CN110535018B - Tunable broadband intermediate infrared laser system - Google Patents
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- GQYHUHYESMUTHG-UHFFFAOYSA-N lithium niobate Chemical compound [Li+].[O-][Nb](=O)=O GQYHUHYESMUTHG-UHFFFAOYSA-N 0.000 claims description 20
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Abstract
The application relates to a tunable broadband mid-infrared laser system, which comprises: the pulse laser generated by the pulse laser is divided into two beams of pulse laser by the beam splitter, the first beam of pulse laser passes through the first pulse stretcher and reaches the coupling mirror, the second beam of pulse laser passes through the broadband signal light generator to obtain broadband signal light, the broadband signal light passes through the second pulse stretcher and reaches the coupling mirror, the first beam of pulse laser and the broadband signal light jointly pass through the coupling mirror and enter the nonlinear crystal to obtain amplified broadband signal light, idler frequency light and residual pump light. And separating the idler frequency light, the amplified broadband signal light and the residual pump light by a spectroscope, and compressing the pulse width of the separated idler frequency light by a pulse compressor to obtain the intermediate infrared ultrashort pulse laser. By the tunable broadband mid-infrared laser system, tunable mid-infrared ultrashort pulse laser can be obtained.
Description
Technical Field
The application relates to the technical field of ultrafast laser, in particular to a tunable broadband mid-infrared laser system.
Background
Due to the lack of a suitable laser gain medium, only a very few lasers of a particular wavelength can be directly generated from the stimulated emission of the laser medium. Currently, commercial lasers of the energy level type are mainly focused in the near infrared band of 1-2 μm. The mid-infrared band, which is between the near-infrared and terahertz (THz), is a very important electromagnetic radiation band. Typical mid-infrared wavelengths are 3-5 μm, corresponding exactly to the second "window" of the atmosphere and the fingerprint spectrum of most molecules. The intermediate infrared ultra-short pulse laser is an important means for researching the dynamic problems of transient transition process between narrow-band semiconductor and superlattice multi-quantum well bands, energy transfer in molecules and between molecules and the like. The energy transfer between the lasers with three different frequencies can be guided by using the optical second-order nonlinear effect. In the Optical Parametric Amplification (OPA), short-wavelength pump light (e.g., titanium sapphire laser) can be nonlinearly down-converted to transfer energy to long-wavelength mid-infrared laser.
OPA is the most common technique for generating mid-infrared pulsed laser by virtue of its advantages of high gain, wide bandwidth, wide tuning, etc. For ultrashort pulse laser, if it is desired to obtain sufficient conversion efficiency in a finite nonlinear crystal, it is necessary to pump with pulse laser with very high peak power, and the peak power of the pump light is limited by the damage threshold of the nonlinear crystal, so that it is difficult to directly generate ultrashort and ultrastrong mid-infrared pulse laser. Optical Parametric Chirped Pulse Amplification (OPCPA) combines the advantages of Chirped Pulse Amplification (CPA) long pulse pumping and OPA high gain to obtain pulsed laser with ultrahigh peak power. In general, the phase matching condition of the OPCPA is optimized based on the center wavelength of the signal light, and the gain to the center wavelength is the highest. However, due to chromatic dispersion of the nonlinear crystal material, it is still difficult to implement broadband phase matching of single-stage optical parametric amplification, which causes an increase in the amount of phase mismatch of spectral components deviating from the center wavelength and a decrease in gain, thereby resulting in a narrowing of OPCPA gain bandwidth and limiting the limit bandwidth of the output ultrashort pulse laser.
Therefore, the prior art is in need of improvement.
Disclosure of Invention
The invention aims to solve the technical problems that optical parametric amplification is limited by a crystal damage threshold value, only lower conversion efficiency can be obtained, and the optical parametric chirped pulse amplification is difficult to realize broadband phase matching.
