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Fiber Optics: How to Avoid Detector Overfill when Working with Fiber Optic Sources

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Fiber Optics: How to Avoid Detector Overfill when Working with Fiber Optic Sources

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This paper could just have easily been titled “estimating spot size at a distance”, since we must calculate the diameter of the output beam of the fiber optic at the approximate distance from fiber tip to the surface of the active area of the detector. We state approximate distance because we must assume that the tip of the fiber connector ferule, or the end face of the bare fiber, is flush to the output surface of the chuck used to mount the fiber, and that the cover glass of the photodiode is at the detector surface position. (Most diode manufacturers consider the distance between the cover glass and the actual photodiode surface to be proprietary.) It is the distance between the end of the ferule, or the end face of the bare fiber, and the cover glass of the photodiode that is used for calculating the output spot size of the fiber at the detector. It is important to determine this spot size in order to know if the detector active area is being overfilled. Overfilling means that some light falls outside the edges of the diameter of the photodiode active area and, therefore, is not detected by the photodiode. This results in an inaccurate and lower than expected power measurement.

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