| Multi-pass Cell?MPC?is one of the core components of a variety of spectral measurement techniques.However,in the measurement methods that affect the characteristics of MPC,such as long optical path and multi-path transmission,which improve the spectral measurement accuracy,there are some problems such as low measurement accuracy,poor measurement timeliness,and system differences between actual use and MPC measurement,which lead to inaccurate measurement.Therefore,this paper proposes a frequency modulation continuous wave?FMCW?forward-emission heterodyne interferometry method to achieve high-precision online real-time visual measurement and adjustment of MPC.The work of this paper mainly includes:Firstly,two frequency tuning linearization correction methods,hardware feedback method and software resampling method,are proposed.According to the frequency tuning characteristics of the laser measured by the auxiliary interferometer,the hardware feedback method calculates the next injection current waveform of the laser by using the inverse function algorithm.The feedback correction method does not have the problem of lock-out.With this method,the linear fitting residual of the frequency tuning characteristics of the laser is reduced from 11.93 GHz to 0.03 GHz,which reduces by three orders of magnitude.The spatial resolution of the MPC system after correction is 3.5 mm under the optical path difference of 3144.9 mm,which is at least three orders of magnitude higher than that of the uncorrected one.The software resampling method does not need to change the hardware conditions such as the laser injection current,and the auxiliary interferometer directly performs the software resampling correction on the interference beat signal generated by the measurement path.Before and after correction,the linear fitting residual of frequency tuning characteristics of the laser was reduced from 11.75 GHz to 7.21×10-4 GHz,which increased by five orders of magnitude.The spatial resolution of MPC system is 4.0 mm when the optical path difference is 7630.4 mm,which is at least four orders of magnitude higher than that of uncorrected system.The noise level of software resampling method is-80 dB,which is two orders of magnitude lower than that of hardware feedback method.Secondly,the measurement method of heterodyne interference forward MPC system using FMCW and feedback correction method is proposed for the first time.Because the feedback correction method is simpler than the software resampling method,and can satisfy the need of regulation for larger interference beat frequency.The software and hardware system of on-line real-time visualization measurement and regulation of the MPC system is built by using the feedback correction method,and the effective on-line visualization measurement and regulation of the MPC system is realized.The multipath reflection exists at4.75 m at an optical path difference of 8.45 m.The formed beat frequency interference,after adjusting the MPC's fine adjustment knob,removes the interference peak at 4.75 m.Thirdly,in order to further improve the spatial measurement resolution,the optical frequency domain tuning?OFDR?is used as the light source for the laser frequency tuning bandwidth.The frequency tuning range of the laser increases from GHz to THz,and the forward emission high resolution measurement of the gas pool is realized for the first time by resampling.Before and after correction,the linear fitting residual of frequency tuning characteristics of the laser is reduced from 15.11 GHz to 1.97×10-4 GHz,which reduces by five orders of magnitude.The spatial resolution of the laser is increased from 45.434 mm to0.071 mm,which is 640 times higher.At the actual optical path position of 1575 mm in the MPC,the spatial resolution of 0.152 mm obtained by forward measurement is more than 2times higher than that of 0.427 mm obtained by OFDR backward measurement,which realizes the high resolution measurement of the MPC.The MPC is adjusted several times,and the effectiveness of the adjustment is verified by the method of measuring gas concentration by wavelength modulated absorption spectroscopy. |
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