High Power Amplifiers
With a growing need for higher power telecommunications amplifiers, doped fiber designs have needed to evolve to handle higher pump power and signal power levels. The requirement to split power over more channels, extract amplifier energy more rapidly, extend the signal range before amplification and the need to blast and split signals into more individual lines pushes the need for higher power capability.
The two discrete groups of doped fibers are core pumped fibers and cladding pumped fibers where typically core pumped fibers are pumped at 5W using Multimode (MM) pump lasers.
Core Pumped Fibers
Core pumped fibers are the most commonly used doped fibers in telecommunications amplifiers with Erbium forming the preferred doping material to give amplification across the C-band and L-band. Due to relatively small core diameters of Erbium Doped Fiber (EDF), gain saturation can occur at relatively low to medium power levels, limiting the maximum power output and efficiency of the amplifier at high power. Subsequently, Fibercore has developed a range high cut-off wavelength “HC” fibers. These fibers give larger core sizes to delay the onset of gain saturation towards higher power capability.
The HC fiber advantage offers significant benefits for the L-band, where the inherently low efficiency L-band amplifiers require high levels of pump power to achieve suitable signal levels.
Cladding Pumped Fibers
For high power amplifiers, many configurations are beginning to take advantage of cladding pumped fibers with dual or even triple cladding. Double and triple cladding fibers are designed to use the cladding as a guiding structure to guide MM pump light.
Figure 1: Diagrams representing the differences between double and triple cladding fibers
These fibers are designed to accept a high level of pump power launched into the inner cladding region, which guides the MM pump light. The inner cladding region typically is shaped in a non-circular structure such as D-shape or an octagon to stimulate mode-mixing with the view of increasing the rate of absorption of the pump light into the core. By launching into a large MM region, the energy is distributed over a larger area, reducing the risks associated with high intensity light, for example thermal failure due to poor splicing or non-linear effects. This subsequently allows the fibers to operate at significantly higher gain levels and higher absolute levels than core pumped EDFs.
The downside to the double and triple cladding structure is the pump light experiences a much lower absorption into the core matrix than a core pump fiber. This is often overcome by co-doping the Erbium core with another active element, Ytterbium, which has a much higher absorption cross section in the 910-980nm region. Ytterbium ions cannot emit in the C-band or surrounding bands, but the energy they absorb can be effectively transferred to excite a nearby Erbium ion, allowing the process of C-band signal amplification. By careful selection of active ion ratios and further doping with other network modifiers an efficient high power amplifier can be achieved through this process.