proposed that uses supercontinuum light as the Stokes light to enable measurement of a wide Raman scattering band (see, for example, Non-Patent Documents 1, 3, and 13). These conventional CARS microscopes use supercontinuum light generated by a highly nonlinear fiber with anomalous dispersion. While this generation method has the advantage of a wide wavelength band, it is known that the spectral shape of each pulse varies significantly and the signal-to-noise ratio (SNR) is low (see, for example, Non-Patent Document 12). To improve this, it is known that by injecting ideal pulses of less than 100 fs (without low-power pedestal components or subpeaks before and after the pulse) into a polarization-maintaining, all-normal-dispersion, highly nonlinear (hereinafter referred to as PM-AND-HNL) fiber, supercontinuum light with small pulse-to-pulse variation and a high SNR can be obtained (see, for example, Non-Patent Documents 10, 11, and 12). However, the supercontinuum light obtained using such PM-AND-HNL fiber has a relatively narrow wavelength band (see, for example, Non-Patent Document 11). In an existing report, a supercontinuum light with a maximum wavelength of 1.39 μm was generated by pump light with a wavelength of 1.049 μm (see, for example, Non-Patent Document 10). However, even when this was used in a CARS microscope, the maximum wavelength was 2300 cm -1 Therefore, the supercontinuum light obtained using the PM-AND-HNL fiber can be used to measure Raman scattering up to 4000 cm. -1 There is a demand for a light source configuration that can provide measurements with stability to a certain extent. [Prior art documents] [Non-patent literature]

[0020] [Non-Patent Document 1] Hiroaki Yoneyama et al., “Invited Article: CARS molecular fingerprinting using sub-100-ps microchip laser source with fiber amplifier”, APL Photonics 3, 092408 (2018) [Non-patent document 2] Shun Kizawa et al., “Ultrahigh-speed multiplex coherent anti-Stokes Raman scattering microspectroscopy using scanning elliptical focal spot”, J. Chem. Phys., 155, 144201 (2021) [Non-patent document 3] Hideaki Kano, "Nonlinear Optical Imaging Using Supercontinuum Light," Applied Physics, Vol. 86, No. 3, pp. 186-193, (2017) [Non-patent document 4] Y. Fu et al. “Characterization of photodamage in coherent anti-Stokes Raman scattering microscopy”, Opt. Express 14, 3942-3951 (2006). [Non-patent document 5] Ian Seungwan Ryu, et al., “Beam scanning for rapid coherent Raman hyperspectral imaging,” Opt. Lett. 40, 5826-5829 (2015) [Non-patent document 6] Z. Zhang et al., “Self-starting mode-locked Cr4+:YAG laser with a low-loss broadband semiconductor saturable-absorber mirror”, Opt. Lett., Vol.24, No.23, pp.1768-1770, (1999) [Non-Patent Document 7] Shota Nuki, "Precise Measurement of Refractive Index of Novel Lasers and Nonlinear Optical Materials," Master's Thesis Abstract, Department of Electrical, Electronics, and Information Engineering, Faculty of Science and Engineering, Chuo University (2013) [Non-patent document 8] E. Sidick et al., “Ultrashort-pulse second-harmonic generation. I. Transform-limited fundamental pulses”, J. Opt. Soc. Am. B, Vol. 12, pp.1704-1712 (1995) [Non-Patent Document 9] Osamu Tadanaga et al., "Highly efficient mid-infrared difference frequency generation using quasi-phase-matched LiNbO3 ridge waveguides," Laser Research, Vol. 36, No. 2, pp. 64-69, (2008) [Non-Patent Document 10] Etienne Genier et al., “Ultra-flat, low-noise, and linearly polarized fiber supercontinuum source covering 670-1390 nm”, Opt. Lett. 46, 1820-1823 (2021) [Non-Patent Document 11] Thibaut Sylvestre et al., “Recent advances in supercontinuum generation in specialty optical fibers.”, J. Opt. Soc. Am. B, Vol. 38, F90-F103 (2021). [Non-Patent Document 12] Mariusz Klimczak et al., “Direct comparison of shot-to-shot noise performance of all normal dispersion and anomalous dispersion supercontinuum pumped with sub-picosecond pulse fiber-based laser.” Sci. Rep. 6, 19284 (2016). [Non-Patent Document 13] Daiki Kaneta et al., “Visualization of water concentration distribution in human skin by ultra-multiplex coherent anti-Stokes Raman scattering (CARS) microscopy”, Appl. Phys. Express 14, 042010 (2021). Summary of the Invention

