Sapphire long-period grating fibers for potential refractive index sensing

Optical fiber sensors compare very favorably to conventional electronic sensors in many of today’s applications. However, due to material limitations, harsh environment sensing remains a challenge for both sensor types. In this work, laser-heated pedestal growth is used to fabricate geometry-modulated long-period gratings (LPGs) in single-crystal sapphire fibers. The grating coupling effect leading to effective transmission loss of approximately 80% at resonance wavelengths between 500-700 nm was clearly observed in preliminary tests with a 90 µm-period grating. The sensing feasibility of such grating was also confirmed in the refractive index measurement test, resulting prominent shifting of resonant wavelength with crossing wavelength varied by ~130 nm. The automated LHPG process incorporates a high degree of system flexibility and cost-effectiveness which potentially allows development of multifunctional harsh environment sensors in single crystal fibers.