Laser device

Abstract

An exemplary laser device according to the present disclosure comprises: a resonator that outputs pulsed laser light; and a movement mechanism. The resonator comprises a nonlinear optical medium and a gain medium disposed in an optical path between a first mirror and a second mirror. The nonlinear optical medium has a periodic polarization inversion structure for mode-locked oscillation of the resonator. The periodic polarization inversion structure is a fan-out structure in which inversion regions and non-inversion regions spread in a fan shape in a direction intersecting the optical path and are alternately arranged in stripes in a direction along the optical path. The movement mechanism varies the position of the nonlinear optical medium in the plane of the medium at least in a direction perpendicular to the optical path.

Description

Laser device

[0001] This disclosure relates to a laser device.

[0002] With the development of pulsed lasers used as light sources, coherent anti-Stokes Raman scattering (CARS) measurements are rapidly advancing. For example, its effectiveness is remarkable when acquiring microscopic images. Because CARS microscopy allows for the acquisition of molecular information from biological samples without staining or invasiveness, it holds great promise for applications in life science research and medicine.

[0003] Hideaki Kano, "Molecular Imaging of Living Cells by Nonlinear Raman Spectroscopy," Biochemistry Vol. 91, No. 6, 2019. Sun, Chi-Kuang, et al. "Higher harmonic generation microscopy for developmental biology." Journal of structural biology 147.1 (2004): pp. 19-30. Lenhardt, F., et al. "888 nm pumped 1342 nm Nd:YVO4 oscillator Kerr-lens mode-locked using cascaded second-order nonlinearities." Applied Physics B 106 (2012): pp. 5-8. Schafer, Christoph, et al. "Parametric Kerr lens mode-locked, 888 nm pumped Nd:YVO4 laser." Optics letters Vol. 36, No. 14 (July 15, 2011): pp. 2674-2676. Xu, Jin-Long, et al. “Generation of 3.3-ps Pulses at 1.34 μm from High-Power Passively Mode-Locked Nd:GdVO4 Laser.” IEEE Journal of Quantum Electronics Vol.48, No.5 (2012): pp.622-627. Iliev, Hristo, et al. “1.34-μm Nd:YVO4 laser mode-locked by SHG-lens formation in “Regularly-poled stoichiometric lithium tantalate.” Optics Express Vol.19, No.22 (2011): pp.21754-21759. Shigeo Ishibashi, et al. “Mode-locked operation of Cr4+:YAG single-crystal fiber laser with external cavity.” Optics Express Vol.22, No. 6 (2014): pp. 6764-6771. Jorg Zimmermann, et al. “Tunable blue laser based on intracavity frequency doubling with a fan-structured periodically poled LiTaO3 crystal,” Optics letters Vol. 27, No. 8 (2002): pp. 604-606.

[0004] One embodiment of the laser device according to the present disclosure is a resonator that outputs pulsed laser light, comprising a gain medium and a nonlinear optical medium disposed in the optical path between a first mirror and a second mirror constituting the resonator, wherein the nonlinear optical medium has a periodic polarization reversal structure for causing the resonator to oscillate in a mode-locked manner, and the periodic polarization reversal structure is a fan-out structure in which the reversal region and non-reversal region spread out in a fan shape in a direction intersecting the optical path and are alternately arranged in a striped pattern in a direction along the optical path, and a moving mechanism for varying the position of the nonlinear optical medium in a direction at least perpendicular to the optical path within the plane of the nonlinear optical medium.

[0005] This is a block diagram showing an example configuration of an existing mode-locked laser. This is a block diagram showing an exemplary configuration of a mode-locked laser, which is an example of a laser device according to Embodiment 1. This is a block diagram showing an exemplary configuration of a mode-locked laser, which is an example of a laser device according to Embodiment 2.

[0006] Embodiments will be described in detail below with reference to the drawings. However, the accompanying drawings and the following description are provided for the benefit of those skilled in the art to fully understand this disclosure and are not intended to limit the subject matter described in the claims. Furthermore, unnecessary details may be omitted. For example, detailed descriptions of already well-known matters or redundant descriptions of substantially identical configurations may be omitted.

