WO2018045701A1 - 一种针对中红外脉冲激光的光谱调控装置 - Google Patents

一种针对中红外脉冲激光的光谱调控装置 Download PDF

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WO2018045701A1
WO2018045701A1 PCT/CN2017/071031 CN2017071031W WO2018045701A1 WO 2018045701 A1 WO2018045701 A1 WO 2018045701A1 CN 2017071031 W CN2017071031 W CN 2017071031W WO 2018045701 A1 WO2018045701 A1 WO 2018045701A1
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mid
infrared
pulsed laser
crystal
infrared pulsed
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French (fr)
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李瑛�
梁宇海
钟亥哲
杨建龙
章礼富
范滇元
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深圳大学
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • 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
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/353—Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
    • G02F1/3544—Particular phase matching techniques
    • 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
    • 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
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0092—Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/353—Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
    • G02F1/3544—Particular phase matching techniques
    • G02F1/3548—Quasi phase matching [QPM], e.g. using a periodic domain inverted structure

Definitions

  • the invention relates to the field of laser technology, in particular to a spectrum control device for a mid-infrared pulsed laser.
  • mid-infrared pulsed lasers are widely used in many different fields.
  • commercial femtosecond lasers still use titanium gems ( Ti:sapphire) is mainly composed of solid-state lasers and erbium-doped ( Er) fiber lasers, and the laser wavelength is generally less than 2 ⁇ m.
  • pulsed laser light of any wavelength can be obtained by nonlinear frequency conversion, such as the mid-infrared band that is difficult to directly generate (3-5) ⁇ m ).
  • nonlinear frequency conversion such as the mid-infrared band that is difficult to directly generate (3-5) ⁇ m ).
  • QPM quasi-phase matching
  • the energy of the pump light is continuously transferred to the signal light.
  • Choosing the appropriate polarization period in theory, can achieve any nonlinear frequency conversion in the range of the nonlinear crystal pass.
  • Periodically polarized lithium niobate crystals PPLN is one of the most typical periodic polarization crystals and is widely used for optical frequency conversion of mid-infrared lasers.
  • non-periodic polarized crystals have been proposed and demonstrated to be suitable for wideband frequency conversion of ultrashort pulsed lasers. By regularly changing the polarization inversion period of the nonlinear crystal, different spectral components of the signal light can satisfy the quasi-phase matching condition in different regions of the crystal, thereby realizing wide-spectrum optical parametric amplification.
  • a non-periodic polarized crystal can provide a gain bandwidth of any width and even shape the spectrum of the signal light.
  • the premise is that during the optical parametric amplification process, the pump light, the signal light, and the idler light are substantially synchronized in time.
  • the group velocity of the interacting pump light, signal light, and idler light in the crystal is not the same, and is generally called group velocity mismatch.
  • Group speed mismatch can cause laser pulse time out of sync.
  • the separation between the pump light and the signal light may cause the signal light to not be continuously amplified, resulting in the non-periodically polarized crystal failing to complete the originally set target, such as regulating the output spectrum.
  • non-periodic polarized crystals were mainly used for helium pulsed lasers, or only very short non-periodically polarized crystals could be used. This greatly increases the complexity of nonlinear frequency conversion systems and sets insurmountable obstacles to their potential applications. especially in In the mid-infrared band of 3-5 ⁇ m, pulse broadening and compression of the ultrashort pulse laser in this spectral region is still an arduous engineering task due to the lack of effective detection means.
  • a spectral control device for a mid-infrared pulsed laser that is suitable for a mid-infrared ultrashort pulse laser of the femtosecond order without the need for it. Broadening, you can directly use the near-infrared pulse laser to spectrally shape it.
  • a spectral control device for a mid-infrared pulsed laser comprising:
  • a temperature control furnace for regulating the operating temperature of the non-periodically polarized crystal
  • the near-infrared pulse laser, the optical coupling mirror, the non-periodically polarized crystal, and the beam splitter are sequentially placed;
  • a near-infrared pulsed laser emitted by a near-infrared pulsed laser passes through the optical coupling mirror, enters the non-periodically polarized crystal together with the mid-infrared pulsed laser, and uses a near-infrared pulsed laser as a pumping light through nonlinear frequency conversion.
  • Differentiating the different spectral components of the mid-infrared pulsed laser; the mixed light of the non-periodically polarized crystal output A spectrally modulated mid-infrared pulsed laser is obtained after separation by the spectroscope.
  • the non-periodically polarized crystal is satisfied.
