Two displacement sensing techniques based on determination of the phase shift induced in an unbalanced interferometer due to laser frequency modulation are described. Experimental results demonstrating the operation of the schemes are presented; optical displacements over a 50 mm range have been recorded with a resolution of 25 microns. Both approaches are suitable for passive remote sensing applications. The authors also discuss the use of this type of sensing approach for the measurement of 'D.C.' parameters, such as quasi-static pressure and structural strain.

A fiber optic Doppler anemometric (FODA) sensor using an optical delay cavity technique and having the advantage of detecting velocity rather than simple speed is outlined. In this sensor the delay in a sensor cavity formed from light back-reflected from a fiber tip (Fresnel reflection) and light back-reflected from particles flowing in a fluid is balanced by the optical delay when light from this sensor cavity passes through a reference cavity formed by a combination of the zero and first diffraction orders produced by a Bragg cell inserted into the optical arrangement. The performance of an experimental sensor based on this scheme is investigated, and velocity measurements using the Doppler shift data from moving objects are presented. The sensitivity of the scheme is discussed, with reference to the other techniques of fluid flow measurement.

A fiber polarizer with an extinction ratio exceeding 72 dB and two high-birefringent fiber depolarizers of the Lyot type with a residual degree of polarization P below -20 dB reduce the long-term bias drift of an interferometric fiber gyroscope based on normal birefringent nonpolarization-maintaining single-mode fiber coil to an equivalent rotation rate of +/- 3 deg/h, while the theoretical limit of polarization-induced offset is at least one order lower. Therefore, the influence of the phase modulation distortion factor and of amplitude and birefringence modulations on the offset value are investigated in order to decrease these effects. A function synthesizer simulating modulated interferometer output signals at various Sagnac phases is used to control the zero-point and scale factor of the electronic unit. An additional second low-frequency phase modulation at a constant amplitude closes an optical feedback loop by compensating for the rotation-caused Sagnac phase shift during specific time periods of the low-frequency wave. The reported methods allow realization of optical gyroscopes using non-expensive standard single-mode fibers with a measuring range of more than 105 deg/h at a stability class below 1 deg/h.

A novel optical-fiber version of Young's interferometer using a low coherent light source and a linear CCD detector is described. With its unique, simple structure, this interferometer greatly reduces the spatial coherence mismatch from which other electronically scanned 'white-light' interferometers suffer. Experimental results are presented for the use of this interferometer as a strain or temperature sensor with large dynamic range.

A novel technique is presented that allows independent simultaneous remote measurement of temperature and magnetic field, and therefore electric current. This technique is based on interferometric phase measurement of temperature combined with magnetic field induced visibility modulation of the interferometric fringes. The technique is demonstrated using a paramagnetic FR-5 glass sensing element, and it is shown that the measurement of temperature enables compensation of the temperature-dependent Verdet constant.

Chemo-optical microsensing systems, whether based on fiber optics or integrated optics, show promising prospects. The physical principles underlying chemo-optical waveguide sensors are discussed with the accent on linear evanescent field sensors. The role of the chemo-optical transduction layer is emphasized. Restriction of the freedom of design by technological uncertainties is briefly dealt with. The structure of a complete micro-sensing system is discussed and illustrated by presenting various design of chemo-optical sensors based on surface plasmon resonance.

A technical description is presented of a device to simultaneously measure fluctuations of water vapor and carbon dioxide in the atmosphere. The emphasis is on the optical features. Field experiments showed that the instrument is suitable for measuring commonly occurring water vapor fluxes, but is limited to high flux cases regarding CO2.

A laser Doppler velocimeter has been developed to measure blood flow velocity in vivo. It consists of a semiconductor laser coupled to a fiber. Laser light is guided into a blood vessel and backscattered light (by red blood cells) is guided back into the laser. The backscattered Doppler shifted light produces an intensity modulation of the laser (self-mixing effect). The beat-frequency of the intensity modulation is related to the Doppler shift of the backscattered light. A model is presented to calculate modulation signals, and results of measurements in vitro and in vivo are shown.

In any kind of refrigerator, frost and ice formation has to be limited by defrosting of the evaporator surface. This paper proposes the use of fiber-optic sensor technology to successfully achieve the goal of sensing and controlling ice formation inside refrigerators, by monitoring ice thickness and servocontrolling the defrosting cycle. The main advantages of such a choice are discussed. Different fiber optic sensors are analyzed. An experimental investigation on a test bench and inside a real refrigerator is performed. The problem of cost of the sensor is also addressed.

