Classifications and Features of Optical Fiber Amplifiers

Figure 1: Optical fiber amplifier model with power monitor option.

An optical fiber amplifier, abbreviated as OFA, refers to a new type of all-optical amplifier used in optical fiber communication lines to achieve signal amplification. Optical amplifiers are mainly divided into three categories: semiconductor optical amplifiers (SOA), optical fiber amplifiers doped with rare earth elements (Er, Tm, Pr, Nd, etc.), and nonlinear optical fiber amplifiers.

The most common type of optical fiber amplifiers is the erbium doped fiber amplifier (EDFA). In addition, there are thulium doped fiber amplifier (TDFA) and praseodymium doped fiber amplifier (PDFA). The nonlinear fiber amplifier refers mainly to a fiber Raman amplifier (FRA). Several types of common fiber amplifiers and their characteristics will be introduced as follows.

1. Optical Fiber Amplifiers

Silica fiber doped with rare earth elements (such as Nd, Er, Pr, Tm, etc.) can form a multi-level laser system, which can directly amplify the input signal light under the action of pump light. The main difference between various types of fiber amplifiers is the type of rare earth doped in them. The performance and characteristics of optical fiber amplifiers vary with the different rare earth doped in them. The following are three common rare earth doped fiber amplifiers.

1.1 Erbium Doped Fiber Amplifiers

The erbium-doped fiber amplifier is doped erbium ions Er3 + in the core through which the signal passes. It is one of the greatest inventions in fiber optic communications. Wavelength-division multiplexing (WDM) technology significantly increases the capacity of optical fiber communication. This makes the erbium doped fiber amplifier the most widely used optical amplifier in current fiber-optic communications. Its advantages are high gain, large bandwidth, high output power, high pumping efficiency, low insertion loss, and insensitivity to polarization states.

Figure 2: 1550nm EDFA Erbium Doped Fiber Amplifier.

1.2 Neodymium Doped Fiber Amplifiers

The operation wavelength of the neodymium doped fiber amplifier is 1060nm and 1330nm. Due to the deviation from the best destination for optical fiber communication and some other reasons, its development and application are limited.

The pump light wavelength of the neodymium doped fluoride fiber amplifier is 1047nm or 1400nm, it can amplify the communication S-band, the wavelength is 1460-1530nm, and it can also amplify the 1650nm light. Therefore, the combination of neodymium doped amplifier can expand the amplifier to a wider wavelength range.

1.3 Praseodymium Doped Fiber Amplifiers

The praseodymium-doped fiber amplifier is a fiber amplifier that operates at about 1300 nm in the second communication window, but its performance is relatively poor compared to the erbium-doped fiber amplifier. Although not commonly used, praseodymium-doped fluoride fibers (PDFA) have their own range of applications: they use pump wavelengths around 1020nm (a less common pump wavelength) or 1047nm (generated by a yttrium lithium fluoride (YLF) laser).

2. Nonlinear Fiber Amplifiers

Due to the nonlinearity of the fiber, stimulated Raman scattering occurs when light is transmitted through the fiber. The fiber Raman amplifier (FRA) uses the stimulated Raman effect to transfer the energy of the strong pump light to the signal beam, so as to achieve the purpose of amplifying the optical signal.
The main advantages of fiber Raman amplifiers are: as long as the desired pump wavelength can be obtained, it can provide gain and low noise for any wavelength. Therefore, the existing usage of placing the fiber Raman amplifier before the fiber amplifier can make the transmission distance between the amplifiers longer. Its main disadvantages are: the need for high pumping and pump polarization.

Figure 3: Performance comparisons of different types of optical amplifiers.

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