Dual wavenumber band light source

Abstract

The present disclosure provides a light source for a coherent Raman scattering microscope at wavelengths of 1,000 nm or more with low scattering in a living body, said light source supporting measurements in both a Raman tag wavenumber range and a high wavenumber range.　Specifically, the present disclosure provides a light source (400) comprising: a mode-locked laser (1) having a function for outputting two different optical pulse trains with different center wavelengths λs; two steady-state oscillating solid-state lasers (3, 4) that output continuous light of two different oscillation wavelengths λp corresponding to the two different optical pulse trains with different center wavelengths λs; and two or one second-order nonlinear optical elements (8, 9) that convert combined light into a picosecond optical pulse train with a center wavelength λc, i.e., a difference-frequency generation conversion optical pulse train. The center wavelengths λs of the optical pulse trains, the oscillation wavelengths λp of the steady-state oscillating solid-state lasers, and the center wavelength λc of the difference-frequency generation conversion optical pulse train satisfy the relation 1 / λc = 1 / λp − 1 / λs.

Description

Dual waveband light source

[0001] The present disclosure relates to a light source used in a coherent Raman scattering (CRS) microscope that detects stimulated Raman scattering (SRS), coherent anti-Stokes Raman scattering (CARS), and the like, primarily targeting cells and biological tissues as measurement targets.

[0002] A widely used research technique in medicine and life sciences involves attaching fluorescent dyes to specific molecules within cells or biological tissues and then capturing fluorescence microscopy images to visualize their distribution. However, the broad emission bands of many fluorescent dyes limit the number of molecules that can be simultaneously observed. Furthermore, fluorescent dyes require relatively large molecular sizes, and attaching them to small molecules alters their properties, preventing their use in small molecule observations. Raman tags address these issues, and alkynes (single triple bond) or polyynes (alternating single and triple bonds) are currently being developed (see Non-Patent Documents 1 and 2). These small molecule tags are expected to enable simultaneous observation of up to 20 different molecules, compared to fluorescent dyes.

[0003] 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, and is also widely used in medicine and industry. Spontaneous Raman scattering is 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 very little power relative to the original incident light power, a light source with high incident power is required to obtain scattered light measurable by a detector. However, most measurement samples have an upper limit on the average 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 long measurement times to obtain a signal with a high signal-to-noise ratio. In contrast, CRS is a nonlinear optical process using a light source with high instantaneous power. Therefore, compared to spontaneous Raman scattering, the power of Raman scattered light is significantly stronger than that of spontaneous Raman scattering when using a light source with the same level of average power, resulting in shorter measurement times.

[0004] The Raman tags of alkynes and polyynes are in the 2000-2300 cm range, a range where other biological organic molecules do not have peaks. －１ When actually obtaining a microscope image, in addition to the above wavenumber bands, a wavenumber of 2800 cm is used, which is a wavenumber at which strong signals from lipids and proteins due to carbon-hydrogen (C-H) bonds, nitrogen-hydrogen (N-H) bonds, or oxygen-hydrogen (O-H) bonds can be obtained. －１ From 3100 cm －１ It is desirable to observe the high wavenumber region of the spectrum and understand the shape of cells and tissues. For this purpose, pulsed light with a time width of a few picoseconds in two wavelength ranges, around 800-850 nm and around 1030 nm, has traditionally been used. The equipment used is a Ti-doped sapphire laser in the 800 nm band and a Yb-doped glass fiber laser in the 1000 nm band.

[0005] FIG. 1 shows the configuration of a conventional laser light source for an SRS microscope, as described in Non-Patent Documents 3 and 4. The conventional light source shown in FIG. 1 includes a Ti-doped sapphire picosecond pulse laser 101 with a wavelength in the 800 nm band and a Yb-doped glass picosecond pulse laser 104 with a wavelength in the 1000 nm band. The Ti-doped sapphire picosecond pulse laser 101 generates a picosecond optical pulse train 103 with a repetition rate of 76 MHz. The Yb-doped glass picosecond pulse laser 104 generates a picosecond optical pulse train 105 with a repetition rate of 38 MHz. The two picosecond pulse lasers are coupled via an electrical signal path 111, and their pulse timing is synchronized. The two pulsed beams are combined by a dichroic mirror 112 and input to a microscope 102 via the same optical path. CRS measurement is performed by simultaneously irradiating the two pulsed beams onto a sample to be measured. The Ti-doped sapphire picosecond pulse laser 101 switches the oscillation wavelength between 790 nm and 843 nm, and can measure the Raman tag wavenumber range of 2000-2300 cm －１ and the high wavenumber range of 2800-3100 cm －１ The Yb-doped glass picosecond pulse laser 104 can achieve a wavelength of 300 cm by sweeping the wavelength between 1014 nm and 1046 nm. －１ This enables measurement of the wavenumber range.

[0006] 2 is a diagram showing an energy diagram of molecules of a sample to be measured when measuring SRS. １ ), Stokes light 107 (angular frequency ω ２ If the correlation between the two lasers is modulated when the laser beam is incident on the measurement sample, modulation via SRS occurs in the transmitted light. Measuring this modulation makes it possible to obtain an SRS microscope image. In the conventional example, the Ti-doped sapphire picosecond pulse laser 101 has a repetition rate of 76 MHz, while the Yb-doped glass picosecond pulse laser 104 has a repetition rate of 38 MHz. Therefore, a 38 MHz modulation via SRS appears in the transmitted light of the Ti-doped sapphire picosecond pulse laser 101. This is detected by a lock-in detection system.

[0007] However, since light with wavelengths in the vicinity of 800-850 nm is significantly scattered by living organisms, it is necessary to use a light source with a longer wavelength that scatters less in living organisms in order to improve measurement sensitivity.

[0008] JP 2014-010171 A International Publication No. 2023 / 243052 A

[0009] Bakthavatsalam, Subha, et al. “A decade of alkyne-tag Raman imaging (ATRI): applications in biological systems.” RSC Chemical Biology 2.5 (2021): 1415-1429.Hu, Fanghao, et al. “Supermultiplexed optical imaging and barcoding with engineered polyynes.” Nature methods 15.3 (2018): 194-200.Ozeki, Yasuyuki, et al. “Multicolor stimulated Raman scattering microscopy with fast wavelength-tunable Yb fiber laser.” IEEE Journal of selected topics in quantum electronics 25.1 (2018): 1-11.Shou, Jingwen, et al. “Super-multiplex imaging of cellular dynamics and heterogeneity by integrated stimulated Raman and fluorescence microscopy.” iScience, 24. 8 (2021): 102832.De la Cadena, Alejandro, et al. “Broadband stimulated Raman imaging based on multi-channel lock-in detection for spectral histopathology.” APL Photonics 7.7 (2022): 076104. Yoshihide Kana. “Molecular imaging of living cells by non-linear Raman spectroscopy.” Journal of Japanese Biochemical Society 91.6 (2019): 820-828.Ripin, D. J., et al.“Generation of 20-fs pulses by a prismless Cr4+: YAG laser.” Optics Letters 27.1 (2002): 61-63.Naumov, S., et al. Nai-ben Ming. “Quasi-phase-matched third-harmonic generation in a quasi-periodic optical superlattice.” Science 278.5339 (1997): 843-846. Zhang, Li, et al. “Second-harmonic and cascaded third-harmonic generation in generalized quasiperiodic poled lithium niobate waveguides.” Optics Letters 48.7 (2023): 1906-1909.

[0010] The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a light source that uses only light having a wavelength of 1000 nm or more and that scatters less in living organisms than conventional technology.

