Focal Ratio Degradation (FRD) is the decrease in focal ratio (decrease in effective F-number) in an optical fiber versus its length. This can be a significant parameter of an optical fiber when matching NAs in telescope-spectrometer systems such as used in astronomy. The ability of a fiber to preserve the angular distribution of the input beam from the telescope to the spectrometer is very important.

The major causes of FRD are mechanical variations in the fiber dimensions with length (under the manufacturer’s control) and the mechanical set-up of the instrumentation (under the control of the user). Small variations in the fiber core diameter or core-clad interface can cause mode stripping, resulting in FRD. Both macrobending and microbending will cause FRD. 

Without proper NA matching between the telescope, spectrometer and the fiber, energy may also be lost from over filling the acceptance cone of the optical fiber or not filling the NA of the spectrometer. Ramsey² found that f-ratios of f/3.0 to f/7.0 were the best matches for glass fibers depending on the fiber diameter. Over-filling or under-filling the optical fiber acceptance cone appeared to produce higher losses. Macrobending did not appear to have any major effect on FRD while microbending did.

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