A basic optical communication link comprises a transmitter and receiver, with an optical fiber cable connecting them. Although signals propagating in optical fiber suffer far less attenuation than in other mediums, such as copper, there is still a limit of about 100 km on the distance the signals can travel before becoming too noisy to be detected.

Before the commercialization of Optical Amplifiers, it was necessary to electronically regenerate the optical signals every 80-100 km in order to achieve transmission over long distances. This meant receiving the optical signal, cleaning and amplifying it electronically, and then retransmitting it over the next segment of the communication link.

While this can be feasible when transmitting a single low capacity optical channel, it quickly becomes unfeasible when transmitting tens of high capacity WDM channels, resulting in a highly expensive, power-hungry and bulky regenerator station, as shown in Figure 1a. Furthermore, the regeneration hardware depends on the number of channels, as well as the bit-rate, protocol, and modulation format of each individual channel, so that any upgrade to the link would automatically require upgrades to the regenerator stations.

In traditional optical communication systems, optoelectronic regenerators are used between terminals to convert signals from the optical to the electrical domain and then back to the optical domain. Since its first report in 1987,1,2 the erbium-doped fiber amplifier (EDFA) has revolutionized optical communications. Unlike optoelectronic regenerators, this optical amplifier does not need high-speed electronic circuitry and is transparent to data rate and format, which dramatically reduces cost.cwdm-mux8a also provide high gain, high power, and low noise figure. More importantly, all the optical signal channels can be amplified simultaneously within the EDFA in a single optical fiber, thus enabling wavelength division multiplexing (WDM) technology.

In the last dozen years, tremendous progress has been made in the development of EDFA components and technology, including erbium-doped fiber, semi conductor pump lasers, passive components, and splicing and assembly technology. In the research area, an EDFA with a bandwidth of 84 nm was recently demonstrated. Using these high-performance amplifiers, long distance transmission at 1 Tb/s was achieved for the first time. In the meantime, an enormous effort has been under way to incorporate EDFAs into commercial optical communication systems. After intensive laboratory research and development, Lucent Technologies (then the communication equipment division of AT&T) conducted the first field trial of a WDM optical communication system in 1989 and supplied the first commercial WDM system, which was deployed in 1995. Since then, the capacity of WDM systems has been increasing at a very fast pace. Lucent’s recently launched WaveStar TM 400G Optical Line System represents the most advanced of such systems. Today, optical amplifiers and WDM technology offer an unprecedented cost-effective means to meet the ever-increasing demand for trans- port capacity, networking functionality, and operational flexibility.

Erbium-doped fiber can be fabricated by several technologies. Figure 1 shows the energy levels of the erbium ion and the associated spontaneous lifetime in the fiber glass host. Erbium-doped fiber is usually pumped by semiconductor lasers at 980 nm or 1480 nm. A three-level model can be used for 980-nm pumps, while a two-level model usually suffices for 1480-nm pumps. 8,9 Complete inversion can be achieved with 980-nm pumping but not with 1480-nm pumping. The quantum efficiency is higher with 1480-nm pumps. The spontaneous lifetime of the metastable energy level ( 4 I 13/2) is about 10 ms, which is much slower than the signal bit rates of practical interest. As a result of the slow dynamics, intersymbol distortion and interchannel crosstalk are negligible—a key advantage of EDFAs.

Figure 2 shows the gain and loss coefficient spectra at different inversion levels for erbium-doped fiber co-doped with aluminum and germanium.Under a homogeneous broadening approximation, the overall gain spectrum of any piece of erbium-doped fiber always matches one of the curves after scaling and does not depend on the details of pump power, signal power, and saturation level along the fiber. The gain spectrum is very important for amplifier design.

A high inversion level provides low noise figure, while a low inversion level yields high efficiency in the conversion of photons from pump to signal. To achieve both low noise figure and high efficiency, two or more gain stages are generally used—the input stage is kept at a high inversion level and the output stage is kept at a low inversion level. Figure 3 shows one such example, in which an amplified spon- taneous emission (ASE) filter is inserted in the middle stage to prevent gain saturation caused by the ASE peak around 1530 nm. For optical amplifiers with two or more gain stages, the overall noise figure is mainly decided by the high-gain input stage, and the output power is basically determined by the strongly saturated output stage. The passive components have minimal impact on noise figure and output power when they are in the middle stage.

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