Synchronous pulse laser light source

Abstract

The purpose of the present disclosure is to provide, in order to solve the problem, a light source for coherent Raman scattering corresponding to the measurement of a fingerprint region, the light source using pulse light in a two-wavelength range through combined use of a Nd-doped gain medium mode synchronous laser and a Cr4+:YAG single crystal fiber laser. One embodiment for achieving such a purpose provides a light source comprising: a first laser configured to output an optical pulse train having a center wavelength λp; and a second laser which includes a resonator formed by a plurality of mirrors and is configured to oscillate in synchronization with one optical pulse train branched by a first optical element, the resonator including a gain medium configured to output oscillation light having a center wavelength λf, and a second-order nonlinear optical medium operating as a Kerr lens that changes a refractive index according to the intensity of the oscillation light.

Description

Synchronous pulsed laser source

[0001] The present disclosure relates to light sources used in nonlinear spectroscopy and nonlinear optical microscopy that detect coherent Raman scattering (CRS), such as Coherent Anti-Stokes Raman Scattering (CARS) and Stimulated Raman Scattering (SRS).

[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. Spontaneous Raman scattering is a phenomenon that produces scattered light whose frequency is 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 high-power incident light source is required to obtain measurable scattered light with a detector. However, most measurement samples have an upper limit on 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, and obtaining a signal with a high signal-to-noise ratio requires significantly longer measurement times than CARS measurements. In contrast, CARS is a nonlinear optical process using a light source with high instantaneous power. Therefore, compared to spontaneous Raman scattering, the power of the Raman scattered light is significantly stronger than that of spontaneous Raman scattering when using a light source of equivalent power, resulting in shorter measurement times. With the development of pulsed lasers used as light sources, CARS measurements have made remarkable progress, and their effectiveness is particularly notable when acquiring microscopic images. When measuring living organisms using Raman scattering spectroscopy, the important wavenumber range is the 500 cm wavenumber range. -1 From 1800 cm -1 Among the CARS microscopes currently reported, the one using a supercontinuum (SC) light source can simultaneously obtain Raman scattering spectra over the widest wavenumber band (see Non-Patent Document 1).

[0003] Figure 1 shows the configuration of a conventional CARS microscope light source described in Non-Patent Document 1. The light source 101 amplifies laser light with a wavelength in the 1 μm band, a pulse width of 50 ps, ​​and a repetition rate of 1 MHz using a Yb-doped glass fiber amplifier, and outputs an average output of approximately 2 W. The amplified light is divided into two parts: one picosecond optical pulse train 103 is used as pump light and probe light, and the other part generates an SC optical pulse train 105 via a photonic crystal fiber 104. Light with a wavelength of 1.1-1.8 μm, from broadband continuous light spanning from visible light to infrared light, is used as Stokes light in the CARS microscope 102 together with the picosecond optical pulse train 103. When measuring CARS spectroscopy as shown in Figure 2, pump light 106 (angular frequency ω1), Stokes light 107 (angular frequency ω2), and probe light 108 (angular frequency ω3) are incident, thereby generating CARS light 109 (angular frequency ωCARS) corresponding to the angular frequency Ω of a vibrational mode 110 possessed by the molecule to be measured. In the conventional example shown in Figure 1, a picosecond optical pulse train 103 is used as the pump light 106 and probe light 108 (ω1 = ω3), and a broadband SC optical pulse train 105 is used as the Stokes light 107, so that a large number of vibrational modes 110 are excited and a broadband CARS light 109 can be measured, which is called a multiplex CARS process (see Non-Patent Document 1).

[0004] Experiments investigating the invasiveness of ultrashort pulses on living organisms using pulsed light with a pulse width of approximately 100 fs have shown that using light with a wavelength of 1.3 μm, longer than the 1 μm wavelength band, raises the threshold for harmful effects on living organisms and is therefore safer (see Non-Patent Documents 2 and 3). The optimal pulse width for CARS microscopy is approximately 3 ps, which is longer than 100 fs. Even in this case, light with a wavelength of 1.3 μm is estimated to be safer than light in the 1 μm wavelength band. While measuring the high wavenumber region of Raman scattering with a pump light wavelength of 1.3 μm is difficult, measuring the fingerprint region, which can yield important information, is not a problem. To achieve non-invasive measurements on living organisms, a CARS light source with a pump light pulse with a wavelength of approximately 1.3 μm and a Stokes light pulse with a wavelength of 1.3-1.8 μm synchronized with the pump light pulse, is required.

