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Optics Letters

Spectrally efficient slow light using multilevel phase-modulated formats

Bo Zhang, Lianshan Yan, Lin Zhang, Scott Nuccio, Louis C. Christen, Teng Wu, Alan Eli Willner

Published Dec 20, 2007• 15 citations
Publisher

Abstract

A phase-preserving and spectrally efficient slow-light scheme has been proposed and demonstrated by utilizing advanced multilevel phase-modulated formats. A 60 ps symbol delay with error-free demodulation of both I and Q channels for 10 Gbit/s return-to-zero differential-quadrature-phase-shift-keyed (DQPSK) signals via a broadband stimulated Brillouin scattering-based slow-light medium is achieved experimentally. Simulation results on 20 Gbit/s DQPSK and 30 Gbit/s D8PSK propose to transmit very high spectrally efficient multilevel formats through a bandwidth-limited slow-light element.

DOI
10.1364/ol.33.000055

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A phase-preserving and spectrally efficient slow-light scheme has been proposed and demonstrated by utilizing advanced multilevel phase-modulated formats. A 60 ps symbol delay with error-free demodulation of both I and Q channels for 10 Gbit/s return-to-zero differential-quadrature-phase-shift-keyed (DQPSK) signals via a broadband stimulated Brillouin scattering-based slow-light medium is achieved experimentally. Simulation results on 20 Gbit/s DQPSK and 30 Gbit/s D8PSK propose to transmit very high spectrally efficient multilevel formats through a bandwidth-limited slow-light element.
Tunable optical delay lines have many applications for high-performance optical switching and signal processing. Slow light has emerged as an enabling technology for achieving continuously tunable optical delays. Delay reconfigurability opens up a whole new field of nonlinear signal processing using slow light. In this paper, the authors review recent advances in slow-light-based optical signal processing, with a focus on the data fidelity after traversing the slow light elements. The concept of slow-light-induced data pattern dependence is introduced and is shown to be the main signal degrading effect. We then propose and experimentally demonstrate phase-preserving slow light by delaying 10 Gb/s differential phase-shift keying (DPSK) signals with reduced DPSK pattern dependence. Spectrally efficient slow light using advanced multilevel phase-modulated formats is further described. With this technique, doubled bit-rate signals can be transmitted through a bandwidth-limited slow light element. We finally show several novel slow-light-based signal processing modules. Unique features such as multichannel operation, variable bit-rate capability, and simultaneous multiple functions are highlighted.
We demonstrate, via simulation and experiment, slowing down of a phase-modulated optical signal. A 10.7-Gb/s NRZ-DPSK signal can be delayed by as much as 42 ps while still achieving error free via broadband SBS-based slow light. We further analyze the impact of slow-light-induced data-pattern dependence on both constructive and destructive demodulated ports. By detuning the SBS gain profile, we achieve 3-dB Q-factor improvement by the reduction of pattern dependence. Performance comparison between NRZ-DPSK and RZ-DPSK shows that robustness to slow-light-induced pattern dependence is modulation format dependent.
Optical tunable delay lines have many applications for high-performance optical switching and signal processing. Slowlight has emerged as an enabling technology for achieving continuously tunable optical delays. The reconfigurable delay opens up a whole new field of nonlinear signal processing using slow light. In this paper, we review recent advances in slow-light-based optical signal processing, with a focus on the data fidelity after traversing the slow light elements. The concept of slow-light-induced data-pattern dependence is introduced and is shown to be the main signal degrading effect. We then propose and experimentally demonstrate phase-preserving slow light by delaying 10-Gb/s differential-phaseshift- keying (DPSK) signals with reduced DPSK data pattern dependence. Spectrally-efficient slow light using advanced multi-level phase-modulated format is further described. With this technique, doubled or even tripled bit-rate signals can be transmitted through a bandwidth limited slow light element. We finally show several novel slow-light-based signal processing modules. Unique features such as multi-channel operation and variable bit-rate capability are highlighted.
We describe a procedure for increasing the fractional delay (or delay-bandwidth product) of a slow-light system. A broadband input signal is sliced into several frequency bands. The light in each band propagates through a separate channel which possesses a highly reduced group velocity over a narrow frequency band. The output of each channel is then combined to form a single output field. For certain discretely distributed values of the group velocity of each channel, the output can replicate the input waveform without the need to adjust the output phase of each channel. Because this scheme makes use of many parallel channels, it can overcome a fundamental limit [D. A. B. Miller, Phys. Rev. Lett. 99, 203903 (2007)] to the delay-bandwidth product for single-channel devices. A practical design is proposed using spectral slicers and stimulated Brillouin scattering as the narrow-band slow-light process. Numerical simulation shows that such a channelized slow-light element can have nearly uniform gain and group index over very large bandwidths.
The manipulation of the group velocity of optical signals (slow and fast light) has many possible applications in telecommunications, sensing, nonlinear optics, and so on. The exploitation of the effect of stimulated Brillouin scattering (SBS) is of special interest for the alteration of the group velocity in optical fibers, since it just requires standard telecom equipment and low-to-moderate optical power. However, each delay in one single, low-gain SBS-based slow-light system is accompanied by a pulse broadening. This is a result of the inherent Kramers–Kronig relations among the gain, the phase change, and the accompanied group velocity. For an ideal flat gain, the phase change is nonideal, and for an ideal phase change the gain curve leads to a broadening. Furthermore, if the gain bandwidth is adapted to the signal by broadening, the delay will be reduced. Thus the signal broadening can be reduced by several methods for one single low-gain slow-light system, but it cannot be zero. Here, we will show how a zero-broadening SBS-based slow light can be achieved with several methods. The basic idea is a reshaping of the original pulse in a second stage by saturation of the SBS system, or by the superposition of two gains. As will be shown, all of these systems show an almost ideal overall gain and phase function over the bandwidth of the pulses. Thus, SBS-based slow light with a delay bandwidth product of 1 bit and zero distortion is possible.
Slow light due to stimulated Brillouin scattering in fibers is a promising technique to realize all-optical data storage and synchronization. For delaying high-speed data, we demonstrate an approach to solve the problem of narrowband Brillouin gain by phase-modulating the Brillouin pump. The bit rate of the phase-modulating signal determines the Brillouin gain bandwidth. A 1.25-Gb/s nonreturn-to-zero pseudorandom binary sequence data is delayed by 520 ps with a phase-modulated pump operating at 2.5-Gb/s bit rate. We have also observed the pattern-dependent delay and distortion of the data caused by the filtering characteristics of Brillouin amplification, and we have suppressed the distortion by detuning the carrier of the signal away from the Brillouin gain peak