Intensity modulated fiber optic sensors have the many distinct advantages associated with fiber optics that makes them suitable for several industrial and military applications. Although, the accuracy of the sensor is far less that for interferometric sensors, the accuracy is more than sufficient for most process control situations. The concepts for intensity modulated fiber optic sensors include: transmissive, reflective, microbending and intrinsic mechanisms. The paper describes the various concepts and applications.
Intensity modulation induced by microbending in multimode fibers is considered as a transduction mechanism for detecting environmental changes such as displacement and pressure. The relevant microbend sensor components such as sensing fiber, light source, leads, and detector have been examined and optimized. Finally, the micro-bend sensor has been tested as a displacement sensor demonstrating good performance.
The uses of fiber optic sensor technology in the process control and aerospace industries are discussed. The operating principles of a number of commercially available fiber optic sensors are analyzed and present application areas are identified. The current state of fiber optic sensor technology is defined and used to predict the critical areas upon which future developments depend. Finally, fiber optic sensor development for aerospace application at Simmonds Precision is covered briefly.
Polarimetric monomode optical-fibre techniques offer many advantages for industrial sensing applications. They are equally applicable to point and spatially-distributed measurement methods. The paper discusses the advantages and provides illustration by describing in some detail several devices and systems which have already been studied. Some pointers for future developments are also discussed.
Fiber Optic Sensors Have Been Developed For Monitoring In-Plant Gaseous Emissions Of Ammonia, Formaldehyde, Organohalides, Hydrogen Sulfide, Carbon Dioxide, Oxygen And Nitrogen Dioxide. In Addition, Industrial, Process Control Sensors For Real-Time Process Or Effluent Monii0R1Ng Have Been Developed For Measuring The Aqueous Concentrations Of Such Ions As H+, Na4, K+, Uov, As Well As Organohalides. Some Of 1Hese Fiber Optic Sensors, Or Optrodes, Are Reversible And Are Engineered To Remain In Place For Periods Up 10 One Year. Other Op1Rodes Are Nonreversible And Require Replacemen1 Of 1He Sensor Tip Every 1-2 Days. The Design And Sensitivity Of Selected Optrodes Are Described As Well As Applications.
The present review is restricted to fiber optic sensors for AC electric fields. Sensors for DC electric fields which use optical fibers are described in the paper by A. R. Johnston and H. Kirkham in these proceedings. Several fiber optic electric field sensor configurations have been recently studied. Most of these configurations have used a piezoactive plastic coating, or ribbon, bonded to the glass fiber. The incident electric field generates strains in the piezoactive plastic, which are transmitted to the glass fiber, and the resultant optical phase shift is detected by making the sensor one arm of a Mach-Zehnder interferometer. In the sections which follow the properties of piezoactive plastics are first reviewed, followed by a review of the fiber optic electric field sensors which have been studied so far. Next, a particular configuration consisting of a concentric piezoactive jacket on the glass fiber, is discussed in detail and the frequency response of this sensor is projected over a wide range of frequencies. Finally, the conclusion section includes a discussion of the advantages of fiber optic electric field sensors.
We review the development of magnetic sensors based on fiber optic technology including magnetostrictive and Faraday rotation sensors. Magnetostrictive sensors utilize fiber interferometers to measure the field-induced change in length of a transducing material. in Faraday rotation sensors, an external magnetic field causes a rotation of linearly polarized light propagating in the fiber itself. The working principles, advantages, and limitations of each technique are presented and current research and development efforts are summarized.
This paper gives an overview on distributed fiber-optic sensors systems (DFOSS), and describes recent advances in the technology. Distributed fiber-optic sensors can use optical radar techniques to access many sensors located along the fiber. Such sensors can be said to employ Fiber Optic Detection and Ranging (FODAR) techniques, similar to those employed in free-space radio frequency and optical radar systems.