Design and performance of an extrinsic guided wave fiber optic magnetic field sensor is reported. The sensor utilizes a substituted YIG (yttrium iron garnet, Y3Fe5O12) thin film as the waveguiding sensing element. A polarization maintaining fiber downlead was used to provide insensitivity to both power and loss fluctuations. The design makes it possible to determine both the magnitude and the sign of the magnetic field. Measurement results indicate a usable measurement range of at least several mT with a noise equivalent magnetic field level of less than 8 nT/(root)Hz.

Proton exchanged LiNbO3 devices are used advantageously for fiber optical sensors because of the high optical damage threshold, extremely high polarization maintenance, and flexibility of waveguide parameters as a consequence of different annealing procedures. For a fiber optical gyroscope operating at a wavelength of 850 nm, a device has been developed integrating the functions of space filter, polarizer, splitter, and modulators on a single chip. This device has been fabricated by annealed proton exchange (LiNbO3:APE); the fabrication technique is described. The optical parameters of the chip and the results

An optical technique to generate a heterodyne carrier frequency suitable for electronic signal processing schemes in sensing applications is described. The technique exploits stimulated Brillouin scattering generated in birefringent optical fiber. Systems based on dual fiber and single fiber topologies are reported which yield 665 MHz and 10.8 MHz carrier frequencies, respectively. Experimental results, indicating the presence of output frequency instabilities, are reported, and the implications for future signal processing and sensing schemes based on stimulated Brillouin scattering are discussed.

A novel three-dimensional interferometric and fiber-optic displacement measuring probe is described. As an important sensing part of a coordinate measuring machine, this compact measuring probe is successfully used in coordinate and surface scanning measurements of several types of complicated workpieces in precision engineering metrology. It is also applicable to three-dimensional vibration measurement. The distinguishing feature of this probe resides in its higher measurement accuracy on a relatively large measuring range, as compared with other conventional electric 3-D measuring probes commonly used in coordinate measuring machines. +/- 0.2 micrometers accuracy is achieved in a +/- 100 micrometers range in any spatial direction. The measuring force is less than 0.33N. This probe is very suitable for continuous surface scanning measurement. This three-dimensional interferometric and fiber-optic measuring probe consists mainly of three parts. The resilient mechanical part provides a universal movement for the measuring tip. The interferometric and fiber-optic small displacement sensing part measures this universal movement in the X, Y, and Z directions simultaneously and precisely. The opto-electronic signal detecting and processing part gives a real-time digital display of the measuring tip spatial displacement.

The detailed topological requirements for each fiber optic sensor system differ, but there are many general features, particularly concerning the power distribution between sensor elements and the collection of the data in the optical domain, which are common to all systems. This paper highlights these general features.

A scheme to frequency multiplex a group of sensors based on all-fiber Michelson interferometers is presented. Lead insensitivity is obtained by using the two fiber leads of the configuration as an extra Michelson interferometer whose differential phase is kept constant by active compensation. Topics concerning the system design, sensor sensitivity, and crosstalk between sensors are investigated. Experimental and numerical computational results are presented.

Frequency-derived distributed optical-fiber sensing is a powerful and convenient method for measuring the spatial distribution of birefringence in a hi-bi fiber. The method relies on the special statistical characteristics of Rayleigh backscatter for its action, and these are analyzed in the context of the system. Some preliminary experimental results are also reported.

Evaluation and experimentation performed on a wavelength division multiplexing (WDM) network system, whose optical architecture has been previously proposed for reflective-based intensity modulation sensors, is described. The key role of WDM devices for accurately transmitting optical signals is outlined, and, in particularly, the effect of wavelength-channel mismatch is discussed. A system for referencing the optical power is discussed. Special effort has been made to determine fiber behavior under common stress, which is then studied as a function of constraints due to the choice of wavelength used for measuring or referencing the light signals passing through the network.

Two novel techniques for multiplexing interferometric optical fiber sensors have been developed. The techniques use spread spectrum modulation of a semiconductor laser and electronic or electro-optic correlation detection of the returned signals from a network of interferometric sensors. A network consisting of two interferometric sensors in parallel has been assembled. Initial results from our experiments show that while the interrogation system monitors a specific sensor, the signals from other sensors in the network can be suppressed by over 25 dB.

