An endoscope, a device which can be inserted into the body and which transmits an image of its surroundings, is generally a bundle of thousands of single-mode optical fibres, each one of which produces a single image pixel. There have been efforts recently to make an endoscope which instead uses a single multimode fibre tens of microns in diameter. The difficulty is that the waveguide modes depend very sensitively on the precise characteristics of the fibre and cannot be predicted, and since any received light will excite multiple modes with different dispersion relations, interpreting the signal is difficult. Commonly, a laser shined through a liquid crystalline spatial light modulator (SLM) is used to excite various combinations of modes, with the image reconstructed from the different optical signals received back. The usual method is to use the feedback from some initial test signals to work out how to change the pattern on the SLM to produce a spot which can be scanned around the object to be imaged [3]. The maximum number of informationbearing pixels is then the number of waveguide modes.
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