A fiber-optic Fabry-Perot interferometer (FPI) is proposed and demonstrated as an electric current sensor based on ohmic heating with temperature self-compensation. The sensing head is fabricated by attaching a silicon pillar on the fiber tip as the FPI and inserting it into a molten Tin (Sn) sphere as the conducting medium. Two copper electrodes are also inserted into the Sn sphere and serve as the electrodes for current measurement. The electric current loaded on the Sn sphere will lead to a temperature rise that is measured with high resolution by the silicon FPI. A fiber Bragg grating (FBG) is incorporated in the vicinity of the sensing head for temperature compensation. Both theoretical analysis and experimental results indicate that the peak or valley wavelengths of the silicon FPI have a linear response to the square of the measured current. The current responsivity of 1.83 nm/A2 has been experimentally obtained, with a resolution of 1.28×10-3 A at around 0.3 A.

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.

Wavelength interrogation has been widely used for fiber-optic sensors based on Fabry-Perot interferometers (FPIs) because of its high measurement resolution. However, phase ambiguity often limits the measurement range to be less than one free-spectral range (FSR) of the FPI. Here, we investigate a simple but effective method that can eliminate the phase ambiguity of a silicon FPI temperature sensor using a fiber Bragg grating (FBG) fabricated on the lead-in fiber close to the silicon FPI. The reflection spectrum of the sensor shows sinusoidal fringes from the FPI and a single peak from the FBG. Assuming the FBG and the silicon FPI have similar temperatures, the FBG peak can be used merely as a marker to identify an FPI fringe peak without ambiguity even when it shifts by more than an FSR. Although the temperature resolution of the FBG is lower than that of the silicon FPI, the measurement range of the silicon FPI can be greatly improved without sacrificing the measurement resolution. Using a temperature sensor based on a 75-μm-thick silicon FPI, we demonstrated temperature measurement over a range of -60 to 140 °C (corresponding to a spectral shift of about five FSRs) with a resolution of 0.003 °C. The measurement range with unambiguous peak recognition is expected to be larger than demonstrated in the experiment and only limited by the spectral bandwidth of the sensor system and the capability of materials used for the sensor construction.

Plasma radiation plays an important role in the operation and control of magnetic confinement fusion and is typically measured by bolometers. Here, we study the influence of mechanical vibration and magnetic field, which are typically present in magnetic confinement fusion devices, on the performance of a fiber-optic bolometer (FOB) based on silicon Fabry-Perot interferometer (FPI) demodulated by a scanning diode laser. It is found that both the vibration and magnetic field could cause large noise due to the birefringence of the silicon FPI. To mitigate the birefringence effects, we demonstrated two effective methods: polarization maintaining FOB and polarization scrambling. We show that two methods can be effective in mitigating the effect caused by the sensor birefringence: (1) replacing the regular single-mode fiber in the FOB system with polarization-maintaining fiber and (2) using a polarization scrambler after the laser source. For both methods, no significant increase in the noise was observed in the vibration and magnetic field tests. Our research highlights the importance of the polarization management for the FOB systems toward their practical applications in magnetic-confinement fusion systems.

A theoretical framework is established to study the interaction between circular-crested Lamb waves (CCLWs) and a fiber-optic ultrasound sensor (FOUS) bonded onto the surface of a plate structure. CCLW is a universal expression of guided Lamb waves, regardless of the source-sensor distance. In the theory developed here, strain transfer and transformation, birefringence of the fiber sensor, and modulation of refractive index by elasto-optic effects are incorporated. Explicit expression for calculating the ultrasound-induced wavelength shift of fiber Bragg gratings (FBGs) is derived. To showcase application of the established theory in a more general situation, a chirped FBG Fabry-Perot interferometer based FOUS is simulated, in combination with transfer matrix method. Results uncover quantitatively the interplay of the following factors governing the sensor response: 1) co-existence of both axial and lateral normal strains in the fiber, especially in the near field; 2) elasto-optic properties of the fiber material; 3) initial polarization state of the fiber sensor; 4) incident angle of the CCLWs; and 5) ratio between the equivalent ultrasonic wavelength and the effective sensor length. The calculated results uncover the underlying mechanisms responsible for several experimental observations reported but inadequately explained in the literature. The theoretical framework is also applicable to other non-FBG-based FOUSs that rely on phase modulation by ultrasounds.

We theoretically and experimentally investigate the performance of an ultrasound detection system based on a fiber ring laser (FRL) whose cavity includes a pair of fiber Bragg gratings. The ultrasonic detection is achieved by the FRL power variations in response to the ultrasound-induced cold-cavity loss modulation of the FRL. The effects of key FRL parameters, including pump power, laser cold-cavity loss, and laser cavity length, on the system signal-to-noise ratio (SNR) performance have been investigated. It is found that the maximum SNR is achieved when the frequency of the ultrasound is the same as that of the FRL relaxation oscillation (RO). Harmonic generations are more prominent when the ultrasonic frequency is at the RO frequency and highly dependent on the strength of the ultrasonic signal. The analysis provides a useful tool for the understanding and optimization of such ultrasonic sensor systems.