When using multimode fibers as high-resolution endoscopes, advanced adaptive optics is needed to overcome the modal dispersion which scrambles the image. Additionally, for non-linear imaging methods, all the wavelengths of a femtosecond laser pulse must be simultaneously focused at the sample plane, with appropriate dispersion compensation, that might vary across the sample area. We investigate the bandwidth of the focused spot for a graded index fiber used as a point scanning imaging device. We demonstrate that with proper compensation for the dispersion of the spatial light modulator this can be <45 nm. We also measure the spectral phase at the sample plane, and demonstrate that this does not vary substantially with spot position.
Multimode fiber imaging using transmission matrix (TM) is a promising way of deep inspection of living objects. These techniques, however, face a problem of obtaining the transmission matrix using phase-shifting interferometry with external reference beam, which requires additional instrumentation and increases space requirements of the experiment. We suggest a method employing input mode represented by a focal spot at the proximal end of the fiber as an internal reference wave. Due to speckle nature of the output, it is necessary to cover blind spots in the transmission matrix arising from the lack of interference signal by several measurements with different input modes used as a reference. The effect of optimized selection of internal references will be analyzed and compared with the external reference approach.