Femtosecond Laser Beam Shaping Based On Compute-Generated Holography

Laser beam shaping is the process of redistributing the irradiance and phase of a laser beam of optical radiation.Laser beam shaping has become a hotspot in the field of optics.With the development of the spatial light modulator,laser beam shaping is no longer limited to a fixed optical parameter design,but can modulate the light field to arbitrary pattern dynamically via the computer-generated hologram(CGH).Otherwise,femtosecond laser is widely used in material processing,information storage,and biomedical imaging benefit from its high peak power,low hot effect and nonlinear effects,which depends on beam shaping.However,femtosecond laser is not strictly monochromatic and has a typical bandwidth of 10 nm,which will introduce dispersion when femtosecond laser diffracted by a spatial light modulator,and deteriorate the effect of beam shaping.This issue is more obvious in femtosecond laser two-photon microimaging.Usually,researchers use prisms or gratings to compensate the dispersion caused by the gratings.The dispersion caused by hologram is not solved so far.In this thesis,we research the dispersion law of femtosecond laser diffracted by the spatial light modulator,and propose a method of high-resolution femtosecond laser beam shaping of arbitrary patterns based on CGH,which is applied to the field of biomedical microimaging.The main works are summarized as follows.(1)The dispersion law of femtosecond laser diffracted by the spatial light modulator is analyzed,which is caused by the inherent two-dimensional grating structure of the spatial light modulator and the hologram,and the hologram can be regarded as the superposition of grating components with different periods and directions.(2)Base on the analysis of the dispersion law of femtosencond laser diffracted by the spatial light modulator,we introduce the Keplerian Dispersion Compensation Module(KDCM)into holography to compensate all the grating components of the hologram simultaneously.So that the dispersion of the hologram is eliminated,and the axial dispersion is compensated to a certain extent at the same time.(3)We develop the method of high-precision femtosecond laser beam shaping.The CGH of arbitrary pattern is generated by Gerchberg-Saxton algorithm.The dispersion caused by the grating structure of the spatial light modulator is compensated by a grating.The dispersion caused by the CGH loaded on the spatial light modulator is compensated by the KDCM.The high-precision and achromatic laser beam shaping of arbitrary pattern is realized.Experimental results confirm that the proposed method can be used to shape femtosecond laser beams into arbitrary patterns in the whole field of view with the resolution near the optical diffraction limit,and the applicable spectral bandwidth is about 200 nm.(4)Appling the method of femtosecond laser beam shaping to bioimaging,we developed a set of three-dimensional random scanning two-photon microscope,which realizes a fast noninertia three-dimensional random scanning with the speed of 19.23 kHz.Using the water immersion objective with magnification of 40 X and numerical aperture of 0.8,the transverse resolution of the microscope is 0.75 ?m,and the axial resolution is 3.22 ?m.In summary,the proposed method in this thesis can achieve high-resolution femtosecond laser beam shaping,and has potential application in many fields such as biomedical microimaging.