Double-pulse difference frequency intermediate infrared laser generating device with automatically overlapped time-space domain

Abstract

The invention belongs to the technical field of infrared laser, and discloses a time-space domain automatically overlapped double-pulse difference frequency intermediate infrared laser generation device, which comprises a synchronous pump light parametric oscillator, a dual-wavelength wave plate and a difference frequency generator, the synchronous pump optical parametric oscillator is designed to output signal pulses and idler frequency pulses from the same output port, wherein time and space domains of the signal pulses and the idler frequency pulses are automatically overlapped. The signal pulse is used as a pumping pulse of the difference frequency generator, and the idler frequency pulse is used as a seed pulse of the difference frequency generator; the dual-wavelength wave plate is used for simultaneously adjusting the polarization states of the signal pulse and the idler frequency pulse so as to meet the polarization condition of phase matching in the difference frequency generation process in the difference frequency generator; the difference frequency generator generates mid-infrared laser based on the difference frequency of the pump pulse and the seed pulse. According to the invention, time and space superposition of the pump and the seed pulse in the difference frequency process can be automatically realized, complex pulse time delay adjustment and space alignment of an optical path are not needed, the complexity and cost of the system can be effectively reduced, and miniaturization and practicability of the device are facilitated.

Description

Technical Field

[0001] The present invention belongs to the field of infrared laser technology, and more specifically, relates to a dual-pulse difference frequency mid-infrared laser generating device with automatic overlap in time and space domains. Background Art

[0002] The mid-infrared band (2-20μm) corresponds to the spectral fingerprint region of molecules and has important research value. Through the absorption spectrum measurement of the mid-infrared band, the molecular components of substances can be accurately identified and quantitatively detected, providing a vital basic tool for applications such as nano-scale chemical analysis, environmental measurement, disease diagnosis, biochemical hazardous product detection and industrial process control.

[0003] Nonlinear frequency conversion is the "classic" technical route for generating mid-infrared lasers. Using a second-order nonlinear crystal as a gain medium, mid-infrared lasers can be generated through an optical parametric oscillator (OPO), an optical difference frequency generator (DFG) or an optical parametric amplifier (OPA). Comparing the three technical solutions, OPA is mainly used to generate high-energy femtosecond pulses, and the repetition rate of the pulses it generates is low, usually less than 1MHz; OPO is limited by crystal materials and coatings, and the wavelength range of long-wave infrared lasers is limited; in comparison, DFG has a greater advantage in generating high-repetition-rate long-wavelength (5-20μm) mid-infrared lasers.

[0004] The seed pulse of a traditional dual-pulse DFG system is usually obtained by frequency conversion of the pump pulse. The process of generating the seed pulse requires frequency conversion through nonlinear media such as optical fiber, which results in a certain time delay difference between the seed pulse and the pump pulse. For ultrashort optical pulses, this time delay difference requires a precise mechanical time delay compensation device to compensate for it to ensure that the dual pulses overlap in the time domain, which will increase the cost of system construction and maintenance, increase the complexity of the system, and reduce the stability of the system. In addition, since the pump and seed pulses of the traditional dual-pulse DFG system do not overlap in space, a beam combining system is required to achieve the alignment and spatial overlap of the dual beams, which further increases the complexity of the system. Summary of the invention

[0005] In view of the above defects or improvement needs of the prior art, the present invention provides a dual-pulse difference-frequency mid-infrared laser generating device with automatic overlap in time and space domains, which aims to reduce the complexity of the dual-pulse difference-frequency mid-infrared laser generating device and improve the stability of the system.

[0006] To achieve the above-mentioned object, according to the first aspect of the present invention, there is provided a dual-pulse difference frequency mid-infrared laser generating device with automatic overlap in time and space domains, comprising: a synchronous pump optical parametric oscillator, a dual-wavelength wave plate and a difference frequency generator; wherein the synchronous pump optical parametric oscillator comprises a nonlinear crystal, a laser reflection mirror group, a laser output coupling mirror and a precision displacement platform; and the nonlinear crystal is a quasi-phase matching crystal or a type of phase matching crystal;

