Precise Gas Temperature Measurement Using a Single Dual-Wavelength Mode-Locked Fiber Laser

A single dual-wavelength comb fiber laser was designed to emit simultaneously two asynchronous ultrashort pulse sequences for gas temperature detection. The interpulse coherence can be easily realized by sharing the same cavity and experiencing the same environment, and complicated feedback control is no longer required. The asynchronous pulses yield an optical sampling for interferogram in the time domain. A heatable cell filled with acetylene and nitrogen was used as the target object. The absorption spectrum of acetylene from 6492.52 to 6496.4 cm-1 was obtained by performing fast Fourier transform (FFT) on the phase-corrected interferograms. The standard deviation (STD) of the residual error between the measured spectra and HITRAN database was within 0.01. A baseline-immune multispectral fitting method for gas absorption estimation was proposed to enable precise temperature calculation. Simulation results demonstrate that compared with two classical methods, i.e., the frequency-domain (FD) multispectral fitting method and time-domain (TD) molecular free-induction decay (mFID) method, the proposed method shows a higher accuracy of gas temperature and concentration measurement and is applicable to various baseline types and noises. In the experiment, compared with the two classical methods, the maximum relative error between the measured and reference temperatures of the proposed method was reduced to 3.2%. Simulation and experiment results verify that the proposed method yields higher accuracy and noise immunity than the two classical methods.

I. Introduction

Gas temperature measurement is essential to combustion diagnosis [1], chemical reaction monitoring, and organic chemical synthesis [2]. Typical methods for gas temperature are either intrusive or nonintrusive. Intrusive methods, such as thermocouples or gas sampling, are widely applied, but they distort turbulent gases for invasive installations [3]. Nonintrusive methods use contactless sensor and has the merits of high sensitivity, fast response, and strong ability of multiparameter simultaneous measurement. These methods are usually realized from laser spectroscopy, e.g., coherent anti-stokes Raman scattering spectroscopy [4], planar laser-induced fluorescence [5], [6], and tunable diode laser absorption spectroscopy (TDLAS) [7], [8], [9]. TDLAS is widely applied in combustion evaluations of industrial and aerospace combustors for its high adaptability, fast response, high accuracy, and versatile parameter detection [10]. In TDLAS, two spectral lines are required to obtain gas temperature and molar concentrations, and these spectral lines are scanned using narrowbanded DFB lasers for fast response and economical cost. A single DFB laser typically only covers one or two spectral lines of target molecules, and a wide spectral coverage requires an increasing number of lasers. In recent years, the dual-comb spectroscopy (DCS) is proposed for wide spectral coverage and applied to gas parameter detection using two optical frequency comb (OFC) lasers. The optical spectral band of OFC laser is usually tens to hundreds of times wider than that of the DFB laser in TDLAS, and they can be used as a reference for HITRAN database calibration [11].