MULTI-MODE FIBER IMAGING WITH SELECTIVE MODE CONTROL

摘要

Single multi-mode fibers are attractive for endoscopy due to their small footprint, high number of degrees of freedom and flexible design. We present an endoscopy system in which the working principle involves calibration of the fiber transmission matrix, calculation of scanning spots in the desired location of the object plane, fluorescence excitation and collection back through the same fiber. Many approaches to multimode fiber imaging have been reported, but a common limitation of all existing methods is the sensitivity of the fiber to environmental perturbations. While, some degree of robustness has been shown in [1] a more methodical control over perturbation resilience is desirable. An analysis of the perturbation effects in a multimode fiber reveals a direct relation to intermodal coupling [2], which suggests that control over the fiber modes can potentially improve fiber robustness. In this presentation we demonstrate a mathematical approach to controlling the fiber modes excited at the distal tip of a multimode fiber. Towards this end, the desired field in the image plane that defines the location of the focal spot, is decomposed in the fiber modes basis and the new mode coefficients, corresponding to the selected set of fiber modes to be excited, are computed by solving a least squares problem. The estimated mode coefficients allow calculation of the optimal phase mask required at the input of the fiber. Selectively exciting fiber modes to reduce intermodal coupling is promising towards improving robustness of multimode fiber endoscopes.