[0007] The pump pulse is focused on the nonlinear crystal to generate a signal pulse and an idler pulse; the laser reflection mirror group and the laser output coupling mirror constitute a laser resonant cavity for positive feedback of the signal pulse; the laser output coupling mirror is used to fully transmit the idler pulse, partially reflect and partially transmit the signal pulse, the reflected signal pulse returns to the laser resonant cavity again to oscillate, and the transmitted signal pulse and the idler pulse are output simultaneously from the laser output coupling mirror; the precision displacement platform is used to adjust the cavity length of the laser resonant cavity to achieve time synchronization of the signal pulse, the idler pulse and the pump pulse;

[0008] The signal pulse and idler pulse simultaneously output from the laser output coupling mirror are adjusted by the dual-wavelength wave plate to have polarization states perpendicular to each other, and then input into the difference frequency generator for difference frequency to generate mid-infrared laser; wherein the signal pulse and idler pulse correspond to the pump pulse and seed pulse in the difference frequency process.

[0009] Further, the laser reflector group includes a dielectric reflector and a metal reflector;

[0010] The pump pulse is focused on the nonlinear crystal through the dielectric reflector, and the generated signal pulse and idler pulse are reflected to the laser output coupling mirror through the metal reflector. The signal pulse reflected by the laser output coupling mirror is partially reflected again by the metal reflector into the laser resonant cavity for oscillation.

[0011] Further, the laser resonant cavity is an X-type resonant cavity composed of a dielectric reflector M1, a metal reflector M2, a dielectric reflector M3 and the laser output coupling mirror;

[0012] Or, the laser resonant cavity is a V-shaped resonant cavity structure composed of a dielectric reflector M1, a metal reflector M2 and the laser output coupling mirror;

[0013] Wherein, the nonlinear crystal is arranged between the dielectric reflector M1 and the metal reflector M2.

[0014] Furthermore, the nonlinear crystal is arranged at the beam waist of the corresponding Gaussian pulse in the laser resonant cavity.

[0015] Further, the difference frequency generator includes a beam focusing element, a difference frequency crystal and a beam collimating element;

[0016] The beam focusing element is used to focus the pump pulse and the seed pulse in the difference frequency process; the difference frequency crystal is used to provide gain for the difference frequency process to generate mid-infrared laser; the beam collimating element is used to collimate the generated mid-infrared laser to obtain collimated mid-infrared laser.

[0017] Furthermore, the synchronous pump optical parametric oscillator also includes: an ultrashort pulse pump source and a pump focusing lens; the ultrashort pulse pump source is used to generate pump pulses; and the pump focusing lens is used to focus the pump pulses on the nonlinear crystal.

[0018] Furthermore, it also includes a half-wave plate and / or an optical isolator arranged between the ultrashort pulse pump source and the pump focusing lens; wherein the half-wave plate is used to adjust the polarization state of the pump pulse generated by the ultrashort pulse pump source to be consistent with the polarization state required by the nonlinear crystal; and the optical isolator is used to isolate the pump pulse returned from the laser resonant cavity.

[0019] According to another aspect of the present invention, there is provided a method for generating mid-infrared laser using the double-pulse difference-frequency mid-infrared laser generating device described in any one of the first aspects.

[0020] Furthermore, the method further comprises: simultaneously changing the phase matching condition of the nonlinear crystal and the cavity length of the laser resonant cavity to achieve wavelength tunability of the signal pulse and the idler pulse;

[0021] By changing the phase matching condition of the difference frequency crystal, the phase matching of the wavelength-tunable signal pulse and the idler pulse is achieved, so as to realize the wavelength-tunable mid-infrared laser output.

[0022] Furthermore, the phase matching condition of the nonlinear crystal is: the pump pulse wave vector is equal to the sum of the signal pulse wave vector and the idler pulse wave vector;

[0023] The phase matching condition of the difference frequency crystal is that the mid-infrared pulse wave vector generated by the difference frequency is equal to the difference between the pump pulse wave vector and the seed pulse wave vector in the difference frequency process.

