SPECIAL REPORTS / Optical Networking/WDM
Optical-amplifier technology has conveniently matured and is well positioned to respond to the soaring demands from future optical networks.
DAVID TEED, BTI Photonics Inc.
The communications industry continues to face challenges stemming from traffic growth and network complexity. Challenges are resulting from both the increasing number of Internet users and new higher-bandwidth-demanding applications. The need for advances in optical technology and functionality in future optical networks has never been greater.
The concept of the all-optical network that will enable carriers to reduce network costs while significantly increasing bandwidth capacity is quickly becoming reality. Amplification technologies that cost-effectively enable the transmission of optical signals over long distances and reduce or eliminate the requirement for signal regeneration stations or optical-electrical-optical (OEO) conversion are a step toward achieving this promise.
Telecommunications industry analyst RHK Inc. (San Francisco) predicts the market for DWDM components to explode, growing from $5 billion in 2000 to $24 billion by 2004. In 2001, the market for amplifiers and other components for the DWDM market will grow an average 48% per year to 2004. RHK says that no product area within the market will grow less than 80% overall.
DWDM increases bandwidth by using many wavelength channels; it is widely used today to exploit the total capacity of optical fiber. DWDM technology has vastly increased fiber capacity by delivering the technology to transform each fiber strand into more than 80 parallel optical wavelengths; 160-wavelength systems are now being introduced.
DWDM would not be viable without optical amplifiers. Cost per bit would be much higher, and this added expense would definitely have limited the growth of the Internet. The Internet's growth is reliant on transmission bandwidth availability and low connection costs.
Advances in optical devices present promising opportunities to meet the challenges due to communications traffic growth. Fiber amplifiers are crucial components for high-performance fiber-optic systems. The function of an optical amplifier is simple: It boosts the power of a photonics network. Optical amplifiers facilitate long-distance transmission, and their ability to directly amplify optical signals without having to convert them to some other form for processing is desirable for communications.
Erbium-doped fiber amplifier (EDFA) amplification occurs when an input signal stimulates emission from excited erbium atoms in the optical fiber (see Figure 1). As erbium atoms drop from their excited state, they release their extra energy as light in the form of spontaneous emission. When directed along the fiber, the light becomes amplified. As with any amplifier, a small amount of noise is generated, in this case by amplified spontaneous emission.
An EDFA can simultaneously amplify weak light signals at wavelengths across their entire operating range. This range varies depending on the amplifier design, with the amplified signals appearing as spikes, typically 30 dB higher than the amplified noise. The signal remains at this high level of energy until it is attenuated in the fiber (80 to 100 km) or until it reaches the next amplifier.
EDFAs are well established and represent a multibillion-dollar-per-year market. They cost-effectively provide high gain and can use existing and new fiber-optic cabling. While EDFAs still comprise about 95% of the total market, they are not the only available amplifiers. Raman amplifiers, erbium-doped waveguide amplifiers (EDWAs), and semiconductor optical amplifiers (SOAs) are currently-or soon will be-commercially available. Some amplifiers may find more applications than others, but all are part of a trend toward increased specialization. Today's optical amplifiers are trying to keep pace with today's networks-not an easy task.
Raman amplifiers use the components similar to those used in EDFAs. They increase the power-handling requirement of passive devices along with creating other demands. Raman amplifiers are rapidly gaining favor in the marketplace because they introduce much less noise than EDFAs. However, they cannot provide high gain, can cost significantly more than EDFAs, and because of their high power, cannot be used with many types of today's existing fiber-optic cable.
Raman amplification requires no special doping in the optical fiber. It is usually accomplished as "distributed amplification," meaning it happens throughout the length of actual transmission fiber rather than all in one place in a small box (as with an EDFA, for example). So a higher-power pump wavelength can be added into the same fiber carrying the signal, which will amplify the signal along many kilometers of fiber until the pump signal eventually fades away.
The signal-to-noise improvement of Ra man amplifiers is also critical to achieving ultra-high-bit-rate systems, whereby additional performance requirements are placed on the system due to dispersion impairments. Not surprisingly, the Ra man amplifier market is forecast at more than $1 billion within a few years.
A major weakness of current EDFAs is their poor signal-to-noise ratio (SNR), but this can be mitigated through Raman amplification. More often than not, many consider the two technologies complimentary and not competing, with one being used in conjunction with the other. EDFAs provide a cost-effective transfer of energy, but can only boost a signal to a certain level before nonlinearities begin to develop in the fiber.
The Raman amplifier acts almost like a preamplifier and provides a better SNR. Ultimately, both types of amplifiers will find their market niches and coexist. Together, they can provide a good solution for many requirements, but as carriers look for much higher returns on investment from their networks-particularly in the rapidly growing but cost-sensitive MAN market-even more economical and efficient amplifier solutions will be required.
With optical-network traffic growing and demand for bandwidth increasing, the requirement for technology to help alleviate network bottlenecks is high. Developers of optical amplifiers have a role to play in successfully addressing this demand. Organizations that hope to become the leaders in this market will need to develop amplifiers with the following characteristics:
- High signal gain.
- High saturated power output.
- Minimal noise generated within the fiber amplifier.
- Ultra-wide bandwidth.
Researchers are now working on innovative new components and proprietary designs to maximize gain and minimize noise generated within the fiber amplifier. These hybrid optical amplifiers will potentially offer the high gain typically associated with EDFAs, with the much lower noise levels of Raman amplifiers.
Hybrid amplifiers could potentially impact the way future optical networks are designed and deployed by allowing much greater distances without regeneration stations. Since regeneration stations are very expensive (millions of dollars each), the cost savings could be huge. The main advantage of a hybrid amplifier would be realized in longer stretches (typically long-haul or very-long-haul networks), but there will also be applications in regional networks and MANs.
Another significant trend in fiber amplifiers over the next couple of years will be much higher levels of integrated functionality. Instead of buying standalone fiber amplifiers, along with numerous other standalone network components, and building them all individually into a network, more and more customers will be looking for additional functionality built into one integrated module. In the very near future, we expect the integration of optical-component functions such as gain-flattening, dispersion compensation, and channel equalization into fiber amplifiers to become much more common. These integrated fiber-amplifier modules will offer very powerful and cost-effective solutions to customers.
Even more exciting and powerful is the ability to offer even more value in the future by integrating emerging all-optical "signal processing" technologies. These technologies will enable dynamic channel equalization, dynamic gain flattening, dynamic dispersion compensation, and dynamic variable optical attenuation into advanced hybrid fiber amplifiers, thus opening up completely new network options and capabilities. That has the potential to allow the emergence of truly dynamic, all-optical networks that will operate much more efficiently.
These networks will allow service providers to offer new cost-effective, revenue-generating services that will take advantage of the flexibility to lease and manage wavelengths dynamically, with far simpler network architectures and without the need for as many (if any) OEO conversions or signal regeneration stations. These dynamic optical building-block functions, integrated into advanced hybrid fiber amplifiers, will create a new generation of "optical regenerator" that can clean up weak input signals, regenerate sharp, correctly timed digital pulses, and eliminate dispersion and distortion effects (see Figure 2).
Optical-amplifier technology has conveniently matured and is well positioned to respond to the soaring demands of future optical networks. EDFAs seem to have the capability to evolve and retain their lead as the premier amplifier for optical networks. Through innovation to improve noise performance and maintain high gain, this approach appears to offer the best value for the emerging all-optical network, while providing an ideal platform for the integration of future value-added optical-component technologies.
David Teed is vice president of product marketing at BTI Photonics Inc. (Ottawa, Ontario). He can be reached at firstname.lastname@example.org.