Analysis and compensation of moire effects in fiber-coupled image sensors

Imaging fiber bundles can relay a curved image surface to a conventional flat focal plane, effectively providing the curved image sensor needed for some high performance lenses. If the fiber bundle period or image sensor pitch are very different, the system resolution is determined by the oversampled fiber or sensor feature. But crosstalk imposes an approximately 2 mu m minimum waveguide pitch, and light collection and fabrication constraints impose a lower limit of 1-2 mu m for the sensor pitch. Maximizing image information leads to some degree of aliasing, which appears in the form of moire pattern on the raw image sensed. For example, a 30 Mpixel 120 degrees field of view imager using a 1.75 mu m Bayer filtered CMOS focal plane with 2.5 mu m pitch fiber bundle yielded images with visible moire. Here we present a study of moire effects in fiber-coupled image sensors, including a method for quantitative analysis of moire, and experimental characterization of the sensors with 1.1 mu m pixel pitch, the highest spatial resolution in commercially available focal plane arrays. We investigate the effect of exposure time of the sensor, angle of incidence of collimated light, and imaging lens F/# on the raw moire pattern strength. This study provides guidelines for optimization and operation of high resolution fiber-coupled imagers.