Coherence of laser radiation traveling along an optical fiber

The degree of coherence of the laser radiation transmitted by a single optical fiber was determined by the Young interference method. It was found that the modulus of the complex degree of coherence decreased with increasing fiber length and also with decreasing diameter (in the case of large diameters). However, for fibers up to 10 cm long this modulus did not fall below ~0.7. The results obtained were explained by a difference between the paths of different rays of the laser beam which appeared during the passage through the fiber; the imperfections in the fiber also played an important role. The main factors responsible for the path difference were considered and their relative importance was estimated.