Novel type brillouin optical time-domain reflectometry for measuring distributed strain and temperature with sub-meter spatial resolution in km-long fiber

A novel Brillouin optical time-domain reflectometry (BOTDR) system, called a double-pulse BOTDR (DP-BOTDR) system, is proposed for measuring distributed strain and temperature in a fiber with a sub-meter spatial resolution. The DP-BOTDR system transmits double-pulsed lights instead of conventional single-pulsed lights into the fiber, and the received Brillouin backscattered signals are passed through a baseband filter matched to the double-pulsed light to cause interference between the backscattered signals of the front and rear pulses. This interference produces broad but oscillatory frequency spectra for the Brillouin backscattered signals that make it possible to measure the Brillouin frequency shift accurately despite the narrow pulse width. Our experiment confirmed that the DP-BOTDR system enables us to measure the distributed Brillouin frequency shift, i.e. the distributed strain and temperature, with a spatial resolution of 20 cm. This spatial resolution is five times better than that provided by the conventional single-pulse BOTDR system.