Scanning Cutback Method for Characterization of Bragg Fibers

In this paper, a scanning cutback method, developed for the investigation of Bragg fibers waveguide characteristics, is presented. The method relies on fiber excitation by a laser beam focused onto the fiber core and cladding with different radial offsets from the fiber axis and on measuring the delivered power. These variable coupling conditions and the cutback technique allow us to determine a set of attenuation coefficients and coupling efficiencies. The method has been employed for the characterization of silica and air core Bragg fibers with core diameters of 26 
$\mu$
m and 56 
$\mu$
m, respectively. Laser radiation delivery was investigated at wavelengths of 1.064 and 1.940 
$\mu$
m. Curves of the attenuation coefficient and coupling efficiency as a function of the radial offset show the effects of excitation and propagation of different optical modes in the Bragg fibers. Spatial profiles of the delivered output beams are also presented, which enables us to determine the part of the fiber structure in which the laser radiation is guided.