Study On Acquisitions And Applications Of Phase Conjugate Beams Based On Digital Holography

Coherent light propagates through the media with the refractive index inhomogeneities,such as the biological tissues,which causes severe wavefront distortion.Therefore,modulating wavefront of the beam and achieving accurate focusing and imaging through biological tissues are of great significance in the field of biomedicine research.The digital optical phase conjugation(DOPC)technique is one of the digital wavefront shaping techniques,which is developed from the optical phase conjugation technique based on traditional optical holography.In recent years,the DOPC technique has attracted a great attention in biomedicine,because it could effectively overcome the wavefront distortion caused by biological tissues and multimode fibers.The DOPC technique is based on the time-reversal principle to overcome the wavefront distortion,which needs two steps:wavefront information of an object beam is acquired based on digital holography and the phase conjugate beam of the original object beam is generated by using the spatial light modulator(SLM).However,the two steps are performed by two distinct devices,which makes significant challenges in the implementation of the DOPC technique,thus limiting its broader application.In addition,the reconstruction of three-dimensional targets and structured light beams through scattering media has not been experimentally demonstrated based on DOPC technique.Considering the above mentioned problems,this thesis develops the study on acquisitions and applications of phase conjugate beams based on digital holography,the main completed works are as follows:(1)According to the laws of geometric optics,the geometrical relationship of the two output beams from the trapezoid Sagnac interferometer is analyzed.On this basis,the lateral shearing common-path digital holographic microscopy imaging system is proposed.Firstly,it is demonstrated that the system shows the high stability and adjustable feature of the off-angle between the object and reference beams.Then,the phase imaging of static and dynamic specimens is performed.After that,based on the trapezoid Sagnac interferometer,the common-path digital holographic interference system is constructed to calibrate the nonlinear phase response of the SLM.And a method is proposed to eliminate phase measurement error caused by the SLM in the working state,thus making the calibrated SLM with the high-accuracy phase modulation.(2)The challenging alignments in the implementation of the DOPC technique are analyzed and the experimental system with a trapezoid Sagnac-like ring feature is designed and constructed to effectively address the pixel matching difficulties in DOPC technique.Firstly,the complex amplitude information of the speckle beam is acquired based on digital holography and the calibrated SLM is used to generate the corresponding phase conjugate beam.Then,light focusing and focal spots scanning through a multimode fiber are demonstrated.At last,to improve the imaging speed through the multimode fibers,a DOPC system for the achievement of wide-field imaging is designed,and the reconstruction of two-dimensional intensity images through a multimode fiber is achieved by recording a single off-axis hologram?(3)It is proposed that the rotational memory effect of the multimode fibers is combined with the DOPC technique.And the range of memory effect affected by the cylindrical symmetry of the multimode fibers itself is analyzed.Employing the designed DOPC system,as a Gaussian beam is coupled deviated from the fiber core axis,digitally rotating the acquired phase conjugate pattern displayed on the SLM around the fiber core axis,rotational scanning through a multimode fiber within a certain rotation angle is achieved by recording a single off-axis hologram.On this basis,the rotational memory effect is combined with the linear superposition principle and the simultaneous generation of two focal spots is achieved by linearly superposing the two acquired phase conjugate patterns.(4)The method for generating structured light beams based on technique is proposed.Firstly,the DOPC system is designed and constructed to generate structured light beams coupled into a multimode fiber.Then,as only the phase of the original scattered beam is time reversed,the reconstruction of a quasi-Bessel beam and vortex beams through a multimode fiber is demonstrated.Moreover,it is demonstrated that he reconstruction results of DOPC are indeed phase conjugate to the original targets.At last,taking advantage of the cylindrical symmetry of multimode fibers,a numerical filtering operator by applying a circular symmetry mask built in the Fourier domain of the acquired speckle beam is proposed.By applying the amplitude and phase annular filtering,the generation of the quasi-Bessel beam and annular beam is achieved at the output facet of the multimode fiber.