Nonlinear optical loop mirrors: investigation solution and experimental validation for undesirable counterpropagating effects in all-optical signal processing

The ultrashort response time of the Kerr effect in the optical fiber suggests the possibility of using nonlinear optical loop mirrors (NOLMs) in applications of ultrafast all-optical signal processing. Roughly speaking, only one of the signal halves in the fiber loop undergoes a phase shift, due to its own power [self-phase modulation (SPM)] or to a copropagating pump light [cross-phase modulation (XPM)]. But even the other signal half is affected by SPM or XPM induced by the mean power of the strong counterpropagating signal. When considering ultrashort pulse trains at low bit rate, the phase shift due to the mean power can be neglected. But as the repetition rate of the pulse train gets higher, as in an optical time-domain multiplexing) frame, the effect of counterpropagating power must be taken into account, as it tends to reduce the efficiency of the NOLM. Up to now, some schemes have been proposed in the literature to eliminate these limitations, but they usually add further complexity. In this paper, we will investigate the impact of the undesirable effects due to the counterpropagating power on the NOLM performance for different duty-cycle values. We will present a simple and low-cost solution to overcome these impairments for both SPM- and XPM-based NOLMs. The solution consists of a proper NOLM design, using non-polarization maintaining) fiber and including a polarization controller into the loop, in order to compensate for the counterpropagating effects. Finally, the proposed solution will be experimentally validated.