Transverse writing of three-dimensional tubular optical waveguides in   glass with slit-shaped femtosecond laser beams
  We report on fabrication of tubular optical waveguides buried in ZBLAN glass based on transverse femtosecond laser direct writing. Irradiation in ZBLAN with focused femtosecond laser pulses leads to decrease of refractive index in the modified region. Tubular optical waveguides of variable mode areas are fabricated by forming the four sides of the cladding with slit-shaped femtosecond laser pulses, ensuring single mode waveguiding with a mode field dimension as small as ~ 4 {\mu}m. 
Yang Liao, Jia Qi, Peng Wang, Wei Chu, Zhaohui Wang, Lingling Qiao, Ya   Cheng
physics.optics