Investigation Of Trace Gas Detection Based On Noise-Immune Cavity-enhanced Optical Heterodyne Molecular Spectroscopy

Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy?NICE-OHMS?is one of the most sensitive laser absorption spectroscopy in the world.It is a combination of frequency modulation spectroscopy?FMS?technology and cavity enhanced absorption spectroscopy?CEAS?technology,the CEAS growth path of the light and absorbing medium by the use of a high finesse cavity,when the laser is coupled into the optical cavity it will be reflect back and forth between the mirrors thereby enhancing the absorption signal.The FMS Technology coding absorption information of the material to high frequency range,then demodulating and filtering the information,suppressing background noise effectively,especially the 1/f noise caused by laser power fluctuation.At the same time,the modulation frequency of the laser is equal to the free spectral range?FSR?of the cavity,any frequency jitter between laser and cavity will produce the same effect to the two side band light,and FMS signal is a result of the beat from two side light,which makes the NICE-OHMS technology have a unique characteristic of noise immune to frequency-amplitude noise,and this noise is the main factor affecting the detection sensitivity of CEAS.Starting from the basic principles of NICE-OHMS,this paper first introduces FMS and CEAS separately,focuses on some parameters and performances of F-P cavity in CEAS.The principle and characteristics of NICE-OHMS are introduced in combination with FMS and CEAS,and the signal type of NICE-OHMS is given.After that,the performance of the laser and the design of the cavity were explained,and the mode matching of laser to high fineness cavity was realized.Further,in order to reflect the frequency locking importance for the system,the paper presents the measurement method of the transfer function of transducer.The design principle of the proportional integral differential circuit?PID?in the servo feedback loop is illustrated combined the transfer function of the transducer,the performance evaluation by lock the laser to cavity.In order to evaluate the NICE-OHMS device,a concentration of NH₃ was measured at 70mTorr pressure,and the measurement results showed that,the Pound-Drever-Hall?PDH?locking bandwidth is 4.3kHz,and DeVoe-Brewer?DVB?locking bandwidth is0.38kHz.The signal-to-noise ratio of NICE-OHMS signal is 43 times large than CEAS signal,the detection sensitivity is 3.7×10^-10cm^-1.What's more,due to the use of high finesse cavity amplified laser power,the NH₃ signal light saturation occurs,the sub Doppler structure assessment by the PDH signal without locked,and assessment result show that Doppler full width at half maximum?FWHM?of 2.05MHz.Finally the high finesse cavity is locked to the sub Doppler absorption line center,corresponding to the laser frequency locked in the sub Doppler absorption line,Allen deviation results shows that the frequency locking precision reach to 1.6×10^-12 in 136s.Based on the NICE-OHMS experiment device,this paper proposes a new method for measuring high finesse cavity finesse named double frequency modulation method.Differ from the cavity ringdown?CRDS?technique,this method is a continuous measurement,obtained signal is the cavity contour,using Lorenz linear fitting can get the cavity information directly.We measured the cavity finesse in the experiment using the technology,the result is 2373.This method not only applies to high finesse cavity,but also the low finesse cavity which ring down time equivalent to the response time of the detector.