Lightguide splice loss — effects of launch beam numerical aperture

An experimental investigation is presented of the effects of launch beam numerical aperture (NA) on optical fiber lightguide splice loss measurements. With a short input fiber preceding the splice, the splice loss sensitivity to transverse offset is shown to be drastically reduced when the launch beam NA is significantly less than the fiber NA. With a launch NA greater than or equal to the fiber NA, the sensitivity to transverse offset is approximately equal to the steady-state (long input fiber) sensitivity for small offsets, but falls below the steady-state value for offsets greater than approximately 0.2 core radii. Displacing or tilting the launch beam relative to the fiber axis increases the measured loss. These observations emphasize the importance of strictly defining the measurement conditions when reporting splice loss results and the need for using a suitable mode filter if the steady-state splice loss is desired.