[0007] Furthermore, in the drawings, identical or corresponding elements are appropriately denoted by the same reference numeral. The drawings are schematic, and the dimensional relationships or ratios of each element may differ from reality. There may also be differences in dimensional relationships or ratios between drawings. When numerical values ​​are given in the following explanation, these values ​​are merely examples, and other values ​​may be used additionally or as substitutes.

[0008] <Overview> 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 of molecules, crystals, amorphous structures, etc., and is also widely used in medicine and industry.

[0009] Spontaneous Raman scattering is a phenomenon in which scattered light is produced at a frequency shifted according to the frequency of molecular or lattice vibrations relative to the incident light. Because this scattered light has very weak power compared to the original incident light power, a high-power light source may be used as the incident light to obtain scattered light that can be measured by a detector.

[0010] However, most samples have an upper limit on the power that can be irradiated per unit area, and if the sample is irradiated with light exceeding this limit, the sample may be altered or destroyed. In many cases, even when using a light source with power equivalent to the upper limit, the scattered light is weak, and obtaining a signal with a high signal-to-noise ratio (SNR) takes significantly longer than CARS measurement.

[0011] In contrast, CARS is a nonlinear optical process using a light source with high instantaneous power. Therefore, compared to spontaneous Raman scattering, when using a light source of equivalent power, the power of the scattered Raman light is significantly stronger, and as a result, the measurement time can be shortened.

[0012] As previously mentioned, CARS microscopy is expected to have great potential applications in life science research and medicine because it can obtain molecular information from biological samples without staining or invasiveness. Among the CARS microscopes reported to date, the configuration using a supercontinuum (SC) light source can obtain Raman scattering spectra with the widest wavenumber band.

[0013] For example, a laser beam with a wavelength of 1 micrometer (μm), a pulse width of 50 picoseconds (ps), and a repetition rate of 1 megahertz (MHz) is amplified by a ytterbium (Yb)-doped glass fiber amplifier. The amplified light is split into two branches; one branch is used as the excitation light (pump light), and the other branch generates SC light using a photonic crystal fiber. The generated SC light is continuous spectral light with a wavelength of 1.1 to 1.8 μm, which is used for Stokes light (for example, Non-Patent Document 1).

[0014] However, CARS microscopy has the following two technical challenges: (1) Past experiments have shown that using 1.2-1.3 μm light, which has a longer wavelength than the 1 μm band, raises the threshold of light power that can have harmful effects on living organisms, making it safer (for example, Non-Patent Document 2).

[0015] (2) By using pulses with high instantaneous intensity, for example, the measurement time for microscope images can be shortened. On the other hand, by using pulses with a narrowed wavenumber bandwidth, spectral images with high wavenumber resolution can be obtained. Therefore, it is expected that the laser light used as the excitation light for CARS can selectively output pulses with a narrow pulse width and high instantaneous intensity, and pulses with low instantaneous intensity but a narrow wavenumber bandwidth and high wavenumber resolution. When the pulse is oscillating at the Fourier limit where energy utilization efficiency is best, such selective pulse output can be achieved, for example, by variable pulse width oscillation.

[0016] As an example of a light source that oscillates with a variable pulse width from 3 ps to 10 ps at a wavelength of 1.3 μm, Figure 1 shows Nd:YVO ４An example of a mode-locked laser configuration using (yttrium orthovanadate doped with neodymium ions) is shown (for example, Non-Patent Document 3).

[0017] The resonator of the mode-locked laser shown in Figure 1 is, for example, Nd:YVO, which is the gain medium (or laser medium). ４ The device comprises a crystal 101, an LBO (lithium triborate) crystal 102 which is a second-order nonlinear optical medium for mode-locked oscillation, a dichroic mirror 103, a thin film polarizer (TFP) 104, a quarter-wave plate 105, an aperture 110, and a total reflection mirror 106.

[0018] The dichroic mirror 103 totally reflects the oscillating light 107 with a wavelength of 1342 nm (nanometers) and transmits the excitation light 108 with a wavelength of 880 nm. Nd:YVO ４ The crystal 101 emits an oscillating light 107 when excitation light 108 is incident on it. The oscillating light 107 repeatedly circulates (or returns) between the dichroic mirror 103 and the total internal reflection mirror 106.

