A Fibre Optic Distributed Differential Displacement Sensor is modelled and experimentally verified to determine
shape. Created using a steel tape, 9/125 μm single mode fibre, and adhesive, the FODDDS can be used to
determine shape or displacement of any object to which it is bonded. A circular shape is examined, and a radius
of curvature comparison yields an error of 2%. The sensitivity of the FODDDS, for the substrate thickness used
in this experiment, is shown to be 1.27 mm between adjacent data points, which corresponds to a radius of
curvature of 103 m.
The design of a phase/frequency detector-based optical phase-locked loop (PFD-OPLL) capable of locking two
commercial semiconductor distributed feedback (DFB) lasers for the purpose of making Brillouin Optical Time-Domain
Analysis (BOTDA) measurements is presented. Due to the aperiodic nature of the PFD transfer characteristic, the PFDOPLL
offers strong acquisition performance without requiring additional acquisition hardware. Design constraints due to
laser linewidths are relaxed by choosing a damping factor of 3.5 instead of 0.707. Loop stability is ensured by reducing
the loop propagation delay by as much as possible in hardware, and choosing the loop natural frequency such that the
loop bandwidth is below the FM phase reversal frequency of the laser. Results show stable lock performance at 11 GHz
with a phase noise of -70dBc/Hz at a 100 Hz offset, a capture range of 2.5 GHz and a tuning range of 3.3 GHz. These
specifications exceed the performance requirements of a BOTDA system.
A Faraday Rotating Brillouin Sensor System (FRoBS) is described and experimentally verified to reduce abrubt
temperature/strain change distortions of the Brillouin Spectrum.This single-ended Brillouin System is created
by modifying a typical Brillouin Optical Time-Domain Analysis system and adding a Faraday Rotating Mirror
(FRM) and a Polarization Beam Splitter (PBS).Both lasers are combined with a PBS and are launched into
the sensing fibre together.The remote end of the sensing fibre has a FRM attached that enables Brillouin
measurement of the outgoing waves as well as the incoming waves, resulting in a mirrored time-domain waveform.
Using weighted averages of the mirrored waveform results in a reduction in the aforementioned distortion.
Recently, strain and temperature measurement results using the first ever spontaneous Brillouin and Raman scattering based fiber optic sensor have been reported (Alahbabi et al., 2004)1. This contribution reports the performance results of a combined Brillouin and Raman sensor used to measure strain and temperature simultaneously. We report on a sensor based on the combination of a BOTDA loss-based Brillouin sensor and a spontaneous Raman scattering based sensor, which has not been previously reported to date. We have implemented the combined sensor system for operation over useful sensing lengths and show significantly improved temperature and strain accuracy along with superior spatial resolution. This combined sensor system is shown to be capable of separating temperature and strain effects which previously limited Brillouin systems in some applications.
Distributed sensors based on time-domain Brillouin scattering have typically had spatial resolutions in the metre range, with some advanced systems improving upon this by an order of magnitude. Resolution in the centimetre range generally has been made possible by using correlation based systems or frequency-domain approaches. Both of these techniques suffer from practical limits on overall sensing length and/or acquisition speed. We present a new technique which uses dark pulses to implement a time-domain sensor system that provides centimetre resolution, short acquisition times and minimal restrictions on sensing length. The method is verified through simulation and results are shown to demonstrate the technique's efficacy in two practical applications.
Brillouin scattering-based distributed fiber optic sensors have been shown to be effective diagnostic tools for monitoring structural health, and detecting fires and hot spots, among other uses. Current research has mainly been focused on improving the spatial, strain and temperature resolutions, and sensing lengths of these systems, generally by the use of better signal processing and improved equipment. In contrast, there has been little published work on optimizing the sensing optical fiber itself. A number of commercially available optical fibers have been measured in order to determine how to optimize their Brillouin characteristics. Some characteristics chosen are the number of Brillouin peaks, the frequency of the peaks, their linewidth, and the temperature and strain coefficients of each peak. It is shown that lowering the intrinsic Brillouin frequency of the fiber can increase the Brillouin strain coefficient and decrease the temperature coefficient of the optical fiber for the main Brillouin peak, among other results.
KEYWORDS: Temperature metrology, Signal to noise ratio, Spatial resolution, Sensors, Scattering, Signal attenuation, Polarization, Pulsed laser operation, Phonons, Fiber lasers
A Brillouin scattering based fiber sensor system has been developed by our Fiber Optics Group for the structural monitoring and civil engineering related applications. In this paper, the Brillouin loss spectrum has been characterized in terms of its center frequency, peak power, line-width and shape. These parameters have been considered as a function of the input pump and probe laser powers, the pump pulse duration, strain and temperature. The measurement accuracy has been studied at different Brillouin frequency steps to study the uncertainty of the Brillouin frequency, line-width, peak power and shape factor vs. signal to noise ratio, so that we can optimize the system performance. Characterization of the Brillouin loss spectrum led to the development of an innovative technique to measure the strain and temperature simultaneously using the strain and temperature dependence on the peak power in conjunction with the Brillouin frequency for the single mode fiber with 3m spatial resolution, 3°C temperature resolution and 200 ?? (?m/m) strain accuracy.
This paper has demonstrated the structural strain measurement of the steel beam with the distributed fiber optical sensor system based on Brillouin scattering. The experiments were conducted both in the lab and in outdoor conditions. When it is in outdoor environment, the temperature compensation must be taken into account for the sunlight radiation effects. The compressive strain can be measured without need of the pre-tension on the fiber. The spatial resolution of the strain measurement is 0.5 m. The strain measurement accuracy is 10 (mu) (epsilon) for the lab environment.
