Pump light parametric oscillator based on composite crystal

Abstract

The invention belongs to the technical field of laser oscillators, and particularly relates to a pump light parametric oscillator based on a composite crystal. The optical parametric oscillator comprises a pumping source and a resonant cavity, the pumping source comprises a laser and a gain medium for receiving initial laser emitted by the laser, the laser and the gain medium are sequentially arranged in the optical path direction of the optical parametric oscillator, and the gain medium is a composite crystal. The composite crystal comprises neodymium-doped yttrium aluminum garnet and yttrium aluminum garnet fixedly connected with the neodymium-doped yttrium aluminum garnet; the resonant cavity comprises a nonlinear crystal, and the nonlinear crystal comprises selenium gallium barium. The optical parametric oscillator provided by the invention can simultaneously realize high-energy output of the pumping source, high-power and high-stability output of the pumping source and high-energy output of the resonant cavity covering the whole middle and far infrared band, and has wide application prospects in the fields of middle infrared confrontation, environment and atmosphere monitoring and remote sensing.

Description

Technical Field

[0001] This invention belongs to the field of laser oscillator technology, specifically relating to a composite crystal-pumped optical parametric oscillator. Background Technology

[0002] Mid-infrared lasers in the 3–5 µm band, due to their location within the atmospheric transmission window and their characteristic absorption peaks for various gas molecules, possess irreplaceable value in fields such as environmental monitoring, medical diagnosis, industrial sensing, and national defense optoelectronic countermeasures. Optical parametric oscillators (OPOs), as an effective means of generating lasers in this band, achieve wide-tunability output through nonlinear frequency conversion. Their performance is fundamentally dependent on the quality of the pump source and the characteristics of the nonlinear crystal.

[0003] Currently, KTA-OPO technology is relatively mature. KTA-OPO uses Nd:YAG crystal as the gain medium of OPO pump source and potassium titanium arsenate (KTiOAsO4, i.e. KTA) crystal as nonlinear crystal in OPO, with mid-infrared laser output.

[0004] A known optical path diagram of an infrared laser in KTA-OPO is as follows: Figure 1 As shown, the pump source is a conventional Nd:YAG laser with an output wavelength of 1064 nm. The pump light is focused by a lens with a focal length of f=750 mm and then enters the optical parametric oscillator (OPO). The OPO resonant cavity consists of a cavity input mirror M1 and a planar output coupling mirror (OC). The cavity input mirror M1 has high reflectivity (HR>99%) for both signal and idler light in the 1.3-5 μm range and high transmittance (HT>95%) for pump light at 1064 nm. The OC has strong reflectivity for both signal and idler light, reaching up to 80% in the 1.3-5 μm range, and good transmittance for pump light. The cavity length is designed to be 35 mm. When the pump light is focused into the KTA through the lens, it generates a signal light in the 1.3-1.6m band and an idler light in the mid-infrared band in the 3-5m band. The signal light oscillates and intensifies between M1 and OC, while the idler light is also amplified and intensified and output from the OPO through OC. The KTA-OPO can directly output narrow-linewidth mid-infrared laser with an output linewidth of approximately 2.5nm (center wavelength 3.5μm).

[0005] However, when Nd:YAG is used as the gain medium for the pump source, during long-term or high-repetition-frequency operation, part of the pump energy is converted into heat, causing a temperature gradient within the Nd:YAG crystal and inducing a change in refractive index, resulting in a significant thermal lensing effect. This severely affects the output energy and stability of the pump light. Meanwhile, the transmittance range of KTA is 0.35~5.3. The KTA laser can only output mid-infrared laser light and cannot output far-infrared laser light. Furthermore, the slope of the KTA output wavelength change with temperature is... Temperature changes have little effect on the output wavelength, making it difficult to achieve a wide range of wavelength tuning by altering the temperature. Summary of the Invention

[0006] The purpose of this invention is to provide a composite crystal-pumped optical parametric oscillator. The optical parametric oscillator provided by this invention can simultaneously achieve high energy output of the pump source, high power and high stability output of the pump source, and high energy output covering the entire mid- and far-infrared band. It has broad application prospects in the fields of mid-infrared countermeasures, environmental and atmospheric monitoring, and remote sensing.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides an optical parametric oscillator, including a pump source and a resonant cavity. Along the optical path of the optical parametric oscillator, the pump source includes a laser and a gain medium that receives the initial laser emitted by the laser, arranged sequentially. The gain medium is a composite crystal, which includes neodymium-doped yttrium aluminum garnet (Nd:YAG) and yttrium aluminum garnet (YAG) fixedly connected to the neodymium-doped yttrium aluminum garnet. The resonant cavity includes a nonlinear crystal, which includes barium gallium selenide (BaGa4Se7, BGSe).

[0008] Preferably, along the optical path direction of the optical parametric oscillator: the yttrium aluminum garnet is fixedly disposed on the upstream end face and / or downstream end face of the neodymium-doped yttrium aluminum garnet.

