The reach and capacity in today’s fiber optical transmission links are limited by the accumulation of noise (originating from optical amplifiers in the link) and by the signal distortion from nonlinear effects in the transmission fiber. In this demonstration, researchers showed that the use of phase-sensitive amplifiers (PSAs) can reduce the affect of both of these disturbances. A fiber optical transmission link of 4000 km was achieved using ultralow-noise PSAs. This is a reach improvement of almost six times what is possible using conventional optical amplifiers. Signal constellation diagrams comparing conventional amplification and phase-sensitive amplification (PSA) in an amplifier-noise-limited regime (−2 dBm launch power) and a fiber-nonlinearity-limited regime (8 dBm launch power). Courtesy of Samuel Olsson. Researchers from Chalmers University of Technology and Tallinn University of Technology used a multichannel-compatible and modulation-format-independent long-haul transmission link with in-line PSAs to demonstrate a reach improvement of 5.6 times at optimal launch powers, with the phase-sensitively amplified link operating at a total accumulated nonlinear phase shift of 6.2 rad. The PSA link transmitted two data-carrying waves, thus occupying twice the bandwidth and propagating twice the total power compared to the phase-insensitively amplified link. The link is both modulation format-independent and multichannel-compatible. The results show that use of in-line PSAs, which offer simultaneous low-noise amplification and nonlinearity mitigation, could provide unprecedented nonlinear tolerance and transmission reach extension. Peter Andrekson, professor of photonics, Chalmers University of Technology, Sweden. Courtesy of Jan-Olof Yxell. The demonstration of long-haul PSA-amplified transmission was performed using a signal with 10 GBd quadrature phase-shift keying (QPSK) data. However, researchers say that, in principle, any modulation format and symbol rate could be used with the copier-PSA scheme. Researchers transmitted a single channel, but they say that with the copier-PSA scheme, multichannel transmission is possible, albeit with increased complexity because of required polarization, delay, and phase alignment of each channel. “While there remain several engineering challenges before these results can be implemented commercially, the results show, for the first time, in a very clear way, the great benefits of using these amplifiers in optical communication,” said professor Peter Andrekson. The amplifiers can provide a significant reach improvement over conventional approaches and could potentially improve the ability of future fiber optical communication systems to serve ever-greater demands for higher transmission capacity. “Such amplifiers may also find applications in quantum informatics and related fields, where generation and processing of quantum states are of interest, as well as in spectroscopy or any other application which could benefit from ultralow-noise amplification,” Andrekson said. The research was published in Nature Communications (doi:10.1038/s41467-018-04956-5).