  • a quasi-polarized crystal that matches the quasi-position, and the polarization inversion period of the non-periodically polarized crystal changes non-periodically along the longitudinal direction.
  • the signal light and the pump light are in the non-periodically polarized crystal
  • the temperature at which the group velocity is the same is the operating temperature.
  • the non-periodically polarized crystal is a non-periodically polarized lithium niobate crystal.
  • the near-infrared pulse laser is 790 nm Ti:Sapphire femtosecond pulsed laser.
  • the beam splitter is an optical element that can separate mixed light of different wavelengths from each other.
  • the present invention provides a spectral control device for a mid-infrared pulsed laser.
  • the near-infrared pulsed laser light emitted by the near-infrared pulse laser passes through the optical coupling mirror, enters the non-periodically polarized crystal together with the mid-infrared pulsed laser, and passes through a nonlinear frequency conversion to
  • the infrared pulsed laser is pumping light, and the different spectral components of the mid-infrared pulsed laser are differentially amplified; the mixed light of the non-periodically polarized crystal output
  • a spectrally modulated mid-infrared pulsed laser is obtained after separation by the spectroscope.
  • the invention adopts class I quasi-phase matching technology to adjust the operating temperature of the non-periodically polarized crystal, Eliminating the group velocity mismatch between the near-infrared pump light and the mid-infrared signal light in the crystal; on the basis of this, the non-periodic polarization crystal is a nonlinear crystal, and the difference of the mid-infrared pulse laser by nonlinear frequency conversion
  • the spectral components are differentially amplified to achieve regulation of their spectra.
  • the technical solution is applicable to a mid-infrared ultrashort pulse laser of the femtosecond order, since the near-infrared pump light and the mid-infrared signal light are always kept in synchronization, so that the mid-infrared ultrashort pulse laser is not required to be widened, and the direct solution can be directly Spectral shaping using near-infrared pulsed laser greatly simplifies the complexity of spectral control devices based on nonlinear frequency conversion.
  • FIG. 1 is a schematic diagram of an optical path of a spectral control device for a mid-infrared pulsed laser provided by the present invention.
  • Figure 2 shows the relationship between the wavelength of the signal light and the crystal temperature under the premise that the pump light and the signal light satisfy the group velocity matching.
  • FIG. 3 is a graph showing the variation of the optical parametric gain with the pulse width of the spectral control device for the mid-infrared pulsed laser provided by the present invention.
  • 4 is a spectrum control device for a mid-infrared pulse laser provided by the present invention, wherein the chirp pulse width is 425fs.
  • 5 is a spectrum adjustment device for a mid-infrared pulse laser provided by the present invention, wherein the chirp pulse width is 700 fs The pulse envelope and spectrum of the amplified mid-infrared pulsed laser.
  • 6 is a spectrum control device for a mid-infrared pulse laser provided by the present invention, wherein the chirp pulse width is 1250fs.
  • a spectrum adjustment device for a mid-infrared pulse laser provided by the present invention, wherein a chirp pulse width is 2500 fs The pulse envelope and spectrum of the amplified mid-infrared pulsed laser.
  • the invention provides a spectroscopic control device for a mid-infrared pulsed laser.
  • the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It is understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
  • the refractive index ( n ) of the crystal changes with temperature, and the group velocity of the pulsed laser in the crystal ( v ) ) is also a physical quantity related to temperature.
  • non-periodic polarized crystals usually need to satisfy the quasi-phase matching condition of class 0 (e+e- ⁇ e ), requires pump light, signal light, and idler light to be e Polarized light.
  • the two pulsed lasers in the same polarization state have a large difference in group velocity, and this difference cannot be simply eliminated by changing the operating temperature of the non-periodically polarized crystal.
  • the present invention provides a spectral control device for a mid-infrared pulsed laser, including a near-infrared pulse laser 1 .
  • the signal light and the pump light are in the The temperature at which the group velocity in the non-periodically polarized crystal is the same is the operating temperature.
  • the near-infrared pulse laser 1 , the optical coupling mirror 2 , the non-periodically polarized crystal 3 , and the beam splitter 7 are sequentially placed;
  • the near-infrared pulsed laser light emitted by the near-infrared pulsed laser 1 passes through the optical coupling mirror 2 and enters the non-periodically polarized crystal together with the mid-infrared pulsed laser light 3
  • differentiating the different spectral components of the mid-infrared pulsed laser by using a near-infrared pulsed laser as a pumping light by nonlinear frequency conversion; and mixing the output light of the non-periodically polarized crystal 3 through the beam splitter 7 After separation, a spectrally modulated mid-infrared pulsed laser is obtained.
