Part 1 of this FAQ began an overview into the arcane world of all-optical amplifiers including where and why they are needed. Part 2 will briefly describe the four types and identify the two most commonly used for optical-fiber link signal boosting.
Q: How “accurate” will this presentation be?
A: Frankly, it will have large-scale simplifications and approximations. The harsh reality is that everything associated with optical amplifiers is heavily dependent on deep-dive principles of optical and atomic-level physics, down to the quantum-physics level. There is no easy way to describe any of it without glossing over many specifics.
Q: What are the four types of optical amplifiers?
A: In brief, they are:
Erbium doped fiber amplifiers (EDFA) which use short lengths (a few meters) of optical fiber doped with the rare-earth element erbium. A pumping laser excites erbium ions in the fiber, which can then give their energy to the optical signals passing through as the ions return to the unexcited state (Figure 1). Amplified wavelength is in the 1550 nm range while the pump-laser wavelengths are usually 980 and/or 1480 nm. EDFAs are widely used with optical fiber links due to their ability to amplify signals at the low loss in the 1550 nm range wavelength range of the fiber. In the past decade, this design has improved and matured and is used for the required booster amplifiers placed every 10-20 km along the long-distance optical cables (both on land and under water) carrying data at very high bit rates.
Semiconductor optical amplifiers (SOA) use a gain block made from group III-V compound semiconductors (such as GaAs/AlGaAs, InP/InGaAs, InP/InGaAsP, and InP/InAlGaAs) as the gain medium in the laser, somewhat analogous to using a transistor to boost a small current base-emitter into larger collector-emitter current (Figure 2). Current is injected into the semiconductor material within a confining waveguide, which then creates new photons as it is stimulated by the input photon stream. The SOA is small and is “pumped” electrically rather than with another laser, and so can be less expensive than the EDFA, but overall the performance is still not as good.
Raman and Brillouin (pumped) amplifiers use nonlinear amplification in which a lower-wavelength pump-laser streams photons while traveling down an optical fiber along with the signal, scatters off atoms in the fiber, loses some energy to the atoms, and then continues its journey with the same wavelength as the signal (Figure 3). Nonlinear fiber is used to increase the intersection between the pump wavelength and the signal to reduce the fiber to the required length. It requires no special doping of the optical fiber but is a distributed amplifier where gain occurs throughout the length of the actual transmission fiber (unlike all in one place as is does with an EDFA).
Optical parametric amplifiers (OPA) use two light beams for its input — one for the signal and one for the pump — and on deliberate induced time variations in a critical parameter. The OPA nonlinear interaction makes the pump beam weaker while amplifying the signal beam, and also creates a new “idler beam” at the sum frequency (analogous to the sum and difference frequency outputs of an RF mixer). It uses parametric nonlinear interactions rather than laser amplification, and is useful for pulsed applications. The OPA allows the wavelengths to be tuned over a modest range, so it is also useful for broader bandwidths and tunable coverage.
Q: This is all somewhat confusing; perhaps you can give me a “road map?”
A: Figure 4 may help:
Q: What are some general limitations of these OAs?
There are many, but one factor is that an OA approach is generally useful or effective only at a specific wavelength or small range of wavelength, and this wavelength is fixed by the basic physics. There is no optical equivalent to a wideband electronic amplifier which can provide gain over a 2:1 frequency range (such as a single RF amplifier covering from 1 to 2 GHz, or an audio amplifier supporting frequencies over several orders of magnitude, typically 200 Hz to 20 kHz or more). Also, some OA approaches are better suited to low-duty cycle pulse operation (such as for a weapon) versus the continuous operation of a data link.
Q: What are the critical performance parameters for these OAs?
A: As with any amplifier or functional block in a signal chain, there are many factors of interest. The relative importance of each one with respect to the others is a function of the application priorities, of course. Among them are: operating center frequency (wavelength), bandwidth and tunability, output power, gain, internal noise, stability, size, cost, efficiency, complexity, sensitivity to component variations, drift and temperature coefficient, among many others.
Q: Where can I find out more?
A: By the nature of this topic, there’s much more to explain to understand how these OAs function. Given their deep-physics underpinnings, there are no quick-and-easy “shortcuts” to describing their principles at a simplified level – and that is without even getting into the intense equations which are critical to their design and implementation.
This general FAQ only touches the surface of the OA topic. There are many references online and in books, ranging from technically valid broad-brush overviews to intense, equation-heavy items. The references below provide some mid-level explanations of theory and operation, key attributes and comparative features, and the relatively strong and weak points of each approach in this fast-moving area. The “right” or “best” OA for a given application is determined by many factors and tradeoffs which must be defined and prioritized. In the case of OAs, however, the inherent physics-based characteristics of each OA structure (such as the limited specific wavelengths for which it works) structure define and limit where it is a possible fit and where it simply cannot be considered.
- Techopedia Inc, “Optical Fiber Amplifier”
- Invocom (Poland), “2 Optical amplifiers: classification, basic configurations and principles of operation”
- University of Arizona, “Section 5: Optical Amplifiers”
- Sergiusz Patela, “Optical amplifiers”
- University of Buffalo, “Inside the optical networks…A closer look to the amplifiers”
- Fowiki, “Understand Fiber Attenuation”
- Engineering 360, “Transatlantic Telegraph Cable: Engineering Innovations Still Used Today”
- Andrew Blum, “Tubes: A Journey to the Center of the Internet”
Plus many of the Wikipedia articles are good, among them: