This PDF file contains the front matter associated with SPIE Proceedings Volume 6770, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Fiber optic grating sensors have been widely used to support the measurement of strain and temperature. By appropriately constructing fiber gratings it is possible to simultaneously measure two or more environmental parameters simultaneously. This in turn results in the realization of a host of new sensing possibilities including multi-dimensional strain, shear strain in bond lines, imaging of damage in composite materials via "strain imaging" and fiber grating sensors that are capable of compensating for temperature while measuring pressure, corrosion and other environmental effects. This paper will review some of these capabilities.

This paper outlines cryogenic Y-joint testing at Langley Research Center (LaRC) to validate the performance of optical fiber Bragg grating strain sensors for measuring strain at liquid helium temperature (-240°C). This testing also verified survivability of fiber sensors after experiencing 10 thermal cool-down, warm-up cycles and 400 limit load cycles. Graphite composite skins bonded to a honeycomb substrate in a sandwich configuration comprised the Y-joint specimens. To enable SHM of composite cryotanks for consideration to future spacecraft, a light-weight, durable monitoring technology is needed. The fiber optic distributed Bragg grating strain sensing system developed at LaRC is a viable substitute for conventional strain gauges which are not practical for SHM. This distributed sensing technology uses an Optical Frequency Domain Reflectometer (OFDR). This measurement approach has the advantage that it can measure hundreds of Bragg grating sensors per fiber and the sensors are all written at one frequency, greatly simplifying fiber manufacturing. Fiber optic strain measurements compared well to conventional strain gauge measurements obtained during these tests. These results demonstrated a high potential for a successful implementation of a SHM system incorporating LaRC's fiber optic sensing system on the composite cryotank and other future cryogenic applications.

For applications over a wide temperature range, it is important to know the temperature effect on certain material parameters, as e.g., Young's modulus, reversibility of elastic deformations, or coefficient of thermal expansion. Especially at cryogenic temperatures, the low temperature dependence of the Bragg wavelength is advantageous for measuring strain effects. It was already demonstrated that fiber Bragg gratings can be used to measure the coefficient of thermal expansion of super-conducting materials [1]. In this paper, we present a method for the measurement of the temperature dependent changes of Young's modulus, down to temperatures of 4.2 K (liquid helium). Deformation samples made of CuBe bronze have been prepared for first tests showing best reversibility.

The fiber optic sensors have grown to a promising technology in the application of oil and gas prospecting. Our group has embarked on the experimental design of fiber optic seismic geophone based on fiber-Bragg-grating (FBG) technology recently. This paper provides the detailed investigation and analysis of the sensor field test result in the seismic reflection survey of the oilfield exploration as a continuing effort to our previous research. Several revisions of previous sensor head design have been implemented which include the carbon fiber composite material based cantilever, auxiliary beam mechanical design, and the moving coil electro-magnetic damping design for an improved seismic signal sensitivity in the field test. The field tests of the redesigned sensing system show that: (1) the FBG seismic geophone has a higher sensitivity than the commercial geophone between 10-70 Hz and equal sensitivity to commercial geophone between 80-140 Hz (2) compared to the conventional geophone, the FBG seismic geophone is significantly immune from the electro-magnetic interference (3) the FBG geophone has a less nonlinear distortion than the conventional geophone and it is totally compatible with various commercial seismic recorders.

An AWG-based interrogator is tested for FBG microphone array. Element microphones composed of FBG bonded on membrane are connected in series in one fiber cable and illuminated by a wideband source. The reflected light is processed with the system consist of a 32-ch AWG, photo diodes and differential amplifiers. First, the sensitivity and the frequency responses of the FBG microphone unit are studied mainly through experiments. Then, the operation point is discussed for the interrogation using AWG. The demonstrations of two-element and four-element arrays are carried out for audible sound waves.

