Investigation of linearity and interference effects in silicon photodiodes coupled to an integrating sphere for laser radiometry working standards.

In laser radiometry, it is essential to evaluate the linearity of the detector and understand how interference effects influence power measurements, as laser light typically has high power and coherence. In this paper, we investigate the linearity and interference effects of widely used silicon photodiodes under the condition that they are coupled to an integrating sphere. For the linearity study, we employed a flux-addition method at 532 nm and found that one photodiode exhibited sufficient linearity, while the other showed supralinearity. This difference was attributed to whether the photodiode was irradiated with light outside its photoactive region. To further explore this, we examined the linearity by scanning the position of the laser irradiation on the photodiodes. In the interference study, we used a narrowband distributed Bragg reflector laser and measured the spectral responsivity by varying the laser wavelength. Due to the cover window, the spectral responsivity of photodiodes can rapidly vary as the wavelength changes under direct laser irradiation, with variations reaching up to tens of percent. However, when the spatial coherence of laser light is sufficiently suppressed by the integrating sphere, these rapid variations decrease to around 0.01% of the smoothly varying fitted values, which is negligible in typical laser power measurements.