Performance of on-axis holography with a flexible endoscope
A new holographic technique based on a numerical reconstruction method is presented and applied to endoscopic holograms. The hologram is obtained by the interference between the light reflected by the object and a reference beam taken as a plane wave travelling back along the axis of the multimode fiber bundle (MMB). The reconstruction of the image is computed numerically, using a fast algorithm to perform the Fresnel transform of the illuminated hologram. The quality of the reconstructed image has been evaluated by simulation and the limitations of the microendoscopic process enlightened: reduced aperture, sampling through the MMB. The resolution limit reachable by a miniaturized endoscope has been predicted by deriving the pseudo 3-D amplitude modulation transfer function (AMTF) of the system, and the noise originating from the twin image. Both AMTF (contrast included) and noise allow the calculation of the SNR (signal to noise ratio) of the reconstructed object. The size of the smallest object observable from the hologram can be established. This size is taken as an image quality index (IQI). Our simulations have shown that this IQI is good enough to identify small size objects (100 micrometers or less) from a small aperture (0.31 mm) endoscopic hologram. The sampling of the on-axis hologram on the tip of the MMB has been shown not to significantly degrade the image.