WO2021068300A1 - 一种可调谐宽带中红外激光系统 - Google Patents

一种可调谐宽带中红外激光系统 Download PDF

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WO2021068300A1
WO2021068300A1 PCT/CN2019/113696 CN2019113696W WO2021068300A1 WO 2021068300 A1 WO2021068300 A1 WO 2021068300A1 CN 2019113696 W CN2019113696 W CN 2019113696W WO 2021068300 A1 WO2021068300 A1 WO 2021068300A1
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light
signal light
broadband signal
pulse
broadband
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French (fr)
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钟亥哲
梁成川
胡斌
梁兆星
李瑛�
范滇元
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深圳大学
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
    • H01S3/1083—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering using parametric generation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/163—Solid materials characterised by a crystal matrix
    • H01S3/1671—Solid materials characterised by a crystal matrix vanadate, niobate, tantalate

Definitions

  • This application relates to the field of ultrafast laser technology, in particular to a tunable broadband mid-infrared laser system.
  • the energy-level commercial lasers are mainly concentrated in the near-infrared band of 1-2 ⁇ m.
  • the mid-infrared band between the near-infrared and terahertz (THz) is a very important electromagnetic radiation band.
  • the typical mid-infrared wavelength is 3-5 ⁇ m, which corresponds to the second "window" of the atmosphere and the fingerprint spectrum of most molecules.
  • Mid-infrared ultrashort pulse laser is an important method for studying dynamic problems such as the transient transition process between narrow band semiconductors and superlattice multiple quantum wells, and the energy transfer between molecules and molecules.
  • OPA Optical Parametric Amplification
  • short-wavelength pump light such as a Ti:Sapphire laser
  • nonlinear down-conversion to transfer energy to a long-wavelength mid-infrared laser.
  • OPA has become the most commonly used technical means to generate mid-infrared pulsed laser due to its advantages of high gain, wide bandwidth, and wide tuning.
  • For ultrashort pulse lasers if you want to obtain sufficient conversion efficiency in a finite-length nonlinear crystal, you need to pump with a pulsed laser with extremely high peak power, and the peak power of the pump light will be limited by the nonlinearity. Due to the damage threshold of the crystal, it is difficult to directly generate ultra-short and ultra-strong mid-infrared pulsed laser.
  • Optical Parametric Chirped Pulse Amplification combines the advantages of Chirped Pulse Amplification (CPA) long pulse pumping and OPA high gain to obtain pulsed lasers with ultra-high peak power.
  • the phase matching condition of OPCPA is optimized based on the center wavelength of the signal light, and the gain to the center wavelength is the highest.
  • the dispersion of nonlinear crystal materials due to the dispersion of nonlinear crystal materials, it is still difficult to achieve the broadband phase matching of single-stage optical parametric amplification, which causes the phase mismatch of the spectral components off the center wavelength to increase and the gain decreases, which in turn leads to the narrowing of the OPCPA gain bandwidth.
  • the technical problem to be solved by the present invention is that the optical parametric amplification is limited by the crystal damage threshold and can only obtain lower conversion efficiency, and the optical parametric chirped pulse amplification is difficult to achieve broadband phase matching.
  • This application provides a tunable broadband mid-infrared Laser system can get tunable mid-infrared ultrashort pulse laser.
  • the present invention provides a tunable broadband mid-infrared laser system.
  • the system includes: a pulse laser, a beam splitter, a first pulse stretcher, a broadband signal light generator, an optical path delayer, a second pulse stretcher, and a coupling Mirror, nonlinear crystal, beam splitter, pulse compressor;
  • the pulsed laser generated by the pulsed laser is divided into two pulsed lasers by the beam splitter, the first pulsed laser passes through the first pulse stretcher and reaches the coupling mirror, and the second pulsed laser passes through the broadband
  • the signal light generator obtains broadband signal light.
  • the broadband signal light passes through the second pulse stretcher and reaches the coupling mirror.
  • the first pulse laser and the broadband signal light pass through the coupling mirror together.
  • the first pulsed laser beam and the broadband signal light emitted by the coupling mirror enter the nonlinear crystal to obtain amplified broadband signal light, idle frequency light and residual pump light, and pass through the beam splitter, Separate the idle frequency light, the amplified broadband signal light, and the residual pump light, and the pulse compressor compresses the pulse width of the separated idle frequency light to obtain mid-infrared ultrashort pulse laser.
  • the first pulse laser is pump light;
  • the optical path delayer is arranged between the first pulse stretcher and the coupling mirror or between the second pulse stretcher and the coupling mirror, and a preset time delay is introduced to enable the first A beam of pulsed laser and the broadband signal light are synchronized in time;
  • the chirped and broadened first pulsed laser light has a chirp direction opposite to that of the broadband signal light that has undergone chirped broadening; the first pulsed laser and the broadband signal light are in the nonlinear crystal Perform optical parametric amplification to obtain the amplified broadband signal light, idle frequency light and residual pump light; the bandwidth of the idle frequency light is determined by the phase matching bandwidth of the nonlinear crystal.
  • the nonlinear crystal is a sector-shaped periodically polarized lithium niobate crystal that meets Type II quasi-phase matching, the first pulsed laser light is o-polarized light, and the broadband signal light is e-polarized light, The idle frequency light is o-polarized light.
  • the center wavelength of the idle frequency light generated is adjusted by one of the following methods:
  • the chirp ratio between the broadband signal light and the pump light is adjusted to optimize the The bandwidth of idle frequency light and the energy conversion efficiency of the tunable broadband mid-infrared laser system.
  • the broadband signal light generator is a supercontinuum generator or an optical parameter generator.
  • the pulsed laser is a 790nm Ti:Sapphire femtosecond laser.
  • the beam splitter is highly transparent to the idle frequency light, highly reflective to the pump light and the amplified broadband signal light, or highly reflective to the idle frequency light, to the The pump light and the amplified broadband signal light are highly transmissive dichroic mirrors.
  • the tunable broadband mid-infrared laser system in this application Through the tunable broadband mid-infrared laser system in this application, the impact of pulse slip caused by group velocity mismatch is reduced, and thanks to the opposite chirp directions of the pump light and the broadband signal light, And, the broadband phase matching characteristic of the periodic polarized lithium niobate crystal (PPLN) that satisfies the type II quasi-phase matching during the parametric amplification process of the reverse chirp mid-infrared light can be compared with that of the pump light and the The idle frequency light with a wider bandwidth of the broadband signal light.
