A double-pass pumping structure of a Gaussian mirror unstable resonator optical parametric oscillator and a building method thereof

Abstract

The present application relates to a kind of Gaussian mirror unstable cavity optical parametric oscillator double-pass pumping structure and its building method, including the OPO input mirror, nonlinear crystal, OPO output coupling mirror, 45 degree beam splitter, concave lens, 0 degree high reflectivity lens placed in order along the light path in sequence;OPO input mirror and OPO output coupling mirror are curved mirror, the curvature of OPO input mirror is R1, the curvature of OPO output coupling mirror is R2, R1 is positive, R2 is negative.The present application is based on the defect of existing unstable cavity Gaussian output mirror coating technology to realize double-pass pumping structure, innovatively proposes cavity mirror combination idea, uses 45 degree beam splitter and 0 degree high reflectivity lens combination, realizes pump light double-pass pumping, innovatively adds concave lens in pump light return light path to compensate the focusing effect caused by unstable cavity lens, improves the output energy of signal light and idler light, expands the application scenario of Gaussian mirror unstable cavity optical parametric oscillator.

Description

Technical Field

[0001] This invention relates to the field of mid- and short-wave infrared laser technology based on optical parametric oscillators (OPOs), and particularly to a two-way pumping structure and its construction method for a Gaussian mirror unstable cavity optical parametric oscillator. Background Technology

[0002] The 1.5μm laser in the eye-safe band is located in the near-infrared window of the atmosphere, exhibiting strong penetrating power through air (including battlefield smoke, especially smoke primarily composed of red and white phosphorus). Therefore, it has broad application prospects in infrared remote sensing, special night vision, spectral measurement, coherent lidar, eye-safe ranging, and biomedicine. The mid-infrared band (3–5μm) is also an atmospheric window band, minimally affected by gas molecule absorption and suspended particle scattering, making it highly valuable for spectral measurement, remote sensing, and military applications. Nonlinear frequency conversion technology based on optical parametric oscillators is one of the main techniques for obtaining lasers in the 1.5μm and 3–5μm bands. Currently commonly used nonlinear crystals include KTiOAsO4 (KTA), KTiOPO4 (KTP), ZnGeP2 (ZGP), and periodically polarized lithium niobate (PPLN).

[0003] For pulsed lasers, pulse energy and laser output beam quality are two key parameters. To achieve high energy output, a two-way pump structure based on a flat-cavity design is often used; however, its output beam quality is poor and cannot meet the requirements of applications such as long-distance measurement. To optimize the output beam quality of optical parametric oscillators (OPOs), unstable cavity structures are widely used in OPOs to improve the brightness and collimation of the output beam. Unstable cavities can be classified into those based on the radial distribution of the reflectivity of the output mirrors, such as those based on uniform output mirrors and those based on non-uniform output mirrors. Unstable cavity structures increase the diffraction loss of the beam oscillation and the oscillation threshold of higher-order modes by changing the radius of curvature of the cavity mirrors, thereby optimizing the output laser beam quality. Compared to the unstable cavity of a uniform output mirror, the unstable cavity of a non-uniform output mirror based on a Gaussian reflectivity mirror (GRM) has a lower oscillation threshold and less loss. It eliminates the on-axis shading and strong ripple phenomenon of the output beam intensity distribution caused by the diffraction effect at the edge of the output mirror, avoids strong points in the beam during propagation, enhances the focusing ability of the output beam, and is more conducive to outputting higher energy.

[0004] Optical parametric oscillators (OPOs) can be classified into single-pump and double-pump Oscillation (DPOs) based on the pump light transmission path. The main difference lies in whether the output mirror is coated with a high-reflectivity film in the pump light band. Due to the low conversion efficiency of the pump light passing through a nonlinear crystal in a single pass, a large amount of pump light remains unused, resulting in energy waste and low utilization efficiency. The output pulse energy is reduced by nearly half compared to a planar cavity structure, limiting its application in high-energy demand fields. Based on this situation, a double-pump structure was proposed to achieve a secondary conversion of the pump light, significantly improving the nonlinear conversion efficiency. Currently, the output energy of the unstable cavity structure is lower than that of the planar cavity structure for two reasons: one is that the unstable cavity structure increases the resonant cavity oscillation loss, raising the light output threshold; the other is that existing unstable cavity Gaussian output mirrors are not coated with a high-reflectivity film for the pump light. Because the process of coating the lens with a Gaussian film is complex, the technical difficulty of coating the output mirror with a film in the pump light band is very high, and current manufacturers have not yet achieved this coating technology. To realize a Gaussian mirror unstable cavity with a two-way pumping structure, two approaches are considered: one is to coat the back of the Gaussian output mirror with a high-reflectivity coating for the pump light. However, the coating process is cumbersome, the lens yield is low, and it has not yet been put into practical use. It is estimated that the pump light coating will result in a loss of about 5% of the parametric light output energy. At the same time, since the unstable cavity mirror is meniscus-shaped, part of the pump light emitted from the back of the output mirror will be reflected by the Gaussian coating. Due to the converging effect of the concave surface, the reflected pump light will gradually focus along the return light path and return to the cavity after passing through the high-reflectivity coating on the back of the lens, which may easily damage the internal components. The other approach is to change the structural layout of the cavity mirror, and use the existing lens to optimize the cavity structure to achieve two-way pumping. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a two-way pumping structure for a Gaussian mirror unstable cavity optical parametric oscillator;