  • the near-infrared pulse laser refers to a pulsed laser having a wavelength in the range of 750 to 2000 nm.
  • the invention adopts class I quasi-phase matching technology to adjust the operating temperature of the non-periodically polarized crystal,
  • the group velocity mismatch between the near-infrared pump light and the mid-infrared signal light in the crystal is eliminated, and the different spectral components of the mid-infrared pulse laser are differentially amplified by nonlinear frequency conversion, thereby realizing the adjustment of the spectrum.
  • Output lasers of different spectral characteristics can also be obtained by using different non-periodically polarized crystals.
  • the direction of the arrow is the direction of light propagation, the near-infrared pulsed laser A and the mid-infrared pulsed laser B.
  • the optical coupling mirror 2 After being coupled by the optical coupling mirror 2, it enters the non-periodically polarized crystal 3.
  • the near-infrared pulsed laser light A is reflected by the optical coupling mirror 2 and enters the non-periodically polarized crystal 3.
  • Mid-infrared pulsed laser B Through the optical coupling mirror 2, it enters the non-periodically polarized crystal 3.
  • the near-infrared pulse laser 3 is a 790 nm titanium gem femtosecond pulse laser, and the output wavelength thereof is 790 nm.
  • Pulse laser A through the optical coupling mirror 2, enters the non-periodically polarized crystal 3 together with a mid-infrared pulsed laser B having a wavelength of 3 ⁇ m, with a pulsed laser having a wavelength of 790 nm A is pump light, which amplifies the mid-infrared pulsed laser B (as signal light) with a wavelength of 3 ⁇ m, and at the same time, achieves effective regulation of its spectrum.
  • the non-periodically polarized crystal 3 is a non-periodically polarized crystal that satisfies the class I alignment.
  • the non-periodically polarized crystal 3 The polarization inversion period varies non-periodically in the longitudinal direction (ie, the direction in which the pulsed laser propagates).
  • the non-periodically polarized crystal 3 is selected from a non-periodic polarization lithium niobate crystal (APPLN). ).
  • the non-periodically polarized crystal 3 The length and the non-periodically changing polarization inversion period are determined according to the spectral characteristics of the input mid-infrared pulsed laser and the desired output spectral characteristics. With this specially designed crystal, the mid-infrared pulsed laser can be ensured. Different spectral components can satisfy the quasi-phase matching conditions in different regions of the crystal, obtain amplification, and can adjust the amplification period of different spectral components of the mid-infrared pulse laser by rationally designing the polarization inversion period of the crystal, thereby realizing the mid-infrared pulse. Regulation of laser spectroscopy.
  • the temperature control furnace 4 is disposed under the non-periodically polarized crystal 3; the temperature control furnace 4 is configured to the non-periodically polarized crystal 3
  • the operating temperature is controlled. Specifically, the temperature control furnace 4 causes the non-periodically polarized crystal 3 to operate at a set temperature, so that the signal light and the pump light are in the non-periodically polarized crystal 6.
  • the group speed is the same.
  • the beam splitter 9 is an optical element that can separate mixed light of different wavelengths from each other; preferably, It is one of a dichroic prism and a dichroic mirror.
  • the mixed light C output from the non-periodically polarized crystal 3 is separated by the beam splitter 7 to obtain a spectrally modulated mid-infrared pulsed laser light D.
  • the transmission limit pulse width of the mid-infrared pulsed laser B with a wavelength of 3 ⁇ m is 100 fs. , spread it to different pulse widths.
  • the pulse width of the near-infrared pulsed laser A with a wavelength of 790 nm is consistent with that of the mid-infrared pulsed laser.
  • the signal light of ⁇ m satisfies the group speed matching, and the temperature of the APPLN crystal is fixed at 127 °C (set temperature) through the crystal temperature control furnace.
  • the length of the APPLN crystal is 10mm
  • the polarization inversion period varies linearly from 34.06 ⁇ m to 34.24 ⁇ m.
  • PPLN PPLN
  • the refractive index formula of the present invention numerically simulates the operation of the spectral control device for the mid-infrared pulsed laser.
  • APPLN that satisfies class 0 quasi-phase matching.
  • the case was also simulated when the crystal length was 2 mm and the polarization inversion period was linearly changed from 21.76 ⁇ m to 21.7 ⁇ m.
