Lightguide Splice Loss - Effects of Launch Beam Numerical Aperture

The power loss in an optical fiber lightguide splice depends on a number of factors, both intrinsic and extrinsic. Intrinsic factors include mismatches of the core radius, r, the maximum relative index difference between core and cladding, A, and the power law profile parameter, a. Extrinsic loss mechanisms include angular misalignment as well as longitudinal and transverse offsets of the two fibers. The measured loss of a given splice also depends on the modal power distribution in the input fiber immediately preceding the splice which, in turn, depends on the nature of the source and the launching conditions used to excite the fiber and the length of fiber between the source and the splice.1,2 A detailed knowledge of the exact measurement conditions is therefore necessary to adequately assess the significance of splice loss data. One method of minimizing the influence of launching conditions on 441 splice loss measurements is to use a long length of input fiber so that mode mixing produces a "steady-state" power distribution which is essentially independent of the launch distribution. It is often inconvenient or impossible, however, to use a long input fiber. Thus many splice loss measurements are made with a short input fiber, and some means is often employed to try to simulate a steady-state condition. On the other hand, there will certainly be cases in most fiber optic systems where short fiber lengths will precede a splice, and where a nonsteady state power distribution will be encountered.