Miniaturized medium-long wave infrared solid-state laser using thulium-doped laser to directly pump ZGP crystal

Abstract

The invention relates to a miniaturized medium-long wave infrared solid-state laser in which a thulium-doped laser directly pumps a ZGP crystal, the thulium-doped laser takes two laser diodes with the central emission wavelength of 793 nm as pumping sources, two Tm: YAP crystals with the same parameters are cascaded and placed in a U-shaped resonant cavity to serve as laser gain media, and an acousto-optic Q crystal controls intra-cavity loss in the resonant cavity, so that the thulium-doped laser directly pumps the ZGP crystal. Pulse laser with the wavelength of 1.94 m is output; the 1.94 m pulse thulium laser serves as pump light and passes through the ZGP-OPO module, and 2.8 m signal light and 6.2 m idler frequency light are obtained. The Tm laser outputs 1.94 m laser through primary oscillation to directly serve as a pump source, the ZGP-OPO outputs medium-long wave infrared laser in a nonlinear mode, the overall light path structure of the medium-long wave laser is simplified, and the electro-optical conversion efficiency is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of high-power laser optics, and in particular relates to a miniaturized medium- and long-wave infrared solid-state laser in which a thulium-doped laser directly pumps a ZGP crystal. Background Art

[0002] Mid-infrared lasers, located in the atmospheric window, are widely used in various fields, especially in remote sensing, medical applications, and laser countermeasures. Nonlinear frequency conversion technology, due to its high conversion efficiency and wavelength tunability, is a powerful means of generating mid- and long-wave infrared pulsed lasers. Among numerous nonlinear crystals, zinc germanium phosphide (ZnGeP2) (ZGP) possesses an extremely high nonlinear coefficient, a high damage threshold, and excellent thermal conductivity, enabling stable operation in both low-repetition-rate, high-energy and high-average-power, high-repetition-rate output states. Therefore, the optical parametric oscillation (ZGP-OPO) technology based on ZGP crystals is currently the most mature in the field of mid-infrared light sources and is an important approach to achieving pulsed lasers in the mid- and long-infrared bands.

[0003] The traditional ZGP-OPO technology uses 2.1µm pulsed laser light emitted by a holmium (Ho) laser as the pump source. Specifically, a Tm laser pumps the 2.1µm laser output of a Ho laser, resulting in a ZGP-OPO pump source. However, the Ho-ZGP-OPO system consists of multiple laser pumping stages, resulting in low overall electro-optical conversion efficiency. Furthermore, the actual optical path is complex and the stability is poor. This makes it difficult to meet the demands of modern applications for miniaturized, highly stable lasers. Furthermore, the thermal effects and beam quality degradation of the Ho laser further limit its overall performance. Summary of the Invention

[0004] The purpose of the present invention is to optimize the traditional ZGP-OPO technology route for outputting medium- and long-wave lasers, such as low overall electro-optical conversion efficiency of the laser, low system stability caused by the complex laser optical path structure, and poor output beam quality. A short-wave to medium- and long-wave infrared all-solid-state laser is proposed. By directly pumping a zinc-germanium-phosphorus (ZGP) optical parametric oscillator (OPO) with a thulium-doped (Tm:YAP) solid-state laser, a compact structure, high electro-optical conversion efficiency, and high beam quality 2.8 µm medium-wave infrared and 6.2 µm long-wave infrared laser output are achieved.

