A device for achieving low-loss operation of long-wave infrared femtosecond optical parametric oscillator

Abstract

The present invention discloses a device for achieving low-loss operation of a long-wave infrared femtosecond optical parametric oscillator, belonging to the field of optical parametric oscillators, and comprising: a pump source, a pump focusing lens, a long-wave infrared nonlinear crystal cut at the Brewster angle, a laser medium dichroic mirror, and an output coupling mirror. The nonlinear crystal satisfies the type I phase matching condition for the optical parametric process, and the front and back surfaces of the crystal are cut at the Brewster angle with respect to the center wavelength of the oscillating signal pulse to reduce the loss of the pulse in the resonant cavity. The present invention overcomes the limitation of the residual loss of the crystal anti-reflection film by cutting the crystal at the Brewster angle, making it possible to construct a femtosecond optical parametric oscillator based on a long-wave infrared nonlinear crystal. This device is expected to overcome the bottlenecks of existing long-wave infrared femtosecond optical parametric oscillators in terms of wavelength tuning range and output power, and provide a practical and low-cost technical solution for the construction and application of long-wave infrared femtosecond optical parametric oscillators.

Description

Technical Field

[0001] The present invention belongs to the field of optical parametric oscillators, and more specifically, relates to a device for realizing low-loss operation of a long-wave infrared femtosecond optical parametric oscillator. Background Art

[0002] The main development direction of mid- and long-wave infrared (3-20μm) femtosecond optical parametric oscillators is to expand the output spectral bandwidth or wavelength tuning range of the laser while improving the wavelength conversion efficiency and output power. In recent years, with the emergence of new nonlinear crystal materials and the advancement of coating technology, the performance of mid- and long-wave infrared femtosecond optical parametric oscillators has been significantly improved. At present, femtosecond optical parametric oscillators have generally achieved watt-level output power in the wavelength range of 3-5μm, and the maximum output power near the central wavelength of 3μm is close to 10W. In terms of wavelength tuning, in 2021, Heriot-Watt University in the UK used a 1040nm pump source and a new OP-GaP crystal to produce a femtosecond laser with a wavelength tuning range of 3.9-12μm. This is the longest wavelength that can be generated by a femtosecond optical parametric oscillator reported so far.

[0003] However, direct generation of femtosecond lasers in the 5-20μm band based on femtosecond optical parametric oscillators (OPOs) still faces numerous practical challenges. Currently, the laser power achieved by femtosecond OPOs in the 5-12μm band remains relatively low, reaching a peak power of only 60mW at 12μm. This difficulty is primarily due to the low nonlinear quality factor of crystals in this wavelength range, resulting in low parametric gain at a given pump intensity. Furthermore, due to technical limitations, the antireflection coatings (AR coatings) in this wavelength range exhibit significant losses, resulting in very high pump intensities required to reach the oscillation threshold of the femtosecond OPO. However, these high-intensity pump pulses can damage the AR coatings. Consequently, the potential of large numbers of crystals for constructing 5-20μm femtosecond OPOs remains unrealized. Existing reported long-wave infrared femtosecond OPOs are based on very small numbers of high-quality crystals, while femtosecond OPOs with a central wavelength of 12-20μm have yet to be reported.

[0004] If it is possible to avoid coating on the surface of long-wave infrared crystals while ensuring that the crystals have high transmittance to the signals and idler pulses of the optical parametric oscillator, it will be possible to use a high-intensity pump source to pump almost all long-wave infrared nonlinear crystals, breaking through the output wavelength limitation of existing femtosecond optical parametric oscillators, achieving a wider range of wavelength tuning and higher-power laser pulse output, and enabling optical parametric oscillators to be more widely used in fields such as spectral detection and microscopic imaging. Summary of the Invention

[0005] In response to the shortcomings of the prior art, the present invention aims to provide a device for achieving low-loss operation of a long-wave infrared femtosecond optical parametric oscillator (LWIR). This device aims to improve the transmittance of the signal and idler pulses while avoiding coating the nonlinear crystal surface. This reduces the pumping threshold of the optical parametric oscillator and increases its output power, making it possible to construct a femtosecond optical parametric oscillator based on nearly any LWIR nonlinear crystal. This device is expected to overcome the bottlenecks of existing LWIR femtosecond optical parametric oscillators in terms of wavelength tuning range and output power, providing a practical, low-cost technical solution for the construction and application of LWIR femtosecond optical parametric oscillators.

