Fully-distributed optical-fiber sensing is a potentially powerful tool for a range of industrial and research applications. Most methods which have been studied hitherto suffer from inadequate spatial resolution, inadequate sensitivity, or both. New methods are being explored in order to overcome these deficiencies. These are primarily polarization-optical methods which rely on birefringent fiber. Both linear and nonlinear optical effects are utilized in backscatter and forward-scatter systems. The methods are described and the future assessed.

Single mode fibers are being installed in nearly all communication lines. For the purpose of maintenance and observation of line, data of the installed fiber condition is needed. Distributed temperature sensing for single mode fiber is expected in this field. Special wavelength light sources are necessary to measure the temperature distribution by Raman ratiometry in order to avoid the single mode fiber's cut-off wavelength. Distributed temperature sensing of the single mode fiber is perceived using fiber Raman laser. Fiber Raman laser is pumped by the LD- pumped solid-state laser source. 1400 nm pulse light is coupled into a single mode fiber and the 1320 Anti-Stokes and 1500 nm Stokes light are detected to calculate the temperature. Temperature accuracy is +/- 2 degree(s)C with the average time of 3 min and the maximum measurable range is 3 km.

Using an optical fiber whose transmission loss depends in a linear way on temperature, a thermometric system can be designed that will allow direct measurement of the temperature averaged along the length of the fiber. This paper describes such a system used to test the feasibility of using rare earth-doped fibers in practical distributed measurements of average temperature.

We have proposed an optical coherence domain reflectometry by synthesis of coherence function (OCDR), for high spatial-resolution measurements of optical waveguide devices. We present the principle of the OCDR and the recent experimental results of the OCDR with spectrum spacing control of many lines. A three-electrode DFB laser diode with wide frequency tunable range is used as the light source to improve the resolution. The spatial resolution of 3 to approximately 4 mm in air has been obtained. Furthermore, overall effects of the performance deterioration factors are theoretically considered.

The Sagnac interferometer can be configured in forms that have position dependent, as well as position independent, response to time varying environmental disturbances. These features allow distributed sensors to be formed by various combinations of Sagnac interferometers and modes of operations. This paper describes a series of Sagnac distributed sensors that allow the determination of the position and amplitude of a time varying environmental disturbance.

Two-mode, elliptical-core optical fibers are demonstrated in weighted, distributed and selective vibration-mode-filtering applications. We show how appropriate placement of optical fibers on a vibrating structure can lead to vibration mode filtering. Selective vibration-mode suppression on the order of 10 dB has been obtained using tapered two-mode, circular-core fibers with tapering functions that match the second derivatives of the modes of vibration to be enhanced. We also demonstrate the use of chirped, two-mode gratings in fibers as spatial modal sensors that are equivalents of shaped piezoelectric sensors.

A new optical fiber capable of sensing distributed forces along its continuous length comprises a small central core and a noncontiguous second light-guiding region of longer optical path length. When interrogated with sufficiently short light pulses launched into the central core at the fiber launch end, mechanical forces acting at different points along the fiber cause the deflection of a fraction of the intensity of the interrogating light pulses propagating along the fiber at each point from the central core to the second light-guiding region, where they generate positive pulsed light signals reaching the fiber distal end separated in the time domain from the interrogating light pulses and from the signals generated at other sensing points along the fiber, and with an intensity several order of magnitude stronger than that of Rayleigh- backscatter signals. In addition to its potential use as a distributed force sensor, the fiber could serve as a telecommunications line allowing the noninvasive coupling of information at many points, simultaneously or in any arbitrary sequence, without the need for time-sharing protocols.

As the use of optical fibers and fiber devices for chemical sensing continues to grow, research and development efforts are beginning to turn to the accomplishment of measurements of spatially averaged chemical concentration, spatial profiles of concentration, the measurement of multiple analyses with a single sensor, and the use of fiber bus structures to carry information from multiple sensors. This paper discusses some of the means by which these measurements can be accomplished, reviews specific examples of each multiplexing or distributed sensing technique, and speculates on the directions such research will take in the future.

We report on the results obtained from testing prototype optical time domain fiber strain sensors on a full scale F-15 test plane at Wright Patterson AFB, OH. The tests included a segmented Optical Time Domain Reflectometry (OTDR) based fiber sensor, as well as a reentrant loop Optical Time Domain strain sensor. The sensors were attached to the underside of one of the plane's wings near the main body of the aircraft. Dynamic loads simulating in-flight conditions were applied through hydraulic actuators mounted on the wing's top and bottom surfaces. Strain levels ranged from 0 to 2500 micron/m during the cyclic load tests. Data was acquired from the high resolution optical time domain system using customized software and a GPIB interface between the system's processor and a personal computer. A new referencing technique allowed quasi-continuous measurements and automatic, periodic reference updating. The strain levels measured with the fiber sensors were compared with those read by conventional strain gages, and good correlation was observed. To our knowledge, these are the first dynamic OTDR picosecond resolution strain measurements ever performed.

This sensor is based on the optical coupling between two optical fibers, submitted simultaneously to the same constraint. Due to the geometrical deformations, photoelasticity, and evanescent field this coupling was studied theoretically and experimentally, and used to build a time multiplexed sensor network by coupling short pulses. This arrangement has high sensitivity and resolution, without some drawbacks of the classical OTDR.

A novel technique for measuring several physical parameters suing a transparent, silicone- rubber optical fiber is described. A discussion of the physical and optical characteristics of the fiber is provided along with preliminary experimental results on various present and future sensor applications. These applications include fiber-optic sensors for detecting and measuring temperature, humidity/moisture, force, and static and dynamic pressure.