[0005] The technical solutions of the present invention are as follows: A miniaturized medium- and long-wave infrared solid-state laser that is directly pumped by a thulium-doped laser into a ZGP crystal. The laser comprises a 793 nm pump beam collimation and focusing module, a Tm:YAP laser resonant cavity module, a 1.94 µm beam conversion module, and a medium- and long-wave ZGP-OPO module. The 793 nm pump beam collimation and focusing module includes: The first laser diode and the second laser diode respectively output 793nm random polarization continuous pump light; Two symmetrically arranged 793 nm collimating and focusing plano-convex lens groups are used to collimate and focus the 793 nm pump light output by the first laser diode and the second laser diode and inject it into the Tm:YAP crystal; Two 45° fully reflective mirrors are distributed within the 793 nm collimating and focusing plano-convex lens group to fold the optical path to compress the laser volume; The Tm:YAP laser resonant cavity module includes a U-shaped resonant cavity and two Tm:YAP crystals placed in series within the U-shaped resonant cavity, with their long sides arranged coaxially with the optical path, to emit 1.94 µm laser light through stimulated emission of radiation. The 1.94µm beam transformation module is used to control the light spot parameters and polarization state; The medium- and long-wavelength ZGP-OPO module includes an OPO four-shaped ring cavity and a ZGP crystal placed in the OPO four-shaped ring cavity. The ZGP crystal is used to output 2.8 µm and 6.2 µm medium- and long-wavelength infrared lasers through nonlinear conversion.

[0006] In the Tm:YAP laser resonant cavity module, two first dichroic mirrors and a second dichroic mirror are placed perpendicularly and symmetrically on both sides of two Tm:YAP crystals placed in series, and the laser resonant cavity presents a "U"-shaped structure. A 1.94 μm total reflection flat mirror serves as a front cavity mirror; a 1.94 μm plano-concave output coupling lens serves as a rear cavity output mirror; an acousto-optic Q crystal is placed between the dichroic mirror and the front cavity mirror; and a Fabry-Perot etalon is placed between the dichroic mirror and the rear cavity mirror.

[0007] The 1.94 µm beam conversion module, comprising a 1.94 µm first plano-concave lens, a 1.94 µm second plano-convex lens, and a 1.94 µm third plano-convex lens, controls the spot diameter and divergence of the 1.94 µm beam to the required sizes before injecting it into the ZGP-OPO. A 1.94 µm half-wave plate converts the polarization state of the pump light into horizontally polarized light. The light then passes through a 45° 1.94 µm polarizer to reflect and filter out the vertically polarized light, leaving the transmitted laser as horizontally polarized light.

[0008] In the medium- and long-wave ZGP-OPO module, a first 45° medium-wave infrared reflector, a second 45° medium-wave infrared reflector, a third 45° medium-wave infrared reflector, and a 45° medium-wave infrared output coupling lens are adjacently and perpendicularly arranged to form an OPO four-shaped ring cavity; a ZGP crystal is horizontally placed between the first 45° medium-wave infrared reflector and the 45° medium-wave infrared output coupling lens with its long side collinear with the laser propagation direction; and a 45° medium-wave infrared spectrometer reflects and outputs 2.8 μm short-wave infrared laser and 6.2 μm long-wave infrared laser, and transmits and separates 1.94 μm short-wave infrared laser for output.

[0009] The present invention has the following advantages: 1. To address the significant impact of thermal lensing on 1.9 μm thulium-doped crystals, a dual-crystal resonator is used. Under the same laser working material excitation length, the larger surface area of ​​the dual crystal compared to a single crystal is utilized to improve the crystal's heat dissipation and reduce the impact of thermal effects, thereby improving laser output efficiency. 2. The ZGP-OPO medium- and long-wavelength laser of the present invention utilizes a thulium-doped laser directly as a pump source. Compared to conventional ZGP-OPO medium- and long-wavelength lasers pumped by Ho lasers, this innovatively simplifies the first-stage Ho laser structure, reduces system complexity, and improves overall electro-optical conversion efficiency. 3. The ZGP-OPO medium- and long-wave laser of the present invention has a streamlined structure. The main body consists of only two parts: a Tm:YAP laser with a first-order oscillation output and a ZGP-OPO structure. The structure is simple and compact. The miniaturized system not only improves the working stability of the laser, but also facilitates the transformation of product achievements and promotes technological iteration, and has high commercial value. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 Optical path diagram of a miniaturized medium- and long-wave solid-state laser using a thulium-doped laser directly pumping a ZGP crystal. DETAILED DESCRIPTION

[0011] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the technical solutions and parameters described in the specific embodiments described herein are only part of the present invention and should not be used to limit the scope of protection of the present invention.