[0006] To achieve the above objectives, the present invention provides a device for achieving low-loss operation of a long-wave infrared femtosecond optical parametric oscillator, comprising: a pump source, a pump focusing lens, a nonlinear crystal cut at the Brewster angle, a laser medium dichroic mirror, and an output coupling mirror.

[0007] The laser medium dichroic mirror and the output coupling mirror together constitute an optical resonant cavity, and the nonlinear crystal is located at the beam waist of the optical resonant cavity; the pump focusing lens is used to focus the pump pulse generated by the pump source to the center of the nonlinear crystal; the pump pulse is converted into a signal pulse and an idler pulse with different frequencies after passing through the nonlinear crystal; the signal pulse is output through the output coupling mirror; and the idler pulse is output through the laser medium dichroic mirror.

[0008] Preferably, the nonlinear crystal satisfies Type I phase matching conditions for the pump pulse, signal pulse, and idler pulse of the optical parametric process. The pump pulse wave vector generated by the pump source is equal to the sum of the signal pulse wave vector and the idler pulse wave vector. The signal pulse and idler pulse have the same polarization state and are both in the p-polarization state relative to the crystal.

[0009] Preferably, the pump pulse and the signal pulse are near-infrared or mid-infrared pulses, and the idler pulse is a long-wave infrared pulse.

[0010] Preferably, the front and rear surfaces of the nonlinear crystal are cut at the Brewster angle to the central wavelength of the signal pulse to avoid Fresnel reflection loss of the signal pulse on the crystal surface and minimize the loss of the idler pulse on the crystal surface.

[0011] Preferably, the surface of the laser medium dichroic mirror is coated with a dielectric film that has high transmittance to pump pulses, high reflection to signal pulses, and high transmittance to idler pulses.

[0012] Preferably, the surface of the output coupling mirror is coated with a dielectric film that partially reflects the signal pulse.

[0013] Preferably, the laser medium dichroic mirror includes a first laser medium dichroic mirror M1, a second laser medium dichroic mirror M2 and a third laser medium dichroic mirror M3; the pump pulse passes through M1 and is incident on a nonlinear crystal to be converted into a signal pulse and an idler pulse with different frequencies; the idler pulse is directly output through M2; the signal pulse is reflected by M2 to the output coupling mirror and then partially output, and the remaining signal pulse is reflected back to M2 by the output coupling mirror, then reflected back to M1 by M2, then reflected back to M3 by M1, and then reflected back to M1 by M3, and finally reflected back to M1 by M3, synchronously meeting the next pump pulse and oscillating back and forth in the cavity.

[0014] Preferably, the third laser medium dichroic mirror M3 is mounted on a translation stage to adjust the length of the resonant cavity to ensure that the time it takes for the signal pulse to propagate one circle in the cavity is equal to the interval between two pump pulses, thereby achieving synchronous pumping.

[0015] Compared with the prior art, the above technical solutions proposed by the present invention can achieve the following:

[0016] Beneficial effects:

[0017] (1) The device proposed in the present invention for realizing low-loss operation of a long-wave infrared femtosecond optical parametric oscillator reduces the loss of p-polarized oscillating signal pulses and non-oscillating idler pulses in the cavity by cutting the crystal at the Brewster angle. Compared with the technical solution of coating an anti-reflection film on the crystal surface, the device can effectively avoid the high residual loss and low damage threshold problems of the crystal anti-reflection film, and is expected to completely solve the oscillation light output problem of the long-wave infrared femtosecond optical parametric oscillator.