[0011] According to one embodiment of the present disclosure, a light source includes a mode-locked laser having a function of outputting two types of optical pulse trains having different center wavelengths λs, a first optical element that splits the optical pulse train into two optical pulse trains, two steady-state oscillation solid-state lasers that output continuous light of two different oscillation wavelengths λp corresponding to the two types of optical pulse trains having different center wavelengths λs, and two optical multiplexers that combine one of the two split optical pulse trains with one of the two continuous light of two different oscillation wavelengths λp corresponding to the center wavelength λs of one of the optical pulse trains. a second optical element for converting the light beams combined by the two second optical elements into a difference frequency generated converted optical pulse train, which is a picosecond optical pulse train with a center wavelength λc; and a third optical element for combining the difference frequency generated converted optical pulse train with the other of the two branched optical pulse trains, wherein the center wavelength λs of the optical pulse trains, the oscillation wavelength λp of the steady state oscillation solid-state laser, and the center wavelength λc of the difference frequency generated converted optical pulse train satisfy the relationship 1 / λc=1 / λp-1 / λs.

[0012] According to one embodiment of the present disclosure, a light source includes a mode-locked laser having a function of outputting two types of optical pulse trains having different center wavelengths λs, a first optical element that branches the optical pulse train into two optical pulse trains, two steady-state oscillation solid-state lasers that output continuous light of two different oscillation wavelengths λp corresponding to the two types of optical pulse trains having different center wavelengths λs, a second optical element that combines one of the two branched optical pulse trains with one of the two continuous light of two different oscillation wavelengths λp corresponding to the center wavelength λs of one of the optical pulse trains, and a difference frequency generation conversion optical pulse that converts the combined light into a picosecond optical pulse train with a center wavelength λc. a second-order nonlinear optical element configured to quasi-phase match with any combination of two different center wavelengths λs output from a mode-locked laser and the oscillation wavelength λp of continuous light corresponding to the two different center wavelengths λs; and a third optical element that combines the converted optical pulse train by difference frequency generation with the other of the two branched optical pulse trains, wherein the center wavelength λs of the optical pulse train, the oscillation wavelength λp of the steady-state oscillation solid-state laser, and the center wavelength λc of the converted optical pulse train by difference frequency generation satisfy the relationship 1 / λc=1 / λp-1 / λs.

[0013] By using a CRS light source that emits only light with a wavelength of 1000 nm or more, which corresponds to measurements in both the wavenumber range and the high wavenumber range of the Raman tag in the present disclosure, highly sensitive measurements with little scattering in living organisms are possible.

[0014] Fig. 1 is a schematic diagram showing a light source for a conventional CRS microscope. Fig. 2 is a diagram showing an energy diagram of a molecule to be measured in stimulated Raman scattering (SRS) that measures multiple wavenumbers. Fig. 3 is a diagram showing an energy diagram of a molecule to be measured in coherent anti-Stokes Raman scattering (CARS) that measures multiple wavenumbers. Fig. 4 is a first schematic diagram showing a light source for a CRS microscope according to an embodiment of the present disclosure. Fig. 5 is a second schematic diagram showing a light source for a CRS microscope according to an embodiment of the present disclosure.

[0015] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The numerical values ​​and materials used in the following description are examples, and the present invention can be implemented using other numerical values ​​and materials without departing from the scope of the present invention.

[0016] The light sources 400 and 500 of this embodiment emit light in the wavenumber range of 2000-2300 cm of the Raman tag in the CRS microscope. －１ and the high wavenumber range of 2800-3100 cm －１ As a result, the light source of this embodiment can be used in both SRS and CARS as nonlinear optical processes in a CRS microscope.

[0017] In the case of the above-mentioned SRS, the pump light 106 (with angular frequency ω １ ) or Stokes light 107 (angular frequency ω ２ When detecting ions, it is possible to use a lock-in detection system with several tens of multi-channels (see Non-Patent Document 5). －１ or less), one is wideband (300cm －１ (or higher), and the wave number is 2000-2300 cm －１ and wave numbers 2800-3100 cm －１ One of the light sources should have a wavelength switching function so that nearby measurements can be changed.