  • PPLN periodic polarized lithium niobate crystal
  • Figure 1 is a schematic structural diagram of a tunable broadband mid-infrared laser system in an embodiment of the present invention
  • 2a is a schematic diagram of the initial bandwidth relationship of pump light, signal light, and idle frequency light in an embodiment of the present invention
  • Figure 2b is a schematic diagram of a broadband phase matching structure in an embodiment of the present invention.
  • FIG. 3 is a schematic diagram of the group velocity characteristic value curve of pump light, signal light of different wavelengths, and corresponding idle frequency light in an embodiment of the present invention
  • Fig. 4 shows the conversion efficiency of 790nm pump light to 1030nm signal light small signal optical parametric amplification and the generated idle frequency light bandwidth of about 3.4 ⁇ m with different chirp ratios ( ⁇ s/ ⁇ p) in the embodiment of the present invention
  • Fig. 5 is a graph showing the conversion efficiency of 790nm pump light to 1100nm-1500nm tunable signal light parametric amplification and the resulting idle frequency light bandwidth as a function of signal light wavelength in an embodiment of the present invention.
  • optical parametric amplification is restricted by the crystal damage threshold and can only obtain lower conversion efficiency
  • optical parametric chirped pulse amplification OPCA
  • the present invention provides a tunable broadband mid-infrared laser system, which uses a wide bandwidth chirped pulsed laser as the pump light.
  • idle frequency light whose initial bandwidth is wider than that of pump light and signal light is obtained.
  • a wide bandwidth can be achieved in an extremely wide wavelength range
  • the effective optical parameter is amplified to obtain a tunable ultra-short pulse laser in the mid-infrared band.
  • optical parametric amplification process of three-wave mixing must satisfy the conservation of energy and momentum, namely
  • the signal light contains a wide-bandwidth spectrum, and there is more or less phase mismatch ( ⁇ k ⁇ 0) for the spectral components that deviate from the center wavelength.
  • phase mismatch the greater the amount of mismatch, the lower the gain.
  • the phase matching bandwidth determines the gain bandwidth of the optical parametric amplification, and also limits the extreme bandwidth of the output ultrashort pulse laser.
  • Equation (3) The first-order terms in equation (3) exactly cancel each other, and ⁇ k is only determined by the second-order and other higher-order dispersion terms. At this time, an extremely wide phase matching bandwidth can be obtained.
  • ⁇ p and ⁇ s represent the linear chirp coefficients of pump light and signal light, respectively.
  • the pump light and signal light are respectively introduced with time chirps in opposite directions, the corresponding relationship of the different spectral components of the acting light (pump light, signal light, idle frequency light) can be adjusted in the time domain.
  • the relatively independent spectral components of the pump light and signal light can meet the phase matching, and realize the perfect phase matching of the whole spectrum and the optical parametric amplification with high conversion efficiency.
  • the chirp ratio ⁇ of signal light and pump light and the group velocity characteristic value of pump light, signal light and idle frequency light are required. equal.
  • the group velocity characteristic value is a fixed value, and to obtain idle frequency light with a wider initial bandwidth, the chirp ratio ⁇ is objectively required to be a negative value, so it is necessary to deal with different optical parameters In the amplification process, a nonlinear crystal with a negative group velocity characteristic value is correspondingly used.
  • the chirp ratio ⁇ of the signal light and the pump light is optimized to make it equal to the group velocity eigenvalue If they are equal, the conversion efficiency of the parametric amplification of the reverse chirp light and the output bandwidth of the idle frequency light can be improved.
  • the group velocity characteristic value determines the chirp ratio between the signal light and the pump light required for broadband phase matching, and further determines the bandwidth ratio of the signal light and the pump light involved in the optical parametric amplification.
  • ⁇ s / ⁇ p is -1
  • the mixing effect is the best, and the optimal idle frequency bandwidth can be obtained (that is, compared with the bandwidth of the incident pump light and signal light, the initial idle frequency light Bandwidth increase ratio).
  • Group velocity eigenvalue It is -1 that is, the group velocity of idle frequency light should be the arithmetic average of signal light and pump light.
  • the linear chirp added to the pump light and signal light has a significant difference, which will inevitably sacrifice part of the signal light or the spectrum of the pump light.
  • the meaning of mixing is also lost. Therefore, in practical applications, whether it is from the perspective of energy utilization or from the perspective of bandwidth improvement, the situation where the group velocity characteristic value is near -1 has more application value.
  • Figure 3 shows the group velocity characteristic value curves of 790nm pump light and signal light of different wavelengths under different crystals and different phase matching modes, and the corresponding idle frequency light, including LN, KTP, LBO, YCOB and PPLN.
  • Linear crystals, and different phase matching methods such as Type I phase matching, Type II phase matching, Type 0 quasi phase matching and Type II quasi phase matching.
  • the periodically polarized lithium niobate crystal (oeo) that meets the Type II quasi-phase matching is not only in the extremely wide spectral range (980nm-1500nm@790nm, that is, 790nm pulsed laser is used as the pump light).
  • the wavelength range is 980nm-1500nm, there is a stable group velocity relationship (gentle group velocity eigenvalue curve), and the corresponding group velocity eigenvalue is still in a relatively ideal range
  • This provides a theoretical basis for periodically polarizing lithium niobate crystals with a sector-shaped structure that meets Class II quasi-phase matching to achieve tunable output pulse laser wavelength.
  • the present invention provides a tunable broadband mid-infrared laser system, as shown in Figure 1, including: pulse laser 1; beam splitter 2; first pulse stretcher 3; broadband signal light generator 6; optical path delayer 4 The second pulse stretcher 7; the coupling mirror 8; the nonlinear crystal 9; the beam splitter 10; the pulse compressor 11.
  • the pulsed laser output from the pulsed laser 1 is divided into two pulsed lasers by the beam splitter 2.
  • the first pulse laser passes through the first pulse stretcher 3 and reaches the coupling mirror 8.
  • the second pulse laser passes through the broadband signal light generator 6 to obtain broadband signal light, and the broadband signal light passes through the second pulse stretcher 7 to reach the coupling mirror 8. Specifically, the second pulse laser enters a supercontinuum generator or an optical parameter generator to generate a tunable broadband signal light, and then the broadband signal light is introduced into the second pulse stretcher 7 to obtain a Tuned broadband chirp signal light.
  • the first pulse laser and the broadband signal light pass through the coupling mirror 8 together.