The optical fiber sensor networking problem is considered from a digital communication viewpoint. A multibit, all-optical temperature sensor design concept is presented. The sensor generates 2M codewords of length M bits (gray coded), each codeword corresponding to a specific temperature threshold within 1-2 deg accuracy. The codewords are generated serially, and therefore only a single output fiber line is required. The proposed sensor design is used in conjunction with a time division multiplexing (TDM) scheme to verify the concept of multibit sensor multiplexing and networking. In addition, sensor array design to multiplex multibit optical sensors is presented, and the trade-offs between the maximum number of sensors, system power margin, bit rates generated by the sensor arrays, and the fiber delay lines necessary for TDM are discussed. Finally, the network architectures (similar to LAN topologies) and the processes necessary at the remote receiver to recover the sensing information from each individual multibit sensor are explained.

Three different techniques for simultaneous measurement of temperature and axial strain applied to a length of optical fiber are reviewed. It is shown theoretically that for large temperature change and high strain, the second-order effect becomes important. The experimental results for two different types of fibers are presented to verify the theoretical predictions, and the cross-sensitivity coefficients were measured.

A fiber optic sensor adapted to the permanent monitoring of oil wells is described, providing a simultaneous determination of pressure and temperature in a down-hole environment. This sensor includes two interferometric transducers in a serial configuration (Fabry-Perot type for the pressure, including a deformable membrane, and birefringent type for the temperature) and uses spectral modulation encoding techniques to recover at large distances both information without cross-talk and with a perfect down-lead insensitivity.

A novel structure incorporating an all-fiber configuration for a fiber-optic sensor of high pressure up to at least 100 MPa is discussed. This configuration is based on the hydrostatic pressure-induced polarization coupling occurring in highly birefringent optical fibers, which the authors reported earlier. The application of two light launching systems--one based on a semiconductor laser diode pigtailed to a single-mode fiber, the other on a similar laser diode pigtailed to a highly birefringent fiber--is reviewed and analyzed. The paper discusses the principles on which the design is based; the construction, instrumentation, and operation of this sensor; and its performance under various conditions, including pressure and temperature coefficients of the output signal. Temperature desensitization procedure is briefly described, and the advantages of the sensor over a sensor utilizing bulk optical polarization controlling elements and a HeNe laser are analyzed.

Large-scale installation of optical fiber sensors in an industrial or military environment is handicapped for several reasons. However, optical fibers are widely accepted to be applied for data transmission. An optically powered sensor system is proposed in which conventional well-proven transducers and data transmission along optical fibers are combined. The fiber is also used to transmit 10 mW of optical power from a remote control unit to the sensor head. In the sensor head the optical power is converted into electrical power to supply the sensor and datatransmission electronics. When a minimum supply voltage level of 4 V is detected, the pressure or temperature is sampled and a pulse position modulated (PPM) signal is transmitted along the fiber to the control unit, where it is demodulated. Feasibility of the system was demonstrated in the laboratory, and field experiments are planned. Alternative system configurations are discussed, in which intelligence is incorporated in the sensor head.

A novel signal processing approach, which results in a significant improvement in the measurement resolution for a ruby-based fiber optic temperature sensor, is proposed. The technique discussed is based on the phase-sensitive detection (lock-in) of the fluorescent signal. As with the other phase-sensitive detection techniques, it has the significant advantage of a high noise-suppression ability, but it differs from existing phase sensitive detection methods in several aspects to yield this improvement. When the system is used to monitor the fluorescence lifetime of ruby, in the range from 40 degree(s)C to 100 degree(s)C, the measurement resolution achievable due to the signal processing scheme is +/- 0.04 degree(s)C, and from 100 degree(s)C to 200 degree(s)C it is +/- 0.2 degree(s)C. At 450 degree(s)C, the resolution is +/- 1 degree(s)C. However, other measurement errors must be reduced to achieve a high accuracy of measurement comparable with this. Finally, the proposed technique can be used not only for the monitoring of fluorescence lifetime, but also for the highly accurate monitoring of the time-constants of other appropriate first-order systems, such as a capacitive sensor, operating under high-noise condition.

