| The development of interventional medicine puts forward higher requirements for endoscopes.Traditional endoscopies based on both fiber bundles and Gradient-Index lens cascade are facing their technique limitations in the aspects of spatial resolution,diameter and length of fiber probe,etc.As a result,they cannot satisfy with higher requirements,such as minimal invasive,noninvasive diagnosis,operation and treatment.Multimode fiber(MMF)is a best choice for high-resolution,miniaturized fiber optic endoscopes owing to its advantage of a small cross section down to tens of microns,the ability of being used as an imaging and transmission device independently,and the ability of parallel information transporting because of thousands of independent spatial modes propagating in it.Unfortunately,mode dispersion and coupling have so far prevented the MMF from being used in endoscopy.Therefore,a key research of the MMF imaging technology is how to eliminate the effect of the mode dispersion and coupling.Lately,a variety of new methods,such as optical transmission matrix and spot scanning imaging,have been proposed and demonstrated the possibility of imaging through an MMF.The optical transmission matrix method can be used to retrieve the object without scanning by calibrating the relationship of MMF input and output.The spot scanning imaging is a method without mechanical scanning,where the phase distortion caused by the mode dispersion is eliminated as the incident wavefront of the MMF is shaped by the spatial light modulator(SLM).As a result,the MMF output can be focused at the target area and thus the object can be retrieved with the imaging model by spot scanning it.The key of the spot scanning imaging method is how to focus and scan the MMF output light quickly.Thus,research on the technique of focusing and scanning light through an MMF with wavefront shaping has important theoretical and practical significance for the development of MMF imaging technology.For this purpose,the main research work in this paper is carried out,including:1.Based on Maxwell's equations and material equations,the optical transmissionproperties of the step index fiber are analyzed.Combined the above analysis withboundary conditions,the mode distribution of the weakly-guided step index fiberis analyzed.Based on scalar diffraction theory,the phase modulation characteristicsand Fourier transform characteristics of the lens are analyzed,which provides atheoretical basis for establishing the mathematical model of the MMF output.Two techniques of wavefront modulation are introduced: the phase compensation technology and the mode selection technology.As a result,the mathematical model of the MMF output is established when the incident wavefront is shaped.The theoretical analysis provides effective guidance for researching the technique of focusing and scanning light through an MMF,experimentally.2.Based on the phase compensation technology,the mathematical model of the MMFoutput is established when the incident wavefront is shaped by the liquid crystalspatial light modulator(LC-SLM),and the relationship of total light intensity within target area and the modulation phase loaded on the sub-region of LC-SLM is analyzed.It is proved that the parallel coordinate algorithm can be used for focusing and scanning light through an MMF based on LC-SLM.Experimentally,the correctness of the theoretical analysis is verified,together with a demonstration of focusing and scanning light through an MMF with a core diameter of 105 um and a length of 5m.The quality of the focused spots is analyzed,too.3.Based on the mode selection technique,a binary amplitude modulation techniqueis proposed for focusing and scanning light through an MMF based on the digitalmicro-mirror device(DMD).The mathematical model of the MMF output isestablished when the incident wavefront is shaped by the binary amplitude modulation technique based on DMD,and the relationship of total light intensity within the target area and the amplitude modulation coefficient of sub-region on DMD is analyzed.As a result,it is possible to focus and scan light through an MMF with global optimal solutions formed by the switching states of the DMD subregions using the binary amplitude modulation based on DMD.Experimentally,we prove the correctness of the theoretical analysis and demonstrate that focusing and scanning light through an MMF with a core diameter of 105 um and a length of 5m.The algorithm reliability is verified by analyzing the quality of the focused spots we obtained. |
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