Optical amplifiers

Saturday, 06 January 2007

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. Therefore, an optical amplifier is provided between a transmitter and a receiver so as to amplify light, thereby compensating for the transmission loss of the light transmitted through the optical transmission line and enabling the light to be transmitted to a farther distance with little error. 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.

An optical amplifier optically amplifies signal light transmitted through an optical transmission line such as an optical fiber transmission line in an optical transmission system to compensate transmission loss in the optical transmission line. Optical amplifiers are commonly used as power amplifiers at the source end of an optical communications link, line amplifiers along the optical signal transmission path, and preamplifiers at the receiving end of the optical communications link, and have other uses as well. Optical communication systems typically use wavelength-division multiplexing to increase transmission capacity. An optical communication system transmits large-capacity information with a high speed in such a manner that signal light having a plurality of channels of different wavelengths from each other propagates through an optical fiber transmission line. A plurality of signal lights each having a different wavelength are multiplexed together into a wavelength division multiplexed (WDM) signal light. The WDM signal light is transmitted over a transmission line, and then demultiplexed at the other end of the transmission line so that the individual signal lights can be individually received. The transmission line is usually a single optical fiber. An optical amplifier is comprised of an amplification optical waveguide, such as an amplification optical fiber or the like, and a pumping light supplying means for supplying pumping light into the amplification optical waveguide. When the signal light is injected into the amplification optical waveguide with supply of the pumping light, the input signal light is amplified in the amplification optical waveguide. Optical amplifiers exhibit low noise, provide a relatively large bandwidth, which is not polarization dependent, and provide low insertion loses at the transmission signal operating wavelengths in the 1550 nm range. The continuous growth of bandwidth requirements in optical-based communication systems has resulted in a large demand for systems able to operate within several optical wavelength ranges including the S-band optical range, the C-band optical range, and the L-band optical range. The optical amplifier installed on an optical transmission line is equipped with an optical waveguide path such as an optical fiber for optical amplification and an exciting means for supplying pump light to the optical waveguide path for optical amplification. When signal light is input from the upstream side of the optical transmission line to the optical wave guide path for optical amplification to which pump light is supplied, the input signal light is optically amplified through the optical waveguide path for optical amplification and is output to the downstream. The pump light used to excite the amplifying fiber can be configured to co-propagate or counter-propagate with respect to the direction of propagation of the transmission signal. The couplers used to provide the pump light to the amplifying fiber have a high coupling ratio at the pump wavelength and a low coupling ratio at the signal wavelength. Many communication systems rely on optical communications because they are less susceptible to noise induced by external sources and are capable of supporting very high speed carrier signals and increased bandwidth. Generally, an optical amplifier includes an optical amplifying medium, such as an erbium-doped fiber (EDF). The WDM signal light travels through the optical amplifying medium. The optical amplifier also includes a light source, such as a laser diode, which provides pump light to the optical amplifying medium. The pump light causes the WDM signal light to be amplified as the light signal travels through the optical amplifying medium. Optical amplifiers often employ electronic feedback arrangements to control the output power from the amplifier. The feedback arrangement may be used to provide a constant gain or a constant output power. An optical amplifier is used to apply a gain to an optical signal. This gain is measured by the power of the signal leaving the amplifier divided by the power of the signal entering the signal. The gain of an optical fiber amplifier depends on a variety of parameters including the rare-earth ion concentration, the length of the doped fiber, the radius of the fiber core, and the power of the pump laser. Within an optical communication system, amplifiers are normally configured in pairs, since the optical transmission signals are bi-directional.

There are several different types of optical amplifiers being used in today's optical communication systems. In general, the two primary types of optical amplifiers are optical fiber based amplifiers, such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers, and semiconductor optical amplifiers (SOAs). Rare-earth doped, optical-fiber amplifiers are used in a wide variety of communications applications to generate appropriate high-power optical signals. Rare-earth doped optical amplifiers use rare-earth ions as the active element. The ions are doped in the fiber core and pumped optically to provide gain. The silica core serves as the host medium for the ions. Erbium doped fiber amplifiers (EDFAs) rely on a pump laser to excite erbium atoms doping several meters of optical fiber. Erbium-doped fiber amplifiers 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. While many different rare-earth ions, such as neodymium, praseodymium, and ytterbium, can be used to provide gain in different portions of the spectrum, erbium-doped optical amplifiers have proven to be particularly attractive because they are operable in the spectral region where optical loss in the silica core is minimal. Erbium-doped fiber amplifiers operate at wavelengths that reduce fiber and component losses and minimize dispersion effects. EDFAs are also attractive from a telecommunications standpoint because they produce high gain with relatively little noise and demonstrate polarization independency. Also, the erbium-doped optical amplifier is particularly useful because of its ability to amplify multiple wavelength channels without crosstalk penalty, even when operating deep in gain compression. EDFAs are widely used in line amplifiers and other applications requiring high output power, high data rates, and low noise. An EDFA can optically amplify signal light in the spectrum band that exhibits the lowest loss of an optical fiber, and also can simultaneously amplify signal light of multi-wavelengths within an amplification spectrum band. Therefore, such an optical fiber amplifier is widely used as an optical amplifier applied to a wavelength division multiplexing (WDM) transmission system. However, EDFAs are quite bulky and require the presence of a separate pumping laser to operate. Hence, EDFAs are difficult to incorporate into confined spaces, and are certainly not amenable to circuit-board-level or chip-level integration.

