Optical fiber sensor technology based on intra-core Bragg gratings has been used in a number of important application areas ranging from structural monitoring to chemical sensing. Practical and cost effective systems are not far in the future judging from advances in grating manufacture and sensor readout instrumentation. Fiber grating technology is not driven by its use in sensors but rather by valuable applications in dense, broadband WDM telecommunications. In this paper, we review the fundamentals of Bragg grating sensors and discuss various means for wavelength-shift demodulation, separation of temperature and strain responses and new directions that will offer additional capabilities.

A simultaneous spatial, time and wavelength division multiplexing topology, with combination of a tunable wavelength filter and an interferometric wavelength scanner, is proposed to interrogate a range of in-fiber Bragg grating (FBG) sensors. An eleven-element FBG sensor network based upon this topology is demonstrated for quasi-static strain sensing. Preliminary experimental results show that a strain resolution of approximately 7 (mu) (epsilon) with an approximately 30 Hz bandwidth (approximately 1.3 (mu) (epsilon) /(root)Hz) for quasi-static strain measurement has been obtained.

A spectral integration method of extracting the monotonic strain distribution along a fiber optic Bragg grating sensor has been developed and tested on a number of applications. Pre-chirping the Bragg grating has been shown to alleviate some of the restrictions on the spectral integration method of acquiring strain profile information.

A novel approach for the interrogation of multiplexed fiber optic Bragg-grating strain sensors is presented. It is a time domain multiplexing technique based on using pulsed radio frequency modulation combined with slow wavelength tuning of a distributed feedback laser diode. A derivative- type signal for each sensor is generated in the receiver and the instantaneous Bragg-wavelength, proportional to the applied strain, is determined from the timing of the zero- crossing of the derivative signal. The Bragg-gratings, all with the same Bragg wavelength, are coupled to the output of a 1 x N fiber optic coupler with appropriate relative fiber delays between each sensor to enable time-division multiplexing. A four channel system has been designed, implemented, and tested in a laboratory environment, and later mounted on the hull of a scaled catamaran model and tested at a ship testing facility. The strain measurements were compared to measurements using standard strain gauges. The system had a dynamic range of about 680 microstrain, limited by the laser diode current tuning, and a resolution of approximately 1 microstrain, limited by interferometric background signals. The bandwidth was limited to about 50 Hz due to the laser diode tuning response.

A volume holographic spectral filter bank formed in photorefractive BaTiO₃ is proposed and demonstrated as a demodulation scheme for in-fiber Bragg grating sensors. A strain range of 2500 microsecond(s) train, with minimum detectable strain of 4 microsecond(s) train/(root)Hz is reported. Extension of the technique to allow intensity referencing and parallel demodulation of an array of sensors is discussed.

We demonstrate a passive fiber Bragg grating sensor demodulator based on the wavelength-dependent transmission of long period grating filters. Strain resolution of the system was 1 (mu) (epsilon) for dc strain in a 3.3 Hz bandwidth. Quasi-static and dynamic operation of the system were investigated.

We report on two possible techniques of short distributed strain measurements along a length of a single fiber Bragg gratings (FBG). The first technique relies on the spectral shape, the second is based on the group delay response of the FBG. A theoretical comparison of the two techniques were made and later applied to determine the strain distribution around a hole in a loaded aluminum plate. Monotonically varying strain measurements were made with a spatial resolution that was dependent on the strain gradient at that point. A spatial resolution as small as approximately 0.8 mm was obtained over the steep parts of the strain variation.

An algorithm has been developed to determine the full deformation field of a cantilever honeycomb plate under free-end loading conditions. The algorithm utilizes strain information from a set of sixteen fiber Bragg grating (FBG) sensors mounted on the plate so that all sensors measure strains along the clamped-free direction. The sensors are interrogated using a wavelength division multiplexing scheme, and measured strains are converted to full-field displacement. The algorithms used to interrogate the sensors, perform the strain-displacement calculations and generate a real-time (approximately 60 Hz) mesh of displacement values are encoded in a C program.

