This PDF file contains the front matter associated with SPIE Proceedings Volume 9098, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

The use of fiber optic sensing in the oil and gas industry has greatly expanded over the past two decades. Since the first optical fiber-based pressure sensor was installed in a well in 1993, the industry has sought to use fiber sensing technology to monitor in-well parameters. Through the years, optical fiber sensing has been used in an increasing number of applications as technical advances have opened the door for new measurements. Today, fiber optic sensors are used routinely to measure temperature throughout the wellbore. Optical sensors also provide pressure measurements at key locations within the well. These measurements are used to verify the integrity of the well, provide feedback during well completion operations, including the actuation of downhole valves, and to monitor the production or injection process. Other sensors, such as seismic monitors and flowmeters, use fiber sensing technology to make in-well measurements. Various optical sensing techniques are used to make these measurements, including Bragg grating, Raman scattering, and coherent Rayleigh scattering. These measurements are made in harsh environments, which require rugged designs for optical cable systems and instrumentation systems. Some of these applications have operating temperatures of 572°F (300°C), and other applications can have pressures in excess of 20,000 psi (1,379 bar). This paper provides a historical perspective on the use of fiber optic sensing in the oil and gas industry from industry firsts to current applications.

For almost three decades, interest has continued to increase with respect to the application of fiber-optic sensing techniques for the upstream oil and gas industry. This paper reviews optical sensing technologies that have been and are being adopted downhole, as well as their drivers. A brief description of the life of a well, from the cradle to the grave, and the roles fiber-optic sensing can play in optimizing production, safety, and protection of the environment are also presented. The performance expectations (accuracy, resolution, stability, and operational lifetime) that oil companies and oil service companies have for fiber-optic sensing systems is described. Additionally, the environmental conditions (high hydrostatic pressures, high temperatures, shock, vibration, crush, and chemical exposure) that these systems must tolerate to provide reliable and economically attractive oilfield monitoring solutions are described.

With the growth in deep-water oil and gas production, condition monitoring of high-value subsea assets to give early warning of developing problems is vital. Offshore operators can then transport and deploy spare parts before a failure occurs, so minimizing equipment down-time, and the significant costs associated with unscheduled maintenance. Results are presented from a suite of tests in which multiple elements of a subsea twin-screw pump and associated electric motor were monitored using a fibre optic sensing system based on fibre Bragg gratings (FBG) that simultaneously measured dynamic strain on the main rotor bearings, pressure and temperature of the lubricating oil, distributed temperature through the motor stator windings and vibration of the pump and motor housings.

The design of an optical fiber to give optimized sensing and lifetime performance for downhole fiber optic seismic sensors is presented. The SM1500SC(7/80)P is designed with an 80μm cladding diameter, pure silica core, high numerical aperture, high cut off wavelength and a polyimide coating to achieve outstanding performance when used in a coiled deployment state and operating in high temperature and hydrogen rich environments.

This paper provides an overview of selected applications of high speed structural monitoring using fiber grating sensors. Rapid and effective diagnostic capabilities are necessary to respond to changes in structural integrity that may affect safety. In the case of aerospace structures operating at high velocity rapid response has the potential to mitigate catastrophic failure. Similar safety issues apply to civil structures where timely decisions are critical to operations of bridges, dams and buildings. Rapid responses for oil and gas, medical and environmental monitoring applications are also highly important. A great deal of progress has been made in improving the quality and capabilities of high speed fiber grating sensor systems. Some of these systems will be discussed.

Recently, we found that by terminating a long length of fiber of up to 2 km with an in-fiber cavity structure, the entire structure can detect vibrations over a frequency range from 5 Hz to 100 Hz. We want to determine whether the structure (including packaging) can be optimized to detect vibrations at even higher frequencies. The structure can be used as a distributed vibration sensor mounted on large motors and other rotating machines to capture the entire frequency spectrum of the associated vibration signals, and therefore, replace the many accelerometers, which add to the maintenance cost. Similarly, it will help detect in-slot vibrations which cause intermittent contact leading to sparking under high voltages inside air-cooled generators. However, that will require the sensor to detect frequencies associated with vibration sparking, ranging from 6 kHz to 15 kHz. Then, at even higher frequencies, the structure can be useful to detect acoustic vibrations (30 kHz to 150 kHz) associated with partial discharge (PD) in generators and transformers. Detecting lower frequencies in the range 2 Hz to 200 Hz makes the sensor suitable for seismic studies and falls well into the vibrations associated with rotating machines. Another application of interest is corrosion detection in large re-enforced concrete structures by inserting the sensor along a long hole drilled around structures showing signs of corrosion. The frequency response for the proposed long-gauge vibration sensor depends on packaging.

Fiber grating pressure sensors have been used to support pressure measurements associated with burn, deflagration and detonation of energetic materials. This paper provides an overview of this technology and serves as a companion paper to the application of this technology to measuring pressure during high speed impacts.

Fiber Bragg gratings (FBGs) are developing into useful sensing tools for measuring high pressure dynamics in extreme environments under shock loading conditions. Approaches using traditional diode array coupled FBG interrogation systems are often limited to readout speeds in the sub-MHz range. For shock wave physics, required detection speeds approaching 100 MHz are desired. We explore the use of two types of FBG sensing systems that are aimed at applying this technology as embedded high pressure probes for transient shock events. Both approaches measure time resolved spectral shifts in the return light from short (few mm long) uniform FBGs at 1550 nm. In the first approach, we use a fiber coupled spectrometer to demultiplex spectral channels into an array (up to 12) of single element InGaAs photoreceivers. By monitoring the detectors during a shock impact event with high speed recording, we are able to track the pressure induced spectral shifting in FBG down to a time resolution of 20 ns. In the second approach, developed at the Special Technologies Lab, a coherent mode-locked fiber laser is used to illuminate the FBG sensor. After the sensor, wavelength-to-time mapping is accomplished with a chromatic dispersive element, and entire spectra are sampled using a single detector at the modelocked laser repetition rate of 50 MHz. By sampling with a 12 GHz InGaAs detector, direct wavelength mapping in time is recorded, and the pressure induced FBG spectral shift is sampled at 50 MHz. Here, the sensing systems are used to monitor the spectral shifts of FBGs that are immersed into liquid water and shock compressed using explosives. In this configuration, the gratings survive to pressures approaching 50 kbar. We describe both approaches and present the measured spectral shifts from the shock experiments.

The Hybrid Sensor Bus is a space-borne temperature monitoring system for telecommunication satellites com­ bining electrical and fiber-optical Fiber Bragg Grating (FBG) sensors. Currently, there is no method available for testing the functionality and robustness of the system without setting up an actual sensor-network implying numerous FBG sensors in which each has to be heated/cooled individually. As a verification method of the mentioned system, FBG reflection based scanning laser interrogator, an FBG­ emulator is implemented to emulate the necessary FBG sensors. It is capable of immediate emulation of any given FBG spectrum, thus, any temperature. The concept provides advantages like emulating different kinds of FBGs with any peak shape, variable Bragg-wavelength λB, maximal-reflectivity τ_max, spectral-width and degradation characteristics. Further, it facilitates an efficient evaluation of different interrogator peak-finding algorithms and the capability of emulating up to 10000 sample points per second is achieved. In the present paper, different concepts will be discussed and evaluated yielding to the implementation of a Variable Optical Attenuator (VOA) as the main actuator of the emulator. The actuator choice is further restricted since the emulator has to work with light in unknown polarization state. In order to implement a fast opto-ceramic VOA, issues like temperature dependencies, up to 200 V driving input and capacitive load have to be overcome. Furthermore, a self-calibration procedure mitigates problems like attenuation losses and long-term drift.

Optical fiber is made of glass, an insulator, and thus it is immune to strong electromagnetic interference. Therefore, fiber optics is a technology ideally suitable for sensing of partial discharge (PD) both in transformers and generators. Extensive efforts have been used to develop a cost effective solution for detecting partial discharge, which generates acoustic emission, with signals ranging from 30 kHz to 200 kHz. The requirement is similar to fiber optics Hydro Phone, but at higher frequencies. There are several keys to success: there must be at least 60 dB signal-to-noise ratio (SNR) performance, which will ensure not only PD detection but later on provide diagnostics and also the ability to locate the origin of the events. Defects that are stationary would gradually degrade the insulation and result in total breakdown. Transformers currently need urgent attention: most of them are oil filled and are at least 30 to 50 years old, close to the end of life. In this context, an issue to be addressed is the safety of the personnel working close to the assets and collateral damage that could be caused by a tank explosion (with fire spilling over the whole facility). This paper will describe the latest achievement in fiber optics PD sensor technology: the use of phase shifted-fiber gratings with a very high speed interrogation method that uses the Pound-Drever-Hall technique. More importantly, this is based on a technology that could be automated, easy to install, and, eventually, available at affordable prices

