1. Basic configuration of an optical fiber amplifier
The schematic diagram of an optical fiber amplifier is shown in Fig. 1. The most important component is rare-earth-doped fibers used as a gain medium, and the other components are the pump LD to excite rare-earth ions, the WDM coupler for multiplexing of the signal and pump lights, and the isolator to prevent laser oscillation resulting from reflected light. The sorts of rare-earth ions are selected to emit the light with specific wavelength, and fiber parameters are designed to optimize the efficiency of amplification. FiberLabs uses two types of the rare earth doped fiber, i.e., a silica fiber and a fluoride fiber to realize various emission wavelength bands including the telecom bands and other bands.
Fig.1 Schematic diagram of basic AMP configuration (*shows doped rare eath ion)
2. Principle of amplification of each wavelength bands
In optical fiber amplifiers, different rare-earth ions, such as Pr, Er, Tm, and Yb, are used to amplify each wavelength band. Since the energy level of each rare-earth ion is different as shown in Fig.2, light of a different wavelength band is emitted according to the energy difference between the excitation level and the lower level. When optical signals pass the doped fibers, the excited ions decay by the stimulated emission process through interaction with the optical signals, and thus optical signals are amplified. Pr ions are suitable for amplification of the 1.3 μm band, Er ions for the 1.5 μm band, Tm ions for the 1.4 μm band, and Yb ions for the 1.0 μm band. Er ions are also available for amplification of the 0.85 μm band and Tm ions are also for the 1.9 μm band. The amplified wavelength bands depends on fiber materials as well as kinds of the doped ions. Amplification of the 1.3 μm band/Pr, the 1.4 μm band/Tm, and the 0.85 μm band/Er can not be realized unless using fluoride fibers. FiberLabs has realized the amplifiers of these wavelength bands by using the originally developed fluoride fibers with optimum fiber design.The optical amplifiers using Pr-, Er- and Tm-doped fibers are called PDFA, EDFA, and TDFA respectively.
Fig.2 Energy level diagram of rare earth ions & wavelength bands of amplification