Preliminary Study On The Method And Application Of Generating Non-diffraction Speckles With Axicon Phase Random Modulation

Non-diffraction field includes non-diffraction beams and non-diffraction speckles,which is characterized by its unique non-diffraction property with propagating in free space.As a member of the family of non-diffraction beams,non-diffraction Bessel beam were first proposed,and have been explored in a wide range of research and application fields.It has important research value and application value in imaging,detection and other fields.The non-diffraction speckles developed on the basis of non-diffraction Bessel beam has not only non-diffracting characteristics,but also random characteristics of speckles,which has better characteristics than ordinary speckles.Because of the unique properties of non-diffraction speckles,it has great prospects and value in the fields of ghost imaging,information encryption and non-destructive testing.In this paper,the method and application of non-diffraction speckles generation are preliminarily studied.The main research contents and innovations are as follows:First,analyzing and summarizing the existing methods with generating non-diffraction speckles in the literature.According to the principles and implementation methods,they are divided into two categories.The second method is to generate a synthetic phase pattern by applying random modulation to the phase function of the axicon with numerical calculation.The synthetic phase pattern is directly displayed on the pure phase spatial light modulator illuminated by collimated beam through optical spatial filtering,and then the non-diffraction speckles is generated,which does not need actual devices and devices to generate actual annular beam,and has flexible adjustment parameters of generating non-diffraction speckles,and improve the utilization of light energy.In this paper,the second method is mainly studied.In the second method,there are two ways to apply random phase modulation to the phase function of the axicon.One is to apply random phase modulation to annular beams through numerical calculation,and then to generate synthetic phase pattern which can generate non-diffraction speckles by Fourier transform.The other is to generate synthetic phase pattern by directly implementing azimuth random modulation the phase function of the axicon,which can generate non-diffraction speckles.Secondly,on the basis of the principle and implementation process of the first kind of the second method,the non-diffracting characteristics and speckle characteristics of the non-diffraction speckles field are studied by numerical simulation,and the influence of the number of random modulation units on the speckle completeness is mainly studied.The principle and implementation process,non-diffraction characteristics and speckle completeness are compared with the second kind of the second method.It is found that:(1)In terms of physical principle,there is no essential difference between the two methods in terms of principle and implementation process.The first kind of the second method is frequency domain modulation,and the second kind of the second method is space domain modulation.The difference is mainly reflected in the realization process of synthesizing phase patterns.(2)In terms of the non-diffracting characteristics of the non-diffraction speckles,the maximum non-diffraction distance and the average size of the speckle particles are determined by the phase gradient parameters of the axicon,the parameters of the spatial light modulator and the wavelength of the illuminating light.Under the same device parameters and wavelength,the speckle fields produced by the two methods have the same non-diffracting characteristics.(3)In the aspect of speckle completeness,the main factor affecting the completeness of the first kind of the second method is the number of random phase modulation units.The more the number of random phase modulation units,the better the completeness of speckle field.The main factor affecting the completeness of the second method is the number of azimuth subdivision.The more the number of azimuth subdivision,the better the completeness of the speckle field.The number of random phase modulation units on the spectral plane is equivalent to the number of azimuth subdivision in spatial domain up to a point.Thirdly,in experiment.(1)the first method is studied by using liquid crystal phase-only spatial light modulator.The experimental results validate the analysis and conclusion in numerical simulation.(2)A preliminary experimental study on non-diffraction speckles generated by phase-only modulation with using DMD is carried out.Fourthly,in the application of non-diffraction speckles,non-diffraction speckles are proposed to be used in digital holographic ghost imaging,and a preliminary experimental study is carried out.A comparison between Gauss speckle and non-diffraction speckles in digital holographic ghost imaging is made.The preliminary results show that,due to the non-diffracting characteristics,the non-diffraction speckles digital holographic ghost imaging improves the resolution and depth of field.The research content of this paper has a certain reference value for the generation,evaluation and application of non-diffraction speckles.