  • Figure 3 The curve of the optical parametric gain with the pulse width is given.
  • the width of the chirped pulse decreases, the time between laser pulses becomes more severe.
  • either Class I or Class 0 QPM can achieve comparable optical parametric gain.
  • Class 0 QPM as the pulse width of the ⁇ is gradually reduced In this case, the optical parametric gain decreases rapidly. More importantly, due to the mismatch between the pump light and the signal light group velocity, the pump light and the signal light will go to each other in time before the short-wavelength component of the signal light is amplified. The spectrum of the mid-infrared pulsed laser that is amplified is severely 'red shifted'.
  • the pulse envelope and spectrum of the amplified mid-infrared pulsed laser at different pulse widths are listed in Figure 4-7, respectively.
  • the solid line indicates the class I quasi-phase matching curve, and the dotted line indicates 0.
  • the quasi-phase-matched curve clearly reveals the trend of the output spectrum as a function of the pulse width.
  • the spectral control device for mid-infrared pulsed lasers uses APPLN that satisfies Class I QPM.
  • the crystal by means of the non-periodically changing polarization inversion period, increases the spectral width of the mid-infrared pulsed laser while substantially retaining its spectral characteristics.
  • the spectral control device for the mid-infrared pulsed laser can be directly applied to the ultra-short pulse laser without flaw width, and the spectrum of the mid-infrared ultrashort pulse laser of the femtosecond order is adjusted to increase the bandwidth thereof. And even plastic.
  • the spectral control device for the mid-infrared pulsed laser can use a longer APPLN.
  • the crystal uses the dispersion relationship between the group velocity of the pulsed laser and the crystal temperature to operate the crystal at a set temperature, thereby eliminating the group velocity mismatch between the near-infrared pump light and the mid-infrared signal light in the crystal.
