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Laser Power Measurement Sphere Briefing

Labsphere LPM Laser Power Measurement Integrating Spheres

LPM integrating sphere exhibits reduced alignment sensitivity compared to general purpose spheres

Integrating spheres are used in many applications in optical test and measurement. Equipped with a photodetector, an integrating sphere provides alignment insensitive measurements of the output of lasers, LEDs and other light sources. When internally illuminated, an integrating sphere emits a field of uniform luminance or radiance for flat fielding and calibrating cameras and array detectors.

Integrating sphere theory tells us that the incident light creates a uniform radiance distribution over the entire interior surface of a sphere after the first reflection from the sphere wall. In reality, this is not the case, and many integrating spheres suffer from some degree of alignment sensitivity. In other words, the radiance distribution in many spheres can depend upon the exact alignment of the light source or beam of light into the sphere. This is highly undesirable, and although the effect lessens with increasing sphere size, it is common for integrating spheres to feature baffles that screen the extended light source from the detector.

The case of collimated or semi-collimated sources, such as the beams from lasers and laser diodes is slightly different. Here, the laser beam creates a “hot spot” (localised zone of disproportionately high radiance) where the beam impinges upon the sphere wall. Integrating spheres designed for use with collimated sources often employ a baffle to screen the detector from the hot spot. This lessens the dependence of the detector signal on the alignment of the laser beam into the sphere. However, crucially, it doesn’t eliminate the effect.

It is however possible to design a sphere for laser power measurements that doesn’t employ baffles and still give readings that are not affected by the beam direction or divergence over a wide range of angles. This is the approach which Labsphere has taken with its LPM (Laser Power Measurement) integrating sphere design. The LPM sphere is designed with the detector port placed in a non-radial position which forces the photodetector to view the radiance of a small patch of sphere wall directly above the input port of the sphere. By limiting the field of view of the detector, we avoid the detector seeing the first strike “hot spot” from the laser beam, which greatly reduces the alignment sensitivity of the detector signal with variations in the direction of the laser beam. The LPM integrating sphere design is more-or-less alignment insensitive over +/- 40 deg horizontally and vertically with respect to the normal to the plane of the input port..

Principle of Operation

The chart below shows the variation in detector signal when a collimated laser was used to illuminate a general purpose as well as the LPM laser power measurement integrating spheres. The chart shows the variation in the detector signal for each sphere as the laser beam was tilted over +/- 40 degrees in the horizontal and vertical directions. As can be clearly seen, the general purpose sphere exhibits a marked sensitivity to the beam direction at certain angles, whereas the LPM sphere's angular response is much more consistent.

Angular Response of General Purpose vs. Laser Power Spheres

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