  • the first pulse laser beam and the broadband signal light emitted through the coupling mirror 8 enter the nonlinear crystal 9 to obtain amplified broadband signal light, idle frequency light and residual pump light, and pass through the
  • the beam splitter 10 separates the idle frequency light, the amplified broadband signal light and the residual pump light, and the pulse compressor 11 compresses the pulse width of the separated idle frequency light to obtain mid-infrared ultrashort pulses Laser, wherein the first pulsed laser light is pump light.
  • the optical path delayer 4 is arranged between the first pulse stretcher 3 and the coupling mirror 8 or between the second pulse stretcher 7 and the coupling mirror 8. By introducing a preset time delay , To synchronize the time of the first pulse laser and the broadband signal light.
  • the chirped and broadened first pulsed laser light has a chirp direction opposite to that of the broadband signal light that has undergone chirped broadening; the first pulsed laser and the broadband signal light are in the nonlinear crystal Perform optical parametric amplification to obtain the amplified broadband signal light, idle frequency light and residual pump light; the bandwidth of the idle frequency light is determined by the phase matching bandwidth of the nonlinear crystal.
  • the nonlinear crystal 9 is a sector-shaped periodically polarized lithium niobate crystal that meets Type II quasi-phase matching, the first pulsed laser light is o-polarized light, and the broadband signal light is e-polarized light , The idle frequency light is o-polarized light.
  • the center wavelength of the idle frequency light generated is adjusted by one of the following methods:
  • the chirp ratio between the broadband signal light and the pump light is adjusted to optimize the The bandwidth of idle frequency light and the energy conversion efficiency of the tunable broadband mid-infrared laser system.
  • the broadband signal light generator 6 is a supercontinuum generator (Supercontinuum Generation, SG) or an optical parametric generator (Optical Parametric Generation, OPG).
  • Supercontinuum Generation SG
  • optical parametric generator Optical Parametric Generation
  • the pulsed laser 1 is a 790nm Ti:Sapphire femtosecond laser.
  • the beam splitter 10 is highly reflective to the idle frequency light, highly reflective to the pump light and the amplified broadband signal light, or highly reflective to the idle frequency light, A dichroic mirror with high transmission for the pump light and the amplified broadband signal light.
  • the first pulsed laser light that is, the pump light
  • the pump light that has undergone chirp expansion
  • the initial bandwidth ratio it is essentially possible to obtain the initial bandwidth ratio.
  • the pump light and the idle frequency light with a wider bandwidth of the broadband signal light.
  • the present invention can be used in an extremely wide wavelength range (980nm-1500nm@790nm , That is, when the 790nm pulsed laser is used as the pump light, the wavelength range is 980nm-1500nm) to complete the wide bandwidth effective optical parametric amplification; on this basis, the sector-shaped structure that meets the type II quasi-phase matching is periodically polarized Lithium niobate crystal can further realize the wide tuning of the output pulse laser wavelength.
  • the pulsed laser 1 is a 790nm Ti:Sapphire femtosecond pulsed laser.
  • the nonlinear crystal 9 is a sector-shaped periodically polarized lithium niobate crystal that meets Type II quasi-phase matching, and its domain length is continuously adjustable in the range of 7.2 ⁇ m-9.1 ⁇ m.
  • the broadband signal light generator 6 is a supercontinuum generator.
  • the beam splitter 10 is a dichroic mirror that highly transmits idle frequency light and highly reflects pump light and amplified broadband signal light.
  • This application provides a specific tunable broadband mid-infrared laser system.
  • the process of obtaining a tunable mid-infrared ultrashort pulse laser from the tunable broadband mid-infrared laser system is as follows:
  • the 790nm Ti:Sapphire femtosecond pulse laser 1 outputs
  • the pulsed laser is divided into two pulsed lasers by the beam splitter 2.
  • One of the pulsed lasers is used as the pump light, and the first pulse stretcher 3 chirps and broadens the pumped light; the other pulsed laser enters
  • the supercontinuum generator 6 obtains a tunable broadband signal light of 980nm-1500nm, and then the second pulse stretcher 7 chirps and broadens the tunable broadband signal light, wherein the chirp broadened
  • the pump light is opposite to the chirp direction of the broadband signal light that has been chirped and broadened; the pump light is introduced into the optical path delayer 4, and an appropriate amount of time delay is introduced to make the pump light and
  • the broadband signal light is synchronized in time, and then enters the coupling mirror 8 respectively.
  • the pump light and the broadband signal light emitted by the coupling mirror 8 enter the periodically polarized lithium niobate crystal 9 with a sector-shaped structure that meets Class II quasi-phase matching, and undergo optical parametric amplification to obtain an amplified Broadband signal light, about 4.1-1.7 ⁇ m tunable mid-infrared broadband idle frequency light and residual pump light.
  • the dichroic mirror 10 the idle frequency light is separated from the amplified broadband signal light and the residual pump light, and then the pulse width of the idle frequency light is compressed by the pulse compressor 11 Finally, a tunable mid-infrared ultrashort pulse laser of about 4.1 ⁇ m-1.7 ⁇ m is obtained.
  • the pump light is a Ti:Sapphire femtosecond pulsed laser with 790nm, and its spectrum is Gaussian.
  • the initial transformation limit pulse width (TL) is 100fs (1/e 2 high half width), and it is broadened by 500 times chirp. The pulse width is stretched to 50ps.
  • the signal light is a supercontinuum light with a center wavelength of 1030nm, a sufficiently broad spectrum, and a super-Gaussian distribution generated by the supercontinuum generator.
  • the initial light intensity of the pump light is 1GW/cm 2 , and the initial light intensity of the signal light is 1% of the initial light intensity of the pump light.
  • Fig. 4 shows the conversion efficiency of the 790nm pump light to the 1030nm signal light small-signal optical parametric amplification in this embodiment and the generated idle frequency light bandwidth of about 3.4 ⁇ m as a function of different chirp ratios ⁇ s/ ⁇ p.
  • the conversion efficiency and the obtained idle frequency bandwidth of about 3.4 ⁇ m will change with the chirp ratio ⁇ s/ ⁇ p, where the maximum conversion efficiency appears at ⁇ s/ ⁇ p ⁇ -0.65.
  • the theoretical value of the optimal chirp ratio ⁇ s/ ⁇ p is about -0.7.
  • the nonlinear crystal 9 in the embodiment of the present invention is a sector structure periodically polarized niobic acid that satisfies Type II quasi-phase matching. Lithium crystals. Compared with a periodically polarized crystal with a single domain length, the fan-shaped periodically polarized crystal can target broadband signal light with different center wavelengths by changing the lateral direction of the broadband signal light and pump light in the fan-shaped periodically polarized crystal. In the active area, the domain length is continuously adjusted to realize the phase matching of broadband signal light with different center wavelengths.
