Michael D. DeMerchant

Characterization of the Brillouin-loss spectrum of single-mode fibers by use of very short (<10-ns) pulses.

The characterization of the Brillouin-loss spectrum of single-mode fibers with very short (<10-ns) pulses has been studied. It was found that the Brillouin-loss signal intensity is linearly related to the duration of the pump pulse used to obtain the spectrum. In contrast with the uniform trend of the signal, three distinct behaviors were observed in the spectral linewidth. At long pulse durations the linewidth was constant at approximately 40 MHz. Pulse durations of the order of the phonon lifetime resulted in a broader spectrum, reaching a maximum width of ~100 MHz at 5 ns. Reducing the pulse duration further resulted in a sudden narrowing of the Brillouin line.

SIMULTANEOUS DISTRIBUTED STRAIN AND TEMPERATURE MEASUREMENT

Brillouin-scattering-based sensors are capable of measuring either the strain or the temperature along the length of an optical fiber in a distributed fashion through measurement of the Brillouin-frequency shift. The cross sensitivity of the frequency shift to these two parameters makes it impossible to differentiate between them by measurement of the frequency shift alone. We report on a new technique that permits the simultaneous measurement of strain and temperature to resolutions of +/-178 microepsilon and +/-3.9 degrees C at a spatial resolution of 3.5 m by incorporation of the Brillouin-loss peak power with the conventional Brillouin-frequency measurement.

Spatial resolution enhancement of a Brillouin-distributed sensor using a novel signal processing method

It is known that the ultimate spatial resolution for a Brillouin-based sensor is limited by the lifetime of the phonons in the fiber that mediate the Brillouin loss process. At optical pulse widths less than 10 ns (corresponding to one meter spatial resolution) the Brillouin line width is considerably broadened, causing a severe penalty in resolving the Brillouin frequency shift. Around 5 ns the Brillouin line width is too broad to allow an accurate frequency determination. The fiber optics group at the University of New Brunswick, Canada, has recently developed an automated system for strain measurements in a distributed sensing system that uses a novel signal processing technique to measure strain at resolutions finer than the Brillouin line width limit. Strain has been resolved to 20 /spl mu//spl epsiv/ at 500 mm and to 40 /spl mu//spl epsiv/ at 250 mm.

Structural monitoring by use of a Brillouin distributed sensor

The testing of a fiber-optic distributed-strain sensor attached to a simple structural member is reported. A Brillouin scattering-based sensor system was used to measure both tensile and compressive strains along the length of a cantilever beam subjected to various loads. The sensing fiber was attached to the beam in such a way that some sections experienced uniform strain, whereas others were subjected to a nonuniform strain distribution. A spatial resolution of 0.4 m was used, and a measurement precision of approximately +/-50 microepsilon was achieved.

Strain measurement in a concrete beam by use of the Brillouin-scattering-based distributed fiber sensor with single-mode fibers embedded in glass fiber reinforced polymer rods and bonded to steel reinforcing bars

The strain measurement of a 1.65-m reinforced concrete beam by use of a distributed fiber strain sensor with a 50-cm spatial resolution and 5-cm readout resolution is reported. The strain-measurement accuracy is +/-15 microepsilon (microm/m) according to the system calibration in the laboratory environment with non-uniform-distributed strain and +/-5 microepsilon with uniform strain distribution. The strain distribution has been measured for one-point and two-point loading patterns for optical fibers embedded in pultruded glass fiber reinforced polymer (GFRP) rods and those bonded to steel reinforcing bars. In the one-point loading case, the strain deviations are +/-7 and +/-15 microepsilon for fibers embedded in the GFRP rods and fibers bonded to steel reinforcing bars, respectively, whereas the strain deviation is +/-20 microepsilon for the two-point loading case.

Brillouin Scattering Based Distributed Sensors for Structural Applications

Over the past four years the Fibre Optics Group at the University of New Brunswick has been developing a distributed sensor for use in smart civil structures. By using Brillouin loss, the sensor is capable of measuring strain or temperature at any region on a sensing fibre, even those that are kilometers in length. This paper outlines the development of the sensor system, and discusses some of the experiments that have been performed with it. New results are presented that demonstrate 100 mm spatial resolution under laboratory conditions. This represents a four-fold improvement in the spatial resolution over previously reported results attained with a Brillouin scattering based distributed sensor.

Pulse width dependance of the Brillouin loss spectrum

Stimulated Brillouin scattering in optical fibers can be used to measure strain and temperature in a distributed manner. To improve the spatial resolution of these measurements, shorter pulses must be used, resulting in reduced signal strengths causing a degradation of strain and temperature resolution. This paper studies the dependance of the Brillouin loss spectrum on the pump pulse width. Theoretical and experimental results display the nonlinear variations in the Brillouin peak power and linewidth over a range of pulse widths from 10 to 4000 ns.

Automated system for distributed sensing

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.

Strain measurement of the steel beam with the distributed Brillouin scattering sensor

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.

Advances in distributed sensing using Brillouin scattering

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.