This paper discusses the use of fiber gratings as sensing devices and the measurement limitations imposed by conventional laboratory instrumentation. Work that is currently in progress on instrumentation development to alleviate some of these limitations to yield practical, robust systems is discussed. Research activities concerning 2 system configurations are addressed in some detail: (1) sensor arrays in a system based on broadband optical transmitters with narrowband optical receivers using conventional optical filters or acoustically-tuned, integrated optic filters, and (2) sensor arrays in a system based on narrowband, optical transmitters with laser diodes or fiber lasers that make use of fiber gratings as wavelength tuning elements in conjunction with simple, broadband optical receivers.

The discussion of Bragg fiber gratings (FBGs) has expanded sharply in the last six years. FBGs were given versatility by the results at the United Technology Research Center when the technique of side-writing was pioneered (1989), which made the Bragg wavelength independent of the writing laser. The promise of economical production of these devices was advanced by the generation of FBGs with a single laser pulse at the Naval Research Laboratory (1992), and this result was expanded by the University of Southhampton in achieving saturated reflectivity with a single-pulse. The basis for manufacturing these excitingly versatile devices is near the point of supporting a new area of applications and industry. The intent of this article is to briefly discuss the conditions and requirements for the practical production of FBGs in support of the technology of distributed sensing.

A portable, rugged, and compact 4-channel Bragg grating fiber laser sensor demodulation system was developed for interrogating an array of 20-intracore Bragg grating sensors that we embedded within several of the concrete girders used to support the deck of a new two span road in Calgary, Alberta. Another unique feature of this bridge is that two types of Carbon fiber based composite prestressing tendons are being tested in a number of the concrete girders. We have instrumented five of the approximately 60 ft long concrete girders with fiber optic Bragg grating strain and temperature sensors in order to monitor the loads on these girders and their response during the construction of the bridge and subsequently over an extended period of time.

Techniques for the detection of wavelength shifts of fiber Bragg grating sensors based on the use of a tunable bandpass filter, interferometric detection, and active laser configurations are described. The methods used to multiplex several sensors along a single fiber using these approaches are also discussed, and results presented.

Polarization mode coupling in hi-bi fibers may be achieved either statically, via the writing of photorefractive gratings, or dynamically, via the optical Kerr-effect of a propagating optical pulse. In each case the coupling can occur under the influence of an external measurand which can then be measured distributedly or quasi-distributedly. Examples are given of a static method and of a dynamic method for implementing good-performance distributed measurement systems.

The mode of operation of selected multiplexing networks for fiber optic interferometric sensors having the potential to be exploited in practical applications is discussed. Particular emphasis is given to the necessity of matching the network to specific types of measurands.

Sensor arrays for multiplexing multimode fiber-based intensity modulated sensors may be constructed in the form of a ladder structure employing 2 X 2 optical fiber couplers. The input/output relationship of a multimode fiber diffused coupler is dependent on the launch conditions of light. The degree of uncertainty between the input/output relationship of a coupler, as a result of the highly unequal split ratio and the state of modal equilibrium in the launched light, is investigated. The power penalties due to this phenomenon are calculated and an optimum solution for multimode fiber-based sensor array design is derived.

The impact of optical fiber sensor technology and optical fiber amplifier technology in acoustic sensor systems is theoretically examined. An attractive feature of the system architecture studied is the fact that a majority of the complex optical components and electrical signal processing are located on the dry-end (land or ship) thus reducing overall system cost and enhancing upgradability. Calculated results are presented for a hybrid time-division and wavelength division multiplexed architecture employing fiber Bragg gratings. The multiplexing scheme, optical schematic, state-of-polarization effects, and noise terms are described.

We describe an approach to the interrogation of fiber sensor arrays over long fiber links which greatly eases the limits on optical power handling capability of the system imposed by Stimulated Brillouin Scattering. The technique relies on matching the coherence properties of the source to the operational characteristics of the interferometric sensors.

