**2. Semiconductor optical amplifier**

Transmitters, receivers, electronic switches and routers limits the capacity of optical communication systems that can exceed 10 Tb/s by the speed of their electronic components [3]. Optical fibers suffers from attenuation (which limits transmission distance) and dispersion (leading to an increase in the system bit error rate, BER). 3R method (reshapingretiming-retransmission) is then used to regenerate the optical signal in optical fibers. It has a number of disadvantages. This involves low optical and electrical transparencies in addition to network unreliability. These limitations may be overcome by using SOAs. The electronic components can be replaced by ultrafast all-optical signal processing components. SOAs are among the most promising candidates for all-optical signal processing devices due to their high speed capability, low switching energy, compactness, and optical integration compatibility. In optical fibers, as the in-line amplifier has only to carry out one function (amplification of the input signal) compared to full regeneration, it is intrinsically a more reliable and less expensive device. Optical amplifiers can also be a useful as power boosters, for example to compensate for splitting losses in optical distribution networks. It can be used as optical preamplifiers to improve receiver sensitivity [3,4].

A semiconductor optical amplifier (SOA) is basically a semiconductor laser (gain medium) with a low feedback mechanism and whose excited carrier amplifies an incident signal but do not generate their own coherent signal. Thus, it operated as a broadband single-pass device for amplification. Each SOA requires some form of external power (a current or optical source) to provide the energy for amplification. An electrical current inverts the medium, by transferring electrons from the valence to the conduction band, thereby producing spontaneous emission and the potential for stimulated emission yields the signal gain. The first studies of SOAs were carried out around the time of the invention of the semiconductor laser in the 1960's. In the 1970's Zeidler and Personick carried out early work on SOAs. Research on SOA device design and modeling gets a lot of importance in 1980's especially for AlGaAs SOAs operating at (830 nm) wavelength and InGaAsP/InP SOAs operating in the (1300-1550 nm) region. In 1989 SOAs designed as devices uses a symmetrical waveguide structures giving much reduced polarization sensitivities [5].