Fiber Bragg grating (FBG) can be multiplexed to realize a quasi-distributed strain or temperature sensing system by use of FBGs of different Bragg wavelength. For a sensing system requiring large number of sensing points, however, this method has difficulties in cost and multiplexable number. On the other hand, we have successfully multiplexed FBGs of the same Bragg wavelength by applying the technique of synthesis of optical coherence function. The measurement range of the technique is limited to the period of the coherence function. Thereafter, we applied Vernier scheme to the system which actively makes use of the periodical coherence function to distinguish the FBGs. In this paper, we propose a new frequency modulation method, time-division phase shift modulation, which solves the FBG-positioning problem in our previous system applying Vernier scheme to the multiplexed FBG sensing with SOCF. Experimental results are reported.

Silica-based fiber Bragg gratings (FBG) sensors are versatile devices that are typically fabricated using UV laser exposure. Their applicability is restricted to temperatures < 600°C because of the erasure of the UV generated grating structure at higher temperature. FBGs made with femtosecond IR lasers and phase masks in standard single mode fiber can have high long-term thermal stability at 1000 °C. Above 1000 °C however, the silica undergoes structural transformations that limit the functionality of the fiber. The most successful optical fiber used for high temperature sensor applications is the single crystal sapphire fiber, which has a glass transition temperature of 2030 °C. Here we present our work on retro-reflective FBGs fabricated in single mode silica and multimode sapphire fiber. For sapphire fiber Bragg gratings (SFBG), no degradation of the grating strength at high temperature was observed when tested up to 1500 °C. The SFBGs have discrete resonant wavelengths that could be used potentially as distributed optical sensor arrays up to 2000 °C. To produce a single mode response, the multimode SFBGs were probed using tapered single mode fibers producing single and low order mode reflection/transmission responses. The taper coupling improved the spectral resolution of the sapphire FBG as compared to its multimode responses.

A technique for creating a temperature insensitive refractometer that utilizes TE and TM modes in an open-top ridge waveguide design is presented. By using the TE mode resonance as a temperature reference, the relative shift of the TM mode can be monitored in order to measure the refractive index of liquids under test. Specifically, the device fabricated here produces a relative resonance shift of 1 pm for every 1×10^-4 of measured index change, with a temperature sensitivity less than 0.2 pm/°C. To increase the sensitivity of these devices, a theoretical model is developed to investigate the performance of some potential waveguide structures. Relationships between the waveguide core size, refractive index distribution, as well as the relative evanescent sensitivity of TE and TM modes are examined.

There is much interest in using optical fiber sensors for strain measurement because they are lightweight and are insensitive to electromagnetic interference. Structural monitoring applications can employ fiber Bragg gratings (FBGs) for point-by-point measurements, whereas continuous sensors, based on Brillouin scattering, can be used to measure strain along entire lengths of un-modified optical fibers. These two strain measurement methodologies, and their practical limitations, are compared. Accurate and reliable static strain measurement with FBGs, a turnkey instrumentation with multichannel detection ability, and embedding techniques with repeatable calibration ability are reported. Embedding techniques that survive high temperature cycling, and high humidity under salt water environments, have been developed.

Given the growing demand for oil and natural gas to meet the world's energy needs, there is nowadays renewed interest in the use of liquefied natural gas (LNG) systems. For LNG to remain in its liquid phase, the gas has to be kept at cryogenic temperatures (< 160°C). And, as part of the LNG supply process, it becomes necessary to transport it using massive carrier tankers with cargo hulls operating at low temperatures and using special insulating double-wall construction. The safe and reliable storage and transportation of LNG products calls for low temperature monitoring of said containers to detect the onset of any potential leaks and possible thermal insulation degradation. Because of the hazardous nature of this cargo, only intrinsically-safe, explosion proof devices can be used. Optical fiber sensors-- such as fiber Bragg gratings-- are ideal for this application given their dielectric nature and multi-point sensing telemetry capability. In this paper, we describe the development of an on-line, multi-point FBG-based low temperature monitoring system based on a network of specially packaged FBG temperature and strain sensors mounted at critical locations within the inner hull, cofferdam and secondary barriers of a LNG carrier tanker. Given the stringent cryogenic operating temperature conditions, pertinent FBG designs, coatings and packaging approaches were formulated along with adequate installation techniques and integration of the interrogating FBG electronics into the tanker's overall SCADA monitoring system. FBG temperature sensors were demonstrated to be stable and sensitive over the 80-480K range. Stability is ± 0.25K or better with repeated calibrations, and long term stability at 480K is ~0.2mK/hour.

