Applications Of Partially Coherent Beams In Quantitative Phase Imaging

In the research fields of biomedical imaging,adaptive optics,and X-ray diffraction optics,the phase information of the object is particularly important.However,human eyes and digital sensing devices,such as camera and CCD,can only record light intensity patterns,and phase cannot be directly measured.Therefore,pure phase objects cannot be directly observed.The common solution is to use fluorescent labeling technology,based on specific labeling and fluorescence excitation to improve the contrast of bright-field imaging,so as to achieve phase observation.However,unable-to-mark may happen in fluorescent labeling technology,and the process of sample preparation,staining,and labeling is likely to cause irreversible damage to the research object,or introduce cytotoxins and genetic modifications that change the function of the protein,thereby affecting the observation conclusion.In comparison,quantitative phase imaging technology can also realize the observation and quantitative measurement of phase objects,and has multiple advantages such as label-free,non-invasive,wider application range and no need for complex preprocessing.Holographic interference,coherent diffraction imaging and transport of intensity equation phase retrieval method are all typical quantitative phase imaging technologies.However,the theories of these existing technologies are originally based on the coherent light source,and the light sources such as X-ray and free electron beam which are commonly used for coherent diffraction imaging,are actually partially coherent light due to the generation mechanism.Previous researches have shown that when the coherence of the light source is degraded,if the method or algorithm is not modified,the reconstruction results will be blurred,disordered,completely wrong or unable to converge.Therefore,how to achieve accurate quantitative phase imaging under partially coherent illumination has become a scientific problem to be solved urgently.Based on the theory of partial coherence,we proposed three non-iterative phase retrieval techniques under partially coherent light illumination,including self-reference holographic technology,pinhole array mask technology and through-focus scanning technology.All three methods can achieve accurate and fast phase retrieval through noniterative means under partially coherent illumination,or even partially coherent light with complex and unknown coherent structures,and they also have their own advantages.Selfreference holographic technology is a real-time imaging technology that can achieve phase reconstruction through only one diffraction intensity.With the help of pinhole array mask,one can reconstruct the phase information of the complete field of view,even when the coherence width is smaller than the size of the object.Through-focus scanning can realize the phase retrieval of the complete field of view under low-coherent light illumination through the acquisition of a series of defocus diffraction intensities without the use of precise phase introduction device.The above-mentioned phase imaging algorithm based on the partial coherence theory can also be used for the measurement of complex wave-front,including the phase distribution of coherent light and the distribution of the 4D cross spectral density of partially coherent light.In the self-reference holographic technology,the reconstruction result will be modulated by the 2D cross spectral density of the illumination.The phase perturbation point corresponds to the reference point of the cross spectral density function.Thus,the result of the structure with an empty window will be the 2D cross-spectral density distribution of the partially coherent light source.Through the shifting of the perturbation point,the measurement of the 4D cross spectral density of the partially coherent light field can be achieved.It is worth mentioning that the above three phase retrieval technologies can all achieve cross-spectral density measurement of partially coherent light with complex coherent structure and the wave-front detection of fully coherent light.The complex wavefront detection technology developed based on the above principles has played a key role in studying the transmission characteristics of vortex beams and topological charge measurement.It is proved that the Fermat spiral slit can realize the longitudinal modulation of on-axis topological charge of the abnormal Bessel vortex beam,for the first time realize the coherent singularity phase measurement of the partially coherent vortex beam,and solve the topological charge measurement of the low-coherence vortex beam.Finally,we proposed a self-reference holographic phase microscopy method to study the phase distribution of normal and abnormal cervical exfoliated cells,which has potential applications in clinical diagnosis.