Ultrafast Spectroscopy Based On Femtosecond Time-stretch Technique

Ultrafast spectroscopy is a crucial tool for understanding the substance composition,molecular evolution,and kinetics in not only fundamental sciences of physics,chemistry,and biomedicine,but also applied domains of gas tracing and leakage warning.The emergence of femtosecond lasers has continuously refreshed the detection speed of spectroscopy because of its ultra-fast repetition frequency and ultra-short pulse duration in the time domain.The femtosecond time-stretch technique mapping the recorded transient phenomena into the time domain,reducing the sampling speed requirement in ultra-fast detection.It plays an important role in the fields of ultra-fast imaging,ranging,single-shot pulse spectrum detection,non-repetitive and rare signal detection,etc.Not only that,the femtosecond laser in the frequency domain exhibits the characteristics of a broad spectrum and equally spaced frequency combs,which brings new opportunities for high-resolution spectral analysis.This thesis combines dual-comb spectroscopy and time-stretch techniques to propose ultra-fast and high-precision spectral analysis methods.The main work of the thesis is as follows:1.The optical frequency comb spectroscopy and time-stretch technique are reviewed,especially the development and application of dual-comb spectroscopy,and the application of time-stretch technique in the field of ultrafast spectroscopy.Dual-comb spectroscopy,realized by two optical frequency combs with slightly different comb spacing,has the ability of spectrum down-conversion from optical frequency comb to radio-frequency comb,while it cannot analysis the spectrum with a single pulse.Spectroscopy based on femtosecond time-stretch technique has the capability of real-time detection with a single pulse,but cannot realize comb-resolved analysis.2.By combining the idea of single-cavity dual-frequency comb spectroscopy and time-stretch technique,a femtosecond imbalanced time-stretch spectroscopy is proposed.A single free-running mode-locked femtosecond laser is utilized,which simplifies the dual-comb system without the requirement of complex phase-locked loops.The dispersive elements with different dispersion coefficients are introduced into the two arms of a fiber Mach-Zehnder interferometer,so that the two pulses with different stretched length are interfered in the interferometer.This spectroscopy not only realizes the mapping of spectral information from optical frequency comb to radio-frequency comb,but also has the ability of single-shot detection.The concept is verified by both modeling analysis in Optiwave software and experiment.A detection speed of 250 ?s and a spectral resolution of 540 MHz are obtained in experiment.3.The wideband radio-frequency spectrum in the femtosecond imbalanced time-stretch spectroscopy is modulated to the sideband of a carrier wave,reducing difficulty of data acquisition and processing.And forming a mapping relationship from optical spectrum to electrical spectrum,and then to optical spectrum.A spectral analysis technique based on wide-spectrum mapping is proposed,which can map the spectrum in low-resolution region to high-resolution region.4.Considering the requirement of wide acquisition bandwidth in common multi-heterodyne detection of single-frequency laser and comb.A comb-referenced optical frequency locking and scanning method is provided,demonstrating a programmed and stable tunable light source.A low-bandwidth multi-heterodyne spectroscopy is demonstrated based on the tunable light source,realizing easy data acquisition,real-time storage,and processing.5.A frequency scanning method based on time-stretch and frequency gating is proposed,and its application in the field of remote sensing of atmospheric spectrum is prospected.