Optical Spatial Filtering And Digital Holographic Imaging Based On Axicon Phase Transformation

Axicons are a type of axis symmetric optical elements that can deflect the light from point sources on the axis to form a long,narrow focal line along the optical axial instead of the usual point focus.Bessel beam is the most widely studied and applied non-diffracted beam in non-diffracted light field,and the axicon has the characteristic of axial symmetry radial linear phase transformation,based on which the zero-order Bessel beam can be generated.Other higher order Bessel beams can be generated by adding other different phase modulation by means of computer-generated holography and spatial light modulators or diffraction optical elements.Bessel beam based on the phase transformation characteristic of axicon has important applications in the fields of autocollimator,laser drilling,laser plasma generation,optical tweezers and optical imaging.The axicon is used in optical imaging system to improve the focal depth and resolution of the imaging system by making use of the non-diffraction characteristics of the zero-order Bessel beam.In this paper,based on the zero-order Bessel beam generated by the axicon phase transformation,the influence of the axicon phase transformation on the spatial spectrum distribution and digital holographic imaging is studied and analyzedIn this paper,the method and application of the non-diffraction Bessel beam generated by the axicon are studied.The main contents and innovations of the study are as followsFirstly,the zero-order Bessel beam and its axicon-lens combination transformation are generated based on the axicon phase transformation.On the basis of introducing the theory of non-diffraction beam,the generation mechanism of zero-order Bessel beam is physically explained in terms of multi-beam interference.A phase type spatial light modulator is used to realize the phase transformation of an axicon and to generate a zero-order Bessel beam.The beam transformation characteristics of the expanded beam collimator laser beam after passing through the axicon-lens combination system are studied.The expanded beam collimator laser beam forms a ring beam after passing through the axicon-lens combination system.The radius and width of the ring beam can be adjusted by changing the focal length of the lens or the Angle of the axicon.Numerical simulation and experimental results of zero-order Bessel beam and ring beam are given.Secondly,optical space filtering based on axicon phase transformation.In the traditional 4f optical spatial filtering system,the axicon phase transform is introduced,the axicon phase transform characteristic and the lens Fourier transform characteristic are combined,the influence of the axicon phase transform on the spatial spectrum distribution and the filtering imaging is studied,and a new optical spatial filtering system is proposed.Different from the traditional optical spatial filtering system,which produces the symmetric spectrum distribution with zero frequency as the center,the symmetric spectrum distribution with circular line and circle is obtained by introducing the phase distribution of the axicon on the spectrum surface,and the zero frequency component is located on the circular line and circle.Therefore,the structure and method of the filter needed to realize the filtering is also different from the traditional optical space filtering.For low-pass filtering,high-pass filtering,band-pass filtering,area filtering,and other forms of filtering operations,the structure of the filters used in this system is different from that in traditional optical spatial filtering systems,and the corresponding output images obtained are also different.In addition,due to the non-diffraction characteristics of Bessel beam,compared with the ordinary optical 4f imaging system,the output image obtained by the axicon-lens system not only increases the depth of field of the system,but also improves the resolution of the system.Thirdly,digital holographic imaging based on axicon phase transformation.Based on the non-diffraction characteristics of Bessel beam,the phase transformation of an axicon is introduced in the process of recording and reproducing digital holography,and the effect of Bessel beam generated by an axicon on the depth of field of the reconstructed image is studied:(1)The phase transformation of the axicon is applied to the hologram in the reconstruction process;(2)In the recording process,the axicon phase transformation is applied to the object light wave,while in the reconstruction process,the axicon phase transformation is applied to the hologram.Based on the above two cases,it can be found that the linear focusing characteristic of the axicon improves the depth of field of digital holographic reconstruction.In this paper,the spectrum structure of the optical space filtering system is obtained by introducing the phase transformation of the axicon,which is different from the traditional optical space filtering system.It is proved that this method can improve the depth of field of digital holographic reconstruction by introducing the phase transformation characteristic of axicon in the process of digital holographic recording and reconstruction.Further research can enrich the optical information processing methods and expand the applications of the Axicons.