  • the center wavelength of the broadband signal light output by the supercontinuum generator or change the delay introduced by the optical path delayer (equivalent to changing the center wavelength of the broadband signal light interacting with the pump light); and correspondingly Fine-tuning the lateral action area of the pump light and broadband signal light in the sector-shaped periodically polarized lithium niobate crystal that meets the type II quasi-phase matching, that is, changing the domain length (meeting the requirements of phase matching), and the second pulse stretcher
  • the chirp extension of the broadband signal light that is, the chirp ratio is changed (to meet the requirements of broadband phase matching), so that the chirp ratio of the broadband signal light and the pump light is optimized, and a tunable mid-infrared ultrashort pulse can be obtained laser.
  • Fig. 5 shows the conversion efficiency of 790nm pump light to 1100nm-1500nm tunable signal light parametric amplification in the embodiment of the present invention and the resulting idle frequency optical bandwidth as a function of signal light wavelength. It is assumed that the length of the periodically polarized lithium niobate crystal of the sector structure that meets the type II quasi-phase matching is 10mm, and the domain length is continuously adjustable in the range of 7.8 ⁇ m-9.1 ⁇ m.
  • the pump light is a Ti:Sapphire femtosecond pulsed laser with 790nm, and its spectrum is Gaussian.
  • the initial transformation limit pulse width (TL) is 100fs (1/e 2 high half width), and it is broadened by 1000 times chirp.
  • the pulse width is stretched to 100ps.
  • the signal light is a supercontinuum light with a tunable center wavelength of 1100nm-1500nm, a wide enough spectrum and a super-Gaussian distribution generated by the supercontinuum generator.
  • the initial light intensity of the pump light is 0.8GW/cm 2
  • the initial light intensity of the signal light is 1% of the initial light intensity of the pump light.
  • the solid line is for broadband signal light with different central wavelengths, and the chirp extension of the second pulse stretcher on the broadband signal light is fine-tuned accordingly, and the chirp ratio of the broadband signal light and the pump light is optimized to their respective optimal values.
  • the dashed line is the characteristic curve under the ideal situation assuming that there is no dispersion in the nonlinear crystal material.
  • the chirp spread of the second pulse stretcher for broadband signal light with different center wavelengths can be fine-tuned, and the chirp ratio of the broadband signal light and the pump light can be optimized to
  • the respective optimal values can further optimize the conversion efficiency of the tunable broadband mid-infrared laser system and the output bandwidth of the mid-infrared idle frequency light.