[0006] The present invention also provides a method for constructing the above-mentioned two-way pump structure;

[0007] This invention overcomes the shortcomings of existing lens coating processes and helps to achieve high energy output under unstable cavity conditions of Gaussian mirrors.

[0008] The technical solution of this invention is as follows:

[0009] A two-way pumping structure for a Gaussian mirror unstable cavity optical parametric oscillator includes an OPO input mirror, a nonlinear crystal, an OPO output coupling mirror, a 45-degree beam splitter, a concave lens, and a 0-degree high-reflectivity lens, arranged sequentially along the optical path; the OPO input mirror, the nonlinear crystal, and the OPO output coupling mirror constitute an optical parametric oscillator.

[0010] According to a preferred embodiment of the present invention, both the OPO input mirror and the OPO output coupling mirror are curved mirrors, with the concave surface of the OPO input mirror facing the nonlinear crystal and the convex surface of the OPO output coupling mirror facing the nonlinear crystal.

[0011] Further preferred, based on the positive and negative values ​​of the optical path parameters, R1 is a positive value for the optical parametric oscillator, and R2 is a negative value for the optical parametric oscillator.

[0012] According to a preferred embodiment of the present invention, the OPO input mirror and the OPO output coupling mirror constitute an unstable cavity, and the magnification m of the unstable cavity is expressed as m = |R1 / R2|, where m is greater than 1.

[0013] According to a preferred embodiment of the present invention, the nonlinear crystal is a cuboid.

[0014] According to a preferred embodiment of the present invention, the coating layer of the OPO input mirror has high transmittance for pump light and high reflectivity for signal light.

[0015] According to a preferred embodiment of the present invention, the coating layer of the OPO output coupling mirror has partial transmittance for signal light and high transmittance for idler light.

[0016] More preferably, the reflectivity distribution of the coating layer on the OPO output coupling mirror conforms to a Gaussian curve.

[0017] According to a preferred embodiment of the present invention, the OPO output coupling mirror is not coated with a pump light high transmittance film.

[0018] According to a preferred embodiment of the present invention, the coating layer of the 45-degree beam splitter has high transmittance for pump light and high reflectivity for signal light and idler light.

[0019] According to a preferred embodiment of the present invention, the focal length f of the concave lens is in the range of R1-R2.

[0020] According to a preferred embodiment of the present invention, the coating layer of the 0-degree high-reflectivity lens has high reflectivity for pump light.

[0021] According to a preferred embodiment of the present invention, both the 45-degree beam splitter and the 0-degree high-reflectivity lens are plane mirrors.

[0022] According to a preferred embodiment of the present invention, the 45-degree beam splitter is placed at a 45-degree angle to the optical path, and the 0-degree high-reflectivity lens is placed at a 90-degree angle to the optical path.

[0023] According to a preferred embodiment of the present invention, the nonlinear crystal is wrapped with indium foil and placed in a fixture for fixation.

[0024] The above-mentioned method for constructing a two-way pump structure includes:

[0025] (1) Fix the OPO input mirror, OPO output coupling mirror, 45-degree beam splitter, concave lens, and 0-degree high reflectivity lens on the lens holder;

[0026] (2) Install and fix the OPO input mirror, nonlinear crystal, OPO output coupling mirror, 45-degree beam splitter, concave lens and 0-degree high reflectivity lens in sequence, so that the center line of all devices coincides with the optical axis.

[0027] (3) The collimated pump light that meets the phase matching condition is incident on the end face of the nonlinear crystal, and a nonlinear effect occurs. The angle of the OPO output coupling mirror is adjusted to output signal light and idler light.

