Fiber Optics: How to Choose an Optic for Free Space Fiber Coupling

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Among the essentials for obtaining optimal transmission through a fiber optic are having a good cleave and end polish, and, if free space coupling light into or out of the fiber, choosing the correct lens. Choosing a coupling optic for a multi-mode fiber is relatively simple. Select an optic whose numerical aperture (NA) is closest to matching that of the fiber. This will result in a focused spot size from the light source that is comparable to the core size of the fiber and whose incident cone angle does not exceed the arcsine of the NA of the fiber, which will allow for maximum coupling efficiency.