Abstract
Using a novel range-resolved interferometric signal processing technique based on the sinusoidal optical frequency modulation of a cost-effective laser diode, a fiber sensing approach termed fiber segment interferometry (FSI) is described. In FSI, a chain of long-gauge length fiber optic strain sensors are separated by identical in-fiber partial reflectors. Targeted at dynamic strain analysis and ultrasound detection for structural health monitoring, this approach allows integrated strain measurements along fiber segments, removing the sensing gaps and sensitivity to inhomogeneities found with localized fiber sensors. In this paper, the multiplexing of six fiber segments, each of length 12.5 cm, is demonstrated. The sensor array can be interrogated at 98 kHz data rate, achieving dynamic strain noise levels ≤ 0.14 nϵ · Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-0.5</sup> . The reflector fabrication is discussed, an analysis of linearity and noise performance is carried out and results from an exemplar experiment to determine the speed-of-sound of a stainless steel rod are shown.
Highlights
L ONG-GAUGE length interferometric fiber optic strain sensors [1] integrate the applied strain field over the sensing fiber, which contrasts with widely-used localised fiber optical strain sensors, such as fiber Bragg gratings (FBGs) [2]
In this paper, expanding work first reported at the 24th Optical Fiber Sensors Conference, OFS24 [27], we present a complete description of the system that was used to interrogate six fiber segment interferometry (FSI) segments of gauge length 12.5 cm at a data rate of 98 kHz
Peff,k is dependent on the mutual polarisation and coherence overlap between the light returned from the FBG in-fiber partial reflector and the local oscillator (LO)
Summary
L ONG-GAUGE length interferometric fiber optic strain sensors [1] integrate the applied strain field over the sensing fiber, which contrasts with widely-used localised fiber optical strain sensors, such as fiber Bragg gratings (FBGs) [2]. The advantages of long-gauge length sensors for structural health monitoring include the reduced dependence on the local properties at the sensor location of measurements made on inhomogeneous substrates [3] and the absence of sensing gaps, whereby the integrating property of the measurement ensures that a localised strain event anywhere along the sensing fiber will not be missed. Long-gauge length fiber optical strain sensors have been researched intensively for use in hydrophones [4]–[7], where high-specification, high cost lasers are used to facilitate low noise operation and where sensor gauge lengths are typically tens of meters.
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