[0009] Preferably, the thickness ratio of the yttrium aluminum garnet and the neodymium-doped yttrium aluminum garnet disposed on the upstream or downstream end face of the yttrium aluminum garnet along the optical path is 1:10~12.

[0010] Preferably, the laser is a fiber-coupled laser diode (LD) laser, the fiber has a core diameter of 400 μm and a numerical aperture of 0.22; the center wavelength of the initial laser emitted by the laser is 808 nm; the wavelength of the pump light output by the pump source is 1064 nm; and the wavelength of the idler light obtained by the resonant cavity is 3.4~4.3 μm.

[0011] Preferably, along the optical path direction of the optical parametric oscillator: the resonant cavity further includes a resonant cavity input mirror disposed upstream of the nonlinear crystal and a resonant cavity idler output mirror disposed downstream of the nonlinear crystal; the resonant cavity input mirror has a transmittance >95% for 1064nm laser and a reflectance >99% for 1.4~1.6μm laser; the resonant cavity idler output mirror has a transmittance >95% for 1064nm laser, a reflectance >99% for 1.4~1.6μm laser, and a transmittance >95% for 3.4~4.3μm laser.

[0012] Preferably, along the optical path direction of the optical parametric oscillator: the pump source further includes a pump source input mirror disposed upstream of the gain medium and a pump source output coupling mirror disposed downstream of the gain medium; the pump side of the pump source input mirror is coated with an 808 nm laser antireflection film, and the other side is coated with a thin film for 1064 nm laser reflection and 808 nm laser transmission; the transmittance of the pump source output coupling mirror at 1064 nm is 5~25%.

[0013] Preferably, along the optical path direction of the optical parametric oscillator: the resonant cavity further includes: a frequency doubling crystal and a resonant cavity signal light output mirror sequentially disposed in the reflected light direction of the resonant cavity idler light output mirror, wherein the frequency doubling crystal is an LBO crystal; and a filter and a Ge filter sequentially disposed in the transmitted light direction of the resonant cavity idler light output mirror.

[0014] Preferably, the end face of the LBO crystal is coated with an antireflection film, the antireflection film having a transmittance of >99% for 1300~1700nm laser and 650~850nm laser; and the resonant cavity signal light output mirror having a reflectance of >99% for 1.4~1.6μm laser.

[0015] Preferably, the filter has a reflectivity >99% for 1064nm laser and 1.35~1.65μm laser, and a transmittance >95% for 3~5μm laser; the Ge sheet completely absorbs lasers below 1.7μm and has a transmittance of 55~58% for 3~5μm laser.

[0016] Preferably, the pump source further includes a cooling unit for cooling the gain medium.

[0017] The optical parametric oscillator (OPO) provided by this invention is a BGSe-based OPO pumped by a composite crystal. Its working principle is as follows: The gain medium of the pump source is a composite crystal, with the YAG crystal within the composite crystal acting as a heat sink to suppress thermal effects during pump source operation. The pump light emitted by the pump source enters the resonant cavity. Under 1.06μm laser pumping, the nonlinear crystal BGSe outputs a 3.5μm idler light and a 1.5μm signal light through type I phase matching. Within the resonant cavity, the nonlinear crystal BGSe converts the pump light into signal light (1.4~1.6μm) and idler light (3.4~4.3μm). The OPO provided by this invention uses a composite crystal to improve the thermal lensing effect of the pump source, achieving a better and more stable pumping method. The OPO provided by this invention uses BGSe as the nonlinear crystal, and the transmission range of BGSe reaches 0.47~18μm, covering the entire mid- and far-infrared band. BGSe crystals not only have a wide transmission range, but also exhibit a high slope for output wavelength variation with temperature, specifically reaching 3.20 nm / ℃. The output wavelength can be continuously tunable by changing the angle temperature of the BGSe crystal, thus overcoming the narrow transmission range limitation of KTA-OPO. In summary, the high-efficiency BGSe optical parametric oscillator provided by this invention can simultaneously achieve high-energy laser output, high-power and high-stability pump source output, and high-energy infrared laser output, showing broad application prospects in mid-infrared countermeasures, environmental and atmospheric monitoring, and remote sensing.

[0018] Furthermore, in this invention, along the optical path direction of the optical parametric oscillator, the resonant cavity further includes: a frequency doubling crystal and a resonant cavity signal light output mirror sequentially disposed in the direction of reflected light from the idler frequency output mirror of the resonant cavity, wherein the frequency doubling crystal is an LBO crystal. This invention uses an LBO frequency doubling crystal to suppress the inverse conversion effect of the optical parametric oscillator, thereby obtaining higher idler frequency conversion efficiency and further improving the problem of low KTA-OPO conversion efficiency.

[0019] Furthermore, in this invention, the yttrium aluminum garnet is fixedly disposed on the upstream and downstream end faces of the neodymium-doped yttrium aluminum garnet; the thickness ratio of the yttrium aluminum garnet and the neodymium-doped yttrium aluminum garnet disposed on the upstream or downstream end faces of the neodymium-doped yttrium aluminum garnet along the optical path is 1:10~12. This invention, by fixing the yttrium aluminum garnet on both end faces of the neodymium-doped yttrium aluminum garnet and simultaneously optimizing the thickness ratio of the yttrium aluminum garnet and the neodymium-doped yttrium aluminum garnet, further improves the high-power stable output performance of the pump source.

