  Multi-mode fiber (MMF) endoscopes are a new type of endoscope that use only a
single optical fiber to transmit images, achieving much higher spatial
resolution for a given diameter than commercial fiber bundle endoscopes.
However, a drawback of MMF endoscopes is that imaging quality degrades
substantially as the MMF is perturbed. We propose a method for improving spot
formation at the distal end of a perturbed MMF, and thus improving imaging
quality, by attaching a partial reflector to the distal end of the MMF. We
experimentally find that the perturbation of the light reflected from the
partial reflector is highly correlated with the perturbation of the light
transmitted through the MMF. We demonstrate a simple method exploiting this
correlation that enables formation of spots at the distal fiber end with
quality 10-15% higher than if the perturbation of the MMF is ignored. In the
future, more advanced algorithms exploiting the correlation may enable further
improvements in spot formation and imaging through perturbed fibers.
