We report a fiber-optic sensor configuration with a cascaded fiber Bragg grating (FBG) and a silicon Fabry-Perot interferometer (FPI) for simultaneous measurement of temperature and strain. The sensor is composed of a 5 mm FBG on a single mode fiber and a 100 μm thick silicon FPI attached to the tip of the optical fiber. The FBG is surface mounted on the host structure, and the FPI tip is suspended. Due to the stress-free, cantilever configuration, the silicon FPI is insensitive to strain, but sensitive to temperature with a sensitivity much higher than the FBG due to the large thermo-optic coefficient of silicon. The sensor is tested from room temperature to 100°C with varying strain up to ∼150 με. The silicon FPI provides high temperature sensitivity of 89 pm/°C unaffected by strain. Since the FBG is attached to the host structure, it is affected by both thermal and mechanical strain; the sensitivity of these was experimentally obtained 32 pm/°C and 1.09 pm/με, respectively. Interrogated with a broadband light source and a high-speed spectrometer, the sensor shows temperature and strain resolutions of 1.9×10−3 °C and 0.042 με, respectively. Due to the small size, enhanced sensitivity, and high resolution, this cascaded FBG-FPI sensor can be used in applications where accurate measurement of temperature and strain is required.
A fiber optic refractive index sensor based on Fabry-Perot interferometer formed by two Chirped Fiber Bragg Gratings on a seven-core fiber is successfully demonstrated. A small part of the fiber cladding is etched to expose the outer 6 cores to the ambient environment. While optical modes supported by the outer 6-cores are affected by both temperature and refractive index changes of the surrounding liquid, the optical mode in the central core is affected by the temperature changes only. Because only a small part of the cladding is removed, the sensor maintains excellent mechanical strength and stability.
Fiber-optic flow sensor based on a laser-heated silicon Fabry-Pérot interferometer (FPI) exhibits a strong directivity owing to the cylindrical shape of the sensor head. In this work, a new sensor structure has been designed to effectively reduce the directivity. The proposed sensor embeds the laser-heated silicon FPI in a Tin microsphere (diameter ~1mm). Due to the circular shape of the outer metal layer, a more symmetric response to flow from different directions is achieved. In the meantime, the high thermal conductivity and small footprint of the metal sphere helps maintain the good responsivity of the silicon FPI to the flow. Directivity of the newly designed sensor has been tested in water flow. Experimental results suggest that deviation in the directional response is reduced to 4% at a speed of ~1.4 ms-1 , in comparison to the 44% for the original sensor without the metal shell. The directivity can be reduced further by improving the fabrication techniques for the metal sphere.
In this paper, we have demonstrated that the spectral bandwidth of a FBG can be modulated by embedding a single Fiber Bragg grating (FBG) sensor in a composite laminate. The bandwidth modulation was achieved by exploiting the birefringence of the embedded – FBG sensor, which was sensitive to the transverse strain and thus varies with temperature or axial load. Combining the bandwidth changes with the Bragg wavelength shift may enable measuring the simultaneous changes in temperature and axial strain using a single FBG sensor.
A hot-wire fiber-optic water flow sensor based on laser-heated silicon Fabry-Pérot interferometer (FPI) has been proposed and demonstrated in this paper. The operation of the sensor is based on the convective heat loss to water from a heated silicon FPI attached to the cleaved enface of a piece of single-mode fiber. The flow-induced change in the temperature is demodulated by the spectral shifts of the reflection fringes. An analytical model based on the FPI theory and heat transfer analysis has been developed for performance analysis. Numerical simulations based on finite element analysis have been conducted. The analytical and numerical results agree with each other in predicting the behavior of the sensor. Experiments have also been carried to demonstrate the sensing principle and verify the theoretical analysis. Investigations suggest that the sensitivity at low flow rates are much larger than that at high flow rates and the sensitivity can be easily improved by increasing the heating laser power. Experimental results show that an average sensitivity of 52.4 nm/(m/s) for the flow speed range of 1.5 mm/s to 12 mm/s was obtained with a heating power of ~12 mW, suggesting a resolution of ~1 μm/s assuming a wavelength resolution of 0.05 pm.