[0020] In this invention, the center wavelength of the initial laser emitted by the laser is 808 nm; the wavelength of the pump light output by the pump source is 1064 nm; the resonant cavity idler output mirror of the resonant cavity obtains signal light (reflected light from the resonant cavity idler output mirror) and idler light (transmitted light from the resonant cavity idler output mirror); the wavelength of the signal light obtained by the resonant cavity idler output mirror is 1.4~1.6 μm, and the wavelength of the idler light is 3.4~4.3 μm. Attached Figure Description

[0021] Figure 1 The optical path diagram of the infrared laser in KTA-OPO; Figure 2 The optical path diagram of the optical parametric oscillator provided in the embodiment of the present invention; Figure 3 A schematic diagram of the structure of the composite crystal provided by the present invention; Figure 4 The optical path diagram of the pump source for the optical parametric oscillator provided by the present invention; Figure 5 The temperature distribution of three laser crystals, Nd:YAG, Nd:YAG / YAG and YAG / Nd:YAG / YAG, was numerically simulated using COMSOL software under LD pumping. In the diagram: M1 - Pump source input mirror, M2 - Pump source output coupler, M3 - Resonant cavity input mirror, M4 - Resonant cavity idler light output mirror, M5 - Resonant cavity signal light output mirror, M6 - Filter, M7 - Ge filter. Detailed Implementation

[0022] This invention provides an optical parametric oscillator, including a pump source and a resonant cavity. Along the optical path of the optical parametric oscillator, the pump source includes a laser and a gain medium that receives the initial laser emitted by the laser, arranged sequentially. The gain medium is a composite crystal, which includes neodymium-doped yttrium aluminum garnet (Nd:YAG) and yttrium aluminum garnet (YAG) fixedly connected to the neodymium-doped yttrium aluminum garnet. The resonant cavity includes a nonlinear crystal, which includes barium gallium selenide (BaGa4Se7, BGSe).

[0023] In this invention, unless otherwise specified, all raw materials / components are commercially available products well-known to those skilled in the art. In this invention, BGSe: BaGa4Se7, barium gallium selenide, is a mid-to-far-infrared nonlinear crystal. YAG: yttrium aluminum garnet, is a near-infrared solid-state laser crystal. Nd:YAG: neodymium-doped yttrium aluminum garnet. It is one of the most widely used solid-state laser materials, primarily emitting 1064nm near-infrared laser light, and features high gain, high stability, and good thermal performance, making it widely used in industrial processing, medical, scientific research, and military fields. OPO: optical parametric oscillator, composed of a nonlinear crystal and an optical resonant cavity, can achieve a wide range of continuous tuning of the output wavelength. The coating treatment of the nonlinear crystal BGSe, pump source input mirror, pump source output coupling mirror, resonant cavity input mirror, resonant cavity idler light output mirror, and resonant cavity signal light output mirror provided in this invention is carried out using conventional materials and conventional methods, and can be outsourced to conventional coating companies in the field. The present invention does not have any special requirements on the source of the filter and the Ge film; commercially available products or those manufactured by conventional coating companies in the field can be used.

[0024] Figure 2 The optical path diagram of the optical parametric oscillator provided by this invention is shown below. Figure 2 The optical parametric oscillator provided by the present invention will be described in detail.

[0025] The optical parametric oscillator provided by this invention includes a pump source disposed outside the resonant cavity and used to emit pump light into the resonant cavity. In this invention, along the optical path direction of the optical parametric oscillator, the pump source includes a laser and a gain medium sequentially arranged to receive the initial laser light emitted by the laser. In this invention, the laser is preferably a fiber-coupled laser diode (LD) laser. The laser pumps the gain medium. The core diameter of the optical fiber is preferably 400 μm, and the numerical aperture (NA) is preferably 0.22. The center wavelength of the laser pump source is preferably 808 nm.

[0026] In this invention, the gain medium is a composite crystal. The composite crystal includes neodymium-doped yttrium aluminum garnet (Nd:YAG) and yttrium aluminum garnet (YAG) fixedly connected to the neodymium-doped yttrium aluminum garnet.

[0027] In this invention, the end face shape of the composite crystal can be circular, elliptical, or square.

[0028] In this invention, the yttrium aluminum garnet can be fixedly disposed on any surface of the neodymium-doped yttrium aluminum garnet, or can be wrapped around the neodymium-doped yttrium aluminum garnet.

[0029] In this invention, along the optical path direction of the optical parametric oscillator: the yttrium aluminum garnet is preferably fixedly disposed on the upstream end face and / or downstream end face of the neodymium-doped yttrium aluminum garnet. Alternatively, the yttrium aluminum garnet can be fixedly disposed on the side surface (the surface through which no light beam passes) of the neodymium-doped yttrium aluminum garnet.

