Research On High-sensitivity Dual-comb Measurement And Application Based On Electro-optic Frequency Comb

Optical frequency combs(OFCs)provide ideal light sources for precision measurement.Dual-comb technology,using two asynchronous OFCs to generate multiheterodyne beatnotes with a single-point detector or array detection devices,enables high temporal and spectral resolution in broadband molecular spectroscopy,precision ranging and high dimensional holographic imaging.These measuring methods based on dual-comb technology can further provide significant tools for basic disciplines such as molecular physics as well as application fields including infrared remote sensing,chemical analysis,biomedical imaging,environmental monitoring,etc.For example,in the molecular spectroscopy,dual-comb spectroscopy(DCS)technology without mechanical scanning and dispersion devices,has shown the benefits of broad spectrum,high resolution,high precision and rapid measurement,providing an effective approach for sub-Doppler molecular spectroscopy and transient spectrum analysis.While in ranging,the dual-comb ranging technology provides a new ranging platform which combines the large dynamic range and high refresh rate of time-of-flight(TOF)method,and high resolution and precision(nanometer scale)of laser interferometry method.However,some factors like the average power of the comb teeth,the coherence of the OFC,the optical detection background and the broadband noise have affected the signal-to-noise ratio and sensitivity of the dual-comb technology,and further limits its applications in trace gas analysis,remote telemetry,industrial gas monitoring and other important fields.In addition,the dual-comb system based on mode-locked laser is not conducive to develop applications,because of its complexity,high cost and sensitivity to environmental disturbance.To solve the above problems,this paper explores new mechanisms and methods of high resolution,high precision and high sensitivity dual-comb measurement,which are demonstrated respectively in OFC source(coherence and phase noise suppression),molecular resonance absorption(infrared molecular vibration transition)and sensitive detection(photoacoustic detection and single-photon detection).Moreover,the dualcomb system based on electro-optic modulation is improved to reduce its complexity,and is applied in gas partial pressure measurement,infrared spectrum analysis,trace analysis and single-photon ranging.The details and innovations are summarized as follows:1.The electro-optic frequency comb(EOFC)based on single modulator was established with low noise and frequency agility,with 4800 narrow linewidth teeth in frequency domain and 260 fs pulse width in time domain,which was close to the Fourier limit.Its central wavelength(1520 nm-1610 nm)and repetition frequency(9khz-1ghz)could be adjusted in a flexible and fast way.A feed-forward control scheme based on an acousto-optic frequency shifter was used to compensate for the phase noise with broadband noise suppression ratio over 20 d B.The system could be employed as coherent optical source for the research on high-SNR dual-comb measurement technology.2.A scheme of gas absorption spectrum measurement with high sensitivity and high resolution was proposed based on electro-optic dual-comb system.A novel spectral interpolation method based on feed-forward self-stabilized frequency technique was developed(5.45 THz/s).Combined with nonlinear optical difference frequency technique and DCS method,the whole system can realize the mid-infrared dopplerlimited spectroscopy with the resolution of 54.5 MHz or 2 pm,and with high sensitivity which was nearly 2 orders of magnitude higher than that of near-infrared spectrum.By using multi-peak fitting and pressure inversion method,the noninvasive,high-precision and high-resolution gas partial pressure measurement was studied realized.3.The concept of linear and nonlinear photoacoustic spectroscopy(PAS)based on photoacoustic effect and quartz resonance enhancement effect was proposed.And the combination mechanism of nonlinear saturation absorption effect,photoacoustic effect and OFC spectroscopy method was employed to break through the limitation of PAS's resolution caused by molecular Doppler effect.Herein,the spectrum resolution reached 10 MHz,and the rapid measurement of uniform collision broadening spectrum line(line width ?45.8 MHz)of rarefied gas molecules without sweeping frequency was experimentally achieved.4.Combining the resonance effect of membrane and cavity enhancement detection method,an ultra-sensitive and wide-band dual-comb PAS rapid detection technique was developed,which effectively expanded the acoustic resonance detection bandwidth(1.4 MHz),made up for the limitation of the dual-comb spectrum width and measurement time due to the bandwidth of microphone(?k Hz)or tuning fork(?Hz).The whole system realized PAS measurement with comb-tooth resolution in the order of microseconds and the ultra-sensitive and fast detection of hydrocarbon gas molecule(1.2 ppb),providing a new way for trace analysis.5.A new sensitive measurement method of single-photon level based on dual-comb was explored.By combining correlated single-photon counting technique and asynchronous optical sampling method,the limitation of time-of-flight ranging accuracy caused by the detector response time and pulse width could be broken through.The system realized high-accuracy ranging with dual-comb TOF and interferometry method under the condition of weak light.The ranging accuracy could reach 2 ?m under the distance of 15.6m.