The invention provides a tunable broadband mid-infrared laser system, which comprises: the device comprises a pulse laser, a beam splitter, a first pulse stretcher, a broadband signal light generator, a light path delayer, a second pulse stretcher, a coupling mirror, a nonlinear crystal, a beam splitter and a pulse compressor;
the pulse laser generated by the pulse laser is divided into two beams of pulse laser by the beam splitter, the first beam of pulse laser passes through the first pulse stretcher and reaches the coupling mirror, the second beam of pulse laser passes through the broadband signal light generator to obtain broadband signal light, the broadband signal light passes through the second pulse stretcher and reaches the coupling mirror, the first beam of pulse laser and the broadband signal light jointly pass through the coupling mirror, the first beam of pulse laser and the broadband signal light emitted by the coupling mirror enter the nonlinear crystal to obtain amplified broadband signal light, idler frequency light and residual pump light, the idler frequency light, the amplified broadband signal light and the residual pump light are separated by the beam splitter, and the pulse compressor compresses the pulse width of the separated idler frequency light to obtain middle-ultra-short pulse laser, the first beam of 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 the first beam of pulse laser light and the broadband signal light are time-synchronized by introducing a preset time delay;
the chirp direction of the first beam of pulse laser subjected to chirp broadening is opposite to that of the broadband signal light subjected to chirp broadening; the first beam of pulse laser and the broadband signal light are subjected to optical parametric amplification in the nonlinear crystal to obtain the amplified broadband signal light, idler frequency light and residual pump light; the bandwidth of the idler is determined by the phase matching bandwidth of the nonlinear crystal.
Optionally, the nonlinear crystal is a periodically poled lithium niobate crystal with a fan-shaped structure meeting the quasi-phase matching of class II, the first beam of pulse laser is o-polarized light, the broadband signal light is e-polarized light, and the idler frequency light is o-polarized light.
Optionally, the center wavelength of the generated idler is adjusted by one of the following methods:
adjusting the central wavelength of the broadband signal light output by the broadband signal light generator;
adjusting the time delay introduced by the optical path time delay device;
correspondingly, in order to ensure the phase matching of the idler frequency optical center wavelength, the transverse acting region of the pump light and the broadband signal light in the sector-structured periodically-polarized lithium niobate crystal meeting the class II quasi-phase matching needs to be adjusted;
on the basis, the chirp broadening amount of the second pulse stretcher to the broadband signal light is adjusted, and the chirp ratio of the broadband signal light to the pump light is adjusted, so that the bandwidth of the idler light and the energy conversion efficiency of the tunable broadband mid-infrared laser system are optimized.
Optionally, the broadband signal light generator is a supercontinuum generator or an optical parametric generator.
Optionally, the pulsed laser is a 790nm titanium sapphire femtosecond laser.
Optionally, the spectroscope is a dichroic mirror that is highly transmissive to the idler light and highly reflective to the pump light and the amplified broadband signal light, or is highly reflective to the idler light and highly transmissive to the pump light and the amplified broadband signal light.
Compared with the prior art, the embodiment of the invention has the following advantages:
through tunable broadband mid-infrared laser system in this application, reduced because the influence that the pulse that the group velocity mismatch caused slided, benefit from moreover the pump light with the chirp direction of broadband signal light is opposite, and, satisfy II type quasi-phase matching periodic polarization lithium niobate crystal (PPLN) in the broadband phase matching characteristic of infrared parametric amplification in-process in the reverse chirp, can obtain than the pump light, broadband signal light bandwidth is wider idle frequency light.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments described in the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a schematic structural diagram of a tunable broadband mid-IR laser system according to an embodiment of the present invention;
FIG. 2a is a diagram illustrating initial bandwidth relationships of a pump light, a signal light, and an idler light according to an embodiment of the invention;
FIG. 2b is a schematic diagram of a wideband phase matching structure according to an embodiment of the present invention;
FIG. 3 is a graph illustrating group velocity characteristic curves of pump light, signal light with different wavelengths, and corresponding idler light according to an embodiment of the present disclosure;
FIG. 4 is a graph showing the conversion efficiency of parametric amplification of a 790nm pump light to a 1030nm signal light small-signal light and the variation of the generated idler bandwidth of about 3.4 μm with different chirp ratios (α s/α p) in an embodiment of the present invention;
fig. 5 is a graph showing the conversion efficiency of parametric amplification of a 790nm pump light with respect to a 1100nm-1500nm tunable signal light and the variation of the generated idler bandwidth with the signal light wavelength in the embodiment of the present invention.
Detailed Description
In order to make the technical solutions of the present invention better understood, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In general, Optical Parametric Amplification (OPA) suffers from crystal damage threshold and can only achieve low conversion efficiency, while Optical Parametric Chirped Pulse Amplification (OPCPA) is difficult to achieve wide bandwidth phase matching.
