This paper describes the principles and recent development of a new technique for measuring temperature and strain distribution using Brillouin backscattering rather than Rayleigh and Raman backscattering. The technique is based on temperature- and strain-induced changes in the Brillouin frequency shift. Experimental sensitivities of 1.2 MHz/K and 5.8 MHz/10^-4 strain are demonstrated at a wavelength of 1.32 micrometers . Two approaches for measuring the weak Brillouin line are discussed.

A design useful for small spatial range distributed fiber optic sensor is presented. This sensor array has advantages of zero dead zone, high spatial resolution, and fast response time, which can be used in the areas of smart skin as well as distributed pressure, force, displacement, and temperature sensors.

Frequency translation or shifting is an important technique in many kinds of optical-fiber reflectometry. For example, high-linearity frequency sweep against time is necessary in coherent-OFDR for achieving a high spatial resolution, and changes in the frequency over a wide range is essential for fading noise reduction in coherent-OTDR. In addition, two mutually coherent lightwaves with a frequency difference of about 10 GHz are required in Brillouin-OTDR to realize coherent detection of Brillouin scattered lightwaves. However, conventional techniques are insufficient for these purposes. In this paper, a new technique for the external frequency translation of lightwaves is proposed. This technique enables the high- linearity sweeping of an optical frequency quasi-continuously over a wide range of about 100 GHz. The frequency translator is composed of an optical pulse modulator and an optical ring circuit containing an acousto-optic frequency shifter, an optical amplifier, and a narrow band filter. Here, the acousto-optic frequency shifter and the pulse modulator are synchronously controlled. The pulse launched into the ring circuit experiences a frequency shift at every circulation around the ring circuit. Therefore, the frequency of the output lightwave from the ring circuit is largely translated from that of the original pulse. Its intensity can be kept nearly constant because of balance between gain and loss in the ring circuit. We also demonstrate Brillouin-OTDR as an application.

Classical LDV techniques permit the recovering of the flow profile at the expense of the movement of projection and recovery optics. This operation limits the performances of the LDV and extends the measurement time. The aim of this communication is to present a novel monobeam distributed laser Doppler velocimeter (DLDV) which permits the continuous recovery of the flow velocity at any point belonging to the light beam, without moving the collimating optics. The DLDV applies the low-coherence multiplexing technique to a backscattering reference-beam LDV scheme. The DLDV uses a very-short coherence source (a superluminescent diode) which feeds an unbalanced interferometer. The sensing arm of the interferometer is launched in the flow while the remote reference arm permits the user to interrogate different flow volumes. The scheme has been implemented by all-fiber components. Preliminary measurements were given on a laboratory hydraulic circuit. The DLDV can be used as a conventional LDV, to measure with high accuracy a flow velocity component in a point of the duct, or in a distributed mode, to recover the flow velocity profile along a duct cross-section. Profiles of the flow were recovered at different average velocity and the first results confirm the exceptional capabilities of the novel instrument.

The extreme sensitivity and time resolution of Geiger-mode avalanche photodiodes (GM- APDs) have already been exploited for optical time domain reflectometry (OTDR). Better than 1 cm spatial resolution in Rayleigh scattering detection was demonstrated. Distributed and quasi-distributed optical fiber sensors can take advantage of the capabilities of GM-APDs. Extensive studies have recently disclosed the main characteristics and limitations of silicon devices, both commercially available and developmental. In this paper we report an analysis of the performance of these detectors. The main characteristics of GM-APDs of interest for distributed optical fiber sensors are briefly reviewed. Command electronics (active quenching) is then introduced. The detector timing performance sets the maximum spatial resolution in experiments employing OTDR techniques. We highlight that the achievable time resolution depends on the physics of the avalanche spreading over the device area. On the basis of these results, trade-off between the important parameters (quantum efficiency, time resolution, background noise, and afterpulsing effects) is considered. Finally, we show first results on Germanium devices, capable of single photon sensitivity at 1.3 and 1.5 micrometers with sub- nanosecond time resolution.