[0012] The following combination Figure 1 This embodiment describes a miniaturized medium- and long-wave solid-state laser that is directly pumped by a thulium-doped laser into a ZGP crystal. Figure 1As shown in the figure, the 1.94 µm pulsed laser output by the Tm:YAP laser is used as the pump light source of the phosphorus-germanium-zinc optical parametric oscillator, and 2.8 µm and 6.2 µm pulsed lasers are output based on the nonlinear frequency conversion technology.

[0013] The Tm:YAP laser uses a commercial laser diode (LD) as its pump light source. The first laser diode 1-1 on the left generates a continuous, randomly polarized laser with a central wavelength of 793 nm. The left-side pump light is collimated by a 793 nm first plano-convex lens 1-3. The 45° 793 nm first fully reflective mirror 1-5 folds the optical path to reduce the overall optical path structure size, and then is focused by a 793 nm second plano-convex lens 1-7. Similarly, the 793 nm laser output by the second laser diode 1-2 on the right is also collimated and focused by a collimating and focusing system consisting of a 793 nm third plano-convex lens 1-4, a fourth 793 nm plano-convex lens 1-8, and a 45° 793 nm second fully reflective mirror 1-6. The 793 nm pump light on both sides passes through the collimating and focusing lens system and is injected into the "U"-shaped resonant cavity where the Tm:YAP double crystal is located, where it excites 1.94 µm laser light through stimulated emission. The Tm:YAP double crystal "U" resonant cavity includes a 1.94 μm total reflection flat mirror 1-13, an acousto-optic Q crystal 1-12, a first dichroic mirror 1-9 and a second dichroic mirror 1-10, a Tm:YAP double crystal 1-11, a Fabry-Perot etalon 1-14, and a 1.94 μm plano-concave output coupling lens 1-15: two Tm:YAP crystals are placed coaxially with the optical path with their long sides and a distance of 2 mm between the contact surfaces; the front cavity mirror is a plano-flat mirror and the rear cavity mirror is a plano-concave mirror, together forming a plano-concave cavity; the acousto-optic Q crystal is placed between the front cavity mirror and the dichroic mirror; the Fabry-Perot etalon is placed between the rear cavity mirror and the dichroic mirror; the 1.94 μm pulsed laser is finally output through the rear cavity mirror for subsequent optical path construction.

[0014] The 1.94 µm pulsed laser passes through the 1.94 µm first plano-concave lens 2-1, the 1.94 µm second plano-convex lens 2-5, and the 1.94 µm third plano-convex lens 2-6 to control the required spot diameter and divergence when the pump light is transmitted into the ZGP crystal; the 1.94 µm half-wave plate 2-2 changes the polarization state of the 1.94 µm pump light and converts it into horizontally polarized light; the 45° 1.94 µm polarizer 2-4 separates the vertically polarized light and the horizontally polarized light of the input pump light, and the horizontally polarized light will continue to be transmitted along the input optical path; the 45° 1.94 µm total reflection mirror 2-3 folds the optical path to make the overall optical path structure compact; considering the safety and stability of the internal devices during laser operation, the half-wave plate, 45° 1.94 µm total reflection mirror, and 45° 1.94 µm polarizer are all placed in the large spot area between the first plano-concave lens and the second plano-concave lens; further, the horizontally polarized 1.94 The 1.94 μm pump light is injected into the ZGP crystal 2-9 for a type of phase matching. The first 45° medium-wave infrared reflector 2-7, the ZGP crystal 2-9, the second 45° medium-wave infrared reflector 2-8, the third 45° medium-wave infrared reflector 2-10, and the 45° medium-wave infrared output coupling lens 2-11 form a zinc-germanium-phosphorus optical parametric oscillator. The 1.94 μm pump light is subjected to nonlinear frequency conversion to achieve 2.8 μm infrared laser output. Finally, the output laser passes through the 45° medium-wave infrared spectrometer 2-12 to transmit and separate the 1.94 μm short-wave infrared laser, resulting in the reflection output of the 2.8 μm short-wave infrared laser and the 6.2 μm long-wave infrared laser.