  • the output bandwidth of the idle frequency light is almost not affected by the change of the center wavelength of the broadband signal light. This characteristic curve (solid line) is quite close to the result (dashed line) under the ideal situation assuming that there is no dispersion in the nonlinear crystal material.

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Abstract

一种可调谐宽带中红外激光系统，脉冲激光器（1）产生的脉冲激光，经分束镜（2）分成两束脉冲激光，第一束脉冲激光经过第一脉冲展宽器（3），到达耦合镜（8），第二束脉冲激光经过宽带信号光产生器（6）得到宽带信号光，宽带信号光经过第二脉冲展宽器（7），到达耦合镜（8），第一束脉冲激光和宽带信号光共同经过耦合镜（8），进入非线性晶体（9），得到放大后的宽带信号光、闲频光和残余泵浦光。通过分光镜（10），将闲频光、放大后的宽带信号光和残余泵浦光分离，脉冲压缩器（11）将分离后的闲频光的脉宽压缩，得到中红外超短脉冲激光。

Description

一种可调谐宽带中红外激光系统 技术领域
本申请涉及超快激光技术领域，特别是涉及一种可调谐宽带中红外激光系统。
背景技术
由于缺乏合适的激光增益介质，只有极少数特定波长的激光能够由受激辐射的激光介质直接产生。目前，能级型的商品化激光器主要集中在1-2μm的近红外波段。介于近红外与太赫兹(THz)之间的中红外波段，是非常重要的电磁辐射波段。典型的中红外波长为3-5μm，正好对应大气的第二“窗口”以及多数分子的指纹谱。中红外超短脉冲激光是研究窄能带半导体和超晶格多量子阱带间瞬态跃迁过程、分子内和分子间能量转移等动力学问题的重要手段。利用光学的二阶非线性效应，能够引导三种不同频率激光之间的能量传递。其中的光参量放大(Optical Parametric Amplification，以下简称为OPA)，可以将短波长的泵浦光(例如钛宝石激光)，通过非线性下转换，把能量传递至长波长的中红外激光。
OPA凭借其高增益、宽带宽、宽调谐等优点，成为产生中红外脉冲激光最常用的技术手段。对超短脉冲激光而言，若想在有限长的非线性晶体中获得足够的转换效率，需要以极高峰值功率的脉冲激光作泵浦，而泵浦光的峰值功率会受限于非线性晶体的损伤阈值，因而难以直接产生超短超强的中红外脉冲激光。光参量啁啾脉冲放大(OPCPA)结合了啁啾脉冲放大(CPA)长脉冲泵浦和OPA高增益的优点，可获得超高峰值功率的脉冲激光。通常情况下，OPCPA的相位匹配条件是基于信号光中心波长优化的，对中心波长的增益最高。但由于非线性晶体材料存在色散，仍难实现单级光参量放大的宽带相位匹配，造成偏离中心波长的光谱分量的相位失配量增大，增益减小，进而导致OPCPA增益带宽的窄化，限制了输出超短脉冲激光的极限带宽。
因此，现有技术有待改进。
发明内容
本发明要解决的技术问题是光参量放大受制于晶体损伤阈值，只能得到较低 转换效率，以及光参量啁啾脉冲放大难以实现宽带相位匹配的问题，本申请提供一种可调谐宽带中红外激光系统，可以得到可调谐的中红外超短脉冲激光。
本发明提供了一种可调谐宽带中红外激光系统，所述系统包括：脉冲激光器、分束镜、第一脉冲展宽器、宽带信号光产生器、光路延时器、第二脉冲展宽器、耦合镜、非线性晶体、分光镜、脉冲压缩器；
所述脉冲激光器产生的脉冲激光，经所述分束镜分成两束脉冲激光，第一束脉冲激光经过所述第一脉冲展宽器，到达所述耦合镜，第二束脉冲激光经过所述宽带信号光产生器得到宽带信号光，所述宽带信号光经过所述第二脉冲展宽器，到达所述耦合镜，所述第一束脉冲激光和所述宽带信号光共同经过所述耦合镜，经所述耦合镜射出的所述第一束脉冲激光和所述宽带信号光，进入所述非线性晶体，得到放大后的宽带信号光、闲频光和残余泵浦光，通过所述分光镜，将所述闲频光、放大后的宽带信号光和残余泵浦光分离，所述脉冲压缩器将分离后的所述闲频光的脉宽压缩，得到中红外超短脉冲激光，其中，所述第一束脉冲激光为泵浦光；
所述光路延时器设置在所述第一脉冲展宽器和所述耦合镜之间或者所述第二脉冲展宽器和所述耦合镜之间，通过引入预设时间延时，使所述第一束脉冲激光和所述宽带信号光时间同步；
经过啁啾展宽的所述第一束脉冲激光，与经过啁啾展宽的所述宽带信号光的啁啾方向相反；所述第一束脉冲激光与所述宽带信号光在所述非线性晶体中作光参量放大，得到所述放大后的宽带信号光、闲频光和残余泵浦光；所述闲频光的带宽，由所述非线性晶体的相位匹配带宽决定。
可选地，所述非线性晶体为满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体，所述第一束脉冲激光为o偏振光，所述宽带信号光为e偏振光，所述闲频光为o偏振光。
可选地，通过以下其中一种方式，对产生的所述闲频光的中心波长进行调整：
调整所述宽带信号光产生器输出的宽带信号光的中心波长；
调整所述光路延时器引入的延时；
相应的，为保证所述闲频光中心波长的相位匹配，需要调整所述泵浦光和所述宽带信号光在所述满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体中 的横向作用区域；
在此基础上，通过调整所述第二脉冲展宽器对所述宽带信号光的啁啾展宽量，对所述宽带信号光与所述泵浦光的啁啾比进行调整，以此优化所述闲频光的带宽以及所述可调谐宽带中红外激光系统的能量转换效率。
可选地，所述宽带信号光产生器为超连续谱产生器或者光学参量发生器。
可选地，所述脉冲激光器为790nm钛宝石飞秒激光器。
可选地，所述分光镜为对所述闲频光高透射、对所述泵浦光和所述放大后的宽带信号光高反射，或者是对所述闲频光高反射、对所述泵浦光和所述放大后的宽带信号光高透射的双色镜。
与现有技术相比，本发明实施例具有以下优点：
通过本申请中的可调谐宽带中红外激光系统，减少了由于群速度失配造成的脉冲滑移的影响，而且，得益于所述泵浦光与所述宽带信号光的啁啾方向相反，以及，所述满足II类准相位匹配周期性极化铌酸锂晶体(PPLN)在反向啁啾中红外光参量放大过程中的宽带相位匹配特性，可以得到比所述泵浦光、所述宽带信号光带宽更宽的所述闲频光。
附图说明
为了更清楚地说明本发明实施例或现有技术中的技术方案，下面将对实施例或现有技术描述中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图仅仅是本发明中记载的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其他的附图。
图1为本发明实施例中一种可调谐宽带中红外激光系统的结构示意图；
图2a为本发明实施例中泵浦光、信号光以及闲频光的初始带宽关系的示意图；
图2b为本发明实施例中宽带相位匹配结构示意图；
图3为本发明实施例中泵浦光、不同波长信号光以及对应闲频光的群速度特征值曲线示意图；
图4为本发明实施例中790nm泵浦光对1030nm信号光小信号光参量放大的转换效率及产生的约3.4μm闲频光带宽随不同啁啾比(αs/αp)的变化曲线；
