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IEEE Journal of Selected Topics in Quantum Electronics

Phase and Amplitude Regeneration of Differential Phase-Shift Keyed Signals Using Phase-Sensitive Amplification

Kevin Croussore, Guifang Li

Published Jan 1, 2008• 103 citations
Publisher

Abstract

Phase-sensitive amplifiers (PSAs) offer numerous advantages over phase-insensitive amplifiers in optical communications. Squeezing of optical phase through PSA can remove accumulated phase jitter, which is a critical functionality for an all- optical, phase-shift keyed network. In recent experiments, reviewed in this report, different implementations of PSA were used for phase regeneration of both return-to-zero differential phase-shift keying and nonreturn-to-zero differential phase-shift keying data. The first demonstration explored the properties and performance of PSA that occurs in nonlinear interferometers. Experiments confirmed that a PSA operating in the depleted pump regime provides simultaneous reduction of amplitude and phase noise (PN). Phase regeneration performance limit was reached as a consequence of pump-wave imperfections, which can be significantly reduced through proper design. PSA that occurs directly in fiber in a traveling-wave configuration through partially degenerate four-wave mixing was also studied. The latter implementation offers stronger phase-matched gain and suppression of amplitude-to-phase noise conversion. Technical issues that remain to be addressed are identified for each implementation. Results characterized using coherent detection offer direct measurements of the phase-regenerative behavior.

DOI
10.1109/jstqe.2007.915397

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Phase sensitive parametric amplifiers and their applications

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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.

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This work is devoted to the characterization of vector phase-sensitive amplifiers and processors. A detailed analysis of degenerate vector phase-sensitive amplifier (PSA) is performed.The gain and phase-sensitive extinction ratio are theoretically analyzed using three-wave theory. Experiments and simulation results confirm the validity of this three-wave theory. The influence of polarization-mode dispersion is also evaluated, showing that aligning the pump polarizations at the fiber input is essential in order to achieve the theoretically predicted results. The scheme is also compared to the degenerate scalar PSA scheme. At the same pump power, the vector PSA has lower gain but also less influence from higher-order idlers and lower pump depletion due to FWM between the pumps. Using the degenerate vector PSA, phase-sensitive (PS) amplification of dual-polarization (DP) binary phase-shift keying (BPSK) signals was demonstrated. To the best of our knowledge, this was the first demonstration of a DP-modulated signal with large net phase-sensitive gain. Furthermore, we also demonstrated that this scheme can phase regenerate the signal. The same scheme was also used for a different purpose: quadrature decomposition into two cross-polarized waves. We demonstrated demultiplexing of quadrature phase-shift keying (QPSK) signal into two cross-polarized BPSK signals by the operating the amplifier in phase-insensitive mode. The design of a novel phase-locked loop scheme enabled stable operation and negligible penalty in the decomposition.

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In optical communication systems, Phase Sensitive Amplifier (PSA) is well known for its superior noise performance in the sub-quantum limit. Therefore, PSAs are highly suitable for long haul transmission systems where amplification and regeneration is critical. In this paper, we analyze the gain performance of a single pump non-degenerate PSA operating in the quadratic gain regime. The variation of gain characteristics with pump power and fiber length has been discussed. It could be seen that for a 5km long fiber, increasing of pump power above 4W subjected to the increasing of amplifier gain while maintaining the relative phase at a stable constant value of 0.001 rad. Therefore, by fine tuning parameters like fiber nonlinearity, pump power and fiber length, a phase stabilized gain performance of the PSA could be achieved when it is operating in the quadratic gain regime.

