Lutz Rapp

Statistics of polarization dependent loss in an installed long-haul WDM system

We have made continual, multiple-day measurements of the polarization dependent loss of multiple C-band channels in an installed 1800 km terrestrial link. The PDLs of individual channels varied on the time-scale of hours, while the temporal variations of the PDLs of adjacent channels often tracked. The probability densities of the field measurements of PDL were not Maxwellian and instead were truncated, consistent with the limited number of elements in the link having appreciable PDL. A new model for the statistics of PDL in systems with few PDL elements is proposed, where a lower bound of the distribution exists if there is a dominant PDL element. The probability distributions from measurement and theory show good agreement.

Impact of Channel Count and PMD on Polarization-Multiplexed QPSK Transmission

Through graphics-processing-unit-based simulations with different numbers of copropagating channels (1--81), the dependence of the nonlinear threshold on channel count, as well as on the fiber polarization mode dispersion (PMD) coefficient, is investigated for both dispersion-managed and DCM-free 40 and 100 Gb/s coherent-detected polarization-multiplexed quadrature phase-shift keying (CP-QPSK) transmission systems. Different fiber types including standard single-mode fiber (SSMF), large effective area fiber (LEAF), and truewave classic fiber (TWC) are considered and compared. Our investigations show that the required number of simulated copropagating channels to correctly simulate the nonlinear penalty caused by interchannel nonlinearities on CP-QSPK modulation is strongly dependent on the fiber type. The generally used assumption of around ten channels for simulating interchannel nonlinearities is only valid for the SSMF with relative low channel input power. For transmission links consisting of fiber types with low dispersion or high nonlinear coefficients, such as the LEAF or TWC, ten copropagating channels are clearly not sufficient. In dispersion-managed systems with DCMs, the required number of simulated copropagating channels is not only dependent on fiber types and data rates but also strongly on PMD present in the links. Our investigations have indicated that for transmission over fibers with very low PMD (this is the case of most new fibers), ten copropagating channels are not sufficient to correctly characterize the interchannel nonlinearities even for high-dispersion fiber types, such as the SSMF, and hence causes a clear underestimation of the nonlinearity penalty. Finally, synchronized and interleaved CP-QPSK is compared. We show that despite the depolarization effect of PMD, there are still some benefits of using interleaved RZ-CP-QPSK systems.

Transient Behavior of EDFA Stages Using Pump Power Splitting or Pump Bypass Technique

Power transients in optical networks impose challenging requirements on the design and control of optical amplifiers. In addition, reducing amplifier cost has become mandatory. Therefore, setups using a single pump to supply pump power to several coils of erbium-doped fibers (EDFS) have become attractive. In most cases, the so-called pump bypass technique is employed, but pump power splitting also constitutes a favorable solution. Both techniques are compared with respect to their transient behavior. The investigations focus on the characteristics of the optical setup representing the control path. Appropriate scenarios are considered to take into account the influence of feedforward and feedback control techniques. Simulation results reveal that the pump power splitting approach is favorable with respect to a feedback control, whereas there are no significant differences with respect to feedforward approaches

Transient performance of erbium-doped fiber amplifiers using a new feedforward control taking into account wavelength dependence

Fast electronic control architectures are currently the most economical solution to stabilize the gain of erbium-doped fiber amplifiers (EDFAs). Commonly, feedback architectures are used since they allow to adjust the gain or output power to given target values and to compensate for control errors. However, purely feedback based controllers cannot meet the transient performance requirements for dynamically reconfigured networks. If improved transient performance is needed, a feedforward controller is added. However, state of the art implementations cannot achieve optimum performance since they do not take into account the shape of the input spectrum.

Dynamics of Spectral Hole Burning in EDFAs: Dependency on Pump Wavelength and Pump Power

Pump wavelength and pump power dependency of the dynamical response of spectral hole burning (SHB) in erbium-doped fiber amplifiers (EDFAs) is investigated. The experimental results show a clear dependency on pump wavelength also in the 1480-nm range. Similar to other effects in EDFAs affecting transient performance, dynamics of SHB speeds up with increasing pump power. We conclude that the pump has a significant contribution to SHB and has to be taken into account in numerical models for SHB.

Dynamics of Spectral Hole Burning in EDFAs: Dependence on Temperature

The dependence of the dynamical response of spectral hole burning (SHB) of an erbium-doped fiber amplifier (EDFA) on temperature is investigated. Experimental results show that gain variations decrease with increasing temperature. The dependence of the transition time on temperature is weak with a trend to larger values at higher temperatures. We also observe that EDFAs using two or three energy levels of the erbium ions show a similar behavior with respect to temperature variation. However, the three-level EDFA presents a higher dependence of the magnitude of induced gain variations on temperature.

Measurements of the polarization dependent loss of multiple WDM channels in an installed, long-haul terrestrial link

We report continuous, multiple-day measurements of polarization-dependent loss (PDL) of several C-band channels on an installed 1800-km terrestrial link. The PDL varies on a time-scale of hours, and the PDL of adjacent channels often track.

Site-Dependent Pumping Effect on Two-Level EDFAs

The impact of site-dependent pumping (SDP) on the performance of erbium-doped fiber amplifiers (EDFAs) pumped at wavelengths around 1480 nm is investigated, both experimentally and numerically. An improved numerical model incorporating the SDP effect for two-level EDFAs is derived and experimentally validated. We confirm by comparing with experimental data that the proposed model accurately describes the dependence of the amplifier gain spectrum on the pump wavelength and pump power. We conclude that SDP has a significant impact on the dynamic response of an EDFA due to input spectral variations and should be considered when simulating high performance links.

Timing Issues in the Feedforward Control of Erbium–doped Fiber Amplifiers

This paper deals with delayed reaction of the feedforward control used for erbium-doped fiber amplifiers (EDFAs). In general, transient performance of a single stage is deteriorated if the pump power is adjusted later than the input power change takes place. For power drops, resulting gain and noise figure overshoots are described as a function of the delay for a setup making use of pump power bypassing. However, a delayed reaction of the feedforward control can also contribute to improve dynamical behavior in multistage configurations. In a setup with two stages, unavoidable overshoots of the gain of the first stage that are due to the population of the third energy level can be compensated by undershoots induced in the second stage. Thus, only a small gain overshoot resembling a needle remains. Pump sharing is a quite often used technique to reduce amplifier cost. However, this technique leads to poor transient performance if the power provided by a common pump is launched into doped fiber coils placed before and after a component afflicted with delay. Results presented in this paper show that the dynamical behavior of such setups can be improved significantly by delaying the reaction of the feedforward control. Furthermore, it turns out that small splitting ratios are favorable with respect to transient performance. In contrast, larger splitting ratios lead to better steady-state performance.

Investigation into the significance of failure rate calculations for erbium—doped fiber amplifiers in view of the reliability of communication networks

This paper deals with the impact of erbium-doped fiber amplifier (EDFAs) on the reliability of communication networks. It is shown that failure rates calculated for EDFAs have only limited significance in view of the reliability of the complete network. Standard amplifier setups using a single pump per stage are compared with setups making use of pump sharing. The analysis reveals that differences with respect to downtime and mean time to repair of networks using these types of amplifiers are much smaller than it is expected from the failure rate calculations. This is due to the fact that standard amplifiers can compensate the failure of an intermediate pump for several operating conditions by increasing the power emitted by the other pumps, whereas compensation capabilities of setups based on pump sharing are strongly constrained. Copyright