[0019] The oscillating light 107, which travels back and forth within the resonator, is stimulated to emit linearly polarized light, but its polarization rotates as it travels back and forth across the quarter-wave plate 105. As a result, the light traveling back and forth between the dichroic mirror 103 and the TFP 104 has a polarization perpendicular to its original polarization, which is selectively coupled to and transmitted by the TFP 104, and output as laser light (pulsed light) 109.

[0020] This laser generates a Kerr lens effect through a cascaded nonlinear optical effect by the LBO crystal 102 and oscillates in a mode-locked manner. The LBO crystal 102 used has a crystal orientation that corresponds to the generation of the second harmonic generation (SHG) by angular phase matching using birefringence.

[0021] As the oscillating light 107 becomes the fundamental wave and passes through the LBO crystal 102, a portion of the fundamental wave is converted into half-wavelength SHG light, and then an inverse conversion occurs in which the SHG light returns to the fundamental wave. This series of conversion processes ends when the light incident on the LBO crystal 102 reaches the opposite end of the LBO crystal 102.

[0022] In SHG optical conversion and inverse conversion, if the phase velocity between the fundamental wave and the SHG light within the LBO crystal 102 is not perfectly matched but adjusted to create a "shift," the resulting phase difference, which depends on the light intensity, produces a phenomenon similar to the Kerr lens effect due to a third-order nonlinear effect.

[0023] In other words, when a light beam having a Gaussian intensity distribution propagates through the LBO crystal 102, the refractive index decreases from the center to the periphery of the LBO crystal 102, thus forming a Kerr lens (convex lens) in the LBO crystal 102. This type of Kerr lens may also be called an SHG lens.

[0024] When an SHG lens occurs, mode-locked oscillation occurs and pulsed light is output as laser light 109. However, if the adjustment is insufficient, steady-state oscillation light may also be output. The output of steady-state oscillation light can be prevented or reduced, for example, by adjusting the position of the LBO crystal 102 and adjusting the aperture diameter of the aperture 110. The direction of changing the position of the LBO crystal 102 is, for example, parallel to the optical axis of the LBO crystal 102, as indicated by arrow 111 in Figure 1.

[0025] Since the beam diameter of the modes within the resonator is dependent on the position in the optical axis direction, changing the position of the LBO crystal 102 in the direction of arrow 111 changes the intensity of the nonlinear effect at the positions before and after the change, thus changing the depth of focus of the SHG lens. In addition, the aperture diameter of the aperture 110 is adjusted so that the optical loss within the resonator is minimized when an SHG lens is generated.

[0026] Here, the phase velocity difference between the fundamental wave and the SHG light is expressed by the coherence length (lc). The value of lc is the wavenumber of the fundamental wave and the SHG light, respectively, when no SHG lens is present, and k １ and k ２ Therefore, in angular phase matching, it is expressed by the following equation (1): l ｃ = π / Δk, Δk = k １ -2k ２ (1)

[0027] When the SHG lens occurs, a component depending on the fundamental wave intensity is added to the relationship of Equation (1), and the coherence length \(l_c\) is expressed as a function of the fundamental wave intensity.

[0028] In this laser, in order to generate stable mode-locked oscillation, by changing \(\Delta k = k\) １ - 2k ２ the relationship represented by the following Equation (2) is established (for example, Non-Patent Document 4). \(l\) ｃ = L / 2m (2)

[0029] In Equation (2), L represents the length of the second-order nonlinear optical medium, and m represents an integer of 1 or more. \(\Delta k = k\) １ - 2k ２ can be changed, for example, by precisely controlling the temperature of the LBO crystal 102 with the temperature controller 112.

[0030] As another configuration example, as an example of oscillating a laser medium doped with Nd ions at a wavelength of 1.3 μm and a pulse width of about 3 ps, there is also a report of constructing a mode-locked laser with an Nd:GdVO ４ (gadolinium orthovanadate doped with neodymium ions) crystal and a semiconductor saturable absorber mirror (SESAM: Semiconductor Saturable Absorber Mirror).

[0031] However, in this case, since a plurality of peaks appear in the oscillation spectrum, it is not suitable for spectroscopic use (for example, Non-Patent Document 5). Therefore, the method of controlling the SHG lens by adjusting the position, aperture, and temperature of the second-order nonlinear optical medium within the optical axis as described above was the only reported example that could be used as a light source for CRS.

[0032] To change the pulse width of the laser beam 109, for example, by rotating the quarter-wave plate 105 to change the output coupling, the intensity of the oscillating light (in other words, the fundamental wave) 107 in the resonator can be varied to control the increase in the oscillation band associated with the cascaded second-order nonlinear effect.

[0033] However, this operation involves adjusting the aperture diameter of the aperture 110 and adjusting the temperature of the second-order nonlinear optical medium. Since the temperature adjustment takes time to stabilize, it is difficult to increase the speed of pulse width change.

[0034] Therefore, hereinafter, exemplary embodiments of a laser device capable of increasing the speed of pulse width change without depending on the temperature adjustment of the second-order nonlinear optical medium will be described.

[0035] <Embodiment 1> Fig. 2 is a block diagram showing an exemplary configuration of a mode-locked laser which is an example of the laser device according to Embodiment 1. The mode-locked laser illustrated in Fig. 2 can be used as a light source for a nonlinear spectroscopy or a nonlinear optical microscope that detects coherent Raman scattering (CRS), such as CARS or stimulated Raman scattering (SRS).

[0036] As shown in Fig. 2, the mode-locked laser of the present embodiment includes, as a resonator (hereinafter also referred to as "laser resonator"), a gain medium (or laser medium) 1, a second-order nonlinear optical medium 2, a dichroic mirror 3, a thin-film polarizer (TFP) 4, a quarter-wave plate 5, an aperture 12, and a total reflection mirror 6.

[0037] The gain medium 1 and the second-order nonlinear optical medium 2 are arranged on the optical path (or optical axis) between the dichroic mirror 3 and the total reflection mirror 6 that constitute the laser resonator, as illustrated in Fig. 2.

[0038] The gain medium 1 is, for example, an Nd:YVO ４ single crystal fiber, and is excited by excitation light (for example, wavelength 880 nm) that passes through the dichroic mirror 3 and is incident from an excitation light source not illustrated in Fig. 2, and outputs oscillation light 7 (for example, wavelength 1342 nm). Note that the use of an Nd:YVO ４ crystal for the gain medium 1 is described, for example, in Non-Patent Document 6. The use of a single crystal fiber as a laser medium is described, for example, in Non-Patent Document 7.

[0039] The second-order nonlinear optical medium 2 is, for example, an Nd:YVO ４This is a periodically polarized reversal lithium tantalate (PPLT) for mode-synchronizing the oscillating light 7 from the single-crystal fiber 1. The use of PPLT as a second-order nonlinear optical medium for mode-synchronized oscillation is described, for example, in Non-Patent Document 6. In this embodiment, PPLT 2 is a fan-out structure PPLT, as illustrated in Figure 2.

[0040] The "fan-out structure," as shown in Figure 2, is a structure in which strip-shaped inverted regions (hatted strip-like areas) and non-inverted regions (unhatted strip-like areas between them) are arranged alternately to form a radial (fan-shaped) striped pattern. The "fan-out structure" may also be referred to as the "fan structure" or "fan-out type."

[0041] Fan-out type periodic polarization reversal second-order nonlinear optical crystals are also described, for example, in Non-Patent Document 8. This type of periodic polarization reversal second-order nonlinear optical medium allows for adjustment of the polarization reversal period (Λ) using a single crystal.

[0042] For example, if the position of PPLT2 is translated in a direction perpendicular to the optical path (see arrow 14), the polarization reversal period (Λ) along the optical path will change continuously in accordance with the translation. In other words, the fan-out structure is an example of a periodic polarization reversal structure that has different polarization reversal periods depending on the position where the light propagates through PPLT2 along the optical path.

[0043] The PPLT2 can be moved, for example, by a movable stage 11 on which the PPLT2 is mounted. The movable stage 11 is an example of a mechanism for moving the PPLT2 and is movable in a direction along the optical axis (see arrow 13) and in a direction perpendicular to the optical axis (see arrow 14).

[0044] For the movement of the movable stage 11, a drive system such as a motor or a piezoelectric element may be used, for example. In order to change the polarization reversal period of the PPLT 2, it is sufficient that the position of the PPLT 2 can be moved in a direction 14 perpendicular to the optical axis at least.

