Scime E E, Freeze J, Gilbert T J, Steinberger T E
Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA.
Rev Sci Instrum. 2024 Aug 1;95(8). doi: 10.1063/5.0219309.
The small signal-to-noise ratio (SNR) of conventional laser induced fluorescence (LIF) measurements using a continuous wave laser, either diode or dye, is typically overcome by amplitude modulating the laser at a specific frequency and then using lock-in amplification to extract the signal from measurement noise. Here, we present LIF measurements of the neutral helium velocity distribution function in an rf plasma using frequency modulated (FM) laser injection. A pulse train of 100% amplitude modulation is generated synthetically with a random sequence of pulse lengths. The FM signal then drives an acoustic optic modulator placed in the path of the injection beam in an LIF measurement. The signal from a fast photomultiplier tube is digitized and cross-correlated with the known modulation signal. The resultant FM-based LIF signal outperforms a conventional lock-in-based LIF measurement on the same plasma in terms of SNR and precision.
使用连续波激光（二极管激光或染料激光）进行传统激光诱导荧光（LIF）测量时，小信噪比（SNR）通常通过在特定频率下对激光进行幅度调制，然后使用锁相放大从测量噪声中提取信号来克服。在此，我们展示了使用调频（FM）激光注入对射频等离子体中的中性氦速度分布函数进行的LIF测量。利用随机脉冲长度序列合成产生100%幅度调制的脉冲序列。然后，FM信号驱动放置在LIF测量注入光束路径中的声光调制器。来自快速光电倍增管的信号被数字化，并与已知调制信号进行互相关。在信噪比和精度方面，基于FM的LIF信号在相同等离子体上的表现优于传统基于锁相的LIF测量。
