Continuous linearity evaluation of photodetectors using a cascaded integrating-sphere system

Traditional nonlinearity measurements are limited by the detector's dynamic range, especially at extremely low light levels. We present a cascaded integrating-sphere system combined with an aperture tube, enabling continuous optical attenuation over more than six orders of magnitude. The system exhibits high stability and reproducibility, ensuring reliable radiometric calibration. Using the laser superposition method, we tested a silicon trap detector, which maintained linearity error below 0.1% from 10^-6 to 10^-10 W . By introducing a single-photon avalanche diode, the system was further extended toward lower power levels while keeping the error below 0.4% down to 10^-12 W. This system offers a robust and scalable approach for precision radiometric calibration over an ultra-wide dynamic range.