A 16-Element Multiplexed Heterodyning Fiber Grating Laser Sensor Array

In this paper, we demonstrate the implementation of a 16-element array of heterodyning fiber grating laser sensors cascaded in a single optical fiber. Each laser consists of a Bragg grating pair inscribed in a short section of Er-doped fiber. The laser offers a RF-domain beat signal between orthogonally polarization modes and can act as a photonic sensor for the measurement of external perturbations, by monitoring the beat frequency. The laser sensors are wavelength multiplexed by inscribing fiber gratings with different pitches, and frequency multiplexed in RF domain by controlling the intracavity birefringence via CO₂ laser side irradiation. The pump power depletion as a result of mode mismatch between the active and passive fibers is a main limitation factor for the multiplexing scale. We can multiplex ten laser sensors at most with a single pump laser with an output power of 130 mW and six more sensors can be multiplexed with an additional pump laser at the other end. The measurement ability of an individual laser sensor can be affected when multiplexed in array, in terms of abrupt frequency jumps and the raised noise level. As a result, the force resolution at 1 kHz is changed from 3 to 6 nN/√Hz, and the minimal detectable static force is changed from 68 to 120 μN. This is attributed to the off-resonance grating couplings and the reflections at the splicing points. This effect can be weakened by selecting mode-matching fibers and optimizing grating profiles, or simply using additional optical isolators.