  Embedding strong photonic stopbands into traditional optical fibre that can
directly access and sense the outside environment is challenging, relying on
tedious nanoprocessing steps that result in fragile thinned fibre. Ultrashort
pulsed laser filaments have recently provided a non contact means of opening
high aspect ratio nanoholes inside of bulk transparent glasses. This method has
been extended here to optical fibre, resulting in high density arrays of laser
filamented holes penetrating transversely through the silica cladding and
guiding core to provide high refractive index contrast Bragg gratings in the
telecommunication band. The point by point fabrication was combined with
post-chemical etching to engineer strong photonic stopbands directly inside of
the compact and flexible fibre. Fibre Bragg gratings with sharply resolved
pi-shifts are presented for high resolution refractive index sensing from n = 1
to 1.67 as the nano-holes were readily wetted and filled with various solvents
and oils through an intact fibre cladding.
