Kellie Brown

Dark-Pulse Brillouin Optical Time-Domain Sensor With 20-mm Spatial Resolution

Brillouin scattering-based distributed fiber-optic sensing is a powerful measurement tool that uses the inelastic scattering of incident light by an acoustic wave (phonon) to determine strain and/or temperature conditions of the fiber. Since the original Brillouin-time-domain-analysis (BOTDA) technique was proposed, several other analysis methods have been introduced to improve sensing performance in four key areas: spatial resolution; measurement accuracy; total sensing length; and measurement-acquisition time. The four factors are generally interrelated and improvements to one factor often come at the cost of one or more of the others. For example, one system might sacrifice spatial resolution for total sensing length, while another might sacrifice accuracy to gain acquisition speed. We present a BOTDA system based on dark-pulse scattering that provides improved resolution, accuracy, and acquisition time over conventional BOTDA systems, without the severe limitations on sensing length often imposed by other high-resolution techniques. Theoretical validation of the method is given, and experimental results are presented that demonstrate 20-mm resolution strain measurements with an accuracy of plusmn20 muepsiv, which is the highest spatial resolution yet reported for a BOTDA system

Distributed sensor based on dark-pulse Brillouin scattering

A novel dark-pulse-based technique has been used for the first time in a Brillouin scattering-based distributed fiber sensor. Experimentally obtained Brillouin spectra demonstrate that the dark-pulse configuration is as capable of strain and temperature measurement as conventional pulse-based systems but at much higher spatial resolution. A spatial resolution of 50 mm is reported with a strain measurement accuracy of 6 /spl mu//spl epsiv/ on a 100-m sensing fiber.

Stimulated Brillouin scattering modeled through a finite difference time domain approach

Stimulated Brillouin scattering (SBS), in an optical fiber, is a three-wave interaction (3WI) resulting from a coupling between light and acoustic waves. In a fiber optic sensing context, SBS results from the interaction between counterpropagating pulsed and continuous fields. We formulate a solution to the time dependant, one dimensional 3WI model in a SBS based fiber optic sensor. It is shown that a low complexity, first order finite difference time domain (FDTD) solution is capable of accurately modelling the dynamics of SBS with little computational effort. A modification to the first order scheme is proposed to combat numerical damping and dispersion, brought on by the low order of the solution. Examples are presented, validating the performance of our modelling technique. The effect of pulse power and risetime on the resulting scattering is examined, along with the effects of γa, an intrinsic fibre parameter related to the linewidth of the Brillouin spectrum. The spatial and temporal evolution of the acoustic field is illustrated; the effect of the steady state value of this field on the 3WI is investigated. The steady state acoustic field strength is related to the extinction ratio of the pulsed source, and it is found that this parameter has a significant influence on the scattering. This type of modelling provides a rapid means of investigating SBS as a tool in fiber optic sensing.

Optical fibers for the application of a fiber radiation sensor

We have measured radiation ((gamma) ray) induced loss of P- doped and Ge-doped fibers with different dopant concentrations and core diameters for different dose rates. The following conclusions can be obtained from our experiment: (1) The fibers we tested (P and Ge doped) have no evident recovery after (gamma) ray radiation. (2) No evident photo-bleaching effect had been observed by irradiating the fiber with diode laser of wavelength 630 micrometer. (3) Optical fiber with higher dopant concentration does not mean more sensitive. (4) For fiber length as short as 0.3 m, the sensitivity of P-doped fiber (0.6 - 0.8 dB/m for total dose of 4 Gy) is sufficient for the measurement of dose range for patients.

Analysis of Brillouin Scattering Based Fiber Optic Sensor Bonding Effects

Brillouin scattering based sensors can measure strain and/or temperature at all points along an optical fiber; first, the fiber must be calibrated. A problem arises because the temperature coefficient of a fiber bonded to a host is changed by the host structure. Using the loose fiber coefficient could lead to large strain and temperature measurement errors. This article studies the change in the Brillouin temperature coefficient due to bonding. The intrinsic Brillouin temperature coefficient of two fibers is also found. This coefficient allows accurate strain and temperature measurements to be made, assuming the coefficient of thermal expansion of the host is known.

Combined Raman and Brillouin scattering sensor for simultaneous high-resolution measurement of temperature and strain

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.

High-resolution distributed sensor using dark Brillouin scattering

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 techniques efficacy in two practical applications.

Optical fiber characterization for optimization of a Brillouin-scattering-based fiber optic sensor

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.

Characterization of Fibers in an Existing Network for High Speed System (10Gb/s or Greater) Compatibility

With OC-192 communications systems being commercially available and higher bit rate systems in development, prudent telecommunications network administrators are testing their installed fibers to determine if they can be successfully used at 10 Gb/s and higher. Together with New Brunswick Telephone (NBTel), the Fiber Optics Group at UNB have tested various installed fibers and cables in the NBTel network for their losses at wavelengths of 1244, 1310, 1550, and 1625 nm, as well as for strain and polarization mode dispersion (PMD). Weather conditions, age, place of installation, and cable types have also been considered. Aging does not seem to affect the performance of the fibers. Although most fibers are high-speed system compatible when looking at attenuation measurements, about 40% of the fibers tested would not meet the 10 Gb/s OC-192 system manufacturers design guidelines concerning PMD.With OC-192 communications systems being commercially available and higher bit rate systems in development, prudent telecommunications network administrators are testing their installed fibers to determine if they can be successfully used at 10 Gb/s and higher. Together with New Brunswick Telephone (NBTel), the Fiber Optics Group at UNB have tested various installed fibers and cables in the NBTel network for their losses at wavelengths of 1244, 1310, 1550, and 1625 nm, as well as for strain and polarization mode dispersion (PMD). Weather conditions, age, place of installation, and cable types have also been considered. Aging does not seem to affect the performance of the fibers. Although most fibers are high-speed system compatible when looking at attenuation measurements, about 40% of the fibers tested would not meet the 10 Gb/s OC-192 system manufacturers design guidelines concerning PMD.

Testing of fibers in an existing network for high-speed system (10 Gb/s or greater) compatibility

With OC-192 communications systems now being commercially available and higher bit rate systems in development, prudent telecommunications network administrators are testing their installed fiber to determine if it can be successfully used at 10 Gb/s and higher. Together with NBTe1, we have tested various installed fibers and cables for their losses at wavelengths of 1244, 1310, 1550 and 1625 nm, as well as for strain and polarization mode dispersion (PMD). Aging effects on different fiber types have been studied. Experimental data has been analyzed to determine the relationships, if any, between loss, strain, weather, fiber age, fiber type and PMD. The suitability of these fibers for high speed systems has been analyzed. Some of the results of this analysis are presented.