连续波腔衰荡吸收光谱技术中的模式匹配研究

Mode Matching in Continuous-Wave Cavity Ring-Down Absorption Spectroscopy

对比了波长调制和腔长调制两种方式实现模式匹配的性能特点,通过测量空腔衰荡时间并进行Allan方差分析,得到两种调制方式的衰荡时间分别为(5.239±0.077)μs和(5.252±0.058)μs,该系统的吸收系数检测限为10^(-10)量级。对于甲烷气体浓度,通过步进式改变入射激光的波长,描绘了甲烷气体的吸收谱线,其检测限可达10^(-9)量级。利用波长调制和腔长调制方式得到环境中甲烷气体的浓度分别为1.868×10^(-6)和1.870×10^(-6)。实验结果表明,虽然波长调制和腔长调制均能实现模式匹配,完成痕量甲烷气体的测量,但是通过对比可知:腔长调制方式具有更高的测量精度和更低的浓度检测限;波长调制方式发挥了可调谐激光器的优势,结构相对简单,无需压电陶瓷等附加的机械结构。因此,在实际应用中可根据需求在测量性能和装置复杂度及成本之间进行取舍。

Objective Continuous-wave cavity ring-down spectroscopy(CW-CRDS)is based on absorption spectroscopy,which can enhance the absorbance of trace gas through an ultralong optical path formed by high reflectivity mirrors to achieve high sensitivity and high precision measurement.The ring-down cavity comprises two high reflectivity mirrors with a distance of 50 cm(reflectivity is 99.99%,radius of curvature is 1 m)and high fineness.The free spectral range(FSR)is the interval between two adjacent modes,and the phase difference is an integral multiple of 2π.According to the characteristics of CW-CRDS,mode matching is crucial to achieve effective detection.The linewidth of a semiconductor laser(3 MHz)is much smaller than that of FSR(300 MHz),and the frequency of the incident light usually falls between two longitudinal modes,so the mode matching cannot occur actively.In this study,to improve the matching efficiency and avoid random matching between incident light and cavity mode,wavelength modulation and cavity length modulation were used to realize resonance between incident light frequency and cavity mode,respectively.Methods In this experiment,the performance characteristics of wavelength modulation and cavity length modulation were introduced and compared.Ideally,there was a matching signal in one acquisition period,so the selection of scanning amplitude depended on the FSR.In wavelength modulated cavity ring-down spectroscopy(CRDS),it was easy to match the cavity mode by scanning the incident light frequency in a small range.According to the measurement results of multiple groups of scanning amplitude(Fig.4,Table 1),15 mV was selected as the best scanning range,so the cavity ring-down time(τ_(0))was more accurate and the standard deviation was smaller.Similarly,in CRDS with cavity length modulation,the piezoelectric ceramics(PZT)changed the cavity length by high-voltage driving,so the longitudinal mode frequency changed and resonated with the incident light frequency and achieved mode matching.The choice of the driving voltage was related to FSR.By measuring the variation of cavity ring-down time(τ_(0))under different driving voltages of PZT,the measured results were closer to the theoretical values when the driving voltage was 50 V(Fig.5,Table 2).Results and Discussions Owing to the instability of the optical cavity and the shift of laser frequency,the measurement the fluctuated in a certain range.Combined with the experimental results,better measurement results could be obtained when the scanning amplitudes of the two modulation modes were slightly larger than that of FSR.In the experiment,the cavity ring-down time(τ_(0))was measured using two modulation methods for a long time(470 s)to compare their adaptability and stability(Fig.6).The statistical distribution of ring-down time was normal,and the statistical distribution of cavity length modulation was narrower,with the standard deviation being 0.058 μs,which had higher measurement stability.In addition,Allan variance analysis was used to analyze the best average times and detection limit of the absorption coefficient of the system under two modulation modes.According to the comparison results(Table 3),when the average time of cavity length modulation was 96,the measurement limit of absorption coefficient was 1.693×10^(-4)μs^(2),which was slightly better than that of wavelength modulation.By measuring the ring-down time with(r)or without(τ_(0))absorption,the concentration of the absorption medium could be quantitatively measured(Fig.7).According to the HITRAN database,the characteristic absorption line of methane in the near-infrared band(1653.72 nm)was selected.The spectral line intensities were 1.455×10^(-21),9.277×10^(-22),and 7.877×10^(-22)cm^(-1)/(mol·cm^(-2)).The absorption lines of methane in Hefei Science Island were measured using two modulation methods.The current of the laser was controlled by software,and the wavelength was varied by changing the current stepwise.To ensure the accuracy of the measurement results,the ring-down time of each wavelength measurement point was obtained by averaging multiple measurements(Fig.8).Finally,the Lorentz fitting of the tracepoint spectrum was performed,and the methane concentration under the two modulation modes was calculated to be 1.868×10^(-6)and 1.870×10^(-6).Conclusions The experimental results show that both wavelength modulation and cavity length modulation can complete the periodic acquisition of ring-down signal and repeated measurement of ring-down time.However,from the comparison,the cavity length modulation method had higher measurement accuracy and lower limit detection of concentration,while the wavelength modulation exploits a tunable laser,andits structure is relatively simple without additional mechanical structures such as PZT.Therefore,in practical application,the selection can be made between measurement performance,device complexity,and cost according to the demand.