Mourad Menif

Gain stabilization in gain clamped EDFA cascades fed by WDM burst-mode packet traffic

This paper studies via simulation the stabilizing effect of all-optical gain-clamping (AOGC) in a chain of erbium-doped fiber amplifiers (EDFA) fed by wavelength-division multiplexing (WDM) burst-mode packet traffic. AOGC is necessary to suppress swings of output power and optical signal-to-noise ratio (OSNR). A case study is selected, in which only the first EDFA in a cascade of six amplifiers is clamped using a ring laser configuration. A numerical model which solves the transcendental equation for the average inversion at each EDFA is used for the analysis. The traffic is generated on the eight WDM channels by ON-OFF time-slotted sources, with statistically independent ON and OFF durations, randomly generated by a truncated Pareto distribution with infinite variance. The simulation model includes the generation of amplified spontaneous emission within each amplifier and the propagation of the lasing power generated in the AOGC EDFA through the cascade. It is shown that the sizable power and OSNR swings arising in an unclamped cascade of EDFAs can be effectively suppressed when a lasing signal a few decibels above the aggregate signal power develops in the AOGC EDFA and propagates along the cascade.

Output power excursions in a cascade of EDFAs fed by multichannel burst-mode packet traffic: experimentation and modeling

A serious problem facing wavelength-division multiplexed (WDM) networks with fiber amplifier cascades is transient cross-gain saturation or gain dynamics of fiber amplifiers. Attention has been focused primarily on circuit-switched scenarios. When the number of WDM channels transmitted through a circuit-switching network varies, channel addition/removal will tend to perturb signals at the surviving channels that share all or part of the route. Even more serious bit error rate deterioration can arise in WDM packet switched burst mode networks. In this paper, we present experimental and theoretical results demonstrating the effect of fast power transients in erbium-doped fiber amplifiers (EDFAs) on packetized traffic transmitted through a chain of five EDFAs. Traffic of a local-area network has been transmitted over three channels. The effect of EDFA cross-gain saturation due to the burstiness of the traffic has been observed in a continuous-wave monitoring channel. The stabilizing effect of gain clamping of the first EDFA in the cascade has been investigated. The experimental results are extended to eight-channel WDM system using a large signal numerical analysis.

Protection of surviving channels in pump-controlled gain-locked Raman fibre amplifier

The authors present results of numerical analysis on transient gain response to channel addition/removal in a counter-directionally pumped Raman fibre amplifier (RFA) with a pump-controlled gain-locking feedback loop. A large signal numerical model which incorporates time variation effects, downstream propagation of multiple signals, upstream propagation of pump and both downstream and upstream propagation of amplified spontaneous emission has been used for the analysis. To lock the gain of the RFA for variable channel loading, the input pump power must be varied. Negative-feedback control derived from a monitoring channel output power has been introduced in the model. It follows from the theoretical investigation that the surviving-channel-power excursion in a 10 dB RFA due to removal/addition of six channels in an eight-channel multiwavelength system can be reduced to less than 0.1 dB when the pump-power-feedback parameters are properly selected.

Application of preemphasis to achieve flat output OSNR in time-varying channels in cascaded EDFAs without equalization

In this paper, we present a new method for optical signal-to-noise ratio (OSNR) equalization of wavelength division multiplexed (WDM) channels at the end of a cascade of several erbium-doped fiber amplifiers (EDFAs) by use of preemphasis, as well as the proper choice of EDFA design parameters. Identical OSNR at the end of the cascade ensures better signal detection and quality of service. The dynamics of the equalizing method have been demonstrated by simulation for single- and double-stage amplifier designs using a numerical model incorporating time variation effects in EDFA. Calculations are based on the solution of a transcendental equation describing the dynamics of the reservoir, i.e., the total number of excited ions, for each EDFA. Traffic on eight WDM channels is modeled as statistically independent ON-OFF time-slotted sources. In addition, we investigate the effect of gain clamping of the first amplifier in the cascade-by implementing a ring laser and propagating the lasing power through the cascade-on the statistics of OSNR variation. We show that it is possible to achieve dynamic OSNR equalization for a WDM system by the use of preemphasis and an appropriate choice of EDFA parameters, without resorting to optical equalization filters. Most previous equalization methods are static with flat gain for a given inversion level in the amplifier. Changes in the input power (due to network reconfiguration or packetized traffic) will lead to a varying inversion level and hence non optimal equalization.

