Leishi Zhang

Combined spatial- and time-division-multiplexing scheme for fiber grating sensors with drift-compensated phase-sensitive detection

A combined spatial- and time-division-multiplexing topology with drift-compensated high-resolution wavelength-shift detection is reported for fiber Bragg grating sensors. An eight-element grating sensor array is demonstrated based on this topology. A resolution of ~1.2 microepsilon over a range of ~1.5 mepsilon with a measurement bandwidth of 30 Hz (~0.22 microepsilon/ radicalHz) has been achieved for quasi-static strain measurement.

ULTRASONIC HYDROPHONE BASED ON SHORT IN-FIBER BRAGG GRATINGS

We investigate the feasibility of using in-fiber Bragg gratings for measuring acoustic fields in the megahertz range. We found that the acoustic coupling from the ultrasonic field to the grating leads to the formation of standing waves in the fiber. Because of these standing waves, the system response is complex and, as we show, the grating does not act as an effective probe. However, significant improvement in its performance can be gained by use of short gratings coupled with an appropriate desensitization of the fiber. A noise-limited pressure resolution of approximately 4.5 x 10(-3) atm/ radicalHz was found.

First in-vivo trials of a fiber Bragg grating based temperature profiling system

We describe the results of in-vivo trials of a portable fiber Bragg grating based temperature profile monitoring system. The probe incorporates five Bragg gratings along a single fiber and prevents the gratings from being strained. Illumination is provided by a superluminescent diode, and a miniature CCD based spectrometer is used for demultiplexing. The CCD signal is read into a portable computer through a small A/D interface; the computer then calculates the positions of the center wavelengths of the Bragg gratings, providing a resolution of 0.2 degree C. Tests were carried out on rabbits undergoing hyperthermia treatment of the kidney and liver via inductive heating of metallic implants and comparison was made with a commercial Fluoroptic thermometry system.

STRAIN SENSING OF MODERN COMPOSITE MATERIALS WITH A SPATIAL/WAVELENGTH-DIVISION MULTIPLEXED FIBER GRATING NETWORK

A spatial/wavelength-division multiplexing topology with combination of a tunable-wavelength filter and an interferometric wavelength scanner is used to interrogate a range of fiber Bragg grating (FBG) strain sensors embedded in modern composite materials. A nine-element FBG sensor system based on this topology is demonstrated for quasi-static strain sensing of a carbon fiber reinforced plastic plate for aerospace applications. Preliminary experimental results show that a strain resolution of ~1.8micro rms with an ~30-Hz bandwidth (~0.32micro/ radicalHz) for quasi-static strain measurement has been achieved.

In-fibre Bragg gratings for ultrasonic medical applications

We investigate the feasibility of using in-fibre Bragg gratings to measure ultrasonic fields for medical applications. Two signal processing schemes for interrogating the gratings are described. Preliminary results for each scheme (one a homodyne approach, the other a heterodyne one) give noise-limited pressure resolutions of and atm respectively, each within a 1 Hz bandwidth. The second scheme, however, gives a more stable response.

Dual-cavity interferometric wavelength-shift detection for in-fiber Bragg grating sensors.

A dual-cavity interrogation method used to enhance the unambiguous measurement range of in-fiber Bragg grating sensors with high-resolution interferometric wavelength-shift detection is described. This novel technique is based on the use of two sets of fringes obtained with a stepped interferometric wavelength scanner with dual cavity lengths. We demonstrate the concept by interrogating an in-fiber grating temperature sensor with a stepped Michelson wavelength scanner.

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.

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.

Extended dynamic range detection system for in-fibre Bragg grating strain sensors based on two cascaded interferometric wavelength scanners

A novel interferometric wavelength-shift detection system based on the use of two cascaded two-beam interferometers with different optical path differences (OPDs) is developed for static and quasi-static strain measurement with in-fibre Bragg grating (FBG) sensors. These two interferometers are frequency-division-multiplexed and used to provide two sets of interferometric fringes induced by strain. The optical phase output from interferometer 1 with the larger OPD gives a high-resolution measurement whilst the phase output from interferometer 2 with the shorter OPD is used to determine the number of the fringes obtained with interferometer 1. Compared with the detection scheme using a single interferometric scanner, the unambiguous measurement range has been considerably improved. An experimental system with enhancement factors between 11 and 40 in the unambiguous measurement range has been demonstrated for absolute static strain measurement. A large range to resolution ratio of has been achieved. Due to the adjustability of the OPDs in these interferometers, the range to resolution required is also variable, making this system suitable for different applications. On the other hand, a range to resolution of makes FBG strain sensors potentially competitive with traditional strain gauges with the well known benefits of fibre-optic sensors. Therefore, it can be anticipated that after further engineering such a system will find important applications in the field of fibre-optic smart structures.