[0015] The 793 nm collimating and focusing lens groups on both sides of the Tm:YAP resonator consist of the first 793 nm plano-convex lens 1-3 and the third 793 nm plano-convex lens 1-4 as collimating lenses, and the second 793 nm plano-convex lens 1-7 and the fourth 793 nm plano-convex lens 1-8 as focusing lenses. Both sides of the lenses are coated with 793 nm anti-reflection coating, with a focal length of 15 mm to 75 mm and a diameter of 10 mm to 15 mm. The reflective surfaces of the 45°793 nm first total reflective mirrors 1-5 and the 45°793 nm second total reflective mirrors 1-6 are coated with a 45°793 nm total reflective film with a diameter of 15 mm; The first dichroic mirror 1-9 and the second dichroic mirror 1-10 are both coated with 793 nm antireflection coating on both sides, and the reflecting surface is coated with 45° 1.94 µm total reflection coating, with a diameter of 15 mm; Both optical surfaces of the acousto-optic Q crystals 1-12 are coated with a 1.94 μm anti-reflection coating; 1.94 μm total reflection flat mirror 1-13 is a flat mirror with 793 nm antireflection coating on both sides and 1.94 μm total reflection coating on the reflecting surface, with a diameter of 15 mm; The tandem Tm:YAP crystals 1-11 consist of two Tm:YAP crystals with the same parameters, placed coaxially with the optical path along their long sides. The distance between the two contact surfaces is 2 mm. Both light-passing surfaces of the single crystal are coated with 793 nm and 1.94 μm antireflection coatings. The crystal size is 4×4×15 mm. 3 , the doping concentration is 2at%, and the tangent direction is b-cut; The thickness of the Fabry-Perot etalon 1-14 is 0.32 mm; The 1.94 µm plano-concave output coupling lens 1-15 is coated with a 793 nm anti-reflection coating on both sides. The reflective concave surface is coated with a 1.94 µm coating with a transmittance of 60% to 70%, and the transmissive surface is coated with a 1.94 µm anti-reflection coating. The radius of curvature of the concave surface is 200 mm and the diameter is 20 mm. The first 1.94 µm plano-concave lens 2-1 is coated with a 1.94 µm anti-reflection coating on both surfaces, with a concave curvature radius of -80 mm and a diameter of 15 mm. The second 1.94 µm plano-convex lens 2-5 and the third 1.94 µm plano-convex lens 2-6 are coated with a 1.94 µm anti-reflection coating on both surfaces, with focal lengths ranging from 80 mm to 200 mm and a diameter of 15 mm. 1.94 µm half-wave plate 2-2 has a 1.94 µm antireflection coating on both sides, 0.1 mm thick, and a diameter of 15 mm. 45° 1.94 µm polarizer, 2-4 reflective surfaces coated with 1.94 µm polarizing film, transmission surfaces coated with 1.94 µm antireflection film, diameter 15 mm; The first 45° MWIR reflector 2-7, the second 45° MWIR reflector 2-8, and the third 45° MWIR reflector 2-10 are all coated with 1.94 µm and 6.2 µm anti-reflection coatings on both sides, and the reflecting surface is coated with a 45° 2.8 µm total reflection coating. The diameter is 15 mm. The 45° MWIR output coupling lens 2-11 is also coated with 1.94 µm and 6.2 µm anti-reflection coatings on both sides, and the reflecting surface is coated with a 2.8 µm film with a transmittance of 40% to 50%. The ZGP crystal is coated with 1.94 µm, 2.8 µm, and 6.2 µm antireflection coatings on both sides and has a size of 6 × 6 × 30 mm. 3 , crystal cutting angle =54.7°; The 45° MWIR beamsplitter 2-12 is coated with a 1.94 µm antireflection coating on both sides, and a 2.8 µm and 6.2 µm total reflection coating on the reflective surface. It has a diameter of 20 mm.

