Compact acousto-optic interrogator for fiber optic Bragg sensors

Fiber optic sensors are widely used to control both strain and temperature by measuring the wavelengths of optical signals reflected from multiple gratings incorporated at different places of a long fiber. Traditionally the expensive tunable lasers or optical spectrum analyzers are used for wavelength interrogation. This paper proposes to replace tunable laser by a broadband optical source incorporated with novel thin linewidth acousto-optic (AO) tunable filter. It utilizes hundreds of times expanding optical beam by multiple reflection from photonic crystal rows of air holes in LiNbO₃ waveguide. New AO interrogator design is numerically studied for a short structure (with 32 photonic crystal rows) by two-dimensional finite difference time domain (FDTD) method. Extrapolation of these results to large structure sizes (around 1 cm) demonstrates the possibility to develop compact AO interrogators with 0.4 pm wavelength resolution and 40 nm tunable range around 1550 nm.