We review recent advancements in making high resolution distributed strain and temperature measurements using swept-wavelength interferometry to observe the spectral characteristics of Rayleigh scatter in optical fibers. Current methods available for distributed strain or temperature sensing in optical fiber include techniques based on Raman, Brillouin, and Rayleigh scattering. These techniques typically employ optical time domain reflectometry and are thus limited in spatial resolution to 0.1 to 1 m. Fiber Bragg gratings can yield higher spatial resolution but are difficult to multiplex in large numbers for applications requiring wide scale coverage. Swept-wavelength interferometry allows the Rayleigh scatter amplitude and phase to be sampled with very high spatial resolution (10s of microns). The Rayleigh scatter complex amplitude can be Fourier Transformed to obtain the Rayleigh scatter optical spectrum and shifts in the spectral pattern can related to changes in strain or temperature. This technique results in distributed strain measurements with 1 με resolution or temperature measurements with 0.1 C resolution. These measurements can be made with sub-cm spatial resolution over a 100 m measurement range or with sub-10 cm resolution over a 1 Km range. A principle advantage of this technique is that it does not require specialty fiber. Thus, measurements can be made in pre-installed single mode or multimode fibers, including those used for telecommunication networks. Applications range from fault monitoring in short range communications networks, structural health monitoring, shape sensing, pipeline and electrical transmission line monitoring, to perimeter security. Several examples are discussed in detail.

The interrogation, via optical fiber, of fiber Fabre Perot interferometers using laser based radio frequency modulation techniques, can provide ultra-sensitive acoustic sensing over very long distances. The benefits over other fiber optical acoustic sensing schemes include; immunity to laser polarization, coherence and intensity noise as well as reduced susceptibility to Rayleigh back scattering. Well defined error signals can be extracted at up to 120 km away. We report on the first multiplexed system, based on RF modulation interrogation techniques, in a 100 km fiber loop. We examine the achievable channel density as well as potential limits to strain sensitivity, such as inter-channel crosstalk, in a multiplexed RF modulated sensor system. The light-weight, small cross-section, intrinsic reliability, sensitivity and remote operation of the fiber sensor array based on RF techniques, enable new applications in hostile environments. The technique is free of electronics in the array part of the system, with all the electronic processing and control located remotely. There are no optical amplifiers or pump lasers - the technique is entirely passive. With appropriate packaging, an array of either hydrophones or geophones may be created with applications in security and defense as well as in geological survey.

The paper describes first completely autonomous measurement system based on transmission-reflection analysis (AMSTRA). The autonomous system utilizes simple optical scheme with 2 mW Fabry-Perot diode laser and original data acquisition and processing system. The location of the loss region is determined from unique relationships between normalized transmitted and Rayleigh backscattered powers for different positions of the disturbance along the test fiber. Accuracy, temporal and thermal stability of the autonomous system with 5.6 km-length test fiber were investigated. The paper also presents the preliminary results of the autonomous measurement system implementation for gasoline leak detection and localization.

We demonstrate distributed measurement of dynamic strain with 1 kHz sampling rate, the highest speed ever reported in Brillouin-based fiber sensors, using the simplified Brillouin optical correlation domain analysis system. Pump and probe waves are generated by direct current modulation of a laser diode applying a pre-compensated rectangular waveform at 100 kHz, and an unbalanced Mach-Zehnder delay line is introduced to the probe wave for the suppression of noise in high speed lock-in detection. Brillouin gain spectrum of single position is acquired at the sampling rate of 1 kHz with position-selectivity, and several kinds of strain variations up to 200 Hz are successfully measured with 10 cm spatial resolution and 20 m measurement range.

