APD Gain Optimizer

Avalanche photodiodes (APDs) amplify photocurrent internally through impact ionization, but this multiplication also introduces excess noise characterized by the McIntyre factor F(M) = kM + (1 − k)(2 − 1/M), where k is the ratio of hole-to-electron ionization coefficients. Because shot noise grows faster than signal at high gain, an optimal multiplication factor M_opt exists that maximizes SNR for a given receiver configuration. This tool scans M from 1 to 500, computing shot noise, dark current noise, and amplifier noise at each step to locate M_opt. Inputs include optical power, unity-gain responsivity, dark current, ionization ratio k (with presets for Si, InGaAs, and Ge), amplifier noise current density, and detection bandwidth. Output compares APD SNR at M_opt against an equivalent PIN receiver (M = 1).