In order to solve the above problems, the present invention provides a tunable broadband mid-infrared laser system, which uses chirped pulse laser with a wide bandwidth as pump light, and obtains idler frequency light with an initial bandwidth wider than the bandwidth of the pump light and the signal light while reducing the influence of pulse slip caused by group velocity mismatch by introducing time chirps with opposite directions to the pump light and the signal light. On the basis, by utilizing the broadband phase matching characteristic of the periodic polarized lithium niobate crystal meeting the II-class quasi-phase matching in the process of amplifying the infrared parametric light in the reverse chirp, the effective optical parametric amplification of wide bandwidth can be completed in an extremely wide wavelength range, and the tunable ultra-short pulse laser in the intermediate infrared band can be obtained.
Various non-limiting embodiments of the present invention are described in detail below with reference to the accompanying drawings.
First, the theoretical basis for generating tunable mid-infrared ultrashort pulse laser by using tunable broadband mid-infrared laser system in the embodiment of the present application is described as follows:
the optical parametric amplification process of three-wave mixing must satisfy energy and momentum conservation, i.e.
ωp＝ωs+ωi (1)
kp＝ks+ki (2)
Wherein, ω isp、ωs、ωiRespectively representing the angular frequencies, k, of the pump, signal and idlerp、ks、kiRespectively representing wave vectors of the pump light, the signal light and the idler frequency light. Phase matching (Δ k ═ k)p-ks-ki0) is the basic requirement for efficient energy conversion for optical parametric amplification. In general, the phase matching is optimized based on the center wavelengths of the pump light and the signal light. Due to the chromatic dispersion of the nonlinear crystal material, the phase matching can only be realized for a single wavelength theoretically. For wide bandwidth optical parametric amplification processes, the signal light comprises a wide bandwidth spectrum with more or less spectral components offset from the center wavelengthThe phase mismatch (Δ k ≠ 0). The larger the amount of mismatch, the lower the gain. Generally, the phase matching bandwidth determines the gain bandwidth of optical parametric amplification and also limits the limit bandwidth of the output ultrashort pulse laser.
For Δ k at center frequency ω0Taylor expansion is carried out by:
wherein the content of the first and second substances,
denotes group velocity, Δ ω ═ α (t-t)0) And represents a time-varying offset from the center angular frequency (when t is t ═ t)0，ω＝ω0，Δω＝0)，
Representing second and other higher order dispersion terms,
linear chirp coefficient, v, representing the chirp pulsep、υs、υiRespectively representing the group velocities, Δ ω, of the pump light, the signal light, and the idler lightp、Δωs、ΔωiThe angular frequency offsets of the pump light, the signal light, and the idler light are respectively expressed.
It can be seen that when the ratio (chirp ratio) σ of the linear chirps of the signal light and the pump light satisfies
The first order terms in equation (3) exactly cancel each other, and Δ k is determined only by the second and other higher order dispersion terms, so that an extremely wide phase matching bandwidth can be obtained. Wherein alpha isp、αsRespectively, the linear chirp coefficients of the pump light and the signal light.
The embodiment of the application adopts reverse chirpThe optical parametric amplification mode, the chirped pump light and the chirped signal light have opposite time chirp directions, i.e. delta omegap、ΔωsThe sign is opposite. Thus, as shown in FIG. 2a, Δ ωi＝Δωp-ΔωsTheoretically, idler lights with wider initial bandwidths than pump lights and signal light bandwidths can be obtained. However, the bandwidth of the idler light that can be finally obtained through optical parametric amplification is still determined by the phase matching bandwidth of the optical parametric amplification. To realize effective optical parametric amplification with wide bandwidth, it is necessary that the center wavelengths of the chirped pump light and the chirped signal light satisfy phase matching, and other one-to-one corresponding spectral components deviating from the center wavelengths also satisfy phase matching as much as possible.