Two complimentary optical fiber strain sensors employing lock-in techniques are presented. The first system interrogates an array of long gauge length sensors, defined by broadband optical reflectors and employs multiplexing in the time domain. The second system operates over shorter gauge lengths using multiple narrowband reflectors and wavelength-division-multiplexing. The first system tracks minima in the amplitude response produced from the superposition of two sinusoidal subcarrier waves. The second uses an acousto-optic-tunable-filter (AOTF) to track the peak reflective wavelength of an array of Bragg gratings. Both systems are constructed using telecommunications components. Together, the systems may be used to examine both line-integrated strain (or temperature) over long gauge lengths and local strain at a number of selected discrete points of particular interest. Lock-in techniques using dithered signals are applicable to sensors having a transfer function containing at least one turning point. This may be a maximum or minimum when observed either in transmission or reflection. The sensor responds to the dither with an amplitude-modulated signal, which permits locking of the interrogation system to the turning point. This provides a real-time response and better noise performance than scanned measurements. High-resolution monitoring of time-varying strain is demonstrated using this method. The long gauge length system has demonstrated a resolution of 3 microstrain over discrete 5 m long sensing sections, with an interrogation time of 0.25 s. When multiplexed to interrogate an array of four sections, intersection crosstalk levels were below minus 50 dB. The short gauge length interrogation system has been demonstrated using both fiber Bragg gratings and an in-line Fabry-Perot cavity as the wavelength selective reflectors. A resolution below 1 microstrain was obtained using the gratings, whereas a resolution of 1.5 multiplied by 10^-6 in optical path-length-difference was obtained when interrogating a Fabry-Perot cavity. Simultaneous monitoring of multiple Bragg gratings has also been demonstrated by multiplexing with different dither frequencies. The versatility and the high resolution make the lock-in systems ideal for smart structures applications.

We have demonstrated the principle of distributed temperature sensing using the Brillouin loss mechanism with a single laser source. The technique involves the use of a stimulated Brillouin scattering (SBS) generator to produce down-shifted light.

A very high resolution distributed optical-fiber temperature sensor system has been demonstrated using a time-resolved photon counting technique. The spatial resolution of the system is 3.5 cm. A temperature sensitivity of 2 degrees Celsius has been achieved with 1 minute integration time when averaging the data points over 10 cm. The system offers a practical solution for life assessment and monitoring of hot spots along the steam pipes in power plants.

We report on the simultaneous measurement of temperature and strain in a single optical fiber. The measurements were made using a combination of a fiber Bragg grating element and a Brillouin scattering interrogation system, operating in different optical wavelength regimes, to determine the two parameters. The system has the potential to provide quasi- distributed determination of strain and temperature along g fiber.

A novel scheme for a Brillouin interaction based distributed sensor is described which increases the practical applicability of the system. The diagnostic apparatus of this sensor requires access to only one end of the sensing fiber while the use of a Faraday rotating mirror as a reflector at the distal end of the fiber renders the sensor signal immune from temporal fluctuations in the birefringence of the fiber.

The paper presents an analysis of the signal to noise ratio of distributed anti-Stokes ratio thermometry (DART) to quantify the effect of varying the gain of the avalanche photodiode (APD). An optimal gain is found based on maximizing the signal to noise ratio of the received circuit output. Good agreement between calculated and measured avalanche gain of APD is obtained.

This paper describes a novel configuration of a distributed optical fiber stress sensor based on a Sagnac ring construction. The principle of operation is based on the FMCW technique. The intensity and location of a stress applied to the fiber can be determined simultaneously by detecting the amplitude and the frequency of the beat signal which is produced by two forward-coupled mode beams in the Sagnac ring. The system was found to have a spatial resolution of 1 meter in a sensing range of 50 meters.

