RSoft Enewsletter

January 2014

Tips on system design using fixed optical output amplifier and fixed optical gain amplifier models in OptSim

Optical amplifiers are an important component of a fiber optic communication system. The most common use of optical amplifiers is to provide amplification of an optical signal. Noise- and nonlinearity-related properties in amplifiers also make them suitable for a wide variety of specialized applications including wideband optical sources and nonlinearity-induced format and wavelength converters. OptSim has a rich library of different types of optical amplifiers. Broadly speaking, the optical amplifier library in OptSim can be divided into two groups: behavioral models and physical models. Physical models include fiber (Raman and parameteric, for example), doped-fiber (Er-, and Yb-, for example) based amplifiers, and semiconductor optical amplifiers (reflective and non-reflective). Behavioral models include fixed-gain, saturable gain and fixed output optical amplifiers.

Here we provide system design tips on two commonly used models: fixed optical output amplifiers and fixed optical gain amplifiers. The most common application of a fixed optical gain amplifier is to compensate for the signal loss in optical fibers. On the other hand, fixed optical power amplifiers help maintain optical power to a desired level by automatically setting its small-signal gain, G. The evaluation of output optical power, Pout, in both cases takes into account internally generated amplified spontaneous emission (ASE) noise, PASE, via the noise figure parameter:


Pout= G Pin+PASE


This has an interesting implication when multiple stages of these amplifiers are used in long-haul transmission systems. Let’s consider a long-haul system with multiple spans of fixed gain optical amplifiers:



long-haul system with multiple spans of fixed gain optical amplifiers

The layout consists of multiple spans of single-mode fibers and fixed-gain amplifiers fully compensating for the span loss, thereby maintaining the channel power value. However, since each of the amplifiers above will add internally generated ASE to the optical signal, total power (signal plus noise) along the link will increase with accumulating noise:

total power (signal plus noise)

Now, let’s consider the same scenario but with fixed optical output amplifier models: 

fixed optical output amplifier models

In this case, each of the amplifiers will keep on adding its ASE while adjusting the small signal gain such that the output power value (signal power plus noise) remains the same. This implies that the channel power reduces along the link with increasing noise to have constant (signal plus noise) output power:

total power (signal plus noise)

The purpose of this article was to bring the above implications to your attention so that you can make appropriate choices of amplifiers while designing your system. We hope you found this tip helpful. If you have any questions, please contact us at