Experimental investigation of high power picosecond 1.06 μm pulse propagation in Bragg fibers

In this paper we present fabrication and characterization of the laboratory-designed Bragg fiber. The fiber consisted of a silica core with the diameter of 26 μm surrounded by 3 pairs of circular Bragg layers with a refractive index contrast up to 0.033. Spectral attenuation was measured using a halogen lamp source and the cut-back method resulting in the attenuation coefficient of 0.175 dB/m at 1064 nm. Experiments on high power laser radiation delivery were performed using a laboratory-designed passively mode-locked Nd:YAG laser system generating pulses with the duration of 20 ps and energy of 15 μJ at the wavelength of 1064 nm. The attenuation coefficient measurements resulted in a value of 0.168 ± 0.005 dB/m. The delivered beam spatial profile was detected in the far-field zone in the distance of ~1 cm from the fiber output face. The beam had a circular symmetry with almost Gaussian intensity profile. Bending losses were measured on 1-m long fiber segments using mandrels with different diameters. The bending losses were equal to zero at mandrel diameters greater than 50 mm. Decreasing the mandrel diameter, the bending losses were slightly increasing to a value of 3.6 dB/turn at the mandrel diameter of 8 mm. Stimulated Raman scattering in the fiber was also investigated. The Raman threshold intensity in the fiber core for a 1-m long fiber segment was 113 GWcm2. In a 48-m long fiber, generation up to the 3rd Stokes component was observed together with the spectral broadening up to 1650 nm.