Phonon-assisted Raman generation laser based on KGW crystal

Abstract

The invention belongs to the field of solid Raman laser, and provides a KGW crystal-based phonon-assisted Raman generation laser, which comprises a pumping source laser, an optical parametric oscillator and a KGd (WO4) 2 crystal Raman resonant cavity, the pumping source laser generates pumping light with the wavelength of lambda, and then the pumping light is divided into two parts through the polarization splitting prism: one part generates green light with the wavelength of lambda / 2 through frequency multiplication, and the green light is injected into the optical parametric oscillator after the pumping light is filtered out to generate near-infrared light with the wavelength of lambda / 2 meeting second-class phase matching; and the other part of the light passes through a lambda / 2 wave plate and then is synchronously injected into the KGd (WO4) 2 crystal Raman resonant cavity with the near-infrared light at a set angle, and finally first-order Stokes light is generated.

Description

Technical Field

[0001] The present invention belongs to the field of solid Raman lasers, and particularly relates to a phonon-assisted Raman generation laser based on a KGW crystal. Background Technique

[0002] The statements in this part only provide background technical information related to the present invention and do not necessarily constitute prior art.

[0003] Stimulated Raman scattering effect is a non-linear effect that has been widely studied and applied. Stimulated Raman scattering has characteristics such as good directivity, high monochromaticity, and high intensity, and has wide applications in the generation of eye-safe lasers, biochemistry, medical diagnosis and treatment, spectral analysis, etc. Currently, the related research on stimulated Raman scattering mainly focuses on the visible light and near-infrared short-wave bands. In recent years, with the emergence of new Raman crystal materials, the research in the eye-safe band has gradually become mature. Nevertheless, the research on stimulated Raman scattering has always been limited to the near-infrared band, and the road to long-wave development is full of difficulties. This is mainly because the Raman gain coefficient in the stimulated Raman scattering process is approximately inversely proportional to the wavelength. Therefore, the longer the wavelength, the higher the threshold for generating stimulated Raman scattering. The usual solution is to use a light source with a longer wavelength and higher power density to pump a Raman crystal medium with a larger Raman frequency shift, but this has more stringent requirements for the pumping laser and the crystal. When applied in engineering, it is often difficult to have a light source with a large enough power density, and the damage threshold of the Raman crystal medium is not high enough. Summary of the Invention

[0004] In order to solve the technical problems existing in the above background technique, the present invention provides a phonon-assisted Raman generation laser based on a KGW crystal, which can generate stimulated Raman scattering light relying on phonon assistance when the pump light power density is insufficient.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: A phonon-assisted Raman generation laser based on a KGW crystal, comprising: A pump source laser, an optical parametric oscillator, and a KGd(WO 4 ) 2 Crystal Raman resonator; The pump source laser generates pump light with a wavelength of λ, and then divides it into two parts through a polarization beam splitter prism: One part generates green light with a wavelength of λ / 2 through frequency doubling. After filtering out the pump light, the green light is injected into the optical parametric oscillator to generate near-infrared light that satisfies type-II phase matching with the λ / 2 wavelength; The other part passes through a λ / 2 wave plate and then is synchronously injected into KGd(WO 4 ) 2In the crystal Raman resonator, first-order Stokes light is finally generated.

[0006] As an implementation manner, the pump source laser is a Q-switched laser with a wavelength of λ pumped by a xenon lamp and Nd:YAG.

[0007] As an implementation manner, the pump light with a wavelength of λ is frequency-doubled through a KTP crystal.

[0008] As an implementation manner, the green light is filtered out of the excess pump light through an optical dichroic mirror and then injected into the optical parametric oscillator.

[0009] As an implementation manner, the optical parametric oscillator includes a λ / 2 wave plate, a front cavity mirror, a KTP crystal, a rear cavity mirror, a colored glass filter, and a long-pass filter arranged in sequence along the optical axis.

[0010] As an implementation manner, the front cavity mirror is coated with an antireflection film; the rear cavity mirror is coated with an antireflection film and a reflection film.