As shown in fig. 2b, since time chirps with opposite directions are respectively introduced to the pump light and the signal light, that is, the corresponding relationship between different spectral components of the working light (pump light, signal light, idler light) can be adjusted in a time domain, so that the spectral components of the relatively independent pump light and signal light can both satisfy phase matching, and perfect phase matching of a full spectrum and optical parametric amplification with high conversion efficiency are achieved. To exactly cancel the first order terms of Δ k and achieve phase matching of wide bandwidth, the chirp ratio σ of the signal light and the pump light and the group velocity eigenvalues of the pump light, the signal light, and the idler light are required
Are equal. For an optical parametric amplification process with a specific wavelength, the group velocity characteristic value is a fixed value, and to obtain an idler frequency light with a wider initial bandwidth, the chirp ratio σ is objectively required to be a negative value, so that a nonlinear crystal with a group velocity characteristic value of a negative value needs to be correspondingly used for different optical parametric amplification processes. On the basis, the chirp ratio sigma of the signal light and the pump light is optimized to the group velocity characteristic value
And the conversion efficiency of the reverse chirp optical parametric amplification and the output bandwidth of the idler frequency light can be improved.
It should be noted that the group velocity characteristic value determines the broadbandThe chirp ratio of the signal light and the pump light required for phase matching determines the bandwidth ratio of the signal light and the pump light participating in optical parametric amplification. Obviously,. DELTA.omega.s/ΔωpThe mixing effect is best at-1, and the optimal idler bandwidth broadening (i.e. the ratio of the initial bandwidth of the idler to the bandwidth of the incident pump and signal light) can be obtained. This requires group velocity eigenvalues
The value is-1, i.e. the group velocity of the idler light should be the arithmetic mean of the signal light and the pump light. If the group velocity characteristic value deviates too much from-1, the difference of the linear chirps applied to the pump light and the signal light is significant in order to realize phase matching of wide bandwidth, and therefore, part of the signal light or the spectrum of the pump light is inevitably sacrificed, and at the same time, the significance of mixing is lost. Therefore, in practical application, whether from the perspective of energy utilization or from the perspective of bandwidth improvement, the case that the group velocity characteristic value is near-1 is more practical.
Fig. 3 shows the group velocity characteristic curves of 790nm pump light, signal light with different wavelengths, and corresponding idler frequency light in different crystal and different phase matching modes, wherein the group velocity characteristic curves include different nonlinear crystals such as LN, KTP, LBO, YCOB, and PPLN, and different phase matching modes such as I-type phase matching, II-type phase matching, 0-type quasi-phase matching, and II-type quasi-phase matching.
Although most bulk material crystals are capable of providing negative group velocity characteristics at a particular wavelength, the applicable wavelength range is quite limited, as shown in fig. 3, and exhibits a steep curve in the negative region. Even if the nonlinear crystals are combined, broadband phase matching can be realized only in a limited discrete narrow waveband. In addition to bulk material crystals, quasi-phase-matched (QPM) based periodically poled crystals are another potential choice, and in order to use the maximum nonlinear coefficient d33 of the crystal, a quasi-phase matching approach (eee) of class 0 is usually adopted. However, as shown in FIG. 3, periodically poled lithium niobate crystals (oeo) satisfying class II quasi-phase matching are not only in an extremely wide spectral range (980nm-1500nm @790nm, i.e., in pulses of 790 nm)When the laser is pumping light, the wavelength range is 980nm-1500nm) has stable group velocity relationship (gentle group velocity characteristic value curve), and the corresponding group velocity characteristic value is in a more ideal interval
The method provides a theoretical basis for realizing the tunability of the output pulse laser wavelength by the periodically poled lithium niobate crystal with the fan-shaped structure meeting the II-type quasi-phase matching.
The invention provides a tunable broadband mid-infrared laser system, which comprises a pulse laser 1; a beam splitter 2; a first pulse stretcher 3; a broadband signal light generator 6; an optical path delayer 4; a second pulse stretcher 7; a coupling mirror 8; a nonlinear crystal 9; a spectroscope 10; a pulse compressor 11.
In the embodiment of the present application, the pulse laser output by the pulse laser 1 is divided into two beams of pulse laser by the beam splitter 2.
The first beam of pulse laser passes through the first pulse stretcher 3 and reaches the coupling mirror 8.
The second beam of 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 and reaches the coupling mirror 8. Specifically, the second beam of pulse laser enters a supercontinuum generator or an optical parameter generator to generate tunable broadband signal light, and then the tunable broadband signal light is introduced into the second pulse stretcher 7 to obtain tunable broadband chirp signal light.