Using a phase-sensitive optical time-domain reflectometer developed at Texas A&M University, this paper reports on recent advances in intruder detection and classificatoin for long perimeters or borders. The system uses light pulses from a narrow linewidth CW laser with low frequency drift to interrogate an optical fiber. The backscattered light is detected, and real-time processing of the received signal is performed. Signatures from single and multiple humans on foot, nearby vehicle traffic on a road, construction-like vehicle activity, and animals have been obtained. Individual footsteps are clearly identified and the cadence readily observed. Time-frequency plots are used to compare the signatures. The detected signal contains information regarding the weight of the intruder as well. An adult weighing around 60kg may produce several π-radian shifts in the optical phase, which is detected by the system. While distances up to 20km have been monitored in previous remote field tests, we report measurements on a local test site with a total fiber length of 12km. A 3-mm diameter fiber cable is buried at a depth of 20-46 cm over a distance of 44m, with a 2km spool of fiber attached prior to the buried fiber and a 10km fiber spool connected in series after the buried section. Recent advances in data acquisition and signal processing allow us to avoid false alarms due to drifts in the laser center frequency and greatly improve the probability of detection. With these advancements, this technology is prime for low-cost perimeter monitoring of high-value and high-security installations such as nuclear power plants and military bases as well as national borders.

Over the last several years, Fiber Optic Sensor (FOS) applications have seen an increased acceptance in many areas including oil & gas production monitoring, gyroscopes, current sensors, structural sensing and monitoring, and aerospace applications. High level optical and mechanical reliability of optical fiber is necessary to guarantee reliable performance of FOS. In this paper, we review recent research and development activities on new specialty fibers. We discuss fiber design concepts and present both modeling and experimental results. The main approaches to enhancing fiber attributes include new index profile design and fiber coating modification.

The authors have been investigating the sensing characteristics of plastic optical fiber (POF) using the optical time-domain reflectometry (OTDR) for various kinds of mechanical disturbances such as bending, clamping, twisting and stretching as well as the effects induced by temperature rise. In this report, methods to fix the POF cable to wooden structures and their performances to detect the deformations are examined. The dimensions of the fixing plate are changed to minimize the undesirable effect of the fixing on the OTDR responses. Life of the memory effect also discussed through experiments. We can detect strain applied to the POF after the event is over through the memory effect of POF, which is caused by the plastic deformation. Our experiments show that the life of memory is over one month after the external force is released. Spatial resolution along the POF is found to be 5 m, and 5-point detection experiments are carried out using a 100-m POF. We discuss the memory effect in the case of multipoint measurements. Three types of POF cables are compared in terms of the reflection and loss at the deformed point.

A number of fiber optic sensors for geophysical applications have been developed over the past two decades at the Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography. These include: a strain sensor to monitor ice flow in Antarctica, a strain sensor to track sediment creep on the ocean bottom, a borehole strain sensor to monitor fault movement during earthquakes, a pressure sensor to detect low frequency acoustic waves, and a seismometer. All of these sensors utilize one of two interrogation techniques. The first is a commercially made electronic distance meter which, by measuring the transit time of light pulses through the sensing fiber, can track changes in a 1000-m-long fiber with a precision of about 1 mm. The second technique is interferometry. For this purpose, a quadrature fringe resolver based on a digital signal processor has been developed. It combines wide dynamic range (centimeters) with high resolution (picometers). Continuous records spanning days to years have been obtained with these instruments.

Optical fiber based sensing has now moved from laboratory demonstrations to actual applications in the real world. This has necessitated an entirely new area of extrusion - the packaging (cabling) of optical fibers and sensor arrays to protect them from the intended environment and installation handling while not masking or attenuating the phenomenon that is being sensed. Although each application presents new and unique challenges, the goal is to create a packaging concept for fiber sensors. Fiber sensing applications can be narrowed down to the five items below: 1. Conventional cable packages 2. Assembled (typically by hand) discrete sensor packages 3. Package enhanced sensors (where the packaging improves the effect of the sensor) 4. Linear sensor installation packaging 5. Scalar packaging (where the cabling adds to the range of the sensor) The above applications can be accomplished in a number of ways, and methods are still being developed in this relatively new science. Some of the new technology methods being explored include: UV cured liquids; Voided space cores; Conventional cable extrusion & its determination of mechanical characteristics. This paper reviews the pluses and minuses of the above methods and how their combination ultimately determines how the fiber or sensor array is to be jacketed in order to meet the specific application requirements. This paper will also review non-standard material characteristics, strength members and their role in measuring strain and stress values along with the overall influence of packaging on optical fibers and sensor arrays.

