M. Karasek

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.

Analysis of channel addition/removal response in all-optical gain-controlled cascade of erbium-doped fiber amplifiers

Application of the all-optical gain-controlled (AOGC) erbium-doped fiber amplifier (EDFA) for protection surviving channels in a multiwavelength network in which several EDFAs are cascaded is studied. The effects of addition and/or dropping of wavelength channels in a network comprising six concatenated EDFAs have been analyzed by numerical simulation. Only the first EDFA in the cascade is gain clamped using a ring laser configuration. A large-signal EDFA model which incorporates time variation numerical effects and the downstream propagation of signal and pump and downstream and upstream propagation of amplified spontaneous emission (ASE) has been used. In particular, the effects of relaxation oscillations from the gain-controlled EDFA on the surviving channel protection are investigated. It is shown that power excursions caused in an eight-channel wavelength division multiplexed (WDM) network by the loss/addition of 6 channels will be lower than 1 dB and free of relaxation oscillations if the gain-controlled EDFA is strongly inverted and the average normalized population density of the metastable level does not drop below 0.74.

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.

Large signal model of TDM-pumped Raman fiber amplifier

In this letter, we study spectral optical signal-to-noise ratio (OSNR) profile of a wide-band Raman fiber amplifier (RFA)with time-division-multiplexed (TDM) pumping. We derive a comprehensive large-signal numerical model which incorporates time variation effects and the downstream propagation of signals, upstream propagation of pumps, and downstream and upstream propagation of amplified spontaneous emission spectral components. We present results for a four-wavelength-pumped discrete RFA with TDM and continuous-wave pumping. Improvement in OSNR flatness of 0.9 dB due to TDM pumping is demonstrated.

High-performance adjustable room temperature multiwavelength erbium-doped fiber ring laser in the C-band

Abstract Experimental realization of a high-power and highly uniform multiwavelength cw erbium-doped fiber ring laser with an intra-cavity frequency shifter and line spacing of 100 GHz is presented. The extremely flat response is achieved by optimizing the laser cavity losses, pump power and length of the active fiber. A simple method of shifting the laser emitting spectral region, which preserves the spectral output flatness, is demonstrated. The laser may be easily switched between two spectral regions of 1538–1548 and 1543–1559 nm. In both ranges, the power uniformity is better than 3 dB.

Chromatic dispersion measurement using a multiwavelength frequency-shifted feedback fiber laser

In this paper, we present the realization of a fiber laser source emitting simultaneously over 17 wavelengths spread over the whole C-band. An acoustooptic frequency shifter is placed in the laser ring cavity to suppress the cross-gain saturation effects of the erbium-doped fiber. The emitted wavelengths are fixed by a set of fiber Bragg gratings (FBGs). A power uniformity reaching 6 dB is achieved by inscribing the FBGs while monitoring the laser output. We demonstrate the reliability of this laser as a source for characterization of optical components and networks by the measurement of optical fiber chromatic dispersion. The measurement is performed over the whole telecommunication C-band (1530-1560 nm) using the time-of-flight method. We perform the measurement on three different fibers with different levels of dispersion, namely a standard fiber, a nonzero dispersion shifted fiber, and a dispersion compensating fiber. The results are compared with measurements obtained using a standard network analyzer. The agreement between the two methods is better than /spl plusmn/1%, thus proving the suitability of the developed laser source for this application.

Channel addition/removal response in cascades of strongly inverted erbium-doped fiber amplifiers

Signal power steady-state and transient fluctuations due to gain cross saturation in erbium-doped fiber amplifiers (EDFAs) in multiwavelength optical networks caused by addition or removal of channels due to network reconfigurations or line failures must be minimized to avoid error bursts in the surviving channels. The effects of addition and/or dropping of wavelength channels in a multiwavelength network comprising six concatenated strongly inverted two-stage EDFAs have been analyzed by numerical simulation. A large-signal numerical model which incorporates time variation effects and the downstream and upstream propagation of signal, pump and amplified spontaneous emission has been used. Power excursions caused in an eight channel wavelength division multiplexing (WDM) network by the loss/addition of one, three, or six channels will be lower than 0.6 dB if the length average normalized population density of the metastable level does not drop below 0.76. No additional measures need to be taken to limit the power excursions of the surviving channels.