[0005] Hideaki Kano, Biochemistry 91.6 (2019): 820-828. Jayne M. Squirrell et al., “Long-term two-photon fluorescence imaging of mammalian embryos without compromising viability.” Nature biotechnology 17.8 (1999): 763-767. Chi-Kuang Sun et al., “Higher harmonic generation microscopy for developmental biology.” Journal of structural biology 147.1 (2004): 19-30. Shigeo Ishibashi and Kazunori Naganuma., “Mode-locked operation of Cr4+:YAG single-crystal fiber laser with external cavity.” Optics express 22.6 (2014): 6764-6771. F. Lenhardt et al., “888 nm pumped 1342 nm Nd:YVO4 oscillator Kerr-lens mode-locked using cascaded second-order nonlinearities.” Applied Physics B 106 (2012): 5-8. Masaki Asobe et al., “All-optical switching by use of cascading of phase-matched sum-frequency-generation and difference-frequency-generation processes in periodically poled LiNbO3.” Optics letters 22.5 (1997): 274-276. G. I. Stegeman, D. J. Hagan, and L. Torner.“χ(2) cascading phenomena and their applications to all-optical signal processing, mode-locking, pulse compression and solitons.” Optical and Quantum electronics 28 (1996): 1691-1740.

[0006] The object of the present disclosure is to solve the above problems by using a mode-locked laser with an Nd-doped gain medium and a Cr 4+ The present invention provides a CRS light source that is compatible with fingerprint area measurement and uses pulsed light in two wavelength ranges by combining a YAG single crystal fiber laser. According to an embodiment for achieving this object, the light source has a central wavelength λ p a first laser configured to output an optical pulse train having a central wavelength λ; a first optical element arranged to split the optical pulse train into two; and a second laser including a resonator configured with a plurality of mirrors, f a second-order nonlinear optical medium that operates as a Kerr lens for the oscillation light; a cavity length adjustment mechanism provided in either or both of the first and second lasers, the cavity length adjustment mechanism repeatedly matching the frequencies of the first and second lasers through a transmission path; a second optical element configured to convert the other of the optical pulse trains branched by the first optical element into a supercontinuum (SC) optical pulse train; and a third optical element configured to combine the oscillation pulse light output from the cavity with the SC optical pulse train and output the combined result.

[0007] According to one embodiment, the second-order nonlinear optical medium in the light source has a center wavelength λ f The present invention provides a light source including a saturable absorber configured in a resonator such that phase matching is achieved to generate a parametric Kerr lens by cascading for second harmonic generation (SHG) from oscillating light having the above formula, and one of the optical pulse trains split by the first optical element and the oscillating light are focused on the same axis and at the same position, and are passed through or reflected.

[0008] According to another embodiment, the second-order nonlinear optical medium in the light source has a length L, and is configured so that one of the optical pulse trains branched by the first optical element and the oscillation light are incident thereon, and Δk=n SFG / λ SFG -n p / λ p -n f / λ f ΔkL defined by the central wavelength λ of the first laser p and the central wavelength of the oscillation light λ f is set to a value that generates a parametric Kerr lens due to cascading of the sum frequency light, and the wavelength of the sum frequency light is λ SFG is 1 / λ SFG = 1 / λ p +1 / λ f and n p , n f , n SFG are the central wavelengths λ p optical pulse train with center wavelength λ f The wavelength of the sum frequency light is λ SFG is the refractive index of the second-order nonlinear optical medium relative to a light source.

[0009] Furthermore, according to another embodiment, the second-order nonlinear optical medium in the light source has a periodic poling with a length L and a period Λ, and Δk=n SFG / λ SFG -n p / λ p -n f / λ f ΔkL defined by −1 / Λ is the center wavelength λ of the first laser. p and the central wavelength of the oscillation light λ f A light source is provided in which the sum frequency light of Λ is set to a value that produces a Kerr lens by cascading, where Λ is the inversion period.

