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.
As seen in the preceding tutorials, receiver sensitivity and the BER of a lightwave system are degraded by many factors that are not always controllable in practice. Depending on details of the system design and objectives, it is entirely possible that a specified BER cannot be achieved. Under such conditions, the use of an error-correction scheme remains the only viable alternative.
The sensitivity of the optical receiver in a realistic lightwave system is affected by several physical phenomena which, in combination with fiber dispersion, degrade the SNR at the decision circuit. Among the phenomena that degrade the receiver sensitivity are modal noise, dispersion broadening and intersymbol interference, mode-partition noise, frequency chirp, and reflection feedback. In this tutorial, we discuss how the system performance is affected by fiber dispersion by considering the extent of power penalty resulting from these phenomena.
With the advent of optical amplifiers, fiber losses can be compensated by inserting amplifiers periodically along a long-haul fiber link as shown below. At the same time, the effects of fiber dispersion (Group Velocity Delay - GVD) can be reduced by using dispersion management. Since neither the fiber loss nor the GVD is then a limiting factor, one may ask how many in-line amplifiers can be cascaded in series, and what limits the total link length. This topic will be covered in another tutorial. Here we focus on the factors that limit the performance of amplified fiber links and provide...
The design of fiber-optic communication systems requires a clear understanding of the limitations imposed by the loss, dispersion, and nonlinearity of the fiber. Since fiber properties are wavelength dependent, the choice of operating wavelength is a major design issue. In this tutorial, we discuss how the bit rate and the transmission distance of a single-channel system are limited by fiber loss and dispersion. In the next tutorial we will discuss multichannel systems. We also consider the power and rise-time budgets and illustrate them through specific examples. The power budget is also called the link budget, and the rise-time budget is sometimes referred to as the bandwidth budget.
