A frequency-domain distributed temperature/strain sensor based on a longitudinally graded optical fiber (LGF) is proposed and evaluated. In an LGF, the Brillouin scattering frequency, ν B , changes (i.e., is chirped) lengthwise monotonically and thus every position along the fiber has a unique ν B . Any change locally (at some position) in the fiber environment will result in a measurable change in the shape of the Brillouin gain spectrum (BGS) near the frequency component mapped to that position. This is demonstrated via measurements and modeling for an LGF with local heating. The LGF is one with ∼100 MHz Brillouin frequency gradient over 16.7 m, with 1.1 and 1.7 m segments heated up to 40 K above ambient. A measurement of the BGS can enable the determination of a thermal (or strain) distribution along a sensor fiber, thus rendering the system one that is in the frequency domain. A sensitivity analysis is also presented for both coherent and pump-probe BGS measurement schemes. The modeling results suggest that the frequency-domain systems based on fibers with a chirped Brillouin frequency are highly suited as inexpensive event sensors (alarms) and have the potential to reach submeter position determination with sub-1-K temperature accuracies at >1 kHz sampling rates. Limitations to the technique are discussed. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
IntroductionThe Brillouin scattering frequency, ν B , is related to the optical wavelength, λ o , and the acoustic velocity, V, by ν B ¼ 2Vn mode ∕λ o . It is well known that both the acoustic velocity and the refractive index of a glass usually are functions of its thermomechanical environment; i.e., temperature and any applied stress or strain to the material. The word "usually" is used as a qualifier because it is possible to realize multicomponent materials of which physical properties, such as the refractive index, are immune to temperature or strain/stress. 1-4 However, in conventional pure silica or GeO 2 -doped silica core optical fibers, the Brillouin scattering frequency is dependent on both the temperature and strain, with the usual dependence being an increasing frequency with increasing temperature or strain. 5 Since the temperature and strain dependencies of the Brillouin frequency (typically ∼1.2 MHz∕K 6 and 500 MHz∕%, 7 respectively, near 1550 nm for conventional fiber 5 ) are not insignificant, it is natural that this phenomenon has been adapted for use in distributed sensor technologies, [8][9][10][11] with very impressive results. High-sensitivity and resolution measurement schemes and configurations have been demonstrated, 12-20 and a commercial market exists for such distributed sensing systems. Resolution is defined in this context to be the ability to resolve two events that are in close proximity to each other, or alternatively, to resolve a certain temperatu...