[0018] (2) The device proposed in the present invention for realizing low-loss operation of a long-wave infrared femtosecond optical parametric oscillator adopts Brewster angle cutting of the crystal, which makes it possible to construct a femtosecond optical parametric oscillator using a long-wave infrared nonlinear crystal with a longer transmission cutoff wavelength but a slightly lower quality factor, and has the potential to generate femtosecond lasers at longer wavelengths.

[0019] (3) The device proposed in the present invention for realizing low-loss operation of long-wave infrared femtosecond optical parametric oscillator adopts Brewster angle cutting for the crystal, which avoids the low damage threshold of the antireflection film. Therefore, higher power pump pulses can be used to pump long-wave infrared nonlinear crystals with better thermal properties to construct femtosecond OPO, which is expected to obtain higher output power in the long-wave infrared band.

[0020] (4) The device proposed in the present invention for realizing low-loss operation of a long-wave infrared femtosecond optical parametric oscillator adopts Brewster angle cutting for the crystal, which can avoid the complex and expensive surface coating process in the crystal production process, save equipment manufacturing costs, save crystal production processing time, and facilitate the practical application of the optical parametric oscillator. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 1 is a schematic structural diagram of a device for achieving low-loss operation of a long-wave infrared femtosecond optical parametric oscillator in an embodiment of the present invention;

[0022] Figure 2 This is a relationship diagram between the idler pulse wavelength and the phase matching angle of the optical parametric oscillator obtained based on the numerical simulation of the parameters in the embodiment of the present invention;

[0023] Figure 3 This is a relationship diagram between the idler pulse wavelength and the group velocity mismatch of an optical parametric oscillator obtained based on the numerical simulation of parameters in an embodiment of the present invention;

[0024] Figure 4 Schematic diagram of the design of the Brewster angle-cut LGS crystal to be used in the embodiments of the present invention.

[0025] Figure 5 These are reflectivity curves of p-light and s-light incident on a lithium gallium sulfide crystal (LGS) at different angles, calculated based on the parameters in the embodiment of the present invention. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0027] To achieve the above objectives, the present invention provides a device for achieving low-loss operation of a long-wave infrared femtosecond optical parametric oscillator, comprising: a pump source, a pump focusing lens, a nonlinear crystal cut at the Brewster angle, a laser medium dichroic mirror, and an output coupling mirror.

[0028] The laser medium dichroic mirror and the output coupling mirror together constitute an optical resonant cavity, and the nonlinear crystal is located at the beam waist of the optical resonant cavity; the pump focusing lens is used to focus the pump pulse generated by the pump source to the center of the nonlinear crystal; the pump pulse is converted into a signal pulse and an idler pulse with different frequencies after passing through the nonlinear crystal; the signal pulse is output through the output coupling mirror; and the idler pulse is output through the laser medium dichroic mirror.

[0029] Specifically, the nonlinear crystal satisfies Type I phase matching conditions for the pump pulse, signal pulse, and idler pulse of the optical parametric process. The pump pulse wave vector generated by the pump source is equal to the sum of the signal pulse wave vector and the idler pulse wave vector. The signal pulse and idler pulse have the same polarization state and are both in the p-polarization state relative to the crystal.

[0030] Specifically, the pump pulse and the signal pulse are near-infrared or mid-infrared pulses, and the idler pulse is a long-wave infrared pulse.

[0031] Specifically, the front and back surfaces of the nonlinear crystal are cut at the Brewster angle with respect to the central wavelength of the signal pulse to avoid Fresnel reflection loss of the signal pulse on the crystal surface and minimize the loss of the idler pulse on the crystal surface.

[0032] Specifically, the surface of the laser medium dichroic mirror is coated with a dielectric film that has high transmittance to pump pulses, high reflection to signal pulses, and high transmittance to idler pulses.

[0033] Specifically, the surface of the output coupling mirror is coated with a dielectric film that partially reflects the signal pulse.