[0021] The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a light source for realizing analysis and observation (particularly, observation using a CARS microscope) using Raman scattering spectroscopy that is highly robust for various types of samples (particularly, biological tissue samples).

[0022] In response to the above-described problems, the present disclosure provides a light source for Raman scattering spectroscopy, which has a central wavelength λ s a mode-locked laser that outputs a femtosecond optical pulse train of λ; a splitter that splits the femtosecond optical pulse train into two systems, a first femtosecond optical pulse train and a second femtosecond optical pulse train, in terms of power; a steady-state solid-state laser that outputs continuous light; a first multiplexer that transmits the continuous light output from the steady-state solid-state laser, reflects the first femtosecond optical pulse train, and outputs the continuous light and the first femtosecond optical pulse train on a coaxial line; and a mode-locked laser that outputs a femtosecond optical pulse train of λ by difference frequency generation between the continuous light and the first femtosecond optical pulse train. ca second-order nonlinear optical element including at least one wavelength conversion element for outputting a difference frequency generation converted optical pulse train consisting of picosecond optical pulses of λ / μ ... s and central wavelength λ c The light source that is set provide. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a diagram conceptually showing the structure of a CARS light source 10 according to the prior art. [Figure 2] FIG. 1 is a diagram showing an energy diagram of sample molecules when measuring CARS spectroscopy. [Figure 3] These are diagrams conceptually illustrating the principle of a CARS microscope using line illumination. Figure 3(a) shows the allocation of information on the spectrometer CCD surface in a CARS microscope, and Figure 3(b) shows the elliptical focus and its scanning direction in a CARS microscope. [Figure 4] 1A and 1B are diagrams conceptually illustrating the structure of a light source 40 according to the present disclosure, in which (a) shows the manner in which input light propagates through a wavelength conversion element 451b in a second-order nonlinear optical element 45, and (b) shows the manner in which input light propagates through a wavelength conversion element 451a in a second-order nonlinear optical element 45. DETAILED DESCRIPTION OF THE INVENTION

[0024] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Materials and numerical values ​​are for illustrative purposes only and are not intended to limit the technical scope of the present disclosure. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present disclosure.

[0025] The light source according to the present disclosure is primarily intended for use as a CARS light source using a line illumination, as shown in Fig. 3, and the use of this line illumination is itself a known technique, as described above. However, it differs from the prior art in that the pulse widths of the pump light (or probe light) and the Stokes light are variable so that the peak power remains below a reference value even when the laser power is increased compared to point sweeping in order to maintain the average power per pixel.