[0011] For example, the front cavity mirror is coated with an antireflection film for 532nm, 913nm, and 1273nm; the rear cavity mirror is coated with an antireflection film for 532nm and 913nm and a partial reflection film for 1273nm with a reflectivity of 50%.

[0012] As an implementation manner, the KTP crystal is a KTP crystal using type-II phase matching.

[0013] As an implementation manner, after the pump light passes through the λ / 2 wave plate, it is then injected into the KGd(WO 4 ) 2 crystal Raman resonator at a set angle through a 45° total reflection mirror.

[0014] As an implementation manner, the KGd(WO 4 ) 2 crystal Raman resonator includes front and rear cavity mirrors and a KGd(WO 4 ) 2 crystal arranged between the front and rear cavity mirrors.

[0015] As an implementation manner, the KGd(WO 4 ) 2 crystal is a standard product with a p-axis tangential cut.

[0016] The beneficial effects of the present invention are: The present invention provides a scheme for generating stimulated Raman scattering light assisted by phonons when the pump light power density is insufficient. It has lower requirements for the pump light source, and the pump light source can generate its first-order Stokes light without a very high power density. The present invention expands the wavelength range of stimulated Raman scattering. Through the method proposed in this scheme, the stimulated Raman scattering effect can be realized in the near-infrared long wave, mid-infrared and far-infrared, and even in the terahertz band which is widely used in the medical communication field at present, breaking through the limitation of the Raman frequency shift of the Raman medium to achieve the effective output of stimulated Raman scattering light.

[0017] Advantages of additional aspects of the present invention will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present invention. Brief Description of the Drawings

[0018] The accompanying drawings forming a part of this specification are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0019] Figure 1 Schematic diagram of a phonon-assisted Raman generation laser based on a KGW crystal according to an embodiment of the present invention.

[0020] Figure 2 Phase matching diagram calculated in a KGW crystal.

[0021] Figure 3 Spectrum diagram of the first-order Stokes light generated by a KGW Raman resonator when the pump light energy at 1064 nm is 45 mJ.

[0022] Figure 4 Spectrum diagram of the Stokes light at 1438 nm generated by a KGW Raman resonator when the pump light energy at 1064 nm is 45 mJ and the energy of the second pump light with a wavelength of 1273 nm injected is 1.03 mJ. Detailed Description of the Embodiments

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0025] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they specify the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0026] In the present invention, terms such as "upper", "lower", "left", "right", "front", "rear", "vertical", "horizontal", "side", "bottom", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. They are only relational terms determined for the convenience of describing the structural relationship of each component or element of the present invention and do not specifically refer to any component or element in the present invention and should not be construed as a limitation to the present invention.

[0027] In the present invention, terms such as "fixed connection", "connected", "connected to" should be understood in a broad sense, which may mean a fixed connection, an integral connection or a detachable connection; it may be directly connected or indirectly connected through an intermediate medium. For those related scientific research or technical personnel in the field, the specific meanings of the above terms in the present invention can be determined according to specific circumstances and should not be construed as a limitation to the present invention.

[0028] Term Explanation: KGW crystal: That is, KGd(WO 4 ) 2 crystal.

[0029] According to Figure 1 shown, the phonon-assisted Raman generation laser based on KGW crystal according to the embodiment of the present invention includes: a pump source laser, an optical parametric oscillator, and a KGd(WO 4 ) 2 crystal Raman resonator; The pump source laser generates pump light with a wavelength of λ, which is then divided into two parts by a polarization beam splitter prism: one part generates green light with a wavelength of λ / 2 through frequency doubling, and after filtering out the pump light, the green light is injected into the optical parametric oscillator to generate near-infrared light that satisfies type-II phase matching with the λ / 2 wavelength; the other part passes through a λ / 2 wave plate and then is synchronously injected into the KGd(WO 4 ) 2 crystal Raman resonator at a set angle, and finally generates first-order Stokes light.

[0030] ; ; θ is KGd(WO 4 )2 Cutting angle of the crystal; = 532.08 nm, = 913 nm, = 1273.38 nm, is the refractive index of light with a wavelength of λ, polarization along the j direction in the KGd(WO 4 ) 2 crystal; j is x or y.

