Propagation of LP01 and LP11 modes in kilometer lengths of optical fibers

We have studied the propagation of subnanosecond light pulses in two different single-mode optical fibers, each being of 1-km nominal length. By operating at a slightly shorter wavelength, from a semiconductor laser (λ = 0.9 μm), compared to the cutoff wavelength (λ
c
≈ 1.2 μm) for single-mode operation, we have observed the propagation of the fundamental LP
01
, mode and a few higher order modes, namely, LP
11
and LP
21
. Calculations made, of the temporal separation between the pulses, after propagation in the kilometer length, agree well with experimentally measured values. The effect of a mode mixer at the input end is studied in individual fibers and also when the two fibers are butt-coupled. In the latter case we have also studied the effect of lateral misalignment at the junction of the coupled fibers, and explain the results in terms of mode excitation.