Potential and practical implementations of phase sensitive amplifiers for all-optical signal regeneration

Phase noise introduced during transmission both from optical amplifiers and nonlinear interactions between channels represents a significant limiting factor to data transmission when advanced modulation formats such as (differential) phase-shift keying, (D)PSK, or quadrature phase shift keying, QPSK, signals are used to increase the network capacity. Consequently, the development of all-optical techniques capable of eliminating phase (and ideally amplitude as well) noise from multi-level phase signals is of great interest. Phase regeneration of (D)PSK signals can be achieved directly by exploiting the phase-squeezing capability of phase sensitive amplifiers (PSAs), which can be operated in the saturation regime to perform simultaneous amplitude regeneration. The main challenge in realising practical PSA-based regeneration is to stabilize and maintain a phase relationship between the PSA pump(s), the signal and any idlers present at the PSA input. This is complicated in practice by the fact that phase-encoded signals have a suppressed carrier. Moreover, even if the carrier can be extracted, it would generally incorporate any noise generated during data transmission. To overcome these issues a regenerative scheme has recently been demonstrated which simultaneously allows carrier recovery and carrier noise suppression. The resulting cleaned carrier is then used to phase-lock the locally generated pumps with the incoming data prior to signal regeneration in a degenerate two-pump PSA. In this scheme two important technologies were used: injection locking of semiconductor lasers to remove high frequency amplitude and phase noise during the carrier recovery process and specialized highly nonlinear fibres (HNLFs) with an alumino-silicate core and a linear strain gradient applied along the length to increase the stimulated Brillouin scattering (SBS) threshold in order to allow higher PSA gains.