Research On The Propagation Characteristics Of Femtosecond Laser Pulses In Random Access Two-photon Microscope

The function of the neuron circuit of the brain is a substantial research area in the lifescience.However,this research is restricted by the fact that the fast activities of large scaleneuron network could not be observed by existing tools.The random access two-photonmicroscopy has favorable advantages such as high spatial and temporal resolution,deepimaging depth and in vivo imaging function,thus is possible for this microscopy to providea new observation tool for the above area.The key feature of the random access two-photon microscopy is fast controlling thedeflection angle of femtosecond laser pulse via the inertial free acousto-optic scanningmethod.However,compensation of the dispersion induced by the acousto-optic deflector(AOD) is very difficult for this method.The propagation characteristics of the femtosecondlaser after angular dispersion is the theoretical basis of this issue,yet traditional theory onthe propagation of the femtosecond laser is no longer adequate for this newly evolved area,which substantially limits the improvement of the random access two-photon microscopyon the spatial and temporal resolution for the fast,accurate observation of the large scaleneuron network.In this paper,the propagation characteristics of the femtosecond laser in random accesstwo-photon microscope was systematically studied and solved theoretically.The couplingbetween the spatial dispersion and temporal dispersion is discovered,which is thetheoretical basis of the new dispersion compensation approach.In which the spatial andtemporal dispersion of AOD are simultaneously compensated by a single angular disperser,and this work facilitated the construction of the random access two-photon microscopy andits application to the neurology research:The beam spot broadening of the femtosecond laser after angular dispersion waselaborated,the spot size expression at arbitrary propagation distance was deduced,thebeam spot broadening of the femtosecond laser after AOD was precisely predicted and thephysical mechanism of beam spot broadening was revealed.The pulse width broadening of the femtosecond laser after angular dispersion wasdescribed,the pulse width expression at arbitrary propagation distance was provided,the pulse width broadening of the femtosecond laser by AOD was precisely predicted,thegeneral theory on pulse width broadening of plane wave,spherical wave and Gaussianbeam was deduced and the physical mechanism of pulse width broadening based on eachabove light beam model was revealed.The induced temporal chirp of femtosecond laser pulse when passing through a classicalpulse stretcher/compressor was analyzed,the essential cause of the inducing of differenttemporal chirp between Gaussian beam and plane wave was analyzed and the propagationcharacteristics of temporal chirp was verified by using two dimensional spectralinterferometry.The feature of the propagation of the spatial chirp and pulse front tilt wasprovided.A device for femtosecond laser pulse measurement was established.The intensityand phase of the pulse were quantificationally measured,and the spatial chirp and pulsefront tilt induced by angular dispersion were qualitatively measured.The principle and method of compensating the dispersion of the random accesstwo-photon microscope was studied.The coupling between the spatial dispersion andtemporal dispersion is discovered,and provided the theory basis for the new idea ofsimultaneously compensating the spatial and temporal dispersion of AOD by a singleangular disperser.Base on the Gaussian beam model,the temporal dispersion was preciselycompensated.This principle and method have now been adopted by some researchers.