We describe a photosensitive capillary waveguide sensor, composed of a capillary structure with a photosensitive cladding ring surrounding the bore. The bore is filled with a fluid having an index of refraction that is sensitive to a measurand of interest, and a grating is inscribed in the cladding ring. We measure the optical interaction between light propagating in the filled capillary waveguide and the grating to determine the parameter of interest. Methods for forming connections between the capillary and single mode optical fiber are also described. We demonstrate the performance of the capillary waveguide as a temperature sensor in which the sensitivity of the Bragg wavelength to temperature is -150 pm/°C. We also describe how to use the capillary waveguide sensor to discriminate between different measurands.

The design and fabrication of an ultra-miniature all-glass pressure sensor with a diameter of 125 μm are presented. The sensor consists of a thin flexible silica membrane fused on a capillary tube section, which is assembled at the tip of a standard multimode fiber, thus forming a Fabry-Pérot air cavity whose length depends on applied pressure. Controlled polishing steps including on-line tuning of the diaphragm thickness during the manufacturing process achieve good repeatability and high sensitivity of the pressure sensor. The prototypes obtained with the described manufacturing method could easily have a sensitivity of ~2 nm/kPa (~0.3 nm/mmHg) with a record, so far, of ~5 nm/kPa (~0.7 nm/mmHg). The relatively simple fabrication technique using common and inexpensive equipments and materials combined with the fact that such sensitive sensors with multimode fiber could be interrogated with low-cost commercial interrogators (such as those using white-light interferometry) make this option very attractive for many applications involving pressure measurement. The sensor significant size reduction is valuable especially for the medical field, for applications such as minimally invasive patient health monitoring and diagnostics or small animals testing. Disposable sensors with ultra-miniature size will certainly open the way for new medical diagnostics and therapies.

Optical measurements of molecular oxygen are based on phosphorescent dyes with decay times dependent on the ambient oxygen content. Additionally, the phosphorescence decay times are affected by the temperature. In this work, we present miniaturized fiber optical probes, which are able to determine both ambient oxygen content and temperature simultaneously.

This paper establishes a procedure for increasing the sensitivity of measurements in integrated ring resonators beyond what has been previously accomplished. This is achieved by a high-frequency phase modulation lock to the ring cavities. A prototyped fiber Fabry-Perot cavity is used for comparison of the method to a similar cavity. The Pound-Drever-Hall (PDH) method is chosen as a proven, ultra-sensitive method with the exploration of a much higher frequency modulation than has been previously discussed to overcome comparatively low finesse of the ring resonator cavities. The high frequency facilitates the use of the same modulation signal to separately probe the phase information of different, integrated ring resonators with quality factors of 5.6 x10⁵ and 2.4 x10⁵. The large free spectral range of small cavities and low finesse provide a challenge to sensing and locking the stability of diode lasers due to the small dynamic range and signal-to-noise ratios (S/N). This can be offset by a calculated increase in modulation frequency using the PDH approach. A distributed feedback (DFB) laser is locked to a ring resonator cavity to demonstrate this sensitivity. This approach using integrated ring resonators is measured to have a refractive index resolution of 1.9x10^-8 that can be compared to other fiber and integrated sensors. The relationship between the signal-to-noise ratio and dynamic frequency range of the cavity error signal is explored with an algorithm to optimize this relationship. The free spectral range and the loss of the cavity provide input parameters to this relationship to determine the optimum S/N and range of the respective cavities used for locking and sensing. The purpose is to show how future contributions to the measurements and experiments of micro-cavities, specifically ring resonators, is well-served by the PDH method with high-frequency modulation.