[0045] The dichroic mirror 3 is an example of a first mirror that constitutes the resonator, and exemplifies totally reflecting the oscillation light 7 of a specific wavelength (e.g., 1342 nm) while transmitting the excitation light 8. The oscillation light 7 repeatedly travels back and forth (or circulates) between the dichroic mirror 3 and the totally reflecting mirror 6, which is an example of a second mirror.

[0046] The TFP4 selectively combines specific linear polarizations of light traveling back and forth between the dichroic mirror 3 and the total internal reflection mirror 6 to output laser light (pulsed light) 9, while reflecting linear polarizations perpendicular to the specific linear polarizations along the optical path.

[0047] The quarter-wave plate 5 rotates the polarization of light traveling back and forth in the optical path between the dichroic mirror 3 and the total internal reflection mirror 6. The aperture 12 has a variable aperture diameter that adjusts the amount of light transmitted back and forth in the optical path between the dichroic mirror 3 and the total internal reflection mirror 6.

[0048] The oscillating light 7, which travels back and forth within the resonator, is stimulated to emit linearly polarized light, and its polarization rotates as it travels back and forth across the quarter-wave plate 5. As a result, the polarization of the light traveling back and forth between the dichroic mirror 3 and the TFP 4, which is perpendicular to the original polarization of the light, is selectively coupled and transmitted through the TFP 4, and output as laser light (pulsed light) 9.

[0049] The laser in this embodiment generates a Kerr lens effect through a cascaded nonlinear optical effect by the PPLT2 and oscillates in a mode-locked manner. The polarization reversal period (Λ) of the PPLT2 is set to correspond to SHG optical conversion by pseudo-phase matching with respect to the wavelength of the emitted light.

[0050] As the emitted light 7 becomes the fundamental wave and passes through the PPLT2, a portion of the fundamental wave is converted into half-wavelength SHG light, and then an inverse conversion occurs in which the SHG light returns to the fundamental wave. This series of conversion processes ends when the light incident on the PPLT2 reaches the opposite end of the PPLT2.

[0051] In SHG optical conversion and inverse conversion, if the phase velocity between the fundamental wave and the SHG light within PPLT2 is not perfectly matched but adjusted to create a "shift," the resulting phase difference, which depends on the light intensity, produces a phenomenon similar to the Kerr lens effect due to a third-order nonlinear effect.

[0052] In other words, when a light beam with a Gaussian intensity distribution propagates through the PPLT2, the refractive index decreases from the center to the periphery of the PPLT2, thus forming an SHG lens in the PPLT2.

[0053] When an SHG lens occurs, mode-locked oscillation occurs, and pulsed light is output as laser light 9. By rotating the quarter-wave plate 5 to adjust the output coupling at TFP 4, the output power of the laser light 9 output from TFP 4 is adjusted. This simultaneously changes the pulse intensity of the oscillating light 7 within the resonator, making it possible to control the pulse width by self-phase modulation.

[0054] In this embodiment, by using a fan-out type PPLT2, the polarization reversal period can be varied by moving the PPLT2 perpendicular to the optical axis and parallel to the substrate plane direction 14. This makes it possible to adjust the pulse width without relying on temperature control of the PPLT2.

[0055] In a second-order nonlinear optical medium 2 having a periodic polarization reversal structure, the coherence length lc is expressed by the following equation (3): lc = π / Δk, Δk = k １ -2k ２ -1 / Λ (3)

[0056] In the fan-out type periodic polarization reversal structure, the optical axis position moves with movement in the direction indicated by arrow 14 in Figure 2, which changes the polarization reversal period Λ in equation (3). Therefore, by the above adjustment, the coherence length lc in the cascade second-order nonlinear effect can be controlled so that equation (3) is satisfied.

[0057] The manner in which the movement of the PPLT 2 is controlled using the movable stage 11 is not particularly limited. As an example of non-limiting methods, the movement of the PPLT 2 may be controlled by providing a control value for a predetermined pulse width stored in a memory device (not shown) to the drive system of the movable stage 11, or the drive system of the movable stage 11 may be controlled so that the pulse width matches the predetermined pulse width based on the results of monitoring the output laser light 9.

