Multiplexing of optical fiber gas sensors with a frequency-modulated continuous-wave technique.

We report on the use of a frequency-modulated continuous-wave technique for multiplexing optical fiber gas sensors. The sensor network is of a ladder topology and is interrogated by a tunable laser. The system performance in terms of detection sensitivity and cross talk between sensors was investigated and found to be limited by coherent mixing between signals from different channels. The system performance can be improved significantly by use of appropriate wavelength modulation-scanning coupled with low-pass filtering. Computer simulation shows that an array of 37 acetylene sensors with a detection accuracy of 2000 parts in 10(6) for each sensor may be realized. A two-sensor acetylene detection system was experimentally demonstrated that had a detection sensitivity of 165 parts in 10(6) for 2.5-cm gas cells (or a minimum detectable absorbance of 2.1 x 10(-4)) and a cross talk of -25 dB.

Fiber-optic tunable loop mirror using Berry's geometric phase.

A method is presented for constructing a fiber-optic loop mirror whose reflectivity is tunable even though a fixed-ratio directional coupler is used. The tunability is polarization and wavelength independent and depends only on the geometry of the fiber-tuning element. This loop mirror can be used to transfer energy from one channel to another in a controllable way. We experimentally demonstrate the tunable reflectivity and present some experimental investigations.

Optical frequency-domain reflectometry for microbend sensor demodulation.

The operation of an incoherent optical frequency-domain reflectometer for monitoring the continuous Rayleigh backscatter in a multimode optical fiber is presented. A simple but effective model to predict the value of beat frequencies arising in the system when excited by a linearly frequency-swept amplitude modulation has been developed. We have verified the model's predictions by experimental measurement of beat frequencies and modulation depth indices of different lengths of standard graded-index multimode optical fiber. Demonstration of the system sensitivity to the detection of microbending loss is then discussed. In particular the detection of loss in a hydrogel-based water-sensing cable allows an alternative interrogation to conventional optical time-domain reflectometry techniques to be implemented. We demonstrate that the incoherent optical frequency-domain reflectometer is capable of detecting and locating sections of increased loss in a multimode optical fiber, and we discuss the fundamental limits on spatial resolution and dynamic range.

Generation and reception of ultrasonic guided waves in composite plates using conformable piezoelectric transmitters and optical-fiber detectors.

A condition monitoring nondestructive evaluation (NDE) system, combining the generation of ultrasonic Lamb waves in thin composite plates and their subsequent detection using an embedded optical fiber system is described. The acoustic source is of low profile with respect to the composite plate thickness, surface conformable, and able to efficiently launch a known Lamb wave mode, at operating frequencies between 100 and 500 kHz, over typical propagation distances of 100 to 500 mm. It incorporates both piezocomposite technology and interdigital design techniques to generate the fundamental symmetrical Lamb wave mode in both metallic and carbon-fiber composite plates. Linear systems and finite element modeling techniques have been used to evaluate the operation of the transducer structure, and this is supplemented by experimental verification of the simulated data. An optical fiber, either bonded to the surface or embedded across the length of the composite plate samples, is used to detect the propagating ultrasonic Lamb waves. Single mode silica fiber has been used in conjunction with a portable 633 nm Mach-Zehnder interferometer for signal demodulation and subsequent data acquisition. This hybrid system is shown to generate and detect the fundamental symmetrical Lamb wave (s(0)) in both carbon-fiber and glass-fiber reinforced composite plates. Importantly, the system signal-to-noise ratio (SNR) associated with the acoustic source compares favorably with s(0) Lamb wave generation using a conventional transducer and angled perspex wedge arrangement.

Mode-hop-free single-longitudinal-mode erbium-doped fiber laser frequency scanned with a fiber ring resonator.

We describe the experimental construction of a single-longitudinal-mode erbium-doped fiber ring laser equipped with a ring resonator filter. The system is designed to avoid mode hopping and can be frequency scanned for spectroscopic sensor applications. We modified a conventional ring resonator filter to have the capability to reduce transmittance of unwanted modes, which is essential for frequency scanning without mode hopping.

Limitation of absorption-based fiber optic gas sensors by coherent reflections.

We report the noise limitation of fiber optic gas sensors with highly coherent laser sources. Interference between signal and reflected waves causes signal fluctuation in the output; this limits the performance of the sensing system. Sensor resolutions limited by coherent reflections are calculated and compared with experimental results.

Fiber-optic backreflectance method for determining the effective focal lengths of optical elements.