图5为本发明实施例中790nm泵浦光对1100nm-1500nm可调谐信号光光参量放大的转换效率及产生的闲频光带宽随信号光波长的变化曲线。
具体实施方式
为了使本技术领域的人员更好地理解本发明方案，下面将结合本发明实施例中的附图，对本发明实施例中的技术方案进行清楚、完整地描述，显然，所描述的实施例仅是本发明一部分实施例，而不是全部的实施例。基于本发明中的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都属于本发明保护的范围。
一般的，光参量放大(OPA)受制于晶体损伤阈值，只能得到较低转换效率，而光参量啁啾脉冲放大(OPCPA)则是难以实现宽带宽的相位匹配。
为了解决上述问题，本发明提供了一种可调谐宽带中红外激光系统，以宽带宽的啁啾脉冲激光作为泵浦光，通过对泵浦光、信号光引入方向相反的时间啁啾，减少由于群速度失配造成的脉冲滑移的影响的同时，得到了初始带宽比泵浦光、信号光带宽更宽的闲频光。在此基础上，利用满足II类准相位匹配的周期性极化铌酸锂晶体在反向啁啾中红外光参量放大过程中的宽带相位匹配特性，可以在极宽的波长范围内完成宽带宽的有效光参量放大，得到可调谐的中红外波段的超短脉冲激光。
下面结合附图，详细说明本发明的各种非限制性实施方式。
首先，对本申请实施例中使用可调谐宽带中红外激光系统产生可调谐的中红外超短脉冲激光的理论依据描述如下：
三波混频的光参量放大过程须满足能量和动量守恒，即
ω p＝ω s+ω i  (1)
k p＝k s+k i  (2)
其中，ω p、ω s、ω i分别表示泵浦光、信号光以及闲频光的角频率，k p、k s、k i分别表示泵浦光、信号光以及闲频光的波矢量。相位匹配(Δk＝k p-k s-k i＝0)是光参量放大有效能量转换的基本要求。通常情况下，相位匹配是基于泵浦光、信号光的中心波长优化的。由于非线性晶体材料存在色散，理论上仅能对单一波长实现相位匹配。对宽带宽的光参量放大过程，信号光包含宽带宽的光谱，对偏离 中心波长的光谱分量或多或少的存在着相位失配(Δk≠0)。失配量越大，增益越低。一般地，相位匹配带宽决定了光参量放大的增益带宽，也限制了输出超短脉冲激光的极限带宽。
对Δk以中心频率ω 0作泰勒展开，有：
Figure PCTCN2019113696-appb-000001
其中，
Figure PCTCN2019113696-appb-000002
表示群速度，Δω＝α(t-t 0)，表示随时间变化的相对于中心角频率的偏移量(当t＝t 0，ω＝ω 0，Δω＝0)，
Figure PCTCN2019113696-appb-000003
代表二阶及其他更高阶色散项，
Figure PCTCN2019113696-appb-000004
表示啁啾脉冲的线性啁啾系数，υ p、υ s、υ i分别表示泵浦光、信号光及闲频光的群速度，Δω p、Δω s、Δω i分别表示泵浦光、信号光及闲频光的角频率偏移量。
可以看出，当信号光与泵浦光的线性啁啾的比值(啁啾比)σ满足
Figure PCTCN2019113696-appb-000005
式(3)中的一阶项恰好相互抵消，Δk仅由二阶及其他更高阶色散项决定，此时，可得到极宽的相位匹配带宽。其中，α p、α s分别表示泵浦光及信号光的线性啁啾系数。
本申请实施例采用的是反向啁啾光参量放大的方式，啁啾泵浦光与啁啾信号光的时间啁啾方向相反，即Δω p、Δω s符号相反。因而，如图2a所示，Δω i＝Δω p-Δω s，理论上，可以得到初始带宽比泵浦光、信号光带宽更宽的闲频光。但是，经过光参量放大，最终能够得到的闲频光的带宽仍由光参量放大的相位匹配带宽决定。要实现宽带宽的有效光参量放大，不仅需要啁啾泵浦光、啁啾信号光的中心波长满足相位匹配，偏离中心波长的其它一一对应的光谱分量也应尽可能的满足相位匹配。
如图2b所示，由于对泵浦光和信号光分别引入了方向相反的时间啁啾，即可以在时间域调整作用光(泵浦光、信号光、闲频光)不同光谱分量的对应关系，使相对独立的泵浦光和信号光的光谱分量都能够满足相位匹配，实现全光谱的完美相位匹配和高转换效率的光参量放大。要使Δk的一阶项恰好相互抵消，实现宽带宽的相位匹配，需要信号光与泵浦光的啁啾比σ与泵浦光、信号光及闲频光的群速度特征值
Figure PCTCN2019113696-appb-000006
相等。对某一特定波长的光参量放大过程，其 群速度特征值为定值，而要得到初始带宽更宽的闲频光，客观上要求啁啾比σ为负值，所以需要针对不同的光参量放大过程，相应的使用群速度特征值为负值的非线性晶体。在此基础上，优化信号光与泵浦光的啁啾比σ，使其与群速度特征值
Figure PCTCN2019113696-appb-000007
相等，即可提高反向啁啾光参量放大的转换效率以及闲频光的输出带宽。
需要说明的是，群速度特征值决定了宽带相位匹配所需要的信号光与泵浦光的啁啾比，进而决定了参与光参量放大的信号光与泵浦光的带宽比。显然，Δω s/Δω p为-1时的混频效果最好，能够得到最优的闲频光带宽增宽(即，相比于入射泵浦光、信号光的带宽，闲频光的初始带宽的提升比例)。这需要群速度特征值
Figure PCTCN2019113696-appb-000008
为-1，即闲频光的群速度应为信号光、泵浦光的算术平均。如果群速度特征值过于偏离-1，为了实现宽带宽的相位匹配，加在泵浦光、信号光上的线性啁啾差别明显，必然会牺牲部分信号光，或者泵浦光的光谱，与此同时，也失去了混频的意义。所以在实际应用中，无论是从能量利用的角度还是从带宽提升的角度，群速度特征值在-1附近的情况更具应用价值。
图3给出了不同晶体、不同相位匹配方式下，790nm泵浦光与不同波长信号光，及对应闲频光的群速度特征值曲线，其中包括LN、KTP、LBO、YCOB和PPLN等不同非线性晶体，以及I类相位匹配、II类相位匹配、0类准相位匹配和II类准相位匹配等不同相位匹配方式。
如图3所示，虽然大多数体材料晶体都能够在特定波长提供负的群速度特征值，但是，适用的波长范围却十分有限，表现为负值区域的陡峭曲线。即便综合了上述多种非线性晶体，也仅能在有限的离散的窄波段实现宽带相位匹配。除了体材料晶体，基于准相位匹配(QPM)的周期性极化晶体为另一种潜在选择，为了能用到晶体最大的非线性系数d33，通常会采用0类的准相位匹配方式(eee)。但是，如图3所示，满足II类准相位匹配的周期性极化铌酸锂晶体(oeo)，不仅在极宽的光谱范围(980nm-1500nm@790nm，即以790nm脉冲激光为泵浦光时，此波长范围为980nm-1500nm)内有着稳定的群速度关系(平缓的群速度特征值曲线)，相应的群速度特征值还处在较为理想的区间
Figure PCTCN2019113696-appb-000009
Figure PCTCN2019113696-appb-000010
这为以满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体实现输出 脉冲激光波长的可调谐提供了理论基础。