[0058] If equation (3) holds true, the oscillating light 7 incident on the fan-out type PPLT2 is converted to SHG light, and the position where the inverse conversion from SHG light to the fundamental wave ends is adjusted to be the output surface of the PPLT2. By controlling the movement of the fan-out type PPLT2, the pulse width of the variable pulse width light source can be changed at high speed.

[0059] Furthermore, adjustments to the resonator in response to changes in the intensity of the car lens can be made by changing the aperture diameter of the aperture 12, or by moving the fan-out type PPLT 2 parallel to the direction 13 along the optical axis.

[0060] The pulse intensity within the resonator may be changed by altering the excitation light intensity instead of adjusting the output coupling at TFP4. In this embodiment, since a single-crystal fiber is used as the gain medium 1, the thermal lensing effect is extremely small, and even when the excitation light intensity is changed, the change in the mode profile accompanying the change in excitation light intensity is minimal. Therefore, it is possible to independently adjust the pulse intensity within the resonator without affecting other elements. It is also possible to use both output coupling control and excitation light intensity control in combination.

[0061] As described above, by using an Nd-doped crystal as the gain medium 1 and a fan-out type PPLT 2, which is a second-order nonlinear optical crystal, as the Kerr lens medium, a mode-locked laser capable of variable pulse width oscillation of approximately 3 ps to 10 ps at a biologically safe wavelength of 1.3 μm can be constructed without temperature control of the PPLT 2.

[0062] This configuration enables oscillation at a biologically safe wavelength of 1.3 μm, making it a practical light source for CARS microscopes, with a minimum pulse width of approximately 3 ps. Furthermore, it allows for very short pulse width changes and achieves a highly reproducible light source.

[0063] <Embodiment 2> Figure 3 is a block diagram showing an exemplary configuration of a mode-locked laser, which is an example of a laser device according to Embodiment 2. The configuration illustrated in Figure 3 differs from the configuration illustrated in Figure 2 in that a dispersion compensation medium 10 is provided in the optical path between the gain medium 1 and the PPLT 2 within the resonator. Other configurations may be equivalent to or similar to those described in Embodiment 1.

[0064] By controlling the dispersion within the resonator using the dispersion compensation medium 10, it is possible to adjust the pulse of the oscillating light 7 to, for example, a Fourier-limit pulse. A non-limiting example of the dispersion compensation medium 10 is the use of a pair of prisms. With a pair of prisms, it is possible to control the dispersion within the resonator without changing the optical path.

[0065] Furthermore, the dispersion compensation medium 10 can be placed anywhere in the optical path of the resonator, as long as it is within the region between the dichroic mirror 3 and the TFP 4, where the emitted light 7 is linearly polarized. Therefore, its placement is not limited to the optical path between the gain medium 1 and the PPLT 2. Depending on the type of dispersion compensation medium 10, it may also be placed in a region other than the region between the dichroic mirror 3 and the TFP 4.

[0066] <Modifications of Gain Medium 1 and Second-Order Nonlinear Optical Medium 2> In the embodiments 1 and 2 described above, the gain medium 1 contains Nd:YVO ４ Although an example using a crystal has been shown, this disclosure is not limited to this, and other laser crystals may be used as the gain medium 1.

[0067] For example, Cr ４＋ One crystal or ceramic (bulk type or fiber type) selected from among YAG (yttrium aluminum garnet doped with tetravalent chromium ions), Cr forsterite, Ti sapphire, Cr:LiSAF (lithium strontium aluminum fluoride doped with chromium ions), Cr:LiCAF (lithium calcium aluminum fluoride doped with chromium ions), Cr:ZnSe (zinc selenide doped with chromium ions), and Cr:ZnS (zinc sulfide doped with chromium ions) may be used as the gain medium 1.

[0068] Furthermore, YAG and YVO are obtained by adding one rare earth ion selected from among Yb (ytterbium), Er (erbium), Nd (neodymium), Tm (thulium), and Ho (holmium). ４ (Yttrium orthobanadate), GdVO ４ Crystals or ceramics (bulk type or fiber type) of gadolinium orthovanadate, YLF (yttrium lithium fluoride), or CALGO (calcium aluminum gadolinium oxide) may be used as the gain medium 1.

[0069] Furthermore, as another example, a glass (bulk type or fiber type) doped with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho may be used as the gain medium 1. In yet another example, a semiconductor crystal may be used as the gain medium 1.

