The design of a fiber-optic communication system involves optimization of a large number of parameters associated with transmitters, optical fibers, in-line amplifiers, and receivers. The design aspects discussed in the preceding tutorials are too simple to provide the optimized values for all system parameters. The power and the rise-time budgets are only useful for obtaining a conservative estimate of the transmission distance (or repeater spacing) and the bit rate. The system margin is used as a vehicle to include various sources of power penalties discussed in the preceding tutorials. Such a simple approach fails for modern high-capacity systems designed to operate over long distances using optical amplifiers.
An alternative approach uses computer simulations and provides a much more realistic modeling of fiber-optic communication systems. The computer-aided design techniques are capable of optimizing the whole systems and can provide the optimum values of various system parameters such that the design objectives are met at a minimum cost.
The following figure illustrates the various steps involved in the simulation process. The approach consists of generating an optical bit pattern at the transmitter, transmitting it through the fiber link, detecting it at the receiver, and then analyzing it through the tools such as the eye diagram and the Q factor.
Each step in the block diagram can be carried out numerically by using the material given in the preceding tutorials. The input to the optical transmitter is a pseudorandom sequence of electrical pulses which represent 1 and 0 bits. The length N of the pseudorandom bit sequence determines the computing time and should be chosen judiciously. Typically, N = 2M with M is in the range of 6 to 10. The optical bit stream can be obtained by solving the rate equations that govern the modulation response of semiconductor lasers. The equations governing the modulation response should be used if an external modulator is used. Chirping is automatically included in both cases.
The most time-consuming part of system simulations is propagation of the optical bit stream through the fiber link which may contain multiple fiber sections with optical amplifiers inserted in between. Changes to the bit stream occurring in each fiber section are calculated by solving the Nonlinear Schrödinger (NLS) equation so that both the dispersive and nonlinear effects are fully included. The noise added by optical amplifiers should also be included at the location of each amplifier.
The optical bit stream is converted into the electrical domain at the photodetector where shot and thermal noises are added. The electrical bit stream is then passed through a pulse-shaping filter whose bandwidth is also a design parameter. An eye diagram is constructed using this filtered bit stream. The effect of varying system parameters can be studied by monitoring the eye degradation or by calculating the Q parameter. Such an approach can be used to obtain the power penalty associated with various mechanisms. It can also be used to investigate trade-offs that would optimized the overall system performance. Numerical simulations reveal the existence of an optimum extinction ratio for which the system penalty is minimum.
Computer-aided design has another important role to play. A long-haul lightwave system may contain may repeaters, both optical and electrical. Transmitters, receivers, and amplifiers used at repeaters, although chosen to satisfy nominal specifications, are never identical. Similarly, fiber cables are constructed by splicing many different pieces (typical length 4-8 km) which have slightly different loss and dispersion characteristics. The net result is that many system parameters vary around their nominal values. For example, the dispersion parameter D, responsible not only for pulse broadening but also for other sources of power penalty, can vary significantly in different sections of the fiber link because of variations in the zero-dispersion wavelength and the transmitter wavelength. A statistical approach is often used to estimate the effect of such inherent variations on the performance of a realistic lightwave system. The idea behind such an approach is that it is extremely unlikely that all system parameters would take their worst-case values at the same time. Thus, repeater spacing can be increases well above its worst-case value if the system is designed to operate reliably at the specific bit rate with a high probability (say 99.9%).
The importance of computer-aided design for fiber-optic communication systems became apparent during the 1990s when the dispersive and nonlinear effects in optical fibers became of paramount concern with increasing bit rats and transmission distances. All modern lightwave systems are designed using numerical simulations, and several software packages are available commercially.