Application Note: SPM Linearity

At low optical signal levels the output of the SPM is proportional to the incident photon flux assuming that the optical signal is uniformly distribution across all microcells of the SPM. For continuous wave (cw) signals the incident photons produce an average photocurrent, I_p, that is proportional to the number of incident photons per second and is given by

where N_ph is the number of incident photons per second, PDE is the SPM photon detection efficiency, G is the gain of the SPM microcells and q is the electronic charge.

The SPM output is linear provided the rate at which the microcells detect photons is much less that the inverse of the microcell deadtime. The microcell deadtime is defined as the total time that the microcell is undergoing quenching and recovery. During the deadtime period an individual microcell cannot detect a photon incident upon it.

Above a certain signal level the SPM response becomes sub-linear. This is because the output pulse of a single microcell is independent of the number of incident photons that initiated the output pulse. As the number of incident photons per microcell per second increases, the probability that two or more photons initiate the output pulse increases. The SPM output begins to saturate when the number of detected photons begins to approach the number of microcells (M),