高功率低噪声中红外飞秒激光频率梳的关键技术

The mid-infrared (mid-IR) wavelength range between 6 and 20 µm, in particular, has been known as the fingerprint region. Spectroscopic information of these vibrational bands reveals the molecular structure and, in turn, identifies the ingredients of the sample under test. This is of particular importance for the detection of explosives and biochemical agents. High power, low noise, tunable mid-IR frequency combs are highly desired from the viewpoint of rapid high-resolution sensing. Such a high-power mid-IR femtosecond comb is usually implemented via difference-frequency generation between an ultrafast near-infrared laser source as the pump and another wavelength-tunable ultrafast source as the signal. Raman soliton arising from Raman-induced frequency shift inside an optical fiber has become the most popular choice of the signal pulse. However, the resulting mid-IR comb exhibits low power and high noise. We will carry out a detailed theoretical and experimental study of the physical mechanisms associated with the power and noise properties of mid-IR combs. We propose a novel approach to implement mid-IR frequency combs with 2-3 orders of magnitude improved power and 1-2 order of magnitude noise reduction, which will facilitate many important applications in physics, chemistry, biology, medicine, industry etc.

中红外光学频率梳，尤其是在6-20微米的“指纹区”波段，具有广阔的应用前景。这样的中红外光学频率梳一般利用泵浦光脉冲和信号光脉冲经过非线性晶体进行光学差频获得。信号光脉冲一般通过传统的、利用孤子自频移技术获得。中红外光学频率梳一直无法避免功率低和噪声高这两大致命缺陷，导致难以获得真正实用。本项目将通过系统的理论和实验研究，全面揭示目前中红外光梳功率过低和噪声过高背后的物理机制，提出全新的思路克服这两个缺陷，获得高功率、低噪声的中红外光梳。尤其在“指纹区”波段，中红外光梳的平均功率将提高2-3个数量级，噪声降低1-2个数量级，从而真正释放中红外光梳在物理、化学、生物、医学以及工业界的应用潜力。

中红外光梳一般利用光学差频产生(DFG)，在该过程中，需要将两个中心波长不同的超短脉冲通过光学晶体进行混频。两个超短脉冲中的泵浦光中心波长固定，信号光波长连续可调，通过调节信号光脉冲的中心波长调节通过差频所产生的中红外脉冲波长。利用传统方案产生的可调谐信号光能量过低，因此所产生的中红外光梳平均功率低，正是这一重大缺陷导致中红外光梳难以获得真正实用。. 根据项目的任务和要求，项目组发展了高功率掺镱光纤激光器系统，利用光纤中的自相位调制（SPM）效应来展宽输入光谱，然后选用合适的滤波器来选择最左和最右的频谱旁瓣（我们将这种技术命名为SESS），从而获得脉冲能量高达105 nJ的波长在940nm至1250nm可调谐的信号脉冲；之后利用 GaSe 非线性晶体实现泵浦光与信号光的差频，获得7-18微米可调谐的中红外光梳。按照计划安排，我们聚焦中红外光源系统的功率可扩展性和噪声特性，系统性地研究了相关关键技术，并与传统的中红外光梳方案进行对比，相对时间抖动低至100as水平。项目组取得的这些研究成果为下一步产生和测量更高功率更低噪声的中红外光梳在实验和理论上打下了坚实的基础，提高了在中红外领域的研究水平，并促进了与国内外其他相关研究单位的合作与交流。