Keith H. Wanser

Fiber devices and sensors based on multimode fiber Bragg gratings

Two categories of multimode fiber Bragg grating (MMFBG) are possible, independent mode and mode coupling, each leads to novel types of fiber devices and sensors. All-mode, wave optics calculations of the reflectivity of independent mode MMFBG are presented, including the effects of individual mode power changes due to microbending transducer loss upstream from the grating. MMFBG offer added flexibility in grating design and performance characteristics compared to single mode FBG, since the reflectivity response may be tuned by the spatial periodicity, length, core size, numerical aperture, and mode coupling characteristics of the grating. New sensing and diagnostic methods that are possible with these gratings and some methods for sensor addressing and multiplexing are described.

Fiber sensors for monitoring structural strain and cracks

Recent structural failure and damage associated with earthquakes and pipeline failures have underscored the need for structural integrity monitoring systems using distributed fiber optic sensors. Such sensors must be low-cost, easily installed on new and existing structures, allow rapid assessment of structural integrity, and be capable of surviving and sensing large displacements associated with cracks and deflections in structural members. Experimental results for multimode fiber crack detection sensors based on the orientation angle approach are reported for both longitudinal and transverse crack displacements. In addition, we report results on a novel all-fiber sensor capable of sensing submillimeter cracks while surviving and sensing larger than 100% strains and cm level displacements.

Fiber-optic strain-displacement sensor employing nonlinear buckling

A new class of intrinsic fiber-optic strain-displacement sensors based on the precisely controlled nonlinear buckling of optical fibers and the resulting optical bend loss is introduced. A multimode fiber version of the sensor is described that exhibits a sensing range convenient for many structural monitoring applications (<100 nm to several millimeters), linear response over a wide range of displacements, and excellent repeatability. It is extremely simple to fabricate and employs inexpensive optoelectronics. A high-temperature version of the sensor is capable of operation at temperatures as high as 600 degrees C.

Crack detection using multimode fiber optical time domain reflectometry

Results of measurements of longitudinal and transverse crack growth using multimode fiber optical time domain reflectometry are presented. Crack detection thresholds less than 0.1 millimeter are readily achieved with OTDR. A sensor package design has been developed to provide controllable directional response characteristics. In particular, a method to eliminate the orientation angle problem and yield omnidirectional sensor response characteristics has been discovered. Results are reported on a novel all-fiber sensor capable of sensing 10 micrometers crack displacements while surviving and sensing 150% strains and displacements of 6 mm. Transmission measurements of the sensor using white light are also reported. The method is suitable for distributed sensing applications covering large areas of structures and adaptable to response enhancements required for real time structural monitoring at rates on the order of hundreds of Hertz. Applications include bridges, buildings, main-steam pipelines, and offshore platforms.

Theory of a Mach Effect Thruster

The Mach Effect Thruster [1, 2, 3, 4] (MET) is a device which uses Mach’s principle in Einstein’s General Relativity to produce a constant acceleration in a device which is undergoing internal energy changes and mass fluctuations. Mach’s principle is a statement that the inertia of a body is the result of the gravitational interaction of the body with the rest of the mass-energy in the universe. The MET device requires no fuel as a propellant needing only electric power of 100-200 Watts to operate. The thrusts at the present time are smallon the order of a few micro-Newtons. The first part of the paper is devoted to experiment and a description of the MET device and apparatus for measuring thrusts. In the second half of the paper, we re-introduce the idea of advanced waves, by summarizing Dirac, Wheeler-Feynman and Hoyle-Narlikar (HN). We show how Woodward’s mass fluctuation formula can be derived from first principles using the HN-theory which is a fully Machian version of Einstein’s relativity. HN-theory reduces to Einstein’s field equations in the limit of smooth fluid distribution of matter and a simple coordinate transformation. PACS codes: 04.20.-q, 04.20.Cv, 04.30.-w, 04.40.Nr Classical GR = 4.20.-q, Fund. problems and general formalism GR = 4.20.Cv, G-waves theory = 4.30,-w Radiation Fields = 4.40.Nr

