Based on current Navy efforts to incorporate fiber optic technology in 21st century Navy ships, the Navy Metrology R&D program has sponsored four R&D projects aimed at the development of new calibration technology to support anticipated Navy shipboard fiber optic systems. New calibration technology is needed to lower the cost and manpower requirements of supporting large shipboard fiber optic systems by conventional calibration methodologies. This paper will present accomplishments of those four R&D projects. The objective of one project was to perform a built-in-calibration design implementation study for fiber optic sensors. The recommended built-in-calibration implementation strategies will be presented. A follow-on project's objective was to develop a polarization based fiber optic temperature sensor utilizing a variable wavelength light source to achieve calibration. The methodology for using wavelength to calibrate a polarization based sensor will be presented. The third project's objective was to develop a fiber optic pulse characterization system for monitoring the health of fiber optic communication systems (both voice and data) by analyzing the characteristics of the optical pulses generated by such systems. The performance characteristics of this device will be presented. The final project's objective was to develop a method to calibrate a fluorescent based fiber optic biosensor capable of detecting the presence of hazardous materials in environmental samples in nanogram per milliliter quantities. A method under investigation of applying built-in-calibration to calibrate this sensor will be presented.

This paper explores an image processing application of optimization techniques which entails interpreting noisy sensor data. The application is a generalization of image correlation; we attempt to find the optimal gruence which matches two overlapping gray-scale images corrupted with noise. Both taboo search and genetic algorithms are used to find the parameters which match the two images. A genetic algorithm approach using an elitist reproduction scheme is found to provide significantly superior results. The presentation includes a graphic presentation of the paths taken by tabu search and genetic algorithms when trying to find the best possible match between two corrupted images.

This paper discusses an automatic method for setting the optimum OTDR measurement conditions for optical fiber lines with Fresnel reflections. An algorithm is proposed that can find the optimum attenuation of the OTDR detector sensitivity by sensing the peak values of Fresnel reflections on OTDR traces. A neural networks software technology is used in order to select the Fresnel reflection at the cable end with great accuracy. The algorithm is applied to an automatic fiber testing system prototype that is composed of couplers, fiber selectors, an OTDR, a personal computer and fiber lines with connectors and filters. The experimental result shows that the best OTDR measurement conditions can be obtained in a faster process time than the conventional method by the automatic and optimum sensing of Fresnel reflections with the algorithm proposed here.

Intensity-based fiber optic sensors are referred to those sensors whose outputs are modulated by intensity levels and not optical phase variations. The attractive features of intensity-based sensors over interferometric sensors include simple in structure, absolute measurement and long-term stability in performance. One important issue for intensity-based fiber sensors is stabilization schemes used to compensate for unwanted disturbances in sensing systems. Two- wavelength ratio (TWR) method is one of the most commonly used compensation technique. In this paper, we study the TWR method in great details by discussing four novel intensity- based sensors developed in our research group. These fiber sensors are: (1) High temperature sensor based on the total internal reflection with temperature measurement up to 1000 degree(s)C; (2) Temperature sensor and distributed sensing system using dielectric filters, achieving temperature measurement of more than 100 degree(s)C and a resolution of 0.5 degree(s)C; (3) Displacement sensor using diffraction grating reflector with a dynamic range of 150 micrometers and 2 micrometers resolution; (4) Displacement sensor using GRIN lens having a displacement range up to 20 micrometers with sub-micrometers resolution. Issues related to the signal transmission in sensing systems are also addressed.

A major problem currently affecting the implementation of grating-based optical fiber devices is the drift in wavelength modulation due to the change in the ambient temperature. For accurate and reliable long term operation of these devices, suitable temperature compensation techniques are a necessity. This paper presents a novel temperature compensation technique for in-fiber refractive index grating-based devices and components. The proposed technique is based on the temperature-dependent spectral characteristics of a dielectric multi-layer thin film interference filter fabricated on the endface of refractive-index grating impressed optical fiber. Temperature compensation is achieved by comparing the reflected intensities at the grating- reflected and the interference filter-reflected wavelengths. The proposed scheme also compensates for any light source fluctuations and lead-in fiber bending losses. Thus a self- calibrated optical fiber sensor configuration is obtained.