The successful utilization of fiber-optic sensors in many areas is dependent upon the ability to multiplex several sensors onto a small number of fiber leads. Several types of multiplexing schemes relevant to interferometric fiber-optic sensors will be reviewed. Performance of these techniques in several key areas will be contrasted. Finally, examples illustrating the use of combinations of techniques and hybrid schemes are given.
Micromachined structures, usually formed by anistropic etchings in silicon, can be fabricated with high precision at low cost and may form the basis of a variety of low power consumption resonant frequency mircotransducers. Such structures are compatible with direct optical excitation of their mechanical vibra, on and as such form an attractive family of fibre optic sensors. This paper reviews the basic features of these micromachined structures and presents an assessment of their performance potential as fibre optic sensors.
Presently, advanced fiber optic sensor development centers upon interferometric or polarization modulated sensing techniques that make use of the high sensitivity achievable only through single mode waveguiding. Each sensing application incorporates a very specialized single mode fiber in order to meet the sensor's specific fiber requirements. These specialized single mode fibers, being much different than standard telecommunication single mode fiber, present challenges to fiber designers and manufacturers. As this technology matures, certain fiber types are becoming standardized for specific sensing applications. An overview of these single mode fibers will be presented along with the effect of fiber coatings relevant to the sensor performance. A brief discussion on future trends in development of single mode fibers for intrinsic sensing applications will also be included.
Abstract: The market for fiberoptic sensors will grow from $20 million in 1983 to $278 million in 1993. This paper presents a model for projecting market growth based on historical market data for the fiberoptics communications industry. A comparison of patents issued for fiberoptic sensor and communications applications is used to derive the fiberoptic sensor market. The results closely parallel market forecasts obtained using other market research techniques.
Two techniques based on the magnetically induced strain in a magnetostrictive sensing element are described in which detection schemes involving high frequency periodic modulation of the sensing element are employed for the recovery of very slowly varying low magnetic field gradients. Measurement resolutions in the range of 0.1 μOe to 0.01 μOe per unit sensing element length for test fields of frequencies in the range 0.1 Hz to 1 Hz are obtained.
Fiber optic magnetometers which require high sensitivity at low frequencies (dc-10 Hz) rely on the nonlinear magnetostriction of materials such as amorphous metallic glass alloys. Typically, fiber is bonded to a magnetostrictive sample to convert strain in the sample to phase shift in a fiber interferometer. We present the results of measurements of the frequency dependence and dc and ac magnetic field sensitivity of both planar and cylindrical transducing elements, and discuss the practical advantages and disadvan-tages of each configuration.
Three types of sensors are discussed which take advantage of the dielectric properties of optical fibers to measure electric field and temperature in electric power systems. An electrically isolated de electric field sensor is described, and analytical and experimental results related to free space field measurement in the presence of space charge currents are presented. Accurate measurements can be made over a dynamic range of 105, and errors less than 10% are observed in space charge environment. A simple, all dielectric fiber electroscope for measurement of high intensity ac fields is briefly described. Concepts for distributed temperature sensing along the length of a power conductor are proposed and constraints due to optical power limitations in an OTDR readout are presented.
We have designed and developed a fiber optic temperature probe system for insitu measurement of temperature-time history of individual coal particles. These coal particles are located in a fixed bed combustor operated at high temperatures from 700 to 1100°C. The temperature range of a particle probed with this system is 250 to 1200°C. Other major features of the system include fast response time, which is of the order of 30 msec. The probe is used in-situ so that real time temperature information is not lost. Because the probe does not contact the target coal particle, the temperature field of the target is not disturbed. With these capabilities, precise temperature-time history of coal combustion may be obtained.
A new robotic tactile sensor based on fiber optics is described. The sensor array is based on a matrix of optical fibers in perpendicular rows and columns separated by an elastomeric pad. The elastomer provides compliant response to normal force applied to the structure, and the magnitude of the force is measured by a change in transmitted light coupling across a gap between the fibers. Spatial structure of the force applied to the pad is obtained by the matrix addressing of rows of fibers individually excited by light emitting diodes (LEDs) and orthogonally arranged columns of signal pickup fibers conducting the sensed light to photodetectors. The structure can be made thin enough to wrap conformally over a curved surface. The sensor array lends itself to sensing and control applications in robotic end effectors and as a tactile imaging platform for part identification and orientation to complement or replace optical vision systems.
We have measured the pressure response of a high-temperature pressure transducer fabricated from fused silica. This transducer uses a microbend fiber-optic sensor to measure diaphragm deflection. The transducer was characterized at pressures up to +690 kPa and tem-peratures to 540°C. The experimental data show approximately a linear dependence on the applied pressure when the microbend sensor is preloaded. This behavior agrees with the calcu-lated results for the linear deflection of a preloaded diaphragm with applied pressure. The experimental results showed a zero offset when the temperature was raised, and also the slope of the pressure response curve decreased with increasing temperature. The observed zero shift to 540°C was 15 times the theoretical predictions. The observed change in slope with temperature was about four times less than the slope at room temperature. These discrepancies have been attributed to the thermally induced radial tensile stress in the fused-silica assembly. These stresses were the result of the original pressure-sealing method that was used for pressure calibration. A modified pressure seal was used to eliminate the thermally induced stresses. Results of the tests performed at various temperatures showed much better agreement of the zero offset and the change in slope of the pressure-response curve with theoretical predictions. The transducer with the original pressure seal was insensitive to vibration over the frequency range from 50 Hz to 2500 Hz. The vibration response was less than 0.25% of the full-scale pressure response. When subjected to three times the design pressure, a permanent offset in the pressure-response curve was observed. This has been attributed to the change in the preload of the clamped fiber.
The effect of a modified cladding on the transmission of light through a step index optical fiber is investigated, using geometrical optics and proper choice of ray cor e-cladding encounters. Measurements and analysis of the light transmission of the optical fiber as a function of the modified cladding refractive index and length are presented for various input illuminations: focused and defocused incident beams, effect of offset, polychromatic illumination and illumination with a single mode fiber. Applications to intensity sensors are discussed.
An all dielectric fiber optic sensor has been developed that has the inherent ability to measure temperature from about 4.1 degrees kelvin to the index inversion point of the fiber. Using spectroscopic techniques, the sensor measures a temperature induced shift in cutoff wavelength of higher order modes in a single mode fiber.
This paper describes a passive interferometric sensor array which uses high duty-cycle time-domain addressing to separate and identify the signals from each sensor. Using the array configuration described here, the source phase-induced intensity noise which limits some other multiplexing methods is substantially eliminated, resulting in high sensitivity. The array configuration facilitates the use of synthetic heterodyne demodulation techniques to prevent environmentally induced signal-fading. An experimental all-fiber implementation of a dual-sensor array was used to investigate practical sensitivity and cross talk levels. A measured sensitivity of approximately 10 μrad/√Hzhas been achieved over a range of signal frequencies.
An all-optical technique for multiplexing and self-referencing a number of intensity modulating fiber optic sensors is described. The optical transducers are fabricated as integral parts of recirculating optical fiber loops connected in parallel between transmit and receive optical fibers. A portion of an input pulse is tapped off by each sensor loop module. Successive fractions are tapped off on each circulation around the loop and transmitted to a detector. These form pulse trains that characterize each sensor's output. The relative magnitudes of the components of the pulse train are insensitive to lead and connector losses between the sensor modules and the source and detector. Time division multiplexing of the sensor return signals is created by the delays introduced by the fiber leads between the sensor modules. This paper details the response of a single sensor loop versus theoretical performance, and a three sensor system is demonstrated.
A simple fiber optic chemical sensor is reported which may be used to detect in situ and in real time concentrations of gases and liquids. The sensor, comprised of a single LED source and a cladding-free multimode fiber, operates on the principle of evanescent field and differential spectroscopy. Exhibiting a linear response to relative humidity, this sensor selectively detects water vapor while remaining insensitive to source power fluctuations.
Fiber optic interferometric sensors are typically susceptible to environmentally induced thermo/mechanical perturbations at low frequencies, which preclude their use as dc sensors. However, by using a fiber/transducer configuration in which strain in a linear transducing element is converted nonlinearly into strain in the fiber, the measurement frequency can be up-converted from dc to a frequency where near shot noise limited performance can be obtained. This technique is applicable to the dc or low frequency measurement of any field variable for which a suitable non-linear displacement to strain configuration can be constructed. In this paper the operation of a dc displacement, acceleration and pressure sensor is described, as well as the multiplexing of these sensors.
This paper reviews the present state of seismometry and discusses a seismometer based on fiber-optic technology. In addition, some actual data taken by a portable laboratory-grade fiber-optic seismometer is presented.
Two optical fiber sensors for robotics are described: a real time operating optical fiber colour recognition sensor and an optical fiber proximity sensor. The concepts of both sensors are specified and some of their characteristics. The paper reports the application of the optical fiber colour recognition sensor in sorting objects of different colours and the application of the optical fiber proximity sensor for collision avoidance. Although both sensors have been developed for robotics they can easily be used in other branches of industry.
This paper describes recent developments in hot-spot detection using distributed fiber-optic sensing systems (DFOSS) and special sensitized polymeric coatings. The work has particular relevance to the electrical power industry for monitoring stator bars, transformers and overhead/underground high-voltage transmission lines. Two types of material are discussed which show interesting optical switching properties. The materials so far investigated are more applicable to hot-spot sensing. However, further work is required to extend the dynamic range so that temperatures may be sensed.
There is great interest for distributed or multiplexed fiber optic sensor networks for process control automation. Optical Time Domain Reflectometry (OTDR) is a potentially very useful method for monitoring several sensors distributed along a fiber, but its actual capabilities, using present concepts, are too restricted to allow competitive industrial performance. We present here a novel scheme of OTDR fiber sensor multiplexing which allows, ,the monitoring of up to 50 ON/OFF switches along a given fiber under practical and realistic conditions. Detection of reflected, rather than backscattered, light makes much better use of the in-line loss introduced by each sensor, yielding a larger dynamic range which explains this very significant improvement in the number of multiplexed sensors. Theoretical considerations about this novel scheme and its limits, taking into account present OTDR technology, are presented together with experimental results of a twenty ON/OFF sensor network feasibility prototype .
Reflection type fiber-optic sensors are discussed. A sensor system developed by Optical Technologies, Inc. that is based upon these devices is described. That systems performance, particularly with respect to the correction of bending loses within the fiber connecting the sensor to its associated components, is reviewed.
By selecting the proper geometry for the reactive element of a photoelastic sensor a considerable improvement in sensitivity over conventional intensity-modulated sensors can be achieved. Other recent photoelastic sensors have consisted of an essentially rectangular prismatic bar of transpqrnt material which is stressed uniformly by a strut connected to a force-summing diaphragml,L. Our alternative is to fabricate the diaphragm itself of a transparent material and use it for both the reactive and force-summing functions. In the basic form reported here the diaphragm consists of a thin glass plate mounted and sealed with zero residual stress on a circular support and subjected to varying differential pressure. Polarized light is directed into one edge of the plate along a diameter and received through an analyzer at the opposite edge. Analysis of the,resulting modulation in received optical power with crossed polarizers shows it to be a sin 4 function of applied differential pressure. The photoelastic phase delay is a linear function of pressure and parameters related to the geometry and material of the plate. For thin plates with maximum sen-sitivity the phase delay is proportional to the cube of the support radius and inversely proportional to the square of the plate thickness. An essentially linear response at zero pressure can be achieved with a quarter wave plate (thick sensor plates) or prestressing (thin sensor plates). Tests on experimental models have shown performance consistent with theory, with a response of 3.68 x 10-4 rad/pbar obtained with a 0.023 cm thick glass plate sealed on a 2.32 cm radius support. Extensive theoretical formulae and experimental data are presented.
Optical and electronic feedback techniques are demonstrated which reduce Rayleigh backscatter noise in semicon-ductor lasers coupled to optical fibers. Low frequency intensity noise in excess of the free running laser intensity noise is observed at the output of 2.2 km of fiber in the absence of Rayleigh backscatter.
Several interferometric fiber-optic sensing applications have been proposed that require extremely high sensitivity. For example, electric field and magnetic field sensing. It has commonly been supposed that for such applications one could simply use extremely long lengths of fiber to obtain the desired sensitivity. It is shown that this is not the case and that one encounters a maximum length beyond which no further increase in sensitivity is obtained for a given frequency, due to the finite propagation speed of light in the fiber. This length is given by ℓmax = (v/2fm ), where v is velocity of propagation in the fiber and fm is the frequency to be sensed. FEr 100kHz this length is approximately one kilometer for fused silica fiber. It is found that linear length scaling projections can be used up to R provided the sensitivity so obtained is reduced by a factor of (2/π) = 0.637. IncrWREing the amount of fiber beyond this maximum length has the surprising result of actually decreasing the sensitivity and eventually one can obtain no sensitivity at all at twice L Implications of these results in terms of minimum required transducer sensitivities are discussed and related to measured short length sensitivities. In addition, methods to circumvent this limitation and some of their consequences are presented.
This report describes a new type of intensity-modulating fiber-optic sensor which has high immunity to the effects of variations in the losses of the fiber-link. A variable-splitting-ratio transducer is used to differentially modulate the intensities of the light which it transmits and reflects. Using a four-fiber optical link, light is impinged onto the transducer from either direction, and, in each case, the transmitted and reflected signals are measured. These four signals are then processed to remove the effects of the fiber and connector losses. Loss-compensated sensors of angular position and displacement are described, and their outputs are shown to be highly stable despite considerable variations in the transmissivities of the fiber-link components.
Optical fibers have been embedded in polymer-matrix fiber reinforced composite materials during fabrication. Subsequent testing has shown that optical coupling to, and transmission through, the embedded fibers can be accomplished. A radio frequency interferometric sensor instrument has been built to detect small changes in the optical fiber path length. Elongations smaller than 1 mm over a 1 m path, corresponding to a distributed strain threshold below 1000 μin./in., have been detected. The instrument has been used to demonstrate strain measurement with embedded fibers in graphite/epoxy test specimens. These measurements are compared with electrical resistance strain gage readings.
In this paper we present results of recent theoretical and experimental investigations of the spectral shape of the phase induced intensity noise at the output of several optical recirculating configurations. These configurations include bulk-optics incoherent Fabry Perots, as well as fiber-optic delay lines. It is shown that whenever the system output consists of an infinite series of time delayed versions of the laser input field (recirculating structures), the RF power spectrum of the phase induced intensity noise is periodic with either notches or peaks at integral multiples of 1/(recirculating time), depending on the specific structure of the device. The relative phases of the output fields are shown to be the important factor which determines the shape of the period.
Biosensors based on specific interaction of the analyte of interest (ligand) with a biochemical agent (receptor) which specifically and tightly binds the analyte are being developed by many groups, agencies, and institutions using a variety of detection technologies. We are studying fluorescent methods using evanescent wave excitation on the surface of planar and cylindrical (fiber optic) optical waveguides. A relatively low ligand-receptor binding constant results in reversibility but low sensitivity. Most biological receptors bind very strongly and specifically--almost irreversibly--which provides an "irreversible" means of detection. Means to regulate ligand-receptor binding constants are discussed in order to provide semi-reversible, sensitive, specific sensors, rather than "one-shot" detectors or dosimeters.
An engineering technique for determining the rail deflection of an Electromagnetic Gun during dynamic operation is described. The method uses a low-powered Helium Neon laser coupled with a fiber optic waveguide as a basis for the measurement system described. In addition to verifying the applicability of the photoeleastic effect in high electro-magnetic field environments, the tests yielded valuable information relating to rail behavior in an experimental barrel configuration. However, the approach described is applicable to most electromagnetic launch rail configurations.