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Fiber Amplifiers

While the low loss of optical fiber allows signals to travel hundreds of kilometers, extremely long haul lines and submarine cables require regenerators or repeaters to amplify the signal periodically. In the beginning, repeaters basically consisted of a receiver followed by a transmitter. The incoming signal was converted from a light signal to an electrical signal by a receiver, cleaned up to remove as much noise as possible, then was retransmitted by another laser transmitter.

Figure 1. Electronic Repeater

These repeaters added noise to the signal, consumed much power and were complicated, which means they were a source of failure. They also had to be made for the specific bit rate of transmission and upgrading required replacing all the repeaters, a really difficult task in an undersea cable!
Since the 1960s, researchers knew how to make fiber lasers. Proper doping of the fiber (introducing small amounts of active elements into the glass fiber) allowed it to be pumped with external light sources until stimulated emission occurred. While making fiber amplifiers was hypothesized early in the stages of fiber optic development, it was not until 1987 that working models were realized. Major contributors to the development included Bell Labs and NTT.
The typical fiber amplifier works in the 1550 nm band and consists of a length of fiber doped with Erbium pumped with a laser at 980. The pump laser supplies the energy for the amplifier, while the incoming signal stimulates emission as the pulse passes through the doped fiber. The stimulated emission stimulates more emission, so there is a rapid, exponential growth of photons in the doped fiber. Gains of >40 dB (10,000X) are possible with power outputs >+20 dBm (100 mW).

Figure 2. Basic Fiber Amplifier

To date, the most efficient fiber amplifiers have been Erbium-Doped Fiber Amplifiers (EDFAs) operating in the 1550 nm range. Since most systems still work at 1310 nm, considerable research has been done to find materials that would work in this range. Praseodymium-doped fluoride fiber amplifiers (PDFFAs) using fibers made from zirconium fluoride or hafnium fluoride have shown some promise, but have not developed the performance needed for widespread applications.
The basic structure of an EDFA is very simple. The amplifier itself emits light energy in a signal wavelength (usually about 1540nm) using energy supplied to it by photons in a pump wavelength (usually 980nm) when stimulated by incoming photons in the signal - the signal which needs amplification. Just like in a laser, the emitted photons then stimulate other emissions, so there is an exponential growth of photons. Supporting the amplifier is a pump laser, which supplies the amplifier's energy, a coupler, which combines the pump laser beams and the signal laser beam and puts them on a single fiber, and an optical filter, which removes the remaining traces of the pump beam so that it doesn't interfere with reception of the signal.

Why Erbium?

Erbium has several important properties that make it an excellent choice for an optical amplifier. Remember that there are several very specific bands (wavelengths) that fiber optic cables can carry. Erbium ions (Er3+) have quantum levels that allows them to be stimulated to emit in the 1540nm band, which is the band that has the least power loss in most silica-based fiber. That gives them the ability to amplify signals in a band where high-quality amplifiers are most needed.
Erbium's quantum levels also allow it to be excited by a signal at either 800nm or 980nm, both of which silica-based fiber can carry without great losses, but aren't in the middle of the signal wavelengths. Those bands are also far enough away from the signal bands that it is easy to keep the pump beam and the signal beam separated.

Figure 3. Energy States of Erbium

When erbium is excited by photons at 800nm or 980nm, it has a non-radiative decay (energy drops without producing light) to a state where it can stay excited for relatively long periods of time - on the order of 10ms. This property is extremely important, because the quantum efficiency of the device is dependent on how long it can stay in that excited state. If it relaxes too quickly, more photons are needed to keep it excited, meaning more input power is needed to make the amplifier work.
Erbium can also be excited by photons at 1480nm, but this is typically undesirable. When excited at that wavelength, both the energy pumping process and the stimulated emission by the signal are happening in the same wavelength and energy band, which can create interactions that lower the efficiency of the device and increase the amplifier noise.
Another important property of erbium for use in a fiber amplifier is that it is fairly soluble in silica, making it easy to dope into mixtures for making silica-based fiber. For many applications, reasonable EDFAs can be made by simply dissolving Er2O3 in a crucible with the SiO2 used to make silica fiber. By using a co-dopant, such as Al2O3, GeO2-Al2O3, or P2O5, the erbium compound's solubility in the silica mixture can be greatly increased, and some of the EDFA's properties can be improved. For example, GeO2-Al2O3 can be used to almost double the time it takes for excited erbium to relax, which therefore almost doubles the quantum efficiency of the EDFA.
EDFAs are not perfect. In practice, you need to have many pump beams along the length of a fiber to provide the energy for EDFAs and these require power and optics (couplers and filters.) EDFAs also have gain that varies with a signal's wavelength which creates problems in many WDM applications. This can be solved by using special optical passive filters that are designed to compensate for the gain variation of the EDFA.

Alternative Designs
The simple diagram of an EDFA shown in Figure 2 is not the only way EDFAs can be made. Pumping can be done in a forward direction as shown, backward from the output end or in both directions. Optical isolators are commonly used at both ends of the EDFA to prevent pump energy from escaping back down the fiber or unwanted reflections that may affect laser stability. Filters, often Bragg gratings (filters fabricated in fibers), are used to flatten the gain over the broadest wavelength range for use in WDM systems.

Other Applications

Besides being used as repeaters, fiber amplifiers are used to increase signal level for CATV systems, which require high power levels at the receiver to maintain adequate signal to noise performance, allowing longer cable runs or using splitters to "broadcast" a single signal through a coupler to many fibers, saving the cost of additional transmitters. In telephony, they combine with DWDM (dense wavelength division multiplexers) to overcome the inefficiencies of DWDMs for long haul transmission.

Future Developments
Fiber amplifiers continue to be developed to support Dense Wavelength Division Multiplexing and to expand to the other wavelength bands supported by fiber optics. Now that fiber manufacturers have all but removed the water bands from the spectrum, there is now a range of 1260 to 1610 nm available for use. Fiber amplifiers and diode lasers will probably be developed within this band to completely fill it with useable bandwidth.

A new device being worked on is the Semiconductor Optical Amplifier (SOA) which basically duplicates the function of a FOA but in an integrated circuit fabricated like a diode laser.

Our thanks to Chris Cox, Craig Metz and Ron Taylor who researched and created much of the material in this page as a project at Virginia Tech.

( c ) 2003, The Fiber Optic Association, Inc.

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