The first beam of pulse laser light and the broadband signal light jointly pass through the coupling mirror 8. The first beam of pulse laser light and the broadband signal light emitted by the coupling mirror 8 enter the nonlinear crystal 9 to obtain amplified broadband signal light, idler frequency light and residual pump light, the idler frequency light, the amplified broadband signal light and the residual pump light are separated by the spectroscope 10, the pulse compressor 11 compresses the pulse width of the idler frequency light after separation to obtain intermediate infrared ultrashort pulse laser, wherein the first beam of pulse laser 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, and the first beam of pulse laser light and the broadband signal light are time-synchronized by introducing a preset time delay.
The chirp direction of the first beam of pulse laser subjected to chirp broadening is opposite to that of the broadband signal light subjected to chirp broadening; the first beam of pulse laser and the broadband signal light are subjected to optical parametric amplification in the nonlinear crystal to obtain the amplified broadband signal light, idler frequency light and residual pump light; the bandwidth of the idler is determined by the phase matching bandwidth of the nonlinear crystal.
Optionally, the nonlinear crystal 9 is a periodically poled lithium niobate crystal with a fan-shaped structure meeting the quasi-phase matching of class II, the first beam of pulse laser is o-polarized light, the broadband signal light is e-polarized light, and the idler frequency light is o-polarized light.
Optionally, the center wavelength of the generated idler is adjusted by one of the following methods:
adjusting the central wavelength of the broadband signal light output by the broadband signal light generator;
adjusting the time delay introduced by the optical path time delay device;
correspondingly, in order to ensure the phase matching of the idler frequency optical center wavelength, the transverse acting region of the pump light and the broadband signal light in the sector-structured periodically-polarized lithium niobate crystal meeting the class II quasi-phase matching needs to be adjusted;
on the basis, the chirp broadening amount of the second pulse stretcher to the broadband signal light is adjusted, and the chirp ratio of the broadband signal light to the pump light is adjusted, so that the bandwidth of the idler light and the energy conversion efficiency of the tunable broadband mid-infrared laser system are optimized.
Optionally, the broadband signal light generator 6 is a Supercontinuum Generator (SG) or an Optical Parametric Generator (OPG).
Optionally, the pulsed laser 1 is a 790nm titanium sapphire femtosecond laser.
Optionally, the spectroscope 10 is a dichroic mirror that is highly transmissive to the idler light, highly reflective to the pump light and the amplified broadband signal light, or highly reflective to the idler light, and highly transmissive to the pump light and the amplified broadband signal light.
Since the first pulse laser beam (i.e., the pump light) subjected to chirp broadening has a time chirp direction opposite to that of the broadband signal light subjected to chirp broadening, the idler light having an initial bandwidth wider than bandwidths of the pump light and the broadband signal light can be obtained substantially. By utilizing the broadband phase matching characteristic of the periodic polarization lithium niobate crystal meeting the II-type quasi-phase matching in the infrared parametric amplification process in the reverse chirp, the invention can complete the effective optical parametric amplification of the broadband in an extremely wide wavelength range (980nm-1500nm @790nm, namely, when 790nm pulse laser is used as pumping light, the wavelength range is 980nm-1500 nm); on the basis, the periodically poled lithium niobate crystal with the sector structure meeting the II-class quasi-phase matching can further realize the wide tuning of the output pulse laser wavelength.
Specifically, in the present embodiment, the pulse laser 1 is a 790nm titanium sapphire femtosecond pulse laser. The nonlinear crystal 9 is a sector-structured periodically poled lithium niobate crystal meeting the II-type quasi-phase matching, and the domain length thereof is continuously adjustable within the range of 7.2-9.1 μm. The broadband signal light generator 6 is a supercontinuum generator. The spectroscope 10 is a dichroic mirror that highly transmits the idler light and highly reflects the pumping light and the amplified broadband signal light.
