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Fiber Bragg gratings (FBGs) are wavelength selective optical reflectors with excellent strain
sensitivity and small sensing footprint, which makes them suitable as diagnostic sensors for structural health
monitoring applications. In this work, we explore the narrowband wavelength selectivity of FBGs for optical
feedback in a tunable fiber ring laser. The fiber ring laser consists of an erbium doped fiber laser that is
pumped with a Raman laser (980 nm) to produce population inversion and amplified spontaneous emission
(ASE) in the C-band. The ASE light is used to illuminate a FBG sensor connected to the ring, and the
reflected light from the sensor is fed back into the laser cavity to produce stimulated emission at the
instantaneous center wavelength of the sensor. As the wavelength of the sensor shifts due mechanical or
thermal strains, the wavelength of the optical output from the ring laser shifts accordingly. By combining the
ring laser with a dynamic spectral demodulator for optical readout, the instantaneous wavelength of the ring
laser is tracked with high temporal resolution. The fiber ring laser system offers several potential advantages
in the diagnostic sensing of mechanical strains for SHM applications including, fully integrated laser and
sensor system, high source power levels at the sensor wavelength, narrow spectral line-width, coherent
spectral demodulation, and low system costs.
In this work, we present experimental results that detail the feasibility of dynamic spectral tuning of
the fiber ring laser at frequencies up to hundreds of kilohertz using a single FBG sensing element. Using
multiple sensing elements, the fiber ring laser system would allow for active monitoring of dynamic strains in
a multi-point sensor array configuration, which is particularly suitable for the localization of high frequency
mechanical strains produced by impact loading and cracking events in structures.
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