[0026] 4A and 4B are diagrams conceptually illustrating the structure of the light source 40 according to the present disclosure, in which (a) shows how input light propagates through the wavelength conversion element 451b in the second-order nonlinear optical element 45, and (b) shows how input light propagates through the wavelength conversion element 451a in the second-order nonlinear optical element 45. As shown in FIG. 4, the light source 40 emits a wavelength λ s a mode-locked laser 41 that outputs a femtosecond pulse laser of wavelength λ; a splitter 42 that splits the femtosecond optical pulse train into two systems in terms of power; p and a continuous wave (hereinafter referred to as CW) solid-state laser 43 that outputs continuous light of wavelength λ . The continuous light output from the CW solid-state laser 43 is transmitted through a splitter 42, which splits one of the femtosecond optical pulse trains into two, and reflects the other femtosecond optical pulse train into a pulse of wavelength λ . p Continuous light with a central wavelength λ s a coupler 44a for coaxially outputting a femtosecond optical pulse train of wavelength λ p Continuous light with a central wavelength λ s By difference frequency generation with a femtosecond optical pulse train, a femtosecond optical pulse train with a center wavelength of λ ca second-order nonlinear optical element 45 for converting the converted optical pulse train into a difference frequency generation converted optical pulse train of λ 1 , an amplifier 46 for amplifying the converted optical pulse train of λ 1 , and another system of the center wavelength λ 1 branched by the demultiplexer 42. s a PM-AND-HNL fiber 47 that converts the femtosecond optical pulse train into supercontinuum light (hereinafter referred to as SC light) having a wavenumber range required for the CARS microscope; a dispersion medium 48 that converts the SC optical pulse train into an SC optical pulse train having a pulse width substantially the same as the pulse width of the difference frequency generation converted optical pulse train output from the amplifier 46; and a combiner 44b that combines the optical pulse trains output from the amplifier 46 and the dispersion medium 48 and inputs the combined pulse to the microscope 15.

[0027] As shown in FIG. 4, in the light source 40 according to the present disclosure, the second-order nonlinear optical element 45 includes wavelength conversion elements 451a and 451b. Although the light source 40 is depicted as including two wavelength conversion elements in the drawing, the number of wavelength conversion elements may be one or more, and if there are more than one, the lengths of the respective elements may differ depending on the design. Furthermore, if there are more than one wavelength conversion elements, the second-order nonlinear optical element 45 converts the input light (wavelength λ p Continuous light with a central wavelength λ s The wavelength conversion element may further include a switch mechanism 452 for directing the combined femtosecond optical pulse trains to a specific wavelength conversion element.

[0028] As shown in FIG. 4 , the light source 40 according to the present disclosure may further include a second dispersive medium 49 that is disposed between the second-order nonlinear optical element 45 and the amplifier 46 and that extends the pulse width by applying chirp to the difference frequency generated converted optical pulse train output from the second-order nonlinear optical element 45.

[0029] In the light source 40 according to the present disclosure, the laser medium of the mode-locked laser 41 is, for example, Cr 4+The laser medium of the mode-locked laser 41 may be YAG, Cr-forsterite, Ti-sapphire, Cr:LiSAF, Cr:LiCAF, Cr:ZnSe, Cr:ZnS, or the like. In another example, the laser medium of the mode-locked laser 41 may be YAG, YVO4, or glass (bulk and fiber) doped with one rare earth ion selected from Yb, Er, Nd, Tm, Ho, or the like. In another example, the laser medium of the mode-locked laser 41 may be a semiconductor crystal.

[0030] In the light source 40 according to the present disclosure, the gain medium comprising the mode-locked laser 41 can be in the form of a rod, a disk, or a fiber.

[0031] In light source 40 according to the present disclosure, splitter 42 may be a half mirror or a beam splitter cube that splits the input light power into two, reflecting one and transmitting the other.

[0032] In the light source 40 according to the present disclosure, the CW solid state laser 43 can be 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.

[0033] In the light source 40 according to the present disclosure, the multiplexers 44a,b may be dichroic mirrors that reflect light of a predetermined wavelength and transmit light of other wavelengths.

[0034] In the light source 40 according to the present disclosure, the wavelength conversion element included in the second-order nonlinear optical element 45 may be a periodically poled lithium niobate (hereinafter referred to as PPLN), a periodically poled lithium tantalate (hereinafter referred to as PPLT), or a periodically poled KTP (KaTiOPO4) crystal, which satisfies the following (Equation 5).

[0035] In the light source 40 according to this embodiment, the amplifier 46 may be a glass fiber amplifier doped with one rare earth ion selected from Yb, Er, Nd, Tm, Ho, etc., or a single crystal fiber amplifier doped with one rare earth ion selected from Yb, Er, Nd, Tm, Ho, etc. in a portion of the amplifier.