Distributed optical-fiber sensing (DOFS) offers full information on the spatial and temporal behavior of a large number of measurand fields. Among potential applications are the monitoring and control of any large structure (including `smart' systems), and a range of environmental monitoring requirements. Methods for realizing DOFS hitherto have relied, almost exclusively, on linear backscatter techniques. A notable exception is the Raman temperature measurement system, which relies on non-linear backscatter. New explorations are concerned to investigate the possibility of utilizing other non-linear effects in either backscatter or forward-scatter arrangements. Forward-scatter methods utilize the non-linear interaction between counter-propagating light signals in a single-mode optical fiber; these hold promise for markedly improved signal levels, with consequent measurement accuracies of approximately 1%, and spatial resolutions approximately 0(DOT)1 m over distances up to 1 km. Attention recently has concentrated on the use of the optical Kerr effect. Two ways in which this effect may be used in DOFS are described. Recent experimental results are presented and remaining problems are defined.

We present a distributed fiber optic acoustic sensor technology that could be used to measure and locate leaks within either fluid- or gas-filled distribution lines. For these applications, the optical fiber sensor would be placed inside the pipe and could potentially locate leaks to within several meters by listening to the acoustic emission produced by the fluid or gas as it escapes from the pipe.

We have constructed a prototype quadrant detector by polishing two sides of the ends of four optical fibers at right angles to each other. The fibers are then brought tightly together. The lines of contact of the fibers then form a crosshair dividing the area of the fibers into four quadrants. Each quadrant transmits a part of a star image respectively. In order to obtain high detecting accuracy and high efficiency, the width of the crosshair should be as thin as possible. We have designed a special structure and used it to fabricate several quadrant fiber assemblies. The width of each crosshair can be made less than 10 micrometers. Using a photomultiplier to detect the light transmitted by each fiber, we then have a high sensitivity quadrant detector for monitoring the position of faint objects.

This review paper covers the field of distributed optical fiber sensors, where measurements may be taken along the length of a continuous section of optical fiber. Such a feature greatly increases the information that can be obtained from a single instrument and hence the cost per sensing point can be more acceptable. The review does not attempt to cover all methods, but gives a selection of some of the more interesting theoretical concepts, describes the current status of research, and indicates where optical sensing methods are being applied in commercial instruments.

This paper reviews optical reflectometry techniques that are capable of achieving spatial resolutions of less than 1 cm. Advantages and disadvantages of these techniques are discussed. A white light interferometry technique known as optical low-coherence reflectometry is emphasized. This technique has been used to obtain spatial resolutions on the order of tens of microns and reflection sensitivities as low as -148 dB.

Distributed temperature sensing by Raman backscattering in optical fibers is unique technology which makes full use of the features of the optical fibers. This paper describes practical applications of the sensor: pipe line maintenance, electric cable maintenance, and road maintenance.

A new approach for temperature sensing between 20 - 300 degree(s)C using a single mode fiber with a depressed cladding is presented. The effective cutoff wavelength (780 nm) is a linear function of temperature for this fiber because of the mismatch of the thermal expansion coefficients between the core and cladding. Consequently, the attenuation coefficient in the region of the cutoff wavelength is linearly dependent upon temperature. The advantage of this sensor is that the sensing wavelength may be selected between 600 - 780 nm.

This work presents the first example of structure mode shapes recovering by means of an embedded quasi-distributed fiber-optic sensor. The developed feed-forward quasi-distributed polarimetric sensor allows a distributed recovery of the structure strain while maintaining low- invasivity, high-sensitivity, and high spatial resolution. The first three modes of a CRFP cantilever have been recovered both in frequency and in modal shape by means of the 8- segment sensor. The excellent agreement between the theoretical datum and the experimental results confirms the suitability of the presented architecture in developing advanced sensing systems for structure application.