  • the polarization inversion period of the non-periodically varying period is determined based on the spectral characteristics of the input mid-infrared pulsed laser and the desired output spectral characteristics.
  • the different spectral components of the mid-infrared pulsed laser can satisfy the quasi-phase matching conditions in different regions of the crystal, obtain amplification, and can control the mid-infrared pulse laser by rationally designing the polarization inversion period of the crystal.
  • the magnification of the different spectral components enables the regulation of the mid-infrared pulsed laser spectrum. Since the pump light and the signal light are always synchronized in time, the spectral control device for the mid-infrared pulse laser can be directly applied to the ultra-short pulse laser without flaw spread, and the mid-infrared ultrashort pulse on the femtosecond level
  • the spectrum of the laser is regulated to increase its bandwidth and even shape it.

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Abstract

一种针对中红外脉冲激光（B）的光谱调控装置。所述光谱调控装置中，由近红外脉冲激光器（1）发出的近红外脉冲激光（A），经过光学耦合镜（2），与中红外脉冲激光（B）一同进入非周期性极化晶体（3），通过非线性频率转换，以近红外脉冲激光（A）为泵浦光，对中红外脉冲激光（B）的不同光谱成分进行差异化放大，以此实现对其光谱的调控。所述光谱调控装置采用I类准相位匹配技术，通过调节非周期性极化晶体（3）的工作温度，消除晶体（3）内近红外泵浦光与中红外信号光间的群速度失配。因而，所述光谱调控装置适用于飞秒量级的中红外超短脉冲激光（B），无需对其啁啾展宽，即可直接利用近红外脉冲激光（A）对其进行光谱整形，极大简化了基于非线性频率转换的光谱调控装置的复杂程度。

Description

一种针对中红外脉冲激光的光谱调控装置
技术领域
本发明涉及激光技术领域，特别涉及一种针对中红外脉冲激光的光谱调控装置。
背景技术
从强场物理的基础研究到医学和工业应用，中红外脉冲激光被广泛应用于众多不同领域。可是，由于缺少合适的激光增益介质和有效的检测方法，商品化的飞秒激光器仍以钛宝石（ Ti:sapphire ）固体激光器和掺铒（ Er ）光纤激光器为主，激光波长一般都小于 2 μ m 。
基于现有脉冲激光器，通过非线性频率转换，可以获得任意波长的脉冲激光，例如难以直接产生的中红外波段（ 3-5 μ m ）。经过多年发展，准相位匹配（ QPM ）已成为实现非线性频率转换的相位匹配的成熟技术。通过周期性地改变非线性晶体的极化方向，使泵浦光的能量持续不断地向信号光转移。选择合适的极化周期，理论上，可以实现非线性晶体通光范围内的任意非线性频率转换。周期性极化铌酸锂晶体（ PPLN ）是其中最为典型的一种周期性极化晶体，被广泛应用于中红外激光的光学频率转换。虽然有很大的有效非线性系数，但 PPLN 有限的相位匹配带宽限制了它在超短脉冲激光中的应用。在实际应用中，人们通常需要在匹配带宽和转换效率之间权衡。为了克服这种限制，非周期性极化晶体被提出并证明适用于超短脉冲激光的宽带频率转换。通过有规律的改变非线性晶体的极化反转周期，信号光的不同光谱成分均能够在晶体的不同区域满足准相位匹配条件，从而实现宽频谱的光参量放大。理论上，非周期性极化晶体可以提供任意宽度的增益带宽，甚至能够对信号光的光谱进行整形。