Combined distributed temperature and strain sensor based on Brillouin loss in an optical fiber.

Long-range distributed optical fiber sensors1-5 exploit the interaction between laser light and the fundamental energy transport mechanisms in the scattering medium. Here we report on the use of Brillouin loss for a combined distributed temperature and strain sensor. Brillouin lossbased distributed sensors exploit the Brillouin interaction between pulsed and cw optical beams counterpropagating in an optical fiber. When the optical frequency of the cw beam is greater than that of the pulsed beam by an amount equal to the Brillouin frequency shift VB at some point in the fiber, the pulsed beam is amplified through the Brillouin interaction, and the cw beam experiences loss. Because distributed sensors based on Brillouin scattering are sensitive to both temperature and strain, unless special precautions are taken when the fiber is deployed it will not be possible to determine whether the system is uniquely sensing a variation in temperature or strain. In a practical system it is virtually impossible to deploy the fiber such that it is not sensitive to temperature variation; however, it is feasible to install the fiber such that the effects of variation in the strain are minimized. Here we report the use of this approach to recover both measurands. Sections of the fiber were deployed such that half of its sensing length was subject to the influence of variations in both strain and temperature, whereas the other half was isolated from the effects of strain and used to determine the temperature variations of the first half of the fiber. The experimental arrangement is illustrated in Fig. 1. Both lasers were solid-state cw diodepumped Nd:YAG ring lasers emitting close to 1319 nm. The maximum launched power of the cw laser was -10 mW. We could adjust the frequency of the laser by temperature tuning the cavity. An acousto-optic modulator was used to provide short optical pulses ranging in time from 50 to 500 ns. The peak launched pulsed power from the firstorder diffracted beam was -5 mW. This signal was monitored at photodetector Dl and was also used to synchronize the start of the data storage. The zero-order beam was mixed with light from the cw beam laser by directional coupler DC1 and detected with a fast (20-GHz) detector, the resultant beat frequency being monitored with a microwave spectrum analyzer. Most of the pulsed power was launched into one end of the 22-km sensing fiber by a 90:10 coupler, DC2, whereas the cw beam was injected into the other end of the fiber by a variable ratio coupler, DC3. The cw light emerging from the sensing fiber was detected with a low-noise photoreceiver with a 125-MHz bandwidth, D2, and monitored with a storage oscilloscope. Most of the sensing fiber (22 km) remained on the original spools as supplied by the manufacturer and was subject to a constant but lowlevel tension. Two separate sections of the sensing fiber were placed in thermally insulated chambers. Chamber 1 contained 300 m of fiber divided into three separate 100-m lengths that were subject to different strain levels. The first 100-m fiber length was wrapped around fiber stretching unit 1, which contained two quartz cylinders, 1 and 2 (diameters 5.8 cm), separated by 0.5 m. Cylinder 1 was fixed to a translation stage, so that its position could be changed to permit stretching of this fiber section. The movement of cylinder 1 was determined with a mechanical dial gauge with 1-Am resolution. The fiber was also glued to the quartz cylinders to ensure