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Optics Communications

Phase-sensitive QPSK channel phase quantization by amplifying the fourth-harmonic idler using counter-propagating Brillouin amplification

Ahmed Almaiman, Yinwen Cao, A. Mohajerin-Ariaei, Morteza Ziyadi, Peicheng Liao, Changjing Bao, Fatemeh Alishahi, Ahmad Fallahpour, Bishara Shamee, Youichi Akasaka, Tadashi Ikeuchi, Steven R. Wilkinson, Joseph D. Touch, Moshe Tur, Alan Eli Willner

Published Apr 16, 2018• 14 citations
Publisher
DOI
10.1016/j.optcom.2018.04.003

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Optical phase-sensitive amplifiers (PSAs) leverage nonlinear interactions to achieve noise figures below the conventional quantum limit. Dual-pump phase-insensitive amplification has been studied in wavelength conversion, phase conjugation and spectral inversion, but phase-sensitive amplification remains challenging due to stimulated Brillouin scattering (SBS) in optical fibers. While pump-phase modulation helps mitigate SBS, it broadens the idler spectrum, complicating practical implementation. We demonstrate a counter-phase pump modulation approach to suppress idler spectral broadening using optical coherent combining and delayed self-heterodyne techniques to achieve excellent noise figure (NF) performance in dual-pump PSAs. Our methods achieve over 30 dB suppression of residual phase modulation tones and significantly improve the idler spectrum. We obtained a 20 dB gain with an NF of 1.45 dB and a minimal NF penalty of 0.2 dB due to pump-phase modulation. This achievement enables PSAs to support optical communication, wavelength conversion, and deep-space communication applications.
We present a novel reconfigurable optical add-drop filter based on phase-sensitive amplifiers (PSAs). By manipulating the relative phase among an optical pump, signals, and idlers, spectral selectivity can be achieved along with low noise amplification. The use of backward Raman amplification enhances the phase-sensitive operation of the parametric process, enabling higher stopband isolation and lower intra-band crosstalk. A Raman-assisted phase-sensitive amplifier (RA-PSA) is characterized in detail, showing a 7.8-dB combined gain extinction ratio enhancement and 10.85-dB gain improvement. For the first time, we demonstrate an RA-PSA enabled optical add-drop filter with less than 0.4-dB penalty using three-channel QPSK signals. The novel scheme provides a simple way to enhance the extinction ratio of the PSA enabled add-drop filter, which can be an essential subsystem in optical links based on phase-sensitive amplification.
Phase sensitive amplification (PSA) is a remarkably powerful tool if implemented correctly - it allows amplification with a noise figure lower than quantum mechanics normally dictates, and also allows the development of systems that coherently process the phase of their optical inputs. PSA at infrared wavelengths can be achieved using second or third order optical nonlinearities - indeed the first demonstrations of PSA utilised degenerate three photon mixing in bulk crystals, allowing the observation of effects such as quantum quadrature squeezing. This PhD research project was aimed at translating the fascinating science of PSAs into applications, based on Kerr nonlinearity in optical fibres, capable of being deployed in modern core optical networks runnning at 40 Gbit/s and higher. This objective was an integral part of the European Union FP7 project PHASORS. Studies, both theoretical and experimental, were carried out on wideband non-degenerate PSAs. The inline cascaded fiber optic parametric amplifier (FOPA), in which a first phase insensitive FOPA is used to generate phase locked signal-idler pairs, followed by a second FOPA in which PSA takes place, was used to experimentally demonstrate PSA gain characteristics in linear and saturated modes of operation, with PSA obtained over 20 nm in the telecom C-band. Focus was then re-directed towards applying a dual pump PSA to DPSK regeneration. The amplitude and phase limiting characteristics of these devices were experimentally studied, in particular revealing that amplitude saturation arises due to an interplay between input phase and nonlinear phase matching along the nonlinear fibre. This feature was used to identify a regime of operation for DPSK regeneration combining simultaneous phase and amplitude regeneration. A practical DPSK regenerator based on a degenerate dual pump phase sensitive FOPA was built. The device advanced the state of the art by incorporating a pump synthesis stage that allowed black-box operation. Detailed measurements using noise sources with varying frequency distributions in both amplitude and in phase, are presented, and the ability of the system to improve the phase and amplitude characteristics of signals at its input was verified. Also presented are results showing the regenerator installed in the middle of an 800 km dark fiber link. Finally, a novel scheme was proposed and demonstrated that utilised parametric mixing to perform arbitrary phase quantization. This relied on the coherent addition of an M-level PSK signal with a conjugated (M-1)th phase harmonic, scaled by a coefficient m, to achieve M-level phase quantization. The concept was successfully demonstrated with QPSK data signals at 56 Gbaud, as well as used to quantize lower bandwidth test signals from 2 to 6 phase levels.