Two modifications of fiber-optic temperature sensors which utilize the temperature-dependent birefringence of optical crystals have been investigated. In a first approach, an additional compensator crystal has been introduced into the customary birefringent filter. The thickness of this crystal was chosen to compensate almost completely the birefringence of the sensor crystal, but the temperature dependence of its birefringence must be negligible. For this purpose the authors used (alpha) -quartz. With this sensor they obtained a high modulation depth of the output signal even for LED light sources. In a second approach the bulk birefringent filter has been completely replaced by a titanium-indiffused waveguide in LiNbO3. Polarization maintaining fibers have been coupled directly to the waveguide ends with appropriate orientation, so that collimating and focusing lenses as well as the two polarizers can be omitted from the sensor head, and thus the compactness is considerably increased.

A new fiber-optic high hydrostatic pressure sensor up to 100 MPa based on extrinsic intensity modulation of light reflected from a cholesteric liquid crystal (ChLC) sensing element is proposed and demonstrated. The sensor exploits the effect of pressure-induced changes in the wavelength of maximum light reflection observed in ChLCs. Three different configurations of the sensor which utilizes multimode optical fibers for communication with the high-pressure region were investigated: a double-source configuration and two configurations with double sensing elements. Results obtained demonstrate that the proposed cost-effective fiber-optic liquid-crystal sensor has a pressure coefficient two orders of magnitude higher than current high-pressure sensors. Depending on the applied configuration, the sensor can display a good linear response and sensitivity for specific ranges of pressure useful in industrial applications.

Distributed Optical-Fibre Sensing (DOFS) is a technique which utilizes the very special properties of the optical fibre to make simultaneous measurements of both the spatial and temporal behaviour of a measurand field [1,2. As such, it provides an extra dimension in the measurement process, leading to finer monitoring and control, and to a new level of understanding, especially in regard to the behaviour of large structures. Thus we may expect to measure spatial distributions with a resolution 0.1 - im over a distance lOOm, to an accuracy —1%. The advantages of optical fibres for general measurement functions are well known: they rely on the insulating, dielectric, passivity of the medium, allied to the sensitivity of its optical propagation properties to xternal influences, over a broad range. DOFS takes advantage of two additional properties of the fibre: its one-dimensional nature, and its mechanical flexibility. With the aid of these, it becomes possible, in principle, to determine the value of a wanted measurand continuously as a function of position, along the length of a suitably-configured fibre, with arbitrarily large spatial resolution; the normal temporal variation is determined simultaneously, from the time-dependence of the signal. Such a facility as this offers many attractive possibilities for industrial and research application. The value of having access to the spatial/temporal behaviour of strain and temperature in, for example, large, critical structures such as dams, aircraft, space-craft, bridges, multi-storey buildings, electrical generators, boilers, chemical pressure vessels etc., are clear, from the points of view both of safety monitoring, and of improved understanding of behaviour under anomalous conditions. The flexibility of the fibre makes it relatively easy to install over the chosen measurement path and thus allows, unlike many other sensor systems, retrospective fitting. Furthermore, again in contradistinction to other sensor arrangements, DOFS offers a facility which is unique: there are no conventional techniques for acquiring the same information. This paper will summarize the principles of DOFS, will give examples of systems which have been studied, and give some indication as to what is in store for the future.

The discovery of high-temperature superconductors (HTS) has opened new opportunities for applications of superconductors in optoelectronics. The HTS perovskites represent a new class of solid-state materials, exhibiting many very interesting and potentially useful electronic, optical, and electro- optical properties. They also operate in the 30-80 K temperature range, where refrigeration is cheap and the parameters of semiconducting devices are optimal. A review of the substrate materials and deposition techniques suitable for fabrication of high-quality epitaxial HTS films for electronic and optoelectronic applications is given. Laser processing techniques of HTS films are presented, with a special emphasis put on the laser writing method, which enables the definition of superconducting and nonsuperconducting regions in the same, epitaxial HTS film. Two possible approaches for the development of a complete optoelectronic system with the elements based on the HTS films and operational at liquid-nitrogen temperatures are presented. The first approach consists of manufacturing the devices made of conventional electro- optic materials and containing HTS transmission lines and electrodes. Design and properties of ultrafast HTS interconnects are discussed, and a new concept of the Mach-Zehnder-type YBa2Cu3/$O(subscript 7-y-on-LiNbO3 optical modulator is introduced. The second, more futuristic approach, is to exploit contrasting properties of the oxygen-poor and oxygen-rich HTS phases to fabricate novel, monolithic devices. Recent experiments, which reveal intriguing optical properties of HTS films, and are most relevant for the development of all-HTS optoelectronic devices are discussed. Several practical devices, such as high-frequency modulators, ultrafast-pulse generators, and sensitive photodetectors will be presented.