The paper presents the state of the art of glass photosensitivity and photoinduced fiber gratings. The techniques for fiber grating fabrication and the basic grating properties are reviewed. Photosensitive fiber glass compositions, techniques to enhance photosensitivity as well as the wavelengths at which gratings can be written are considered. The most important applications of Bragg and long-period gratings are discussed.
An active wavelength demodulation approach with high- resolution for fiber Bragg grating sensor is proposed and demonstrated for multiplexed temperature measurement in this paper. A tunable fiber laser controlled by computer is used as the scanning light source. And the computer synchronously performs data gathering and processing. Here Gaussian-Newton curve fitting approach is used to determine the Bragg wavelength of the grating sensors. A resolution approximately +/- 0.1 pm, corresponding to approximately +/- 0.01 degree(s)C has been demonstrated using this approach.
In this paper we propose a novel matrix method for measuring the internal distributed structure within fiber Bragg grating. It bases on the reflection characteristics of short segment of grating and OCDR (optical coherence domain reflectometry) theory, and shows the relationship of the global spectrum characteristics and local internal structure of grating mathematically. Simulation results show that higher reconstruction resolution for FBG can be obtained with the new theory.
Strain, deformation and temperature measurements constitute the most interesting parameters to be monitored in smart structures. White-light interferometry offers these performances even for long-term measurements. Based on the Michelson interferometer, fiber optic quasi-distributed strain-sensing techniques are demonstrated in this paper. Related problem about the spatial division limitation of fiber optic sensors is discussed.
Distributed sensing system is not necessary for locating breakpoint along fiber as only the first breakpoint need to be cared of. Thus, we modify the phase shift which is used to measure distance, and proposed a cost-effective method for locating breakpoint.
An optical fiber sensing system for biomedical and biochemical sensing was developed. The system was designed for multiparameter sensing to pH, SO2, NH3/NH4+. In the developed system, two light emitting diodes are used as light sources, a silicon photo-diode is used as light signal detector, a multimold stepped bifurcate optical fiber bundle with great aperture is used as light transmissive medium, the dyestuff is coupled to fiber by different methods. The new design idea is double beam compensation. The system is small, cheap but stable and sensitive, its response for pH is about 0.01 pH, the detective limits are 2 micrometers ol/L for NH+4 in water, 2 X 10-4 mol/L for SO32- in water, respectively. The system is powered with battery, so can be used for fieldwork.
Integrating fiber optic sensors offer the potential to monitor large spatial extents due to their geometry flexibility. To date, this potential has not been widely exploited. By increasing the length of an integrating fiber optic sensor, its gain and/or gauge length can be increased. In addition, by configuring the sensor to a spatial distribution of some parameter field of interest, the antenna gain of the sensor will provide selectivity for that particular distribution. In this paper we analyze the application of very long gauge length integrating fiber optic sensors to high sensitivity/highly directional seismic sensing. The ability to focus a seismic sensor on particular faults or locations such as underground nuclear test sites offers the potential of enhanced warning of impending earthquakes or detection of the occurrence of nuclear device testing. We provide an analysis of the application of very long gauge sensing to seismic detection and present some experimental results demonstrating antenna gain.
We report on the use of a frequency-modulated continuous wave (FMCW) technique for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity from a broadband source by a linear swept- frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in frequency-domain and demodulated using a tunable optical filter. The potential and limitation of the technique are discussed. A 3-sensor FMCW multiplexed FBG array of parallel topology and a 6-sensor hybrid FMCW/WDM system were experimentally demonstrated with -30 dB crosstalk between sensors and 2 (mu) (epsilon) resolution in terms of root-mean-square strain value.
The results of an investigation of the performance of a TDM FBG sensor array using a tunable laser source are reported. The system performance is found to be limited by the extinction ratio of the optical pulse modulator used for pulse amplitude modulation. Formulas that relate the crosstalk to the extinction ratio of the optical pulse modulator, the modulation parameters of the tunable laser, and the optical path differences among sensing channels are derived. Computer simulation shows that an array of 20 FBG sensors with 3 (mu) (epsilon) resolution can be realized with a commercially available single Mach-Zehnder type optical pulse modulator of -35 dB extinction ratio. Experiments with a 4-FBG sensor array demonstrated a strain resolution of about 3.98 (mu) (epsilon) with an optical pulse modulator of about -24 dB extinction ratio.