但是 , 其前提是在光参量放大过程中，泵浦光、信号光和闲频光在时间上基本保持同步。但是，通常情况下，相互作用的泵浦光、信号光和闲频光在晶体中的群速度并不相同，一般称之为群速度失配。群速度失配会造成激光脉冲时间上的不同步。其中，泵浦光与信号光间的走离会造成信号光不能持续地放大，导致非周期性极化晶体无法完成原先设定的目标，例如调控输出光谱。为了减小时间走离造成的影响，过去，非周期性极化晶体主要用于啁啾脉冲激光，或者只能使用很短的非周期性极化晶体。这极大地增加了非线性频率转换***的复杂性，并对其潜在应用设置了难以逾越的障碍。特别是在 3-5 μ m 的中红外波段，由于缺乏有效的检测手段，对处于这个光谱区域的超短脉冲激光进行脉冲展宽和压缩仍然是一项艰巨的工程任务。
现有技术还有待改进和提高。
发明内容
鉴于上述现有技术的不足之处，本发明的目的在于提供一种针对中红外脉冲激光的光谱调控装置，所述装置适用于飞秒量级的中红外超短脉冲激光，无需对其啁啾展宽，即可直接利用近红外脉冲激光对其进行光谱整形。
为了达到上述目的，本发明采取了以下技术方案：
一种针对中红外脉冲激光的光谱调控装置，包括：
近红外脉冲激光器；
光学耦合镜；
非周期性极化晶体；
用于对所述非周期性极化晶体的工作温度进行调控的控温炉；
分光镜；
所述近红外脉冲激光器，光学耦合镜，非周期性极化晶体，分光镜 依次放置；
由近红外脉冲激光器发出的近红外脉冲激光，经过所述光学耦合镜后，与中红外脉冲激光一同进入所述非周期性极化晶体，通过非线性频率转换，以近红外脉冲激光为泵浦光，对中红外脉冲激光的不同光谱成分进行差异化放大；所述非周期性极化晶体输出的混合光 经所述分光镜分离后得到经过光谱调控的中红外脉冲激光 。
所述的针对中红外脉冲激光的光谱调控装置中，所述非周期性极化晶体为满足 I 类准位相匹配的非周期性极化晶体，所述非周期性极化晶体的极化反转周期沿纵向方向呈非周期性变化。
所述的针对中红外脉冲激光的光谱调控装置中，以信号光与泵浦光在所述非周期性极化晶体 中的群速度相同时的温度为所述工作温度 。
所述的针对中红外脉冲激光的光谱调控装置中，所述非周期性极化晶体为非周期性极化铌酸锂晶体。
所述的针对中红外脉冲激光的光谱调控装置中，所述近红外脉冲激光器为 790nm 钛宝石飞秒脉冲激光器。
所述的针对中红外脉冲激光的光谱调控装置中，所述分光镜为可以将不同波长的混合光相互分离的光学元件。
相较于现有技术，本发明提供一种针对中红外脉冲激光的光谱调控装置。所述光谱调控装置中，由近红外脉冲激光器发出的近红外脉冲激光，经过所述光学耦合镜后，与中红外脉冲激光一同进入所述非周期性极化晶体，通过非线性频率转换，以近红外脉冲激光为泵浦光，对中红外脉冲激光的不同光谱成分进行差异化放大；所述非周期性极化晶体输出的混合光 经所述分光镜分离后得到经过光谱调控的中红外脉冲激光 。本发明采用 I 类准相位匹配技术，通过调节非周期性极化晶体 的工作温度， 消除晶体内近红外泵浦光与中红外信号光间的群速度失配；在此基础上，以非周期性极化晶体为非线性晶体，通过非线性频率转换，对中红外脉冲激光的不同光谱成分进行差异化放大，以此实现对其光谱的调控。所述技术方案适用于飞秒量级的中红外超短脉冲激光，由于近红外泵浦光与中红外信号光始终保持同步，因此，无需对中红外超短脉冲激光啁啾展宽，即可直接利用近红外脉冲激光对其进行光谱整形，极大简化了基于非线性频率转换的光谱调控装置的复杂程度。
附图说明
图 1 为本发明提供的针对中红外脉冲激光的光谱调控装置的光路示意图。
图 2 为 泵浦光与信号光满足群速度匹配的前提下，信号光波长随晶体温度的变化曲线 。
图 3 为本发明提供的针对中红外脉冲激光的光谱调控装置中， 光参量增益随啁啾脉冲宽度的变化曲线。
图 4 为 本发明提供的针对中红外脉冲激光的光谱调控装置中， 啁啾脉冲宽度为 425fs 时的放大后的中红外脉冲激光的脉冲包络与光谱。
图 5 为 本发明提供的针对中红外脉冲激光的光谱调控装置中， 啁啾脉冲宽度为 700fs 时的放大后的中红外脉冲激光的脉冲包络与光谱。
图 6 为 本发明提供的针对中红外脉冲激光的光谱调控装置中， 啁啾脉冲宽度为 1250fs 时的放大后的中红外脉冲激光的脉冲包络与光谱。
图 7 为 本发明提供的针对中红外脉冲激光的光谱调控装置中， 啁啾脉冲宽度为 2500fs 时的放大后的中红外脉冲激光的脉冲包络与光谱。
具体实施方式
本发明提供一种针对中红外脉冲激光的光谱调控装置。为使本发明的目的、技术方案及效果更加清楚、明确，以下参照附图并举实施例对本发明进一步详细说明。 应当理解，此处所描述的具体实施例仅用以解释本发明，并不用于限定本发明。
