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 o f the point spread function (PSF) in multicore fibers (MCFs) used as a n image guide in microendoscopy. In the first method, the coupling int ensities between the fiber cores in a MCF are measured at three differ ent wavelengths (632, 520, and 488 nm) by scanning the input face of t he MCF with an illuminated pinhole. In the second method, the intensit y coupled into a fiber core from all the neighboring fiber cores is me asured by obturating the fiber core and measuring the output light int ensity, The nonnegligible contribution of leaky modes in the large int ercore distance coupling is demonstrated. The two methods are fairly w ell correlated. The investigations have been carried out for two MCFs presenting well-differentiated characteristics. The major coupling mec hanisms between fiber cores have been identified and their roles quant ified as functions of the coupling distance. At short distance, crosst alk dominates, with a maximum coupling intensity of 15% at 632 nm for the nearest neighbor fiber core. At larger distances (ranging from 3.8 to 5 intercore distances), leaky modes play a predominant role in the coupling, which remains nonnegligible.