本发明提供了一种可调谐宽带中红外激光系统，如图1所示，包括，脉冲激光器1；分束镜2；第一脉冲展宽器3；宽带信号光产生器6；光路延时器4；第二脉冲展宽器7；耦合镜8；非线性晶体9；分光镜10；脉冲压缩器11。
在本申请实施例中，所述脉冲激光器1输出的脉冲激光，经分束镜2分成两束脉冲激光。
第一束脉冲激光经过所述第一脉冲展宽器3，到达所述耦合镜8。
第二束脉冲激光经过所述宽带信号光产生器6得到宽带信号光，所述宽带信号光经过所述第二脉冲展宽器7，到达所述耦合镜8。具体地，所述第二束脉冲激光进入超连续谱产生器或者光学参量发生器，产生可调谐的宽带信号光，随后，将所述宽带信号光引入所述第二脉冲展宽器7，得到可调谐的宽带啁啾信号光。
所述第一束脉冲激光和所述宽带信号光共同经过所述耦合镜8。经所述耦合镜8射出的所述第一束脉冲激光和所述宽带信号光，进入所述非线性晶体9，得到放大后的宽带信号光、闲频光和残余泵浦光，通过所述分光镜10，将所述闲频光、放大后的宽带信号光和残余泵浦光分离，所述脉冲压缩器11将分离后的所述闲频光的脉宽压缩，得到中红外超短脉冲激光，其中，所述第一束脉冲激光为泵浦光。
所述光路延时器4设置在所述第一脉冲展宽器3和所述耦合镜8之间或者所述第二脉冲展宽器7和所述耦合镜8之间，通过引入预设时间延时，使所述第一束脉冲激光和所述宽带信号光时间同步。
经过啁啾展宽的所述第一束脉冲激光，与经过啁啾展宽的所述宽带信号光的啁啾方向相反；所述第一束脉冲激光与所述宽带信号光在所述非线性晶体中作光参量放大，得到所述放大后的宽带信号光、闲频光和残余泵浦光；所述闲频光的带宽，由所述非线性晶体的相位匹配带宽决定。
可选地，所述非线性晶体9为满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体，所述第一束脉冲激光为o偏振光，所述宽带信号光为e偏振光，所述闲频光为o偏振光。
可选地，通过以下其中一种方式，对产生的所述闲频光的中心波长进行调整：
调整所述宽带信号光产生器输出的宽带信号光的中心波长；
调整所述光路延时器引入的延时；
相应的，为保证所述闲频光中心波长的相位匹配，需要调整所述泵浦光和所述宽带信号光在所述满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体中的横向作用区域；
在此基础上，通过调整所述第二脉冲展宽器对所述宽带信号光的啁啾展宽量，对所述宽带信号光与所述泵浦光的啁啾比进行调整，以此优化所述闲频光的带宽以及所述可调谐宽带中红外激光系统的能量转换效率。
可选地，所述宽带信号光产生器6为超连续谱产生器(Supercontinuum Generation，SG)或者光学参量发生器(Optical Parametric Generation，OPG)。
可选地，所述脉冲激光器1为790nm钛宝石飞秒激光器。
可选地，所述分光镜10为对所述闲频光高透射、对所述泵浦光和所述放大后的宽带信号光高反射，或者是对所述闲频光高反射、对所述泵浦光和所述放大后的宽带信号光高透射的双色镜。
由于经过啁啾展宽的所述第一束脉冲激光(即所述泵浦光)，与经过啁啾展宽的所述宽带信号光的时间啁啾方向相反，所以，本质上可以得到初始带宽比所述泵浦光、所述宽带信号光带宽更宽的所述闲频光。利用满足II类准相位匹配的周期性极化铌酸锂晶体在反向啁啾中红外光参量放大过程中的宽带相位匹配特性，本发明可以在极宽的波长范围内(980nm-1500nm@790nm，即以790nm脉冲激光为泵浦光时，此波长范围为980nm-1500nm)，完成宽带宽的有效光参量放大；在此基础上，所述满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体可进一步实现输出脉冲激光波长的宽调谐。
具体地，在本申请实施例中，所述脉冲激光器1为790nm的钛宝石飞秒脉冲激光器。所述非线性晶体9为满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体，其畴长在7.2μm-9.1μm范围内连续可调。所述宽带信号光产生器6为超连续谱产生器。所述分光镜10为对闲频光高透射，对泵浦光和放大后的宽带信号光高反射的双色镜。
本申请提供一种具体的可调谐宽带中红外激光系统，由可调谐宽带中红外激光系统得到可调谐的中红外超短脉冲激光的过程如下：所述790nm的钛宝石飞秒脉冲激光器1输出的脉冲激光经分束镜2分为两束脉冲激光，其中一束脉冲激 光作为泵浦光，由所述第一脉冲展宽器3对所述泵浦光作啁啾展宽；另一束脉冲激光进入所述超连续谱产生器6，得到980nm-1500nm可调谐的宽带信号光，再由所述第二脉冲展宽器7对所述可调谐的宽带信号光啁啾展宽，其中，经过啁啾展宽的所述泵浦光，与经过啁啾展宽的所述宽带信号光的啁啾方向相反；将所述泵浦光引入光路延时器4，引入适量的时间延时，使所述泵浦光与所述宽带信号光时间同步，再分别进入所述耦合镜8。
经所述耦合镜8出射的所述泵浦光和所述宽带信号光进入所述满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体9，经过光参量放大，得到放大后的宽带信号光、约4.1-1.7μm可调谐的中红外宽带闲频光以及残余泵浦光。通过所述双色镜10，将所述闲频光与所述放大后的宽带信号光，以及，所述残余泵浦光分离，再由所述脉冲压缩器11压缩所述闲频光的脉宽，最终得到约4.1μm-1.7μm可调谐的中红外超短脉冲激光。
进一步地，基于全维度的耦合波方程，对所述可调谐宽带中红外激光系统的运行情况作了详细的数值仿真。
理论上，在泵浦光时间啁啾不变的前提下，只有将宽带信号光的时间啁啾调整至合适数值，使得宽带宽的啁啾泵浦光和宽带宽的啁啾信号光每一时刻的瞬时频率都恰好满足相位匹配，才能实现最优的宽带相位匹配。
假定满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体的长度为5mm，畴长固定在7.5μm。泵浦光为790nm的钛宝石飞秒脉冲激光，其光谱呈高斯分布，初始的变换极限脉宽(TL)为100fs(1/e 2高半宽)，经500倍啁啾展宽，泵浦光的脉宽被展宽至50ps。信号光为超连续谱产生器产生的中心波长为1030nm、光谱足够宽且为超高斯分布的超连续光。泵浦光初始光强为1GW/cm 2，信号光初始光强为泵浦光初始光强的1％。
图4给出了本实施例中790nm泵浦光对1030nm信号光小信号光参量放大的转换效率及产生的约3.4μm闲频光带宽随不同啁啾比αs/αp的变化曲线。如图4所示，转换效率及得到的约3.4μm闲频光带宽都会随啁啾比αs/αp的变化而改变，其中，转换效率的最大值出现在αs/αp≈-0.65。相比较的，由图2给出的结果，可简单推导出最优啁啾比αs/αp的理论值约为-0.7。其中的细微差别主要是因为图2给出的理论值仅考虑了非线性晶体中的一阶色散，忽略了二阶及其他更高阶色 散的影响。而约3.4μm闲频光的带宽则是在αs/αp≈-0.75达到最大。
综合理论推导与数值模拟的结果，确信，调整宽带宽的啁啾信号光和宽带宽的啁啾泵浦光的啁啾比，使其处于最优值，可以提高所述可调谐宽带中红外激光系统的转换效率以及中红外闲频光的输出带宽。