[0034] Specifically, the laser medium dichroic mirror includes a first laser medium dichroic mirror M1, a second laser medium dichroic mirror M2 and a third laser medium dichroic mirror M3; the pump pulse passes through M1 and is incident on a nonlinear crystal to be converted into a signal pulse and an idler pulse with different frequencies; the idler pulse is directly output through M2; the signal pulse is reflected by M2 to the output coupling mirror and then partially output, and the remaining signal pulse is reflected back to M2 by the output coupling mirror, then reflected back to M1 by M2, then reflected back to M3 by M1, and then reflected back to M1 by M3, and finally reflected back to M1 by M3, synchronously meeting the next pump pulse and oscillating back and forth in the cavity.

[0035] Specifically, the third laser medium dichroic mirror M3 is mounted on a translation stage to adjust the length of the resonant cavity.

[0036] The present invention also provides a method for achieving low-loss operation of a long-wave infrared femtosecond optical parametric oscillator, comprising the following steps:

[0037] S1. Determine the type and phase matching angle of a long-wave infrared nonlinear crystal that meets the Type I phase matching condition based on the pump pulse wavelength and the desired signal and idler pulse wavelengths.

[0038] S2. Calculate the refractive index of the crystal at the center wavelength of the signal pulse according to the Sellmeier equation of the nonlinear crystal, and use this to calculate the Brewster angle corresponding to the wavelength.

[0039] S3. Determine the direction of the crystal optical axis according to the phase matching angle of the nonlinear crystal in step S1.

[0040] S4. Cutting the front and back surfaces of the nonlinear crystal according to the Brewster angle calculated in step S2.

[0041] S5. Build an optical parametric oscillator resonant cavity for feedback signal light.

[0042] S6. Focus the pump pulses generated by the ultrafast pump source into the center of the nonlinear crystal.

[0043] S7. Adjust the resonant cavity length to meet the pulse synchronous pumping conditions and realize signal and idler pulse output.

[0044] In order to further illustrate the device and method for reducing cavity loss in an optical parametric oscillator provided by the present invention, a detailed description is given below with reference to the accompanying drawings and specific embodiments.

[0045] Example 1

[0046] The schematic diagram of the device structure of the long-wave infrared femtosecond optical parametric oscillator based on the Brewster angle cut LGS crystal provided in this embodiment is as follows: Figure 1 As shown in the figure, it includes: an ultrafast pulse pump source, a pump focusing lens, a first laser medium dichroic mirror M1, a second laser medium dichroic mirror M2, an LGS crystal cut at the Brewster angle, a third laser medium dichroic mirror M3 mounted on a translation stage, and an output coupling mirror.

[0047] LGS crystals have a relatively high thermal conductivity of approximately 7 W / (m·K), enabling phase matching in the 5-10 μm wavelength range under 1.05 μm pump conditions. Furthermore, high-power 1 μm femtosecond laser pump source technology is relatively mature. Therefore, this example proposes using LGS crystals to construct a femtosecond OPO, generating high-power long-wave infrared femtosecond lasers at a central wavelength of 6 μm.

[0048] The transparent wavelength range of LGS crystal is 0.32-11.6μm. The relationship between the idler pulse wavelength and the phase matching angle of the optical parametric oscillator based on 1.05μm pump source and type I (e→oo) phase matching is as follows: Figure 2 When the idler pulse center wavelength is 6 μm, the corresponding phase matching angle θ is 53.1°. Figure 3 A graph showing the relationship between the idler wavelength and group velocity mismatch in an optical parametric oscillator (OPO) is presented. At a central wavelength of 6 μm, the temporal walkoff between the signal and idler pulses is 49.4 fs / mm, while the temporal walkoff between the pump and signal pulses is 8.3 fs / mm. Therefore, for a 2 mm crystal, a pump pulse width greater than 100 fs is considered to have no significant walkoff effect. Furthermore, due to the different polarization states of the pump and signal / idler light, a spatial walkoff angle exists, which is calculated to be approximately 20 mrad.

