UV Bragg Grating Inscription in Germanium-doped Photonic Crystal Fibers

The design flexibility of photonic crystal fibers has accelerated the development of specialty optical fibers for a wide variety of applications. Optical fiber sensor applications for instance can benefit from this fiber technology. Fiber Bragg grating inscriptions in photonic crystal fibers have been reported with inscription setups that go from continuous-wave to femtosecond pulsed laser sources. However, the compatibility of the microstructures in these fibers with conventional ultraviolet inscription techniques was never before investigated in a broad range of (Germanium doped) fibers. We present UV laser induced dynamics of Bragg gratings growths in photonic crystal fibers with a hexagonal arrangement of 6 rings of airholes around a Germanium doped core region. The average refractive index increase and the refractive index modulation by the grating inscription process are compared for microstructures with several doping levels, airhole filling factors, airhole pitch distance and fiber orientation. We show how the parameters of the microstructure can influence the Bragg grating inscriptions. In addition we expand the range of fibers in which Bragg gratings, with reflection strengths that are useable for sensing purposes, can be inscribed to fibers with Germanium doping concentrations as low as 1.36 and 0.45 mol%.