Research On Key Technologies Of Electro-optic Frequency Comb Based Dual-comb Interferometer

Optical frequency combs(OFCs)provide equidistantly coherent frequency comb teeth in broad bandwidth,and each comb tooth corresponds to an absolute optical fre-quency,which have become important sources in optical metrology and measurement technology.Coherent dual-comb interferometers(DCIs)may fully utilize the excellent characteristics of OFCs in terms of frequency accuracy,frequency resolution,spectral range and pulse width,which stand out among OFC-based measurement techniques.In the frequency domain,DCI is multi-heterodyne detection of two OFCs with a small dif-ference in line-spacings;in the time domain,DCI is equivalent time sampling of two pulse sequences with a small difference in repetition periods.In spectroscopy,DCI,known as dual comb spectroscopy(DCS),may achieve high refresh rate not limited by the me-chanical scanning rate and high sensitivity,which has been widely used in atomic and molecular line determination,infrared spectroscopy,Raman spectroscopy and gas com-position monitoring.In time domain,DCI with ultra-high time resolution,also known as linear optical sampling(LOS),has become the foundation of the applications includ-ing ultra-fast optical phenomenon observation,optical signal monitoring,time-frequency transmission,and absolute distance measurement.For more than ten years,various novel OFC sources have gradually developed and matured to be flexible or small-scaled alternatives of femtosecond frequency comb.Among them,the electro-optic modulated frequency combs(EOFCs)provide line-spacing without limitation of cavity length,adjustable center wavelength,and intrinsic mutual coherence in DCI,which attract the attention of researchers.However,existing DCIs based on EOFCs are relatively weak in key performances,and fail to fully exploit the advantages of EOFC in flexibility and cost-effectiveness.Therefore,the research on the EOFC-based DCI have important academic and practical significance.This paper focuses on the research on key technologies of the EOFC-based DCI.The specific research contents contain high-performance EOFC generations,key performance breakthroughs in spectroscopy,and sampling technique explorations.The innovations and main research achievements of this paper may be summarized into the following three points:(1)This paper propose DCIs based on dense EOFC,which broaden the measure-ment bandwidth and increase the number of demodulated comb lines.Based on the anal-ysis of existing EOFC generation methods and performances,this paper propose two DCI schemes that can combine the advantages of over-driven EOFC and period-diven EOFC.First,we propose broadband and dense EOFC based on two-stage modulation,which can adjust the line-spacing with same bandwidth.Based on two-stage modulated EOFC,DCI with flexible resolution and repetition rate relation is realized.The quality factor and refresh rate of high-resolution DCI working in quasi-integer-ratio mode are improved.Combined with nonlinear spectral broadening,totally 150000 EOFC lines(10MHz resolution,1.5 THz bandwidth)are simultaneously demodulated.Secondly,using broadband and equidistant frequency reference provided by over-driven master EOFC,we combine the seed line selected by injection locking technique and high refresh rate DCI to realize wideband and high resolution spectroscopy.With low locking dead time,the proposed method can demodulate 20 channels with total 360000 comb lines(1 MHz resolution,360 GHz bandwidth)in 26 ms,and the quality factor reaches 3.4×10~7.(2)We propose a spectral interleaving DCS based on EOFCs.Different from reduc-ing the line-spacing,the proposed method realizes the spectral interleaving by sweep-ing the probe EOFC to improve the spectral resolution,in which the local EOFC is also sweeping to produce stable dual-comb interferograms.The stabilized seed laser is exter-nal modulated to generate high-linearity fast sweeping lightwave.The proposed method realizes an ultra-high spectral resolution of 20 k Hz in 90 ms measurement time,and record over 2×10~7spectral points in 400 GHz bandwidth under the Nyquist-limited spectral sam-pling rate.We theoretically analyze the limitations and trade-offs of key performances(including measurement time,spectral resolution,and measurement signal-to-noise ra-tio).Then,the proposed method realizes 1.6 ms single-shot measurement time(1 MHz spectral resolution),and 30.6 d B measurement signal-to-noise ratio in 50 ms averaging time(50 MHz spectral resolution).(3)For researches on dual-comb waveform sampling,based on bandwidth compres-sion and frequency mapping,we respectively use the EOFCs as the generalized sampling source and coherent wavelength division multiplexing(WDM)signal carrier.First,using the complex coefficients of the EOFC lines,we recover the intensity and phase waveforms of the signal under test from the interferograms of non-pulse EOFC sampling source.With higher average line power,EOFC as sampling source may improve signal-to-noise ratio and reduce minimum detection power.Then,we use DCI to sample WDM signal based on EOFC carrier,in which each comb line channel containing pseudo-random bit is bandwidth compressed and mapped to different radio frequency.The proposed method simultaneous samples 40-channel 20 Gb/s differential phase shift keying signals,which significantly reduces system complexity.In summary,this paper focuses on EOFC-based DCI techniques,improves the key performance,and proposes low-complexity schemes,which shows great prospects in re-searches and applications.