Device for realizing low-loss work of long-wave infrared femtosecond optical parametric oscillator

Abstract

The invention discloses a device for realizing low-loss work of a long-wave infrared femtosecond optical parametric oscillator, which belongs to the field of optical parametric oscillators and comprises a pumping source, a pumping focusing lens, a long-wave infrared nonlinear crystal cut at a Brewster angle, a laser medium dichroscope and an output coupling mirror. Wherein the nonlinear crystal meets the I-type phase matching condition for the optical parameter process, and the front and rear surfaces of the crystal cut the central wavelength of the oscillated signal pulse at a Brewster angle so as to reduce the loss of the pulse in the resonant cavity. According to the long-wave infrared femtosecond optical parametric oscillator, the limitation of residual loss of a crystal antireflection film is broken through by adopting Brewster angle cutting on the crystal, so that the construction of the femtosecond optical parametric oscillator based on the long-wave infrared nonlinear crystal becomes possible, and the bottleneck of the existing long-wave infrared femtosecond optical parametric oscillator in the aspects of wavelength tuning range, output power and the like is expected to be broken through; and a practical low-cost technical scheme is provided for construction and application of the long-wave infrared femtosecond optical parametric oscillator.

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 produced by a femtosecond optical parametric oscillator reported so far.

[0003] However, there are still many practical difficulties in directly generating femtosecond lasers in the 5-20μm band based on femtosecond optical parametric oscillators: At present, the laser power obtained by femtosecond optical parametric oscillators in the 5-12μm band is still low, and the highest power at 12μm is only 60mW. The main reason for the dilemma is that the nonlinear quality factor of the crystal in this band is low, resulting in low parametric gain under a specific pump intensity; at the same time, the anti-reflection film in this band has a large loss due to technical conditions, resulting in a high pump intensity required for the femtosecond optical parametric oscillator to reach the oscillation threshold, and high-intensity pump pulses will cause damage to the anti-reflection film. Therefore, the potential of a large number of crystals in constructing 5-20μm femtosecond optical parametric oscillators cannot be realized. The long-wave infrared femtosecond optical parametric oscillators reported so far are all based on a very small number of high-quality factor crystals, and femtosecond optical parametric oscillators with a central wavelength of 12-20μm have not yet been reported.

[0004] If we can 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, we can 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 making optical parametric oscillators more widely used in spectral detection, microscopic imaging and other fields. Summary of the invention

[0005] In view of the defects of the prior art, the purpose of the present invention is to provide a device for realizing low-loss operation of a long-wave infrared femtosecond optical parametric oscillator, aiming to avoid coating the surface of a nonlinear crystal while increasing its transmittance to signals and idler pulses, thereby reducing the pumping threshold of the optical parametric oscillator and increasing its output power, making it possible to construct a femtosecond optical parametric oscillator based on almost all long-wave infrared nonlinear crystals. The present invention is expected to break through the bottlenecks of existing long-wave infrared femtosecond optical parametric oscillators in terms of wavelength tuning range, output power, etc., and provide a practical and low-cost technical solution for the construction and application of long-wave infrared femtosecond optical parametric oscillators.

[0006] To achieve the above objectives, the present invention provides a device for realizing 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 a 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 transformed 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 the type I phase matching condition for the pump pulse, signal pulse and idler pulse of the optical parametric process, and 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 the 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 idle frequency 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 comprises 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 is incident on a nonlinear crystal after passing through M1 and is transformed 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 then 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 a 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 conceived by the present invention can achieve the following

[0016] Beneficial effects:

[0017] (1) The device for realizing low-loss operation of a long-wave infrared femtosecond optical parametric oscillator proposed in the present invention 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 surface of the crystal, the high residual loss and low damage threshold of the crystal anti-reflection film can be effectively avoided, and it is expected to completely solve the oscillation light output problem of the long-wave infrared femtosecond optical parametric oscillator.

[0018] (2) The device for realizing low-loss operation of a long-wave infrared femtosecond optical parametric oscillator proposed in the present invention 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 for realizing low-loss operation of long-wave infrared femtosecond optical parametric oscillator proposed in the present invention adopts Brewster angle cutting for the crystal, thereby avoiding the low damage threshold of the anti-reflection 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 for realizing low-loss operation of a long-wave infrared femtosecond optical parametric oscillator proposed in the present invention 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 It is a schematic structural diagram of a device for realizing low-loss operation of a long-wave infrared femtosecond optical parametric oscillator in an embodiment of the present invention;

[0022] Figure 2 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 is a graph showing the relationship 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 embodiment of the present invention.

