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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 (λc ≈ 1.2 μm) for single-mode operation, we have observed the propagation of the fundamental LP01, mode and a few higher order modes, namely, LP11 and LP21. 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.