The application provides a specific tunable broadband mid-infrared laser system, and the process of obtaining tunable mid-infrared ultrashort pulse laser by the tunable broadband mid-infrared laser system is as follows: pulse laser output by the 790nm titanium gem femtosecond pulse laser 1 is divided into two beams of pulse laser by a beam splitter 2, wherein one beam of pulse laser is used as pump light, and the first pulse stretcher 3 is used for chirping and stretching the pump light; another beam of pulse laser enters the supercontinuum generator 6 to obtain tunable broadband signal light of 980nm-1500nm, and the tunable broadband signal light is subjected to chirp broadening by the second pulse stretcher 7, wherein the chirp broadened chirp direction of the pump light is opposite to the chirp broadened chirp direction of the broadband signal light; and introducing the pump light into an optical path delayer 4, introducing a proper amount of time delay, so that the pump light and the broadband signal light are time-synchronized and then respectively enter the coupling mirror 8.
The pump light and the broadband signal light emitted by the coupling mirror 8 enter the periodically poled lithium niobate crystal 9 with the fan-shaped structure meeting the quasi-phase matching of the class II, and the amplified broadband signal light, the tunable intermediate infrared broadband idler frequency light with the wavelength of about 4.1-1.7 microns and the residual pump light are obtained through optical parametric amplification. Separating the idler frequency light, the amplified broadband signal light and the residual pump light through the dichroic mirror 10, and compressing the pulse width of the idler frequency light through the pulse compressor 11 to finally obtain the tunable intermediate infrared ultrashort pulse laser with the wavelength of about 4.1-1.7 μm.
Furthermore, the operation condition of the tunable broadband mid-infrared laser system is subjected to detailed numerical simulation based on a full-dimensional coupling wave equation.
Theoretically, on the premise that the time chirp of the pump light is not changed, optimal broadband phase matching can be realized only by adjusting the time chirp of the broadband signal light to an appropriate value so that the instantaneous frequencies of the broadband chirp pump light and the broadband chirp signal light at each moment just meet the phase matching.
Assuming that the length of the sector-structured periodically poled lithium niobate crystal satisfying the class II quasi-phase matching is 5mm, the domain length is fixed at 7.5 μm. The pump light is 790nm titanium gem femtosecond pulse laser, the spectrum is Gaussian distribution, the initial transformation limit pulse width (TL) is 100fs (1/e)2High half width), the pulse width of the pump light is broadened to 50ps by 500 times of chirp broadening. The signal light is super-continuum light which is generated by a super-continuum spectrum generator, has a central wavelength of 1030nm, has a sufficiently wide spectrum and is super-Gaussian distributed. Pump light is firstInitial light intensity is 1GW/cm2The 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 parametric amplification of 790nm pump light to 1030nm signal light small-signal light and the variation curve of the generated idler frequency bandwidth of about 3.4 μm with different chirp ratios α s/α p in this embodiment. As shown in fig. 4, the conversion efficiency and the resulting idler bandwidth of about 3.4 μm both vary with the chirp ratio α s/α p, where the maximum of the conversion efficiency occurs at α s/α p ≈ -0.65. In contrast, from the results given in fig. 2, it can be easily deduced that the theoretical value of the optimal chirp ratio α s/α p is about-0.7. The nuance is mainly because the theoretical values given in fig. 2 only consider first-order dispersion in nonlinear crystals, neglecting the effects of second-and other higher-order dispersion. While the bandwidth of the idler light of about 3.4 μm is maximized at α s/α p ≈ -0.75.
By combining the results of theoretical derivation and numerical simulation, it is believed that adjusting the chirp ratio of the wide-bandwidth chirped signal light and the wide-bandwidth chirped pump light to be at an optimal value can improve the conversion efficiency of the tunable broadband mid-infrared laser system and the output bandwidth of the mid-infrared idler light.
In order to realize the tunability of the wavelength of the mid-infrared ultrashort pulse laser output by the tunable broadband mid-infrared laser system, the nonlinear crystal 9 in the embodiment of the invention is a sector-structured periodically poled lithium niobate crystal meeting the quasi-phase matching of class II. Compared with a periodic polarized crystal with a single domain length, the sector-structure periodic polarized crystal can continuously adjust the domain length of the sector-structure periodic polarized crystal by changing the transverse acting areas of broadband signal light and pump light in the sector-structure periodic polarized crystal aiming at broadband signal light with different central wavelengths, so that the phase matching of the broadband signal light with different central wavelengths is realized.