为了实现所述可调谐宽带中红外激光系统输出的中红外超短脉冲激光波长的可调谐，本发明实施例中的非线性晶体9为满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体。扇形结构周期性极化晶体相较于单一畴长的周期性极化晶体，可以针对不同中心波长的宽带信号光，通过改变宽带信号光与泵浦光在扇形结构周期性极化晶体中的横向作用区域，对其畴长进行连续调整，实现对不同中心波长的宽带信号光的相位匹配。
基于满足II类准相位匹配的周期性极化铌酸锂晶体在反向啁啾中红外光参量放大过程中的宽带相位匹配特性。其不仅在极宽的光谱范围(980nm-1500nm@790nm，即以790nm脉冲激光为泵浦光时，此波长范围为980nm-1500nm)内有着稳定的群速度关系(平缓的群速度特征值曲线)，相应的群速度特征值还处在较为理想的区间
Figure PCTCN2019113696-appb-000011
在满足对宽带信号光中心波长相位匹配的基础上，相应的微调第二脉冲展宽器对宽带信号光的啁啾展宽量，使宽带信号光与泵浦光的啁啾比最优，即可得到可调谐的中红外超短脉冲激光。
具体的，调整超连续谱产生器输出的宽带信号光的中心波长，或者改变光路延时器引入的延时(等同于改变与泵浦光相互作用的宽带信号光的中心波长)；并相应的微调泵浦光、宽带信号光在满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体中的横向作用区域，即改变畴长(满足相位匹配的要求)，以及第二脉冲展宽器对宽带信号光的啁啾展宽量，即改变啁啾比(满足宽带相位匹配的要求)，使宽带信号光与泵浦光的啁啾比最优，即可得到可调谐的中红外超短脉冲激光。
图5是本发明实施例中790nm泵浦光对1100nm-1500nm可调谐信号光光参量放大的转换效率及产生的闲频光带宽随信号光波长的变化曲线。假定满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体的长度为10mm，畴长在7.8μm-9.1μm范围内连续可调。泵浦光为790nm的钛宝石飞秒脉冲激光，其光谱呈高斯分布，初始的变换极限脉宽(TL)为100fs(1/e 2高半宽)，经1000倍 啁啾展宽，泵浦光的脉宽被展宽至100ps。信号光为超连续谱产生器产生的中心波长为1100nm-1500nm可调谐，光谱足够宽且为超高斯分布的超连续光。泵浦光初始光强为0.8GW/cm 2，信号光的初始光强为泵浦光初始光强的1％。
其中，实线为针对不同中心波长的宽带信号光，相应微调第二脉冲展宽器对宽带信号光的啁啾展宽量，将宽带信号光与泵浦光啁啾比优化至各自最优值情况下的特性曲线。虚线为假定非线性晶体材料不存在色散的理想情况下的特征曲线。由于满足II类准相位匹配的周期性极化铌酸锂晶体在1100nm-1500nm的光谱范围内有着稳定的群速度关系(如图2所示)，即便在固定宽带信号光与泵浦光啁啾比不变的情况下，依然能够在上述波段得到平缓的效率、带宽调谐曲线。在此基础上，如果能针对不同中心波长的宽带信号光，相应微调第二脉冲展宽器对不同中心波长的宽带信号光的啁啾展宽量，将宽带信号光与泵浦光啁啾比优化至各自最优值，即可进一步优化所述可调谐宽带中红外激光系统的转换效率以及中红外闲频光的输出带宽。特别是在1250nm-1500nm的光谱范围内，闲频光的输出带宽几乎不受宽带信号光中心波长变化的影响。此特征曲线(实线)已相当接近于假定非线性晶体材料不存在色散的理想情况下的结果(虚线)。
以上实施例的各技术特征可以进行任意的组合，为使描述简洁，未对上述实施例中的各个技术特征所有可能的组合都进行描述，然而，只要这些技术特征的组合不存在矛盾，都应当认为是本说明书记载的范围。
以上所述实施例仅表达了本申请的几种实施方式，其描述较为具体和详细，但并不能因此而理解为对发明专利范围的限制。应当指出的是，对于本领域的普通技术人员来说，在不脱离本申请构思的前提下，还可以做出若干变形和改进，这些都属于本申请的保护范围。因此，本申请专利的保护范围应以所附权利要求为准。

Claims (6)

  1. 一种可调谐宽带中红外激光系统，其特征在于，所述系统包括：脉冲激光器、分束镜、第一脉冲展宽器、宽带信号光产生器、光路延时器、第二脉冲展宽器、耦合镜、非线性晶体、分光镜、脉冲压缩器；
    所述脉冲激光器产生的脉冲激光，经所述分束镜分成两束脉冲激光，第一束脉冲激光经过所述第一脉冲展宽器，到达所述耦合镜，第二束脉冲激光经过所述宽带信号光产生器得到宽带信号光，所述宽带信号光经过所述第二脉冲展宽器，到达所述耦合镜，所述第一束脉冲激光和所述宽带信号光共同经过所述耦合镜，经所述耦合镜射出的所述第一束脉冲激光和所述宽带信号光，进入所述非线性晶体，得到放大后的宽带信号光、闲频光和残余泵浦光，通过所述分光镜，将所述闲频光、放大后的宽带信号光和残余泵浦光分离，所述脉冲压缩器将分离后的所述闲频光的脉宽压缩，得到中红外超短脉冲激光，其中，所述第一束脉冲激光为泵浦光；
    所述光路延时器设置在所述第一脉冲展宽器和所述耦合镜之间或者所述第二脉冲展宽器和所述耦合镜之间，通过引入预设时间延时，使所述第一束脉冲激光和所述宽带信号光时间同步；
    经过啁啾展宽的所述第一束脉冲激光，与经过啁啾展宽的所述宽带信号光的啁啾方向相反；所述第一束脉冲激光与所述宽带信号光在所述非线性晶体中作光参量放大，得到所述放大后的宽带信号光、闲频光和残余泵浦光；所述闲频光的带宽，由所述非线性晶体的相位匹配带宽决定。
  2. 根据权利要求1所述的系统，其特征在于，所述非线性晶体为满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体，所述第一束脉冲激光为o偏振光，所述宽带信号光为e偏振光，所述闲频光为o偏振光。
  3. 根据权利要求2所述的系统，其特征在于，通过以下其中一种方式，对产生的所述闲频光的中心波长进行调整：
    调整所述宽带信号光产生器输出的宽带信号光的中心波长；
    调整所述光路延时器引入的延时；
    相应的，为保证所述闲频光中心波长的相位匹配，需要调整所述泵浦光和所述宽带信号光在所述满足II类准相位匹配的扇形结构周期性极化铌酸锂晶体中的横向作用区域；
    在此基础上，通过调整所述第二脉冲展宽器对所述宽带信号光的啁啾展宽量，对所述宽带信号光与所述泵浦光的啁啾比进行调整，以此优化所述闲频光的带宽以及所述可调谐宽带中红外激光系统的能量转换效率。
  4. 根据权利要求1所述的系统，其特征在于，所述宽带信号光产生器为超连续谱产生器或者光学参量发生器。
  5. 根据权利要求1所述的系统，其特征在于，所述脉冲激光器为790nm钛宝石飞秒激光器。
  6. 根据权利要求1所述的系统，其特征在于，所述分光镜为对所述闲频光高透射、对所述泵浦光和所述放大后的宽带信号光高反射，或者是对所述闲频光高反射、对所述泵浦光和所述放大后的宽带信号光高透射的双色镜。
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