A multiplexed temperature sensing system is constructed by cascading three temperature sensors along one multimode fiber such that each individual sensor responds to its local temperature disturbance. The sensing element of each sensor is a dielectric edge filter with a specific cutoff wavelength. White light serves as the light source. The performance of this sensor is based on the temperature dependence of the reflection or transmission spectrum of each filter. The reflected or transmitted light from the filter is then sent to two dielectric bandpass filters, which are selected for each particular edge filter and referred to as the sensing and reference filters, respectively. A photometer is placed behind each bandpass filter. The ratio of the sensing filter power to the reference filter power is a function of temperature. Since the cutoff wavelengths of these edge filters (sensors) along the fiber are well separated, the multiplexed signals are divided by different pairs of bandpass filters. In the corresponding experiments, three edge filters were cascaded and 100/104 micrometers graded index fibers were used. A resolution of each temperature sensor was determined to be +/- 0.2 degree(s)C over the temperature range of 30 degree(s)C to 100 degree(s)C.

A wide range of multiplexing techniques for fiber optic sensors have been proposed and demonstrated over the past 10 years. In many cases, systems utilizing multiplexed sensors have undergone field trials which have successfully proven the technology. This paper reviews this technology and discusses recent research efforts in the area.

A time division multiplexed topology has been implemented using four Michelson sensor simulators and a transceiver link. The topology ensures that no light returning from the network is injected back into the laser diode. The phase sensitivity of the sensors is limited by the phase noise of the laser. Coherent noise due to Rayleigh backscattered light mixing with the signal is significantly smaller than this phase noise. A demodulated sensitivity of 25 (mu) rad/(root)Hz at 1 kHz has been achieved using the differentiate-cross-multiply technique.

Passive birefringence compensation using Faraday rotator and mirror combinations in Michelson interferometer systems has been shown to produce an interferometric output signal which is free of the effects of polarization induced fading. This technique promises to have a strong impact on the development of practical interferometric sensor systems, and is particularly important in the area of multiplexed sensors where the problem of polarization fading in conventional arrays is a serious technical issue. In this paper we describe various topologies for implementing passive birefringence compensation to achieve polarization independent operation in multiplexed sensor arrays. Results obtained on a time-division multiplexed system with 4 sensor elements are presented.

Spectral slicing of a broadband LED source is an efficient method for providing multiple wavelength channels for WDM sensor networking. However, optical power budgetary considerations limit the numbers that can be accommodated even with the use of power efficient topologies. Very recently, extended spectral width broadband LEDs have become available which provide spectral widths greater than 120 nm. Such LED devices exhibit both power output and spectral width capabilities comparable with the combined effect of two or three conventional LEDs. This paper reports on an investigation of the performance of such devices in relation to their utilization within WDM point-sensor networks. Furthermore, specific available device characteristics are utilized within a WDM sensor network modeling process in order to determine the maximum number of sensors that can be accommodated on various power efficient topologies. Hence, it is demonstrated that the use of these very broadband LED sources can facilitate significantly larger numbers of sensors than may be accommodated by employing spectral slicing of conventional LEDs.

The advantages of using spatial light modulators (SLMs) as the multiplexing elements are that low crosstalk levels and high signal to noise ratios (SNR) are attainable. In addition, a substantial number of sensors can be incorporated in the network without serious loss in performance. SLMs are flexible multiplexers which allow a range of multiplexing methods to be used. In this paper time division multiplexing (TDM), frequency division multiplexing (FDM), and code division multiplexing (CDM) are considered. The advantages and limitations of the three multiplexing systems are discussed and their relative performances are compared. The paper concludes with a report on a network incorporating twelve discrete strain sensors using TDM for which the average SNR was 60 dB and the average crosstalk level was better than -70 dB.

