This document discusses modeling photodetector sensitivity in OptiSystem. It provides four examples of setting up and measuring receiver sensitivity for different photodetector configurations: 1) an ideal PIN photodetector limited by shot noise, 2) a PIN photodetector limited by thermal noise, 3) an APD photodetector with both thermal and shot noise, and 4) a PIN photodetector with optical pre-amplification where the dominant noise is signal-ASE beat noise. For each example, the document outlines the system parameters and reference, and how OptiSystem can be used to analyze eye diagrams and calculate sensitivity metrics like bit error rate.