[0016] The above description is only a preferred embodiment of the present invention. Those skilled in the art will easily understand that it should not limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A miniaturized medium- and long-wave infrared solid-state laser directly pumped by a thulium-doped laser into a ZGP crystal, characterized in that: include: 793 nm pump beam collimation and focusing module, Tm:YAP laser resonator module, 1.94 µm beam transformation module, and medium- and long-wavelength ZGP-OPO module; The 793 nm pump beam collimation and focusing module includes: The first laser diode (1-1) and the second laser diode (1-2) respectively output 793nm random polarization continuous pump light; Two symmetrically arranged 793 nm collimating and focusing plano-convex lens groups are used to collimate and focus the 793 nm pump light output by the first laser diode (1-1) and the second laser diode (1-2) and inject it into the Tm:YAP crystal; Two 45° fully reflective mirrors are distributed within the 793 nm collimating and focusing plano-convex lens group to fold the optical path to compress the laser volume; The Tm:YAP laser resonant cavity module includes a U-shaped resonant cavity and two Tm:YAP crystals placed in series within the U-shaped resonant cavity, with their long sides arranged coaxially with the optical path, to emit 1.94 µm laser light through stimulated emission of radiation. The 1.94µm beam transformation module is used to control the light spot parameters and polarization state; The medium- and long-wavelength ZGP-OPO module includes an OPO four-shaped ring cavity and a ZGP crystal placed in the OPO four-shaped ring cavity. The ZGP crystal is used to output 2.8 µm and 6.2 µm medium- and long-wavelength infrared lasers through nonlinear conversion.

2. The miniaturized medium- and long-wave infrared solid-state laser according to claim 1, characterized in that: The U-shaped resonant cavity comprises a first dichroic mirror (1-9), a second dichroic mirror (1-10), an acousto-optic Q crystal (1-12), a 1.94 µm total reflection flat mirror (1-13), a Fabry-Perot etalon (1-14) and a 1.94 µm plano-concave output coupling lens (1-15). The first dichroic mirror (1-9) and the second dichroic mirror (1-10) are vertically symmetrically placed on both sides of the two Tm:YAP crystals (1-11) placed in series, and the first dichroic mirror (1-9) and the second dichroic mirror (1-10) are coated with a 793 nm anti-reflection film and a 1.94 µm total reflection film. The 1.94 µm total reflection flat mirror (1-13) serves as a front cavity mirror, and the 1.94 The μm plano-concave output coupling lens (1-15) serves as a rear cavity mirror, and together they form a U-shaped plano-concave resonant cavity; the acousto-optic Q crystal (1-12) is arranged between the front cavity mirror and the first dichroic mirror (1-9) for controlling intracavity loss and achieving pulse output; and the Fabry-Perot etalon (1-14) is arranged between the rear cavity mirror and the second dichroic mirror (1-10) for spectral mode selection.

3. The miniaturized medium- and long-wave infrared solid-state laser according to claim 1 or 2, characterized in that: The parameters of the two Tm:YAP crystals (1-11) are the same, and the distance between the contact surfaces is 2-20 mm.

4. The miniaturized medium- and long-wave infrared solid-state laser according to claim 1 or 2, characterized in that: The two Tm:YAP crystals (1-11) placed in series are coated with a 793 nm anti-reflection film and a 1.94 μm anti-reflection film on both sides.

5. The miniaturized medium- and long-wave infrared solid-state laser according to claim 2, characterized in that: Both light-passing surfaces of the acousto-optic Q crystal (1-12) are coated with a 1.94 μm anti-reflection film.