[0010] Furthermore, according to another embodiment, the second-order nonlinear optical medium is a first second-order nonlinear optical medium, and one optical pulse train is irradiated with a central wavelength λ p a second second-order nonlinear optical medium configured to convert the center wavelength λ 1 / 2 into a second harmonic wave;f Phase matching is achieved to generate a parametric Kerr lens by cascading second harmonic generation (SHG) from an oscillating light having a center wavelength λ p / 2 second harmonic to center wavelength λ f A light source is provided that is phase matched to the optical parametric amplification that converts the light into

[0011] FIG. 1 is a diagram showing the configuration of a conventional CARS microscope light source described in Non-Patent Document 1. FIG. 2 is a diagram showing an energy diagram of molecules of a sample to be measured when measuring CARS spectroscopy. FIG. 3 is a configuration diagram showing the relationship between two mode-locked lasers that constitute a synchronized pulsed laser light source according to the present disclosure. FIG. 4 is a configuration diagram showing an embodiment in which the pulse timing of a follower laser is synchronized with the pulse timing of a primary laser. FIG. 5 is a configuration diagram showing another embodiment in which the pulse timing of a follower laser is synchronized with the pulse timing of a primary laser. FIG. 6 is a configuration diagram showing another embodiment in which the pulse timing of a follower laser is synchronized with the pulse timing of a primary laser.

[0012] 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.

[0013] The light source of this embodiment is a CRS microscope with a wave number of 500 cm in the fingerprint region. -1 From 1800 cm -1 It can be used as a light source for observing the nonlinear optical processes of the CRS microscope, both SRS and CARS.

[0014] (Primary Laser) Fig. 3 is a schematic diagram showing the configuration of a light source for a CRS microscope in this embodiment. Although not shown in Fig. 3, specifically, the primary laser 1 has a Cr 4+It is a mode-locked laser equipped with 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 primary laser 1 is an external resonator type consisting of a single crystal fiber waveguide and a spatial optical system (see Non-Patent Document 4).

[0015] In the present disclosure, the primary laser 1 has a pulse width of about 100 fs and a central wavelength (λ p ) outputs a 1.5 μm optical pulse train 3. The optical pulse train 3 output from the primary laser 1 is split into two optical pulse trains by a first optical element 7, such as a semi-transparent mirror or a beam splitter, formed on the output side. One of the optical pulse trains, optical pulse train 10, passes through a pulse width adjustment mechanism 8, such as a group delay dispersion medium, and is converted into picosecond pulse light 10, the pulse width of which is expanded to approximately 3 ps, similar to the pulse width of oscillation pulse light 9 of the follower laser 2, which will be described later. However, pulse width adjustment by the pulse width adjustment mechanism 8 may not be necessary depending on the pulse width of the laser used, and is therefore optional. In addition, the picosecond pulse light 10, whose optical path has been adjusted by the dichroic mirror 11a, is introduced into the resonator of the follower laser 2 as gate light for synchronizing the pulse timing between the primary laser 1 and the follower laser 2.

[0016] The other optical pulse train split into two by the first optical element 7 is converted into an SC optical pulse train 5 generated by a highly nonlinear glass fiber 4, and is incident as Stokes light on a CARS microscope 14 coaxially and simultaneously with the oscillation pulse light 9 from the follower laser 2 via a third optical element 13 (e.g., a dichroic mirror).

[0017] (Follower Laser) In the present disclosure, the follower laser 2 has a pulse width of about 3 ps and a center wavelength (λ f ) a mode-locked laser (see Non-Patent Document 5) with a 1.3 μm Nd:YVO crystal as the gain medium, which can be configured to output a train of optical pulses to be used as pump and / or probe light in the microscope 14.

[0018] The oscillation pulse light 9 output from the follower laser 2 passes through a timing delay device 12 and is then incident as pump light and / or probe light on a CARS microscope 14 coaxially and at the same timing as the other branched optical pulse train by a third optical element 13. However, there are cases where the timing adjustment of the oscillation pulse light 9 by the timing delay device 12 is unnecessary and is therefore optional.

