How do I couple light into a fiber using a lens / ball lens?

To couple light from a source into a fiber, a Ball Lens or lens with a positive focal length can be used. If the light is collimated, the lens must have a f/# greater than or equal to 0.5/NA, where NA is the numerical aperture of the fiber. The f/# of the lens is equal to f/D where f is the lens focal length and D is the beam diameter, not the diameter of the lens. This insures that the cone of light entering the fiber is within the fiber's acceptance angle. It should be placed away from the fiber at a distance equal to the lens focal length. A ball lens placed in contact with the fiber can also help couple light into the light guide. Its NA must be less than or equal to the NA of the fiber. This insures that the light is effectively coupled into the light guide. If the source is not collimated, i.e. light emitting from another fiber with a similar numerical aperture, one ball lens can be used to first produce a more collimated output beam and then another identical ball lens can be used to couple that light into the other fiber. The ball lenses should be placed in near contact to maximize efficiency.