Makowski M A, Victor B S
Akima/Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
Rev Sci Instrum. 2021 Mar 1;92(3):033537. doi: 10.1063/5.0043325.
A Fast Fourier Transform (FFT) based method has been developed, which improves the frequency response of the Motional Stark Effect (MSE) system by about a factor of 10 over the conventional analog lock-in method. The method uses fits to rigorously derived analytic expressions for the FFT spectral components of the MSE signal to accurately obtain the amplitudes and phases of the 2f and 2f photo-elastic modulator (PEM) frequencies that encode the polarization angle. Since no frequency filtering is used in the FFT method, the frequency response is limited by fundamental measurement properties: the frequency response of the detector, photon statistics, sample rate, and the ability to resolve the spectral components. In contrast, the frequency response of the analog lock-in is limited by a low pass filter with a cutoff of around 500 Hz. In the case of the DIII-D MSE system, the output of the photo-multiplier tube detector was sampled at 500 kHz and FFTs with as few as 100 points were used to obtain the amplitudes of the 2f and 2f PEM frequency components. This corresponds to a frequency response of 5 kHz, about ten times faster than the analog lock-in amplifier system. Details of the FFT method will be presented and compared to those of the analog lock-in system.
已经开发出一种基于快速傅里叶变换（FFT）的方法，与传统的模拟锁相方法相比，该方法将运动斯塔克效应（MSE）系统的频率响应提高了约10倍。该方法通过拟合严格推导的MSE信号FFT谱分量的解析表达式，来准确获取编码偏振角的2f和2f光弹调制器（PEM）频率的幅度和相位。由于FFT方法中未使用频率滤波，其频率响应受基本测量特性限制：探测器的频率响应、光子统计、采样率以及分辨谱分量的能力。相比之下，模拟锁相的频率响应受截止频率约为500 Hz的低通滤波器限制。在DIII-D MSE系统中，光电倍增管探测器的输出以500 kHz采样，并且使用少至100个点的FFT来获取2f和2f PEM频率分量的幅度。这对应于5 kHz的频率响应，比模拟锁相放大器系统快约10倍。将介绍FFT方法的细节，并与模拟锁相系统的细节进行比较。
