ABSTRACT

Optical time-domain reflectometry (OTDR) underpins essentially all of the distributed optical fibre sensor (DOFS) approaches. This chapter describes some of the basic techniques of OTDR; they are used in various combinations in the DOFS systems. The OTDR was initially used in optical telecommunications and fibre quality assurance to measure uniformity and to check the attenuation profile of optical fibres for installation defects or quality control. OTDRs provide information on the continuity of a fibre; i.e. they inform the user of the existence and location of any break or high point-loss. The double-ended technique allows the backscatter factor and the local attenuation to be determined independently of one another. Cross-sensitivity, whilst often overlooked in scientific papers, is a serious issue for practical application of the technology. The total noise follows the multiplied shot noise curve initially until that falls below the preamplifier noise line.