The development of optical fibers has revolutionized telecommunications by enabling long-distance broad-band transmission with minimal loss. In turn, the ubiquity of high-quality low-cost fibers enabled a number of additional applications, including fiber sensors, fiber lasers, and imaging fiber bundles. In this work, we show that a multimode optical fiber can also function as a spectrometer by measuring the wavelength-dependent speckle pattern formed by interference between the guided modes. In practice, the wavelength-dependent speckle patterns are recorded in a transmission matrix. After calibration an arbitrary input spectra can be reconstructed based on the speckle pattern it produces. The spectral resolution is dictated by the change in wavelength required to produce an uncorrelated speckle pattern, which scales inversely with the length of the fiber. Using a 100 meter long multimode fiber, we were able to resolve two lines separated by merely 1 pm at a wavelength of 1500 nm. Broad-band operation is also possible by using a shorter fiber with lower resolution. We showed that a 4 cm fiber can provide 350 nm of bandwidth across the visible spectrum with 1 nm resolution. The fiber spectrometer consists only of a multimode fiber and a monochrome camera used to record the speckle patterns. Since the fiber can be coiled into a small volume, the entire spectrometer can be compact, lightweight, and low cost while providing ultrahigh resolution, broad bandwidth, and low loss.

Optical fiber interferometers (OFIs) have been extensively utilized for precise measurements of various physical/chemical quantities (e.g., temperature, strain, pressure, rotation, refractive index, etc.). However, the random change of polarization states along the optical fibers and the strong dependence on the materials and geometries of the optical waveguides are problematic for acquiring high quality interference signal. Meanwhile, difficulty in multiplexing has always been a bottleneck on the application scopes of OFIs. Here, we present a sensing concept of optical carrier based microwave interferometry (OCMI) by reading optical interferometric sensors in microwave domain. It combines the advantages from both optics and microwave. The low oscillation frequency of the microwave can hardly distinguish the optical differences from both modal and polarization dispersion making it insensitive to the optical waveguides/materials. The phase information of the microwave can be unambiguitly resolved so that it has potential in fully distributed sensing. The OCMI concept has been implemented in different types of interferometers (i.e., Michelson, Mach-Zehnder, Fabry-Perot) among different optical waveguides (i.e., singlemode, multimode, and sapphire fibers) with excellent signal-to-noise ratio (SNR) and low polarization dependence. A spatially continuous distributed strain sensing has been demonstrated.

Nowadays, the interferometric sensors belong to the one of the most accurate, thanks to its great sensitivity. With their help we are able to measure temperature, strain, level, flow, vibration, stress, etc. For its operation the Michelson interferometer consist of the two arms terminated by mirrors, by which is possible to measure generated phase shift in the individual arms. Furthermore, polarization maintaining fibers were used. With this setup we will examine the effects of vibration and also how is this sensor influenced by the different setup arrangement and how it will manifest its frequency response. It is important to isolate the reference arm to increase the sensitivity of the measurement and the subsequent effect on the maximum phase shift and maximum frequencies response. In this work, we are going to describe various combinations of the arrangement of the measuring and reference arm and their effect on the sensitivity of different measured phenomena. Subsequently obtained frequency bands are evaluated for these various configurations and materials.

It has been proposed that fast-light optical phenomena can increase the sensitivity of an optical gyroscope of a given size by several orders of magnitude. MagiQ Technologies is developing a compact fiber-based fast light Inertial Measurement Unit (IMU) using Stimulated Brillouin Scattering (SBS) in optical fibers with commercially mature technologies. We have demonstrated repeatable fast-light effects in the lab using off-the shelf optical components. Numerical analysis has revealed the requirements for stable, sensitive operation of gyroscopes, accelerometers or other sensors, as well as identified methods for optimizing efficiency, size, and reliability with known optical technologies. By using photonic integrated circuits and telecom-grade components along with specialty fibers, our design would be appropriate for mass production. We have eliminated all free-space optical elements or wavelength dependent elements such as atomic vapor cells in order to enable a compact, high sensitivity IMU stable against environmental disturbances. Results of this effort will have benefits in existing applications of IMUs (such as inertial navigation units, gyrocompasses, and stabilization techniques), and will allow wider use of RLGs in spacecraft, unmanned aerial vehicles or sensors, where the current size and weight of optical IMUs are prohibitive.