Theory of thermal phase noise in Michelson and Sagnac fiber interferometers

The intrinsic thermal phase noise spectra of the Michelson, Michelson with phase conjugate mirrors, and Sagnac fiber optic interferometers are presented. The thermal phase noise is maximum at dc and zero at the loop frequency for the Michelson with ordinary mirrors. Thermal phase noise eliminates the factor of two transducer phase sensitivity advantage of the Michelson over a comparable length Mach Zehnder. The Michelson with two phase conjugate mirrors exhibits identical noise to a Sagnac interferometer with the same total fiber length. The Sagnac exhibits common mode phase noise rejection at low frequencies and a broad maximum in the phase noise at the loop frequency of approximately 1 (mu) rad rms/(root)Hz for a 1 km fiber loop at 1319 nm.

Single-mode fiber diameter measurements using Lloyd’s mirror

An interferometric method for measurement of the fiber outside diameter with an uncertainty of <1 microm is proposed and experimentally demonstrated. The fringe pattern produced by placing the fiber in contact with a reflecting surface is utilized.

Interferometric Measurement And Calibration Of Dc Strain Of A Fiber Optic Embedded Graphite Epoxy Composite Panel

Measurements of DC strain produced by transverse deflection of a fiber optic embedded graphite/ bismaleimide composite panel in cantilever geometry were made using a Mach Zehnder interferometer and active homodyne demodulation technique. The measured strain ranged from an average longitudinal strain of 2.0 x 10-4 to 6.6 x 10-7, corresponding to a maximum transverse deflection of 0.3 inches (3%) and a minimum transverse deflection of 0.001 inches respectively. We find that the strain optic coefficient of the embedded fiber ranges from 30% to 40% less than the value for uniaxial strain. In addition, these measurements reveal hysteresis and or creep during the loading and unloading cycle of the composite. Experiments were performed to determine sources of DC drift, and the main sources of DC drift identified. Calibration of the results is discussed as well as future modifications that will permit four orders of magnitude increase in tracking range, greatly improved interferometer stability, and elimination of the effects of piezoelectric hysteresis on the measurement of strain.

High Spatial Resolution Measurement Terminal For Monitoring Short Structures Using Fiber Optics

Recent interest in distributed fiber optic sensing systems utilizing optical reflectometry has generated a need for simulation tools to study the effects of the many system variables on performance, as well as assess the performance claims of various Optical Time Domain Reflectometry (OTDR) instruments and the suitability for their use in a particular sensing application. Computer simulation tools have been developed to enable the optoelectronics system designer to study the fundamental limitations of instrument and sensing system performance based on Rayleigh backscattering. A standalone program utilizing a three segment optical fiber model has been developed that allows calculation of the absolute backscattered power as a function of time, both the total power and the time dependent power from a particular fiber segment. In addition, comprehensive PC based spreadsheet tools have been developed to allow for modeling of signal to noise ratios,including receiver design, in all types of coherent and incoherent optical time domain reflectometry systems, for both single mode and multimode systems. Several examples of the applications of these programs are presented and discussed, with application to the design and development of a high spatial resolution measurement terminal for monitoring short structures. Finally, we present experimental data on a two sensor system based on periodic microbending that shows effects of sensor interaction, and demonstrates the need for careful interpretation of OTDR results as well as further experimental and theoretical work on the effects of microbending induced leaky mode coupling and backscattering in the presence of such modes.

High accuracy optical inverse square law experiment using inexpensive light to frequency converters

In this paper we report on the use of two different light to frequency converters, four different light sources, three of which are novel and inexpensive, and a hand held digital multimeter with a frequency counter, suitable for making accurate and rapid determination of the optical inverse square law exponent of − 2 to better than ± 0.005 over more than a factor of 100 in distance and a factor of 5 × 105 variation of light intensity. Accurate data sets can be taken in 20–40 min, typically one minute per data point or less. The wide range of intensities measured makes a convincing demonstration of the optical inverse square law, and forms the basis for students learning how to plot log–log graphs over multiple orders of magnitude, extract power law exponents and learn about coefficients of determination. The extremely low cost of this high accuracy experiment and the ease of its implementation are appropriate for even the most modest budgets and least equipped high school physics laboratories.