We demonstrate a fiber-optic acoustic emission (AE) sensor system that is capable of performing AE detection, even when the sensor is experiencing large quasi-static strains. The sensor is a Fabry–Perot interferometer formed by cascaded chirped fiber-Bragg gratings (CFBGs). The reflection spectrum of the sensor features a number of narrow spectral notches equally spaced within the reflection bandwidth of the CFBG. A semiconductor laser whose wavelength can be fast tuned through current injection is used to lock the laser line to the center of a slope of a spectral notch. When the notch is knocked out of the tuning range of the laser, a neighboring notch moves into the range. Through a smart feedback control scheme, the laser is unlocked from the current spectral lock and relocked to the desired point of the new notch. The fast speed of the unlocking/relocking process ($<1\mathrm{ms}$) ensures that the AE signal is monitored without significant disruption.

This paper presents a fiber Bragg gating (FBG) sensor that can be surface mounted for simultaneous strain and temperature measurements. By embedding a conventional FBG sensor in a composite laminate, local birefringence is introduced, which causes the bandwidth of the FBG spectrum to vary with strain as well as temperature. As such, temperature and strain can be simultaneously determined from two FBG spectral parameters, i.e. the spectral bandwidth and the Bragg wavelength. Techniques for improving the spectrum of the FBG-composite sensor and for inversely determining the strain and temperature from the measured FBG spectral parameters are discussed. Thermal–mechanical testing of the FBG-composite sensor was carried out to validate the sensor performance. The measurement errors, within one standard deviation, for the strain and temperature measurements were found to be ±62 με and ±1.94 °C, respectively.

We propose and demonstrate a modified phase-generated carrier (PGC) demodulation scheme optimized for detection of ultrasound using interferometric sensors with sinusoidal fringes. The sensor used in demonstration is made from a pair of weak fiber Bragg-gratings at the ends of a coiled fiber that form a low-finesse Fabry-Perot interferometer. The phase of the laser source is modulated using an electro-optic phase modulator to generate the carrier signal and obtain 2 quadrature (the sine and cosine) terms at the first and the second order carrier frequencies. The signal of interest (ultrasound) has much higher frequency than the environmental perturbation but a very small amplitude that causes only small phase shift. Using small-signal approximation, for each of the 2 quadrature terms, we separate the contributions from the environmental perturbations (quasi-DC component) and from the ultrasound (AC component). The AC components that contain the information of the ultrasound signal are then further amplified with a large gain. The signal of interest is constructed by simple algebraic operations on the 2 quasi-DC components and the 2 amplified AC components involving multiplying and summing. This work provides a simple and robust demodulation method with potentially high sensitivity for fiber-optic interferometric ultrasound sensors.

The work presented in this paper shows that Fiber Bragg Grating (FBG) optical fiber sensors can potentially be used as receivers in a long-range guided wave torsional-mode pipe inspection system. Benefits over the conventional pulse-echo method arise due to reduced total travel distance of the ultrasonic guided wave reflections, since reflections from defects and structural features do not need to propagate a full round trip back to the transmitting collar. This is especially important in pipe configurations with high attenuation, such as coated and buried pipelines. The use of FBGs as receivers instead of conventional piezoelectric or magnetostrictive elements also significantly reduces cabling, since multiple FBG receivers can be placed along a single optical fiber which has a diameter on the order of only around 100 μm. The basic approach and sample results are presented in the paper. Additionally, a brief overview of some topics in ultrasonic guided waves is presented as a background to understand the inspection problem presented here.

We report a sensitive and mechanically robust fiber-optic refractometer with temperature compensation based on a Fabry-Perot interferometer formed by two chirped fiber Bragg gratings on a seven-core multicore fiber. A small part of the fiber cladding at the sensing region is etched to expose the outer cores to the environment. While the optical modes supported by the outer cores are affected by both temperature and refractive index (RI) changes of the surrounding liquid, the optical mode in the central core of the fiber is affected by the temperature changes only. The wavelength shift due to RI change is obtained by subtracting the wavelength shift of the central core from that of one of the outer cores so that the temperature effect is removed. The demodulation can also remove the errors caused by the wavelength drift of laser. Due to the narrow spectral features of the sensor, the sensor shows a low detection limit of 2.4×10-6 at a background RI of 1.316 at wavelength of ~1550 nm. Because only a small part of the cladding is removed, the sensor maintains excellent mechanical strength and stability.

We report a high-sensitivity fiber-optic ultrasonic sensor system using a self-injection-locked distributed-feedback (DFB) diode laser where a $\pi$-phase-shifted fiber Bragg grating ($\pi \mathrm{FBG}$) serves as both the locking resonator and the sensing element in a fiber ring feedback loop. By controlling the delay time of the feedback light through a fiber stretcher, the laser wavelength is locked to an external cavity mode on the spectral slope of the $\pi \mathrm{FBG}$, and the ultrasound-induced wavelength shifts of the $\pi \mathrm{FBG}$ are converted to laser intensity variation. The ultrasonic sensing scheme simplifies the feedback control because the self-injection locking automatically pulls the laser wavelength to the $\pi \mathrm{FBG}$ resonant wavelength. In addition, it improves the detection sensitivity because the frequency noise of the DFB laser is drastically reduced. We show that the sensor system achieves a strain sensitivity of $78\mathrm{f\epsilon }/{\mathrm{Hz}}^{1/2}$ at around 200 kHz.