Based on the broadband phase matching characteristic of the periodic polarized lithium niobate crystal meeting the II-class quasi-phase matching in the infrared parametric amplification process in the reverse chirp. It has stable group velocity relationship (flat group velocity relationship) in extremely wide spectral range (980nm-1500nm @790nm, that is, wavelength range of 980nm-1500nm when 790nm pulse laser is used as pumping lightGroup velocity characteristic curve), the corresponding group velocity characteristic is still in a more ideal interval
On the basis of meeting the requirement of matching the central wavelength phase of the broadband signal light, the chirp broadening amount of the broadband signal light by the second pulse stretcher is correspondingly finely adjusted, so that the chirp ratio of the broadband signal light to the pump light is optimal, and the tunable intermediate infrared ultrashort pulse laser can be obtained.
Specifically, the center wavelength of the broadband signal light output by the supercontinuum generator is adjusted, or the delay introduced by the optical path delay unit is changed (which is equivalent to changing the center wavelength of the broadband signal light interacting with the pump light); and correspondingly trimming the transverse action area of the pump light and the broadband signal light in the periodically poled lithium niobate crystal with the fan-shaped structure meeting the II-class quasi-phase matching, namely changing the domain length (meeting the requirement of phase matching), and changing the chirp broadening quantity of the second pulse stretcher to the broadband signal light, namely changing the chirp ratio (meeting the requirement of broadband phase matching), so that the chirp ratio of the broadband signal light and the pump light is optimal, and the tunable intermediate infrared ultrashort pulse laser can be obtained.
FIG. 5 is a graph showing the conversion efficiency of 790nm pump light to 1100nm-1500nm tunable signal light parametric amplification and the variation of the generated idler bandwidth with the signal light wavelength in the embodiment of the present invention. The length of the sector-structured periodically poled lithium niobate crystal which meets the II-type quasi-phase matching is assumed to be 10mm, and the domain length is continuously adjustable within the range of 7.8-9.1 μm. The pump light is 790nm titanium gem femtosecond pulse laser, the spectrum is Gaussian distribution, the initial transformation limit pulse width (TL) is 100fs (1/e)2High half width), the pulse width of the pump light is broadened to 100ps by 1000 times of chirp broadening. The signal light is generated by a super-continuum spectrum generator, the center wavelength of the signal light is 1100nm-1500nm tunable, the spectrum is wide enough, and the signal light is super-continuum light with super-Gaussian distribution. The initial light intensity of the pump light is 0.8GW/cm2The initial light intensity of the signal light is 1% of the initial light intensity of the pump light.
The solid line is a characteristic curve obtained by correspondingly fine-tuning the chirp broadening amount of the second pulse stretcher to the broadband signal light aiming at the broadband signal light with different central wavelengths and optimizing the chirp ratio of the broadband signal light and the pump light to respective optimal values. The dashed line is the characteristic curve for an ideal case assuming that the nonlinear crystal material does not have dispersion. Because the periodically poled lithium niobate crystal satisfying the class II quasi-phase matching has a stable group velocity relationship in the spectral range of 1100nm to 1500nm (as shown in fig. 2), even under the condition that the chirp ratio of the fixed broadband signal light to the pump light is not changed, a gentle efficiency and bandwidth tuning curve can be obtained in the above waveband. On the basis, if the chirp broadening amount of the second pulse stretcher to the broadband signal light with different center wavelengths can be correspondingly finely adjusted aiming at the broadband signal light with different center wavelengths, and the chirp ratio of the broadband signal light to the pump light is optimized to respective optimal values, the conversion efficiency of the tunable broadband mid-infrared laser system and the output bandwidth of the mid-infrared idler light can be further optimized. Especially in the spectral range of 1250nm-1500nm, the output bandwidth of the idler is hardly affected by the change of the central wavelength of the broadband signal light. This characteristic curve (solid line) has been quite close to the result (dashed line) in the ideal case assuming that the nonlinear crystal material is free of dispersion.