A three dimension sensing network, which is composed of periodically etched multi-mode optical fibers, is embedded in the laminated composite material structure of vertical tail-wing of plane for nondestructive evaluation of structural states in smart composite material and structures. The composite specimens in which sensing network is embedded are tested with stretching, bending and impacting. It is exhibited in the experiment that the responses of fiberoptic sensor are linear, repeatable, have a high sensitivity and no measurable hysteresis. The experimental results have shown the feasibility of embedded fiberoptic sensor network to be used to measure the structural states such as strain, stress and damage. The signal processing for the parallel and distributing fiberoptic sensing network by using artificial neural network is also described. In this paper, the BP network which is suitable for distributing sensing signal processing is studied, and the simulation has demonstrated that the BP network's ability for identifying the position of stress, strain and damage is above 90%.
The possibility of the development of fiber-optics measurement network for reconstruction of 2D distributions of physical fields is shown. The structure of the network is developed and special algorithm of its signal processing allowing in four time to decrease the number of necessary fiber measurement lines is tested. Experimental model of the network is successfully used for reconstruction of intensity distribution of transversal vibrations of ship fuel tank surface.
A problem of tomography reconstruction of functions of spatial distribution of vector physical fields by using 2D measuring network based upon fiber-optics measuring lines having integral sensitivity is discussed. The problem solution was found for cases (1) output signal from measuring lines is formed under action of vector projection, (2) output signal is formed under action of the derivative of vector projection along measuring lines axis, (3) output signal is formed under action of square of vector projection.
Main advantages of neural networks are the flexibility of architecture and ability to training. These advantages allow using neural networks for solving many difficult problems. One of them is the processing of data collected by fiber- optical measuring systems. We offered universal neural-like system for optical data processing in optical measuring system.
The main principles of the operation of fiber-optic tomographic sensors are formulated. The ways in which they differ from the known tomographic analogues are considered. Potential capabilities, future development trends, and applications of these sensors are discussed. A brief review is given of the literature on fiber-optic tomographic sensors. The main parameters of the constructed sensors and some results of experimental investigations of scalar and vector physical fields are given.
This paper presents multimode fiber optic sensors suitable for large smart structures where distributed sensing needed. By many observations of speckle patterns from different multimode fibers, it seems that each speckle pattern has its own signature and uniquely defines its fiber. This prompted us to superimpose (multiplexed) the speckle patterns generated by two different fibers, and detect the resultant pattern. A two-stage feature extraction algorithm is then applied on the image, which reduces the dimension of the pattern vector. Next, a backpropagation neural network with single hidden layer is trained in mapping the feature vectors to the stress applied on each fiber. Series of experiments are conducted on two fiber sensors glued on a square shape plate under different point loads. The sensors successfully estimated that which fiber was under stress with light or heavy weight.
Adaptive correlation filter recorded in photorefractive crystal as an effective instrument for processing signals of fiber-optical measuring network is considered in this paper. Some basic properties and parameters of the filter are experimentally and theoretically studied. We proposed a number of fiber-optical measuring techniques based on using this filter that can be used for different physical parameters monitoring.
In certain applications, such as microelectronic manufacturing, monitoring and controlling small floor vibrations can be essential. This paper describes how an array of distributed fiber optic sensors can be used to monitor micro floor vibrations as part of an adaptive control strategy. The performance target is to limit vibrations in the 10 Hz frequency range to less than 10 microinches peak to peak. The distributed fiber optic sensors are of an interferometric design that uses a single mode to multi mode to single mode configuration. The fibers can be arranged in a grid so as to localize vibration hot spots. Data comparing the performance of the vibration sensing array with that of conventional point piezoelectric accelerometers, as well as sensing and control strategies will be presented.