By emitting an encoded light signal into a fiber optic sensor the application of the AC/DC compensation technique can be extended to stabilize sensors in static measurement. The source coding is a process that light signal is modulated, and the sensor functions as a decoder to the corresponding encoded light signal. Modulation depth of the decoded light signal can be changed by the DC measurand. A phase-encoded light source is used to demonstrate this technique where the white light interferometer is employed as the decoding sensor.

The intensity-based reciprocity-insensitive structure (IRIS) can effectively compensate the birefringent and lossy influences in low-birefringence fiber-based current sensors. Theoretical model and analysis for different Lo-Bi fiber sensors are given based on the Jones matrix technique, and it is shown that the stability of the sensor can be maximized by using the IRIS and choosing a suitable initial birefringence. Experimental results agree well with those from simulations.

Considerable interest has been shown in the use of fiber gratings as multiplexed sensor elements for civil and aerospace structures and for underwater acoustics. Fiber gratings can also act as transducer elements for many generic sensor types such as temperature, strain, pressure, and E-field for electric utility power plant applications. Properly annealed grating sensors can be used under load conditions to temperatures of 400 degree(s)C with no significant loss in reflectivity or wavelength changes over long periods of time. New fiber grating sensor demodulation schemes also look attractive for decoding the sensor's wavelength encoded output. There are also new developments using fiber gratings in short cavity fiber lasers for ultra high sensitivity sensors.

We propose the use of modal interferometers to detect changes in the transmitted signal in high-finesse extrinsic Fabry-Perot interferometric cavities for real-time, absolute strain and temperature measurements. The short length of the cavity ensures a large free spectral range of the resulting output Airy pattern and by tracking the wavelength shift of one peak, the applied perturbation may be completely characterized. The same principle is also proposed to detect the signal reflected from fiber Bragg gratings for strain and temperature sensing. The relative merits and demerits of this demodulation scheme are discussed and preliminary experimental results are presented.

In this paper, the fabrication of fiber Bragg grating by holography interference is described. A fourth harmonic Nd:YAG laser, acting as an ultraviolet light source, has been used in the experiment. To improve the coherence effect of the laser beam, an F-P etalon and a diaphragm with a little aperture are applied. The sensor system to measure one-dimension strain of cantilever beam or pier based fiber Bragg grating is established and developed. The focus of this development is due to two facts: utility of a tunable outer cavity semiconductor laser in sensing system and the design of new arrangement of sensors. The laser acts as both broadband optical source and demodulator to encoding wavelength. The arrangement of sensors including two fiber Bragg gratings with different Bragg wavelengths, can be applied to discriminate the effect of strain and temperature.

Cross-sensitivity refers to the `cross-talk' between the perturbation being monitored (strain, etc.) and the unwanted ambient physical variables. These cross-sensitivity considerations need to be resolved for realizing accurate, self-calibrated fiber sensor configurations. This paper presents an evaluation of the cross-sensitivity existing between physical variables typically measured by Bragg grating based fiber sensors and the extrinsic Fabry-Perot interferometer (EFPI) based sensors. Cross-sensitivities were investigated between the desired measurand, axial strain, and undesirable sources of error, lateral strain and temperature fluctuation. It is shown that the EFPI sensors are less susceptible to cross-sensitivity induced error than Bragg grating based optical fiber sensors. From the results obtained, it is concluded that the extrinsic Fabry-Perot sensor is a better candidate for certain applications requiring minimized cross- sensitivities, as compared with the Bragg grating-based optical fiber sensor.

This paper presents two new approaches to distributed optical fiber stress sensors whose principle of operation is based on the FMCW technique. In each case, the sensor utilizes a single length of birefringent fiber which has a mirror at one end to act as the sensing element. The two forward-coupled beams which derive from the incident light beam and reflected light beam in the fiber are used to determine the position and intensity of the stress. In addition, the second sensor uses a polarization maintaining optical fiber coupler instead of the bulk optical elements so that it is more compact than the first one. These experiments have shown that both sensors have many advantages, including large signal intensity, good signal contrast, high resolution and long length of sensing arm.

