Increasing transmission capacity of installed fiber optical communication systems by multiband (MB) transmission offers large advantages compared with other options if physical access to all ...intermediate amplification sites is granted. This is typically the case in terrestrial networks or unrepeatered submarine links without remote optically pumped amplifier (ROPA). Challenges for introducing this technology and solutions for overcoming them are detailed with a focus on energy efficiency, transmission performance, system installation, amplifier manufacturing, and device control. However, only few systems will be prepared for such a capacity upgrade such that traffic interruption is unavoidable. Splitting an incoming wavelength-division multiplexing (WDM) signal into different bands for amplification, which is mandatory when using rare-earth doped fiber amplifiers, leads to performance degradation and reduced power conversion efficiency. Based on a hypothetical energy level diagram, it is explained why using lanthanides other than erbium results in reduced power conversion efficiency. Due to the intrinsic wavelength characteristics of the cross sections of thulium, efficiency is further significantly reduced when designing gain-flattened thulium-doped fiber amplifiers for operation beyond 1510 nm. Simulation results show that gain and tilt adjustment in reaction to power transients will take at least 200<inline-formula><tex-math notation="LaTeX">\,\mu \text{s}</tex-math></inline-formula>.
Parallel data transmission in several wavelength bands over a single optical fiber imposes divergent requirements on the employed optical amplifiers. The focus of the investigations is on the maximum ...output power and the maximum gain each of the amplifiers needs to be designed for. The results for a C+L-band system reveal that the maximum output power provided by a long-wavelength band (L-band) amplifier is limited and any further increase of the total power launched into a fiber span needs to be realized by higher total powers in the conventional band (C-band). Based on these findings, the resulting noise performance is determined for operation at maximum number of channels for a typical design of a commercial erbium-doped fiber amplifier. Output power and gain requirements are finally considered when further extending the used bandwidth by the short-wavelength band (S-band). For this scenario, noise figure is determined by means of a generic optical amplifier model making the results independent of the current level of technology. It is shown that designing amplifiers for making the optical transmission system tolerant to power transients leads to noise figure degradation of the long-wavelength amplifiers when all bands are fully populated with channels.
Codirectional Raman amplification with higher-order pumping is a promising solution for improving performance of commercial unrepeatered submarine links and thus to allow for an upgrade to advanced ...modulation formats offering larger capacity. However, power fluctuations of the involved high-power pump induce phase shifts in phase modulated signals via the nonlinear Kerr effect that destroy the theoretically possible performance improvement. A technique for reducing the impact of this effect on system performance by jointly processing signals of neighboring channels and evaluating the phase of both channels at a time for symbol detection is presented and evaluated.
Opening new wavelength bands is the most economic step for further increasing the capacity of optical transmission links. Characteristics of different amplifier technologies for signal amplification ...in different wavelength bands are detailed. In particular, the suitability of these technologies for short-term and mid-term implementation is considered. An important criterion is the availability of qualified components, notably the required pump laser diodes. On this basis, solutions for the near-term and the mid-term are discussed.
Single-fiber bidirectional transmission systems using the same wavelength for both directions suffer from reflection induced interference. Experimental results demonstrate the dependence of system ...performance and of the threshold of the forward error correction (FEC) on signal waveform variations induced by fiber effects. Larger signal distortions lead to slightly larger impact of the interference, but reduce the shift of the FEC threshold to smaller bit error ratios.
Feedforward control of optical amplifiers is mandatory in wavelength routed networks to avoid traffic interruption resulting from fast input power variations. Effects limiting the efficiency of this ...control technique are discussed and an overview over techniques compensating for the most important ones is presented. Detailed insight into the wavelength dependence of the optimum pump power setting is provided by a mathematical model. Taking this dependence into account requires to filter the input signal. Three different techniques are explained that make use either of an additional filter placed in front of the input monitor, an embedded gain–flattening filter, or even the erbium–doped fiber itself. Timing is another important issue. Besides introducing a delay in front of the first doped fiber, sophisticated control schemes working without additional optical components by modifying the waveform of the pump signal are analyzed. Different setups such as cascaded amplifier stages with intermediate dispersion compensating fiber are considered. The stages are either controlled individually or are coupled via a pump splitter. Finally, solutions complementing the pump power control by fast adaptation of the attenuation of a variable attenuator are contemplated.
Feedforward control of optical amplifiers is mandatory in wavelength routed networks to avoid traffic interruption resulting from fast input power variations. Effects limiting the efficiency of this ...control technique are discussed and an overview over techniques compensating for the most important ones is presented. Detailed insight into the wavelength dependence of the optimum pump power setting is provided by a mathematical model. Taking this dependence into account requires to filter the input signal. Three different techniques are explained that make use either of an additional filter placed in front of the input monitor, an embedded gain–flattening filter, or even the erbium–doped fiber itself. Timing is another important issue. Besides introducing a delay in front of the first doped fiber, sophisticated control schemes working without additional optical components by modifying the waveform of the pump signal are analyzed. Different setups such as cascaded amplifier stages with intermediate dispersion compensating fiber are considered. The stages are either controlled individually or are coupled via a pump splitter. Finally, solutions complementing the pump power control by fast adaptation of the attenuation of a variable attenuator are contemplated.
Higher-order pumping schemes enhance the performance improvement provided by codirectional Raman amplifiers, which is achieved by pushing the maximum of the signal power deeper into the transmission ...fiber. This paper deals with the performance limits inherent to this technology.
Simulation results reveal that the performance of an unrepeatered link operated with 10Gbit/s NRZ signals increases with growing distance between the location of the maximum power and the fiber input up to distances of 70km. For larger distances, the system performance starts to decrease. The amount of dispersion precompensation providing optimum system performance increases with increasing distance of the maximum to the fiber input. Furthermore, it is shown that the tolerance to variations of the precompensation is almost independent of the position of the maximum and, thus, the order of the pumping scheme. Nevertheless, the operation of commercial systems employing this kind of technologies becomes more complex since the system becomes more sensitive to variations of the residual dispersion. If the maximum of the signal power is reached after more than ≈50km of propagation in the fiber, deviations of the residual dispersion from its optimum value exceeding ≈20ps/nm/km lead to a performance degradation larger than 0.5dB. However, it is also shown that the sensitivity to deviations of the residual dispersion can be reduced at cost of the resulting performance benefit.
Existing unrepeatered submarine links are increasingly upgraded to the most advanced modulation format currently available for commercial applications. Quite often the use of third-order ...codirectional Raman amplifiers is necessary. Power fluctuations of the involved high-power pump induces phase shifts in phase modulated signals via the nonlinear Kerr effects. An approach for reducing the impact of this effect on system performance is explained in this document.
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.
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