6. The miniaturized medium- and long-wave infrared solid-state laser according to claim 1, characterized in that: The 1.94 µm beam conversion module comprises a 1.94 µm first plano-concave lens (2-1), a 1.94 µm half-wave plate (2-2), a 45° 1.94 µm full-reflection mirror (2-3), a 45° 1.94 µm polarizer (2-4), a 1.94 µm second plano-convex lens (2-5) and a 1.94 µm third plano-convex lens (2-6) arranged in sequence along the optical path; wherein the 1.94 µm first plano-concave lens (2-1), the 1.94 µm second plano-convex lens (2-5) and the 1.94 µm third plano-convex lens (2-6) are used to control the spot diameter and divergence of the 1.94 µm beam to the required size before injecting it into the ZGP-OPO; the 1.94 µm half-wave plate (2-2) is used to convert the polarization state of the 1.94 µm pump light into horizontally polarized light; the 45° 1.94 The µm polarizer (2-4) is used to reflect and filter out vertically polarized light, and transmit the laser as horizontally polarized light.

7. The miniaturized medium- and long-wave infrared solid-state laser according to claim 6, characterized in that: The OPO four-shaped ring cavity is composed of a first 45° medium-wave infrared reflector (2-7), a ZGP crystal (2-9), a second 45° medium-wave infrared reflector (2-8), a third 45° medium-wave infrared reflector (2-10) and a 45° medium-wave infrared output coupling lens (2-11), wherein the ZGP crystal (2-9) is horizontally placed between the first 45° medium-wave infrared reflector (2-7) and the 45° medium-wave infrared output coupling lens (2-11).

8. The miniaturized medium- and long-wave infrared solid-state laser according to claim 2, characterized in that: The Fabry-Perot etalon (1-14) has a thickness of 0.32 mm. The 1.94 µm plano-concave output coupling lens (1-15) is coated with a 793 nm anti-reflection film on both sides. The concave surface is coated with a 1.94 µm transmittance 60% to 70% film with a curvature radius of 200 mm, and the flat transmission surface is coated with a 1.94 µm anti-reflection film with a diameter of 10 mm.

9. The miniaturized medium- and long-wave infrared solid-state laser according to claim 6, characterized in that The 1.94 µm first plano-concave lens (2-1) is coated with a 1.94 µm anti-reflection film on both sides, with a concave curvature radius of -80 mm and a diameter of 15 mm; the 1.94 µm second plano-convex lens (2-5) and the 1.94 µm third plano-convex lens (2-6) are coated with a 1.94 µm anti-reflection film on both sides, with a focal length of 80 mm-200 mm and a diameter of 15 mm; the 1.94 µm half-wave plate (2-2) is coated with a 1.94 µm anti-reflection film on both sides, with a thickness of 0.1 mm and a diameter of 15 mm.

10. The miniaturized medium- and long-wave infrared solid-state laser according to claim 7, characterized in that: The 45°1.94 µm total reflection mirror (2-3) is coated with 793 nm anti-reflection film on both sides, and the reflection surface is coated with 1.94 µm total reflection film, with a diameter of 15 mm. The 45°1.94 µm polarizer (2-4) is coated with 1.94 µm polarization film on the reflection side, and 1.94 µm anti-reflection film on the transmission side, with a diameter of 15 mm. The first 45° medium-wave infrared reflector (2-7), the second 45° medium-wave infrared reflector (2-8) and the third 45° medium-wave infrared reflector (2-10) are coated with 1.94 µm anti-reflection film and 6.2 µm anti-reflection film on both sides, and the reflection surface is coated with 45°2.8 µm total reflection film, with a diameter of 15 mm. The ZGP crystal (2-9) is coated with 1.94 µm anti-reflection film, 6.2 µm anti-reflection film and 2.8 µm anti-reflection film on both sides, with a length of 30 The 45° mid-wave infrared output coupling lens (2-11) is coated with a 1.94 µm anti-reflection film and a 6.2 µm anti-reflection film on both sides, and a 2.8 µm film with a transmittance of 40% to 50% is coated on the reflective surface, with a diameter of 15 mm.

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