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

[0026] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is 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 used to limit the present invention.

[0027] To achieve the above objectives, the present invention provides a device for realizing 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 a 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 transformed 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 the type I phase matching condition for the pump pulse, signal pulse and idler pulse of the optical parametric process, and 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 the 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 rear 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 to minimize the loss of the idle frequency 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 is incident on a nonlinear crystal after passing through M1 and is transformed into a signal pulse and an idler pulse with different frequencies; the idler pulse is directly output through M2; the signal pulse is reflected to the output coupling mirror through M2 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 then 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 cavity 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 the long-wave infrared nonlinear crystal that meets the type I phase matching condition according to the pump pulse wavelength and the required signal and idler pulse wavelengths.

[0038] S2. Calculate the refractive index of the crystal at the central wavelength of the signal pulse according to the Sellmeier equation of the nonlinear crystal, and use it 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 rear 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 to the center of the nonlinear crystal.

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

[0044] In order to further illustrate a device and method for reducing cavity loss in an optical parametric oscillator provided by the present invention, a detailed description is given below in conjunction with 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, 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 a Brewster angle, a third laser medium dichroic mirror M3 mounted on a translation stage, and an output coupling mirror.

[0047] The thermal conductivity of LGS crystal is relatively high, about 7W / (m·K), and phase matching in the wavelength range of 5-10μm can be achieved under 1.05μm pumping conditions; at the same time, high-power 1μm femtosecond laser pump source technology is relatively mature. Therefore, this embodiment intends to use LGS crystal to construct a femtosecond OPO to obtain a high-power long-wave infrared femtosecond laser at a central wavelength of 6μm.

[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 shown in the figure below: Figure 2 When the central wavelength of the idler pulse is 6 μm, the corresponding phase matching angle θ is 53.1°. Figure 3 The relationship between the wavelength of the idler pulse of the optical parametric oscillator and the group velocity mismatch is given. At the central wavelength of 6μm, the time walk-off between the signal and the idler pulse is 49.4fs / mm, and the time walk-off between the pump and the signal pulse is 8.3fs / mm. Therefore, for a 2mm crystal, it can be considered that there is no obvious walk-off effect when the pulse width of the pump pulse is greater than 100fs. In addition, there will be a spatial walk-off angle between the pump light and the signal / idler light due to the different polarization states. According to calculations, the walk-off angle is about 20mrad.

[0049] The design diagram of the LGS crystal cut at the Brewster angle is shown in the figure. Figure 4 As shown. According to the crystal refractive index equation, the refractive index of the crystal at the signal pulse 1.27μm (corresponding to the idle pulse 6μm) is 2.1. Substituting this result into the Fresnel refraction-reflection law, 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 based on LGS crystal adopts a single resonance configuration, and the signal light is the oscillating light. The pump light is focused on the center of the LGS crystal through a focusing lens. The optical parametric oscillator adopts an X-shaped cavity structure, including a pair of curved mirrors M1-M2, a high-reflection mirror M3 and an output coupling mirror. The curved mirror M1 uses a fused silica substrate, and the coating is highly transparent to the pump light (1.05μm) and a low-dispersion and high-reflection film for the signal light (1.27μm); the curved mirror M2 uses a ZnSe substrate, and the coating is a low-dispersion and high-reflection film for the signal light (1.27μm) and a high-transmittance for the idler light (6μm). The plane mirror M3 uses a fused silica substrate, and is coated with a low-dispersion and high-reflection film for the signal light band (1.27μm). The output coupling mirror uses a fused silica substrate, and is coated with a low-dispersion and partially reflective film for the signal light band (1.27μm).

[0051] It will be easily understood by those skilled in the art that the above description is only 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 protection scope of the present invention.

Claims

1. A device for realizing 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 idle frequency 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 a p-polarization state relative to the nonlinear crystal.

4. The device according to claim 1, characterized in that The pump pulse and the signal pulse are near-infrared or mid-infrared pulses, and the idle 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 medium film which has high transmission for pump pulses, high reflection for signal pulses and high transmission for idle frequency 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 comprises 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 is incident on a nonlinear crystal after passing through M1 and is transformed into a signal pulse and an idler pulse with different frequencies; the idler pulse is directly output through M2; the signal pulse is reflected to an output coupling mirror after passing through M2 and is 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 then 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 displacement stage to adjust the length of the resonant cavity to ensure that the time for a 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 sulfide crystal.

Images