Philip J. Henderson

Vibration: history and measurement with an extrinsic Fabry–Perot sensor with solid-state laser interferometry

We have studied the history of vibration and demonstrate a laser-based noncontact interferometric vibration sensor. The sensor promises the measurement of microdisplacement by using a Fabry-Perot cavity formed between a partially coated gradient-index lens and a movable reflector. Displacement is determined by the detection of interference fringes caused by phase modulation within the cavity. The sensor was tested in conjunction with both multimode and single-mode fiber transmission. Calibration with multimode fiber produced a fringe-contrast function that decreased monotonically with displacement. This calibration allowed at least 30 fringes to be discriminated, giving a displacement resolution of 0.034 microm across a range of 10.2 microm. Dynamic tests demonstrated a working range of at least 3.74 microm at frequencies as high as 2 kHz. Similar tests in which single-mode fiber was used indicated a dynamic working range of at least 4.29 microm.

The interrogation of a conventional current transformer using an in-fibre Bragg grating

An alternative approach for measuring large currents at high potentials is presented. The output signal from a conventional current transformer is measured by transducing the voltage developed across the transformer secondary into a shift in the reflected wavelength from an in-fibre Bragg grating using a piezoelectric element. A pseudo-heterodyne detection scheme was used to determine this shift. We demonstrate the schemes performance over a current range of 700 A. The current resolution was about over a frequency range of about 50 to .

Highly-multiplexed grating-sensors for temperature-referenced quasi-static measurements of strain in concrete bridges

A fibre-optic system for monitoring bridges with multiplexing support for >64 grating-based strain sensors is presented. Differential wavelength-demultiplexing controlled via LabView™ results in a system with 1με resolution, in-built temperature-referencing and readiness for commercial field-trials.

Vibration monitoring by using a dynamic proximity sensor with interferometric encoding

An optical sensor for the dynamic measurement of proximity is presented. The sensor combines an extrinsic geometric transducer with interferometric encoding for high vibration sensitivity. Static calibration showed a unique variation in interference contrast over at least 60 fringes, leading to a measurement range of 20 mum and a resolution of at least 0.033mum. Dynamic excitation by using low-amplitude vibrations at 3.6 kHz showed a similar contrast variation, verifying fringe discrimination up to the sixth order. With verification of dynamic performance over all 60 fringes, the sensor should offer a low-cost approach to vibration monitoring in electrical switchgear.

High-resolution vibration measurements using wavelength-demultiplexed fibre Fabry-Perot sensors

A fibre-optic system based upon fibre Fabry-Perot strain sensors, capable of measuring deformations and modes of vibration of a composite panel, is presented. Wavelength-division signal demultiplexing allows for the simultaneous interrogation of the strain sensors.

Simultaneous multi-parameter monitoring using a serial fibre-Fabry-Perot array with low-coherence and wavelength-domain detection

A sensor multiplexing scheme for the simultaneous monitoring of dynamic strain and temperature is presented. The scheme combines low-coherence and wavelength-division signal processing with a serial array of fibre-Fabry-Perot (FFP) interferometric sensors defined by in-fibre grating mirrors of matched Bragg wavelength. Heterodyne measurements of signal phase from a 39 cm long FFP interferometer result in a dynamic strain resolution of about at acoustic frequencies. Temperature measurements via the induced changes in Bragg wavelength have a sensitivity of about . The cross-talk between the strain and temperature channels is within the system noise.

High-resolution vibration measurements using wavelength-demultiplexed fiber Fabry-Perot sensors

A fiber-optic system capable of measuring deformations, impacts and modes of vibration of a composite panel is presented. Wavelength-division signal de-multiplexing allows for the simultaneous interrogation of 8 strain sensors.

Prospects for speckle-pattern based vibration sensing in electromechanical equipment

Condition monitoring of heavy electromechanical equipment such as electrical switchgear is increasingly reliant on vibration sensing technology. Of applicable optical methods, electronic speckle-pattern interferometry (ESPI) is established as a sensitive whole-field technique for monitoring surface vibration. Such systems incorporate video signal processing to offer automated real-time measurement of vibrational amplitude and phase. Speckle fluctuations, often a source of noise, may also be utilized within multimode fibres for fast one-dimensional sensing. This paper discusses the optoelectronic processing behind such speckle metrologies, and reviews technological developments suited to vibration monitoring in extreme electromechanical environments. Advances in the technology, particularly in the areas of transient and multiple-frequency response, will be driven by the application and the ability to fabricate industry-compatible system prototypes.

In situ substrate temperature measurements during radio frequency sputtering of ZnO thin film using fiber Bragg grating

Several research groups throughout the world are currently working on fiber acousto-optic components, principally phase modulators, made by depositing a film of piezoelectric materials such as zinc oxide (ZnO) directly onto the optical fiber. It is well known that the quality of the resulting film is highly dependent on the temperature of the substrate (i.e., the fiber), but this has not been directly measured in situ. We present for the first time in situ measurements of the fiber temperature during the deposition process using a fiber Bragg grating temperature sensor. We show that the fiber temperature may rise substantially above that of its immediate surroundings. We also present a simple model for the heat flow for the fiber and the holder based on the heat gain from the plasma and the radiation loss between the various components in the sputtering chamber.

Simultaneous strain, temperature, and vibration measurement using a multiplexed FBG/FFPI sensor system

A novel technique for simultaneous measurement of static- strain, temperature and vibration for structural health monitoring is demonstrated using a wavelength-multiplexed in-fiber-Bragg-grating (FBG)/fiber-Fabry-Perot- interferometer (FFPI) sensor system, which combines the advantages of both FBGs and fiber-optic low-coherence interferometry. An experimental system, including two 1m long FFPIs with central wavelengths of 1531nm and 1534nm and a FBG with a central wavelength of 1555nm, is demonstrated. A static strain resolution of better than 1 (mu) (epsilon) , a temperature sensitivity of 0.1 degrees C and a vibration amplitude sensitivity of better than 1nm/(root) Hz have been obtained. The system cross-talk measured is less than -50dB.