The ability to detect, localize and characterize impacts in real time is of critical importance for the safe operation of aircraft, spacecraft and other vehicles, particularly in light of the increasing use of high performance composite materials with unconventional and often catastrophic failure modes. Although a number of systems based on fiber optic sensors have been proposed or demonstrated, they have generally proved not to be useful due to difficulty of implementation, limited accuracy or high cost. In this paper, we present the results of an investigation using two spatially weighted distributed fiber optic sensors to detect, localize and characterize impacts along an extended linear region. By having the sensors co-located with one having sensitivity to impacts ranging from low to high along its length while the other sensor has sensitivity ranging from high to low along the same path, impacts can be localized and their magnitudes determined using a very simple algorithm. A theoretical description of the techniques is given and compared with experimental results.

Techniques for distributed optical fiber chemical sensor development were investigated and a model system for pH measurement was developed and, as a result, discrete, distributed signals were obtained. Fluorescein sodium was chosen as a pH indicator for this work because of its well known properties and high fluorescent intensity. A low temperature sol-gel glass manufacturing process was utilized to immobilize the indicator onto the optical fiber core. Thin (approximately 1 micrometer) porous glass films were deposited on the surface of the optical fiber core with indicator molecules entrapped in the matrix. An OTDR technique was employed to obtain the signal at specific positions along the fiber. A dye laser, pumped by a N₂ laser, produced blue light pulses at 440 nm which were launched into a 1 multiplied by 2 optical fiber coupler. A fiber with eight sensitive sections was splice to a 50:50 coupler. The indicator molecules were excited by the blue light via the evanescent wave. Part of the fluorescent light from the indicator molecules was coupled back into the fiber and transmitted back to the coupler. A fast PMT tube was attached to the other arm of the 1 by 2 coupler to detect the fluorescent light. Results were obtained for solutions of various pH value. The system appears to have potential for applications in environmental and safety monitoring.

An application of a linear fiberoptic sensor for detection and measurement of structural bending is described. Design, development and fabrication of the sensor is described, along with details of static bench testing and static structural bending tests of an actual aircraft wing. Knowledge of inflight aircraft surface flexure may be important for certain surveillance sensor and weapon delivery computational applications.

A problem of tomography reconstruction of vector physical fields by using the sets of contour integrals from a vector projection and from projection of a vector derivative with respect to direction is investigated. A necessity of using nonrectilinear measuring lines for tomography reconstruction of vector field parameters distribution is shown. If the measuring line output signal is proportional to integral from projection of a researched vector it is possible to use the measuring line in the form of a narrow loop and the decision of a problem can be received by use of integral theorems. If the measuring line output signal is proportional to integral from projection of a vector derivative with respect to direction (e.g. problem of deformed two-dimensional object), it is possible to use the measuring line of step form and potential component of a vector field can be reconstructed. The method can be widely applied to research distribution of an electromagnetic, deforming and other vector fields inside elements of technical and technological objects constructions and also can become a basis of systems of vector physical fields monitoring.

Multiplexed and distributed sensor systems are generally employed when the number of sensing points makes the use of an individually addressed sensor array prohibitive based upon some system cost function. Fiber optic sensing techniques offer great potential for the creation of multiplexed, quasi-distributed and distributed sensor systems. In addition, fiber optic sensors can be spatially weighted and configured for maximum sensitivity to particular patterns of extended parameter fields. This allows such sensors to perform a patten recognition preprocessing function, reducing system cost and processing overhead. In this paper, the appropriate use of multiplexed sensor systems, distributed sensor systems and long gauge length sensor systems with pattern matching capability are discussed as a function of system size and purpose. Design options for long gauge length sensors in terms of preprocessing functionality also are discussed. Finally, a specific example of the use of a spatially weighted sensor for vehicle identification is covered.

We describe new fiber-optic vibrational and acoustic sensor systems developed for airport ground traffic monitoring. The theoretical background is derived and results of laboratory experiments as well as initial field tests in an experimental surface movement guidance and control system (ESMGCS) at the Braunschweig airport are reported.

