FIBER  AMPLIFIERS

      A solid-state amplifier used extensively in the communications field to boost weak signals in long runs of fiber optic cables is the erbium-doped glass fiber amplifier. This amplifier consists of a long (10- to 20-m) section of glass fiber doped with erbium ions (Er³⁺‏), making an Er: glass medium. A pump laser at 980 nm is coupled to the amplifier fiber via a coupler. This pump radiation is absorbed by the erbium atoms in the fiber (which have a strong absorption peak at this wavelength), exciting them to an upper level which rapidly decays to a level 0.80 eV above ground state. This level, which has a relatively long spontaneous radiative lifetime (τ_sp, about 10 ms long) can amplify incoming signals via stimulated emission, producing a net optical gain at 1549 nm. A diagram of this amplifier is shown in Figure 1.1.

      Where no input signal is present, erbium ions eventually emit spontaneous radiation which is amplified by the fiber and appears as broadband noise in the output [called amplified spontaneous emission (ASE)]. Where an input signal is present, the extraordinarily long lifetime of the upper lasing level gives the ions a good chance of emitting by stimulated instead of spontaneous emission, so the amount of broadband noise in the signal is reduced drastically, as the ions are now coaxed to produce output on the single wavelength of the signal. The output from such

Figure 1.1. Erbium fiber amplifier.

Figure 1.2. Fiber amplifier output.

 an amplifier, as analyzed on an optical spectrum analyzer (OSA), is depicted in Figure 1.2.

         Although the fiber amplifier is just that an amplifier, not a laser Er: glass will laser if provided with a suitable feedback mechanism. Er: glass may also be fabricated in the form of a rod and used in the same manner as an Nd: YAG laser. Because the wavelength is considered eye-safe (meaning that it will not penetrate the eye to reach the retina), applications include rangefinders, where ocular exposure is likely (the U.S. military M-1 tank reportedly uses an Er: glass laser in the rangefinder, replacing the more dangerous Nd: YAG lasers used previously).