Fiber loop ringdown technique has shown promise in biomedical applications in recent studies. In the present work, fiber loop ringdown sensors using the evanescent field as the sensing mechanism have been fabricated and tested in actual human urines for the first time. In order to evaluate the sensors’ performance, the sensors were comparatively tested in healthy human urines, synthetic urine solutions, and diabetic urines. Due to different features or chemical compositions of each urine sample, the sensors experience different optical losses, equivalently, different ringdown times. The comparative results show that evanescent field-fiber loop ringdown glucose sensors can discriminate the three different urine samples by displaying different ringdown times. The evanescent field-fiber loop ringdown glucose sensors had fast response, good reproducibility, and high sensitivity. The promising results imply that the evanescent field-fiber loop ringdown sensors have potential for near real-time detection of diabetic urines.

We have explored the use of a fiber-optic probe with surface-enhanced Raman scattering (SERS) sensing modality for early, noninvasive and, rapid diagnosis of potential renal acute rejection (AR) and other renal graft dysfunction of kidney transplant patients. Multimode silica optical fiber immobilized with colloidal Ag nanoparticles at the distal end was used for SERS measurements of as-collected urine samples at 632.8 nm excitation wavelength. All patients with abnormal renal graft function (3 AR episodes and 2 graft failure episodes) who were clinically diagnosed independently show common unique SERS spectral features in the urines collected just one day after transplant. SERS-based fiber-optic probe has excellent potential to be a bedside tool for early diagnosis of kidney transplant patients for timely medical intervention of patients at high risk of transplant dysfunction.

Extraction of intrinsic fluorescence in a turbid medium such as tissue is a challenging problem due to the various interrelationships between different constituents of tissue. If the fluorescence happens to be in the visible region, predominant absorption of hemoglobin can quench the emission making the intrinsic extraction and hence the actual measurement of the respective fluorophore concentration erroneous. This work explores to understand the effect of hemoglobin quenching in layered skin tissue phantoms. Monte Carlo simulations on layered phantoms were performed with varying concentrations of FAD (Flavin Adenine Dinucleotide) and hemoglobin and the results are correlated with experimental fluorescence measurements.

Layer-by-layer (LbL) self-assembly via hydrogen bonding is one of the primary mechanisms to achieve stimuliresponsive polyelectrolyte thin films for a variety of applications. The ability to monitor the individual LbL steps in situ is of great significance in the development and understanding of hydrogen-bonded LbL systems of new design functions and properties. Long-period gratings in single-mode fiber (SMF-LPG) has the potential for such application by virtue of its high index sensitivity. We report a theoretical and experimental investigation to sort out the coupled cladding mode that is most sensitive to molecular adsorption events during LbL deposition. We have undertaken a combined theoretical and experimental investigation to illustrate and demonstrate the strong correlation between the order of the coupled cladding modes in long-period gratings (LPG) in conventional single mode fiber and their sensitivity to the process of layer-by-layer (LbL) assembly of polyelectrolyte multi-layers. We show that high-order cladding modes such as LP_0,10 in LPG are significantly more sensitive than their lower-order counterparts such as LP_0,2, with LP_0,10 yielding a shift of 1.575 nm in resonance wavelength per polyelectrolyte repeating unit of poly(vinyl pyrrolidone)/poly(methacrylic acid) bi-layer. We illustrate the potential utility of SMF-LPG integrated with stimuli-responsive LbL thin films as sensors. The integrated LPG/LbL strategy as a robust LbL test-bed has broad ramifications in exploring and exploiting sensors and devices enabled by versatile, stimuli-responsive polyelectrolyte thin films.

Optical microresonators have been proven as an effective means for sensitive chemical sensors development. The changes in refractive index near the resonator surface lead to the effective refractive index change and thus a shift at certain resonance wavelength. The high quality (Q) whispering gallery modes (WGMs) contributed by the rotationally symmetric structures will interact with the local circumstances through the evanescent field. The high sensitivity in detection was achieved by the long photon lifetime of the high-Q resonator (thus the long light-environment interaction path). In this paper, we present our recent research on using fiber pigtailed capillary coupler for WGM resonator excitation and its sensing applications. Capillary tube with wall thickness of several microns was used as the waveguide. The PMMA microsphere and porous glass microsphere (PGM) were integrated with the etched capillary tube for different sensing purposes. The Q-factors and free spectrum ranges (FSR) of different types of microspheres were measured by coupling light into the microsphere using novel fiber pigtailed capillary coupler. Chemical vapor at different concentrations were tested using PGM microresonator. This alignment free structure provides a new sensing probe based on WGM resonator concept.