[0070] Regardless of which of the material systems listed above is applied to the gain medium 1, it is possible to obtain the same effects and advantages as in Embodiment 1 and / or Embodiment 2.

[0071] Furthermore, while the embodiments 1 and 2 described above show examples in which a fan-out type PPLT2 is used as the second-order nonlinear optical medium 2, this disclosure is not limited thereto, and other fan-out type second-order nonlinear optical media may be used.

[0072] For example, instead of the fan-out type PPLT2, a fan-out type periodically polarized reversal lithium niobate (PPLN) or periodically polarized reversal KTP (potassium titanyl phosphate) (PPKTP) may be used. When these alternative material systems are used in the second-order nonlinear optical medium 2, it is possible to obtain the same effects and advantages as in Embodiment 1 and / or Embodiment 2.

[0073] <Note> Any reference to elements with designations such as "first..." or "second..." does not limit the quantity or order of those elements. These designations are merely a convenient way to distinguish between two or more elements. For example, references to the first and second elements do not mean that only two elements can be adopted, nor do they mean that the first element must take precedence over the second element in any physical quantity.

[0074] While the Disclosure has been described in detail above, it will be apparent to those skilled in the art that the purpose and scope of this Disclosure are not limited to what has been described herein. This Disclosure can be implemented in modified and altered forms without exceeding the purpose and scope of this Disclosure as defined by the claims. Therefore, the descriptions in this Disclosure are for illustrative purposes only and are not intended to be restrictive in any way to the purpose and scope of this Disclosure.

[0075] This disclosure is useful as a light source that can be used in nonlinear spectroscopy or nonlinear optical microscopy for detecting CRS, such as CARS or SRS.

[0076] 1. Gain medium 2. Second-order nonlinear optical medium 3. Dichroic mirror 4. Thin-film polarizer (TFP) 5. Quarter-wave plate 6. Total internal reflection mirror 7. Oscillation light 8. Excitation light 9. Laser light 10. Dispersion compensation medium 11. Movable stage 12. Aperture

Claims

1. A laser device comprising: a resonator that outputs pulsed laser light, the resonator comprising a gain medium and a nonlinear optical medium disposed in the optical path between a first mirror and a second mirror constituting the resonator, wherein the nonlinear optical medium has a periodic polarization reversal structure for causing the resonator to oscillate in a mode-locked manner, and the periodic polarization reversal structure is a fan-out structure in which the reversal region and the non-reversal region spread out in a fan shape in a direction intersecting the optical path and are alternately arranged in a striped pattern in a direction along the optical path; and a moving mechanism for varying the position of the nonlinear optical medium in a direction at least perpendicular to the optical path within the plane of the nonlinear optical medium.

2. The laser apparatus according to claim 1, wherein the resonator further comprises: a polarizer that outputs pulsed laser light by coupling a specific linear polarization of light traveling back and forth along the optical path to the optical path between the nonlinear optical medium and the second mirror, and reflects a linear polarization orthogonal to the specific linear polarization along the optical path; a quarter-wave plate that rotates the polarization of light traveling back and forth along the optical path; and an aperture having a variable aperture diameter for adjusting the amount of light transmitted back and forth along the optical path.

3. The laser apparatus according to claim 1 or 2, wherein the resonator includes a dispersion compensation medium provided in the optical path.

4. The gain medium is Cr ４＋ : One bulk-type or fiber-type crystal or ceramic selected from YAG, Cr forsterite, Ti sapphire, Cr:LiSAF, Cr:LiCAF, Cr:ZnSe, and Cr:ZnS, or YAG, YVO with one rare earth ion selected from Yb, Er, Nd, Tm, and Ho added. ４ GdVO ４ The laser apparatus according to claim 1 or 2, wherein the material is a bulk-type or fiber-type crystal or ceramic of YLF, CALGO, or a bulk-type or fiber-type glass or semiconductor crystal to which one rare earth ion selected from Yb, Er, Nd, Tm, and Ho is added.

5. The laser apparatus according to claim 1 or 2, wherein the nonlinear optical medium is a periodically polarized reversing lithium tantalate with a fan-out structure, a periodically polarized reversing lithium niobate with a fan-out structure, or a periodically polarized reversing potassium titanyl phosphate with a fan-out structure.

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