[0031] Wherein, the wavelength λ can be specifically set according to the actual situation. The following are specific embodiments: Taking a Q-switched laser with a wavelength of 1064.15 nm pumped by a xenon lamp pumped Nd:YAG as the pump source laser as an example, its pulse width is 12.5 ns and the pulse repetition frequency is 1 Hz. A half-wave plate (HWP1) at 1064 nm works together with an optical isolator (ISO) to prevent the laser beam at 1064.15 nm from returning to the laser cavity. A convex lens L1 (f = 250 mm) and a concave lens L2 (f = -100 mm) together constitute a beam shrinking device, and the beam shrinking ratio of the beam is 2.5:1, and the spot diameter of the shrunk beam is 1.7 mm. The beam is split into two beams by a polarization beam splitter prism (PBS).

[0032] One beam of laser is reflected by mirrors M1 and M2 at 1064 nm to pump the Raman laser. The Raman cavity consists of a front cavity mirror H1 and a rear cavity mirror H2. H1 has a high transmittance (e.g., T > 99.0%) at 1064.15 nm and a high reflectance (e.g., R > 99.5%) at 1177.19 m. H2 has a high transmittance (e.g., T = 99.0%) at 1064.15 nm and a partial reflectance (e.g., R = 76.0%) at 1177.19 nm. The KGW Raman crystal is cut along the p-axis and has a size of . Its inlet and outlet surfaces are coated with an antireflection film at all frequencies of and . A half-wave plate HWP3 is used to adjust the polarization direction of the 1064.15 nm pump beam so that it is parallel to the m-axis of the KGW crystal. So the SRS excitation mode is p[mm]p. The corresponding Raman frequency shift is 901 cm -1 , the Raman gain coefficient is 3.5 cm / GW, and the wavelength is 1064.15 nm. M2 is installed on a control platform composed of an electronically controlled rotation platform and a two-dimensional translation platform, and the adjustment accuracy of the electronically controlled rotation platform is 0.00125°. M2 can be used to accurately adjust the angle β between the 1064.15 nm beam pumped and the axis of the Raman cavity.

[0033] The second pump beam at 1064.15 nm reaches the frequency doubling crystal KTiOPO after being reflected by the 1064 nm mirrors M3 and M4 4 (KTP1). The generated 532.12 nm laser has a pulse width of approximately 10 ns and a beam diameter of 1.1 mm. M5 is an optical dichroic mirror with high reflectivity at 532.12 nm (e.g., R>99%) and high transmittance at 1064.15 nm (e.g., T>99%). A 532.12 nm laser is used to pump an optical parametric oscillator composed of an OPO front cavity mirror H3 and an OPO rear cavity mirror H4. The coating of H3 has high transmittance at the pump and high reflectivity at shorter and longer parametric light wavelengths. The coating of H4 has a relatively high transmittance at the pump and shorter parametric light wavelengths and partial reflection at longer parametric light wavelengths, e.g., R is approximately 50% at 1273 nm. The nonlinear KTP2 crystal is cut at , and has a size of . As shown in Figure 2 , under type-II phase matching conditions (o+e→o), parametric light in the spectral ranges of 855.77~945.33 nm and 1216.74~1406.13 nm can be generated. In this experiment, 1273.38 nm laser is used as the secondary pump source for the KGW Raman crystal. This laser is reflected by M7 and M8 after passing through the long-pass filter M6. A convex lens L3 (f = 200 mm) is used to focus the beam spot size of the 1273.38 nm beam to 1.7 mm.

[0034] In the nonlinear interaction of a pump beam and a Stokes beam, phase matching is not required. However, for phonon-assisted Raman generation, since the phonon wave Q wave must interact with , , , simultaneously, the wave vectors of the four-frequency optical waves must satisfy phase matching.

[0035] As shown in Figure 3 , the 1064.15 nm pump light is incident obliquely into the Raman resonator of the KGW crystal at an external angle of 2.55°. When the energy of 1064.15 nm reaches 30 mJ, axial Stokes light at 1177.19 nm can be observed. When the energy of 1064.15 nm reaches 60 mJ, the measured energy of the 1177.19 nm Stokes light reaches 15.8 mJ, and the conversion efficiency can still reach 26.3%. In this case, a large number of stimulated phonons will be generated inside the crystal due to stimulated polarization.