[0028] (4) Adjust the angle of the 0-degree high reflectivity lens so that the transmission path of the reflected pump light is consistent with the incident light and is incident on the end face of the nonlinear crystal.

[0029] (5) The concave lens expands the pump light through the OPO input mirror and the OPO output coupling mirror, avoiding beam return to focus and damage to the lens and crystal, thus realizing the Gaussian mirror unstable cavity two-way pumping.

[0030] Compared with existing technical solutions, the significant advantages of this invention are:

[0031] 1. In the case that existing coating technology cannot achieve the coating layer required for double-pass pumping of Gaussian mirror, this paper overcomes the technical shortcomings and innovatively adopts a combination of a beam splitter for pump light and parametric light and a high reflectivity lens for pump light to design a double-pass pumping structure for unstable Gaussian mirror cavity.

[0032] 2. A concave lens is innovatively added to the pump light return path to compensate for the focusing effect caused by the unstable cavity mirror, optimize the spot size of the returned pump light, improve the conversion efficiency, and avoid damage to the device;

[0033] 3. Without affecting the output beam quality, the output energy and conversion efficiency of the Gaussian mirror unstable cavity are improved, expanding the application scenarios of the optical parametric oscillator built on the Gaussian mirror unstable cavity. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the two-way pumping structure based on the Gaussian mirror unstable cavity of the present invention;

[0035] Figure 2 This is a schematic diagram of the reflectance distribution curve of the Gaussian mirror coating of the present invention;

[0036] Figure 3 This is a schematic diagram showing the output energy and conversion efficiency under single-pass pumping and double-pass pumping in an embodiment of the present invention;

[0037] Figure 4(a) is a schematic diagram of the signal beam quality results under single-pass pumping in an embodiment of the present invention;

[0038] Figure 4(b) is a schematic diagram of the signal beam quality results under two-way pumping in an embodiment of the present invention.

[0039] Among them, 1. Pump light, 2. OPO input mirror, 3. Nonlinear crystal, 4. OPO output coupling mirror, 5. 45-degree beam splitter, 6. Concave lens, 7. 0-degree high reflectivity lens. Detailed Implementation

[0040] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments, but is not limited thereto.

[0041] Example 1

[0042] A two-way pumping structure for a Gaussian mirror unstable cavity optical parametric oscillator, such as Figure 1 As shown, the optical path includes an OPO input mirror 2, a nonlinear crystal 3, an OPO output coupling mirror 4, a 45-degree beam splitter 5, a concave lens 6, and a 0-degree high-reflectivity lens 7, arranged sequentially along the optical path. The OPO input mirror 2, the nonlinear crystal 3, and the OPO output coupling mirror 4 constitute an optical parametric oscillator. The nonlinear crystal 3 is wrapped with indium foil and fixed in a fixture.

[0043] A combination of a 45-degree beam splitter 5, a concave lens 6, and a 0-degree high-reflectivity lens 7 enables two-way pumping of the pump light 1. After collimation, the pump light 1 is incident on the surface of the nonlinear crystal 3, where a nonlinear effect occurs. By finely adjusting the angles of the OPO input mirror 2 and the OPO output coupling mirror 4, signal and idler light outputs are achieved. Placing the 45-degree beam splitter 5 separates the pump light 1, signal light, and idler light at a 90-degree angle. The separated pump light 1 is reflected back sequentially through the concave lens 6 and the 0-degree high-reflectivity lens 7, achieving beam expansion and preventing beam focusing damage to optical components. The returned pump light 1 again undergoes a nonlinear effect with the nonlinear crystal 3, achieving two-way gain for the signal and idler light. This invention creatively employs a combination of the 0-degree high-reflectivity lens 7 and the 45-degree beam splitter 5 to realize a Gaussian mirror unstable cavity optical parametric oscillator with a two-way pumping structure, increasing the output energy of the signal and idler light without affecting the output beam quality.

[0044] Example 2

[0045] The difference between the two-way pumping structure of the Gaussian mirror unstable cavity optical parametric oscillator described in Example 1 and the following is:

[0046] Both the OPO input mirror 2 and the OPO output coupling mirror 4 are curved mirrors. The concave surface of the OPO input mirror 2 faces the nonlinear crystal 3, and the convex surface of the OPO output coupling mirror 4 faces the nonlinear crystal 3.

[0047] According to the standard of positive and negative values ​​of optical path parameters, R1 is a positive value for the optical parametric oscillator, and R2 is a negative value for the optical parametric oscillator.