[0018] 3 is a diagram showing an energy diagram of a molecule of a sample to be measured when CARS is measured. As shown in FIG. 3, when CARS is measured, the pump light 106 (with an angular frequency ω １ ), Stokes light 107 (angular frequency ω ２ ), probe light 108 (angular frequency ω ３ ) is incident on the target molecule, the CARS light 109 (with angular frequency ω CARS ) occurs (see Non-Patent Document 6). As in the case of SRS, the light source is a narrow-band (5 cm) light source that serves as both the pump light 106 and the probe light 108. －１ Stokes light 107 is broadband (300 cm －１(or higher), and the wave number is 2000-2300 cm －１ Measurements in the vicinity and wavenumbers 2800-3100 cm －１ One of the light sources is provided with a wavelength switching function so that nearby measurements can be changed.

[0019] That is, the light source of this embodiment can be used for both SRS and CARS. However, when used for SRS, the pump light 106 (with angular frequency ω １ ) and Stokes light 107 (angular frequency ω ２ ) must be modulated before it is incident on the measurement sample.

[0020] 4 shows a measurement system (e.g., a spectroscopic microscope) including a light source 400 according to an embodiment of the present disclosure, and in particular, the light source 400 according to this embodiment is indicated within a dashed line. The light source 400 includes a mode-locked laser 1 capable of switching between and outputting two types of femtosecond optical pulse trains having different center wavelengths λs, two steady-state (CW) solid-state lasers 3 and 4 that output continuous light with different oscillation wavelengths λp and steady powers, two second-order nonlinear optical elements 8 and 9 for generating a difference frequency corresponding to the difference frequency between the switched femtosecond optical pulse train with the center wavelength λs and the continuous light with the corresponding oscillation wavelength λp, and an optical amplifier 15 that amplifies the difference frequency generated optical pulse train output from the second-order nonlinear optical elements.

[0021] Hereinafter, a configuration will be described in which two types of femtosecond optical pulse trains with different center wavelengths λs output from a mode-locked laser are converted into pump light with the same center wavelength λc.

[0022] (Mode-locked laser) The mode-locked laser 1 is a Cr laser whose central wavelength λs can be switched between a predetermined wavelength, in this embodiment, around 1380 nm and around 1550 nm. ４＋ The mode-locked laser 1 is a YAG single crystal fiber mode-locked laser (see Non-Patent Documents 7 and 8). The mode-locked laser 1 may have an internal switching mechanism for switching between a dispersion compensation medium and a saturable absorber mirror set to enable the center wavelength λs to be changed.

[0023] Although not shown in FIG. 4, the mode-locked laser 1 specifically includes a Cr４＋ The mode-locked laser 1 comprises a YAG single crystal fiber, a dispersion compensation medium (specifically, a silica glass rod and a dispersion compensation mirror), and a saturable absorber mirror (specifically, a semiconductor saturable absorber mirror). The mode-locked laser 1 is an external resonator type composed of a single crystal fiber waveguide and a spatial optical system, and an optical path switching mirror is installed midway through the optical path of the spatial optical system. A saturable absorber mirror is arranged on one optical path from the switching mirror to generate pulses with a central wavelength, for example, 1380 nm. A dispersion compensation mirror and a saturable absorber mirror are arranged on the other optical path from the switching mirror to generate pulses with a central wavelength, for example, 1550 nm. The switching mirror can be switched by an electrical signal as required. The laser configuration is similar to that described in Patent Document 1, ４＋ It is similar to a YAG single crystal fiber laser.

[0024] Cr ４＋ The oscillation pulse width of the YAG single crystal fiber mode-locked laser 1 can be set to 50 fs or less, and in this embodiment, it is about 40 fs. This pulse light has a wavenumber width of 300 cm －１ It has an energy band roughly corresponding to

[0025] A femtosecond optical pulse train 5 output from a mode-locked laser 1 is split into two optical pulse trains by a first optical element 10, such as a semi-transparent mirror or a beam splitter, formed on the output side. One of the optical pulse trains is used for difference frequency generation, and the generated difference frequency is amplified by an optical amplifier to become pump light. The other optical pulse train has its pulse width adjusted to become Stokes light. Finally, both optical pulse trains (pump light and Stokes light) are combined and output to a CRS microscope 2.