The technical features of the above embodiments can be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the above embodiments are not described, but should be considered as the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present application, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the concept of the present application, which falls within the scope of protection of the present application. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (3)
1. A tunable broadband mid-infrared laser system, the system comprising: the device comprises a pulse laser, a beam splitter, a first pulse stretcher, a broadband signal light generator, a light path delayer, a second pulse stretcher, a coupling mirror, a nonlinear crystal, a beam splitter and a pulse compressor;
the pulse laser generated by the pulse laser is divided into two beams of pulse laser by the beam splitter, the first beam of pulse laser passes through the first pulse stretcher and reaches the coupling mirror, the second beam of pulse laser passes through the broadband signal light generator to obtain broadband signal light, the broadband signal light passes through the second pulse stretcher and reaches the coupling mirror, the first beam of pulse laser and the broadband signal light jointly pass through the coupling mirror, the first beam of pulse laser and the broadband signal light emitted by the coupling mirror enter the nonlinear crystal to obtain amplified broadband signal light, idler frequency light and residual pump light, the idler frequency light, the amplified broadband signal light and the residual pump light are separated by the beam splitter, and the pulse compressor compresses the pulse width of the separated idler frequency light to obtain middle-ultra-short pulse laser, the first beam of 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 the first beam of pulse laser light and the broadband signal light are time-synchronized by introducing a preset time delay;
the nonlinear crystal is a fan-shaped structure periodically polarized lithium niobate crystal meeting the II-type quasi-phase matching, the first beam of pulse laser is o-polarized light, the broadband signal light is e-polarized light, and the idler frequency light is o-polarized light;
the pulse laser is a 790nm titanium gem femtosecond laser, and the first beam of pulse laser is a 790nm titanium gem femtosecond laser; the chirp ratio sigma of the pump light and the signal light, the group velocity upsilonp of the pump light, the group velocity upsilos of the signal light and the group velocity upsiloi of the idler frequency light basically satisfy the relation of sigma (1/upsilop-1/upsilon i)/(1/upsilon s-1/upsilon i);
the chirp direction of the first beam of pulse laser subjected to chirp broadening is opposite to that of the broadband signal light subjected to chirp broadening; the first beam of pulse laser and the broadband signal light are subjected to optical parametric amplification in the nonlinear crystal to obtain the amplified broadband signal light, idler frequency light and residual pump light; the bandwidth of the idler is determined by the phase matching bandwidth of the nonlinear crystal;
adjusting the center wavelength of the generated idler by one of the following methods:
adjusting the central wavelength of the broadband signal light output by the broadband signal light generator;
adjusting the time delay introduced by the optical path time delay device;
correspondingly, in order to ensure the phase matching of the idler frequency optical center wavelength, the transverse acting region of the pump light and the broadband signal light in the sector-structured periodically-polarized lithium niobate crystal meeting the class II quasi-phase matching needs to be adjusted;
on the basis, the chirp broadening amount of the second pulse stretcher to the broadband signal light is adjusted, and the chirp ratio of the broadband signal light to the pump light is adjusted, so that the bandwidth of the idler light and the energy conversion efficiency of the tunable broadband mid-infrared laser system are optimized.
2. The system of claim 1, wherein the broadband signal light generator is a supercontinuum generator or an optical parametric generator.
3. The system of claim 1, wherein the beam splitter is a dichroic mirror highly transmissive to the idler, highly reflective to the pump and amplified broadband signal light, or highly reflective to the idler, highly transmissive to the pump and amplified broadband signal light.
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| CN114486788B (en) * | 2021-09-29 | 2024-02-13 | 华东师范大学重庆研究院 | Mid-infrared frequency up-conversion imaging technology and device with large visual field and ultrasensitive function |
| CN114156727A (en) * | 2021-10-26 | 2022-03-08 | 西安电子科技大学 | High-power intermediate infrared tunable femtosecond laser generating device |
| CN114284853B (en) * | 2021-12-29 | 2022-12-02 | 四川大学 | Intermediate infrared dual-wavelength tunable femtosecond pulse laser |
| CN114744478B (en) * | 2022-06-14 | 2022-09-30 | 中国科学技术大学 | Laser light source system suitable for helium resonance fluorescence laser radar |
| CN115275741A (en) * | 2022-07-22 | 2022-11-01 | 深圳技术大学 | Pulse stretching device, pulse stretching system and laser |
| CN115656042B (en) * | 2022-10-18 | 2024-05-24 | 中国科学院沈阳自动化研究所 | Large-rotation-angle tuning medium-and-long-wave infrared coherent light source device with stable light beam direction |
| CN115579723B (en) * | 2022-11-25 | 2023-04-07 | 武汉中科锐择光电科技有限公司 | Time domain and spectrum shape controllable pulse train generation system and method |
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