[0019] (Resonators for Pulse Timing Synchronization) As described above, a light source with synchronized pulse timing between the Stokes light from the primary laser 1 and the pump and / or probe light from the follower laser 2 is needed for use in a CARS microscope. Accordingly, this disclosure describes and provides three types of resonator configurations (40a, 40b, 40c) for synchronizing the lasers.

[0020] To synchronize the primary laser 1 and the follower laser 2, a cavity length adjustment mechanism 24 is provided in both lasers or in one of the lasers to electronically control the cavity length of the lasers, and the repetition rates of the primary laser 1 and the follower laser 2 are referenced via a transmission line 6 provided between the two lasers to precisely match the two values. As an example, the cavity length adjustment mechanism 24 can be installed to move one mirror, such as the output coupling mirror 18, in each of the resonators (40a, 40b, 40c) described below (see Figures 4 to 6). The repetition rates of the two lasers are synchronized by driving a piezoelectric element that controls the cavity length based on a signal corresponding to the repetition rates of the two lasers. Here, the primary laser 1 serves as the primary (reference oscillator) and the follower laser 2 serves as the follower (follower oscillator), but the roles of the two can also be reversed. The transmission line 6 can be either wired or wireless.

[0021] Various pulse lasers can be used for the primary laser 1 and the follower laser 2. For example, the primary laser 1 and the follower laser 2 may be Cr 4+The primary laser 1 and the follower laser 2 may be configured using a mode-locked laser with a gain medium that is a crystal or ceramics (bulk and fiber) selected from the group consisting of Cr:YAG, Cr-forsterite, Ti-sapphire, Cr:LiSAF, Cr:LiCAF, Cr:ZnSe, and Cr:ZnS. Furthermore, the primary laser 1 and the follower laser 2 may be configured using a mode-locked laser with a gain medium that is a crystal or ceramics (bulk and fiber) of YAG, YVO, YLF, or CALGO doped with a rare earth ion selected from the group consisting of Yb, Er, Nd, Tm, and Ho.

[0022] In other embodiments, the primary laser 1 can be configured using a mode-locked laser with a gain medium being glass (bulk and fiber) doped with one rare earth ion selected from Yb, Er, Nd, Tm, Ho, etc. In other implementations, the primary laser 1 may also be configured using a mode-locked laser with a semiconductor crystal as a gain medium.

[0023] (Embodiment 1) Hereinafter, one embodiment of a resonator for synchronizing the pulse timing of a primary laser 1 and a follower laser 2 will be described with reference to Fig. 4. Fig. 4 is a diagram showing a follower laser 2 including a resonator 40a equipped with components for synchronizing the pulse timing. As shown in Fig. 4, the resonator 40a includes an Nd:YVO crystal as a gain medium 15, a mirror 11a for adjusting the optical path of picosecond pulsed light 10 from the primary laser 1, a second-order nonlinear optical medium 16 (e.g., an LBO crystal) for mode-locking, a semiconductor saturable absorber mirror (SESAM) 23 equipped with a saturable absorber 17 and a mirror 20, and an output coupling mirror 18.

[0024] 4, the saturable absorber 17 and the mirror 20 are integrated, but they may also be separated by a certain distance and arranged so that the oscillation light 19 passes through the saturable absorber 17 and reaches the mirror 20. In addition, in the present disclosure, a semiconductor is used for the saturable absorber 17, but other saturable absorbers such as carbon nanotubes and graphene may also be used. Furthermore, the resonator 40a is formed by the output coupling mirror 18 and the mirror 20, and outputs the oscillation light 19 from the gain medium 15 after making a round trip.

[0025] Oscillating light 19 generated by the gain medium 15 is input to a second-order nonlinear optical medium 16 arranged on the output side of the gain medium 15. The second-order nonlinear optical medium 16 operates as a Kerr lens, as a refractive index change proportional to the intensity of the oscillating light 19 occurs in the second-order nonlinearity (CSO) phenomenon twice via second harmonic generation (SHG) of the oscillating light 19 (see Non-Patent Document 5).

[0026] This is called a parametric Kerr lens due to SHG cascading. For example, by installing an aperture in the optical path within the resonator, the resonator is designed to reduce circular loss throughout the entire resonator when a Kerr lens occurs. In addition, the second-order nonlinear optical medium 16 is controlled by temperature control or the like so that it is slightly shifted from a state in which the SHG phase is perfectly matched to the oscillating light 19. This results in mode-locked oscillation with a pulse width of 3 ps (see Non-Patent Document 5).