The optical fiber has the features of low loss and wide bandwidth; it has replaced the coaxial cable as the mainstream of the communication system in recent years. Because of its high sensitivity characteristic, the interferometer is usually applied to long distance, weak signal detection. In general, if the area to be monitored is located far away, the weak signal will make it uneasy to detect. An interferometer is used for phase detection. Thus, the hydrophone which is based on interferometric fiber optic sensor has extremely high sensitivity. Sagnac interferometric hydrophone has low noise of marine environment, which is more suitably used to detect underwater acoustic signal than that of a Mach-Zehnder interferometer. In this paper, we propose the configuration of dual Sagnac interferometer, and use the mathematical methods to drive and design optimal two delay fiber lengths, which can enlarge the dynamic range of underwater acoustic detection. In addition, we also use software simulation to design optimal two delay fiber lengths. The experimental configuration of dual Sagnac interferometer with two optical delay line is shown as Fig. 1. The maximum and minimum measurable phase signal value of dual Sagnac interferometer (L₂=2 km, L₄=222.2 m), shown in Fig. 3. The fiber optic sensor head is of mandrel type. The acoustic window is made of silicon rubbers. It was shown that we can increase their sensitivities by increasing number of wrapping fiber coils. In our experiment, the result shows that among all the mandrel sensor heads, the highest dynamic range is up to 37.6 ± 1.4 dB, and its sensitivity is -223.3 ±1.7 dB re V / 1μ Pa. As for the configuration of the optical interferometers, the intensity of the dual Sagnac interferometer is 20 dB larger than its Sagnac counterpart. Its dynamic range is above 66 dB where the frequency ranges is between 50 ~ 400 Hz, which is 24 dB larger than that of the Sagnac interferometer with the sensitivity of -192.0 dB re V / l μPa. In addition, by using software simulation to design optimal lengths of delay fibers, we can increase the dynamic range of interferometer on underwater acoustic detection. This paper verifies that, by means of adjusting the length of these two delay fibers, we can actually increase the dynamic range of acoustic signal detection.

Experimental measurements of the strain and pressure of rotor blades are important for understanding the aerodynamics and dynamics of a rotorcraft. This understanding can help in solving on-blade problems as well as in designing and optimizing the blade profiles for improved aerodynamics and noise attenuation in the next generation rotorcraft. The overall goal of our research is to develop a miniature wireless optical sensor system for helicopter on-blade pressure and strain measurements. In this paper, leveraging past and current experiences with fiber optic sensor development, a proof-of- concept of fiber optic pressure/strain sensor system with wireless data acquisition and transfer capability is demonstrated. The recently developed high-speed, real-time fiber optic sensor demodulation techniques based on low coherence interferometry and phase-shifting interferometry is used. This scheme enables a Spatial Division Multiplexing configuration that consists of multiple Fabry-Perot strain and pressure sensors. Calibration of the strain and pressure sensors is carried out by using commercially available sensors as references. Spin chamber testing of the sensor system for simultaneous on-blade pressure and strain field measurements is also performed. It is expected that such a sensor system will result in enhanced robustness and performance for on-blade pressure and strain field measurements.

In Taiwan, many areas suffer from the debris flow due to the earthquakes, typhoon, and many flood disasters in recent years. Traditional geophones were applied to detecting underground sound induces by debris flow. They usually used the coaxial cable as a signal transmission line. However, they are limited by the length and interfering phenomenon by noise. Because the optical fiber has the features of low loss and wide bandwidth, it has replaced the coaxial cable as the mainstream of the communication system recently. Due to its high sensitivity characteristic, the interferometer is usually applied to detect weak signal. In general, if the debris flow is happened far away in the mountain area, the weak signal would be uneasy to monitor. In this paper, we study the debris flow monitoring system. Two kinds of sensing system are studied and compared with their performance. The first sensor is a traditional geophone, while the second one is a fiber optic sensor. This sensing system is composed of a fiber optic sensor and a Sagnac interferometer. The optical sensor is constructed by a mandril wrapped with fiber. We compare the characteristic of those two sensor heads. The results indicate that the fiber optic interferometric sensor head (fiber optic geophone) has high resolution and its frequency response highly match with the traditional geophone. The results of experiments show that the performance of fiber optic geophone sensing system work well for low frequency range. According to our preliminary test, the dynamic range detected by the optic fiber sensor is 45 dB, while the frequency ranges is between 10 ~ 300 Hz.