Moratuwa Engineering Research Conference (MERCon)•Jul 1, 2020•0 citations
Phase-sensitive amplifiers (PSAs) offer numerous advantages over phase-insensitive amplifiers in optical communications. Squeezing of optical phase through PSA can remove accumulated phase jitter, which is a critical functionality for an all- optical, phase-shift keyed network. In recent experiments, reviewed in this report, different implementations of PSA were used for phase regeneration of both return-to-zero differential phase-shift keying and nonreturn-to-zero differential phase-shift keying data. The first demonstration explored the properties and performance of PSA that occurs in nonlinear interferometers. Experiments confirmed that a PSA operating in the depleted pump regime provides simultaneous reduction of amplitude and phase noise (PN). Phase regeneration performance limit was reached as a consequence of pump-wave imperfections, which can be significantly reduced through proper design. PSA that occurs directly in fiber in a traveling-wave configuration through partially degenerate four-wave mixing was also studied. The latter implementation offers stronger phase-matched gain and suppression of amplitude-to-phase noise conversion. Technical issues that remain to be addressed are identified for each implementation. Results characterized using coherent detection offer direct measurements of the phase-regenerative behavior.
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.
All-optical regeneration of differential phase-shift keyed signals based on phase-sensitive amplification in a nonlinear fiber Sagnac interferometer is described. Nearly ideal phase regeneration can be achieved in the undepleted pump regime, with output differential phase noise limited only by fast fluctuations of the pump phase relative to the DPSK signal. Operating in the depleted pump regime offers the possibility of simultaneously regenerating both phase and amplitude information of DPSK signals while providing low noise, phase-sensitive gain.
This work is devoted to the characterization of vector phase-sensitive amplifiers and processors. A detailed analysis of degenerate vector phase-sensitive amplifier (PSA) is performed.The gain and phase-sensitive extinction ratio are theoretically analyzed using three-wave theory. Experiments and simulation results confirm the validity of this three-wave theory. The influence of polarization-mode dispersion is also evaluated, showing that aligning the pump polarizations at the fiber input is essential in order to achieve the theoretically predicted results. The scheme is also compared to the degenerate scalar PSA scheme. At the same pump power, the vector PSA has lower gain but also less influence from higher-order idlers and lower pump depletion due to FWM between the pumps. Using the degenerate vector PSA, phase-sensitive (PS) amplification of dual-polarization (DP) binary phase-shift keying (BPSK) signals was demonstrated. To the best of our knowledge, this was the first demonstration of a DP-modulated signal with large net phase-sensitive gain. Furthermore, we also demonstrated that this scheme can phase regenerate the signal. The same scheme was also used for a different purpose: quadrature decomposition into two cross-polarized waves. We demonstrated demultiplexing of quadrature phase-shift keying (QPSK) signal into two cross-polarized BPSK signals by the operating the amplifier in phase-insensitive mode. The design of a novel phase-locked loop scheme enabled stable operation and negligible penalty in the decomposition.
We propose and experimentally demonstrate a phase-sensitive optical processor, capable of generating two codirectional and π-phase-shifted phase-sensitive amplifiers (PSAs) in a single device. Phase-sensitive operation is obtained by polarization mixing a phase-locked signal/idler pair generated in a degenerate dual-pump vector parametric amplifier based on four-wave mixing in a highly nonlinear fiber. We refer to this configuration as a polarization assisted PSA and demonstrate some of the applications that it may find. First, we experimentally demonstrate the regeneration of a binary phase shift keying signal in a system that requires only a very low nonlinear phase shift of 0.35 rad. Second, we decompose a quadrature phase shift keying (QPSK) signal into its in-phase and quadrature components. While application to a QPSK signal is shown in our demonstration, we demonstrate numerically that any complex modulation format signal can be decomposed using this approach. Finally, we use our processor to regenerate QPSK signals in a single nonlinear device.
We demonstrate numerically phase regeneration of a star-8QAM signal with two amplitude and four phase states in a phase-sensitive amplifier. In a dual-stage setup, two phase-conjugated idlers are generated in a first stage consisting of two fiber-optic parametric phase-insensitive amplifiers operated in highly nonlinear gain regime. These are used as pumps in the second, phase-sensitive amplification stage which enables efficient phase regeneration via a degenerate four-wave-mixing process. The latter can be operated in two different operation modes: without format conversion or with phase-shifted amplitude levels. In both regimes, we observe high phase-regeneration efficiency for all amplitude levels: the initial phase noise with 0.2 rad standard deviation is reduced by a factor of 5.
In optical communication systems, Phase Sensitive Amplifier (PSA) is well known for its superior noise performance in the sub-quantum limit. Therefore, PSAs are highly suitable for long haul transmission systems where amplification and regeneration is critical. In this paper, we analyze the gain performance of a single pump non-degenerate PSA operating in the quadratic gain regime. The variation of gain characteristics with pump power and fiber length has been discussed. It could be seen that for a 5km long fiber, increasing of pump power above 4W subjected to the increasing of amplifier gain while maintaining the relative phase at a stable constant value of 0.001 rad. Therefore, by fine tuning parameters like fiber nonlinearity, pump power and fiber length, a phase stabilized gain performance of the PSA could be achieved when it is operating in the quadratic gain regime.