[0027] Pulsed light 10 from the primary laser 1 passes through a dichroic mirror 11a arranged on the optical path of the resonator 40a and is irradiated onto the SESAM 23 so as to be focused coaxially with the oscillating light 19 and at the same position on the mirror surface. However, it is also possible to design a resonator in which the pulsed light 10 and oscillating light 19 are irradiated coaxially and at the same focus without using the dichroic mirror 11a. The oscillating pulse of the follower resonator passes through the mirror 20 in time with the saturable absorber 17 of the SESAM 23 being saturated by the irradiation of the pulsed light 10, thereby increasing the reflectivity of the mirror 20, thereby enabling the timing of both lasers 1 and 2 to be synchronized. The oscillating light 19 from the gain medium 15 is extracted as picosecond oscillating pulsed light 9 by the output coupling mirror 18.

[0028] (Embodiment 2) Hereinafter, another embodiment of a resonator for synchronizing the pulse timing of the primary laser 1 and the follower laser 2 will be described with reference to Fig. 5. Fig. 5 is a diagram showing a follower laser 2 including a resonator 40b equipped with components for synchronizing the pulse timing. As shown in Fig. 5, the resonator 40b of the follower laser 2 includes an Nd:YVO crystal as a gain medium 15, a mirror 11a for adjusting the optical path of pulsed light 10 from the primary laser 1, a second-order nonlinear optical medium 16 for mode locking, and an output coupling mirror 18 and a mirror 20 that form the resonator 40b. Oscillated light 19 from the gain medium 15 travels back and forth within the resonator 40b before being output.

[0029] The main difference between the resonator 40b in this embodiment and the resonator 40a shown in the first embodiment is that the pulse timing is synchronized using sum-frequency generation (SFG) in the second-order nonlinear optical medium 16 instead of the semiconductor saturable absorber 17.

[0030] A method for synchronizing pulse timing in the second-order nonlinear optical medium 16 will be described with reference to Figure 5. As shown in Figure 5, the second-order nonlinear optical medium 16 is disposed in the resonator 40b of the follower laser 2, and the picosecond pulsed light 10 from the primary laser 1 introduced into the resonator 40b of the follower laser 2 has its optical path adjusted by a dichroic mirror 11a. Thereafter, the picosecond pulsed light 10 from the primary laser 1 is incident on the second-order nonlinear optical medium 16 coaxially with the follower oscillation light 19. However, it is also possible to design a resonator in which the pulsed light 10 and the oscillation light 19 are irradiated coaxially onto the second-order nonlinear optical medium 16 without using the dichroic mirror 11a.

[0031] The second-order nonlinear optical medium 16 is a second-order nonlinear optical medium having a length L, and the refractive index of the second-order nonlinear optical medium 16 with respect to the center wavelength and the incident polarization direction of the picosecond pulse light 10 from the primary laser 1 is defined as λ p , n p Similarly, λ f , n f , and SFG light generated by the primary light and follower light are similarly expressed as λ SFG , n SFG In angular phase matching, the phase mismatch amount Δβ or Δk is Δβ=2πΔk=2π(n SFG / λ SFG -n p / λ p -n f / λ f ) can be defined as follows.

[0032] When ΔkL is close to but not zero, a second-order nonlinear optical effect via sum frequency generation (SFG) occurs twice, resulting in a phenomenon equivalent to a third-order nonlinear optical effect (see Non-Patent Documents 5, 6, and 7). In the first embodiment, a parametric Kerr lens of a CSO via SHG was used, but here a parametric Kerr lens is used due to cascading of SFG. The resonator is designed so that a parametric Kerr lens is generated due to cascading of SFG, resulting in mode-locked oscillation, only when the picosecond pulse light 10 from the primary laser 1 and the follower oscillation light 19 are simultaneously incident on the second-order nonlinear optical medium 16.

[0033] This achieves timing synchronization between the follower oscillation light 19 and the picosecond pulse light 10 from the primary laser 1. Furthermore, instead of the LBO crystal, periodically poled lithium niobate (PPLN) or the like, which can achieve phase matching of SFG, can also be used for the second-order nonlinear optical medium 16.