[0030] In this embodiment of the invention, the yttrium aluminum garnet is fixedly disposed on the upstream and downstream end faces of the neodymium-doped yttrium aluminum garnet, and is referred to as YAG / Nd:YAG / YAG. The yttrium aluminum garnet is fixedly disposed only on the upstream end face of the neodymium-doped yttrium aluminum garnet, and is referred to as YAG / Nd:YAG.

[0031] In this invention, the thickness ratio of the yttrium aluminum garnet and the Nd:YAG disposed on the upstream or downstream end face of the Nd:YAG along the optical path is preferably 1:10~12, and in some embodiments it can be 1:11. In a specific embodiment of this invention, the Nd doping concentration in the Nd:YAG can be 1.1%. The size of the Nd:YAG can be... 5×110mm; the dimensions of the YAG disposed on the upstream or downstream end face of the Nd:YAG can be 5×110mm. 5×10mm.

[0032] In this invention, the pump source preferably further includes a cooling unit for cooling the gain medium. By incorporating the cooling unit, this invention can promptly remove the heat generated by the gain medium. In this invention, the cooling unit is preferably a water-cooled copper heat sink. In this invention, the water-cooled copper heat sink is made entirely of copper, and its overall structure is a copper block. In this invention, the water-cooled copper heat sink has a through-hole structure along the optical path of the optical parametric oscillator for mounting the gain medium. The gain medium is installed in the through-hole structure, with one end receiving the initial laser emitted by the laser and the other end outputting the pump light. The water-cooled copper heat sink also includes a microchannel structure through which the cooling medium flows. Preferably, the gain medium is wrapped in indium foil and placed in the through-hole structure of the water-cooled copper heat sink. The temperature of the cooling unit is preferably 15°C. The water-cooled copper heat sink cools the gain medium using a cooling medium (water).

[0033] In this invention, the preparation method of the composite crystal preferably includes the following steps: photopolymerizing the Nd:yttrium aluminum garnet and the Nd:yttrium aluminum garnet to obtain a photopolymerized crystal. In this invention, prior to photopolymerization, the end faces of the Nd:yttrium aluminum garnet and the Nd:yttrium aluminum garnet are preferably precision-machined to achieve a surface roughness ≤0.5 nm and a flatness lower than λ / 10. This invention preferably performs photopolymerization in a cleanroom. The photopolymerized surface of the crystal must be free of defects such as pitting and bubbles.

[0034] After obtaining the photoresist crystal, the present invention sintersects the photoresist crystal to obtain the composite crystal. In this invention, the sintering process is carried out in a bonding furnace. Preferably, Al2O3 insulating bricks are used to keep the photoresist crystal at a constant temperature, resulting in a more uniform temperature distribution on the bonding surface during heating and cooling. In this invention, the holding temperature for the sintering process is preferably 1300~1400℃, and the holding time is preferably 2~10h. The heating rate from room temperature to the holding temperature for the sintering process is preferably 5~10℃ / min. The cooling rate from the holding temperature for the sintering process to room temperature after the sintering process is preferably 5~10℃ / min. This invention fixes the Nd:YAG and Nd:YAG garnets through sintering.

[0035] In this invention, the pump light output by the pump source is a laser with a wavelength of 1064nm.

[0036] In this invention, along the optical path of the optical parametric oscillator: the pump source further includes a pump source input reflector M1 disposed upstream of the gain medium and a pump source output coupling mirror M2 disposed downstream of the gain medium. The pump source input reflector M1 is disposed between the laser and the gain medium. The pump side of the pump source input reflector M1 is preferably coated with an 808 nm laser antireflection film, and the other side is preferably coated with a 1064 nm laser reflection and 808 nm laser transmission film. The transmittance of the pump source output coupling mirror M2 at 1064 nm is preferably 5-25%, more preferably 10-20%, and in this embodiment, it can be 15%. By controlling the transmittance of the pump source output coupling mirror at 1064 nm to preferably be 15%, this invention can output higher power laser light.

[0037] The optical parametric oscillator provided by this invention includes a resonant cavity. The resonant cavity includes a nonlinear crystal, specifically barium gallium selenide (BaGa4Se7, BGSe). In this invention, the nonlinear crystal BGSe converts pump light into signal light (1.4~1.6μm) and idler light (3.4~4.3μm) within the resonant cavity. In this invention, the nonlinear crystal can be cuboid in shape, with dimensions of 6mm × 8mm × 15mm. The preferred cutting angle of the nonlinear crystal along the optical path direction is (56.3°, 0°). Preferably, the surface of the nonlinear crystal BGSe is polished. In this invention, the light transmission range of BGSe reaches 0.47~18μm, covering the entire mid- and far-infrared band. The BGSe crystal not only has a wide light transmission range but also a high slope for its output wavelength change with temperature, specifically reaching 3.20 nm / ℃. The output wavelength can be continuously adjusted by changing the angle temperature of the BGSe crystal, covering the entire mid- and far-infrared band.

