The Method And Technique Of Time-resolved Coherent Anti-Stokes Raman Scattering Based On The Supercontinuum

As a spectroscopic analyzing method based on the inherent molecular vibrations, the coherent anti-Stokes Raman scattering (CARS) spectroscopic and microscopic imaging technique is a noninvasive spectroscopic and microscopic imaging method with high temporal and spatial resolution, high sensitivity, high chemical specificity and 3D sectioning imaging capability.In order to quickly and effectively distinguish the various components of samples with different kinds of components and unknown components, and real-time monitor the change of environment and the interaction courses between different molecules based on the molecular vibrational spectrum, the capability of quickly obtaining the whole molecular vibrational spectrum with high spectral resolution is necessary with the CARS spectrography. In this dissertation, we present an ultra-broadband time-resolved CARS (T-CARS) spectrography based on the supercontinuum. In the ultra-broadband T-CARS, the output of one locked-mode femtosecond laser is split into two beams. One beam of femtosecond laser pulses is introduced into a photonic crystal fiber (PCF) to generate the supercontinuum used as the pump and Stokes simultaneously. The other is used as the probe after passing through a narrow-band-pass filter. The whole molecular vibrational spectrum can be simultaneously obtained. The nonresonant background (NRB) noise accompanying with resonant CARS signals can be effectively suppressed by changing the delay time between pulses of the probe and supercontinuum. The sensitivity and spectral resolution can be improved. Otherwise, the relaxation courses of all molecular vibrational modes in the simultaneously detectable spectral coverage can be recorded by the time-resolved method. The dephasing time of each molecular vibrational mode can be obtained by numerical fitting.In this dissertation, the main completed research work has been shown below.(1) Based on the extensive research, the developments and present state of the CARS spectroscopic and microscopic imaging technique are introduced.(2) We studied the mechanism of CARS process. The CARS course, in which three beams of laser pulse with different frequencies are used, is analyzed by using half-classic theoretical analysis. The methods of suppressing the NRB noise are presented and analyzed in detail. The T-CARS is studied by the numerical simulation method. The physical explanation is given for the suppressing NRB noise by the time-resolved method. The feasibilities are theoretically analyzed for achieving an ultra-broadband T-CARS spectrography.(3) In order to simultaneously obtain the whole molecular vibrational spectrum, besides an enough broad bandwidth, the spectral continuity, simultaneity, uniformity and stability of various spectral components in superconyinuum are all necessary. The theoretical analysis and numerical simulation are carried out for obtaining the experimental conditions to improve the supercontinuum. The main optical parameters of existing PCF, such as the dispersion curve, zero dispersion wavelength, are obtained by the finite element method. The distributions of the supercontinuum in the frequency and time domain are explored under various conditions with the split step Fourier method. And the temporal distributions of various spectral components in supercontinuum are analyzed with the cross-correlation frequency-resolved optical gating method. The experimental conditions for improving supercontinuum are obtained with numerical simulation. It is the guidance for achieving the required supercontinuum source.(4) Based on the theoretical analysis and numerical simulation, an ultra-broadband T-C ARS spectrometer system is successfully achieved. The main parameters of system are calibrated by measuring some samples with the known Raman spectrum, such as the spectral resolution, simultaneously detectable spectral range. The molecular vibrational spectrums in 387-4092cm-1 of a variety of organic samples are obtained by experiments. The relationships between the intensity of CARS signals and pump laser are studied. The simultaneous measurement of the dephasing time of various molecular vibrational modes is carried out.The main innovations are:(1) The experimental conditions for improving supercontinuum are obtained with numerical simulation. The supercontinuum laser source is achieved with the good spectral continuity, simultaneity, uniformity and stability. The requirements on the supercontinuum laser source of the ultra-broadband T-C ARS spectrography are met.(2) The ultra-broadband T-CARS spectrography based on the supercontinuum is achieved. The organic molecular vibrational spectrums with high spectral resolution can be simultaneously obtained in 387-4092cm-1. It has the capabilities of quickly and effectively distinguishing different components.(3) The relaxation courses of various molecular vibrational modes in the spectral coverage can be simultaneously measured to obtain the dephasing time. It has the capabilities of real-time observing the molecular interactions and monitoring the change of surrounding environment.