[0026] In order to wavelength-convert two types of femtosecond optical pulse trains 5 having different center wavelengths λs output from a mode-locked laser 1 into pump light having the same center wavelength λc, two steady-state oscillation solid-state lasers 3 and 4 having different oscillation wavelengths λp and two corresponding second-order nonlinear optical elements 8 and 9 for difference frequency generation are used. The steady-state oscillation solid-state lasers 3 and 4 and the second-order nonlinear optical elements 8 and 9 will be described below.

[0027] (Steady-State Oscillation Solid-State Laser) In order to convert femtosecond optical pulse trains 5 near 1380 nm and 1550 nm into pump light of the same center wavelength λc (1052 nm in this embodiment), two steady-state oscillation solid-state lasers 3, 4 are provided, each corresponding to a respective center wavelength λs. Of the two steady-state oscillation solid-state lasers 3, 4, one (first) steady-state oscillation solid-state laser 3 outputs continuous light 6 with an oscillation wavelength λp of 597 nm, and the other (second) steady-state oscillation solid-state laser 4 outputs continuous light 7 with an oscillation wavelength λp of 627 nm. In this embodiment, a diode-pumped solid-state (DPSS) laser is used as the steady-state oscillation solid-state laser.

[0028] The continuous light beams 6 and 7 output from the steady-state oscillation solid-state lasers 3 and 4 are multiplexed with the femtosecond optical pulse train 5 output from the mode-locked laser 1 to generate a difference frequency optical pulse train. Each of the steady-state oscillation solid-state lasers 3 and 4 is equipped with a mechanical shutter or the like configured to selectively output continuous light in accordance with the center wavelength λs of the femtosecond optical pulse train 5 output from the mode-locked laser 1.

[0029] Furthermore, two second optical elements 11 and 12 are arranged on the output sides of the steady-state oscillation solid-state lasers 3 and 4 to combine the continuous light and the femtosecond optical pulse train 5 output from the mode-locked laser 1 and input the combined light to the second-order nonlinear optical elements 8 and 9. As an example, a dichroic mirror can be used for the optical element 11, and an optical path switching mirror can be used for the optical element 12.

[0030] When the femtosecond optical pulse train 5 has a center wavelength of 1380 nm, the femtosecond optical pulse train 5 passes through an optical element 12 arranged on the output side of the second steady-state oscillation solid-state laser 4. The transmitted femtosecond optical pulse train 5 is then multiplexed with continuous light 6 having a wavelength of 597 nm by an optical element 11 arranged on the output side of the first steady-state oscillation solid-state laser 3, and the multiplexed light is input coaxially to a first second-order nonlinear optical element 8.

[0031] Furthermore, when the femtosecond optical pulse train 5 has a central wavelength of 1550 nm, the optical element 12 arranged on the output side of the second steady-state oscillation solid-state laser 4 is switched to reflection for the femtosecond optical pulse train 5. The reflected femtosecond optical pulse train 5 is multiplexed with the continuous light 7 having a wavelength of 627 nm, and the combined light is input coaxially to the second second-order nonlinear optical element 9.

[0032] (Second-order nonlinear optical element) The difference frequency optical pulse train 6, which is obtained by combining the femtosecond optical pulse train 5 and the continuous light from the first steady-state oscillation solid-state laser 3, is incident on a first second-order nonlinear optical element 8 and can be converted into a difference frequency generated converted optical pulse train 13 having a center wavelength of 1052 nm as a difference frequency. The first second-order nonlinear optical element 8 generally uses poled lithium niobate (PPLN), but is not limited to PPLN. In this case, the pulse width of the difference frequency generated converted optical pulse train 13 is 2.5 ps.

