Yun Jiang Rao

Recent progress in fibre optic low-coherence interferometry

A review of recent progress in signal processing, sensor developments and multiplexing techniques for fibre optic low-coherence interferometry is presented, with an emphasis on the new developments in this field which have the potential to be exploited for practical applications.

In-fiber Bragg-grating temperature sensor system for medical applications

A novel quasidistributed in-fiber Bragg grating (FBG) temperature sensor system has been developed for temperature proving in vivo in the human body for medical applications, e.g., hyperthermia treatment. This paper provides the operating principle of FBG temperature sensors and then the design of the sensor head. High-resolution detection of the wavelength-shifts induced by temperature changes are achieved using drift-compensated interferometric detection while the return signals from the FBG sensor array are demultiplexed with a simple monochromator which offers crosstalk-free wavelength-division-multiplexing (WDM). A strain-free probe is designed by enclosing the FBG sensor array in a protection sleeve. A four FBG sensor system is demonstrated and the experimental results are in good agreement with those obtained by traditional electrical thermocouple sensors. A resolution of 0.1/spl deg/C and an accuracy of /spl plusmn/0.2/spl deg/C over a temperature range of 30-60/spl deg/C have been achieved, which meet established medical requirements.

Synthesized source for white-light sensing systems

We report a novel technique in which by combining the output from two multimode laser diodes a synthesized source with an extremely short coherence length is produced for use in white-light interferometric sensing systems. Experimentally and theoretically it is demonstrated that, by summing the autocorrelation function of two multimode laser diodes with a wavelength difference of 108 nm, a synthesized source with an equivalent coherence length of ~4 microm is generated, greatly reducing the signal-to-noise ratio required to identify the central fringe position.

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.

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.

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.

Simultaneous spatial, time and wavelength division multiplexed in-fibre grating sensing network

Abstract A simultaneous spatial, time and wavelength division multiplexing topology, with combination of a tunable wavelength filter and an interferometric wavelength scanner, is proposed to interrogate a range of in-fibre Bragg grating (FBG) sensors. An eleven-element FBG sensor network based upon this topology is demonstrated for quasi-static strain sensing. Preliminary experimental results show that a strain resolution of ∼7 μϵ with a ∼30 Hz bandwidth (∼1.3 μϵ⧸ Hz for quasi-static strain measurement has been obtained.

A multiplexed optical fibre-based extrinsic Fabry-Perot sensor system for in-situ strain monitoring in composites

The detection of impact damage in fibre reinforced composites is of significant concern because such damage can reduce the load-bearing ability of the composite. A number of factors can influence the nature and extent of impact damage development in composites including: (a) the type of reinforcing fibre and resin system; (b) the magnitude of the residual (fabrication) stresses; (c) the lay-up sequence; and (d) other factors such as the nature of the impactor, impact velocity, impact energy, temperature, moisture content in the composites, etc. From a structural health monitoring point of view, it is necessary to investigate the distribution of damage through the thickness of the composite. This paper reports on a simple, partially multiplexed optical fibre strain sensor system for in-situ strain and residual strain measurements in a carbon fibre reinforced epoxy composite. An extrinsic Fabry-Perot interferometric (EFPI) sensor design was used along with single-mode fibres. The multiplexing scheme was based on wavelength division via the use of two super luminescent diodes (SLDs) at different wavelengths. A low-cost fibre optic CCD spectrometer was used as the detector. The multiplexing scheme was demonstrated using two EFPI sensors. In principle, a number of EFPI sensors can be multiplexed using the proposed scheme provided that each sensor is illuminated at a specified and different wavelength. The feasibility of detecting the residual strain in the composite was demonstrated successfully at two specified positions within a 16-ply carbon fibre reinforced composite panel. Preliminary results indicated that the sensor system was also capable of detecting the effects of a 3.2 J impact. Excellent correlation was obtained between the EFPI sensor output and that obtained using surface mounted strain gauges.

Universal fiber-optic point sensor system for quasi-static absolute measurements of multiparameters exploiting low coherence interrogation

A universal fiber-optic high-resolution point sensor system, based upon signal recovery by dual-wavelength low coherence interferometry, has been developed and demonstrated for quasistatic absolute measurements of multiparameters. This system is capable of multiplexing up to 32 fiber-optic point sensors which can be fiber optic interferometers or fiber Bragg gratings or any combination of the two. The topology of this system is based on a spatially multiplexed scheme with low coherence signal recovery that we have reported previously. A range of multiparameter point sensors, including a medium pressure sensor, a high pressure sensor, a miniature temperature sensor, a displacement sensor, and a fiber Bragg grating strain sensor with drift-compensation, have been developed and incorporated into this network and demonstrated A range to resolution of better than 10/sup 4/:1 and 2/spl times/10/sup 3/:1 has been achieved for the interferometric sensors and the Bragg grating strain sensor, respectively. The interchangeability of the sensors has also been demonstrated, allowing the sensors to be replaced in the event of damage. Due to the universality of the signal interrogation, the instrument can be compatible with any interferometric point sensor which has a similar optical path difference with the transmitting interferometer or any fiber grating sensor whose normal wavelength is within the spectral range of the light source. In addition, as the total sensor number which can be multiplexed is quite large, the average cost for each sensor is reduced considerably. Therefore, this system allows optical fiber sensors to compete with conventional sensors with the additional benefits of fiber-optic sensors.

Development of prototype fiber-optic-based Fizeau pressure sensors with temperature compensation and signal recovery by coherence reading

A range of prototype fiber-optic-based Fizeau interferometric pressure sensors with temperature compensation and signal recovery by dual-wavelength coherence reading have been developed. A separate fiber-optic-based Fizeau temperature sensor with similar cavity length was incorporated into the pressure sensor to allow the pressure measurement to be corrected for the temperature dependence of the pressure probe. The pressure and temperature probes were multiplexed spatially. For the low-pressure sensor, the obtained range to resolution ratio and the accuracy were /spl sim/6.7/spl times/10/sup 3/:1 and better than /spl plusmn/1 percent over a pressure range of 0-0.48 bar, respectively. For the medium pressure sensor, the achieved range to resolution ratio and the overall accuracy were 3.6/spl times/10/sup 4/:1 and /spl plusmn/0.15 percent over a full-pressure range of /spl sim/10 bar. For the high pressure sensor, a range to resolute ratio of /spl sim/1.67/spl times/10/sup 4/:1 and an overall measurement accuracy of /spl plusmn/0.69 percent over a pressure range of /spl sim/1000 bar have been achieved. Due to the universality of the signal-processing scheme based on the dual-wavelength coherence-reading technique, the signal-processing box can be compatible with a range of sensors illuminated by the sources with similar central wavelengths. This study would be readily used to develop a range of commercial fiber-optic pressure sensors with similar optical path differences, interrogated by a universal signal-processing box, for different applications. >