[0024] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0025] (1) The dual-pulse difference frequency mid-infrared laser generating device with automatic overlap in time and space in the present invention realizes the time synchronization of the signal pulse, idler pulse and ultrashort pulse pump source through the designed synchronous pump optical parametric oscillator; quasi-phase matching or a type of phase-matched nonlinear crystal is used to ensure that the output signal pulse and the idler pulse have the same polarization state, thereby avoiding the spatial walk-off of the pulse due to the birefringence effect and ensuring the spatial overlap of the dual pulses. Therefore, the signal pulse and the idler pulse output simultaneously from the same laser output coupling mirror automatically overlap in both the time domain and the space domain. Afterwards, the polarization state is adjusted by a dual-wavelength wave plate so that the dual pulses meet the polarization conditions of phase matching in the difference frequency generation process. In this way, the signal pulse is used as the pump pulse in the difference frequency process, and the idler pulse is used as the seed pulse in the difference frequency process, and the difference frequency can generate mid-infrared laser. Compared with the traditional DFG system, there is no need to use a complex beam delay adjustment device, and there is no need to use a complex dual-beam beam combining device, which reduces the system complexity and device cost and improves the robustness of the system.

[0026] (2) Preferably, the nonlinear crystal is disposed at the beam waist of the corresponding Gaussian pulse in the laser resonant cavity to increase the peak power of the pump pulse and the signal pulse, thereby improving the efficiency of the nonlinear conversion process.

[0027] In summary, the dual-pulse difference-frequency mid-infrared laser generating device with automatic overlap in the time and space domain of the present invention avoids the introduction of complex time delay adjustment and spatial beam alignment structures in the DFG system, and can significantly reduce the complexity of the DFG system while improving its performance stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a double-pulse difference-frequency mid-infrared laser generating device with automatic overlap in time and space domains in an embodiment of the present invention.

[0029] Figure 2 It is a spectrum tuning diagram of a signal pulse output by a synchronous pump optical parametric oscillator obtained by numerical simulation based on the parameters in the embodiment of the present invention.

[0030] Figure 3 It is a spectrum tuning diagram of the idler pulse output by the synchronous pump optical parametric oscillator obtained by numerical simulation based on the parameters in the embodiment of the present invention.

[0031] Figure 4 This is a relationship diagram between the phase matching angle of the difference frequency crystal and the central wavelength of the optical parametric oscillator signal pulse in the difference frequency generation process obtained by numerical simulation based on the parameters in the embodiment of the present invention.

[0032] Figure 5The diagram is a variation diagram of the idler center wavelength in the difference frequency generation process obtained by numerical simulation based on the parameters in the embodiment of the present invention with the synchronous pump optical parametric oscillator signal pulse and the idler center wavelength.

[0033] Figure 6 It is a mid-infrared spectrum tuning diagram of the difference frequency generation process obtained by numerical simulation based on the parameters in the embodiment of the present invention. DETAILED DESCRIPTION

[0034] 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 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. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0035] like Figure 1 As shown, an embodiment of the present invention provides a dual-pulse difference frequency mid-infrared laser generating device with automatic overlap in time and space domains, comprising: a synchronous pump optical parametric oscillator, a dual-wavelength wave plate and a difference frequency generator;

[0036] The synchronous pump optical parametric oscillator is designed to output signal pulses and idler pulses that automatically overlap in time and space domains from the same output port; the signal pulse serves as the pump pulse of the difference frequency generator, and the idler pulse serves as the seed pulse of the difference frequency generator; the dual-wavelength wave plate is used to simultaneously adjust the polarization states of the signal pulse and the idler pulse to meet the polarization conditions of phase matching in the difference frequency generation process in the difference frequency generator; the difference frequency generator uses the pump pulse and the seed pulse to generate mid-infrared laser by difference frequency.

[0037] Specifically, the synchronous pump optical parametric oscillator includes an ultrashort pulse pump source, a pump focusing lens, a nonlinear crystal, a laser reflection mirror group, a laser output coupling mirror and a precision displacement platform.

[0038] An ultrashort pulse pump source is used to generate pump pulses.

[0039] The pump focusing lens is used to focus the pump pulse onto the nonlinear crystal so that the nonlinear crystal generates a signal pulse and an idler pulse.

[0040] The nonlinear crystal is arranged in a resonant cavity formed by a laser reflector group and a laser output coupling mirror, and the nonlinear crystal is a quasi-phase matching or a type of phase matching crystal to ensure that the output signal pulse and the idler pulse have the same polarization state to avoid spatial walk-off and ensure the spatial overlap of the double pulses; wherein the resonant cavity formed by the laser reflector group can be an "X" type resonant cavity, a "V" type resonant cavity, etc. Preferably, the nonlinear crystal is arranged at the beam waist of the corresponding Gaussian pulse in the resonant cavity to increase the peak power of the pump pulse and the signal pulse, thereby improving the efficiency of the nonlinear conversion process.