[0033] Furthermore, the difference frequency optical pulse train 7, which is obtained by combining the femtosecond optical pulse train 5 and the continuous light from the second steady-state solid-state laser 4, is incident on a second second-order nonlinear optical element 9 and can be converted into a difference frequency generated converted optical pulse train 14 having a center wavelength of 1052 nm as a difference frequency. Similarly, the second second-order nonlinear optical element 9 generally uses poled lithium niobate (PPLN), but is not limited to PPLN. In this case, the pulse width of the difference frequency generated converted optical pulse train 13 is 2.5 ps. Note that the inversion period and crystal length of the PPLN for appropriate difference frequency generation are set using the same method as in Patent Document 2.

[0034] The central wavelength λs of the femtosecond optical pulse train 5, the oscillation wavelength λp of the steady state oscillation solid state lasers 3 and 4, and the central wavelength λc of the difference frequency generation converted optical pulse trains 13 and 14 are numerical values ​​that satisfy the following formula 1.

[0035] The difference frequency generation converted optical pulse train 14 output from the second second-order nonlinear optical element 9 passes through a total reflection mirror 20 and reaches an optical path switching mirror 22. The optical path switching mirror 22 transmits the femtosecond optical pulse train 5 when the central wavelength is 1380 nm, and reflects it when the central wavelength is 1550 nm, and in either case, emits the difference frequency generation converted optical pulse train 13 or 14 from the same position and in the same direction.

[0036] The difference frequency generated converted optical pulse train 13 or 14 is amplified by a Yb glass fiber optical amplifier 15 to an average output of about 10 mW to 1 W, and is input to the microscope 2 as pump light 17 .

[0037] A configuration in which a femtosecond optical pulse train 5 having two central wavelengths λs is input to the microscope 2 as Stokes light will be described below.

[0038] The other optical pulse train, which is branched by the first optical element 10 from the femtosecond optical pulse train 5 output from the mode-locked laser 1, is chirped by a dispersion medium 16 arranged in the propagation direction of the mode-locked laser 1 and converted into Stokes light 18 with a pulse width of about 2.5 ps.

[0039] The Stokes beam 18 passes through a total reflection mirror 21 arranged in the optical path, is combined with the pump beam 17 by a dichroic mirror 19, and is output coaxially toward the CRS microscope 2. At this time, an appropriate adjustment mechanism for matching the timing of the pulses of the pump beam 17 and the Stokes beam 18 may be installed in the optical path.

[0040] The light source shown in embodiment 1 enables highly sensitive measurements with little scattering in living organisms by using a CRS light source with a wavelength of 1000 nm or more that is compatible with measurements in both the wavenumber range of the Raman tag and the high wavenumber range.

[0041] (Embodiment 2) In the above-described embodiment 1, a light source using multiple second-order nonlinear optical elements 8 and 9 was described in order to obtain a difference frequency generated optical pulse train with the same center wavelength λc from a mode-locked laser that outputs two types of optical pulse trains with different center wavelengths λs. However, in the case of embodiment 1, optical elements 12 and 22 that switch the optical path in accordance with the center wavelength λs are required, which may impair the stability of the pump light 17. Therefore, in this embodiment, a part of the configuration of embodiment 1 is modified to provide a light source with higher output stability.

[0042] FIG. 5 illustrates a measurement system including a light source 500 in accordance with one embodiment of the present disclosure, in particular the light source 500 in this embodiment shown within a dashed line.

[0043] 4 is the difference frequency generating section, and the second-order nonlinear optical element 8 employs a polarization inversion structure (a superlattice structure in this embodiment) designed to achieve quasi-phase matching whether the central wavelength λs of the femtosecond optical pulse train 5 from the mode-locked laser 1 is 1380 nm or 1550 nm. The polarization inversion structure of the second-order nonlinear optical element 8 used in this embodiment is the same as the quasi-periodic structure disclosed in Non-Patent Documents 9 and 10.

