Fiber Optic Tutorials
Optical Signal-To-Noise Ratio (OSNR)
The ASE noise added by each amplifier to the signal reduces the SNR of the amplified signal. The performance of a lightwave system is dictated by the electrical SNR obtained after the signal has been converted into an electric form using a photodetector. However, the concept of optical SNR, defined as the ratio of optical power to the ASE power (within a certain bandwidth), is also useful as it can provide design guidelines. In this section we focus on the optical SNR and consider electrical SNR in the next section.
Raman Amplifiers
Fiber-based Raman amplifiers make use of stimulated Raman scattering (SRS) occurring in silica fibers. The following figure shows how a fiber can be used as a Raman amplifier in the forward-pumping configuration. The pump and signal beams at frequencies ωp and ωs are injected into the fiber through a fiber coupler. During the SRS process, a pump photon gives up its energy to create another photon of reduced energy at the signal frequency; the remaining energy is absorbed by the silica material in the form of molecular vibrations (optical phonons). The energy is transferred continuously fro the pump to the signal through SRS as the two beams co-propagate inside the fiber. The pump and signal beams counter-propagate in the backward-pumping configuration often used in practice.
Erbium-Doped Fiber Amplifiers (EDFA)
An important class of lumped optical amplifiers makes use of rare-earth elements as a gain medium by doping the fiber core during the manufacturing process. Although doped-fiber amplifiers were studied as early as 1964, their use became practical only 25 years later, after their fabrication techniques were perfected. In such amplifiers, properties such as the operating wavelength and the gain bandwidth are determined by the dopants while silica plays the role of a host medium. Among the rare-earth elements, erbium is the most practical element to realize fiber amplifiers operating in the wavelength region near 1.5 μm, and erbium-doped fiber amplifiers (EDFAs) have been studied extensively. Their deployment in WDM systems after 1995 revolutionized fiber-optic communications and led to lightwave systems with capacities exceeding 1 Tb/s.
Compensation of Fiber Losses
The transmission distance of any fiber-optic communication system is eventually limited by fiber losses. Until 1995, this loss limitation was mostly overcome using optoelectronic repeaters, devices in which the optical signal is first converted into an electric form using a receiver and then regenerated using a transmitter. Such regenerators become quite complex and expensive for WDM systems because they require demultiplexing of individual WDM channels. An alternative approach to loss management makes use of optical amplifiers, which amplify the entire WDM signal directly without requiring conversion of each channel to the electric domain. Several kinds of optical amplifiers were developed during the 1980s, and the use of optical amplifiers for lightwave systems became widespread during the 1990s. By 1996, optical amplifiers became a part of the fiber-optic cables laid across the Atlantic and Pacific oceans. This tutorial is devoted to the management of fiber losses in long-haul systems. In the first tutorial we discuss the common technique in which optical amplifiers are used periodically along a fiber link and identify the two schemes known as lumped and distributed amplification schemes.
Code-Division Multiplexing (CDM)
A multiplexing scheme, well known in the domain of wireless communications, makes use of the spread-spectrum technique. It is referred to as code-division multiplexing (CDM) because each channel is coded in such a way that its spectrum spreads over a much wider region than occupied by the original signal. Although spectrum spreading may appear counterintuitive from a spectral point of view, this is not the case because all users share the same spectrum. In fact, CDM is used often for cell phones as it provides the most flexibility in a multiuser environment. It is also relatively secure because it is difficult to jam or intercept the signal in view of its coded nature. The term code-division multiple access (CDMA) is often employed in place of CDM to emphasize the asynchronous and random nature of multiuser connections.