[0038] In this invention, the end face of the nonlinear crystal BGSe is preferably coated with an anti-reflection film for pump light, signal light, and idler light. The end face of the nonlinear crystal BGSe receiving the pump light is coated with an anti-reflection film for pump light, and the transmittance (T) of the anti-reflection film coated on the nonlinear crystal BGSe is preferably >95%. The end face of the nonlinear crystal BGSe transmitting the signal light and idler light is coated with an anti-reflection film for signal light and idler light, and the transmittance (T) of the anti-reflection film coated on the nonlinear crystal BGSe for signal light and idler light is preferably >95%.

[0039] In this invention, along the optical path of the optical parametric oscillator: the resonant cavity further includes a resonant cavity input mirror M3 disposed upstream of the nonlinear crystal, and a resonant cavity idler output mirror M4 disposed downstream of the nonlinear crystal. In this invention, the resonant cavity input mirror M3 preferably has a transmittance (T) >95% for 1064nm laser and a reflectance (R) >99% for 1.4~1.6μm laser. The resonant cavity idler output mirror M4 is preferably tilted, and the tilt angle of the resonant cavity idler output mirror M4 is preferably 45°. The resonant cavity idler output mirror M4 preferably has a transmittance (T) >95% for 1064nm laser, a reflectance (R) >99% for 1.4~1.6μm laser, and a transmittance (T) >95% for 3.4~4.3μm laser.

[0040] In this invention, along the optical path direction of the optical parametric oscillator, the resonant cavity preferably further includes: a frequency doubling crystal and a resonant cavity signal light output mirror sequentially disposed in the direction of reflected light from the idler light output mirror of the resonant cavity, wherein the frequency doubling crystal is an LBO (LiB3O5) crystal. In this invention, the LBO is a frequency doubling crystal that doubles the frequency of the signal light. The end face of the LBO crystal is preferably coated with an antireflection film, and the transmittance (T) of the antireflection film coated on the end face of the LBO crystal for 1300~1700nm laser and 650~850nm laser is preferably >99%. In this invention, when the 1.35~1.65μm signal light laser passes through the LBO crystal, frequency-doubled red light (675~825nm) can be generated under type I matching conditions, thereby suppressing the inverse conversion effect of the signal light and idler light recombining into pump light and improving the conversion efficiency.

[0041] In this invention, the reflectivity (R) of the resonant cavity signal light output mirror M5 for 1.4~1.6μm laser light is preferably >99%.

[0042] In this invention, along the optical path direction of the optical parametric oscillator, the resonant cavity preferably further includes a filter M6 and a Ge plate M7 sequentially disposed in the transmission light direction of the idler output mirror of the resonant cavity. In this invention, the filter M6 is a 1064nm filter. Preferably, the filter M6 is placed at an angle. The reflectivity (R) of the filter M6 for 1064nm laser (pump light) and 1.35~1.65μm laser is preferably >99%, and the transmittance (T) for 3~5μm laser is preferably >95%. In this invention, the Ge plate (i.e., germanium plate) is used to completely filter out residual pump light (1064 nm) and signal light (1.4~1.6μm). The Ge plate completely absorbs laser light below 1.7μm, and the transmittance for 3~5μm laser is preferably 55~58%, and in the embodiment, it can be 56%. In this invention, the output beam is filtered by filter M6 to remove residual 1064 nm light, and then by Ge plate M7 to further filter stray light and attenuate 55-58% (approximately 56%) of the transmitted mid-infrared light, resulting in a pure 3.4-4.3 μm (approximately 3.5 μm) laser, i.e., idler light. The energy of the idler light can then be measured by an energy meter. In this invention, filter M6 and Ge plate M7 can be replaced with other devices, such as lenses with high reflectivity for 1064 nm and signal light, and high transmittance for idler light.

[0043] In this invention, the optical parametric oscillator preferably further includes a measuring instrument disposed downstream of the Ge wafer. The measuring instrument measures the output wavelength of the laser. The measuring instrument is used to measure the energy of the idler light (3~5μm laser, which can be a 3.5μm laser). The measuring instrument can be an energy meter. The measuring instrument can be one or more combinations of a grating spectrometer, a computer, a mid-infrared detector, and an oscilloscope.

[0044] The optical parametric oscillator provided by this invention has an L-shaped optical path. The L-shaped optical path of the optical parametric oscillator provided by this invention can be modified, and a narrow-linewidth output mid-infrared laser can be achieved by using a similar L-shaped optical path.

[0045] The BGSe optical parametric oscillator scheme based on composite crystal pumping provided by this invention has the advantages of good beam quality and stable performance of YAG / Nd:YAG / YAG pump source; wide output range and large temperature tuning range of BGSe crystal laser; and high conversion efficiency of LBO frequency doubling crystal in L-shaped optical path to suppress inverse conversion effect. It improves the conversion efficiency of output laser while improving the thermal effect of pump source and the stability of laser output. This type of laser has broad application prospects in mid-infrared countermeasures, environmental and atmospheric monitoring and remote sensing.

