光强非线性响应下的光致热弹光谱光强修正

Light intensity correction for light-induced thermoelastic spectroscopy based on nonlinear response of light intensity

提出了基于光强非线性响应的光致热弹光谱光强修正方法,实现了激光光强的准确修正。控制DFB激光器工作于波长调制模式,设置激光调制频率为16369.75 Hz,通过光纤放大器增强其出射光强非线性项的幅值,光束经多次反射池后汇聚于石英音叉根部激发光热信号,采用数字锁相放大器解调得到对应的谐波信号,通过多项式拟合谐波信号背景,反演得到与浓度及光强分别对应的谐波信号。实验结果表明,当光强从22.03 mW变化至3.16 mW,谐波信号背景幅值与光强具有良好的线性关系,线性相关系数大于0.998,归一化后的谐波信号幅值波动小于0.37%。针对甲烷测量,系统在较大的浓度范围内具有良好的线性响应,谐波信号信噪比表明系统的最低检测限达0.22×10-6。该研究为光致热弹光谱的光强修正提供了一种新的方法,可有效提高LITES系统在长期测量应用中的稳定性。

This study presents a method for correcting light intensity in light⁃induced thermoelastic spec⁃troscopy(LITES)using a nonlinear light intensity response,enabling precise light intensity corrections.The DFB laser,set to a wavelength modulation mode at a frequency of 16369.75 Hz,benefits from en⁃hanced nonlinear light intensity due to a fiber amplifier.The laser beam traverses a multi⁃pass cell before fo⁃cusing on the quartz tuning fork(QTF)base,generating a thermoelastic signal.A lock⁃in amplifier demod⁃ulates the harmonic signal,and polynomial fitting of the harmonic signal's baseline extracts harmonics relat⁃ed to both concentration and light intensity.Experimental data reveal a strong linear correlation(coeffi⁃cient>0.998)between the baseline amplitude of the harmonic signal and light intensity as it varies from 22.03 mW to 3.16 mW,with normalized harmonic signal amplitude variation under 0.37%.In methane detection,the system demonstrates a robust linear response across a broad concentration range,with harmonic signal noise ratios indicating a detection threshold as low as 0.22×10-6.This research offers a novel approach for LITES light intensity correction,significantly enhancing system stability for pro⁃longed measurements.