If you are working on your project with OptiSystem, then the modulation index you are looking for is the essential characteristic of the laser source that you are using at the transmitter. In any laser based system, the optical modulation index (OMI) is a measure of how much the modulation signal affects the light output, and is measured in %. It serves for verification of optimal operation point of this laser. Typical OMI values depend on the type of laser being used.
For example, DFB lasers are the preferred lasers for high-speed fiber-optical telecommunication systems today because of their spectral purity, linearity and excellent power output levels. They have the highest modulation index as well. The typical OMI for DFB is in the range of 19 to 25%.