Temperature microstructure in the ocean can lead to localized changes in the index of refraction and can distort underwater electro-optical (EO) signal transmission. A similar phenomenon is well-known from atmospheric optics and generally referred to as “optical turbulence”. Though turbulent fluctuations in the ocean distort EO signal transmission and can impact various underwater applications, from diver visibility to active and passive remote sensing, there have been few studies investigating the subject. To provide a test bed for the study of impacts from turbulent flows on underwater EO signal transmission, and to examine and mitigate turbulence effects, we set up a laboratory turbulence environment allowing the variation of turbulence intensity. Convective turbulence is generated in a large Rayleigh- Bénard tank and the turbulent flow is quantified using high-resolution Acoustic Doppler Velocimeter profilers and fast thermistor probes. The turbulence measurements are complemented by computational fluid dynamics simulations of convective turbulence emulating the tank environment. These numerical simulations supplement the sparse laboratory measurements. The numerical data compared well to the laboratory data and both conformed to the Kolmogorov spectrum of turbulence and the Batchelor spectrum of temperature fluctuations. The controlled turbulence environment can be used to assess optical image degradation in the tank in relation to turbulence intensity, as well as to apply adaptive optics techniques. This innovative approach that combines optical techniques, turbulence measurements and numerical simulations can help understand how to mitigate the effects of turbulence impacts on underwater optical signal transmission, as well as advance optical techniques to probe oceanic processes.
Temperature measurement is one of the key quantifies in ocean research. Temperature variations on small and large scales are key to air-sea interactions and climate change, and also regulate circulation patterns, and heat exchange. The influence from rapid temperature changes within microstructures are can have strong impacts to optical and acoustical sensor performance. In this paper, we present an optical fiber sensor for the high-resolution and high-speed temperature profiling. The developed sensor consists of a thin piece of silicon wafer which forms a Fabry-Pérot interferometer (FPI) on the end of fiber. Due to the unique properties of silicon, such as large thermal diffusivity, notable thermo-optic effects and thermal expansion coefficients of silicon, the proposed sensor exhibits excellent sensitivity and fast response to temperature variation. The small mass of the tiny probe also contributes to a fast response due to the large surface-tovolume ratio. The high reflective index at infrared wavelength range and surface flatness of silicon endow the FPI a spectrum with high visibilities, leading to a superior temperature resolution along with a new data processing method developed by us. Experimental results indicate that the fiber-optic temperature sensor can achieve a temperature resolution better than 0.001°C with a sampling frequency as high as 2 kHz. In addition, the miniature footprint of the senor provide high spatial resolutions. Using this high performance thermometer, excellent characterization of the realtime temperature profile within the flow of water turbulence has been realized.
Flowmeters have been finding vast applications in all kinds of industrial processes, such as process control, food quality surveillance, wind turbines, environment monitoring, etc. In this paper, we propose a new anemometer which consists of a Fabry-Pérot interferometer (FPI) implemented using a thin silicon mounted on the tip of an optical fiber. The anemometer takes advantage of the superior thermal and optical properties of silicon. Silicon is transparent to infrared wavelength, while it absorbs visible light. Thus, the silicon FPI can be heated by a beam injected from a red diode laser while the infrared signals go through it without any interference from the heating light. The heat loss from the silicon film will increase when the sensor is placed in stronger flow (wind), which induces a decrease in the optical path of the silicon FPI, which lead to blueshifts the output spectrum. A higher wind speed corresponds to a larger wavelength shift. By tuning the heating power, the response range and sensitivity of the anemometer is changed. Experimental results demonstrate that a wavelength shift -0.574 nm was observed for a wind speed of 4 m/s. Better sensitivity is to be expected when stronger heating applied. The proposed sensor also features simple structure, low cost and fast response.
KEYWORDS: Fiber Bragg gratings, Sensors, Ultrasonics, Signal detection, Fiber lasers, Signal attenuation, Signal to noise ratio, Laser resonators, Temperature metrology, Acoustic emission
We develop a novel ultrasonic sensor system using a fiber ring laser (FRL) to detect acoustic emissions. The sensor
system incorporates two fiber Bragg gratings (FBGs) in the FRL cavity, a short and strong FBG as the sensing element
and a long and weak FBG as the adapting element. The reflection spectra of both FBGs are matched such that the
reflection peak of the long FBG is positioned at the linear slope of the short FBG’s reflection spectrum. Ultrasonic waves
impinging onto the FBGs are to modulate the FRL cavity loss, which leads to laser intensity variations that can be
detected directly by photodetectors. The two FBGs are placed side-by-side in close proximity so that the sensor system is
able to adapt to the ambient temperature drift. We demonstrate that the ultrasonic sensor system can operate normally
within approximately 15ºC temperature change. In addition, the performance of signal-to-noise ratios is investigated as a
function of the FRL cavity loss. The proposed temperature-insensitive sensor system is attractive in practical applications
where temperature change is unavoidable.
