Photographs of periodic void formation in a waveguide made of Yb-doped bismuthate glass

see Fig. 14 in

R. Mary, D. Choudhury and A. K. Kar:
“Applications of fiber lasers for the development of compact photonic devices”,
Selected Topics in Quantum Electronics, IEEE Journal of,
(Volume:PP, Issue: 99, 2014).
10.1109/JSTQE.2014.2301136