精准线性跳频工作模式的高重频、窄线宽中红外激光光源研究

A precise linear-frequency-agile, narrow-linewidth mid-infrared laser generates periodical single-frequency pulses with stepped central frequency in the mid-infrared wavelength band. This unique operation mode is favorable to precise differential-absorption-based gas sensing in complicated field environments. The difference frequency generation (DFG) between a frequency-stabilized pump laser and a linear-frequency-agile signal laser is the most common method to produce such laser emission. However, there are several drawbacks in the ever-reported laser sources, including poor wavelength repeatability, uneven frequency spacing, low tuning speed, and insufficient sampling wavelengths, making them unsuitable for fast and precise sensing applications. In order to tackle these crises, an acousto-optic modulator (AOM) and fiber amplifier based loop is proposed to periodically shift the central frequency of the injected single-frequency seed laser. Precise linear-frequency-agile pump and signal lasers are developed, respectively, according to this configuration. Furthermore, the central frequency of the pump laser is ramp-up shifted while that of the signal laser is ramp-down shifted by selecting +1st or -1st diffraction order for the corresponding AOMs, respectively. As a result, the frequency difference between adjacent pulses of the precise linear-frequency-agile idler produced from DFG combines the total frequency shifts of the pump and signal. In addition, cascaded lithium niobate super-lattices with two lateral sections are introduced to recycle part of the amplified signal wave in the 1st DFG section to further amplify the idler wave via optical parametric amplification process in the succeeding section so as to boost the total conversion efficiency. Consequently, the novel precise linear-frequency-agile, high-repetition-rate, and narrow-linewidth mid-infrared laser sources could be developed based on the aforementioned methods and techniques.

精准线性跳频的窄线宽中红外激光能输出中红外波段，中心频率以固定频差精确跳变的周期性单频脉冲序列，独特的工作模式使其非常适合外场复杂条件下气体的高精度差分吸收探测。使用稳频泵浦光和线性跳频信号光差频产生(DFG)是构建此类光源的常用手段。但目前基于该原理的光源存在波长重复性差、跳频频差不稳定、调谐速度慢、跳频波长稀疏等不足，无法满足高速精密测量的需要。为此，本项目使用基于声光调制器(AOM)和光纤放大器的环路对注入的单频种子脉冲循环移频，分别产生精准线性跳频的高重频泵浦和信号光；分别利用AOM的﹢1级和-1级衍射，实现泵浦光的增频跳频和信号光的降频跳频，并通过DFG获得频差累加的精准线性跳频闲散光；通过引入分段级联式铌酸锂光学超晶格，将第一级DFG过程中放大的信号光通过第二级光参量放大过程继续转为闲散光，实现转换效率的提升；并最终实现新型精准线性跳频工作模式的高重频、窄线宽中红外激光输出。

对温室气体和有毒有害气体的精确测量有助于研判全球气候变化，保证人民生命和财产安全。差分吸收激光雷达作为一种常用的气体探测设备，一般使用两个波长的单频激光作为光源，其中一个波长位于气体的吸收峰，另一个位于临近的吸收谷，通过分析两个波段的回波偏差来反演待测气体的浓度。但在实际探测中，待测气体的温度、压强、多普勒频移等多种因素都会引起吸收线型的改变从而导致测量不准。为此，本项目提出了精准跳频的激光工作模式，使用单频的稳频激光器作为种子，将斩波后的脉冲种子激光注入放大的光纤移频环路，通过环路的循环移频形成了中心频率跳变的激光脉冲串。当该跳频的激光脉冲串覆盖整个气体吸收线时即可通过脉冲串的时域——光频域对应关系，仅使用时域探测就能拟合出整条吸收线，从而感知由于外界因素导致的吸收线改变并消除其影响。利用该原理，本项目使用不同的环内移频结构研制了单通单频差跳频、双通单频差跳频和双通双频差跳频三种不同的工作模式，并通过倍频和差频等非线性光学过程将常见的掺铒或掺镱跳频激光器的工作波长转换到待测气体的吸收峰。使用项目研制的精准跳频激光器，在气体池实验中成功测得了高压和常压氧气、空气以及常压和低压二氧化碳氮气混合气等不同状态的多种气体的吸收线型。在所有测试条件下，测得的气体吸收线与理论计算的吸收线均吻合很好，每个数据点的残差均小于0.5%，测试带宽内的RMS误差均在10e-3量级，展示了该光源的良好应用前景。为了给后期基于跳频激光器的实用化大气激光雷达研究奠定基础，项目组还开展了激光雷达收发系统的验证工作，分别验证了可见光波段、红外波段的收发系统，并采用了单元探测器、线阵探测器以及面阵探测器等多种器件验证距离分辨、光谱和距离同时分辨以及距离与空间同时分辨等不同的雷达探测体制。针对目前近红外——中红外单光子灵敏度探测器的不足，项目组开展了基于PPLN晶体的视场展宽型腔内上转换探测的机理与实验研究。通过非共线和晶体温度拉偏两种方法结合使得基于PPLN晶体的腔内上转换探测系统的视场提升了约2倍，有望应用于近红外——中红外跳频激光的探测系统中，实现单光子灵敏度的跳频激光回波探测。