[0041] The laser reflector group includes a dielectric reflector and a metal reflector; wherein the dielectric reflector is used for fully transmitting the pump pulse generated by the ultrashort pulse pump source and fully reflecting the signal pulse oscillating in the resonant cavity, and the metal reflector is used for fully reflecting the signal pulse and the idler pulse; specifically, the pump pulse generated by the ultrashort pulse pump source is sequentially incident on the nonlinear crystal through the pump focusing lens and the dielectric reflector, the nonlinear crystal generates the signal pulse and the idler pulse, the signal pulse and the idler pulse are fully reflected by the metal reflector to the laser output coupling mirror, the laser output coupling mirror is a dielectric reflector, and the idler pulse is fully reflected. Transmission, transmit part of the signal pulse, reflect the other part of the signal pulse, realize the idle pulse and part of the signal pulse to be output simultaneously through the laser output coupling mirror, that is, realize the output of double pulses from the same output port, used as the pump pulse and seed pulse of the difference frequency generator; and the other part of the signal pulse reflected by the laser output coupling mirror returns to the resonant cavity, reflects to the dielectric mirror through the metal reflector, and oscillates between the dielectric reflector and the metal reflector until it encounters the next pump pulse generated by the ultrashort pulse pump source, repeats the above process again, and outputs the next double pulse from the same output port. In this process, the precision displacement platform adjusts the cavity length of the resonant cavity to achieve the time synchronization of the signal pulse, the idle pulse and the ultrashort pulse pump source, that is, the time for the signal pulse reflected back to the laser resonant cavity to go back and forth in the laser resonant cavity matches the repetition frequency of the pump pulse generated by the ultrashort pulse pump source. Generally, the laser reflector is mounted on the precision displacement stage, and the cavity length is adjusted by moving back and forth.

[0042] The dual-wavelength wave plate is used to adjust the polarization states of the signal pulse and the idler pulse simultaneously output from the laser output coupling mirror to be perpendicular to each other. Preferably, the dual-wavelength wave plate introduces a relative phase delay of an odd multiple of half a wavelength to the pump and seed pulses that originally have the same polarization state, so that the two pulses have orthogonal polarization directions to meet the polarization conditions for phase matching in the difference frequency generation process.

[0043] The difference frequency generator performs difference frequency on the signal pulse and the idler pulse whose polarization states are adjusted by the dual-wavelength wave plate to generate mid-infrared laser; wherein the signal pulse serves as the pump pulse in the difference frequency process, and the idler pulse serves as the seed pulse in the difference frequency process.

[0044] Preferably, the difference frequency generator includes a beam focusing element, a difference frequency crystal and a beam collimating element; the beam focusing element is used to focus the signal pulse serving as a pump pulse and the idler pulse serving as a seed pulse; the difference frequency crystal is used to provide gain for the difference frequency process of generating mid-infrared laser, wherein the difference frequency crystal satisfies the phase matching condition for the pump pulse and the seed pulse to realize the difference frequency process; the beam collimating element is used to collimate the generated mid-infrared laser to obtain the collimated mid-infrared laser.

[0045] Preferably, for synchronously pumped optical parametric oscillators, wavelength tuning of signals and idler pulses can be achieved by simultaneously changing the phase matching conditions of the nonlinear crystal and the length of the resonant cavity. For difference frequency generators, phase matching of wavelength tunable signals and idler pulses can be achieved by changing the phase matching conditions of the difference frequency crystal, thereby achieving wavelength tunable mid-infrared laser output.

[0046] In the embodiment of the present invention, the phase matching condition of the nonlinear crystal is: the pump pulse wave vector k generated by the ultrashort pulse pump source pump1 Equal to the signal pulse wave vector k signal and idler pulse wave vector k idler The sum is:

[0047] k pump1 =k signal +k idler ,Right now,

[0048] Where n is the refractive index, ω is the angular frequency of the optical pulse, and c is the speed of light; the subscripts pump1, signal, and idler correspond to the pump pulse, signal pulse, and idler pulse generated by the ultrashort pulse pump source, respectively.