In this work, we present research in using confocal optical techniques with femtolitre focal volumes and obtain very high signal-to-noise and signal-to-background ratios for single molecule detection (SMD). We were able to achieve improved signal strength by using highly fluorescent quantum dots and nanopatterned substrates to obtain plasmon induced resonant fluorescence enhancement. A method to simultaneously using multiple excitation spots without the use of confocal apertures and an array of single photon sensitive Geiger mode avalanche photodiodes was used to increase the throughput of the detection system. Using this highly sensitive SMD system, we detect small quantities of synthetic DNA through hybridization eliminating the need of polymerase chain reaction.

Modulated optical fiber laser sources have proven useful in applications ranging from fiber communications systems to optical fiber sensors. As with many other coherent light sources, fiber lasers can easily be modulated by means of electro-optical devices; several modulation schemes have been successfully reported. In this paper, we report on a fiber laser with modulation regimes achieved by means of polarization feedback. The all-fiber laser cavity is based on a Fabry-Perot arrangement in which single or dual polarization output can be obtained upon adjusting the intra-cavity birefringence. Feedback of one polarization yields several effects on the population dynamics of the fiber laser, thereby providing a simple way of achieving a dual polarization modulated output. Monitoring of both polarization modes show that during modulation, polarization dynamics can exhibit in-phase and anti-phase behavior, and for pump power levels well above threshold, chaotic-like behavior can also be observed. Experiments show that the modulated regime obtained with the fiber laser depends on operating parameters such as pump power level, intra-cavity birefringence and the phase of the polarized mode used as a feedback signal. As shown through radio-frequency spectral analysis, the proposed arrangement could provide a simple polarimetric sensing scheme with radio-frequency readout for fiber optic sensors. Finally, we discuss the use of the proposed fiber laser in applications such as polarization-switched sensing and self-mixing interferometry.

We are conducting research to confirm the performance of a semiconductor fiber-optic ring laser gyroscope (S-FOG) featuring a semiconductor in its laser cavity. This S-FOG consists of a semiconductor optical amplifier (SOA) as a gain medium, a polarization-maintaining fiber to make a ring cavity, and a directional coupler to take part of the optical power out of the cavity. One of the advantages of the S-FOG is the adaptability of the laser cavity, which allows us to examine many cases of S-FOG applications easily. In the first case, we observed that the S-FOG generated Sagnac beat signals whose peak frequency was proportional to the rotation rate when it rotated. In the second case, we changed the area surrounded by the ring cavity (the fiber) and its perimeter and maintained a near-fixed oscillation wavelength of the ring laser. In this case, all of our experimental results were in good agreement with theoretical calculations, within a few percent. In the third case, we changed the oscillation wavelength and fixed the shape of the ring cavity. In this case, our results were also in good agreement with theoretical calculations. In the fourth case, we examined the Sagnac beat spectrum generated by the S-FOG in detail. The linewidth of the Sagnac beat spectrum increases as the area bounded by the optical path in the ring cavity becomes smaller, or as the length of the cavity becomes shorter. Our experimental results show that the S-FOG works as a gyro and that there exists practical potential for a semiconductor ring laser gyro.

Power cables should be operated at an adequate temperature level. Therefore, numerous power utilities have installed optical distributed temperature sensing (DTS) systems to measure the temperature of underground cables. Protection and metering systems used in power systems require measurements of the current flowing in the high-power conductors as well. Optical current sensors achieve increasing attention and acceptance for this application due to their inherent electrical insulation, high bandwidth, and immunity to EMI. DTS systems are based on spontaneous Raman scattering or Brillouin scattering, which use spectral information in the reflected light, whereas optical current sensors are based on the Faraday effect, which changes the intensity of transmitted light. This paper proposes a novel design of a combined optical temperature and current measurement system, using both physical effects. A first measurement setup is described, and first results are discussed. Thereby, the specifications for the combined data acquisition and data processing unit are analyzed in order to optimize the accuracy and the reliability of each subsystem and the whole system.