[0044] In the second embodiment, when the femtosecond optical pulse train 5 has a center wavelength of 1380 nm, the continuous light 6 from the first steady-state oscillation solid-state laser 3 having a wavelength of 597 nm passes through the dichroic mirror 23, and is combined by the second optical element 11 with one of the optical pulse trains branched by the first optical element 10 formed on the output side of the mode-locked laser 1, and then input to the second-order nonlinear optical element 8.

[0045] Furthermore, when the femtosecond optical pulse train 5 has a central wavelength of 1550 nm, the continuous light 7 from the second steady-state oscillation solid-state laser 4 having a wavelength of 627 nm is reflected by the dichroic mirror 23, and is combined by the second optical element 11 with one of the optical pulse trains branched by the first optical element 10 formed on the output side of the mode-locked laser 1, and then input to the second-order nonlinear optical element 8.

[0046] As a result, similar to the first embodiment, the femtosecond optical pulse train 5 can be converted into a difference frequency generated converted optical pulse train 13 with a center wavelength of 1052 nm in either case where the center wavelength is 1380 nm or 1550 nm. Therefore, there is no need to selectively move the switching mirror as described above, and a light source that can provide a more stable output than the first embodiment can be expected.

[0047] In embodiments 1 and 2, Cr ４＋ The light source shown here is a combination of a YAG single crystal fiber mode-locked laser 1 and a Yb glass fiber optical amplifier 15. However, this is only an example, and the mode-locked laser 1 can be replaced with a Cr ４＋: YAG, Cr-forsterite, Ti-sapphire, Cr:LiSAF, Cr:LiCAF, Cr:ZnSe, and Cr:ZnS, or YAG doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho, or YVO4 doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho ４ The mode-locked laser 1 may be a laser medium made of a crystal, ceramics, or glass doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho, or a semiconductor crystal. The laser medium of the mode-locked laser 1 may be a rod, disk, or fiber. While the pulse train output from the mode-locked laser 1 has a pulse width of about 40 fs, this is merely an example, and the laser may have a shorter pulse width or a longer pulse width, including a pulse width of 1 picosecond or more.

[0048] In addition, in the first and second embodiments, the optical amplifier 15 may be either a glass fiber optical amplifier doped with one rare earth ion selected from Yb, Er, Nd, Tm, Ho, etc., or a single crystal fiber optical amplifier doped with one rare earth ion selected from Yb, Er, Nd, Tm, Ho, etc., as needed.

[0049] In addition, in embodiments 1 and 2, the steady-state oscillation solid-state lasers 3 and 4 can be one steady-state oscillation solid-state laser selected from 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.

[0050] Furthermore, in the first and second embodiments, the second-order nonlinear optical element 8 is made of PPLN, but it may be made of other materials such as periodically poled lithium tantalate and periodically poled KTP.

[0051] Additional Considerations The foregoing description of embodiments of the present invention has been presented for purposes of illustration and is not intended to be exhaustive or to be limited to the precise form disclosed. Those skilled in the art will recognize that many modifications and variations are possible in light of the above disclosure.

[0052] Finally, the language used herein has been selected primarily for readability and instructional purposes, and may not have been selected to delineate or limit the subject matter of the invention. Accordingly, it is intended that the scope of the invention be limited not by this detailed description, but rather by the appended claims. Accordingly, the disclosure of embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the claims.