[0036] In the light source 40 according to the present disclosure having such a configuration, two types of optical pulse trains, namely, the difference frequency generation converted optical pulse train amplified by the amplifier 46 and the SC optical pulse train output from the dispersion medium 48, are output in a combined state and input to the microscope 15. Then, in the microscope 15, the light obtained by combining the two types of optical pulse trains is incident on the sample as incident light, and CARS measurement is performed. In the light source 40 according to the present disclosure, the difference frequency generation converted optical pulse train corresponds to the pump light (or probe light), and the SC optical pulse train corresponds to the Stokes light. λ is set so that the required CARS measurement can be performed. c is the wavelength of the pump light (or probe light) and the SC light is the wavelength of the Stokes light. s and λ p is selected.

[0037] As can be understood from the above, the light source 40 according to the present disclosure has variable pulse widths for the pump light (or probe light) and the Stokes light, which are both nonlinear optical elements and dispersive media. Therefore, in CARS measurements, it is possible to prevent unnecessary reduction in CARS signal intensity while suppressing sample damage. This is particularly effective when performing high-precision measurements on samples, such as biological tissue, where suppressing damage during observation is important.

[0038] Hereinafter, an embodiment of a light source according to the present disclosure will be described in detail with specific examples. In the description of this embodiment, as an example, the light source is a light source having a wave number of 400 cm in CARS spectroscopy of a multimodal nonlinear optical microscope. -1 From 4000cm -1 The mode-locked laser is used as a light source for observing the region. 4+The mode-locked laser oscillator uses a YAG laser as the laser medium, the wavelength conversion element of the second-order nonlinear optical element is PPLN, the amplifier is a Yb-doped glass fiber amplifier (hereinafter referred to as YbFA), and the CW solid-state laser is a Yb:YLF laser oscillator oscillating at a single wavelength of 0.607 μm. 4+ The YAG laser is a femtosecond pulse laser with a central wavelength of 1.42 μm, a pulse width of 100 fs, and a repetition rate of 80 MHz (see, for example, Non-Patent Document 6). The CW solid-state laser is an oscillator of CW light with a single wavelength of 0.607 μm.

[0039] As described above, in the light source according to the present disclosure, the femtosecond optical pulse train output from the mode-locked laser is branched into two by a branching filter. One of the branched femtosecond optical pulse trains is input to a PPLN as signal light for generating difference frequency generation with the picosecond optical pulse train output from the CW solid-state laser. Meanwhile, the CW light output from the CW solid-state laser is input to the PPLN as pump light for difference frequency generation.

[0040] In such cases, the extraordinary ray refractive index of the PPLN used is calculated using Equation 5, where λ is the wavelength (μm) (see, for example, Reference 7).

[0041]

number

[0042] Furthermore, when generating a picosecond optical pulse train by difference frequency generation using CW light as the pump light and a femtosecond optical pulse train as the signal light, the usable length of the PPLN is limited by the difference in group velocity resulting from the difference in wavelength between the two optical pulse trains in the PPLN. The usable length Lτ of the PPLN can be calculated using Equation 6, with reference to the method for calculating the usable length Lτ in the case of SHG (see, for example, Non-Patent Document 8).

[0043]

number

[0044] where τ c is the pulse width (full width at half maximum) of the converted light (difference frequency generated converted light pulse train), and v gc is the group velocity of the converted light, v gs is the group velocity of the signal light.

[0045] On the other hand, the relationship of (Equation 7) is satisfied between the wavelengths of the converted light, the signal light, and the pump light.

[0046]

number

[0047] where λ c , λ s , and λ p are the wavelengths of the converted light, the signal light, and the pump light, respectively.

[0048] From (Equation 6) and (Equation 7), in this example, the wavelength of the signal light is 1.42 μm, so the wavelength of the converted light is 1.06 μm, and the pulse width of the converted light caused by the difference in group velocities of the two light beams in the PPLN is 2.5 ps, so Lτ is calculated to be 0.03 m.

[0049] Furthermore, in order to maximize the conversion efficiency of PPLN, it is necessary to satisfy the phase matching condition expressed by (Equation 8) for the inversion period Λ (see, for example, Non-Patent Document 9).

