An optical amplifier is one of key components realizing the long distance and large capacity of optical communication system. Technologies associated with the communication of information have evolved rapidly over the last several decades. Optical information communication technologies have evolved as the technology of choice for backbone information communication systems due to their ability to provide large bandwidth, fast transmission speeds and high channel quality. A typical communications system includes a transmitter, an optical fiber, a receiver, multiplexers and demultiplexers, amplifiers, switches and other components. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal. A multiplexer combines the individual optical signals from each optical fiber into a multiple channel optical signal and launches the multiple channel optical signal into an optical fiber. A demultiplexer separates the channels out of the multiple-channel optical signal and launches them into separate fibers. Then each receiving portion of a transceiver accepts an optical signal from a fiber and converts it to an electric signal. In an optical communication system, light emitted from a transmitter that is transmitted through an optical transmission line suffers transmission loss that reduces the signal arriving at a receiver. When the power of light arriving at a receiver is smaller than a predetermined value, the receiving error prevents normal optical communication from being performed. Optical communication networks, in particular long-haul networks of lengths greater than 600 kilometers, inevitably suffer from signal attenuation due to variety of factors including scattering, absorption, and bending.
Before the advent of optical amplifiers, regenerators were used to refresh or strengthen the weakened optical signals. Regenerators convert the optical signal to an electrical signal, clean the electric signal, and convert the electrical signal back to an optical signal for continued transmission in the optical communication network. Regenerators, however, can typically only amplify one channel or a single wavelength. Optical amplifiers are an improvement to regenerators because optical amplifiers can amplify light signals of multiple wavelengths simultaneously. Optical amplifiers provide a valuable tool for optical communication systems because of their ability to amplify, regenerate, or otherwise control optical energy to be communicated to a next destination. Optical amplifiers are superior to regenerators because they are not as sensitive to bit rates and modulation formats as regenerators. Optical amplifiers can also be used with multiple wavelengths while regenerators are often specific to a particular wavelength
high coupling efficiency, isolatorsSOA can be operated in saturation, or unsaturated. gain clampingthe optical fiber itself acts as a gain medium that transfers energy from pump lasers to the optical data signal travelingtherethrough. In semiconductor optical amplifiers, an electrical current is used to pump the active region of a semiconductor device. A typical semiconductor optical amplifier (SOA) is a waveguide structure with a semiconductor gain medium (either bulk or multi-quantum well), similar to a semiconductor laser. An SOA has multiple layers formed from compound semiconductor materials that are grown on a semiconductor substrate. Semiconductor gain medium is sandwiched between a substrate and a semiconductor layer. These two layers have a lower index of refraction than gain medium and tend to confine the optical mode within gain medium, as does passivation layer. Passivation layer serves to protect the waveguide and substrate surfaces and reduce surface leakage currents, as well as to act as a cladding layer.
EDFA has revolutionized optical communicationsOnly disadv : cant b used at ckt board lvl.
EDFAs rely on a pump laser to excite erbium atoms doping several meters of optical fiberEDFAs typically comprise at least one pump laser whose output is optically coupled to the input of one or more serially connected coils of erbium-doped optical fiber. When a light signal passes through the excited doped fiber, the erbium reverts to its unexcited energy state and gives up the pump energy as a photon of the same wavelength as the light signal triggering the reversion. The pump light usually has a wavelength of 980 or 1480 nm. When a transmission signal, using having a wavelength in the 1550 nm range, propagates through the amplifying fiber, this light stimulates the erbium atoms to release their stored energy as additional 1550 nm light waves which continues as the transmission signals propagates through the amplifying fiber.
The principal difference between C- and L-band amplifiers is that a longer length of doped fibre is used in L-band amplifiers. The longer length of fibre allows a lower inversion level to be used, thereby giving at longer wavelengths (due to the band-structure of Erbium in silica) while still providing a useful amount of gain.EDFAs have two commonly-used pumping bands - 980 nm and 1480 nm. The 980 nm band has a higher absorption cross-section and is generally used where low-noise performance is required. The absorption band is relatively narrow and so wavelength stabilised laser sources are typically needed. The1480 nm band has a lower, but broader, absorption cross-section and is generally used for higher power amplifiers. A combination of 980 nm and 1480 nm pumping is generally utilised in amplifiers
In a Raman amplifier, the signal is intensified by Raman amplification. stimulated raman scattering (SRS), Unlike the EDFA and SOA the amplification effect is achieved by a nonlinear interaction between the signal and a pump laser within an optical fibre.Wavelength set by pump light wavelength used.
The pump light may be coupled into the transmission fibre in the same direction as the signal (co-directional pumping), in the opposite direction (contra-directional pumping) or both. Contra-directional pumping is more common as the transfer of noise from the pump to the signal is reduced.The pump power required for Raman amplification is higher than that required by the EDFA, with in excess of 500 mW being required to achieve useful levels of gain in a distributed amplifier. Lumped amplifiers, where the pump light can be safely contained to avoid safety implications of high optical powers, may use over 1W of optical power.
The principal source of noise in DFAs is Amplified Spontaneous Emission (ASE), which has a spectrum approximately the same as the gain spectrum of the amplifier. Noise figure in an ideal DFA is 3 dB, while practical amplifiers can have noise figure as large as 6-8 dB.As well as decaying via stimulated emission, electrons in the upper energy level can also decay by spontaneous emission, which occurs at random, depending upon the glass structure and inversion level. Photons are emitted spontaneously in all directions, but a proportion of those will be emitted in a direction that falls within the numerical aperture of the fibre and are thus captured and guided by the fibre. Those photons captured may then interact with other dopant ions, and are thus amplified by stimulated emission. The initial spontaneous emission is therefore amplified in the same manner as the signals, hence the term Amplified Spontaneous Emission. ASE is emitted by the amplifier in both the forward and reverse directions, but only the forward ASE is a direct concern to system performance since that noise will co-propagate with the signal to the receiver where it degrades system performance. Counter-propagating ASE can, however, lead to degradation of the amplifier's performance since the ASE can deplete the inversion level and thereby reduce the gain of the amplifier.
SSE – SIGNAL SPONTANEOUS EMISSIONS
Gain is achieved in a DFA due to population inversion of the dopant ions. The inversion level of a DFA is set, primarily, by the power of the pump wavelength and the power at the amplified wavelengths. As the signal power increases, or the pump power decreases, the inversion level will reduce and thereby the gain of the amplifier will be reduced. This effect is known as gain saturation - as the signal level increases, the amplifier saturates and cannot produce any more output power, and therefore the gain reduces. Saturation is also commonly known as gain compression.To achieve optimum noise performance DFAs are operated under a significant amount of gain compression (10 dB typically), since that reduces the rate of spontaneous emission, thereby reducing ASE. Another advantage of operating the DFA in the gain saturation region is that small fluctuations in the input signal power are reduced in the output amplified signal: smaller input signal powers experience larger (less saturated) gain, while larger input powers see less gain.The leading edge of the pulse is amplified, until the saturation energy of the gain medium is reached. In some condition, the width(FWHM) of the pulse is reduced.
Optical Amplifiers and its Future Uses By :- Paul SouryaChatterjee ECE – 1034 6th Sem. Academy Of Technology
INTRODUCTION The optical fiber amplifier was invented by H. J. Shaw and Michel Digonnet at Stanford University, California (1980s).
Optical communication A typical communications system includes a transmitter, an optical fiber, a receiver, multiplexers and demultiplexers, amplifiers, switches and other components. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal.