Optical Amplifiers

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Definition

Optical amplifiers are the device that can amplifie optical signal directly, without the need to first convert it to an electrical signal. They are a key enabling technology for optical communication networks. Together with wavelength-division multiplexing (WDM) technology, which allows the transmission of multiple channels over the same fiber, optical amplifiers have made it possible to transmit many terabits of data over distances from a few hundred kilometers and up to transoceanic distances, providing the data capacity required for current and future communication networks. Optical amplifiers are important in optical communication and laser physics.

Main Amplifier Technologies in Use Today

Today’s common used optical amplifiers include Erbium-Doped Fiber Amplifier (EDFA), Raman Amplifier, and Silicon Optical Amplifier (SOA).

Erbium-Doped Fiber Amplifier (EDFA)
EDFA works through a trace impurity in the form of a trivalent erbium ion that is inserted into the optical fiber’s silica core to alter its optical properties and permit signal amplification. The trace impurity is known as a dopant, and the process of inserting the impurity is known as doping or being doped. Pump lasers, known as pumping bands, insert dopants into the silica fiber at a 980 or 1480 nanometer (nm) wavelength, resulting in a gain, or amplification, in the 1550 nm range, which is the optical C-band. The 1480 nm band is usually used in amplifiers with greater power. Pump lasers operate bidirectionally. This action amplifies a weak optical signal to a higher power, effecting a boost in the signal strength. However, EDFAs are usually limited to no more than 10 spans covering a maximum distance of approximately 800 kilometers (km) and also cannot amplify wavelengths shorter than 1525 nanometers (nm). The EDFA was the first successful optical amplifier and a significant factor in the rapid deployment of fiber optic networks during the 1990s.

Raman Amplifier
In a Raman amplifier, the signal is amplified due to stimulated Raman scattering (SRS). Raman scattering is a process in which light is scattered by molecules from a lower wavelength to a higher wavelength. When sufficiently high pump power is present at a lower wavelength, stimulated scattering can occur in which a signal with a higher wavelength is amplified by Raman scattering from the pump light. SRS is a nonlinear interaction between the signal (higher wavelength; e.g. 1550 nm) and the pump (lower wavelength; e.g. 1450 nm) and can take place within any optical fiber. In most fibers however the efficiency of the SRS process is low, meaning that high pump power (typically over 1 W) is required to obtain useful signal gain. Thus, in most cases Raman amplifiers cannot compete effectively with EDFAs. Raman amplification provides two unique advantages over other amplification technologies. The first is that the amplification wavelength band of the Raman amplifier can be tailored by changing the pump wavelengths, and thus amplification can be achieved at wavelengths not supported by competing technologies. The other more important advantage is that amplification can be achieved within the transmission fiber itself, enabling what is known as distributed Raman amplification (DRA). Raman amplifiers are most often used together with EDFAs to provide ultra-low NF combined amplifiers, which are useful in applications such as long links with no inline amplifiers, ultra-long links spanning thousands of kilometers, or very high bit-rate (40/100 Gb/s) links.

Silicon Optical Amplifier
SOAs are amplifiers which use a semiconductor to provide the gain medium. They operate in a similar manner to standard semiconductor lasers (without optical feedback which causes lasing), and are packaged in small semiconductor “butterfly” packages. Compared to other optical amplifiers, SOAs are pumped electronically (i.e. directly via an applied current), and a separate pump laser is not required. However, small size and potentially low cost due to mass production, SOAs suffer from a number of drawbacks which make them unsuitable for most applications. In particular, they provide relatively low gain (

Properties of Optical Amplifiers
  • Gain, Input Power and Output Power – The gain is typically measured in dB, and is in the range of 10-30 dB. A gain of 10 dB means the input optical signal is amplified by a factor of 10, while a gain of 30 dB means the input optical signal is amplified by a factor of 1000. WDM amplifiers require a relatively high saturated
    output power, typically in the range 17-23 dBm.
  • Noise – The noise performance of an optical amplifier is characterized by its noise figure (NF), which is defined as the ratio of the signal-to-noise ratio (SNR) at the amplifier output to an ideal SNR at the input. Noise figure in an ideal DFA is 3 dB, while practical amplifiers can have noise figure as large as 6–8 dB.
  • Dynamic Properties
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