Hi Mukesh,
Thermal power density corresponds to the thermal noise that affects the signal processed in the photodetector. It is the power density of the famous unavoidable thermal noise which is caused by the random thermal motion of electrons. To see the mathematical relationship of how the noise is based on the power density, please have a look at the help section of the photodetector component. And the possible range of thermal power density in Optisysetm is between 0 and 1e+100.
Regards