I. Introduction
Long-period gratings (LPGs) have been known for over a decade. LPGs are a periodic modulation of the refractive index (RI) along the length of an optical fiber [1]. Under special phase-matching conditions, the grating couples the fundamental core mode to discrete cladding modes that are attenuated due to absorption and scattering. The coupling is wavelength-dependent, so a spectrally selective loss can be obtained. A number of sensors based on the LPGs has been proposed for temperature, strain, hydrostatic pressure, bending and RI sensing (e.g., [2], [3] ). It has also been shown that high RI sensitivity can be successfully applied other purposes including label-free biosensing [4], [5].