[0034] (Embodiment 3) Hereinafter, another embodiment of a resonator that synchronizes the pulse timing of the primary laser 1 and the follower laser 2 will be described with reference to Fig. 6. Fig. 6 is a diagram showing a follower laser 2 that includes a resonator 40c equipped with components for synchronizing the pulse timing. This embodiment has the advantage that a light source can be configured with a primary laser 1 that is weaker in power than in embodiment 2.

[0035] 6, the resonator 40c of the follower laser 2 comprises an Nd:YVO crystal as the gain medium 15, a second-order nonlinear optical medium 16 (hereinafter referred to as the first second-order nonlinear optical medium) for mode locking, a second second-order nonlinear optical medium 21 for converting the picosecond pulse light 10 from the primary laser 1 into a second harmonic wave 22, a mirror 11a for adjusting the optical path of the second harmonic wave 22, and an output coupling mirror 18 and a mirror 20 that form the resonator 40c. The oscillating light 19 from the gain medium 15 travels back and forth within the resonator 40c before being output.

[0036] The picosecond pulse light 10 from the primary laser 1 is incident on the second second-order nonlinear optical medium 21, which has a center wavelength λ p A picosecond pulsed light 10 having a wavelength of λ p For example, in this embodiment, the picosecond pulsed light 10 is converted into a second harmonic 22 having a wavelength of λ / 2. p is 1.5 μm, and the center wavelength of the second harmonic 22 is 0.75 μm. In order to generate parametric light in the first second-order nonlinear optical medium 16 described later, the center wavelength λ of the oscillating light 19 from the gain medium 15 must be 1.5 μm. pThis is because the wavelength must be shorter than 1.3 μm in this embodiment. The second second-order nonlinear optical medium 21 is made of a nonlinear crystal capable of generating second harmonics by angular phase matching, such as LBO, KTP, lithium niobate, or lithium tantalate. Alternatively, a quasi-phase matching crystal, such as PPLN, PPLT, or PPKTP, may be used instead of angular phase matching.

[0037] The parametric light emission process is briefly explained below. Parametric light emission occurs at an angular frequency ω p Intense light and angular frequency ω s When weak light of ω is simultaneously incident on a material with second-order optical nonlinearity, p =ω s +ω i Three lights (e.g., pump light, signal light, idler light) that satisfy the energy condition ω p and ω s By difference frequency mixing with ω i Light of ω p and ω i By difference frequency mixing with ω s In this case, the energy of the pump light is divided into the signal light and the idler light, resulting in the generation of ω s and ω i By utilizing the wavelength conversion and amplification functions in this optical parametric process, it is possible to synchronize the timing of two short pulse light sources with different wavelengths.

[0038] The first second-order nonlinear optical medium 16 is a second-order nonlinear optical medium (e.g., PPLN) having a length L and a periodically poled structure, and is disposed within the resonator 40c of the follower laser 2. The optical path of the second harmonic wave 22 is adjusted by the dichroic mirror 11a and enters the first second-order nonlinear optical medium 16. Here, the second harmonic wave 22 is adjusted so that it enters the first second-order nonlinear optical medium 16 coaxially with the follower oscillation light 19. However, it is also possible to design a resonator in which the second harmonic wave 22 and the oscillation light 19 are irradiated coaxially onto the second-order nonlinear optical medium 16 without using the dichroic mirror 11a.

[0039] The incident second harmonic wave 22 with a central wavelength of 0.75 μm is further split into pulsed light with wavelengths of 1.3 μm and 1.77 μm by the first second-order nonlinear optical medium 16. At the timing when the first second-order nonlinear optical medium 16 generates pulsed light of 1.3 μm, oscillation light 19 (with a central wavelength λ f The first second-order nonlinear optical medium 16 with a length L is phase-matched to generate a parametric Kerr lens by cascading the SHG of the follower oscillation light, and the center wavelength λ of the second harmonic 22 of the primary light is synchronized. p / 2 to the center wavelength λ of the follower light f This is because phase matching is achieved for the optical parametric amplification that is converted into

[0040] As a result, when the follower resonator oscillates in Kerr-lens mode locking, the optical parametric amplification pulse from the second harmonic pulse of the primary light becomes the seed light, thereby realizing timing synchronization between the follower pulse light and the primary pulse light. Furthermore, the second-order nonlinear optical medium 16 can also be made of periodically poled lithium tantalate (PPLT) or periodically poled KTP (PPKTP) instead of PPLN.