[0046] This invention uses a YAG / Nd:YAG / YAG composite crystal as the gain medium of the pump source and a barium gallium selenide (BaGa4Se7, or BGSe) crystal as the nonlinear crystal in the OPO. High-energy laser output is achieved by inserting a frequency-doubling crystal into the OPO. High-power, high-stability pump source output is achieved by changing the thickness of the YAG segments in the composite crystal; the output wavelength is adjusted by changing the angle and temperature of the BGSe crystal; and high-energy mid-infrared laser output is achieved by adjusting parameters such as the thickness, flatness, and tilt angle of the frequency-doubling crystal.

[0047] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0048] Example 1 The optical path diagram of the optical parametric oscillator provided in this embodiment is as follows: Figure 2 As shown, the gain medium in the pump source is a YAG / Nd:YAG / YAG composite crystal, and the YAG / Nd:YAG / YAG composite crystal is pumped by an optical fiber coupled LD with a center wavelength of approximately 808 nm.

[0049] Among them: pump source input mirror M1, the pump side is coated with an 808 nm anti-reflection film, and the other side is coated with a 1064 nm high reflectance and an 808 nm high transmittance film; The pump source output coupling mirror M2 has a transmittance of 15% at 1064nm; BGSe crystal is a nonlinear crystal; The resonant cavity input mirror M3 has high transmittance for 1064nm laser (transmittance T>95%) and high reflectivity for 1.4~1.6μm (reflectivity R>99%).

[0050] The resonant cavity idler output mirror M4 has high transmittance at 1064nm (T>95%), high reflectance at 1.4~1.6μm (R>99%), and high transmittance at 3.4~4.3μm (T>95%). The resonant cavity signal light output mirror M5 has high reflectivity (reflectivity R>99%) for 1.4~1.6μm. LBO is a frequency doubling crystal that doubles the frequency of signal light. The end face of the LBO crystal is coated with an anti-reflection film with high transmittance (T>99%) for 1300~1700nm and 650~850nm. The 1064nm filter M6, placed at a small angle, has high reflectivity (R>99%) for pump light (1064nm) and signal light (1.35~1.65μm), and high transmittance (T>95%) for idler light (3~5μm). The Germanium wafer M7 is used to completely filter out residual pump light and signal light. The germanium wafer completely absorbs laser light below 1.7μm and has a transmittance of about 56% for laser light in the 3~5μm range. An energy meter is used to measure the energy of a 3.5μm laser. It can also be used to measure the output wavelength of a laser by combining a grating spectrometer, a computer, a mid-infrared detector, and an oscilloscope.

[0051] The working principle of the optical parametric oscillator provided by this invention is as follows: The gain medium of the pump source is a YAG / Nd:YAG / YAG composite crystal, with the YAG crystals at both ends of the composite crystal serving as heat sinks to suppress thermal effects during pump source operation. An 808 nm laser is emitted using an optical fiber-coupled LD. A 1064 nm laser is generated by the YAG / Nd:YAG / YAG composite crystal within the resonant cavity formed by the pump source input reflector M1 and the pump source output coupling mirror M2. The YAG / Nd:YAG / YAG composite crystal is temperature-controlled by a water-cooled heat sink to maintain stable output.

[0052] When a 1064 nm laser is used as the pump light into the resonant cavity (BGSe-OPO), the nonlinear crystal is a BGSe crystal. Under 1.06 μm laser pumping, the BGSe outputs a 3.5 μm idler light and a 1.5 μm signal light through type I phase matching. The end faces of the BGSe crystal are coated with an antireflection film that provides high transmittance (T>95%) for the pump light, signal light, and idler light. The resonant cavity input mirror M3 provides high transmittance (T>95%) for the 1064 nm laser and high reflectivity (R>99%) for the 1.4~1.6 μm range. The resonant cavity idler light output mirror M4, placed at 45°, provides high transmittance (T>95%) for the 1064 nm laser, high reflectivity (R>99%) for the 1.4~1.6 μm range, and high transmittance (T>95%) for the 3.4~4.3 μm range. The resonant cavity signal light output mirror M5 provides high reflectivity (R>99%) for the 1.4~1.6 μm range. The nonlinear crystal BGSe converts pump light into signal light (1.4~1.6μm) and idler light (3.4~4.3μm) within the resonant cavity.

[0053] A LiB3O5 (LBO) crystal is placed between the resonant cavity idler output mirror M4 and the resonant cavity signal output mirror M5. When the 1.35~1.65μm signal light passes through the LBO crystal, it can generate frequency-doubled red light (675~825nm) under type I matching conditions, thereby suppressing the inverse conversion effect of the signal light and idler light recombining into pump light and improving the conversion efficiency. The output beam is filtered by a 1064nm filter M6 to remove residual 1064nm light, and then further filtered by a Ge plate to remove stray light and attenuate about 56% of the transmitted mid-infrared light, resulting in a pure laser of about 3.5μm (i.e., idler light 3.4~4.3μm). Finally, its energy is measured by an energy meter.