The strain distribution in a 1.65m long reinforced concrete beam was measured using the distributed fiber optic sensing system developed by Dr. Bao's Fiber Optic Group at the University of New Brunswick (UNB) with center point and two point loading pattern. A spatial resolution of 0.5m was used. Past experience has shown that the bare optical fiber is too fragile to act as a sensor in a reinforced concrete structure. Therefore, in this experiment, two methods of protecting the fibers were incorporated into the concrete beam to increase the fibers' resistance to mechanical damages and prevent chemical reaction from occurring between the fibers and the concrete. The fibers were either embedded in pultruded glass fiber reinforced vinyl ester (GFRP) rods or bonded to the steel reinforcing bars with an epoxy adhesive. The strain at midspan of the beam as measured by the distributed sensing system was compared with the readings of electrical resistance strain (ERS) and mechanical strain (MS) gauges. The experimental results showed that the pultruded GFRP rods effectively protected the fibers, but the strain readings from the GFRP rods did not agree with the strain measurement of the ERS on the steel reinforcing bars due to the possible slippage of the rods in the concrete. However, the fiber bonded to steel reinforcing bars produced more accurate results and confirmed the potential of this technology to accurately measure strain in a reinforced concrete structure. As expected, the fiber with direct contact to the concrete and steel reinforcing bar, can effectively measured the strain under center point or two point loading.
In preparation for the construction of the Rollinsford bridge in Rollinsford, NH, a test specimen of the bridge deck was
constructed at UNH. The testing of this slab was also used as a trial run for an experimental distributed strain sensor. The
slab was equipped for strain measurement using a Brillouin-scattering based fibre optic sensor, along with more conventional
strain and displacement gauges. Some of the results and the difficulties encountered during this investigation will be
presented. Additionally, further investigations into measuring strain on structural members done in preparation for
instrumenting the actual bridge are presented.
Distributed fiber optic sensors based on Brillouin scattering are capable of measuring the strain on an arbitrary fiber section. Their small cross section and ability to sense over a long distance makes them ideally suited for use as a sensing component in smart civil and aerospace structures. Over the past few years, the sensing range and spatial resolution of Brillouin systems have been improved considerably. It has been speculated that linewidth broadening and diminishing signal strength for optical pulses less than 10 ns would limit the spatial resolution of a Brillouin sensor to about 1 m. While this is suitable for some applications, others would benefit from improved spatial resolution. Through numerical simulation we have determined the contributions that linewidth broadening and reduced signal strength have on sensing accuracy. Experimentally, we have discovered that while the signal strength does decrease linearly with pulse widths, the linewidth does not increase correspondingly. Instead, it was observed that at pulse widths below about 5 ns the linewidth decreases dramatically. By improving the signal to noise ratio in our system we have achieved a spatial resolution of 100 mm. At this resolution the Brillouin linewidth is approximately 50 MHz, about the same as the steady state linewidth.
Recent improvements to Brillouin scattering based distributed sensors have reduced both the spatial and strain resolutions to the point where they are acceptable for many smart structures applications. This type of optical fiber sensor can measure both strain and temperature as both parameters produce a change in the optical fiber's Brillouin frequency. Since both measurands have the same observed effect it is impossible to determine which measurand is responsible for the shift in frequency. This problem must be overcome for these sensors to be suitable for many smart structures applications. Techniques have recently been developed for Brillouin scattering based distributed sensor systems to separate strain and temperature information. However, these methods are limited theoretically to spatial resolutions approaching 5 - 10 meters. This paper reports on a new technique that was used at a shorter spatial resolution. The Brillouin loss spectrum peak power was determined as a function of strain and temperature at a spatial resolution of 3.5 meters. By combining this information with the conventional Brillouin frequency measurement, strain and temperature were successfully differentiated.
Distributed strain sensors based on Brillouin scattering in optical fibers are attractive as structural monitoring systems due to their unmatched measurement flexibility. Despite their potential, very little research has been conducted on these sensors under the types of conditions that would be experienced in practical applications. This paper presents the results of a simple study in which a Brillouin sensor was used to measure the strain along the length of a cantilever beam subjected to two loading patterns. A spatial resolution of 400 mm was used to perform the measurements and a precision of approximately plus or minus 50 (mu) (epsilon) was achieved. The experimental results were found to be in excellent agreement with theoretical predictions, demonstrating that this type of sensor system is well suited to use in structural monitoring applications.
KEYWORDS: Sensors, Signal attenuation, Digital filtering, Sensing systems, Spatial resolution, Software development, Interference (communication), Scattering, Data processing, Signal processing
A distributed strain and temperature sensor system based on the Brillouin loss principle has been constructed. Computer control software has been developed to automate the measurement process for this system. This paper describes the data processing performed by the software and the rationale behind its development. Several methods of measuring Brillouin loss signals are described with a discussion of the merits of each. Results from a simple experiment are presented to demonstrate the capabilities of the automated system.
Fiber optic distributed sensors based on Brillouin scattering can measure strain and temperature in arbitrary regions of a sensing fiber. The fiber optics group at the University of New Brunswick has recently developed an automated system for strain measurements in a distributed sensing system. Under a computer control program, strain measurements are taken using Brillouin Optical Time Domain Analysis. The computer takes a series of measurements of Brillouin loss in the fiber as a function of the frequency difference between the two lasers in the system. By fitting the returned data to a predetermined model, accurate determination of the Brillouin frequency and hence strain in the fiber can be made. An experiment was conducted to test the sensor system in which fiber was stretched by use of dead weights hanging on a system of pulleys. Determination of strain to within 17 (mu) (epsilon) was realized. Spatial resolutions of better than 1 m were obtained through standard BOTDA methods and resolutions of better than 500 mm were realized using our compound spectrum analysis method.
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