Research On Near-infrared Optical-feedback Linear Cavity-enhanced Absorption Spectroscopy

Since its birth,laser technology has developed rapidly due to its high stability,good monochromaticity,and strong coherence.Among them,laser absorption spectroscopy technology（LAS）is based on the interaction of electric fields and gas-phase atoms,and has been widely used in basic scientific research fields such as polar exploration,scientific research on the Qinghai-Tibet Plateau,large-scale wind tunnel experiments,as well as medical pathological diagnosis,environmental pollution detection and other application fields.In the past 30 years,laser absorption spectroscopy technology has been regarded as the primary development technology for a new generation of trace gas detection due to its high selectivity,high sensitivity,and real-time detection.Among them,the most widely used LAS technology is Tunable Diode Laser Absorption Spectroscopy（TDLAS）.The detection sensitivity is usually between 10^-3and 10^-4cm^-1/Hz^1/2,but it is far from meeting the needs of some fields.Therefore,in order to further improve the sensitivity,people have developed Cavity Enhanced Absorption Spectroscopy（CEAS）technology,which is based on an optical cavity composed of high-reflection mirrors.When the laser is coupled into the cavity,the laser will be reflected back and forth between the cavity mirrors,thereby increasing the length of interaction with gaseous medium,the equivalent optical length can reach several kilometers,which greatly enhances the absorption signal.However,as the reflectivity of the cavity mirror increases,the cavity mode linewidth becomes very narrow,and it is difficult for general data collection to accurately capture the cavity mode amplitude;In addition,the semiconductor laser has a wider linewidth and only a small part of the laser frequency components can resonate with the optical cavity at a specific moment,so the cavity mode amplitude is low.Due to the above two reasons,direct CEAS not only has high detection noise,but also is difficult to achieve high-sensitivity measurement of absorption spectrum.In order to solve the above problems,the optical-feedback cavity-enhanced absorption spectroscopy technology（OF-CEAS）came into being.OF-CEAS is based on the optical feedback effect.The resonant light field in the cavity is transmitted through the cavity front mirror,and the original path is injected back into the semiconductor laser.Under proper feedback rate and feedback phase conditions,the laser frequency will be locked to the optical cavity mode frequency.The frequency noise of the laser is greatly suppressed,and the coupling efficiency of the laser to the cavity is significantly improved.Therefore,the amplitude of the observed optical cavity transmission signal is increased and stable.In order to avoid the direct reflection from the optical cavity to cause optical feedback,the traditional OF-CEAS employs a three-mirror V-shaped resonator.However,we have found that when the feedback phase is properly controlled,the light directly reflected by the optical cavity will not affect the optical feedback,and the laser can be locked to the resonant light of the optical cavity.Therefore,we proposed OF-CEAS based on linear FP cavity,and designed a set of dynamic servo loop that accurately controls the phase of the optical feedback,using the symmetry of the transmission cavity mode to calculate the error signal of the feedback phase control.Then,101 consecutive cavity transmission modes with stable amplitude and broad width have been observed.Finally,we detected methane standard gas with 32 ppm and obtained the OF-CEAS absorption signal.Based on signal to noise ratio,the detection sensitivity down to 0.54ppm（1σ）was estimated.