[0050]

number

[0051] Furthermore, the efficiency of difference frequency generation (η) (% / W) is expressed as follows: c ,P s , and P p If (W), it can be calculated using (Equation 9).

[0052]

number

[0053] From the above, the efficiency η (% / W) of difference frequency generation when (Equation 7) is satisfied can be expressed by (Equation 10).

[0054]

number

[0055] where C LN is a constant for PPLN, L is the length of PPLN (m), A eff are the beam cross sections (μm 2 ) In the existing reports, λ c = 2.3 μm, λ s = 1.58 μm, λ p =0.937μm, L=0.05m, A eff =8.6×13μm 2 It has been reported that the efficiency of difference frequency generation η is 100% / W in this case (see, for example, Non-Patent Document 9). Using these values, C LN is 2.28 x 10 8 It is calculated as follows.

[0056] In this embodiment, the length of the first PPLN is set to 0.030 m as calculated above. In this case, a 2.5 ps pulse is generated in difference frequency generation in the PPLN due to the difference in group velocities between the signal light and the converted light.

[0057] The wavelength of the converted light, 1.06 μm, is included in the gain band of the YbFA that constitutes the amplifier. By combining multiple (e.g., two or three) YbFAs in the amplifier, a gain of about 60 dB can be obtained, so Cr 4+ Amplified picosecond optical pulse trains with a pulse width of 2.5 ps are output, synchronized with a :YAG mode-locked laser.

[0058] Here, the second-order nonlinear optical element further includes a second PPLN with Lτ = 0.060 m and a switch mechanism for switching between two optical paths of different lengths. When the optical path is switched to the second PPLN with Lτ = 0.060 m, an optical pulse train with a pulse width of 5 ps is output as difference frequency generated light.

[0059] Furthermore, if a dispersive medium is included between the second-order nonlinear optical element and the amplifier, it is possible to chirp each pulse of the difference frequency generation converted optical pulse train, stretching the pulse width to 10 ps or 20 ps before passing it. If chirp is not necessary, the optical path can be switched so that the difference frequency generation converted optical pulse train does not pass through the dispersive medium. The dispersive medium consists of at least one of an optical fiber, a dispersion compensation mirror, or a prism pair. With an optical fiber, two different lengths or types can be selected, with a dispersion compensation mirror the number of bounces can be changed, and with a prism pair the position can be changed.

[0060] One of the two branches of a femtosecond optical pulse train output from a mode-locked laser is converted into an SC optical pulse train by a PM-AND-HNL fiber. As previously reported, when clean femtosecond optical pulses without pedestals are coupled into a PM-AND-HNL fiber, SC optical pulses with a good signal-to-noise ratio and no spectral peaks can be obtained (e.g., Non-Patent Documents 10 and 11). Compared with SC light generated using anomalous dispersion fiber or fibers with zero dispersion, SC light from an all-normal dispersion fiber improves the signal-to-noise ratio of a CARS microscope, enabling high-resolution, high-speed measurements.

[0061] For example, in an existing report, a simulation of SC light generated by a PM-AND-HNL fiber is presented (see, for example, Non-Patent Document 11), which shows that SC light is generated in the range of approximately 0.8 μm to 1.4 μm for a pump light of 1.04 μm. This is approximately 5000 cm -1 Assuming that the central wavelength of the wavelength band of the SC light and the pump light wavelength are the same, the wavelength band is 5000 cm-1 In this case, if the wavelength of the pump light (or probe light) is 1.06 μm, the central wavelength λ of the pump light pulse for SC generation is s should be set in the wavelength range from 1.26 μm to 1.53 μm. s is 1.42 μm, which is an appropriate value.

[0062] The SC optical pulse train output from the PM-AND-HNL fiber passes through a dispersive medium and is adjusted to have approximately the same pulse width as the picosecond pulse train. Note that the picosecond pulse train and the femtosecond optical pulse train can be synchronized by adjusting the optical path length of the other branched femtosecond optical pulse train or the optical path length of the picosecond pulse train between the demultiplexer and the microscope.

