1. INTRODUCTION
The Brillouin backscattering process in a fiber has been widely investigated as a basis for measuring distributed strain and temperature since the Brillouin frequency shift has a linear dependence on both strain and temperature. A Brillouin optical time-domain reflectometer (BOTDR) [1] enables us to perform measurements by accessing only one end of the sensing fiber. However, the spatial resolution of the conventional BOTDR system is 1m at best [2]. This is because there is a trade-off between spatial resolution and Brillouin frequency shift resolution when measuring with the conventional BOTDR system [3]. The spatial resolution can be improved by using short pulses, but the frequency spectrum of the Brillouin backscattered signal broadens rapidly as the pulse width decreases below 10 ns, which corresponds to a 1 m spatial resolution. This makes it difficult to measure the Brillouin frequency shift accurately and limits the spatial resolution to 1 m. To overcome this limitation, we have proposed a novel BOTDR system, called a double-pulse BOTDR (DP-BOTDR) system, and successfully achieved 20-cm spatial resolution [4], [5] on a laboratory bench.