[0036] When ω 1p and ω 2pWhen the pump pulse energies at [the specific location] are 45.0 mJ and 1.03 mJ respectively, the spectral diagram at 1438.51 nm is measured using a YOKOGAWA AQ6370C spectrometer, as Figure 4 shown.

[0037] Finally, when the pump energy at 1064.15 nm is 60 mJ and the pump energy at 1273.38 nm is 1.26 mJ, the maximum output energy at 1438.51 nm is 122 μJ, and the optical-to-optical conversion efficiency from 1273.38 nm to 1438.51 nm can reach 10%.

[0038] In this embodiment, the green light of 532 nm finally outputs near-infrared light of 1273 nm after passing through an optical parametric oscillator. After passing through two high-reflection mirrors in the near-infrared band, it is synchronously injected into the Raman resonator of KGW together with the pump light of 1064 nm. The pump light of 1064 nm with a higher power density generates first-order Stokes light in the resonator, causing a large number of stimulated phonons to be generated inside the crystal, providing additional gain for 1273 nm whose power density is not sufficient to generate first-order Stokes light, and finally realizing the generation of its first-order Stokes light at 1438 nm.

[0039] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A phonon-assisted Raman generation laser based on KGW crystal, characterized in that: include: Pump source laser, optical parametric oscillator and KGd(WO4)2 crystal Raman resonator; The pump source laser generates pump light with a wavelength of λ, which is then divided into two parts by a polarization beam splitter prism: A portion of the green light is generated by frequency doubling with a wavelength of λ / 2, and the green light is injected into an optical parametric oscillator after filtering out the pump light to generate near-infrared light that satisfies the second type phase matching with the wavelength of λ / 2; The other part passes through the λ / 2 wave plate and is then injected into the KGd(WO4)2 crystal Raman resonant cavity at a set angle synchronously with the near-infrared light, ultimately generating first-order Stokes light.

2. The phonon-assisted Raman generation laser based on KGW crystal according to claim 1, characterized in that: The pump source laser is a xenon lamp pumped Nd:YAG Q-switched laser with a wavelength of λ.

3. The phonon-assisted Raman generation laser based on KGW crystal according to claim 1, characterized in that: The pump light with wavelength λ is frequency doubled by a KTP crystal.

4. The phonon-assisted Raman generation laser based on KGW crystal according to claim 1, characterized in that: The green light is injected into the optical parametric oscillator after filtering out redundant pump light through an optical dichroic mirror.

5. The phonon-assisted Raman generation laser based on KGW crystal according to claim 1, characterized in that: The optical parametric oscillator comprises a λ / 2 wave plate, a front cavity mirror, a KTP crystal, a rear cavity mirror, a colored glass filter and a long-pass filter which are sequentially arranged along an optical axis.

6. The phonon-assisted Raman generation laser based on KGW crystal according to claim 5, characterized in that: The front cavity mirror is coated with anti-reflection film; the rear cavity mirror is coated with anti-reflection film and reflective film.

7. The phonon-assisted Raman generation laser based on KGW crystal according to claim 5, characterized in that: The KTP crystal is a KTP crystal using type II phase matching.

8. The phonon-assisted Raman generation laser based on KGW crystal according to claim 1, characterized in that: After the pump light passes through the λ / 2 wave plate, it is injected into the KGd(WO4)2 crystal Raman resonant cavity at a set angle through a 45° total reflection mirror.

9. The phonon-assisted Raman generation laser based on KGW crystal according to claim 1, characterized in that: The KGd(WO4)2 crystal Raman resonant cavity comprises front and rear cavity mirrors and a KGd(WO4)2 crystal arranged between the front and rear cavity mirrors.

10. The phonon-assisted Raman generation laser based on KGW crystal according to claim 1, characterized in that: The KGd(WO4)2 crystal is a standard product tangential to the p-axis.

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