[0048] Changing the distance between the OPO input mirror 2 and the OPO output coupling mirror 4 can realize both non-confocal unstable cavity and confocal unstable cavity. The non-confocal unstable cavity has higher output energy, while the confocal unstable cavity has a better effect on improving beam quality.

[0049] The OPO input mirror 2 and the OPO output coupling mirror 4 constitute an unstable cavity. The amplification m of the unstable cavity is expressed as m = |R1 / R2|, where m is greater than 1. To increase the loss of the resonant cavity on the oscillating light spot and optimize the beam quality of the signal light, m ​​is usually greater than 1.

[0050] The coating layer of OPO input mirror 2 has high transmittance for pump light (transmittance T>99.5%) and high reflectance for signal light (R>99.5%). The coating layer of OPO output coupling mirror 4 has partial transmittance for signal light (transmittance lower than that of high-transmittance coating layers) and high transmittance for idler light.

[0051] like Figure 2 As shown, the reflectivity distribution of the coating layer on the OPO output coupling mirror 4 conforms to a Gaussian curve. It is a k-th order Gaussian, where k is an even number. The center of the lens has high reflectivity, while the edges have low reflectivity. The coating radius depends on the specific application.

[0052] The expression for the reflectivity distribution of the coating layer on the OPO output coupling mirror 4 for signal light is: Where r is the radius, R r R0 is the reflectivity, and w is the central reflectivity. m Let be the coating radius, and k be the order (even number).

[0053] OPO output coupling mirror 4 is not coated with a high-transmittance pump light film.

[0054] Both the 45-degree beam splitter 5 and the 0-degree high-reflectivity lens 7 are plane mirrors. The coating on the 45-degree beam splitter 5 has high transmittance for pump light and high reflectivity for signal light and idler light. The coating on the 0-degree high-reflectivity lens 7 has high reflectivity for pump light.

[0055] Example 3

[0056] The difference between the two-way pumping structure of the Gaussian mirror unstable cavity optical parametric oscillator described in Example 2 and the one described in Example 2 is as follows:

[0057] The 45-degree beam splitter 5 is placed at a 45-degree angle to the optical path, and the 0-degree high-reflectivity lens 7 is placed at a 90-degree angle to the optical path. The nonlinear crystal 3 is a cuboid. It is suitable for high-energy pumping conditions and has a high damage threshold.

[0058] The focal length f of concave lens 6 ranges from R1 to R2. Its surface is coated with a high-transmittance pump light film.

[0059] Pump light 1 will be focused after passing through OPO input mirror 2 and OPO output coupling mirror 4. Therefore, concave lens 6 is placed directly on 45-degree beam splitter 5 and 0-degree high reflectivity lens 7 to expand and collimate pump light 1, and avoid the focus of the beam hitting the optical device and causing damage.

[0060] In this embodiment, the output energy and conversion efficiency under single-pass pumping and double-pass pumping are as follows: Figure 3 As shown in Figure 4(a), the signal beam quality results under single-pumping are shown in Figure 4(b); in Figures 4(a) and 4(b), the horizontal axis represents the beam spot measurement position, and the vertical axis represents the beam spot radius at the corresponding measurement position. The beam quality M is obtained by fitting the beam propagation equation. 2 Numerical value, M x 2 M y 2 These represent the beam quality in the x and y directions, respectively. At a pump energy of 260 mJ, the highest output energy of the two-pass pump structure is 57 mJ, with a conversion efficiency of 21.9%. Removing the concave lens 6 and the high-reflectivity lens results in a Gaussian mirror unstable cavity single-pass pump structure, with a highest output energy of 42 mJ and a conversion efficiency of 16.1%. Under the same pump energy conditions, the output energy and conversion efficiency of the two-pass pump are improved by 36% compared to the single-pass pump. In the single-pass structure, the signal beam quality is M. x 2 =9.96,M y 2 =6.65; Under the dual-pass structure, the signal beam quality is M x 2 =9.70,M y 2 =6.73. Under the same pumping conditions, the beam quality factors of single and double passes are basically the same, and the fluctuation range can be considered as measurement error.

[0061] Example 4

[0062] The method for constructing a two-way pumping structure for a Gaussian mirror unstable cavity optical parametric oscillator according to any one of Examples 1-3 includes:

[0063] (1) Fix the OPO input mirror 2, OPO output coupling mirror 4, 45-degree beam splitter 5, concave lens 6, and 0-degree high reflectivity lens 7 on the lens holder;

[0064] (2) Install and fix the OPO input mirror 2, nonlinear crystal 3, OPO output coupling mirror 4, 45-degree beam splitter 5, concave lens 6, and 0-degree high reflectivity lens 7 in sequence, so that the center line of all devices coincides with the optical axis.

