Picosecond pulse distortion in optical fibers
Low-loss glass fibers are a prospective transmission medium for optical communication systems. To study their dispersion characteristics, we investigated the propagation of short optical pulses in up to 10 m of cladded fiber. The pulses, 7 ps long and 6 THz in spectral width, were derived from a mode-locked Nd:glass laser by frequency doubling (0.53-μm wavelength). We measured the broadened pulses emerging from the fiber by a sampling technique that employed an optical Kerr shutter of 10 ps gating time. The fiber had a core diameter of 11 μm and an index difference of 1 percent between core and cladding. The loss was about 1.8 dB/m. Our measurements showed satisfactory agreement with a theory that takes material dispersion and mode dispersion into account and assumes strong attenuation of the high-order modes in the particular fiber tested. At the end of 10 m, the 7-ps pulse reached a width of 110 ps, haft of which can be accounted for by material dispersion. When we extrapolate these results to practical systems, we conclude that pulses from a light-emitting diode transmitted through a fiber with the above design would broaden by at least 2.6 ns/km (because of material dispersion), and possibly by as much as 33 ns/km, if all modes were excited and reached the detector with comparable attenuation and negligible coupling.
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