Abstract

The idea of transmitting images through multi-mode fibers (MMF) was proposed in 1976 [1]. An experimental demonstration of an image reproduced at the proximal end of the fiber after propagating through a MMF and phase conjugated at the distal (far) end was reported in 1982 using photorefractive crystals to carry out the phase conjugation [2]. Image transmission through a cascade of two separate fibers with a phase conjugating crystal in the middle was demonstrated in 1996 [3]. Recently a flurry of new activity in imaging with MMFs has taken place [4]-[9]. There are two reasons for this resurgence in interest. The first is the emergence of digital phase conjugation and wavefront shaping. The replacement of the holographic recording medium (such as a photorefractive crystal) with the combination of a CCD or CMOS sensor and a spatial light modulator facilitated enormously the practical implementation of phase conjugation. The second reason for the renewed interest in imaging with MMF is the discovery of ways to perform useful tasks with only a single MMF rather than a matched pair, as was a proposed in reference [1]. The simplest example is light focusing at the far (distal) end of the fiber by illuminating the near (proximal) end of the fiber with the proper light distribution. This can be accomplished for example by illuminating the far end with light from a point source, recording holographically the resulting complex field at the near end and then launching back the phase conjugate field which comes to a focus at the far end. We can store sequentially in the computer the holograms corresponding to all the possible points in the field of view at the far end. The process of recording the holograms can be thought of as a training phase to learn the fiber. Once the system is trained, we can choose digitally where to focus the light at the far end of the fiber simply by selecting the right hologram to display on the spatial light modulator that shapes the field being launched at the near end. Endoscopic fluorescence imaging was demonstrated [6] by scanning the focused light spot at the far end of the MMF and collecting the generated fluorescence with the same fiber.

© 2015 Optical Society of America