Performance analysis and small signal identification of time-resolved stand-off Raman spectroscopy system

Over the last decade, the time-resolved stand-off Raman spectroscopy has attracted considerable attention due to its daytime measurement model and fluorescent suppression. Suffering from the small Raman scattering efficiency, this system perusing large detection distance requires both large power consumption and optical system, which blocks its applications. In order to seek for high signal collection efficient, in this work, we designed a 2-inch refractive telescope system, and obtained 20 m detection distance using a frequency-doubled 532 nm Nd: YAG laser with pulse energy of 30 mJ. By adjusting the excitation energy through a variable attenuator, we investigate the interrelations of input power, the signal and the noise consisting of the internal and signal-induced noises. For both main peak and sub peak, the signal-induced noises are subject to a power function of the signal intensity, and can be filtered and smoothed by the norm-distribution-function weighted moving average method (WMAM) without the loss of the signal information. After transforming the correlation spectrum, defined by the multiplication of the noise and signal spectrum, iteratively by WMAM, we can identify the small signal of about 1 count beyond the internal noise limitation. With help of the iteration method, the requirement of integration time as well as the excitation energy can be greatly reduced.