Doped fiber amplifier (The typical representative: EDFA)
Erbium-doped fiber amplifier (EDFA) is the most widely used fiber-optic amplifiers, mainly made of Erbium-doped fiber (EDF), pump light source, optical couplers, optical isolators, optical filters and other components. Among them, a trace impurity in the form of a trivalent erbium ion is inserted into the optical fiber’s silica core to alter its optical properties and permit signal amplification.
The working principle of the EDFA is to use the pump light sources, which most often has a wavelength around 980 nm and sometimes around 1450 nm, excites the erbium ions (Er3+) into the 4I13/2 state (in the case of 980-nm pumping via 4I11/2), from where they can amplify light in the 1.5-μm wavelength region via stimulated emission back to the ground-state manifold 4I15/2.
Advantages & Disadvantages of EDFA
• EDFA has high pump power utilization (>50%)
• Directly and simultaneously amplify a wide wavelength band (>80nm) in the 1550nm region, with a relatively flat gain
• Flatness can be improved by gain-flattening optical filters
• Gain in excess of 50 dB
• Low noise figure suitable for long haul applications
• Size of EDFA is not small
• It can not be integrated with other semiconductor deviecs
Fiber Raman amplifier (FRA)
Fiber Raman Amplifier (FRA) is also a relatively mature optical amplifier. In a FRA, the optical signal is amplified due to stimulated Raman scattering (SRS). In general, FRA can is divided into lumped type called LRA and distributed type called DRA. The fiber gain media of the former is generally within 10 km. In addition, it requires on higher pump power, generally in a few to a dozen watts that can produce 40 dB or even over gains. It is mainly used to amplify the optical signal band of which EDFA cannot satisfy. The fiber gain media of DRA is usually longer than LRA, generally for dozens of kilometers while pump source power is down to hundreds of megawatts. It is mainly used in DWDM communication system, auxiliarying EDFA to improve the performance of the system, inhibiting nonlinear effect, reducing the incidence of signal power, improving the signal to noise ratio and amplifing online.
The principle of FRA is based on the Stimulated Raman Scattering (SRS) effect. The gain medium is undoped optical fiber. Power is transferred to the optical signal by a nonlinear optical process known as the Raman effect. An incident photon excites an electron to the virtual state and the stimulated emission occurs when the electron de-excites down to the vibrational state of glass molecule. The Stokes shift corresponding to the eigen-energy of a phonon is approximately 13.2 THz for all optical fibers.
Advantages & Disadvantages of FRA
• Variable wavelength amplification possible
• Compatible with installed SM fiber
• Can be used to extend EDFAs
• Can result in a lower average power over a span, good for lower crosstalk
• Very broadband operation may be possible
• High pump power requirements, high pump power lasers have only recently arrived
• Sophisticated gain control needed
• Noise is also an issue