We demonstrate the use of a novel integrated-optic device for multiplexing and passive demodulation of interferometric sensors. A device combining an amplitude and phase modulator is used to create both a phase carrier and the pulses for multiplexing using time division. A time sampling technique is successfully used to account for the optical cross-talk in TDM processing and a very high frequency phase carrier is demonstrated.

The use of intensity based fiber optic senors has been limited to this point due to two factors. First, referencing the sensors is usually difficult because sensor modulation is indistinguishable from variable signal loss in other parts of the system. Secondly, the cost of the opto- electronics required for multiplexing numbers of these sensors has been prohibitive up to this time. In this paper, we present the results of an investigation into the use of wavelength multiplexing for intensity based fiber optic sensors as a solution to these problems. A demonstration system was fabricated using a transmissive star architecture and having fiber sensors plus a directly connected intensity reference channel. Three proximity sensors were multiplexed together with a fiber optic switch and fluid presence sensor. A theoretical basis is given for the system architecture chosen and performance details of the prototype system, which use cost effective off-the-shelf components for signal conditioning, are provided.

The operation of new quasi-distributed interferometric sensor is discussed. The sensor is based on the array of unbalanced interferometers formed by point polarization couplers along birefringent fiber. Simultaneous interrogation of the sensing interferometers was achieved by using spectroscopic methods and a simple signal processing. Results of investigation of sensor performance and analysis of cross-talk problem are presented.

A novel, multiplexed optical fiber differential-pressure transducer is described for the real-time pressure measurement of airflow in applications involving actuator- and SMA-controlled airfoils and multi-parameter skin friction measurements. The design of the pressure transducer is based upon extrinsic Fabry-Perot interferometry (EFPI) and uses a micromachined silicon diaphragm to modulate the sensing cavity. The pressure transducer was designed to operate from minus 10 to 10 psi_g and have a resolution of greater than 0.01 psi. Ten pressure transducers were spatially multiplexed and tested for smart wing applications. Results are also reported for an integrated skin friction balance/optical fiber pressure transducer tested in Virginia Tech's Supersonic Tunnel (VTSST).

We discuss the design and performance of a system of two coherence-multiplexed sensors based on highly birefringent fibers for measuring hydrostatic pressure. Both sensors are temperature compensated. 3M polarizing fiber is used as a linking fiber ensuring the maximum contrast (0.5) for the interference pattern associated with each sensor in the system. A Wollaston prism with step delay line is applied as a decoding interferometer. The results of testing of pressure response within the range of 0 - 30 MPa, temperature stability, and long term stability are presented.

The OPTONET system monitors up to 64 on/off fiber optic sensors. It is based on optical time domain reflectometry (OTDR). An optical pulse emitted by a pulsed laser diode is splitted between the sensors through an optical star network including delay lines to separate the pulses reflected back from the sensors. A dual wavelength emission board is implemented to guarantee line surveillance and optical path loss compensation, in order to minimize the error rate, and information discrepancy. These characteristics also combined with the well known advantages of fiber optic sensors make this system attractive in military or harsh environment. Various reflective sensors can be connected to the OPTONET system; we describe here sensors developed for a French Navy application.

One missing component in the next generation optical communications link is an ultra-fast tunable filter. This is a crucial element in dense WDM systems. Here, a novel fiber delay line configuration is proposed, which operates as such filter. The filter implementation is based upon currently available technologies. It is shown that with the suggested technique, optical selective elements can be generated with a tuning speed of less than a nanosecond. This is three orders of magnitude faster than other currently available filters. The filter can be designed compact, and is immunized to external instabilities. Using compound configurations, the filter can be designed with a very wide range of filter finesse and free spectral range.

