Comparison of characteristics of Bragg fibers with silica and air cores

摘要

This paper presents the comparison of experimental characteristics of two types of Bragg fibers, one with a silica core and second with an air core. The claddings of the fibers consist of three pairs of Bragg layers. Each pair is composed of one layer with a high and one layer with a low refractive index with a refractive-index contrast of about 0.03. The diameter of the silica core is of about 30 μm and that of the air core is of about 55 μm. Preforms of the Bragg fibers in the form of a tube have been prepared by the MCVD method by using germanium dioxide, phosphorous pentoxide and fluorine as silica dopants. In the case of the fiber with the air core only Bragg layers were applied inside a substrate tube while in the case of the fiber with the silica core one additional silica layer was deposited over the Bragg layers. The fibers were drawn from the preforms under controlled temperatures in order to obtain fibers with air or solid cores. Results of characterization of prepared fibers by optical microscopy are presented. Several laser sources including a pulse and continuous-wave Nd:YAG lasers at 1064 nm and a laser diode at 1550 nm were used for testing the transmission of laser radiation through the fibers. Transmittances, attenuation coefficients, bending losses and spatial profiles of output beams from the fibers were determined from such measurements.