[0053] 101 Ti-doped sapphire picosecond pulse laser 102 Microscope 103, 105 Picosecond optical pulse train 104 Yb-doped glass picosecond pulse laser 106, 17 Pump light 107, 18 Stokes light 108 Probe light 109 CARS light 110 Vibration mode 111 Electrical signal path 112 Dichroic mirror 400, 500 Light source 1 Mode-locked laser 2 Microscope 3, 4 Steady-state oscillation solid-state laser 5 Femtosecond optical pulse train 6, 7 Continuous light 8, 9 Second-order nonlinear optical element 10-12, 19-23 Optical element 13, 14 Difference frequency generation converted optical pulse train 15 Optical amplifier 16 Dispersion medium

Claims

1. A light source comprising: a mode-locked laser capable of outputting two types of optical pulse trains having different center wavelengths λs; a first optical element that splits the optical pulse train into two optical pulse trains; two continuously oscillating solid-state lasers that output continuous light of two different oscillation wavelengths λp corresponding to the two types of optical pulse trains having different center wavelengths λs; two second optical elements that respectively combine one of the two split optical pulse trains with one of the continuous light of the two different oscillation wavelengths λp corresponding to the center wavelength λs of one of the optical pulse trains; two second-order nonlinear optical elements that convert the light combined by each of the two second optical elements into a difference frequency generation converted optical pulse train that is a picosecond optical pulse train with a center wavelength λc; and a third optical element that combines the difference frequency generation converted optical pulse train with the other of the two split optical pulse trains, a central wavelength λs of the optical pulse train, an oscillation wavelength λp of the steady-state oscillation solid-state laser, and a central wavelength λc of the difference frequency generated converted optical pulse train satisfy the relationship 1 / λc=1 / λp-1 / λs.

2. A light source comprising: a mode-locked laser capable of outputting two types of optical pulse trains having different center wavelengths λs; a first optical element that splits the optical pulse train into two optical pulse trains; two steady-state oscillation solid-state lasers that output continuous light of two different oscillation wavelengths λp corresponding to the two types of optical pulse trains having different center wavelengths λs; a second optical element that combines one of the two split optical pulse trains with one of the continuous light of the two different oscillation wavelengths λp corresponding to the center wavelength λs of one of the optical pulse trains; and a second-order nonlinear optical element that converts the combined light into a difference frequency generated optical pulse train that is a picosecond optical pulse train with a center wavelength λc, the second-order nonlinear optical element being configured to quasi-phase match any combination of the two different center wavelengths λs output from the mode-locked laser and the oscillation wavelength λp of the continuous light corresponding to the two different center wavelengths λs. a third optical element that combines the converted optical pulse train by difference frequency generation with the other of the two branched optical pulse trains, wherein a central wavelength λs of the optical pulse train, an oscillation wavelength λp of the steady-state oscillation solid-state laser, and a central wavelength λc of the converted optical pulse train by difference frequency generation satisfy the relationship 1 / λc=1 / λp-1 / λs.

3. A light source according to claim 1 or 2, comprising an optical amplifier for amplifying the difference frequency generated converted optical pulse train.

4. A light source according to claim 1 or 2, wherein the optical pulse train is a femtosecond optical pulse train.

5. A light source according to claim 1 or 2, further comprising a dispersive medium that chirps the other of the two branched optical pulse trains to extend the pulse width.

6. The light source according to claim 1 or 2, wherein the mode-locked laser uses one laser medium selected from the group consisting of: a crystal or ceramic selected from Cr4+:YAG, Cr-forsterite, Ti-sapphire, Cr:LiSAF, Cr:LiCAF, Cr:ZnSe, and Cr:ZnS; a YAG crystal or ceramic doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho; a YVO4 crystal or ceramic doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho; a glass doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho; or a semiconductor crystal.

7. The light source according to claim 6, wherein the shape of the laser medium is one selected from the group consisting of a rod, a disk, and a fiber.

8. The light source according to claim 3, wherein the optical amplifier is one selected from the group consisting of: a glass fiber optical amplifier doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho; or a single crystal fiber optical amplifier doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho.

9. The light source according to claim 1 or 2, wherein the steady-state solid-state laser is one selected from the group consisting 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.

10. The light source according to claim 1 or 2, wherein the second-order nonlinear optical element is one second-order nonlinear optical element selected from the group consisting of periodically poled lithium niobate, periodically poled lithium tantalate, and periodically poled KTP.

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