Abstract: In order to identify a fiber optic cable (10) a beam (14) of polarised light is caused to pass down the cable to a first site (A) at which an electromagnetic field (24) is applied to the cable (10). The electromagnetic field (24) traverses the cable (10) in an essentially transverse direction and has a time-varying component orientated along the length of the cable (10) at the first site (A), with the component varying so that the line integral thereof along the cable (10) is non-zero. This results in a variation in the polarisation of the light, which can then be detected by a polarisation discriminator (20) at a second site (B), thereby to identify the cable (10).

Abstract: In order to identify a fiber optic cable (10) a beam (14) of polarised light is caused to pass down the cable to a first site (A) at which an electromagnetic field (24) is applied to the cable (10). The electromagnetic field (24) traverses the cable (10) in an essentially transverse direction and has a time-varying component orientated along the length of the cable (10) at the first site (A), with the component varying so that the line integral thereof along the cable (10) is non-zero. This results in a variation in the polarisation of the light, which can the be detected by a polarisation discriminator (20) at a second site (B), thereby to identify the cable (10).

Abstract: A Sagnac or loop type interferometer is disclosed which uses a single broadband Erbium-doped fibre source (10) and a single IngaAs detector (150) together with a 40 km long sensor loop (90). A wavelength division multiplexer (50) spectrally slices the broadband light from the source (10) into two sub-bands, with different optical paths being defined between the source (10) and detector (150) for the light in the different spectrally sliced sub-bands. The two optical paths include separate phase modulators (70, 130) which modulate the two signals at different frequencies, and also separate delay loops (60, 130) at different places relative to the sensor loop (90). Effectively, two separate Sagnac loops are provided with a single sensor loop (90), source (10) and detector (150). Standard phase locked loop techniques can be used to extract information from signals that have passed through the two loops and to determine the location of a mechanical or thermal perturbation applied to the sensor loop (90).

Abstract: In order to identify a fiber optic cable (10) a beam (14) of polarized light is caused to pass down the cable to a first site (A) at which an electromagnetic field (24) is applied to the cable (10). The electromagnetic field (24) traverses the cable (10) in an essentially transverse direction and has a time-varying component orientated along the length of the cable (10) at the first site (A), with the component varying so that the line integral thereof along the cable (10) is non-zero. This results in a variation in the polarization of the light, which can then be detected by a polarization discriminator (20) at a second site (B), thereby to identify the cable (10).