Laser Beam Shaping Technology Based On Piezoelectric Deformable Mirror

Gaussian beam is output by the conventional laser.With the development of technology,the Gaussian beam is unable to meet requirements of beam with special energy distribution in practical application.The laser beam shaping technology has become a hot research topic.Among many beam shaping techniques,adaptive optics beam shaping technology not only generates modulation phase of beams of different shapes with high precision,but also correct the aberrations of the optical system.This method can effectively improve the output spot quality by beam shaping.Beam shaping technology based on adaptive optics has important development prospects due to its flexible control and high light transmission efficiency.As a crucial component in adaptive optics,electrostatic film deformation mirror,electromagnetic deformation mirror,piezoelectric stack deformation mirror and unimorph/biomorph piezoelectric deformation mirror are common types in adaptive optics system.As a kind of DM,piezoelectric DM is widely used in adaptive optics due to their large strokes,low cost and high damage threshold.In this paper,the annular beam,the line focused beam,the rectangular beam,the zero-order Bessel beam and the Airy beam are used as the target beam.A unimorph piezoelectric DM is used for the beam shaping.The main research work is as follows:(1)A focal spot shaping technique based on adaptive optics.The modulated phase used for generating target focal spot is simulated iteratively by combining the Fresnel diffraction theory and the stochastic parallel gradient descent algorithm(SPGD).Then the modulated phase is reproduced using a deformable mirror with the feedback of the measured wavefront by a wavefront sensor,realizing the shaping of the focal spot.The annular focal spots with different diameters(0.32 mm?0.4 mm?0.6 mm)and widths(0.05 mm?0.08 mm?0.1 mm)were set as the target.After obtaining the modulated phase corresponding to the annular focal spot,a beam shaping system based on a unimorph deformable mirror with 61 actuators was established.The far-field focal spot was measured by a CCD placed on the focal plane of the system.The annular focal spots were generated by the experiment is closely match the target,and the experiment has achieved the desired result.(2)A focal spot shaping technique based on wavefront sensorless adaptive optics.In order to realize the beam shaping experiment more simply and effectively,a closed-loop beam shaping experimental platform based on wavefront sensorless feedback is set up.The experiment uses a SPGD algorithm combined with a 61-element DM,according to the actual spot shape and the shape of the target spot.The error automatically adjusts the voltage value of each actuator on the DM,and realizes the experiment of shaping the incident Gaussian beam into different annular spot and line focused spot and rectangular spot.It is emphasized that the above two methods are not limited to ring spot,line focus.Experiments with rectangular spots are theoretically applicable to other shape spot shaping.(3)Generation of Bessel beams with an unimorph DM.An experiment using a 61-element DM to generate Zero-order Bessel beam.The DM not only corrects the wavefront aberration of the system,but also reduces the residual error RMS of the system to 16 nm,and produces a high-quality conical wavefront with adjustable cone angle.At the same time,when the DM reconstructs the conical wavefront,the effect of the rounded tip of conical wavefront on the effective propagation distance of the Bessel beam is obtained.Finally,the Fourier transform experiment based on Bessel beam with cone angle of 2 mrad~6 mrad is carried out.The measured beam profile of Fourier transform plane is basically consistent with the theoretical result,and the radius of the annular beam is linear with the cone angle.(4)Generation of Airy beam with an adaptive DM.In addition to the long focal depth and self-repairing characteristics,Airy beam has other characteristics such as curve and self-acceleration in the propagation process.An effective method for generating high-quality adjustable Airy beam by using a unimorph DM is proposed.The 32 element unimorph DM used in the experiment not only reconstructs the amplitude-adjustable cubic phase with an error of less than 2.5%,but also corrects the aberration of the system.The residual error RMS of the corrected system is 40 nm.The beam shape and propagation trajectory of the beam are almost identical to the theoretical simulation results.Finally,the cubic phase of different amplitude and rotation angle is generated by experiments.The experimental intensity profiles of the Airy beams coincides with the theoretical results,demonstrating the ability to readily control the angle of the Airy beam by the DM device.