liquid filling of photonic crystal fibres reduces the scattering from air–glass interfaces during Bragg grating writing in many layered photonic crystal fibres. Within experimental uncertainty, the grating index modulation of a grating written in germanium-doped photonic crystal fibre with 10 rings of holes was more than doubled for a fixed fluence.
Sørensen, H. R., Canning, J., Lægsgaard, J., Hansen, K. P., & Varming, P. (2007). Liquid filling of photonic crystal fibres for grating writing. Optics Communications, 270(2), 207-210. https://doi.org/10.1016/j.optcom.2006.09.009