Surging internet video and cloud services rapidly fill up existing fibre networks. Operators eye unused spectrum within fibre as the most cost-effective way to expand capacity before burying new cables. But transmitting data outside optimized 1550nm wavelengths encounters challenges. Signals simply fade away without adequate use of an optical amplifier.
Fortunately, optical amplifiers can selectively strengthen light across wide wavelength ranges. Erbium-doped varieties have amplified systems for three decades by emitting photons matching signals passing through. Germanium-doped fibre also extends its range by capturing pump light and then shifting energy to information signals via nonlinearity.
This article surveys attributes of key amplifier species to assess readiness providing gain across alternate wavelength bands.
Motivations and Challenges
With relentless growth in internet bandwidth demand, existing fibre network capacity centred around optimized 1550nm C-band wavelengths nears exhaustion. Further increasing single-mode fiber data carriage beyond emerging PAM4, faster electronics and denser wavelength packing becomes progressively more difficult and expensive. However, additional unconventional bands spanning from 1400-1650nm exist offering potential capacity relief if only signals propagating therein could be amplified to extend range.
Erbium-doped fibre amplifiers (EDFAs) have reliably amplified C-band systems for over two decades by emitting photons precisely matching passing signals. Raman amplification has similarly extended reach by using high-power pumps to stimulate parametric wavelength conversion governed by fibre’s molecular gain resonance. However, realizing capacity growth by migrating into fresh wavelength territory requires replicating C-band amplification performance across this wider spectral expanse.
Fortunately, optical amplifier physics allows customizing gain by engineering interactions between light, matter and charge carriers across broad wavelength regimes. For example, rare earth minerals other than erbium can potentially stimulate emission in bands like S, E and O when doped into silica fibre. Properly designed semiconductor optical amplifiers also induce optical gain through electronic transitions. Even the complex behaviour of recently discovered bismuth-doped media shows promise for bandwidth expansion.
Key Optical Amplifier Approaches
This paragraph surveys various amplifier and pumping approaches assessing readiness to provide gain across alternate wavelength division multiplexed bands.
Rare Earth Doped Fiber Amplifiers
These amplifiers employ silica or fluoride glass optical fibres doped with rare earth elements such as erbium, thulium, praseodymium and neodymium embedded in the core. These trivalent metallic ions share similar atom-like electron configurations that enable useful optical properties. Optical pumping raises electrons to metastable states outside the 4f subshell. Transitions between these energy states provide a mechanism for stimulated signal amplification.
Erbium particularly provides efficient amplification centred at 1530nm because higher energy transitions are strongly quenched while the lower state exhibits a long-lived metastable level around 10ms. This easily populates the upper laser transition. Sharp emission cross section also concentrates gain narrowly around peak wavelength.
By contrast, non-erbium dopants exhibit shorter metastable state lifetimes that limit inversion. More pathways compete with the desired optical transition reducing gain efficiency. These non-radiative transitions require suppression sometimes needing low phonon energy glass hosts. Overall heavier engineering lift makes extending rare earth amplification inherently challenging.
Distributed Raman Amplification
Unlike rare earth schemes, Raman amplification leverages the innate nonlinearity of Germanium-doped silica fibre. Pump photons scattering off the optical fibre lattice transfer energy to signals propagating at longer wavelengths prescribed by fiber’s Raman gain spectrum. Peak gain occurs when pump wavelength 13.2THz above signal.
Backwards pump propagation is preferred since the backwards scattering probability is higher. Counterpropagation also reduces relative noise. Matching multiple pump wavelengths allows a cascading optical gain downshifting spectrum. But low intrinsic nonlinearity limits efficiency, so high pump powers up to 1W are needed. Relative intensity noise also rises with a large Raman shift. Still, Raman amplification provides indispensable flexibility extending standard EDFAs.
Semiconductor Optical Amplifiers
Semiconductor optical amplifiers stimulate electrons occupying conduction bands of III-V compound semiconductors to recombine with holes in the valence band emitting photons by spontaneous and stimulated emission. Electrical current injection maintains carrier population in bands. Tensely confining optical mode and carrier diffusion in thin active regions increase gain through high inversion levels.
