M.G. Xu

Modeling and performance analysis of a fiber Bragg grating interrogation system using an acousto-optic tunable filter

The performance of a fiber grating interrogation system using an acousto-optic tunable filter (AOTF) is analyzed. An equivalent linear circuit is presented to describe the system, which tracks the wavelength of a fiber Bragg grating (FBG) sensor in a FBG sensor array. From the model, the tracking resolution of a shot-noise-limited system has been determined for both transmissive and reflective interrogation of FBGs. An optimum ratio between the bandwidths of the FBG and the AOTF has been derived. Experimental results of a tracking system are presented to validate the theoretical predictions. The theory should also be suitable for interrogation systems using other forms of tunable bandpass filters for tracking the FBG wavelength.

Simultaneous measurement of strain and temperature using fibre grating sensors

Spectral behavior of strain and thermal sensitivities of two superimposed fiber gratings of two different Bragg wavelengths has been studied. This involves monitoring the Bragg wavelength as a function of strain and temperature on the gratings. The results show that the ratio of sensitivity at two different Bragg wavelengths (850 nm and 1300 nm) is different with strain and temperature, which can be used for simultaneous measurement of these parameters in fiber grating sensors.

Multiplexed point and stepwise-continuous fiber grating based sensors: practical sensor for structural monitoring?

The paper describes recent progress on developing a complimentary pair of fiber grating-based sensor methods for structural monitoring. Particular emphasis is placed, firstly, on interrogating arrays of point-sensors based on fiber gratings using an acousto-optic tunable filter and, secondly, on determining the optical path length between these point sensors using optical time domain reflectometry. The combined system is capable of measuring both local and spatially-averaged strain. A new approach which permits the simultaneous measurement of strain and temperature, using superimposed fiber gratings, is presented. Potential applications in aerospace composites are also discussed.

Novel frequency-agile interrogating system for fiber Bragg grating sensors

We have demonstrated a new frequency-agile system for the interrogation of in-fiber Bragg grating sensors. The scheme involves frequency shift keying (FSK) of the rf drive to an acousto-optic tunable filter (AOTF) to track the wavelength shifts of a Bragg grating. Theoretical studies to derive the optimum frequency deviation for achieving maximum sensitivity are given, and experimental results are presented for temperature measurement. This technique is capable of rapid, random access and very wide tuning range, showing the potential for interrogating multiplexed arrays of Bragg-grating-based sensors.

Novel hollow-glass microsphere sensor for monitoring high hydrostatic pressure

Laboratory prototypes of a novel pressure sensor have been produced using a hollow glass microsphere, bonded, in an on-axis position, to the end of a monomode optical fiber. The sphere surfaces form a low finesse Fabry-Perot interferometer. The construction of the probe is simple in concept, yet the sensing element is intrinsically hermetically sealed. Experimental trials, under the influence of hydraulic pressure have been carried out and show a good match with predicted behavior. The observed shift in wavelength with pressure was -0.93 nm/MPa, two orders to magnitude higher than that we have measured with a in-fiber-grating sensor under similar conditions. The ratio of the pressure sensitivity to the temperature sensitivity for our microsphere sensor was more than two orders of magnitude better than the in-fiber-grating type, so therefore less compensation is necessary to correct for temperature changes. This new form of sensing probe has potential for many high-pressure sensing applications.

Multiplexed measurements of strain using short- and long-gauge length sensors

The paper discusses two complimentary optical fiber sensing techniques which have been researched for structural monitoring applications. The short-gauge-length sensor system is based on fiber gratings and has achieved a strain resolution below 1 microstrain. The long-gauge-length sensor system has achieved 100 micron spatial resolution using a new OTDR technique. Results are presented for surface-mounted and embedded sensors. Both sensors systems can be multiplexed to make more efficient use of the interrogating unit. Their system designs should be capable of being developed to meet real engineering applications.

Structural bending sensor using fiber gratings

A new thermally-compensated bending gauge using surface-mounted fiber Bragg gratings is demonstrated. The gauge configuration involves two fiber Bragg gratings, surface-mounted on opposite surfaces of a bent mechanical structure. Experimental results using a cantilever beam are presented, showing a strain resolution of 9 microstrain.

Lock-in techniques for interrogation of long- and short-gauge length optical fiber sensor arrays

Two complimentary optical fiber strain sensors employing lock-in techniques are presented. The first system interrogates an array of long gauge length sensors, defined by broadband optical reflectors and employs multiplexing in the time domain. The second system operates over shorter gauge lengths using multiple narrowband reflectors and wavelength-division-multiplexing. The first system tracks minima in the amplitude response produced from the superposition of two sinusoidal subcarrier waves. The second uses an acousto-optic-tunable-filter (AOTF) to track the peak reflective wavelength of an array of Bragg gratings. Both systems are constructed using telecommunications components. Together, the systems may be used to examine both line-integrated strain (or temperature) over long gauge lengths and local strain at a number of selected discrete points of particular interest. Lock-in techniques using dithered signals are applicable to sensors having a transfer function containing at least one turning point. This may be a maximum or minimum when observed either in transmission or reflection. The sensor responds to the dither with an amplitude-modulated signal, which permits locking of the interrogation system to the turning point. This provides a real-time response and better noise performance than scanned measurements. High-resolution monitoring of time-varying strain is demonstrated using this method. The long gauge length system has demonstrated a resolution of 3 microstrain over discrete 5 m long sensing sections, with an interrogation time of 0.25 s. When multiplexed to interrogate an array of four sections, intersection crosstalk levels were below minus 50 dB. The short gauge length interrogation system has been demonstrated using both fiber Bragg gratings and an in-line Fabry-Perot cavity as the wavelength selective reflectors. A resolution below 1 microstrain was obtained using the gratings, whereas a resolution of 1.5 multiplied by 10-6 in optical path-length-difference was obtained when interrogating a Fabry-Perot cavity. Simultaneous monitoring of multiple Bragg gratings has also been demonstrated by multiplexing with different dither frequencies. The versatility and the high resolution make the lock-in systems ideal for smart structures applications.

Multiplexed fibre optic system for both local and spatially averaged strain monitoring

An optical fiber sensor system to interrogate point sensors (Bragg gratings) and optical path length between point sensors is discussed. The paper describes interrogation schemes capable of measurement resolutions better than 100 microstrain based on simple optics, telecommunications electronics and sophisticated signal processing.