Cross-gain modulation in Raman fiber amplifier: experimentation and modeling

The effects of channel loss on the performance of Raman fiber amplifiers (RFA) are investigated both experimentally and theoretically. Signals from one distributed-feedback (DFB) laser and an external cavity laser (ECL) were transmitted through counterdirectionally pumped RFAs consisting of 15.6 km of dispersion compensating fiber (DCF). The ECL light was square-wave modulated at 500 Hz. At the output of the RFA, the signal of the modulated channel was eliminated with an optical band pass filter, and power fluctuations of the surviving channel were recorded with a high-speed digital oscilloscope. Power fluctuations as high as 0.45 dB with typical saturated amplifier overshoots were observed. The experimental results were confirmed by a large signal numerical analysis.

Error-free transmission for incoherent broad-band optical communications systems using incoherent-to-coherent wavelength conversion

A new method is presented to exploit the cross-gain modulation mechanism in semiconductor optical amplifiers to reduce the intensity noise of spectrum-sliced communications systems via conversion of the modulated incoherent optical signal into a coherent signal before the detection process. Wavelength-division-multiplexed networks using broad-band sources have performance limited by intensity noise; conversion from an incoherent to a coherent signal reduces the intensity noise and enables the elimination of the bit-error-rate floor.

Static lightpath establishment with transmission impairments consideration in WDM all-optical networks

Providing ultra high-speed end-to-end connectivity in core optical networks while satisfying the requirements for quality of service is a topic of intense research. In an optical network, a connection is set up to carry a data signal via an all-optical channel (lightpath) from its source to destination nodes. The analog nature of the signal transmission through the network links and nodes leads to signal quality degradation due to impairments accumulation. Such degradation should be taken into account when considering the Routing and Wavelength Assignment (RWA) problem. Most of preceding studies carried out on RWA neglect the impact of the physical layer impairments. In this paper, three physical layer impairment aware RWA algorithms are developed. We explicitly consider the effect of four physical layer impairments namely Chromatic Dispersion (CD), Polarization Mode Dispersion (PMD), Optical Signal to Noise Ratio (OSNR) and Nonlinear Phase Shift (φΝι). Taking into account physical impairments when solving the RWA problem intuitively should affect the computed solution: the selection of a suitable path and suitable wavelength may fail to meet the minimum transmission requirement. To the best of our knowledge, this is the first attempt to include, simultaneously, the four aforementioned transmission impairments when dealing with the RWA problem. We here propose a function that computes the Q-factor based on Eye-Opening Penalties (EOP) instead of using variances. This should lead to a better Bit Error Rate (BER) estimation. Few studies considered EOP in the last years when dealing with such a problem. The performance of the proposed algorithms is demonstrated to be promising through illustrative numerical examples.

Beat noise effects on spectrum-sliced WDM

In this paper, we review first, the different noise terms involved in amplified and filtered spontaneous emission detection using the standard beating theory (or the semi-classical field expansion theory) of the noise in optical amplifiers. We derive BER calculations using the following approaches: 1) non-ideal electrical and optical filters combined with a Gaussian approximation for the resultant noise process and an ideal integrate and dump electrical filter and an ASE source combined with non-Gaussian Erlang noise statistics.

An Encoder/Decoder Device Including a Single Reflective Element for Optical Code Division Multiple Access Systems

Summary In this paper, we present an encoding/decoding device for OCDMA communications. The device uses a single reflecting element to perform both the encoding of the outgoing signal and the decoding of the incoming signal. A directional optical setup allows differentiating the origin of the signals to forward the outgoing signals after encoding to the network and the incoming signals after decoding to a receiver.

Optimization of Optical Flat Comb Source based on Dual-Arm Mach-Zehnder modulator for flexgrid terabit superchannel WDM-Nyquist systems

In this paper, we investigate the generation of ultra-flat Optical Flat Comb Source (OFCS) using an asymmetrically driven Dual Arm Mach-Zehnder modulator (DA-MZM). A modified simulated annealing-based optimization method is applied to derive the necessary ranges for which the comb source is ultra-flat (less than 1 dB ripple) over 2, 4, 8, 16 and 32 carriers where the driving signals are well within the nominal operational range of a typical LiNbO3 MZM, and an acceptable output optical power is reached.