32-km distributed temperature sensor based on Brillouin loss in an optical fiber.

Among the hierarchy of sensors, the distributed fiberoptic temperature sensor is unique, as it offers continuous sensing over tens of kilometers with good temperature accuracy and high spatial resolution. Distributed temperature sensors (DTS's) that use Raman scattering as the sensing mechanism have already been reported.`-4 More recently, the variation with temperature of the Stokes frequency shift in Brillouin scattering has been proposed as a suitable mechanism for a DTS.5 A system based on this technique has been demonstrated by Kurashima et al. Those authors reported a temperature resolution of 30C combined with a spatial resolution of 100 m over 1.2 km of fiber for this technique, which they have termed Brillouin opticalfiber time-domain analysis (BOTDA). Higher spatial resolution and temperature accuracy combined with a significant increase in the sensing range have recently been reported6'7 with BOTDA. Our research7 has indicated that the maximum range when BOTDA is used at 1.3 4um (without optical amplifiers) is restricted to -20-25 km because of system noise (combined electronic and optical) and the input cw beam power limit, which is governed by the pulsed power depletion that occurs through the Brillouin interaction. Although at 22 km the sensing length of the DTS described in Ref. 7 is the largest yet reported to our knowledge, there are several applications for which a longer sensing length combined with higher spatial resolution is desirable. To this end we have explored the possibility of using Brillouin loss rather than Brillouin gain. We have found that significant improvements in the performance of DTS's based on Brillouin scattering are possible with this approach. In this Letter we describe a DTS system based on Brillouin loss that has demonstrated a temperature resolution of 10C with a spatial resolution of 5 m and a sensing length of more than 32 km. The system operates as follows: light from a tunable-frequency cw laser at a frequency VLi (the L in the subscript represents the Brillouin loss process) is launched into one end of the sensing fiber. The output from a pulsed laser at frequency VL2 is injected into the other end of the sensing fiber. When 'L1 = VL2 + VB ('B is Brillouin frequency), the counterpropagating beams interact through the Brillouin gain mechanism.8 The pulsed beam is amplified at the expense of the cw beam; hence the intensity of the cw beam will be reduced as a result of the Brillouin interaction. If the intensity of the cw beam emerging from the fiber is monitored following the launch of the pulsed beam, a decrease in the intensity will be observed whenever Brillouin loss occurs. The time delays between the launch of the pulsed beam and those regions where power is transferred from the cw beam correspond to round-trip times for light traveling to and from the regions of Brillouin loss. These times provide the positional information. If the laser frequency difference is adjusted, then the cw light will experience loss in parts of the fiber at a different temperature. Hence by slowly scanning one of the laser frequencies it is possible to map out the temperature distribution of the whole fiber. In our previous system7 the cw laser frequency was lower than that of the pulsed laser and therefore experienced gain. When depletion of the pulsed light can be neglected, the two approaches are equivalent for sensing purposes, giving signals that differ only in sign and not in magnitude. However, the Brillouin loss approach is superior when much of the fiber is at the same temperature, as may often be the case. In such a situation, when Brillouin gain is used, the pulsed beam is depleted by the interaction, resulting in a very weak signal from the end of the fiber most distant from the pulsed source. Conversely, with Brillouin loss the pulsed beam is increased as a result of the interaction, resulting in a much stronger signal from this end of the fiber and thus permitting the realization of longer sensing lengths. The experimental arrangement is illustrated in Fig. 1. Both lasers were solid-state cw diode-pumped Nd:YAG ring lasers emitting close to 1319 nm. The maximum launched power of the cw beam laser was 10 mW. The frequency of the laser could be adjusted by temperature tuning the cavity. A Bragg cell was used to provide short optical pulses ranging in time from 50 to 400 ns. The peak launched pulsed power from the first-order diffracted beam was -5 mW. This signal is monitored at photodetector DI and is also used to synchronize the start of the data storage. The zero-order beam was mixed with