[0063] The light source of this embodiment may be used for spectroscopic measurements and spectroscopic microscope measurements using coherent Raman scattering (e.g., stimulated Raman gain, stimulated Raman loss, etc.) other than CARS measurement microscopes. Also, the light source may be used as a multimodal nonlinear optical microscope by measuring SHG and THG together with the CARS signal. [Industrial Applicability]

[0064] As described above, the light source disclosed herein allows for variable pulse widths of the pump (or probe) and Stokes beams, thereby maintaining the average power per pixel. Furthermore, 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 to the wavelength range required for CARS measurements. Therefore, this method is expected to be applied in the medical and industrial fields as an analytical and observation technique that applies Raman scattering spectroscopy, which has high robustness for various types of samples (especially biological tissue samples).

Claims

1. 1. A light source for Raman scattering spectroscopy, comprising: center wavelength λ ｓ a mode-locked laser that outputs a femtosecond optical pulse train of a demultiplexer that splits the femtosecond optical pulse train into two lines, a first femtosecond optical pulse train and a second femtosecond optical pulse train, in terms of power; a steady-state oscillation solid-state laser that outputs continuous light; a first combiner that transmits the continuous light output from the steady-state oscillation solid-state laser, reflects the first femtosecond optical pulse train, and outputs the continuous light and the first femtosecond optical pulse train coaxially; The difference frequency generation between the continuous light and the first femtosecond optical pulse train generates a pulse having a center wavelength λ ｃ a second-order nonlinear optical element including at least one wavelength conversion element for outputting a difference frequency generated converted optical pulse train consisting of picosecond optical pulses of an amplifier that amplifies the difference frequency generated converted optical pulse train; a polarization-maintaining all-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 supercontinuum optical pulse train into a pulse width that is substantially the same as the pulse width of the difference frequency generation converted optical pulse train output from the amplifier; a second combiner that combines the difference frequency generation converted optical pulse train output from the amplifier and the supercontinuum optical pulse train output from the first dispersion medium and outputs the combined signal; Equipped with The center wavelength λ 1 is set so that coherent anti-Stokes Raman scattering measurement can be performed using the difference frequency generation converted optical pulse train and the supercontinuum optical pulse train output from the amplifier. ｓ and the central wavelength λ ｃ The light source to be set.

2. The second-order nonlinear optical element is A plurality of the wavelength conversion elements having different lengths; a switch mechanism that is installed on the input side of the plurality of wavelength conversion elements and guides input light to a specific wavelength conversion element; The light source of claim 1 further comprising:

3. 3. The light source according to claim 1, further comprising a second dispersion medium disposed between the second-order nonlinear optical element and the amplifier, for extending the pulse width of the difference frequency generated converted optical pulse train output from the second-order nonlinear optical element by applying chirp to the difference frequency generated converted optical pulse train.

4. The laser medium of the mode-locked laser is Cr ４＋ : YAG, Cr forsterite, Ti sapphire, Cr:LiSAF, Cr:LiCAF, Cr:ZnSe, or Cr:ZnS, YAG or YVO doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho, etc. ４ 10. The light source of claim 1, which is either glass (bulk and fiber) or semiconductor crystal.

5. 2. The light source of claim 1, wherein the gain medium constituting the mode-locked laser is in the form of a rod, a disk, or a fiber.

6. 2. The light source of claim 1, wherein the amplifier is either a glass fiber amplifier doped with one rare earth ion selected from the group consisting of Yb, Er, Nd, Tm, and Ho, or a single crystal fiber amplifier partially doped with one rare earth ion selected from the group consisting of Yb, Er, Nd, Tm, and Ho.

7. 2. The light source of claim 1, wherein the steady-state solid-state laser is any one of 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, and a semiconductor laser.

8. 2. The light source of claim 1, wherein the wavelength conversion element included in the second-order nonlinear optical element is one of periodically poled lithium niobate, periodically poled lithium tantalate, and periodically poled KTP crystal.

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