An Adaptive Beam-Steering-Distortion-Compensated TDLAS Method for Precise and Robust Temperature Measurement

Tunable diode laser absorption spectroscopy (TDLAS) is a popular technique for noncontact temperature measurement but is highly susceptible to beam-steering distortion due to the time-varying along-path inhomogeneity of refractive index in dynamic process monitoring. An adaptive beam-steering-distortion compensation method is proposed to eliminate the distortion in TDLAS measurement. A high-frequency modulated laser at a nonabsorbing wavelength is coupled with a conventional low-frequency scanning laser at an absorbing wavelength and passes through the target field along an identical path. A synchronous demodulation method is proposed to demodulate the transmitted hybrid laser signal into the distorted absorption and distortion signals. The distortion signal is utilized to adaptively compensate the distortion parasitic on the distorted absorption signal. Experimental results obtained on a vibrated water thermostat show that the maximum deviation of temperature derived by the proposed method is 4.97 K, while that by the traditional DAS method is 34.64 K. Experimental results on a turbulent flame show that the relative standard deviation of temperature obtained by the proposed method has decreased from 36.7% (DAS) to 4.20%. Also, the temperature fitting success rate has increased from 88.0% to 100% in the turbulent flame case. The results show that the proposed method is more precise and robust than the traditional method. The proposed method will potentially play an important role in laser-based dynamic combustion monitoring and diagnosis.