The causes of SNR fluctuation and nonlinear distortion, uncovered during field trials of PFM optical video transmission systems, are investigated theoretically and experimentally. Degradations are found to be significant in long distance transmission at a wavelength around 0.85 μm. This is because PFM pulse jitter closely relating to the degradation is caused by laser mode competition and its interaction with fiber material dispersion. However, at a wavelength around 1.3 μm, where fiber material dispersion is negligible, only small nonlinear distortion due to feedback light from connectors is observed. Thus, good picture quality is maintained over long distance transmission.