Based on placements, for a DWDM link there are three types of amplifiers:
- Booster Amplifier
- Pre-Amplifier
- In-Line Amplifier
Booster Amplifier
Main purpose of booster amplifier is to boost the power transmitted.
A booster amplifier is used to amplify the signal channels exiting the transmitter to the level required for launching into the fiber link. In most applications this level is in the range of 0-5 dBm per channel, however, it can be higher for more demanding applications. A booster is not always required in single channel links, but is essential in a WDM link where the multiplexer attenuates the signal channels. A booster amplifier typically has low gain (in the range of 5-15 dB) and high output power, typically about 20dBm for a 40 channel WDM system. The NF of a booster amplifier is not usually a critical parameter. At the other end of a link a pre-amplifier may be required to amplify the optical signal to the level where it can be detected over and above the thermal noise of the receiver.
Pre-Amplifier
These amplifiers are commonly used to improve the receiver sensitivity. Transmission distance can also be increased by putting an amplifier just before the receiver to boost the received power.
A pre-amplifier should provide high gain, often in the range of 30 dB, and have a low NF in the range of 4-5.5 dB, in order to assure error-free detection of the signal channels. The output power of the pre-amplifier need not be very high.
For links up to about 150 km, a booster and/or pre-amplifier are usually sufficient to ensure error-free transmission. However, for links above 150 km the performance deteriorates to such an extent that the signal becomes undetectable.
In-Line Amplifier
These amplifiers are used for compensating distribution losses in local-area networks. replace electronic regenerators. An in-line amplifier is characterized by large gain and low noise to amplify an already attenuated signal so that it can travel an additional length of fiber.
In-line amplifiers are placed every 80-100 km to ensure that the optical signal level remains above the noise floor. In-line amplifiers typically require moderate gain in the range of 15-25 dB, and NF in the range of 5-7 dB. Output power requirements are similar to those of booster amplifiers. While in the early days of optical amplifiers different amplifier models had to be specifically tailored for each of the above functions, today the technology has advanced so that a single well designed amplifier model can perform many of the functions for typical applications. However, there still remain challenging applications which require specially designed amplifiers, such as very high output power boosters, or ultra-low noise pre-amplifiers.