[0054] The method for preparing an optical parametric oscillator provided by this invention specifically includes the following steps: Step 1: Cut one Nd:YAG crystal and two YAG crystals from the Nd:YAG crystal and the YAG crystal respectively. The Nd:YAG crystal has an Nd doping concentration of 1.1% and a size of [missing information]. 5×110mm; YAG crystal size is 5×10mm. The Nd:YAG crystal and the YAG crystal end face are precision machined to achieve a roughness of less than 0.5nm and a flatness of less than λ / 10.

[0055] Step 2: In a cleanroom, photoresist is applied to Nd:YAG crystal and two YAG crystals to prepare YAG / Nd:YAG / YAG photoresist crystals. The photoresist surface must be free of defects such as pits and bubbles. Figure 3 This is a schematic diagram of the structure of a YAG / Nd:YAG / YAG composite crystal.

[0056] Step 3: Place the YAG / Nd:YAG / YAG photoresist crystal in a bonding furnace for high-temperature sintering. Use Al2O3 insulating bricks to keep the crystal at a constant temperature to ensure a more uniform temperature distribution on the bonding surface during heating and cooling. Set the maximum temperature to 1400℃, the holding time to 8 hours, and the heating / cooling rate to 10℃ / min. Once cooled to room temperature, remove the crystal to obtain the YAG / Nd:YAG / YAG composite crystal. Figure 3 As shown.

[0057] Step 4: Based on the YAG / Nd:YAG / YAG composite crystal from Step 3, construct a pump source to output a 1064nm laser. The optical path of the pump source is as follows: Figure 4 As shown, Figure 4 This is a pump source based on a YAG / Nd:YAG / YAG composite crystal. The gain medium of the pump source is a YAG / Nd:YAG / YAG composite crystal, which is pumped using an optical fiber-coupled laser (LD) with a center wavelength of approximately 808 nm. The fiber core diameter is 400 μm, and the numerical aperture (NA) is 0.22. To effectively dissipate the heat generated by the YAG / Nd:YAG / YAG composite crystal during laser experiments, the crystal is wrapped in indium foil and placed in a water-cooled copper heat sink with a microchannel structure, maintaining the temperature at 15°C. The pump source outputs a 1064 nm laser beam.

[0058] Step 5: A BGSe crystal was selected as the nonlinear conversion crystal in the resonant cavity. The BGSe crystal is a cuboid with dimensions of 6mm × 8mm × 15mm and a cut angle of (56.3°, 0°). The surface of the BGSe crystal was polished, and antireflection coatings were deposited on the pump light, signal light, and idler light.

[0059] Step Six: Based on the pump source from Step Four and the BGSe crystal from Step Five, construct an optical parametric oscillator. The optical path diagram is as follows: Figure 2 As shown.

[0060] Test example: This invention uses COMSOL software to numerically simulate the temperature distribution of three laser crystals—Nd:YAG, Nd:YAG / YAG, and YAG / Nd:YAG / YAG—under LD pumping. The results are as follows: Figure 5 As shown in the figure. Simulation results show that, under the same pumping conditions, the composite crystal structure can effectively improve heat dissipation: compared with the maximum temperature of 351K for a single Nd:YAG crystal, the maximum temperatures of Nd:YAG / YAG and YAG / Nd:YAG / YAG are reduced by 13K and 15K, respectively; and at the crystal end face, the temperature reduction of the composite crystal is more significant, reaching 30K and 33K, respectively. Analysis shows that the undoped YAG portion acts as a heat sink, effectively reducing the pump end face temperature and dispersing the heat zone, thereby reducing the temperature gradient and thermal stress. As the pump power increases from 5W to 80W, the maximum temperature rise of the YAG / Nd:YAG / YAG composite crystal is reduced by 23.88% and 27.75% compared with Nd:YAG, respectively, proving that the composite structure can significantly suppress thermal effects and improve the thermal management capability of the crystal under high power.

[0061] As can be seen from the above embodiments: (1) When Nd:YAG is used as the gain medium of the pump source, during long-term or high-repetition-frequency operation, the pump energy is partially converted into heat energy, resulting in a temperature gradient inside the crystal and causing a change in refractive index, which will produce a significant thermal lensing effect. This will seriously affect the output efficiency, beam quality and stability of the pump light. In this invention, in response to the thermal lensing effect of the Nd:YAG pump source, YAG crystals are bonded to both ends of the Nd:YAG crystal to prepare a YAG / Nd:YAG / YAG composite crystal. The YAG crystal acts as a heat sink to reduce the waste heat generated by the Nd:YAG crystal during the operation of the pump source. At the same time, the YAG / Nd:YAG / YAG composite crystal is cooled by constant-temperature circulating water cooling. (2) The transmittance range of KTA is 0.35~5.3. m, can only output mid-infrared laser and cannot output far-infrared laser. In addition, the slope of the KTA output wavelength with temperature is 0.077 nm / ℃, and the temperature change has little effect on the output wavelength, making it difficult to obtain a wide range of wavelength tuning by changing the temperature. In view of the disadvantage of the narrow light transmission range of KTA crystal, this invention uses BGSe crystal as a substitute. The light transmission range of BGSe reaches 0.47~18μm, which can cover the entire mid- and far-infrared band. BGSe crystal not only has a wide light transmission range, but its output wavelength also has a high slope with temperature, specifically up to 3.20 nm / ℃, and the output wavelength can be continuously tunable by changing the temperature. (3) The thermal lensing effect of Nd:YAG pump source will change the spot size and wavefront curvature of the pump light in the nonlinear crystal, destroying the precise matching between it and the signal light mode in the OPO resonant cavity, resulting in a decrease in nonlinear conversion efficiency. In view of the problem of low KTA-OPO conversion efficiency, this invention uses LBO frequency doubling crystal to suppress the inverse conversion effect of optical parametric oscillator, thereby obtaining a higher idler light conversion efficiency. In summary, the wide-tunable-range BGSe optical parametric oscillator scheme based on composite crystal pumping provided by this invention has the advantages of good beam quality and stable performance of YAG / Nd:YAG / YAG pump source; wide output range and large temperature tuning range of BGSe crystal laser; and an L-shaped resonant cavity. The L-shaped optical path constructed with LBO frequency doubling crystal suppresses the inverse conversion effect and has high conversion efficiency. It improves the conversion efficiency of the output laser while improving the thermal effect of the pump source and the stability of laser output. This type of laser has broad application prospects in the fields of mid-infrared countermeasures, environmental and atmospheric monitoring, and remote sensing.