The paper describes how active fiber devices and special fibers can enhance the performance of multiplexed and distributed sensing systems. The use of active fibers, such as rare-earth-doped types, can act as powerful CW or pulsed sources, as wavelength-tunable sources, high-power amplifiers and as low-noise detector preamplifiers. Thus many arrangements for sensor-multiplexing, or distributed sensing, which may appear unattractively lossy on first consideration, may become perfectly viable with the insertion of active devices. The paper also reviews possible applications of special fibers in multiplexed and distributed sensors. The use of such fibers can greatly enhance the performance of sensor systems and even allow the construction of new types of optical sensor which were not previously possible with conventional fibers.

Practical considerations in the design of an optically multiplexed ladar fiber-optic linear position sensing system are discussed including network architecture, bus fiber count, fault location, sensor separation and network efficiency. The results of a six channel multiplexing experiment using a single laser diode are presented.

The different multiplexing techniques are reviewed and their aeronautical applications are discussed. A new TDM approach is presented: a second pulse at another wavelength is used as a clock signal. The application for a network of position sensors is investigated.

Wavelength division multiplexing can be applied efficiently to build optical fiber sensors networks.

The advantages of networking fiber sensors using optical telemetry techniques to form all-optical arrays of sensor-elements are well recognized. Interest in this general area began in the early 1980s, and over the past six years at NRL we have attempted to experimentally and theoretically characterize various different array configurations for interferometric sensor arrays based on frequency-division, time-division and coherence multiplexing schemes. Recently a number of tests by the U.S. Navy have successfully demonstrated the operation of multiplexed sensors at sea. Work has also been conducted on other novel multiplexing schemes for non-linear phase transduction based sensors and intensity-modulation based schemes. This paper will review the progress made in this area in recent years, and discuss some aspects of current research interest.

A reflective array of optical fiber sensors multiplexed in time and with their status read using coherence sensing associated with directly modulated multimode laser diode illumination is investigated. Sensor sensitivity as determined by primary noise sources is evaluated and numeric results are presented. The concept is demonstrated with two all fiber Michelson interferometers and applied to the measurement of periodic and quasi-static parameters. It is shown that the effect of feedback light into the laser cavity on the level of the system noise floor is negligible, making unnecessary the use of source optical isolation.

The results are presented of the first comprehensive experimental investigation of the effects of the state of polarization (SOP) in the common input of the multielement array of sensors on the interferometric signal visibilities of the individual sensors in the array. In one case of a 10-sensor array with randomly set initial visibilities, an empirical determination of the optimum input SOPs is shown to maintain the visibilities of all sensors larger than 0.55, or signal fade less than 6 dB. A theoretical model is developed which predicts the minimum visibility obtained from controlling the input SOP in N-element arrays. The results agree very well with experimental results obtained from a 10-sensor array, including the worst case situation in which all sensors are initially set to the complete polarization-fade condition.

Bragg reflection gratings and out-coupling taps for sensors can be written holographically within the core of many commercial fibers available today. The gratings appear to be permanent and have been tested to temperatures in excess of 500 degree(s)C. Quasi-distributed temperature, strain, pressure, chemical, and interferometric type sensors can be made with the wavelength selective, reflection gratings, and taps. The fiber gratings, and the different types of sensors they can make, conveniently lend themselves to WDM, TDM, and FDM types of multiplexing schemes. Instrumentation to detect the multiple sensors and measure their spectral shift for localized and quasi-distributed sensing is currently under development.

A digital, all-optical temperature sensor design concept based on optical sampling and digital encoding is presented. The proposed sensor generates 2M binary digital codewords of length M bits. The codewords are generated serially and, therefore, only a single output fiber line is required. A multiplexing scheme, which minimizes the power requirement per sensor array and facilitates a cost-effective digit regeneration for remote monitoring over long distance, is presented. The sensor arrays are used as building blocks to configure large scale sensor networks based on LAN topologies.

We report a modified all-fiber Mach-Zehnder interferometer configuration incorporating recirculating fiber paths in both the signal and reference arms. This system can be operated using a time-division based interrogation approach to yield a self phase-noise compensated sensor.

The paper deals with some of the major issues concerned with the provision of wavelength division multiplexed multiple point-sensor networks. In particular it addresses the provision of multiple wavelength channels by the spectral slicing of an LED source. Consideration through the development of analytical models is given to slice separation, optical crosstalk, slice width, power in the slice, and, hence, the number of slices that may be obtained from a particular broadband LED source. These results are then applied to specific sensor network topologies using commercially available component parameters in order to determine the number of sensors that can be accommodated. Furthermore two novel WDM sensor network topologies are proposed which facilitate the operation of an increased number of distributed point-sensors in comparison with more conventional network topologies.

Leaks in dielectric fluid-filled, high-voltage distribution lines can cause significant problems for the electric power industry. Often, these lines run over long distance and are difficult to access. Operators may know that a leak exists because additional fluid is required to maintain pipe pressure; however, locating the leak is often a significant challenge. A system that could monitor and locate leaks within the electrical distribution pipe lines would be highly desirable. We present a distributed fiber optic acoustic sensor technology that could be used to measure and locate leaks within fluid-filled, high-voltage distribution lines. In this application, the optical fiber sensor is placed inside the fluid-filled pipe and can potentially locate leaks to within several meters. The fiber optic acoustic sensor is designed such that it can listen to the sound produced by the fluid as it escapes from the pipe into the surrounding soil. The fluid inside the pipe is typically maintained at a pressure of 200 psi and escapes at high velocity when a leak occurs. The distributed fiber optic sensing system being developed is based upon the Sagnac interferometer and is unusual in that range information is not obtained by the more common method of optical time domain reflectometry or optical frequency domain reflectometry, but by essentially a CW technique which works in the frequency domain. It is also unusual in that the signal processing technique actually looks for the absence of a signal.