We present a study on surface-enhanced Raman scattering (SERS) utilizing unclad single crystal sapphire fiber with Ag nanoparticles (NPs) immobilized either at the fiber distal end for direct excitation or on the fiber surface for evanescent-field interaction. The dependence of SERS intensity on the coverage density of Ag NPs was investigated. We demonstrated robust SERS sensitivity in both cases. For direct excitation-based sensing, we found that a sensitivity maximum exists with increased particle coverage beyond which the sensitivity starts to decline. More importantly though, for evanescent-field based measurements, we revealed that multimode sapphire fiber can accommodate Ag NPs at a far higher particle coverage density than single-mode fiber while maintaining the dominance of SERS gain despite competitive absorption and scattering loss by Ag NPs with a limit of detection of 10^-9 M Rhodamine 6G solution.

Recent progress in combing multiple materials with distinct optical, electronic, and thermomechanical properties monolithically in a kilometer-long fiber drawn from a preform offers unique multifunctionality at a low cost. Here, we summarize the application of this nascent concept of multimaterial fibers to infrared fiber optics.

With advancements in optical fiber technology, the incorporation of multiple sensing functionalities within a single fiber structure opens the possibility to deploy dielectric, fully distributed, long-length optical sensors in an extremely small cross section. To illustrate the concept, we designed and manufactured a multicore optical fiber with three graded-index (GI) multimode (MM) cores and one single mode (SM) core. The fiber was coated with both a silicone primary layer and an ETFE buffer for high temperature applications. The fiber properties such as geometry, crosstalk and attenuation are described. A method for coupling the signal from the individual cores into separate optical fibers is also presented.

We report a balanced PIN-TIA photoreceiver integrated with a 3 dB fiber coupler for distributed fiber optic sensors. This detector demonstrates -3 dB bandwidth >15 GHz and coupled conversion gain >65 V/W per photodiode through either input port of the 3 dB coupler, and can be operated at local oscillator power of +17 dBm. The combined common mode rejection of the balanced photoreceiver and the integrated 3 dB coupler is >20 dB. We also present measurement results with various optical stimuli, namely impulses, sinusoids, and pseudo-random sequences, which are relevant for time domain reflectometry, frequency domain reflectometry, and code correlation sensors, respectively.

In this paper, we analyze the performance of the electro-optic hybrid optical current transformer (HOCT) proposed by ourselves for high-voltage metering and protective relaying application. The transformer makes use of a fast variable optical attenuator (FVOA) to modulate the lightwave according to the voltage from the primary current sensor, such as low-power current transformer (LPCT). In order to improve the performance of the transformer, we use an optic-electro feedback loop with the PID control algorithm to compensate the nonlinearity of the FVOA. The linearity and accuracy of the transformer were analyzed and tested. The results indicate that the nonlinearity of the FVOA is completely compensated by the loop and the ratio and phase errors are under 0.07% and 5 minutes respectively, under the working power of less than 1 mW power. The transformer can be immune to the polarization and wavelength drift, and also robust against the environmental interference.

A fiber optic DC voltage sensor based on an intensity modulation scheme is proposed. Fiber optic voltage sensors have the advantage over conventional voltage sensors in that they offer voltage isolation and can easily be incorporated in telecommunication systems. The intensity modulation approach to sensing is a less costly and simpler measurement system compared to other available fiber optic voltage sensor techniques. Intensity modulation is achieved using a piezoelectric ceramic which produces a displacement on application of a voltage varying the transmitted optical power in a fiber to fiber coupling system. A critical analysis was performed on the theory behind the intensity modulation scheme for fiber voltage sensing. Simulations and experimental investigations based on this concept showed good linearity between the applied voltage and optical power in the fiber. The feasibility of obtaining a single valued relationship for voltage sensing purposes was also observed. The constructed voltage sensor produced useful results with the sensor exhibiting good linearity in forward and reverse voltages over a DC voltage of 0V to 100V but exhibited hysteresis. A linearity of 92% and 88.8% was measured for the forward and reverse voltages respectively and a dynamic range of approximately 0.3dB over the 100V range was achieved with a resolution of 1.9V. The hysteresis in the sensor was measured at 20%. Based on the results obtained recommendations have been made on a more linear, lower hysteresis and stable sensor of this type.