The fused taper polarization maintaining optical fiber couplers have been fabricated with Bow- Tie fiber. The principal axis of Bow-Tie fiber are accurately aligned using laser back scattering method. The extinction ratios are more than 15 dB, the insertion loss is less than 0.5 dB. The splitting ratios are about 1:1 for Y polarized input light and 10:1 for X polarized input light at 1.3 micrometers .

This article proposes a new type of fiber, a novel bow-tie polarization-maintaining fiber for fused-taper coupler. Based on a stress analysis of bow-tie optical fiber, a special requirements of coupling between two fibers for fused-taper coupler are presented. The optimum fiber structure has also been given. The fiber has been fabricated using MCVD. The fiber performance has also been analyzed and measured. Using this fiber, polarization-maintaining fiber coupler has been made successfully. The coupler performance: excess loss < 0.5 dB. extinction ratio > 20 dB. coupling ratio (%): 50%.

A computer controlled optical thermometer has been built to demonstrate a self-calibrating optical sensor. The self-calibrating thermometer records the temperature with a fiber-optic polarimetric temperature sensor. The wavelength sensitivity of the polarimetric sensor is used to facilitate the recalibration. The system contains an optical source which can be tuned over approximately a 9 nm wavelength range, and a monochromator to measure any shifts in the wavelength of the laser. The monochromator is calibrated with the spectrum of a neon discharge lamp.

The applicability of fiber-optic strain and temperature sensors to monitor power plant structures was evaluated on a super-heated steam pipe operating at 1000 degree(s)F at the Tennessee Valley Authority power plant in Kingston, Tennessee. The potential applications of these fiber-optic sensors include health monitoring of high-temperature structures such as boilers, tube headers, and steam pipes, as well as many other power plant structures exposed to less severe environments. The sensor selected for this application is based on a white-light interferometric technique. The key features of this sensor include its ability for absolute measurements that are not affected by light loss along the fiber cable due to, for example, microbending effects and coupler loss, its compatibility with off-the-shelf fiber-optic components, and its low cost. The glass fiber-optic strain sensors were packaged in a rugged metal housing and were spot welded to the high-temperature steam pipe. Another set of gages was placed inside a thermowell for steam temperature measurement. Data collected during a routine start-up is very encouraging and the details are presented in this manuscript.

A theoretical analysis and experimental evaluation of a novel Mach-Zehnder interferometer (MZI), used as a recovery interferometer in an electronically-scanned white-light interferometer is reported. This modified version of a MZI has the advantages of being very simple and compact, with high stability and does not introduce any spatial `misoverlapping' of the beams. In addition, since only one beamsplitter is employed in the interferometer (instead of the usual two used in a conventional MZI), the system is relatively easy to implement and at low cost. Furthermore, this configuration can be simply set to cover a wide range of optical path difference values by simply changing the angle and position of one of the mirrors. This gives the interferometer the advantage of being easily adapted for a range of different measurement applications.

We present modifications in the design and operation of the conventional optical fiber extrinsic Fabry-Perot interferometric (EFPI) sensor to obtain real-time, self-calibrated, on-line and absolute strain measurements. The absolute EFPI (AEFPI) system utilizes the concept of white light interferometry to interrogate a fiber Fabry-Perot cavity and demodulation of the output signal may be carried out using a number of simple techniques like path matching or optical spectrum analyzer detection. The limitations of the conventional EFPI strain sensors are listed and it is shown that the modified AEFPI system overcomes most of these drawbacks by virtue of the fact that the information is wavelength-encoded. The AEFPI system is used to determine the strain during loading tendons that are commonly utilized in pre-stressed concrete. Other major applications include strain measurements in high-performance aerospace materials and structures under extreme mechanical vibrations and temperature variations. Preliminary experimental results are presented and applications to smart structures are proposed.

