 Detector saturation

Detector saturation occurs when increasing the input no longer produces the response expected from the detector's lower-input calibration. The limitation may arise in the sensing element, its load circuit, an amplifier or the digitizing stage of the measuring chain. A nearly flat output then represents many different inputs, so the lost amplitude information cannot be recovered by applying the original linear conversion.

A primary physical example is Thorlabs' silicon-photodiode saturation study. It varies reverse bias and load resistance and examines the resulting saturation limit and noise floor. Its stated limitations include testing one FDS100 photodiode with a continuous-wave source; those observations do not establish a universal limit for every detector or pulsed illumination condition.^[ref]

For an original idealized output-limit model, let incident optical power be $P\ge 0$ andThe linear branch reaches its limit at $P=0.5\mathrm{mW}$. Inputs $0.5$, $1$ and $2\mathrm{mW}$ all produce $2.5\mathrm{V}$ in this model. Interpreting that output as $P=V/a$ therefore reports $0.5\mathrm{mW}$ for each, hiding the differences. The abrupt limit is a teaching model; real response can depart from linearity gradually.

An optical attenuator placed before the detector can provide a check while retaining usable input information. For transmission $T=1/4$ and original $P=1\mathrm{mW}$, the detector receives $0.25\mathrm{mW}$ and gives $1.25\mathrm{V}$ on the linear branch. Correcting the known attenuation recovers $P=V/(aT)=1\mathrm{mW}$. Scaling a clipped stored value after detection cannot perform the same recovery.

To test detector linearity, vary calibrated attenuation or source power while monitoring stability and keeping geometry and wavelength fixed. Compare output ratios, including the background contribution, rather than assuming every output plateau is a constant source. Thorlabs also identifies reverse bias, load and bandwidth as relevant system conditions.^[ref] In the flat part of the ideal response, transducer sensitivity is zero, and the inverse measurement model is no longer unique.