[0049] The phase matching condition of the difference frequency crystal is: the difference frequency generates the mid-infrared pulse wave vector k MIR Equal to the pump pulse wave vector k in the difference frequency process pump2 Subtract the seed pulse wave vector k seed ,Right now:

[0050] k MIR =k pump2 -k seed ,Right now,

[0051] Where n is the refractive index, ω is the angular frequency of the optical pulse, and c is the speed of light. The subscripts MIR, pump2, and seed correspond to the infrared pulse, pump pulse, and seed pulse in the difference frequency process, respectively.

[0052] Preferably, the dual-wavelength wave plate has a certain working bandwidth, which can introduce a relative phase delay of an odd multiple of half a wavelength into the dual pulses within the wavelength tuning range of the pump and signal pulses, so that the two pulses have orthogonal polarization directions to meet the phase matching conditions of the difference frequency generation process of pulses with different central wavelengths.

[0053] Preferably, along the optical path, the laser reflectors between the nonlinear crystal and the laser output coupling mirror are all metal reflectors to prevent the signal pulse and the idler pulse from leaking from other ports.

[0054] Preferably, the invention further comprises a half-wave plate, or / and an optical isolator, arranged between the ultrashort pulse pump source and the pump focusing lens; wherein the half-wave plate is used to adjust the polarization state of the pump pulse generated by the ultrashort pulse pump source to be consistent with the polarization state required by the nonlinear crystal. The optical isolator is used to isolate the pump pulse that may return from the laser resonant cavity to avoid damage to the ultrashort pulse pump source.

[0055] The dual-pulse difference frequency mid-infrared laser generating device with automatic overlap in time and space domains in the present invention realizes the time synchronization of the signal pulse, the idler pulse and the ultrashort pulse pump source through the designed synchronous pump optical parametric oscillator; the output signal pulse and the idler pulse have the same polarization state by using quasi-phase matching or a type of phase-matched nonlinear crystal, so as to avoid the spatial walk-off of the pulse due to the birefringence effect, and ensure the spatial coincidence of the dual pulses. Therefore, the signal pulse and the idler pulse output simultaneously from the same laser output coupling mirror automatically overlap in both the time domain and the space domain. Afterwards, the polarization state is adjusted by a dual-wavelength wave plate so that the dual pulses meet the polarization conditions of phase matching in the difference frequency generation process. In this way, the signal pulse is used as the pump pulse in the difference frequency process, and the idler pulse is used as the seed pulse in the difference frequency process, and the difference frequency can generate mid-infrared laser. Compared with the traditional DFG system, there is no need to use a complex beam delay adjustment device, and there is no need to use a complex dual-beam beam combining device, which reduces the system complexity and device cost, and improves the robustness of the system.

[0056] The device of the present invention is further described below by taking an "X"-shaped resonant cavity structure formed by a laser reflector group in a synchronously pumped optical parametric oscillator as an example.

[0057] The laser reflector group includes laser reflectors M1, M2 and M3. M1, M2, M3 and the laser output coupling mirror together constitute an "X" type laser resonant cavity for positive feedback of the signal pulse. Among them, M1 is a dielectric plano-concave laser reflector with a curvature radius of 100mm. The surface is coated with a dielectric film that has high transmittance to the pump pulses generated by the ultrashort pulse pump source and high reflection to the signal pulse. M2 is a metal plano-concave laser reflector with a curvature radius of 100mm. The surface is coated with a broadband reflective metal silver film, which can achieve high reflection of signal pulses and idler pulses. M3 is a dielectric plane laser reflector with the same coating as M1. It is installed on a precision displacement platform and is used to adjust the cavity length of the optical parametric oscillator to meet the synchronous pumping requirements of different wavelengths. The laser output coupling mirror is a dielectric plane mirror, and the surface is coated with a dielectric film with a transmittance of 40% for signal pulses and full transmittance for idler pulses. The nonlinear crystal used to provide gain in the resonant cavity is a fan-out periodically poled lithium niobate (PPLN) crystal with a length of 3mm and a polarization period gradually changing from 31.41-31.90μm. Its quasi-phase matching gain bandwidth covers the signal light band of 1700-2000nm. The ultrashort pulse pump source is a yttrium-doped fiber master oscillator-amplifier system based on nonlinear polarization rotation mode locking. The system outputs a pump pulse train with a central wavelength of 1050nm, a time domain pulse width of 300fs, a maximum average power of 8W, a pulse train repetition frequency of 80MHz, and a horizontal polarization state. A 1050nm central wavelength half-wave plate and an optical isolator are installed after the pump source to achieve average power adjustment and isolation protection of the pump source.

