Research On The Application Of Optical-optical Modulated Dual-comb Spectroscopy In Laser Diagnostics

The diagnostics of high temperature flow field enables experimental multi-parameter measurements with high performances in thermodynamic flow fields.These measurements not only assist in some scientific researches on heat transfer,aerodynamics,chemical kinetics and other physical and chemical fileds in combustion process,but also serve as validated parameters for a high-fidelity simulation of the real high temperature flow field.As such,many application fields such as electrical power generation,industrial manufacturing and propulsion engineering have been developed by the diagnostics.Currently,non-intrusive laser-based technology is a powerful tool for the diagnostics of high temperature flow field.In particular,through precise measurements of molecular absorption lines with a tunable diode laser or Fourier transform spectrometer,precision measurements of gas species,temperature and flow velocity can be achieved by laser absorption spectroscopy.However,there exists a trade-off between the optical bandwidth,spectral resolution and refresh rate in present spectroscopies,which restricts the measuring performance and applicable system of laser diagnostics.Dual-comb spectroscopy(DCS),as a novel laser absorption spectroscopy,enables a precise spectral measurement with a broad spectral bandwidth over dozens of THz,high spectral resolution of MHz level and short temporal resolution on the scale of microseconds.This indicates that the DCS has a high potential to realize a diagnosis of high temperature flow field.Therefore,this thesis focus on a research on the diagnosis of high temperature flow field based on dual-comb absorption spectroscopy.Near-and mid-infrared broadband passive-coherent dual-comb spectrometers with high spectral performances are studied and achieved firstly.Researches on the sensitive sensing of gas species,precise measurements of gas velocity and temperature based on molecular absorption principle are then demonstrated and performed,respectively.These can provide theoretical models and experimental demonstrations for a practical combustion diagnosis based on dual-comb absorption spectroscopy.The details and innovations are summarized as follows:1.Near-and mid-infrared broadband passive-coherent dual-comb spectrometers are achieved.Firstly,as an optical-optical modulated technique is combined with the nonlinear spectral broadening and precise dispersion compensation,a broadband pulse laser that has an average power exceeded 150 m W,repetition frequency of 108 MHz,spectral coverage ranging from 1200 nm to 1800 nm,pulse duration of 15.7 fs(3optical cycles)is achieved.Besides,the transport processes of optical pulse in the optical parametric amplification,polarization-maintaining fiber and highly nonlinear optical fiber are simulated in time-frequency domain,respectively.Based on the above broadband laser,a near-infrared dual-comb spectrometer with an adjustable measuring speed from 1 Hz to 4.8 k Hz,spectral coverage ranging from 1500 nm to1600 nm,spectral resolution of 108 MHz is realized.When an adaptive sampling technique is introduced in the spectrometer,a comb-tooth-resolved dual-comb spectrum can be obtained.Furthermore,a mid-infrared dual-comb spectrometer with an adjustable measuring speed from 1 Hz to 50 k Hz,spectral coverage ranging from4000 nm to 4250 nm,spectral resolution of 160 MHz is realized for the dual-comb thermometry,which is loaded on a platform with a size of 120 cm×60 cm×100 cm(length×width×height).2.The researches on sensitive sensing of gas species in DCS are performed.As interference-suppression technique and spectrum-encoded technique are introduced into DCS for the the first time,respectively,detection sensitivity of gas species in DCS has been improved.The method of optical interference suppression is used in dual-comb spectroscopy to obtain a background-free dual-comb molecular spectrum.At the same response power of the detector,the maximum intensity of absorption lines in the background-free spectrum is 123.62 which is improved 46.8 times compared to that of 2.64 in conventional dual-comb spectrum.In addition,a spectrum-encoded technique is introduced into dual-comb spectroscopy and a parallel sensitive sensing of multiple molecules(C₂H₂?HCN?CO?CO₂)is presented.Compared to conventional dual-comb spectroscopy,the measurement speed is increased from 778 Hz to 4.8 k Hz(corresponding to a measurement time of 208?s)and the short-term signal-to-noise ratio(SNR)is improved by a factor of 5.68 while a broadband spectral coverage ranging from 1520 nm to 1580 nm is reserved.3.The researches on the diagnostics of high temperature flow field based on DCS are performed.It is the first time that a Doppler velocimeter based on DCS is demonstrated.Besides,a high temperature measurement is achieved by the DCS.Since the uncertainty on a fitting absorption line center can be quantified by the SNR of the line,a model for multi-line Doppler frequency determination in broadband DCS is proposed and validated experimentally.The flow field of C₂H₂ gas with flow velocity of 44.87 m/s is measured successfully and the measuring uncertainty is 0.67m/s in 1 s measurement time.The measuring uncertainty is improved with an increasing measuring time and the uncertainty reaches 0.19 m/s in 15 s.Furthermore,a continuous dynamic monitoring of the flow velocity with a measuring uncertainty down to the submeter per second over the range from 8.7 m/s to 44.8 m/s at a measuring speed of 1 Hz is demonstrated by the DCS.In addition,CO₂ molecule in a standard high-temperature gas cell is measured by a mid-infrared dual-comb spectrometer at nearly 4.2?m and absorption lines of high-temperature CO₂ molecule are obtained.After measured absorption lines in the region of?₃ band R-branch band head are fitted successfully by simulated absorption lines with adjusted lineshape parameters,temperature measurements in the conditions at a low pressure of 100mbar and atmospheric pressure with a high temperature ranging from 765 K to 1259K are achieved with a measuring accuracy below 1.04%by the DCS.