Fiber Optic Tutorials
Dispersion Problem and Its Solution - Dispersion-Compensating Fibers
Optical amplifiers solve the fiber-loss problem but, at the same time, make the dispersion problem worse because dispersive effects keep accumulating along the entire chain of amplifiers. Indeed, long-haul WDM systems making use of amplifiers are often limited by the dispersive and nonlinear effects rather than fiber losses. However, the dispersion problem can be managed in practice through a suitable dispersion-compensation scheme. The following series of tutorials focus on several such techniques.
Periodically Amplified Lightwave Systems
Many experiments performed during the early 1990s employed a chain of cascaded in-line optical amplifiers for increasing the length of long-haul fiber links. These experiments showed that fiber dispersion becomes the limiting factor in periodically amplified lightwave systems. Indeed, the experiments were feasible only because the system was operated close to the zero-dispersion wavelength of the fiber link. Moreover, the residual dispersion was tailored along the link in such a way that the total dispersion over the entire link length was quite small at the operating wavelength. By 1992, total system length could be increased to beyond 10,000 km with such dispersion-management techniques. In a 1991 experiment, an effective transmission distance of 21,000 km at 2.5 Gb/s and of 14,300 km at 5 Gb/s was realized using a recirculating fiber loop. By 2010, 96 channels, each channel operating at 100 Gb/s, could be transmitted over 10,600 km by using a phase-based modulation format.
Role of Dispersive and Nonlinear Effects
So far we have considered amplifier noise without paying attention to how this noise interacts with the dispersive and nonlinear effects that also occur as an optical signal propagates along the fiber link. In reality, ASE noise propagates with the signal and is affected by the same dispersive and nonlinear mechanisms that affect the optical signal. This tutorial shows that ASE noise can be enhanced considerably if the conditions are suitable for modulation instability to occur. Moreover, ASE noise affects optical pulses and induces not only energy fluctuations but also timing jitter.
Receiver Sensitivity and Q Factor
We have considered input signals with constant power. Optical signals in any lightwave systems are in the form of a pseudo-random bit stream consisting of 0 and 1 bits. This tutorial focuses on such a realistic situation and evaluates the effects of amplifier noise on the BER and the receiver sensitivity.
Electrical Signal-To-Noise Ratio (SNR)
Optical SNR, although useful for design purposes, is not what governs the BER at the receiver. In this tutorial, we focus on the electrical SNR of the current generated when an ASE-degraded optical signal is incident on a photodetector. For simplicity of discussion, we use the configuration shown in the figure below and assume that a single optical amplifier is used before the receiver to amplify a low-power signal before it is detected. This configuration is sometimes used to improve the receiver sensitivity through optical pre-amplification.