[0058] Specifically, the optical path of the signal pulse and the idler pulse in the "X" type laser resonant cavity composed of M1, M2, M3 and the laser output coupling mirror is as follows: after the signal pulse and the idler pulse are generated in the nonlinear crystal, they are totally reflected by M2 to the laser output coupling mirror. After that, the idler pulse is fully transmitted and output, and the signal pulse is partially transmitted and output. The remaining signal pulse is reflected by the output coupling mirror to M2, then reflected by M2 to M1, then reflected by M1 to M3, and then reflected back to M1 by M3, and this is a cycle, oscillating back and forth in the cavity.

[0059] It should be noted that, in other embodiments, a "V"-shaped resonant cavity for positive feedback of the signal pulse composed of M1, M2 and a laser output coupling mirror can also be used, M1 is a plane dielectric reflector, M2 is a plano-concave metal reflector, and the laser output coupling mirror is a plane dielectric reflector. The nonlinear crystal is placed between M1 and M2 as close to the side of the plane mirror M1 as possible. The pump pulse generated by the ultrashort pulse pump source is focused by the pump focusing lens L1, and after being fully transmitted through M1, it is focused and converged on the nonlinear crystal to generate a signal pulse and an idler pulse. The signal pulse and the idler pulse are reflected by M2 to the laser output coupling mirror, and the idler pulse is fully transmitted and output. A part of the signal pulse is transmitted and output, and the other part is reflected to M2, and then reflected to M1 through M2, and this is a cycle, oscillating back and forth between M1, M2 and the output coupling mirror.

[0060] In order to make it easier for relevant technical personnel in the field to understand the characteristics of the signal pulse and idler pulse output by the optical parametric oscillator in the embodiment of the present invention, the three-wave coupling equation is numerically solved by the distributed Fourier algorithm to respectively give the tunable spectrum diagrams of the signal pulse and idler pulse output by the optical parametric oscillator, as shown in FIG. Figure 2 , 3 It can be seen that the wavelength tuning range of the signal pulse covers 1700-2000nm, and the corresponding wavelength tuning range of the idler pulse is 2746-2210nm.

[0061] Since the optical parametric oscillator in the embodiment of the present invention operates in the quasi-phase matching mode of e=e+e type polarization, the generated signal pulses and idler pulses do not have spatial walk-off caused by the birefringence effect in the crystal. The two are highly overlapped in space, and the polarization state is consistent with the pump light, both of which are horizontal polarizations.

[0062] The signal pulse and idler pulse output by the optical parametric oscillator then pass through the broadband dual-wavelength wave plate in the embodiment of the present invention, which provides a flat λ phase shift for the signal pulse with a central wavelength of 1700-2000nm and a flat 1 / 2λ phase shift for the idler pulse with a central wavelength of 2746-2210nm. Therefore, the polarization state of the signal pulse remains unchanged after passing through the wave plate; the polarization state of the idler pulse changes to vertical polarization after passing through the wave plate.

[0063] The signal with orthogonal polarization states and the idler pulse are focused onto the difference frequency crystal through the plano-convex lens L2 (focal length 50mm). In the embodiment of the present invention, the difference frequency crystal is a 2mm long GaSe crystal fixed on a precision 6-axis adjustment frame. Under different signal and idler wavelength conditions, the relationship between the phase matching angle corresponding to the crystal that satisfies Type I (o=eo) phase matching and the central wavelength of the infrared laser generated by the difference frequency and the change of the central wavelength of the signal and idler pulse are as follows: Figure 4 , 5 shown. Figure 6 The spectrum of the mid-infrared laser obtained by simulation in the difference frequency generation process is given. It can be seen that in the embodiment of the present invention, the tunable mid-infrared laser output is achieved without using a beam delay adjustment device and a dual beam combining device.