A new fiber-optic current sensor (FOCS) is described which employs phase shifting algorithms to process the optical signal. In this approach the sensing element consists of a coil low birefringence fiber placed between one polarizer and four analyzers. In the polarimeter layout, the output light from the current sensing element is divided into four beams through three nonpolarizing beamsplitters, and in each beam path is placed an analyzer and a detector. This paper discusses the characteristics of the optical current sensors, specifically for relaying applications were measurement of fault current is required. The design of the sensor, results and shift algorithms for electrical current characterization will be presented.

Current and Voltage sensors have existed as a special field for more than 100 years in Power Systems. In the last two decades there has been a fast development for systems based on fiber optical technology. As consequence, groups of IEEE- PSRC, IEC TC57, CIGRE SC A3, and some others are involved in developing activities in the area of optical systems for protective relaying. This paper provides an overview on those activities discussed by international groups. The document is divided into the four major sections. The first one explains the interaction of sensors within a protection scheme. The second section deals with the designing and components of fiber-optic current sensors (FOCS). Once presented their description, there are divulged general tests identified as relevant. Finally a brief comparison with conventional sensors is presented.

A fiber-optic PH sensor is developed based-on the long period grating (LPG). The LPG is fabricated by using CO₂ laser with a point-by-point technique. Then the grating portion is coated with PH sensitive hydrogel. The hydrogel, made of PVA/PAA, swells its volume in response to the PH change in the surrounding environment and results in a change in the refractive index. As a result, the LPG can response to the refractive index change in the coating by shifting its wavelength. Therefore, change in refractive index can be measured by tracking the wavelength shift using an optical spectrum analyzer (OSA). In this research, the LPG is dip-coated by the hydrogel. A chemostat is designed to simulate the marine environment. The PH in the chemostat is varied by controlling the CO₂ concentration in the sea water. A PH resolution 0.046/nm using the OSA has been obtained. This sensor is designed to monitor the sea water PH change in a long term basis.

This paper brings a short view into the hybrid communication-sensing fiber analysis. Commonly used telecommunication fibers are designed to be independent of external conditions. The transmitted signal should not be influenced by the external conditions. On the other side there are optical fiber sensors which are used for detecting or measuring the external quantities. In our research we are trying to include both properties in one optical fiber. A special type of fiber (a hybrid fiber), which preserves character of the standard communication fiber, was designed and maintained for this purpose. We use a microscope with a high-resolution camera to watch the power redistribution between individual modes at the output end of the fiber. The coupled power equations are used to predict changes in the optical power redistribution after determination of the coupling coeffcients. The key problem is how to excite individual modes for determination of the coupling coeffcients because of a very small diameter of the fiber. The Fourier and wavelet analysis is used to find out the significant points in the progression of the optical power. There are changes in the Fourier spectra and changes in wavelet coeffcients. From the Fourier analysis we can predict the progression, wavelet analysis enables us to find out singularities. It is expected, that every change to the fiber has its own "fingerprint" in the redistribution of the optical power. So we believe that we will be able to say, what affects the fiber, if it is heat, pressure, tension etc.

This paper describes a cantilever-based photoacoustic spectrometer that is capable of gas detection at ppm levels. The use of optical fiber components enables the first demonstration of modulation frequency divisional multiplexing (MFDM) using tunable diode lasers and the photoacoustic spectrometer. The best-case sensitivity of 3.4 × 10^-10 cm^-1 W Hz^-1/2 (3σ) for measurements using a CO2-containing gas standard are consistent with a previously published value of 1.7 × 10^-10 cm^-1 W Hz^-1/2 (1σ). To the best of our knowledge we have presented the first demonstration of MFDM using a cantilever-based photoacoustic spectrometer. An erbium doped fiber amplifier (EDFA) has been used to amplify simultaneous two distributed feedback tunable diode lasers of closely spaced wavelengths as demonstrated by recording photoacoustic (PA) signals of CO and CO₂.