In this paper, we present a temperature-insensitive refractive index sensor based on π-phase-shifted Bragg gratings fabricated on side-hole fibers processed by wet chemical etching technique. The reflection spectrum of the π-phase shifted gratings on etched side-hole fiber features two notches with large spectral separation, which was used for refractive index (RI) detection in our application. The relative spectral notch separation exhibited a RI sensitivity of −278.5 pm/RIU (RIU: RI unit). Theoretical simulation obtained the temperature sensitivity of −0.00241 pm/°C, and experimental results also showed little sensitivity to temperature of our RI sensor.
Fiber-optic ultrasonic transducers are an important component of an active ultrasonic testing system for structural health monitoring. Fiber-optic transducers have several advantages such as small size, light weight, and immunity to electromagnetic interference that make them much more attractive than the current available piezoelectric transducers, especially as embedded and permanent transducers in active ultrasonic testing for structural health monitoring. In this paper, a distributed fiber-optic laser-ultrasound generation based on the ghost-mode of tilted fiber Bragg gratings is studied. The influences of the laser power and laser pulse duration on the laser-ultrasound generation are investigated. The results of this paper are helpful to understand the working principle of this laser-ultrasound method and improve the ultrasonic generation efficiency.
We demonstrate a fiber-optic Raman probe based on single-crystal sapphire fibers to overcome the large background signal of Raman probes based on standard glass fibers. Using 514.5 nm and 785 nm excitation lasers, Raman spectroscopy was performed on samples of calcite, aqueous sodium carbonate solution, and silicon wafer using a single crystal sapphire fiber and a silica fiber of similar lengths and numerical apertures. The single-crystal sapphire fiber exhibits narrow Raman peaks and low background signal, allowing for the detection of weaker Raman signals or Raman signals located in the lower wavenumber region, while the traditional silica fiber has a strong broadband Raman spectrum of its own.
A reliable and low-cost two-wavelength quadrature interrogating method has been developed to demodulate optical
signals from diaphragm-based Fabry-Perot interferometric fiber optic sensors for multipoint partial discharge detection
in power transformers. Commercial available fused-silica parts (a wafer, a fiber ferrule, and a mating sleeve) and a
cleaved optical single mode fiber were bonded together to form an extrinsic Fabry-Perot acoustic sensor. Two lasers
with center wavelengths separated by a quarter of the period of sensor interference fringes were used to probe acousticwave-
induced diaphragm vibration. A coarse wavelength-division multiplexing (CWDM) add/drop multiplexer was used
to separate the reflected two wavelengths before two photo detectors. Optical couplers were used to distribute mixed
laser light to each sensor-detector module for multiplexing purpose. Sensor structure, detection system design and
experiment results are presented.
We present a high-sensitivity fiber-optic ultrasonic sensor based on a π-phase-shifted fiber Bragg grating (πFBG). A π-
phase-shift at the center of a traditional fiber Bragg grating results in the formation of a narrow-bandwidth notch in the
reflection spectrum, leading to much higher sensitivity than standard fiber-Bragg-grating (FBG) sensors. A method of
fabrication is introduced, including a method of removing introduced polarization dependence. A tunable-cavity diode
laser is used to interrogate the πFBG by locking the wavelength to the linear slope of the spectral notch. High-sensitivity
detection of ultrasonic waves is demonstrated. The directivity of the grating is characterized. The effect of strain
induced polarization dependence is investigated. Preliminary tests using ultrasonic pressure waves in water are
performed.
This paper gives a review of a proposed fully-distributed fiber-optic sensing technique based on a traveling long-period
grating (LPG) in a single-mode optical fiber. The LPG is generated by pulsed acoustic waves that propagate along the
fiber. Based on this platform, first we demonstrated the fully-distributed temperature measurement in a 2.5m fiber. Then
by coating the fiber with functional coatings, we demonstrated fully-distributed biological and chemical sensing. In the
biological sensing experiment, immunoglobulin G (IgG) was immobilized onto the fiber surface, and we showed that
only specific antigen-antibody binding can introduce a measurable shift in the transmission optical spectrum of the
traveling LPG when it passes through the pretreated fiber segment. In the hydrogen sensing experiment, the fiber was
coated with a platinum (Pt) catalyst layer, which is heated by the thermal energy released from Pt-assisted combustion of
H2 and O2, and the resulted temperature change gives rise to a measurable LPG wavelength shift when the traveling LPG
passes through. Hydrogen concentration from 1% to 3.8% was detected in the experiment. This technique may also
permit measurement of other quantities by changing the functional coating on the fiber; therefore it is expected to be
capable of other fully-distributed sensing applications.