[0041] Therefore, by using any of the resonators of the first, second, and third embodiments described above, a non-invasive CARS microscope light source can be configured using pump light with a wavelength of 1.3 μm and Stokes light with a wavelength of 1.3 to 1.8 μm.

[0042] In the first and second embodiments, other second-order nonlinear media than LBO, such as KTP, lithium niobate, or lithium tantalate, may be used. Also, instead of angle phase matching, quasi-phase matching crystals, such as PPLN, PPLT, or PPKTP, may be used. In the second embodiment, in the case of quasi-phase matching, the inversion period is set to Λ, and instead of the above formula, the phase mismatch amount Δβ or Δk is expressed as Δβ=2πΔk=2π(n SFG / λ SFG -n p / λ p -n f / λ f -1 / Λ).

[0043] 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.

[0044] 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 present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the claims.

[0045] REFERENCE SIGNS LIST 101 Light source 102 Microscope 103 Picosecond optical pulse train 104 Fiber 105 SC optical pulse train 106 Pump light 107 Stokes light 108 Probe light 109 CARS light 110 Vibration mode 1 Primary laser (first laser) 2 Follower laser (second laser) 3 Optical pulse train 4 Highly nonlinear glass fiber (second optical element) 5 SC optical pulse train 6 Transmission path 7 Semi-transmitting mirror (first optical element) 8 Pulse width adjustment mechanism 9 Oscillation pulse light 10 Picosecond pulse light 11a, b Dichroic mirror 12 Timing delay device 13 Dichroic mirror (third optical element) 14 Microscope 15 Gain medium 16 Second-order nonlinear optical medium 17 Saturable absorber 18 Output coupling mirror 19 Oscillation light 20 Mirror 21 Nonlinear optical medium 22 Second harmonic wave 23 Semiconductor saturable absorber mirror 24 Resonator length adjustment mechanism

Claims

1. Center wavelength λ p a first laser configured to output an optical pulse train having a central wavelength λ; a first optical element disposed to split the optical pulse train into two; and a second laser including a resonator configured with a plurality of mirrors, wherein the resonator has a central wavelength λ f a gain medium that outputs an oscillating light having a center wavelength λ; and a first second-order nonlinear optical medium that operates as a Kerr lens for the oscillating light, wherein the first second-order nonlinear optical medium outputs an oscillating light having a center wavelength λ f a second laser comprising: a first second-order nonlinear optical medium capable of phase matching to produce a parametric Kerr lens by cascading for second harmonic generation (SHG) from the oscillation light having the formula: and a saturable absorber configured to allow one of the optical pulse trains branched by the first optical element and the oscillation light to focus coaxially and at the same position, and to pass or reflect the light; a cavity length adjusting mechanism provided in either or both of the first laser and the second laser, the cavity length adjusting mechanism matching the repetition frequency of one of the first laser and the second laser with the repetition frequency of the other laser through a transmission path; a second optical element configured to convert the other of the optical pulse trains branched by the first optical element into a supercontinuum (SC) optical pulse train; and a third optical element configured to combine the oscillation pulse light output from the resonator with the SC optical pulse train and output the combined light.

