hi gherbi mohamed…
Brief info was already provided by Ravil about it and thud I am quoting the same as…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….
with regards