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Two types of multimode fiber couplers made from plastic-clad fibers are described: (1) cross type couplers with coupling efficiencies from 30 dB to 50 dB depending on the crossover angle; and (2) parallel type couplers with low transmission loss and coupling efficiencies from 15 dB to 30 dB depending on the coupling length. These experimental results are in good agreement with theory.

Transmission losses versus fiber end offset separation, and angular misalignment of graded index fibers excited by an incoherent source, have been measured in two independent experiments. The measurement setup, fiber diameter, and length were different in the two experiments, yet the measurement results are strikingly similar. The loss measurements clearly show that transverse offset is much more critical in connector and splice design than angular misalignment and end separation. Two‐tenths of the fiber core radius in transverse offset alone may cause 0.5 dB loss while one fiber core radius in axial separation combined with 1° in angular misalignment may cause 0.5 dB loss.