Short Coherence Length Interferometer for the Measurement of the Absolute Fiber Facet Angle

Fiber optical systems showed an increasing interest in the past few years, especially in the development of fiber lasers and amplifiers as well as for high power beam delivery. For many applications such as splicing, cleaving or polishing, the inspection of the fiber facets becomes unavoidable. In this paper we present a Twyman-Green white light interferometer with phase-shift technique for the characterisation of the absolute angle of the fiber facets. The short coherence length of the used LED allows to focus on the sample by simply maximizing the fringe contrast. With a proper alignment of the interferometer (has to be done only once) the maximum fringe contrast coincides with sharp imaging of the fiber facet. The setup includes an exchangeable magnification optic, allowing a quick AOI change between 400 µm and 1000 µm. By recording and analyzing the interferograms obtained from two positions (initial position and 180° rotated) one can calculate the absolute angle of the fiber facet. A new high precision fiber rotator was designed and produced to allow an extremely precise axial rotation. Several experiments have been performed with an excellent reproducibility and stability.