[0064] 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 dual-pulse difference frequency mid-infrared laser generating device with automatic overlap in time and space domains, characterized in that: include: A synchronously pumped optical parametric oscillator, a dual-wavelength wave plate and a difference frequency generator; wherein the synchronously pumped optical parametric oscillator comprises a nonlinear crystal, a laser reflector group, a laser output coupling mirror and a precision displacement platform; and the nonlinear crystal is a quasi-phase matching or a type-I phase matching crystal; The pump pulse is focused on the nonlinear crystal to generate a signal pulse and an idler pulse; the laser reflection mirror group and the laser output coupling mirror constitute a laser resonant cavity for positive feedback of the signal pulse; the laser output coupling mirror is used to fully transmit the idler pulse, partially reflect and partially transmit the signal pulse, the reflected signal pulse returns to the laser resonant cavity again to oscillate, and the transmitted signal pulse and the idler pulse are output simultaneously from the laser output coupling mirror; the precision displacement platform is used to adjust the cavity length of the laser resonant cavity to achieve time synchronization of the signal pulse, the idler pulse and the pump pulse; The signal pulse and idler pulse simultaneously output from the laser output coupling mirror are adjusted by the dual-wavelength wave plate to have polarization states perpendicular to each other, and then input into the difference frequency generator for difference frequency to generate mid-infrared laser; wherein the signal pulse and idler pulse correspond to the pump pulse and seed pulse in the difference frequency process.

2. The double-pulse difference frequency mid-infrared laser generating device according to claim 1, characterized in that: The laser reflector group includes a dielectric reflector and a metal reflector; The pump pulse is focused on the nonlinear crystal through the dielectric reflector, and the generated signal pulse and idler pulse are reflected to the laser output coupling mirror through the metal reflector. The signal pulse reflected by the laser output coupling mirror is reflected again by the metal reflector to the laser resonant cavity for oscillation.

3. The double-pulse difference frequency mid-infrared laser generating device according to claim 2, characterized in that: The laser resonant cavity is an X-type resonant cavity composed of a dielectric reflector M1, a metal reflector M2, a dielectric reflector M3 and the laser output coupling mirror; Or, the laser resonant cavity is a V-shaped resonant cavity structure composed of a dielectric reflector M1, a metal reflector M2 and the laser output coupling mirror; Wherein, the nonlinear crystal is arranged between the dielectric reflector M1 and the metal reflector M2.

4. The double-pulse difference frequency mid-infrared laser generating device according to any one of claims 1 to 3, characterized in that: The nonlinear crystal is arranged at the beam waist of the corresponding Gaussian pulse in the laser resonant cavity.

5. The double-pulse difference frequency mid-infrared laser generating device according to claim 4, characterized in that: The difference frequency generator includes a beam focusing element, a difference frequency crystal and a beam collimating element; The beam focusing element is used to focus the pump pulse and the seed pulse in the difference frequency process; the difference frequency crystal is used to provide gain for the difference frequency process to generate mid-infrared laser; the beam collimating element is used to collimate the generated mid-infrared laser to obtain collimated mid-infrared laser.

6. The double-pulse difference frequency mid-infrared laser generating device according to claim 1, characterized in that: The synchronous pump optical parametric oscillator further comprises: an ultrashort pulse pump source and a pump focusing lens; the ultrashort pulse pump source is used to generate pump pulses; the pump focusing lens is used to focus the pump pulses on the nonlinear crystal.

7. The double-pulse difference frequency mid-infrared laser generating device according to claim 6, characterized in that: It also includes a half-wave plate and / or an optical isolator arranged between the ultrashort pulse pump source and the pump focusing lens; wherein the half-wave plate is used to adjust the polarization state of the pump pulse generated by the ultrashort pulse pump source to be consistent with the polarization state required by the nonlinear crystal; and the optical isolator is used to isolate the pump pulse returned from the laser resonant cavity.

8. A method for generating mid-infrared laser using the double-pulse difference frequency mid-infrared laser generating device according to any one of claims 1 to 7.

9. The method according to claim 8, characterized in that Also includes: By simultaneously changing the phase matching condition of the nonlinear crystal and the cavity length of the laser resonant cavity, the wavelength of the signal pulse and the idler pulse can be tuned; By changing the phase matching condition of the difference frequency crystal, the phase matching of the wavelength-tunable signal pulse and the idler pulse is achieved, so as to realize the wavelength-tunable mid-infrared laser output.

10. The method according to claim 9, characterized in that The phase matching condition of the nonlinear crystal is: the pump pulse wave vector is equal to the sum of the signal pulse wave vector and the idler pulse wave vector; The phase matching condition of the difference frequency crystal is that the mid-infrared pulse wave vector generated by the difference frequency is equal to the difference between the pump pulse wave vector and the seed pulse wave vector in the difference frequency process.

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