Determination of refractive index by fiber-loop cavity ring-down spectroscopy and a long-period fiber grating

ABSTRACT

The combination of fiber-loop ring-down spectroscopy and a long-period fiber grating were employed in an intensity-interrogated refractive index sensing method using a broad band light source and the specifically designed long-period fiber grating. Compared with the traditional long-period-fiber-grating based refractive index sensors, this approach monitored the decay of the optical signal passing through the loop of the long fiber grating. The decay time changed regularly when the grating was immersed in surrounding media with varying refractive indices. This instrumentation offered reduced cost and eliminated the influence of source fluctuation. The sensitivity and resolution were 2421 ns/RIU and 3 × 10⁻⁴ for refractive index values from 1.3330–1.3737.

KEYWORDS:

• Cavity ring-down spectroscopy
• fiber-loop ring-down spectroscopy technology
• long-period fiber grating
• refractive index