We present an intrinsic Fabry-Perot interferometric fiber sensor for high density quasi-distributed temperature and strain
measurement. The two internal partial reflection mirrors in such a sensor are formed by a pair of ultra-short fiber Bragg
gratings. We experimentally demonstrate the multiplexing of 56 such sensors in a single fiber using a frequency-division
multiplexing scheme. We show that theoretically as many as 500 sensors can be multiplexed. We demonstrated the experimental results of the multiplexed sensors for quasi-distributed temperature and strain measurement and the temperature compensation of strain sensors.
Pressure sensors are the key elements for industrial monitoring and control systems to lower equipment maintenance cost, improve fuel economy, reduce atmospheric pollution, and provide a safer workplace. However, the testing environment is usually harsh. For example, inside the turbine engine, temperatures might exceed 600°C and pressures might exceed 100psi (690kPa), where most current available sensors cannot survive. Moreover, due to the restricted space for installation, miniature size of the sensor is highly desirable. To meet these requirements, a novel type of all fused silica optic fiber tip pressure sensor with a 125μm diameter was developed. It is a diaphragm based pressure sensor in which a Fabry-Perot interferometer is constructed by the end face of an optical fiber and the surface of a diaphragm connected by a short piece of hollow fiber. The FP cavity length and the interference pattern will change according to ambient pressure variation. Its main improvement with respect to previously developed optical sensors, such as those utilizing techniques of wet etching, anodic bonding and sol-gel bonding, is the fact that no chemical method is needed during the cavity fabrication. Its dynamic pressure performance was verified in a turbine engine field test, demonstrating not only that it can safely and reliably function near the fan of a turbine engine for more than two hours, but also that its performance is consistent with that of a commercial Kulite sensor.
A new structure for diaphragm-based extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensors is presented. This structure introduces a through hole in a conventional diaphragm-based EFPI sensor and significantly reduces the effect of operating point drift due to the background pressure and fluctuations. This structure also potentially has high temperature stability.
Pressure measurements at various locations of a gas turbine engine are highly desirable to improve the operational performance and reliability. However, measurement of dynamic pressure (1psi (6.9kPa) variation superimposed on the static bias) in the operating environment of the engine, where temperatures might exceed 600°C and pressures might exceed 100psi (690kPa), is a great challenge to currently available sensors. To meet these requirements, a novel type of fiber optic engine pressure sensor has been developed. This pressure sensor functions as a diaphragm-based extrinsic Fabry-Perot interferometric (EFPI) sensor. The structure of the sensor head, composed entirely of fused silica, allows a much higher operating temperature to be achieved in conjunction with a low temperature dependence. The sensor head and the fiber tail have been packaged in a metal fitting connected to a piece of metal extension tubing, which improves the mechanical strength of the sensor and facilitates easy sensor installation. The sensor exhibited very good performance in an engine field test, demonstrating not only that the sensors' package is robust enough for engine operation, but also that its performance is consistent with that of a commercial Kulite sensor.
Sapphire (single crystal alumina) has superior optical and mechanical properties. With a very high melting point of about 2050°C, sapphire fiber is an excellent candidate in optical fiber sensing area for high-temperature measurements. This paper presents a new type of sapphire-fiber-based extrinsic Fabry-Perot interferometric (EFPI) temperature sensor. The spectral interference pattern is generated by a sapphire diaphragm placed in front of the sapphire fiber. The sensing element is interrogated by a white-light source. Temperature is demodulated from the spectral change of interference pattern. Prototype sensor is tested at high temperature up to 1545°C. Both theoretical and experimental analysis are presented. Preliminary data shows the sensor is very promising for measuring ultra-high temperature.
In this paper, we present a miniature fiber optic pressure sensor. The sensor is extrinsic Fabry-Perot interferometer (EFPI) based with its FP cavity directly fabricated on the tip of the fiber by fusion splicing and chemical etching. The processes are simple, with no other materials but silica fibers involved. The sensor has the same dimension as the fiber itself, only 125μm in diameter. The length of the FP cavity and the interference pattern will change in response to ambient pressure variation. The signal is demodulated by tracing the spectrum shift. Sensitivity can be adjusted for different applications with low or high pressure range. Prototype sensors were fabricated and tested for static response. Dynamic measurements were performed in a turbine engine. Theoretical and experimental analysis of the sensor response are also presented.
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