A spread spectrum multiplexing (SSM) system for an array of reflective, frequency-out fiber optic sensors of identical characteristics has been developed. This system allows accessing of a large number of sensors with high sampling frequency of up to a few MHz and simultaneously permits a considerable improvement of the system optical power budget over single pulse time-division multiplexing methods. An experimental SSM system has been assembled and tested and the experiments have successfully demonstrated the passive multiplexing of eight reflective frequency-out fiber optic sensors with a vibration frequency of 17.36 kHz. Crosstalk of approximately -22 dB has been observed, which is in agreement with theoretical results expected from consideration of the code length of the pseudorandom bit sequence used and the sensor number in the network. Additionally, a novel digital PRBS exhibiting zero auto-correlation at non-zero delays has been modelled and experimentally tested, yielding promising results for sensor multiplexing.

Two-mode fiber sensors based on elliptical-core fiber are of interest in a variety of strain sensing applications particularly in embedded fiber sensor systems. In recent papers we have reported passive interrogation techniques for such sensors configured in a lead-insensitive mode of operation. In this paper, we describe a time division multiplexed array of two-mode elliptical-core fiber sensors, based on the use of reflectometric sensor elements and passive laser FM based phase shift interrogation. Results obtained with a 4 channel system are presented.

We have examined various fiber sensor multiplexing techniques, e.g., frequency-, time-, coherence-multiplexing, in an attempt to ascertain the method best suited for interrogation of multiple sensors scattered throughout a modern civil structure. Based on our embedded fiber sensor results conducted at the Stafford Biotechnology Complex at the University of Vermont, a 65,000 square foot, multistory reinforced concrete structure, where more than fifty single- mode and multimode fiber optic sensors have been embedded into the structure, we have determined that in many instances a radio telemetry method of interrogating the sensors is optimal. Many real-world factors such as architectural details, lighting, power, and HVAC design requirements influence the overall nature of the use of multiplexed fiber sensors in civil structures. In instances where we have multiplexed intensity-modulating fiber sensors onto a single transmit/receive fiber, radio telemeterized command and data acquisition from the fiber sensor `network' may be achieved. The development of the interrogation of the multiplexed fiber optic sensors is presented, as are experimental results obtained from fiber optic vibration sensors.

A group of sensors in a ladder OFS network, in general, employs two main optical fibers, the feed fiber and the return fiber. As another feed fiber, called nth feed fiber, is introduced into the ladder network, another group of sensors which uses the original return fiber path and the nth feed fiber as its feed path can be identified with address code n. On the other hand, as another return fiber, called mth return fiber, is introduced into the ladder network, a new sensor group which uses original feed fiber path and the mth return fiber as its return path also can be identified with the address code m. When both the nth feed fiber and the mth return fiber are introduced into a ladder network, a 3-dimensional OFS network is formed. In a basic ladder OFS network, the individual sensor is identified by a time coded system with the detected kth pulse. Therefore, the 3-dimensional network can be identified by 3 integer variables (k, m, n). When k, m, n take the values from 1 to N, it can be achieved in a 3- dimensional OFS network that the N X N X N sensors are monitored by means of 2 X N optical fibers. A temperature sensor system using the 3-D network is being implemented in our lab.

Transverse stress applied to a highly birefringent fiber at an arbitrary angle (other than 0 or 90 degrees) to the fiber birefringence axes causes rotation of the birefringence axes and changes the beat length of the fiber in that section. If one of the polarization modes is excited at the input, coupling of light from one mode to the other will be observed at a stress point. The presentation describes a method for determining the locations of discrete mode coupling points spaced along a polarization maintaining fiber using a pump-prob architecture based on the optical Kerr effect. Probe light experiences coupling at different stress locations. Counterpropagating strong pump light also experiences coupling while inducing additional birefringence, and changing the polarization state of the probe at the output. This system may be made temperature independent by introducing a phase tracking/triggering system. The advantages and limitations of this technique are described.

This paper describes a digital approach to interferometric demodulation having capabilities of high sample rates and low self noise. Using this approach, a single demodulator can be used to demodulate many interferometric sensors where data is presented to the demodulator in a time domain multiplexed format. This paper presents the basic demodulation algorithm, theoretical performance predictions, and experimental data.