The multiplexing of fiber optic sensors has been a subject of considerable interest for a number of years. The majority of the techniques presented to date, however, have depended upon a division of the power input to the system among a number of sensors. In this paper, a technique is presented whereby all of the power input to the system is sequentially routed through each sensor in turn. This is done through the use of electrically passive photoactivated optical switches with known time constants. An analysis of the technique is carried out and incorporated into a computer model to predict system performance. A comparison of the technique with other multiplexing concepts is also made.

In this paper, we investigate the use of the advanced fiber optic multiple-access networks in future high-performance commercial aircrafts to support real time on-board flight control, sensing, voice communications, and TV distributions. Novel tunable optical code division multiple-access (OCDMA) networks using prime-code sequences are designed, which can be quickly programmed to generate the required OCDMA sequences. Using the newly proposed OCDMA technique can efficiently build high-speed sensor data distribution networks which can guarantee parallel data communications and multiple connections to support real-time sensor data processing and distributed control. Due to the channel independence and protocol transparence of wavelength-division multiplexing (WDM), the WDM-based hybrid multiple- access techniques are very attractive to future avionics systems integration, which can achieve high communication efficiency and network utilization.

Application of electronic scanning to a sequence of coherence-multiplexed strain sensors is proposed in this paper. The sensors consist of a cascade of HB fibers spliced with their polarization axes rotated by certain angles. We theoretically analyzed the behavior of such chains as a function of their alignment and found the optimal alignment ensuring readout of every sensor with the maximum S/N ratio. The performance of parallel and serial multiplexing configurations was experimentally studied. As a receiving interferometer we used a Wollaston prism followed by a CCD camera and an image processing system while a stepped delay line made of calcite and crystalline quartz was used to match the group delays between sensing and receiving interferometers.

A new system of multipoint temperature sensor using fiber-optic ring resonators is proposed. It is demonstrated that a multipoint sensor consisting of two ring resonators can measure temperature with resolution of 0.5 K.

Fiber-optic sensor technology is a rapidly growing field, and over 200 companies world wide are marketing optical fiber based sensors for the measurement of a multitude of physical and chemical parameters. The potential exists for broad base use of distributed fiber-optic sensor systems in process monitoring and control applications. This paper describes distributed sensor technological developments along with the important issues which must be resolved in order for utilization of these distributed sensor systems in process environments.

Quasi-distributed optical fiber sensors using multiple frequency shifters (FSs) in two-mode elliptical core fiber (TMECF) are presented. Sensing measurand is the phase shift between the two propagating modes in the fiber. In the present experimental configuration, four FSs provide three independent sensing regions along a single TMECF. We construct one sensor in each region to measure localized strain and/or bending simultaneously. Cross talk among the sensors is theoretical and experimentally evaluated. Measurements were carried out using a computer system that allows a real time data acquisition and display.

A new arrangement is described for mapping transverse stress regions along a highly- birefringent fiber, using the optical Kerr effect in a pump-probe system. This arrangement provides a passively-biased detection system immune from thermal variations in sensitivity and, in principle, enables information to be extracted with only one laser pulse. It also enables real time location of transverse stress points which are moving along the fiber. As the laser sources employed are very compact, this system is potentially easily portable.

This paper presents the results of studies on a distributed temperature sensor (DTS) based on Brillouin gain and loss. The relationship between Brillouin gain/loss and input laser power and the effect of temperature non-uniformities on the system have been studied experimentally and theoretically. Finally we describe results from a combined distributed temperature and strain sensor based on the Brillouin loss mechanism.

Some new studies involving the fabrication of organic photochromic fibers and measurement of their photochromic characteristics, the analysis of electric field characteristics of liquid crystal clad fiber and the strain characteristics of plastic fibers provided with cuts, are presented for the purpose of the distributed sensing of UV light, electric field and strain, respectively.