Raman amplifiers take advantage of stimulated raman scattering (SRS), a non linear effect that can cause broadband optical gain in optical fibers. SRS can be used to amplify an optical signal at a certain wavelength by the use of a strong radiation at a lower wavelength, called the pump radiation. Raman gain results from the interaction of intense light with optical phonons of the glass constituting an optical fiber. An EDFA operates by passing an optical signal through an erbium-doped fiber segment, and "pumping" the segment with light from another source such as a laser. The pump source excites erbium atoms in the doped segment, which then serves to amplify the optical signal passing through. In contrast, Raman amplification is more distributed and occurs throughout an optical transmission fiber when it is pumped at an appropriate wavelength or wavelengths. Each Raman amplifier may contain one or more pumps. Gain is achieved over a spectrum of wavelengths longer than the pump wavelength through the process of stimulated raman scattering. In such an amplifier, the output of a pair of orthogonally polarized pump-diode lasers provides backward propagating pump power in the transmission fiber. Forward-propagating signals achieve gain in the fiber because higher-energy (shorter wavelength) pump photons scatter off the vibrational modes of the optical fiber's lattice matrix and coherently add to the lower-energy (longer wavelength) signal photons. Raman amplifiers may be one of two types, depending upon the type of the gain fiber used therein. The distributed parameter Raman amplifier is the one in which pumping light is introduced into a transmission path (for example, a silica-based fiber and the like) of an optical communication system to Raman amplify distributively an optical signal being propagated in the transmission path, so that a part of transmission losses is compensated. On the other hand, the concentrated type Raman amplifier is the one in which the pumping light is introduced concentratingly into a medium having higher non-linearity (for example, a silica-based fiber having a smaller effective cross-sectional area) to Raman amplify the optical signal efficiently. Raman amplifiers are naturally tunable and capable of providing amplification at wavelengths in a broad optical band. The Raman amplifier can easily adjust an amplification band by properly setting the wavelength of the pumping light for the Raman amplification, and has a low noise figure. Raman amplification is becoming increasingly important in optical communication systems, in particular in high-bit rate wavelength division multiplexing systems and dense wavelength division multiplexing (DWDM) systems. An important advantage of raman amplification is that the effective optical signal-to-noise ratio is significantly lower than that of an erbium-doped fibre amplifier (EDFA) having the same gain. In such an amplifier, an amplification wavelength is simply selected by tuning a pump laser to produce a wavelength capable of producing stimulated Raman emission at the selected wavelength. Raman amplifiers can cover a much wider spectral range than rare-earth based amplifiers. Furthermore, Raman amplifiers have effectively lower noise levels than rare-earth amplifiers. These advantages make Raman amplifiers desirable for long haul WDM systems where the transmission bandwidth may be broad. However, the Raman amplifier not only has very low optical amplification efficiency but also needs a high-priced pumping light source, thereby increasing the entire size of the optical amplifier module and the price of the optical amplifier module. While the maximum gain levels that can be achieved with Raman amplifiers are typically less than those achievable by EDFA amplifiers, Raman amplifiers are more economical since they require no specially doped optical fiber and can act as a low noise pre-amplifier before the EDFA.

Semiconductor optical amplifiers (SOAs) are another type of optical amplifier. In erbium-doped fiber amplifiers (EDFAs) and Raman amplifiers, the optical fiber itself acts as a gain medium that transfers energy from pump lasers to the optical data signal traveling therethrough. 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. Contact layer provides reduced contact resistance with contact and provides a ready supply of carriers to be pumped into gain medium during operation giving rise to a population inversion. Stimulated radiative recombination of carriers in gain medium leads to coherent amplification of optical signals passing through the SOA. The ends of the waveguide are usually treated to avoid optical feedback. The facets of the SOA are formatted by cleaving the semiconductor wafer along a crystal plane, thus forming a mirror. An anti-reflective coating is often applied to the facets to decrease the facet reflection. When an optical signal is injected from the input facet, the light is amplified by the gain of the active layer. The optical signal is input to the SOA from the optical fiber where it experiences gain due to stimulated emission as it passes through the active region of the SOA. Semiconductor optical amplifiers contain a semiconductor active region and an electrical current typically is used to pump the electronic population in the active region. An optical signal propagating through the active region experiences gain due to stimulated emission. Semiconductor optical amplifiers (SOAs) have been proposed as a means of reducing this cost in many system applications.The SOA uses gain characteristics of a semiconductor and can adjust its amplification band according to a semiconductor band gap. SOAs can be fabricated similar to the fabrication of edge emitting lasers, for example, forming a waveguide by cleaving and/or etching of vertical facets in semiconductor materials to form entry and exit points for the amplifying waveguide. Due to their compact size, reduced power consumption and reduced cost of fabrication, semiconductor optical amplifiers have begun to replace EDFAs in short to intermediate reach, narrow band gain applications. The disadvantages of SOAs include much narrower wavelength bands, reduced amplification, and higher noise figure than erbium-doped optical amplifiers.

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