The interferometric type of optical fiber sensor is an important class of high precision sensors. The multiplexing technique of optical fiber sensor is also an important developing direction of optical fiber sensor. In this paper, the multiplexing optical fiber displacement sensor based on the principles of frequency-modulation continuous wave (FMCW) and heterodyne interference is presented. Since using the frequency modulation of single-longitudinal mode laser diode, the FMCW heterodyne interference system of optical fiber can be easily realized. The basic sensing subsystems are comprised of a Fizeau type interferometer, so the noncontact displacement sensing system has very good reliability. In this paper, a new multiplex method, a four-channel (or 2 X N-channel) multiplexing optical fiber displacement sensors using the scheme of frequency-intensity-division multiplexing is put forward and studied. Many points or multi-dimension displacements or many physical measurands can be measured at the same time. The measurement accuracy of each channel is smaller than 0.1 micrometers , the highest resolution of displacement is 0.01 micrometers . In this paper, also, the influences of the feedback light, the temperature drift on laser diode, the light intensity and cross-talk between the channels for the measurement accuracy are discussed.

Phase fading, intensity noise and zero sensitivity at small-signal are the three limitations of a Sagnac interferometer-based fiber optic current sensor. In this paper, a birefringent Sagnac interferometer is designed to maximize the small signal sensitivity, balance the intensity noise, and compensate the polarization coupling-induced sensitivity degradation in a birefringent Sagnac interferometer. By locking the birefringence of the sensing fiber to certain values, the spun fiber-based intrinsic fiber optic current sensor can achieve almost the same performance as an ideal sensor with zero birefringence.

The new method for straightforward spectral signal processing in interferometric fiber-optic sensors is presented. Its main idea is to analyze the output spectrum of sensing interferometer and separate it from different noise sources in the domain of Fourier images. The absolute and accurate value of OPD can be found directly from the spectral dependence of the analytical signal of interferometer's output signal. The technique doesn't suffer from power and spectral fluctuations of LED and sufficiently noise resistant. It is also insensitive to the influence of parasitic interferometers appearing in the optical connectors and other parts of the sensor. Devices based on such principles don't need recalibration after switching on and further operation. The method has a good potential for combining sensors in a network.

In this article, we describe a fiber-optic catheter-type pressure-sensing system that has been successfully introduced for medical diagnostic applications. We present overall sensors and optoelectronics designs, and highlight product development efforts that lead to a reliable and accurate disposable pressure-sensing system. In particular, the incorporation of an intelligent on-site self-calibration approach allows limited sensor reuses for reducing end-user costs and for system adaptation to wide sensor variabilities associated with low-cost manufacturing processes. We demonstrate that fiber-optic sensors can be cost-effectively produced to satisfy needs of certain medical market segments.

A new photoelastic pressure sensor characterized by the self loss-calibrating by means of the two-wavelength polarization coding as well as by using a corner cube prism as a retarder is proposed and its usefulness is experimentally demonstrated.

Techniques for distributed optical fiber chemical sensor development were investigated and a model system for pH measurement was developed and, as a result, discrete distribution signals were obtained. pH indicator fluorescein sodium was chosen 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. Robust porous glass thin film (approximately 1 micrometers ) was grown on the surface of the optical fiber core with indicator molecules entrapped in the matrix. OTDR was employed to obtain the signal at specific positions along the fiber. A dye laser, pumped by a short pulse N₂ laser, produced blue light pulses at 440 nm which were launched into a 1 X 2 optical fiber coupler. A fiber with four sensitive sections was spliced 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 X 2 coupler to detect the fluorescent light. Results were obtained for solutions of various pH value. The system appears to have great potential due to the wide applications in chemical, biochemical, environmental and safety monitoring.

A novel compensation technique for polarization-modulated optical fiber sensors is proposed, which employs a single LED to provide the measurement and the reference signals. In the sensor head, a polarization beam splitter is used not only as a polarizer, but also as an analyzer, and the reflective structure is used so that the sensor head is simple, compact and reliable. A loss-compensated temperature sensor based on the thermal effect of birefringent crystal is demonstrated. The experimental results show that this compensation technique can stabilize the sensor output with a high accuracy and better long-term stability.

The optical fiber sensor network and its applications are described in this paper. This sensor network system includes two parts: data acquisition part and data transportation part. The data acquisition part is used for measuring the parameters of gasoline tanks level, temperature, flowrate, pressure, and interface between gasoline and water. The data transportation part is a multi-parameters and multi-points fiber sensor network. The specifications of the system are given in this paper. The advantages of the sensor heads are digital type, that is the output signals are digital. By this way the system has high stability for long distance propagation and long term use. This sensor network system has continually worked in laboratory for one year and now is installed in the field of South China for field test about one year. The constructs and experiment results of this system in the field are described in detail.