2. Center wavelength λ p a first laser configured to output an optical pulse train having a central wavelength λ; a first optical element disposed to split the optical pulse train into two; and a second laser including a resonator configured with a plurality of mirrors, wherein the resonator has a central wavelength λ f and a first second-order nonlinear optical medium that operates as a Kerr lens for the oscillation light, the first second-order nonlinear optical medium having a length L, and configured to receive one of the optical pulse trains branched by the first optical element and the oscillation light, and wherein Δk=n SFG / λ SFG -n p / λ p -n f / λ f ΔkL defined by the central wavelength λ of the first laser p and the central wavelength λ of the oscillating light f is set to a value that generates a parametric Kerr lens due to cascading of the sum frequency light, and the wavelength λ of the sum frequency light is SFG is 1 / λ SFG = 1 / λ p +1 / λ f and n p , n f , n SFG are the central wavelengths λ p the optical pulse train having a central wavelength λ f The wavelength λ of the oscillation light and the sum frequency light SFG a second laser comprising: a first second-order nonlinear optical medium, wherein the refractive index of the first second-order nonlinear optical medium is Rf / Rf; a cavity length adjusting mechanism provided in both or one of the first laser and the second laser, the cavity length adjusting mechanism matching the repetition frequency of one of the first laser and the second laser with the repetition frequency of the other laser through a transmission path; a second optical element configured to convert the other of the optical pulse trains branched by the first optical element into a supercontinuum (SC) optical pulse train; and a third optical element configured to combine and output oscillation pulse light output from the resonator with the SC optical pulse train.

3. The first second-order nonlinear optical medium has a periodic polarization reversal of length L and a reversal period Λ, and Δk=n SFG / λ SFG -n p / λ p -n f / λ f ΔkL defined by −1 / Λ is the center wavelength λ of the first laser. p and the central wavelength λ of the oscillating light f 3. The light source according to claim 2, wherein the sum frequency light of said first and second frequencies is set to a value that causes a Kerr lens effect by cascading.

4. Center wavelength λ p a first laser configured to output an optical pulse train having a central wavelength λ; a first optical element disposed to split the optical pulse train into two; and a second laser including a resonator configured with a plurality of mirrors, wherein the resonator has a central wavelength λ f a gain medium that outputs oscillating light having a center wavelength λ; a first second-order nonlinear optical medium that operates as a Kerr lens for the oscillating light; and a second optical medium that splits one of the optical pulse trains branched by the first optical element into a pulse train having a center wavelength λ. p a second second-order nonlinear optical medium configured to convert the first second-order nonlinear optical medium into a second harmonic wave having a center wavelength λ / 2; f Phase matching is achieved to generate a parametric Kerr lens by cascading for second harmonic generation (SHG) from the oscillation light having a center wavelength λ p / 2 to the second harmonic of the center wavelength λ f a second laser that is phase-matched to optical parametric amplification that converts an optical pulse train into a supercontinuum (SC) optical pulse train; a cavity length adjustment mechanism provided in both or one of the first laser and the second laser, the cavity length adjustment mechanism matching the repetition frequency of one of the first laser and the second laser with the repetition frequency of the other laser through a transmission path; a second optical element configured to convert the other of the optical pulse trains branched by the first optical element into a supercontinuum (SC) optical pulse train; and a third optical element configured to combine and output the oscillating pulse light output from the resonator with the SC optical pulse train.

5. A light source according to any one of claims 1 to 4, further comprising a pulse width adjusting mechanism configured to adjust the pulse width of said one of said light pulse trains.

6. The light source according to claim 1, wherein the saturable absorber is one of a semiconductor, a carbon nanotube, and graphene.

7. A light source according to any one of claims 1 to 3, characterized in that the first second-order nonlinear optical medium is any one of LBO, KTP, lithium niobate, lithium tantalate, periodically poled lithium niobate (PPLN), periodically poled lithium tantalate (PPLT), and periodically poled KTP (PPKTP).

8. The light source according to claim 4, wherein the first second-order nonlinear optical medium and the second second-order nonlinear optical medium are either periodically poled lithium niobate (PPLN), periodically poled lithium tantalate (PPLT), or periodically poled KTP (PPKTP), or the second second-order nonlinear optical medium is either LBO, KTP, lithium niobate, or lithium tantalate.

9. The gain medium contains Cr. 4+ 5. The light source according to claim 1, wherein the light source is made of a crystal or ceramics (bulk and fiber) of YAG, Cr-forsterite, Ti-sapphire, Cr:LiSAF, Cr:LiCAF, Cr:ZnSe, or Cr:ZnS, or a crystal or ceramics (bulk and fiber) of YAG, YVO4, YLF, or CALGO doped with one rare earth ion selected from Yb, Er, Nd, Tm, Ho, or the like, or a glass (bulk and fiber) or a semiconductor crystal doped with one rare earth ion selected from Yb, Er, Nd, Tm, Ho, or the like.

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