Toward a practical use of phase-sensitive optical amplifiers
Optical amplifiers are essential in optical communication systems as they compensate loss induced by transmission fibre, thus ensuring the signal integrity of the information being transmitted. Because phase-sensitive amplifiers (PSA) are coherent, the phase relationship among the optical waves at input play a key role. PSAs have unique and superior properties compared with all other optical amplifiers, most notably the potential for noiseless amplification, very broad optical bandwidth and enabling a range of ultrafast, all-optical functionalities. In communication, there is an urgent need to develop new technologies that can break the ‘nonlinear Shannon capacity limit’ – a serious barrier to achieving the continued capacity increase needed to meet the growing demand for bandwidth. Although the use of PSAs is expected to be an essential part of this development, before they can be used for telecommunication and other potential applications, several important challenges must first be addressed. Overcoming challenges Researchers with the EU-funded PSOPA project set out to address these challenges. For example, one challenge is the undesired nonlinear phenomena of Stimulated Brillouin scattering that limits the amount of optical power that can be launched into an optical fibre. ‘While techniques exist to increase the threshold of this effect, they come with associated performance penalties,’ says project coordinator Peter Andrekson. ‘We developed an approach based on stretching sections of fibre with optical isolators in between, which allowed for more than an order of magnitude threshold increase with no associated penalty.’ Another challenge that PSOPA overcame was the need for a low noise, high power pump wave at each PSA in a transmission system. ‘Here we implemented an all-optical technique to recover a weak pump wave for the PSA by using so-called injection locking, which demonstrates that this is a very promising technique to maintain the PSA performance in a system,’ explains Andrekson. PSOPA also overcame engineering challenges. ‘We made progress toward a new and compact platform for the implementation of PSA using nonlinear silicon-nitride waveguides,’ says Andrekson. ‘Although nonlinear amplification in these chips still must be demonstrated, this remains a promising platform as it can be implemented in a wide range of operating wavelengths and can facilitate new non-telecom related applications.’ Andrekson adds that the combination of PSA’s ultralow noise and transmission fibre nonlinearity mitigation capabilities, which were discovered in this project, were experimentally investigated in long-haul fibre optic transmission system for the first time. ‘These experiments showed a very significant data transmission reach extension of three times what is capable when using conventional amplifiers,’ he says. Commercial potential Andrekson also notes that these approaches allowed PSOPA to use PSA in a record-breaking demonstration of high sensitivity in free-space optical links. ‘This was achieved very late in the project and is something we continue to pursue quite intensely,’ says Andrekson. ‘This may be the project’s most promising commercial opportunity, with potential applications including very long reach free-space optical communication – necessary for communication to the moon or Mars.’ The project is currently in discussions with companies and space agencies, including the European Space Agency (ESA).
Keywords
PSOPA, PSA, optical communication, phase-sensitive amplifiers