[0065] (3) The collimated pump light 1 that meets the phase matching condition is incident on the end face of the nonlinear crystal 3, and a nonlinear effect occurs. The angle of the OPO output coupling mirror 4 is adjusted to output signal light and idler light.

[0066] (4) Adjust the angle of the 0-degree high reflectivity lens 7 so that the transmission path of the reflected pump light 1 is consistent with the incident light and is incident on the end face of the nonlinear crystal 3.

[0067] (5) The concave lens 6 expands the pump light 1 through the OPO input mirror 2 and the OPO output coupling mirror 4 to avoid the beam returning to focus and damaging the lens and crystal, thus realizing the Gaussian mirror unstable cavity two-way pumping.

Claims

1. A two-way pumping structure for a Gaussian mirror unstable cavity optical parametric oscillator, characterized in that, The device includes an OPO input mirror, a nonlinear crystal, an OPO output coupling mirror, a 45-degree beam splitter, a concave lens, and a 0-degree high-reflectivity lens, which are placed sequentially along the optical path; the OPO input mirror, the nonlinear crystal, and the OPO output coupling mirror constitute an optical parametric oscillator. Both the OPO input mirror and the OPO output coupling mirror are curved mirrors, with the concave surface of the OPO input mirror facing the nonlinear crystal and the convex surface of the OPO output coupling mirror facing the nonlinear crystal. The curvature of the OPO input mirror is R1, and the curvature of the OPO output coupling mirror is R2. R1 is a positive value for the optical parametric oscillator, and R2 is a negative value for the optical parametric oscillator. The OPO input mirror and the OPO output coupling mirror constitute an unstable cavity. The magnification m of the unstable cavity is expressed as m=|R1 / R2|, where m is greater than 1. The focal length f of the concave lens has a value range of R1-R2.

2. The two-way pumping structure of a Gaussian mirror unstable cavity optical parametric oscillator according to claim 1, characterized in that, The coating layer of the OPO input mirror has high transmittance for pump light and high reflectance for signal light; the coating layer of the OPO output coupling mirror has partial transmittance for signal light and high transmittance for idler light.

3. The two-way pumping structure of a Gaussian mirror unstable cavity optical parametric oscillator according to claim 2, characterized in that, The reflectivity distribution of the coating layer on the OPO output coupling mirror conforms to a Gaussian curve.

4. The two-way pumping structure of a Gaussian mirror unstable cavity optical parametric oscillator according to claim 1, characterized in that, The 45-degree beam splitter is placed at a 45-degree angle to the optical path, and the 0-degree high-reflectivity lens is placed at a 90-degree angle to the optical path.

5. The two-way pumping structure of a Gaussian mirror unstable cavity optical parametric oscillator according to claim 1, characterized in that, Both the 45-degree beam splitter and the 0-degree high-reflectivity lens are plane mirrors; the coating of the 45-degree beam splitter has high transmittance for pump light and high reflectivity for signal light and idler light; the coating of the 0-degree high-reflectivity lens has high reflectivity for pump light.

6. The two-way pumping structure of a Gaussian mirror unstable cavity optical parametric oscillator according to claim 1, characterized in that, The nonlinear crystal is a cuboid.

7. The two-way pumping structure of a Gaussian mirror unstable cavity optical parametric oscillator according to any one of claims 1-6, characterized in that, The nonlinear crystal is wrapped with indium foil and placed in a fixture for fixation.

8. The method for constructing the two-way pump structure according to any one of claims 1-7, characterized in that, include: (1) Fix the OPO input mirror, OPO output coupling mirror, 45-degree beam splitter, concave lens, and 0-degree high reflectivity lens on the lens holder; (2) Install and fix the OPO input mirror, nonlinear crystal, OPO output coupling mirror, 45-degree beam splitter, concave lens, and 0-degree high reflectivity lens in sequence, so that the center line of all devices coincides with the optical axis. (3) The collimated pump light that meets the phase matching condition is incident on the end face of the nonlinear crystal, and a nonlinear effect occurs. The angle of the OPO output coupling mirror is adjusted to output signal light and idler light. (4) Adjust the angle of the 0-degree high reflectivity lens so that the transmission path of the reflected pump light is consistent with the incident light and is incident on the end face of the nonlinear crystal. (5) The concave lens expands the pump light through the OPO input mirror and the OPO output coupling mirror, avoiding beam back focusing and damage to the lens and crystal, thus realizing the Gaussian mirror unstable cavity two-way pumping.

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