SOAs support very wide bandwidths up to 100nm since emission energy primarily depends on materials rather than atomic states. However, performance remains inferior to EDFAs due to higher noise figures and lower saturation output power. On-chip reflective elements like Bragg gratings help overcome limitations by clamping gain. Monolithic integration also enables compact packaging and mass production, though expensive epitaxial growth constrains scalability today.
Bismuth-Doped Fiber Amplifiers
Still early in development, bismuth displays unusual amplification characteristics compared to rare earths. As the lone pnictogen group element amenable to glass doping, outer electron levels readily interact with hosts making for highly tunable emission wavelengths and bandwidths. Oxide and fluoride fibers produce broad photoluminescence spanning 1350nm to 1700nm although precise atomic mechanisms remain unclear.
This environment sensitivity causes bismuth-doped media efficiency and reliability to currently fall short of rare earths. But measured internal fibre gain now approaches 30% in E and O bands – double early demonstrations. And broad 110nm amplification recently achieved rivals EDFAs. So while atomic nature and poor reliability still undergoing investigation, bismuth-doped fiber amplifiers exhibit significant promise and rapid improvement.
Each technology displays strengths and weaknesses providing gain across extended wavelength division multiplexing (eWDM) spans. Performance metrics like efficiency, gain flatness and bandwidth differ in each wavelength range. And commercial availability varies widely.
Practical Network Deployment Scenarios
Grouping low-loss telecom wavelength regimes into bands labelled O, E, S, C, L and U allows d capacity expansion. Short, medium and long term time horizons provide realistic milestones balancing commercial availability, cost and reliability.
Short Term Strategy – Leverage Working Infrastructure
The short-term strategy foresees to use of only thoroughly proven amplifiers like EDFAs and Raman pumps that qualified for stringent Telcordia reliability tests.
Distributed Raman backpropagating 1400-1500nm pumps offer the widest spectral coverage – perhaps 120nm bandwidth at low gain ripple. Counterpropagating pumps also reduce relative intensity noise. So given better efficiency and supply maturity, hybrid EDFA and distributed Raman mixes enable easily achievable capacity growth on existing fiber plants.
Mid Term Approach – EDFA Extension
Once amplifier reliability is confirmed through accelerated lifetime testing, optical gain extending beyond L-band becomes viable. For example, thulium-doped fibre amplifiers (TDFAs) offer direct L-band extension when paired with 1400nm pumps. 50% efficiency makes TDFAs attractive for high-reliability links. S-band distributed Raman amplification can also be engineered by cascading longer wavelength pumps.
Combining both TDFAs and Raman amplification could cover 1450-1640nm if challenges interconnecting dissimilar amplifier species are solved. Bandpass filters must isolate different gain media while output power balancing prevents band cross-talk. Employing gain-flattened EDFA preamplifiers also manages noise figures and nonlinear penalties.
Longer Term Vision – Unconventional Bands
Even emerging bismuth-doped fibres demonstrably claim over 40nm bandwidth in higher loss E and O bands today, with rapid ongoing improvements through atomic engineering. Optimized semiconductor optical amplifier constructs show promise in approaching EDFA performance given adequate isolation. So prudent planning bets on consistent incremental upgrades rather than disruptive revolution.
Conclusion: Roadmaps to Terabit Capacity
Savvy engineering balancing commercial availability, performance and reliability will illuminate optimal eWDM migration paths. True, no single amplifier species serves all wavelength ranges or network locations. But judiciously combined Raman, EDFA and TDFA amplification promises to unlock abundantly available dark fibre capacity for decades of network growth ahead.
The technology mosaic enabling economic capacity scaling exists within the industry’s grasp. Realization hinges on meticulous multidisciplinary engineering – and stakeholders willing to deeply understand unfamiliar amplifier species until performance metrics prove compatibility with operational needs. Testing combinations by focused specialists willing to methodically quantify pros and cons prepare next-generation network advancements when commercial constraints inevitably demand cost-optimized rollouts.
For now, economics confines capacity enlargement to erbium’s sliver of the vast glass bandwidth. But patient pragmatic synthesis promises prosperous ubiquitous amplification across optical telecom’s wavelength estate in due season.