关键词: 腔衰荡光谱技术/
双重锁定/
PDH锁频/
Kalman滤波
English Abstract
Research and data processing of double locked cavity ringdown absorption spectroscopy
Jia Meng1,2,Zhao Gang1,2,
Hou Jia-Jia1,2,
Tan Wei1,2,
Qiu Xiao-Dong1,2,
Ma Wei-Guang1,
Zhang Lei1,2,
Dong Lei1,2,
Yin Wang-Bao1,2,
Xiao Lian-Tuan1,2,
Jia Suo-Tang1,2
1.Institute of Laser Spectroscopy, State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, Taiyuan 030006, China;
2.Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Fund Project:Project supported by the National Basic Research Program of China (Grant No. 2012CB921603), the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (Grant No. IRT13076), the National Natural Science Foundation of China (Grant Nos. 11434007, 61475093, 61378047, 61275213, 61475093), the National Science and Technology Support Program, China (Grant No. 2013BAC14B01), the Shanxi Natural Science Foundation for Young Scientists, China (Grant Nos. 2013021004-1, 2012021022-1), the Shanxi Scholarship Council of China (Grant Nos. 2013-011, 2013-01), and the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, China.Received Date:29 January 2016
Accepted Date:12 April 2016
Published Online:05 June 2016
Abstract:A continuous wave cavity ringdown spectroscopy based on a double-locking loop is proposed to improve the shortcoming of low acquisition rate of concentration in traditional scheme. A small portion of laser is separated to pass through a C2H2 reference cell, used to lock the laser frequency to the 1+3 band P(9)e absorption line of C2H2 at 6534.3634 cm-1 by the 1st harmonic demodulation of the frequency modulation spectroscopy. The remaining portion is incident on a high finesses cavity to observe the ringdown events. Meanwhile, the reflected light of cavity is used to extract the error signal to lock the laser based on the PDH frequency locking technique. As a consequence, the frequency drift of the laser and the jitter of the cavity length are improved, therefore a more relatively accuracy result is expected. The laser light is dual frequency modulated by a fiber coupled electro optic modulator (FEOM)in the above system. In order to optimize, to some extent, the asymmetry of the error signal caused by the residual amplitude modulation due to the inconsistency of the laser polarization direction with the extraordinary axis of the FEOM, the demodulation phase is adjusted carefully until the error signal is smoothed up and close to symmetry. Then, the effect of locking loop is examined. The frequency of laser, based on the measurement by a wavelength meter, is more stable and the relative frequency discrimination between the laser and the longitudinal mode of cavity is about 9.8 kHz. In addition, the PDH locking, ensuring the efficient coupling of the laser with the cavity, can gain a high acquisition rate of the concentration information. In order to obtain a complete ringdown event, the frequency of square wave to the fiber coupled acoustic optical modulator (FAOM) is limited to 30 kHz with the duty cycle of 85%, which is determined by the ringdown time and re-lock time. However, there exists a relatively large random noise in a series of ringdown time measurements of empty cavity, which is mainly caused by the errors of fitting and measurement. For the further improvement of the accuracy of experiment, an efficient digital filter, Kalman filter which can suppress the noise considerably at no expense of real-time capability, is used. The standard deviation of the ringdown time is reduced from 0.00333 to 0.00153. According to Allan variance analysis, the detection limit can reach 410-9 cm-1 for a 2 s integration time. Finally, the C2H2 gases with different concentrations from 100 ppb to 5 ppm are measured to demonstrate the linear response of this system.
Keywords: cavity ringdown spectroscopy/
double-locking loop/
PDH frequency locking/
Kalman filter