The study on trichromatic optical fiber radiation thermometers is especially useful for the non- contact accurate temperature measurement of those materials with the emissivities changing dramatically with wavelength. On the basis of the trichromatic thermometry principle, the operation wavelengths of the thermometer are selected properly so as to compensate the emissivity's influence on measurement result effectively. Using 1 X 3 branching fiber coupler and narrow band filter significantly improves the instrument's reliability. The instrument is self-calibrated, multi-functional and easy to use due to the embodied microcontroller and some other techniques.

For non-contact measurement of temperature under strong reradiation with the objects having nearly equal temperature levels conventional methods are not accurate. Method in which measurement of two heat fluxes, one of that is descending on surface of object and second is ascending from object, have been developed. Temperature of the object surface is computed in real time by sequential approximation method using the calibration curves that are received by preliminary experimental investigation of corresponding materials. Method was tested for wide range temperature measurement. Comparison of this method with thermocouple measurements shows a very good accuracy even for low temperature range. Original sensors for high temperature and high heat flux application were developed. One of the advantages of sensors is their capability for accurate measurement of temperature under strong convection influence. The system includes also microprocessor and special software for calibration and temperature measurement. Main application of the system is ovens and furnaces for metallurgy, environmental testing chambers and object temperature monitoring.

In this paper, optimal parameter selection method is proposed to determine two working wavelengths, and selection of their bandwidths is also studied in detail. It's of great significance in practical applications.

An all optical fiber temperature sensor with polarization modulation is described in this paper. The principle of the sensor is based on the thermal effect of the propagation constant of the orthogonal polarization modes in a high birefringence fiber. The construction of the sensor is as follows. A linear polarization light beam is injected into a high birefringence fiber and the output light beam is split into two beams, the two beams pass through the analyzer into detector respectively. In this way not only the magnitude of the temperature but also the variation direction of the temperature can be estimated. This sensor is designed for measuring the average temperature in large area, such as in an oil tank or in reservoir. The construct and measurement results are described.

A novel method of measuring machinery vibration linear or angular is presented. A piece of quartz crystal wedge is put in a HeNe laser cavity, which results in laser mode (frequency) splitting. One mode is split into two frequencies. The difference between the two frequencies, beat frequency, is dependent on the thickness of the wedge in the intracavity laser beam. When the wedge is vibrated, its thickness in the intracavity laser beam is changed, which leads to beat-frequency changing. The variation of beat-frequency represents the amplitude of vibration. Each instantaneous value of the beat-frequency is directly proportional to the displacement of the wedge. A transducer of frequency versus voltage is used to transfer the beat-frequency into a voltage signal which is similar to that capacitance transducers have.

A folded Sagnac interferometer is proposed in this paper by inserting two (lambda) /4 waveplates and one reflector into a conventional Sagnac interferometer. These waveplates can be arranged in a certain way such that the optical path difference between the two orthogonal polarized light beams with opposite transmitting directions can be dramatically reduced after they travel through the sensing loop twice.

An optic fiber interferometric system for measuring absolute distance up to 1 meter is presented. Two interferometers are employed in this system. A fiber Mach-Zehnder structured interferometer using a laser diode as source is set up to position the target and tuning the optic paths. Another fiber Mach-Zehnder structured interferometer with He-Ne laser source is used to measure the displacement. To expend the dynamic range of the system, the method of multiplexing the optic path by introducing accurately calibrated optic fibers into the system is illustrated. In addition, the signal processing methods used in the system are also discussed in the paper.

An optical fiber fluorescence sensor system capable of compensating fiber bending loss is presented. The system utilized a modulated light-emitting diode and digital-signal processing chips to enhance the measurement of fluorescence signals. A fiber-optic oxygen sensor system suitable for measuring oxygen levels in gas and in aqueous media was developed, and the capability of the system to alleviate fiber bending loss was demonstrated. The signal-to-noise ratio of the system was found to exceed 30 dB using inexpensive components.