<title>Transient characterization of modal dispersion in an optical fiber using 250-ps shuttered MCPII</title>

Abstract

A gate MicroChannel-Plate Image Intensifier (MCPII) was used as an optical shutter with a 250 ps exposure time to record the transient image of the modally dispersed output of a 30 meter length of 600 micron diameter step index optical fiber. The fiber input was pulsed light from a Raman shifted, frequency-doubled, Nd:YAG laser having a wavelength of 559 nm and a width of 20 ps FWHM. The MCPII was synchronously gated on in time to the laser pulse Time Of Arrival. The laser light from the fiber was spatially resolved with a 50 mm fl Nikon camera lens and the fourier plane was imaged onto a MCPII. The output from the MCPII was then recorded with a RS-170 video camera and a PC based video digitizer. The relative timing between the input laser pulse and the shuttering of the MCPII was precisely adjusted to capture transient images at successive times. These images reveal not only meridional rays arriving in one to one correspondence of delay time versus angle (or distance in the fourier plane) but also the time and numerical aperture broadening due to the skew rays. Indeed one 250 ps snapshot reveals delayed emission at angles that exceed the critical angle in the fiber, i.e., a picture of skew rays leaving the fiber at an angle to the z-axis greater than the meridional critical angle after all the lower order meridional rays have gone. Experimental data will be presented.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.