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.
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.
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.
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.
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.
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