An alternative and simple fiber-optic backreflectance method for indirectly determining the effective focal lengths of optical elements by the spatial location of three specific points (the focal point and two object points) is presented. The basic optical element of the method is the single-mode optical fiber. It serves simultaneously as a point light source, an object for projecting, and a point receiver that is highly sensitive to spatial displacements for focused backreflectance laser emission. The method provides high accuracy for both locating the spatial points (1 µm) and determining the effective focal length (less than 0.5%).

Surface-bonded and embedded optical fibers as ultrasonic sensors.

The effectiveness of surface-bonded and embedded optical fibers for the detection of ultrasonic Lamb waves in 2-3-mm-thick steel, carbon-fiber-reinforced plastic (CFRP) and glass-reinforced plastic (GRP) plates are compared. A novel integrating ultrasonic sensor was achieved using the signal arm of an actively stabilized 633-nm homodyne Mach-Zehnder fiber interferometer which was either bonded directly to the plate surface or spliced to single-mode fibers embedded within a composite plate during manufacture. An embedded fiber is shown to be about 20 times more sensitive to Lamb wave motions than a surface-bonded fiber. However, the latter may be more practical.

Distributed sensor for water and pH measurements using fiber optics and swellable polymeric systems.

We report on the design, construction and test of a generic form of sensor for making distributed measurements of a range of chemical parameters. The technique combines optical time-domain reflectometry with chemically sensitive water-swellable polymers (hydrogels). Initial experiments have concentrated on demonstrating a distributed water detector; however, gels have been developed that enable this sensor to be readily adapted to distributed pH measurements.

Simultaneous recovery of strain and temperature fields by the use of two-moded polarimetry with an in-line mode splitter/analyzer.

We report the simultaneous recovery of temperature and strain through measurement of polarization changes in the LP(01) and LP(11) modes of Andrew Corporation elliptical-core fiber. This technique required the development of an in-line mode splittery/analyzer, which is also described. The scheme has been demonstrated to be sensitive to changes of 1 degrees C and 5 microepsilon and is compatible with white-light multiplexing methods to provide quasidistributed information.

Source-noise limitation of fiber-optic methane sensors.

The effect of source (LED) noise on the sensitivity of fiber-optic methane sensors is discussed. Once the source is dominant, the system sensitivity cannot be improved by increasing the source power further.

Direct readout of dynamic phase changes in a fiber-optic homodyne interferometer.

A novel time-domain method is described in which the dynamic phase modulation index applied to an interferometer is directly measured from the waveform of the output photocurrent of the interferometer. A dynamic phase deviation as high as 78 rad has been experimentally demonstrated with a minimum detectable phase deviation of ~0.3 rad.

Thin-metal-clad waveguide polarizers: analysis and comparison with experiment.

We present an analysis of a thin-metal-film in-line fiber polarizer and compare the results with experimentally observed TE- and TM-mode losses. It is shown that the analysis predicts reasonably well the experimentally observed behavior. The present analysis can be used as a design aid for obtaining initial design parameters such as buffer thickness, type of metal, metal thickness, and superstrate refractive index for such polarizer structures.

Programmable fiber/integrated-optics bipolar tap.

A programmable bipolar tap implementation is described. The proposed scheme utilizes fiber and integrated-optics technology in conjunction with twos complement number representation and digital multiplication by an analog convolution algorithm. Simple experimental verification of the scheme is presented.

Explanation of errors inherent in the effective-index method for analyzing rectangular-core waveguides.

It is shown that use of the effective-index method for a rectangular-core waveguide is equivalent to analysis of a pseudorectangular-core waveguide, the dielectric constant of which is higher in some of the cladding regions than that of the actual waveguide. This explains why the effective-index method gives higher values for the propagation constants for the various guided modes than other methods and different results depending on whether one starts with the longer dimension or the shorter dimension to construct the effective-index waveguide.

Fiber-optic bipolar tap implementation using an incoherent optical source.

A fiber-optic bipolar tap implementation is described. The proposed scheme utilizes 2's complement-number representation and digital multiplication by an analog convolution algorithm. Simple experimental verification of the scheme is presented. The addition of programmability to the taps results in the realization of a fast, flexible processor for manipulating binary quantities.

Fiber-optic matrix multiplier using a two-dimensional systolic-array architecture.

An optical matrix multiplier based on a two-dimensional systolic-array architecture is described, exploiting the parallelism offered by optical methods. Experimental verification of multiplier performance is presented. A technique for multiple matrix multiplications is suggested.