[0062] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. An optical parametric oscillator, comprising a pump source and a resonant cavity, characterized in that, Along the optical path of the optical parametric oscillator, the pump source includes a laser arranged in sequence and a gain medium that receives the initial laser emitted by the laser. The gain medium is a composite crystal, which includes neodymium-doped yttrium aluminum garnet and yttrium aluminum garnet fixedly connected to the neodymium-doped yttrium aluminum garnet. The resonant cavity includes a nonlinear crystal, which includes barium gallium selenide.

2. The optical parametric oscillator according to claim 1, characterized in that, Along the optical path of the optical parametric oscillator: the yttrium aluminum garnet is fixedly disposed on the upstream end face and / or downstream end face of the neodymium-doped yttrium aluminum garnet.

3. The optical parametric oscillator according to claim 2, characterized in that, The thickness ratio of the yttrium aluminum garnet and the neodymium-doped yttrium aluminum garnet disposed on the upstream or downstream end face of the yttrium aluminum garnet along the optical path is 1:10~12.

4. The optical parametric oscillator according to claim 1, characterized in that, The laser is a fiber-coupled laser diode laser, the fiber has a core diameter of 400 μm and a numerical aperture of 0.22; the center wavelength of the initial laser emitted by the laser is 808 nm; the wavelength of the pump light output by the pump source is 1064 nm; and the wavelength of the idler light obtained by the resonant cavity is 3.4~4.3 μm.

5. The optical parametric oscillator according to claim 1 or 4, characterized in that, Along the optical path of the optical parametric oscillator: the resonant cavity further includes a resonant cavity input mirror disposed upstream of the nonlinear crystal and a resonant cavity idler output mirror disposed downstream of the nonlinear crystal; the resonant cavity input mirror has a transmittance of >95% for 1064nm laser and a reflectance of >99% for 1.4~1.6μm laser; the resonant cavity idler output mirror has a transmittance of >95% for 1064nm laser, a reflectance of >99% for 1.4~1.6μm laser, and a transmittance of >95% for 3.4~4.3μm laser.

6. The optical parametric oscillator according to claim 1 or 4, characterized in that, Along the optical path of the optical parametric oscillator: the pump source further includes a pump source input mirror disposed upstream of the gain medium and a pump source output coupler disposed downstream of the gain medium; the pump side of the pump source input mirror is coated with an 808 nm laser antireflection film, and the other side is coated with a thin film for 1064 nm laser reflection and 808 nm laser transmission; the transmittance of the pump source output coupler at 1064 nm is 5~25%.

7. The optical parametric oscillator according to claim 5, characterized in that, Along the optical path of the optical parametric oscillator: the resonant cavity further includes: a frequency doubling crystal and a resonant cavity signal light output mirror, which are sequentially arranged in the direction of reflected light of the idler frequency output mirror of the resonant cavity, wherein the frequency doubling crystal is an LBO crystal; and a filter and a Ge filter, which are sequentially arranged in the direction of transmitted light of the idler frequency output mirror of the resonant cavity.

8. The optical parametric oscillator according to claim 7, characterized in that, The LBO crystal has an anti-reflection coating on its end face, and the anti-reflection coating has a transmittance of >99% for 1300~1700nm laser and 650~850nm laser; the resonant cavity signal light output mirror has a reflectance of >99% for 1.4~1.6μm laser.

9. The optical parametric oscillator according to claim 7, characterized in that, The filter has a reflectivity of >99% for 1064nm laser and 1.35~1.65μm laser, and a transmittance of >95% for 3~5μm laser; the Ge sheet completely absorbs lasers below 1.7μm, and has a transmittance of 55~58% for 3~5μm laser.

10. The optical parametric oscillator according to claim 1, characterized in that, The pump source also includes a cooling unit for cooling the gain medium.

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