The sensitivity of a large area photodiode detector at a given wavelength is dependent on various factors such as beam size, angle of incidence and the position on the active area illuminated by the laser beam.
The positional dependence is mainly due to etalon effects in the protection window of the photodiode. This dependence can be minimized by covering the photodiode with a suitable diffusing material, as is the case for most large area photodiode detectors from Artifex Engineering. This has the effect of homogenizing the beam to some extent.
A typical positional dependence is shown in the following diagram for an OPM150UVS detector head.
The relatively flat region on top is the active area of the detector. The steep sidewalls occur when the laser beam is cut off by the aperture of the detector.
Considering only the active area, we typically find a positional dependence of up to ±1% as shown here:
Now consider the effect of the beam diameter on this measurement. If we scan across the surface of the detector with a laser beam of various diameters, we observe the following:
As the beam diameter decreases, the dependence on the beam position becomes more erratic and the deviation from the calibration value increases. Note in particular (see Ø0.25mm graph) that relatively large deviations from the calibration value occur for quite small shifts in the position of the laser beam. Thus, the measurement becomes relatively irreproducible.
This effect occurs because at very small beam diameters, the granular structure of the diffuser is no longer significantly smaller than the beam, which reduces the effectivity of the homogenization of the diffuser.
Note that the beam diameter does not affect the measurement results when using integrating sphere type detectors. In such detectors, the beam experiences multiple reflections from the diffuse surface spreading the beam over the full inner surface of the sphere before being sampled by the implemented photodiode.
In conclusion: for all large area photodiode detector heads in the OPM150 series, Artifex Engineering recommends use of a beam diameter of at least Ø1mm to ensure accuracy and reproducibility of measurement.
About the Author:
Dr. Steven Wright
CEO & Doctor of Natural Sciences (Dr. rer. nat.)
Dr. Steven Wright is the founder and CEO of Artifex Engineering. Born in Canada, he holds a Ph.D. in Natural Sciences and has worked in photonics for more than 30 years, learning the field from the ground up and applying that experience to building Artifex. Still hands-on today, he regularly works alongside the team in the lab, staying close to the science.