In this paper, we describe a sensor configuration based on optical coupling between two optical fibers submitted simultaneously to the same constraint. Theoretical and experimental results are reported on photoelasticity and the evanescent field of these coupled fibers due to geometrical deformations. Our purpose was to build a time multiplexed sensor network by coupling short pulses. This arrangement exhibits high sensitivity and resolution without any drawbacks of the classical OTDR.

Techniques are being investigated to spatially weight the strain sensitivity of a two-mode fiber strain sensor by mechanically altering the shape of the fiber along its length. The goal of this process is to obtain fibers that will be single-mode (V < 2.4) over a well-defined sensing length and dual-mode (2.4 < V < 3.8) over the remaining section of fiber. The single- mode section will be relatively insensitive to applied strain, while the dual-mode section will detect a distributed strain field associated with structural vibration mode shapes. The integrated strain will be over the two-mode section of the fiber, which will change with the changing wavelength of the interrogation beam.

A simple model of coherent noise produced by a random scattering process in fiber systems is presented. The temporal and spectral predictions of this model are compared to experimental results.

Today multidimensional signal processing (MSP) methods are of great interest. Tomography principles of data obtaining and processing can be very useful for these methods. A distributed tomography-like fiber-optic measuring network for multidimensional physical fields parameter distribution functions reconstruction is suggested. Network performances, reconstruction accuracy, and reconstruction procedure requirements are discussed and tested by simulations. An experimental model of suggested network is made and tested. Experimental results show a good coincidence with theoretical predictions.

A novel approach for the interrogation of multiplexed fiber optic Bragg grating sensors is described. Signal recovery is achieved by matching a receiving grating to a corresponding sensor grating. As a demonstration, the technique is applied to strain sensing.

An all fiber Fabry-Perot interferometer is proposed, using coherence demultiplexing and fiber loop reflectors, whereby the inherent disadvantages of the interferometer can be overcome. Disadvantages include the limited sensing range and the non-linear output. The limited sensing range is overcome by the use of coherence demultiplexing and the bilinear transform is used for linearizing the output. Coherence demultiplexing is achieved using a double cavity Fabry- Perot interferometer consisting of two cavities formed between the three fiber loop reflectors. An added advantage using two cavities, is the ability to introduce a phase generated carrier demodulation system.

This paper presents a frequency division multiplexing (FDM) design that is compatible with 3 X 3 coupler terminated Mach-Zehnder interferometric sensors. Unlike the FDM technique that is presently used with phase generated carrier (PGC) demodulation schemes, this technique does not require modulation of the wavelength of the laser source or the introduction of optical path differences (OPD) in the interferometer in order to generate a phase carrier modulation in the interferometer. The technique described in this paper relies simply on amplitude modulation (AM) of the light intensity and uses the three outputs of an interferometer to unambiguously recover the phase modulation in the interferometer.

We have demonstrated a new frequency-agile system for the interrogation of in-fiber Bragg grating sensors. The scheme involves frequency shift keying (FSK) of the rf drive to an acousto-optic tunable filter (AOTF) to track the wavelength shifts of a Bragg grating. Theoretical studies to derive the optimum frequency deviation for achieving maximum sensitivity are given, and experimental results are presented for temperature measurement. This technique is capable of rapid, random access and very wide tuning range, showing the potential for interrogating multiplexed arrays of Bragg-grating-based sensors.

Optical rib waveguides have been formed in silicon-on-insulator structures. The effects of rib waveguide dimensions on mode and loss characteristics have been studied. Waveguides with dimensions compatible with single mode optical fibers have exhibited single mode behavior with low loss. The compatibility of these silicon-on-insulator waveguide structures with optical fibers, combined with the ability to form active optical modulators in silicon based on free carrier injection/depletion is encouraging for the future use of silicon guided optics in fiber optical and integrated optical sensors. The use of SOI active waveguide circuits for closed loop AC dither fringe detection in interferometric sensors is a possible application area.