晶体折射率（ n ）会随温度的不同而变化，晶体内脉冲激光的群速度（ v ）也是与温度有关的物理量。为了利用非线性晶体最大的非线性系数（ d33 ）， 非周期性极化晶体通常都需要满足 0 类的准相位匹配条件（ e+e- › e ），要求泵浦光、信号光和闲频光均为 e 偏振光。处于同一偏振态的两种脉冲激光，其群速度存在比较大的差别，而且，这种差异并不能简单地通过改变非周期性极化晶体的工作温度来消除。但是，当这两束激光以正交的方式在晶体中传播（例如，信号光为 o 偏振光、泵浦光为 e 偏振光），在特定温度下， 它们会有相同的群速度。为了消除泵浦光与信号光间的群速度失配，所述针对中红外脉冲激光的光谱调控装置采用 I 类准相位匹配技术，利用脉冲激光的群速度与晶体温度之间的色散关系，通过调控晶体的工作温度，使信号光与泵浦光满足群速度匹配（ GVM=0 ）。在这种时间同步的工作状态下，理论上，即便使用很长的非线性晶体，它们也不会在相互作用的过程中走离。在这样的物理条件下，无须啁啾展宽，即可将非周期性极化晶体直接应用于飞秒量级超短脉冲激光的非线性频率转换，并对其光谱进行调控。
进而，请参阅图 1 ，本发明提供一种 针对中红外脉冲激光的光谱调控装置 ， 包括 近红外脉冲激光器 1 、 光学耦合镜 2 、非周期性极化晶体 3 、 用于对所述 非周期性极化晶体 3 的工作温度进行调控的控温炉 4 和 分光镜 7 。其中，以信号光与泵浦光在所述 非周期性极化晶体 中的群速度相同时的温度为所述 工作温度 。
所述近红外脉冲激光器 1 ，光学耦合镜 2 ，非周期性极化晶体 3 ，分光镜 7 依次放置； 由近红外脉冲激光器 1 发出的近红外脉冲激光，经过所述光学耦合镜 2 后，与中红外脉冲激光一同进入所述非周期性极化晶体 3 ，通过非线性频率转换，以近红外脉冲激光为泵浦光，对中红外脉冲激光的不同光谱成分进行差异化放大；所述非周期性极化晶体 3 输出的混合光 经所述分光镜 7 分离后得到经过光谱调控的中红外脉冲激光 。所述近红外脉冲激光是指波长在 750 ～ 2000nm 范围内的脉冲激光。
本发明采用 I 类准相位匹配技术，通过调节非周期性极化晶体 的工作温度， 消除晶体内近红外泵浦光与中红外信号光间的群速度失配，且通过非线性频率转换，对中红外脉冲激光的不同光谱成分进行差异化放大，从而实现对其光谱的调整。还可通过使用不同的非周期性极化晶体获得不同光谱特性的输出激光。
请参阅图 2 ，图 2 给出了泵浦光与信号光满足群速度匹配的前提下，信号光波长随晶体温度的变化曲线。从图中可以看出，配合 3 种不同波长的商品化脉冲激光器，这种群速度匹配方案可以应用于 2-5 μ m 的中红外波段，因此，所述中红外脉冲激光的波长为 2-5 μ m 。以 3 μ m 的典型中红外波长为例，通过以下实施例，对本发明做进一步详细说明。
图 1 所示的光谱调控装置中，箭头方向为光传播的方向，近红外脉冲激光 A 与中红外脉冲激光 B 经光学耦合镜 2 耦合后进入到非周期性极化晶体 3 中。近红外脉冲激光 A 经光学耦合镜 2 反射后进入到非周期性极化晶体 3 中。中红外脉冲激光 B 透过所述光学耦合镜 2 ，进入到非周期性极化晶体 3 中。
进一步的，所述近红外脉冲激光器 3 为 790nm 钛宝石飞秒脉冲激光器，其输出的波长为 790nm 的脉冲激光 A ，经过所述光学耦合镜 2 ，与波长为 3 μ m 的中红外脉冲激光 B 一同进入所述非周期性极化晶体 3 ，以波长为 790nm 的脉冲激光 A 为泵浦光，对波长为 3 μ m 的中红外脉冲激光 B （作为信号光）进行放大，与此同时，实现对其光谱的有效调控。
所述非周期性极化晶体 3 为满足 I 类准位相匹配的非周期性极化晶体。所述非周期性极化晶体 3 的极化反转周期沿纵向方向（即脉冲激光的传播方向）呈非周期性变化。在本实施例中，所述非周期性极化晶体 3 选用的是非周期性极化铌酸锂晶体（ APPLN ）。
所述非周期性极化晶体 3 ，其长度以及非周期性变化的极化反转周期是根据输入中红外脉冲激光的光谱特性和想得到的输出光谱特性来确定的，借助这种特别设计的晶体，既能保证中红外脉冲激光的不同光谱成分能够在晶体的不同区域满足准相位匹配条件，获得放大，又能通过合理设计晶体的极化反转周期，调控中红外脉冲激光的不同光谱成分的放大倍数，从而实现对中红外脉冲激光光谱的调控。
所述控温炉 4 设置在非周期性极化晶体 3 的下方；所述控温炉 4 用于对所述非周期性极化晶体 3 的工作温度进行调控，具体的，所述控温炉 4 令所述非周期性极化晶体 3 工作在设定温度，使信号光与泵浦光在所述非周期性极化晶体 6 中的群速度相同。
所述分光镜 9 为可以将不同波长的混合光相互分离的光学元件；优选的， 为分光棱镜、双色镜中的一种。从所述非周期性极化晶体 3 输出的混合光 C 经所述分光镜 7 分离后得到经过光谱调控的中红外脉冲激光 D 。
