Optical Feedback Raman Spectroscopy Gas Detection Technology in Complex Environment
High-precision monitoring of multi-component gases is crucial in diverse fields such as environmental monitoring,agricultural development,industrial safety,and medical diagnosis.Laser Raman spectroscopy provides a unique capability to simultaneously detect multiple gases by exploiting the distinct Raman frequency shift characteristics of each gas.However,the sensitivity of gas detection remains suboptimal due to the limited Raman scattering cross-section and weak scattering effect,posing a barrier to widespread adoption.To overcome these limitations,this study utilizes cavity enhancement technology to augment the detection sensitivity of Raman spectroscopy.However,the resonance of the Fabry-Perot(F-P)cavity is vulnerable to external environmental factors such as vibration and temperature drift.Therefore,frequency locking technology is necessary to maintain cavity resonance stability in practical applications,ensuring the generation of stable intra-cavity high power.Optical feedback frequency locking technology achieves this by locking the semiconductor laser into a high-precision external cavity using a slow servo system,eliminating the need for strict linewidth matching conditions.Nonetheless,in complex environments,perturbations from machinery or airflow can induce phase changes that exceed the adjustment range of the servo loop.This rapidly diminishes the capability to identify phase deviations,resulting in poor performance of the PID controller and unstable Cavity Transmission Signals(CTS).To tackle these challenges,this paper proposes a novel feedback phase adjustment approach.Firstly,utilizing the established laser cavity coupling model,a comprehensive analysis of the optical feedback phase's impact on cavity transmission signal is conducted.Theoretical findings suggest that compensating for deficiencies in the shape of the cavity transmission signal as a phase deviation indicator can be achieved by considering the signal strength.Subsequently,the entire 2π period is divided into four distinct regions based on CTS asymmetry and maximum value.Each region employs different error signals for phase adjustment.When the feedback phase significantly deviates while being close to the peak,a proportional coefficient,contingent upon the degree of asymmetry,is multiplied to serve as the feedback phase error control,thus readjusting the phase to the optimal position.The phase deviation tolerance of the phase-locked system spans the entire 2π interval,simplifying the process of obtaining the initial phase either manually or through algorithmic means.An optical feedback Raman spectroscopy gas detection platform was designed and constructed.Experimental validation demonstrates that the proposed optical feedback phase-locked method exhibits superior stability and robust recovery capability in scenarios involving strong and rapid airflow disturbances.This significantly enhances the potential of optical feedback cavity-enhanced Raman spectroscopy gas detection technology for real-time,in-situ detection scenarios.The detection limits for O2 and N2 in air were measured at 4.8 Pa(48.83 μL/L)and 5.8 Pa(57.43 μL/L),respectively.The minimum detection limit for a 10%CO2 standard gas concentration is 4.4 μL/L under the conditions of a gas chamber pressure of 3 atm and an integration time of 200 s.
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