There is currently considerable interest in the development of distributed fiber sensors based on Brillouin scattering in optical fibers as this approach has been shown to offer the possibility of long range sensing of temperature (and strain) with good spatial resolution. The accuracy with which temperature (or strain) can be measured is not only related to the system signal to noise ratio but also on the frequency separation between sampling points over the Brillouin spectrum. In our experimental systems, the Brillouin frequency is measured by manually adjusting the laser frequency difference until the Brillouin interaction is optimized at the fiber segment of interest. However, in an automated system, it is likely that the Brillouin interaction would be monitored while the laser frequency difference is scanned over a number of discrete values and the Brillouin frequency determined by curve fitting to this data. In this paper we describe a statistical method for analyzing the accuracy in measuring the Brillouin frequency when such an automated routine is used. We use Gaussian statistics to simulate a noisy Brillouin spectral profile and fit a Lorentzian line shape to the noisy data. We compare error distributions in the Brillouin frequency from two fitting algorithms. The first is a numerical approach using an iterative algorithm based on the Newton-Raphson method. In the second method an analytic approach is followed that involves the transformation of the Lorentzian line-shape to a linear function; fitting then being carried out using the least squares technique. Results from both approaches are compared with a very simple analytic expression.

Despite advantages of distributed fiber optic sensor systems, the stability, reliability and limitation of physical parameters detected by the systems are still the bottlenecks of the applications in industrial control fields. In this paper, we compare different structures of fiber optic sensor networks based on special needs for manufacturing control and monitoring. A double-bus fiber optic sensor network directly used in industrial scene is presented. The multiplexing methods, such as time division multiplexing, frequency division multiplexing and code division multiplexing are analyzed. The result demonstrates that the double-bus fiber optic sensor network is adapted to use as manufacturing control systems in industrial environment, which offer low crosstalk levels, high signal to noise ratio and low optical path loss.

This paper describes a new distributed optical fiber stress sensor system which consists of a piece of birefringent fiber with a mirror at one end and whose principle of operation is based on the frequency modulation continuous wave (FMCW) technique. The intensity and the location of an applied stress can be determined simultaneously by detecting the amplitude and the frequency of the beat signal which is produced by two forward-coupled mode beams. The system was found to have the advantages of large signal intensity, good signal contrast, 1 meter resolution and up to 100 meters measurement range.

The distributed optical fiber sensing technique has significant application in the real-time monitoring and damage detection of the large and critical engineering structures. In this paper, a distributed optical fiber sensor based on the optical time domain reflectometry technique has been used for monitoring structural deformations, where a novel structure of the microbend sensor was developed for measuring of both tensile and compressive strain in structures.

A novel interferometric technique is described to detect and locate perturbations along an optical fiber. This distributed sensor has a position dependent response to time-varying disturbances such as strain or temperature. These disturbances cause a phase shift which is detected and converted to spatial information. The sensor consists of a Sagnac interferometer merged with a Michelson interferometer. This is achieved by a frequency selective mirror in the center of the Sagnac-loop. The sensor is illuminated by two light sources with wavelengths lambda₁ and lambda₂, respectively. The mirror reflects lambda₁ and transmits lambda₂, therefore causing the interferometer to operate as a Michelson at wavelength lambda₁ and as a Sagnac at wavelength (lambda) ₂. Any time-varying perturbation on the fiber will result in a signal at lambda₂ proportional to the product of the rate of phase change caused by the perturbation and the distance of the perturbation relative to the position of the mirror. The output of the Michelson interferometer at wavelength lambda₁ is proportional to the phase change caused by the unknown perturbation. By dividing the output of the Sagnac interferometer by the time rate of change of the Michelson interferometer signal, the position of the disturbance relative to the mirror is located. Results obtained with a 200 m distributed fiber sensor are discussed.

The use of an in-situ optical fiber array system in conjunction with a dye tracer for monitoring the production water discharge from an oil platform in the North Sea is reported. The dye tracer properties mimic those of the produced water. The optical fiber array fluorosensor is capable of detecting fluorescent dye tracer particles down to 2.5 multiplied by 10⁶ p/l. The optical fiber array fluorosensor was able to detect the dye tracer plume from the produced water effluent discharge of the CLYDE platform approximately 100 m from the platform, but was unable to detect tracer further away from the platform. The results suggest that the produced water dilution is greater than previously believed. The use of these combined techniques is shown to be a feasible method for production water monitoring.