The Generation And Applications Of The High-resolution Dual-comb System

The dual-comb system,developed from the optical frequency comb?OFC?technology,uses two combs with slightly different repetition rates to realize the optical heterodyne detection.The dual-comb system has become a research hotspot with serval attractive advantages of high speed,high resolution and high accuracy,and shows great potential in applications of precision spectroscopy,optical ranging and optical imaging in recent years.The high-resolution measurements rely on highly coherent dual-comb sources.Nowadays,using high-speed modulators to control the two degrees of freedom in mode-locked fiber lasers is the main method to optimize the coherence.On the one hand,it is the basis of the stabilized dual-comb system to study the noise characteristics of mode-locked lasers and acquire low-noise femtosecond pulses.On the other hand,developing precise phase locking approaches is important for the suppression of system noise and the generation of mutually coherent dual-comb sources.Moreover,developing the compact and integrated system is the key to improving the practicability and expanding the applications of dual-comb technology.This paper focuses on the generation and applications of the high-resolution dual-comb system and discusses the low-noise mode-locked fiber laser,the precise phase locking approach and the dual-comb system.Based on the homemade all-polarization-maintaining mode-locked fiber laser,the noise characteristics have been analyzed in both theory and experiment.The influences on noise of the pulse parameters,including pulse energy,pulse duration and nonlinearity,have been proved and low-noise pulses are obtained by optimizing the pump energy eventually.The phase locking of OFCs is discussed and the practical OFC generated from an all-PM-fiber laser with a tunable repetition rate is demonstrated.Moreover,an effective phase locking approach with high bandwidth based on electro-optic modulators is demonstrated and the ultralow-noise OFC is realized by the optical reference.At last,the comb-resolved dual-comb source with high resolution is developed,based on the relative phase locking with the narrow linewidth continuous wave laser.The molecular absorption spectroscopy and the 3-D imaging are realized then by applying the dual-comb source in another detection system.The details and innovations are summarized as follows:1.The Pump-PN balancing mechanism and an all-polarization-maintaining mode-locked fiber laser with low noise are demonstrated.The simple and robust all-fiber ring laser,mode-locked by SESAM with a new cavity structure,has the excellent ability to resist environmental disturbances.The output pulses have a repetition rate of101 MHz with a 12.4-nm spectrum centered at 1560 nm.The noise evolution of the mode-locked laser is analyzed in both theory and experiment to prove the relationship between the noise characteristics and the pulse parameters in our laser.Finally,the timing jitter of the mode-locked fiber laser is suppressed to a low level of 269 fs?from10 Hz to 10 MHz?by optimizing the pump energy.2.A programmed adjustment scheme of cavity length is designed for the scanning of the repetition rate and an all-fiber optical frequency comb with a tunable repetition rate is realized.By inserting an electrically-controlled fiber delay line in the homemade low-noise fiber laser,a tunable range of 342 kHz?at a repetition rate of 101MHz?is achieved.The repetition rate f_rep and the carrier-envelope offset frequency f_ceoeo are both stabilized with another all-fiber amplifier and two phase-locked loops,and a compact OFC system with practicality is obtained then by the integrated package design.The standard deviations of the locked f_rep and f_ceo are only 165?Hz and 1.1 mHz.The stabilized f_rep and f_ceo also show the fractional instability of 1.49×10^-12 and 5.74×10^-18at a 1-s averaging time respectively,which is limited by the noise characteristics of the reference signals from the Rb clock,and the relative residual phase noise are determined to be 336?rad and 713 mrad?integrated from 1 Hz to 1 MHz?.3.A comprehensive noise suppression technology for optical frequency comb is proposed.By utilizing electro-optic modulators with high feedback bandwidth as fast actuators of the oscillator and improving the cavity design,the bandwidth of amplitude and phase modulations in the fiber laser is extended to more than 150 kHz.The amplitude modulation is employed for locking carrier-envelope offset frequency f_ceo,while the phase modulation is for locking the optical beat signal f_beat.Then,f_beat and f_ceo are both well controlled for the generation of the ultralow-noise OFC:the fully stabilized f_beat and f_ceo show frequency offsets with the standard deviations of only 206?Hz and 265?Hz?in the continuous locking over 20 hours?,and show the fractional instability of 6.31×10^-19 and 9.15×10^-19 at a 1-s averaging time.Meanwhile,the residual phase noises of f_beat and f_ceo,integrated from 1 Hz to 1.5 MHz,are respectively suppressed to 21.8 mrad and 86.1 mrad,corresponding the timing jitter of 18.1 as and71.3 as.4.We build the comb-resolved dual-comb source with high resolution.The optical beat signals f_beat and carrier-envelope offset frequencies f_ceo of two mode-locked fiber lasers are fully stabilized with the same references by high-bandwidth phase locking.Their residual phase noises are all reduced below 100 mrad and the high coherence of the dual-comb source is obtained.The two combs have well-controlled repetition rates of 100150315 Hz and 100150175 Hz.The minimum relative linewidth?limited by the computation system?of 120 mHz is also demonstrated,corresponding to a spectral resolution of 85.7 kHz.5.The dual-comb detection system with high resolution and high accuracy is constructed.By applying a reference arm with the signal arm,the spectral analysis and the absolute distance caculation can be simultaneously realized by analyzing the interferograms of the system.In our following work,the gas analysis in the transmission path and the 3-D imaging of the sample are achived by another 2-D scanning with the acquisition time of 0.5 s on each spot.The accurate absorption specrum of CO₂ and CO is obtained with the spectral resolution of 1.43 MHz and the SNR of 2230,while the 3-D imaging shows a horiziontal?vertical?resolution of 40?m with a depth precision of 0.68?m.