The mid-infrared atmospheric window of 3-5.5  μm holds valuable information regarding molecular composition and function for fundamental and applied spectroscopy. Using a robust, mode-locked fiber-laser source of <11  fs pulses in the near infrared, we explore quadratic (χ^{(2)}) nonlinear optical processes leading to frequency comb generation across this entire mid-infrared atmospheric window. With experiments and modeling, we demonstrate intrapulse difference frequency generation that yields few-cycle mid-infrared pulses in a single pass through periodically poled lithium niobate. Harmonic and cascaded χ^{(2)} nonlinearities further provide direct access to the carrier-envelope offset frequency of the near infrared driving pulse train. The high frequency stability of the mid-infrared frequency comb is exploited for spectroscopy of acetone and carbonyl sulfide with simultaneous bandwidths exceeding 11 THz and with spectral resolution as high as 0.003  cm^{-1}. The combination of low noise and broad spectral coverage enables detection of trace gases with concentrations in the part-per-billion range.

中文翻译：

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具有少量周期脉冲和χ^ {（2）}非线性光学的中红外频率梳的产生和光谱分析。

3-5.5μm的中红外大气窗口包含有关基本光谱和应用光谱的分子组成和功能的有价值的信息。使用近红外中<11 fs脉冲的稳健的锁模光纤激光源，我们研究了二次（χ^ {（2）}）非线性光学过程，该过程导致了整个整个中红外大气窗口中的频率梳生成。通过实验和建模，我们证明了脉冲内差分频率的产生，该频率在单次通过周期性极化的铌酸锂时可产生几个周期的中红外脉冲。谐波和级联的χ^ {（2）}非线性特性还可以直接访问近红外驱动脉冲序列的载波包络偏移频率。利用中红外频率梳的高频稳定性对丙酮和羰基硫进行光谱分析，同时带宽超过11 THz，光谱分辨率高达0.003 cm ^ {-1}。低噪声和宽光谱范围的结合使检测痕量气体的浓度达到了十亿分之几。