One of Ophirs clients was trying to measure transmission loss in fiber optic cables, using a PD300 sensor. He was obtaining results that vary strongly with the distance between the fiber and the sensor. For example, the power meter read 10 microwatts with the fiber very close, and 12 microwatts with the fiber a millimeter or two away from the face of the sensor. When measuring laser power of a fiber optic, how close to the sensor should the fiber tip be placed?
The PD300 sensors are not designed to measure with the fiber pushed up right against the detector surface. It may be reading lower in such a case due to saturation of the detector from the concentrated beam or higher due to back reflections off the detector and back again from the fiber tip. The optimal reading will be where the beam is expanded to a size of 2-5mm diameter. Therefore, you should back off the fiber to a distance where the beam has expanded somewhat. Do not back off too far, otherwise if the nominal beam size is larger than given above, you may lose some of the beam off the edges of the detector.
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