[0049] The design diagram of the LGS crystal cut at the Brewster angle is shown in the figure below. Figure 4 As shown. According to the crystal refractive index equation, the refractive index of the crystal at the signal pulse of 1.27μm (corresponding to the idler pulse of 6μm) is 2.1. Substituting this result into the Fresnel law of refraction and reflection, the relationship between the incident angle and reflectivity of the p-wave and s-wave is further calculated as follows: Figure 5 As shown in the figure, the Brewster angle θ corresponding to this wavelength is B=64.5°. Accordingly, the surface reflection loss of the pump pulse (s-wave) is about 40%.

[0050] The optical parametric oscillator (OPO) based on an LGS crystal uses a single-resonance configuration, with the signal light acting as the oscillator. The pump light is focused onto the center of the LGS crystal by a focusing lens. The OPO employs an X-shaped cavity structure, consisting of a pair of curved mirrors M1 and M2, a highly reflective mirror M3, and an output coupling mirror. Curved mirror M1 is constructed from a fused silica substrate and coated with a low-dispersion, highly reflective coating for pump light (1.05μm) and a low-dispersion, highly reflective coating for signal light (1.27μm). Curved mirror M2 is constructed from a ZnSe substrate and coated with a low-dispersion, highly reflective coating for signal light (1.27μm) and a high-transmission coating for idler light (6μm). Plane mirror M3 is constructed from a fused silica substrate and coated with a low-dispersion, highly reflective coating for the signal light band (1.27μm). The output coupling mirror is constructed from a fused silica substrate and coated with a low-dispersion, partially reflective coating for the signal light band (1.27μm).

[0051] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to 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 device for achieving low-loss operation of a long-wave infrared femtosecond optical parametric oscillator, characterized in that: include: Pump source, pump focusing lens, nonlinear crystal cut at Brewster angle, laser medium dichroic mirror, output coupling mirror; The laser medium dichroic mirror and the output coupling mirror together form an optical resonant cavity, and the nonlinear crystal is located at the beam waist of the optical resonant cavity; the pump focusing lens is used to focus the pump pulse generated by the pump source to the center of the nonlinear crystal; The pump pulse is transformed into a signal pulse and an idler pulse with different frequencies after passing through a nonlinear crystal; the signal pulse is output through an output coupling mirror; and the idler pulse is output through the laser medium dichroic mirror.

2. The device according to claim 1, characterized in that The nonlinear crystal satisfies the following phase matching condition: the pump pulse wave vector generated by the pump source is equal to the sum of the signal pulse wave vector and the idler pulse wave vector.

3. The device according to claim 1, characterized in that The signal pulse and the idler pulse have the same polarization state, and are both in the p-polarization state relative to the nonlinear crystal.

4. The device according to claim 1, characterized in that The pump pulse and signal pulse are near-infrared or mid-infrared pulses, and the idler pulse is a long-wave infrared pulse.

5. The device according to claim 1, characterized in that The surface of the laser medium dichroic mirror is coated with a dielectric film that has high transmission for pump pulses, high reflection for signal pulses, and high transmission for idler pulses.

6. The device according to claim 1, characterized in that The surface of the output coupling mirror is coated with a dielectric film that partially reflects the signal pulse.

7. The device according to claim 1, characterized in that The laser medium dichroic mirror includes a first laser medium dichroic mirror M1, a second laser medium dichroic mirror M2 and a third laser medium dichroic mirror M3; the pump pulse passes through M1 and is incident on a nonlinear crystal to be converted into a signal pulse and an idler pulse with different frequencies; the idler pulse is directly output through M2; the signal pulse is reflected by M2 to the output coupling mirror and then partially output, and the remaining signal pulse is reflected back to M2 by the output coupling mirror, then reflected back to M1 by M2, then reflected back to M3 by M1, and then reflected back to M1 by M3, and finally reflected back to M1 by M3, synchronously meeting the next pump pulse and oscillating back and forth in the cavity.

8. The device according to claim 7, characterized in that The third laser medium dichroic mirror M3 is mounted on a translation stage to adjust the length of the resonant cavity to ensure that the time it takes for the signal pulse to propagate one circle in the cavity is equal to the interval between two pump pulses, thereby achieving synchronous pumping.

9. The device according to claim 1, characterized in that The nonlinear crystal is a lithium gallium sulfur crystal.

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