Here we report a novel, to the best of our knowledge, method of active intracavity intensity modulation for cavity-enhanced photoacoustic spectroscopy (PAS) without the need for any external optical modulators. Based on the Pound-Drever-Hall (PDH) locking technique, a dither is added to the PDH error signal to periodically vary the locking point between the laser frequency and optical cavity within a sub-MHz frequency range. While significantly enhancing the intracavity laser intensity, the optical cavity also acts as an intensity modulator. As a proof-of-principle, we demonstrated the PAS of ${{\rm C}_2}{{\rm H}_2}$C2H2 by placing a photoacoustic cell ($Q$Q-factor $\sim{10}$∼10) inside a Fabry-Perot cavity (finesse $\sim{628}$∼628) and adopting the proposed intracavity intensity modulation scheme. By detecting the weak ${{\rm C}_2}{{\rm H}_2}$C2H2 line at ${6412.73}\;{{\rm cm}^{ - 1}}$6412.73cm-1, the sensor achieves a normalized noise equivalent absorption (NNEA) coefficient of ${1.5} \times {{10}^{ - 11}}\;{{\rm cm}^{ - 1}}{{\rm WHz}^{ - 1/2}}$1.5×10-11cm-1WHz-1/2. This method enables the continuous locking of laser frequency and optical cavity, and it achieves the intracavity intensity modulation with an adjustable modulation depth as well.

中文翻译：

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利用Pound-Drever-Hall锁定对光声光谱进行腔内激光强度的主动调制。

在这里，我们就我们所知，报告了一种新颖的有源腔内强度调制方法，用于腔增强型光声光谱法（PAS），不需要任何外部光学调制器。基于磅-霍尔-霍尔（PDH）锁定技术，将抖动添加到PDH误差信号中，以在亚MHz频率范围内周期性地改变激光频率与光学腔之间的锁定点。在显着提高腔内激光强度的同时，光学腔还充当强度调制器。作为原理的证明，我们通过放置光声电池（$ Q $ Q因子$ \ sim {10}，展示了$ {{\ rm C} _2} {{\ rm H} _2} $ C2H2的PAS在法布里-珀罗（Fabry-Perot）腔内（精细度\\ sim {628} $〜628）中的约10美元），并采用建议的腔内强度调制方案。通过在$ {6412.73} \;处检测弱的$ {{\ rm C} _2} {{\ rm H} _2} $ C2H2线； {{\ rm cm} ^ {-1}} $ 6412.73cm-1，传感器达到标准化噪声等效吸收（NNEA）系数$ {1.5} \ times {{10} ^ {-11}} \; {{\ rm cm} ^ {-1}} {{\ rm WHz} ^ {- 1/2}} $ 1.5×10-11cm-1WHz-1 / 2。这种方法可以连续锁定激光频率和光腔，并且还可以通过可调的调制深度实现腔内强度调制。