The paper presents a proximity sensor based on plastic optical fiber as tilt sensor. Discrete and continuous response of the sensor against change in tilt angle of the setup is studied. The sensor can detect tilt angles up to 5.7⁰ and the achieved sensor sensitivity is 97mV/⁰ .

In this paper new algorithms and procedures are reported which enable miniaturization and optimization of the thickness of a diaphragm for an all-glass extrinsic Fabry-Perot interferometer (EFPI)-based pressure sensor. Diaphragm etching improves the EFPI sensors ability to detect relatively small changes in pressure (0.1mmHg) and the resulting sensor exhibits excellent stability over time (drift < 1 mmHg / hour) for measurement in air and liquid. The diaphragm etching procedure involves fiber polishing followed by etching in hydrofluoric (HF) acid. An additional Ion-beam etching technique was investigated separately to compare with the HF-etching technique. A sensitivity better than 10 10 nm/kPa, which provides a pressure resolution of 0.05mmHg, is achieved by reducing the EFPI diaphragm thickness down to less than 2μm for the miniature pressure sensor used in this investigation (overall diameter of 200μm). The techniques reported is also applicable for the fabrication of high sensitivity sensors using a smaller fiber diameter e.g. 80μm.

Researcher’s teams were dealing with the microwave emitter’s inhomogeneity problem since the microwaves were used. One possible way, how to measure electromagnetic field is the measurement on inhomogeneous temperature distribution on the irradiated sample, which can cause problems as in other material processing, so in the undesirable change of properties and even security. Inhomogeneity of electromagnetic field is specific by creating spots with higher or lower temperature called “hot spots”. This inhomogeneity strongly affects the temperature distribution in the cross section of the material and its resultant heating. Given the impossibility of using classical electronic devices with metal temperature sensors were various indirect methods used in the past. This paper deals with experimental measurement of the microwave emitter’s inhomogeneity (2.45 GHz) using the optical fiber DTS. The greatest advantage of this sensor system is just in using of the optical fiber (electromagnetic resistance, small size, safety using in inflammable and explosive area, easy installation). Due to these properties of the optical fiber sensor it’s possible to measure the temperature of the sample in real time. These sensor are able to measure the temperature along the fiber, in some cases they use nonlinear effect in optical fiber (Raman nonlinear effect). The verification of non-homogeneity consists in experimental measuring of the temperature distribution within the wooden sample. The method is based on heat exchange in an isolated system where wooden sample serves as an absorber of the irradiated energy. To identify locations with different power density was used DTS system, based on nonlinear phenomena in optical fibers.

We present an experimental method for straight forward dual wavelength Erbium doped fiber linear cavity laser characterization based in laser line spectrum behavior due to the Hi-Bi FOLM transmission spectrum wavelength displacement by temperature variations in the fiber loop. The laser operation is for a single and dual mode, obtained through the adjustment of the cavity losses by the Sagnac interferometer spectrum wavelength displacement due to the temperature variation of the fiber loop. The method allow determine the laser operation from a single emission line and a two emission lines simultaneously through the Sagnac transmittance spectrum optical power variations measurement due to wavelength spectrum shifting for each laser wavelength generated separately and overlapping these obtained spectrums.

In this paper a linear cavity Erbium doped fiber (EDF) laser based in a fiber Bragg grating (FBG) and a fiber optical loop mirror with a high birefringence fiber in the loop (Hi-Bi FOLM) is used as a strain sensor. The Fabry-Perot cavity is formed by the FBG and the Hi-Bi FOLM, used as a measurement system of strain variations produced on the FBG, used as a strain sensor device. Usually, fiber laser sensor experimental setups determine the measured variable magnitude by using of an optical spectrum analyzer (OSA). Hi-Bi FOLM transmission spectrum wavelength displacement by fiber loop temperature variations measurement can be an attractive application exploiting the characteristics of FOLM transmission spectrum behavior due to Hi-Bi fiber loop temperature variations to determine the FBG strain applied through the maximal optical power monitoring by simple use of a photodetector and a temperature meter.

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.