Measurements of the point spread function for multicore fibers used as image guides in microendoscopy

Two different methods are presented and discussed for the evaluation of the point spread function in multicore fibers (MCFs) used as an image guide in microendoscopy. In the first method, the coupling intensities between the fiber cores in a MCF are measured at three different wavelengths (632, 520, and 488 nm) by scanning the input face of the MCF with an illuminated pinhole. In the second method, the intensity coupled into a fiber core from all the neighboring fiber cores is measured by obturating the fiber core and measuring the output light intensity. The nonnegligible contribution of leaky modes in the large intercore distance coupling is demonstrated. The two methods are fairly well correlated. The investigations have been carried out for two MCFs presenting well-differentiated characteristics. The major coupling mechanisms between fiber cores have been identified and their roles quantified as functions of the coupling distance. At short distance, crosstalk dominates, with a maximum coupling intensity of 15% at 632 nm for the nearest neighbor fiber core. At larger distance (ranging from 3.8 to 5 intercore distances), leaky modes play a predominant role in the coupling, which remains nonnegligible.