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Phase-sensitive optical parametric amplifiers

Final Report Summary - PSOPA (Phase-sensitive optical parametric amplifiers)

Optical amplifiers are essential in optical communication systems as they compensate loss induced by the transmission fiber ensuring signal integrity of the information being transmitted. The objective of this project was to explore the potential of so-called phase-sensitive amplifiers (PSA) for telecommunication and other potential applications. These amplifiers are coherent, meaning that the phase relationship among the optical waves at the input play a key role. This can be used for a variety of operations aside from amplification, for example squeezing of the phase, potentially useful for signal regeneration.

A unique aspect of these amplifier is the possibility of noiseless amplification. In this project, we have evaluated the use of PSAs instead of conventional optical amplifiers in optical fiber transmission systems. We find a significant improvement of the system performance, partly because of the better noise performance, but equally important, because of the ability of PSAs to mitigate the degradation caused by the transmission fiber nonlinearities. The latter ability was discovered in this project. An extension of the transmission reach, compared to the used of conventional amplifiers, of up to factor of five was experimentally confirmed.

As PSAs can operate at the minimum possible noise level (quantum noise), we were able to demonstrate the most sensitive optical receiver ever reported. This improvement may be of significant merit in very long reach free-space optical links, for example in deep-space missions.

The physics behind the PSA amplification relies on a nonlinear phenomenon known as four-wave mixing which is highly dependent of the state of the polarization of the light wave. We explored the possibility to make the amplifiers independent of the polarization by inventing a new concepts based on so-called vector PSAs, and quantified this ability experimentally.

While in the project we mainly used a special nonlinear optical fiber as the key element in the PSAs, we also invested much effort in developing a nonlinear element based on very compact silicon-nitride chips fabricated in our in-house clean room. The purpose of this is not only the small size, but also the fact that this platform can operated over a wide range of optical wavelengths potentially enabling new, non-telecom applications of PSAs. These have been evaluated for nonlinear applications such as ring resonators for optical comb generation. We have now reached a loss around 0.4 dB/cm at 1550 nm and significant steps toward achieving parametric amplification based on these chips have been taken.