具体的，假设波长为 3 μ m 的中红外脉冲激光 B 的传输极限脉宽为 100fs ，将其啁啾展宽至不同脉冲宽度。对应的，波长为 790nm 的近红外脉冲激光 A 的脉宽与中红外脉冲激光保持一致。为了使波长为 790nm 的泵浦光与波长为 3 μ m 的信号光满足群速度匹配，通过晶体控温炉，将 APPLN 晶体的温度固定在 127 ℃（设定温度）。 APPLN 晶体的长度为 10mm ，极化反转周期从 34.06 μ m 到 34.24 μ m 线性均匀变化。基于 HCP 公司的 5% 掺杂 MgO ： PPLN 的折射率公式，本发明对所述针对中红外脉冲激光的光谱调控装置的运行情况进行了数值模拟。为了证明其优越性，我们对使用满足 0 类准相位匹配的 APPLN （晶体长度为 2mm ，极化反转周期从 21.76 μ m 到 21.7 μ m 线性均匀变化）时的情况也进行了模拟。如图 3 所示，图 3 中给出了光参量增益随啁啾脉冲宽度的变化曲线。随着啁啾脉冲宽度的减小，激光脉冲间的时间走离越发严重。当中红外脉冲激光的啁啾脉冲宽度大于 2ps 的时候，无论是 I 类还是 0 类 QPM ，都能得到相当的光参量增益。随着啁啾脉冲宽度的逐渐减小， 0 类 QPM 情况下的光参量增益迅速下降，更重要的是，由于泵浦光与信号光群速度的不匹配，在信号光的短波长成分获得放大之前，泵浦光与信号光会在时间上相互走离，导致放大后的中红外脉冲激光的光谱严重'红移'。 4 种不同啁啾脉冲宽度下的放大后的中红外脉冲激光的脉冲包络与光谱分别列在图 4- 图 7 ，实线表示 I 类准相位匹配的曲线，虚线表示 0 类准相位匹配的曲线，清楚揭示了输出频谱随啁啾脉冲宽度的变化趋势。相比之下，所述针对中红外脉冲激光的光谱调控装置使用满足 I 类 QPM 的 APPLN 晶体，借助非周期性变化的极化反转周期，增加中红外脉冲激光频谱宽度的同时，基本保留了其频谱特征。利用这样的设计，所述针对中红外脉冲激光的光谱调控装置可以直接应用于无啁啾展宽的超短脉冲激光，对飞秒量级的中红外超短脉冲激光的光谱进行调控，增加其带宽，甚至整形。
由此可见，所述针对中红外脉冲激光的光谱调控装置可以使用较长的 APPLN 晶体，利用脉冲激光的群速度与晶体温度之间的色散关系，令晶体工作在某设定温度，以此消除晶体内近红外泵浦光与中红外信号光间的群速度失配。根据输入中红外脉冲激光的光谱特性和想得到的输出光谱特性，确定非周期性变化的极化反转周期。借助这种特别设计的晶体，中红外脉冲激光的不同光谱成分均能够在晶体的不同区域满足准相位匹配条件，获得放大，又能通过合理设计晶体的极化反转周期，调控中红外脉冲激光的不同光谱成分的放大倍数，从而实现对中红外脉冲激光光谱的调控。由于泵浦光与信号光在时间上始终保持同步，所述针对中红外脉冲激光的光谱调控装置可以直接应用于无啁啾展宽的超短脉冲激光，对飞秒量级的中红外超短脉冲激光的光谱进行调控，增加其带宽，甚至整形。
可以理解的是，对本领域普通技术人员来说，可以根据本发明的技术方案及其发明构思加以等同替换或改变，而所有这些改变或替换都应属于本发明所附的权利要求的保护范围。

Claims (6)

  1. 一种针对中红外脉冲激光的光谱调控装置，其特征在于，包括：
    近红外脉冲激光器 ；
    光学耦合镜；
    非周期性极化晶体；
    用于对所述非周期性极化晶体的工作温度进行调控的控温炉；
    分光镜；
    所述近红外脉冲激光器，光学耦合镜，非周期性极化晶体，分光镜 依次放置；
    由近红外脉冲激光器发出的近红外脉冲激光，经过所述光学耦合镜后，与中红外脉冲激光一同进入所述非周期性极化晶体，通过非线性频率转换，以近红外脉冲激光为泵浦光，对中红外脉冲激光的不同光谱成分进行差异化放大；所述非周期性极化晶体输出的混合光 经所述分光镜分离后得到经过光谱调控的中红外脉冲激光 。
  2. 根据权利要求 1 所述的针对中红外脉冲激光的光谱调控装置，其特征在于，所述非周期性极化晶体为满足 I 类准位相匹配的 非周期性极化晶体，所述非周期性极化晶体的极化反转周期沿纵向方向呈非周期性变化。
  3. 根据权利要求 1 所述的针对中红外脉冲激光的光谱调控装置，其特征在于，以信号光与泵浦光在所述非周期性极化晶体中的群速度相同时的温度为所述工作温度 。
  4. 根据权利要求 1 所述的针对中红外脉冲激光的光谱调控装置，其特征在于，所述非周期性极化晶体为非周期性极化铌酸锂晶体。
  5. 根据权利要求 1 所述的针对中红外脉冲激光的光谱调控装置，其特征在于，所述近红外脉冲激光器为790nm 钛宝石飞秒脉冲激光器。
  6. 根据权利要求 1 所述的针对中红外脉冲激光的光谱调控装置，其特征在于，所述分光镜为可以将不同波长的混合光相互分离的光学元件。
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