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Acta Physica Sinica

Design and optimization of phase regenerator with photonic crystal fiber

Lixia Xi, Tang Xian-Feng, Shaokang Wang, Xiaoguang Zhang, 北京邮电大学信息光子学与光通信教育部重点实验室, 北京邮电大学信息光子学与光通信研究院，北京 100876

Published Jan 1, 2009• 2 citations
Publisher

Abstract

Amplification and simultaneous phase regeneration of differential phase-shift keying （DPSK） signals is demonstrated using a phase regenerator. Phase regeneration is achieved in a Sagnac fiber interferometer comprised of photonic crystal fiber operating in an un-depleted pump regime. The formula of signal after phase regenerator is derived. For different incident power of signal, the relations between signal-pump phase difference and signal gain are discussed, as well as the influence of fiber length on signal output intensity. The optimum parameters of the phase regenerator are given. The distributions of phase and intensities for both the non-regenerated and regenerated signals are simulated. Simulations show that nearly ideal regeneration can be achieved for the DPSK signals.

DOI
10.7498/aps.58.6243

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 1, 2019•0 citations
Amplification and simultaneous phase regeneration of differential phase-shift keying （DPSK） signals is demonstrated using a phase regenerator. Phase regeneration is achieved in a Sagnac fiber interferometer comprised of photonic crystal fiber operating in an un-depleted pump regime. The formula of signal after phase regenerator is derived. For different incident power of signal, the relations between signal-pump phase difference and signal gain are discussed, as well as the influence of fiber length on signal output intensity. The optimum parameters of the phase regenerator are given. The distributions of phase and intensities for both the non-regenerated and regenerated signals are simulated. Simulations show that nearly ideal regeneration can be achieved for the DPSK signals.
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.
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.
The polarization-division multiplexing (PDM) technology is a practical method to double the transmission capacity, and the corresponding phase regeneration (PR) for PDM signals is meaningful and necessary to extend the transmission distance and increase the transparency for the phase-encoded PDM system. Those reported PDM PR schemes either utilized polarization-diversity technique or required special PDM format. In order to overcome these issues, the PR for the PDM phase-modulated signals is proposed and theoretically demonstrated in this paper, based on the vector dual-pump nondegenerate phase sensitive amplification (PSA). The theoretical model is established and the detailed characteristics are investigated to optimize the PR performance. Results show an obvious phase squeezing for the degraded 80 Gbit/s PDM differential phase-shift keying (DPSK) signals, and the error vector magnitude (EVM) of the regenerated signals on dual polarization states can be improved from 22.58% and 21.39% to 4.57% and 4.63%, respectively. Furthermore, the applicability of the proposed scheme for PDM quaternary-phase shift keying (QPSK) signals is investigated. The proposed scheme can be useful and promising in current PDM based coherent fiber-optic communication.
Optical amplification in optical fibers by means of degenerate four-wave mixing (DFWM) is studied by classical equations. A balanced interferometer is used to mix and separate the signal and the pump. Exact solutions are obtained even with pump depletion. If the cross-phase modulation is small, the signal power gain varies as the square of the pump self-phase shift. This gain can be sizable for moderate fiber lengths and laser powers, before the onset of stimulated Raman scattering. We have experimentally verified these calculations with a Sagnac interferometer, comprising a 300 m long polarization-preserving fiber, emitting pulses with 100-200 W peak power and 100-200 nsec duration. We have measured a signal power gain of 22 (13.3 dB); this value correlated well with a pump self-phase shift of 3π/2 measured in a separate two-wavelength experiment. This system can be used for squeezing fluctuations of the signal quadrature which is not amplified. We have performed a classical derivation of squeezing, based upon a geometrical study of the transformation of the signal phasor. The results are in agreement with those of more elaborate quantum-mechanical derivations.
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.
Summary form only given. When a signal propagates through a transmission line, it is deteriorated by amplitude and phase noise, which reduces its signal-to-noise ratio (SNR) [1]. According to Shannon's theory, the SNR determines the maximum spectral efficiency. It is therefore necessary to limit the noise [3]. For this purpose, one can use phase sensitive amplifiers (PSA) instead of phase insensitive amplifiers (PIA) like Erbium-doped fiber amplifiers. PSAs amplify only one phase quadrature of the signal while deamplifying the other, thanks to a required phase relationship between the pump and the signal. This unique property, known as phase-squeezing, allows to obtain noise figures (NF) as low as 0 dB whereas PIAs, based on stimulated emission, cannot be go below the 3 dB quantum limit [2]. Moreover, the phase-squeezing property of PSAs can be used for quantum communication or even in metrology [1]. PSA can be realized with χ(2)material through degenerate parametric amplification [2]. The large spectral separation between the pump and the signal makes it very easy to filter out the pump after amplification. A ± 11 dB amplification with a sub-quantum NF and a regeneration of a DPSK signal was experimentally performed in LiNbO3 [3]. A fiber based approach would however be preferable for telecommunication purposes. PSA's based on four wave-mixing have been demonstrated, but the long length and the difficulty to separate pump from the signal [1] hinders its practical implementation. Here we present the first demonstration of a PSA based on periodically poled silica fiber (PPSF). This design has the advantage of all-fiber devices as well as simple pump filtering. The experimental set-up is depicted in Figure 1. A 20 mW CW fiber laser at 1550 nm is sent through a 99:1 coupler to be divided into a signal and a pump. The pump is modulated into 1 ns wide pulses with a repetition rate of 50 MHz. It is then amplified and frequency doubled in a PPLN crystal. The peak power of the 775 nm pump pulses reaches 10W. The pump and CW signal are then combined in a WDM and sent through a 30 cm long PPSF [4] where the signal undergoes parametric amplification. A second WDM is used to separate the pump from the amplified signal. The wave plates and the polarization controller in the pump path, allow to simultaneously control the polarization of the pump and the power in the fundamental mode of the PPSF (at 775 nm the fibre is slightly multimode). The relative phase between the pump and signal is controlled by fine tuning the frequency of the laser via a built-in piezo.