An apparatus for locating the position of an event, comprising: a light source; a waveguide (10, 12, 13) adapted to receive light from the light source from a first end of the waveguide and a second end of the waveguide so that the light is caused to propagate in both directions along the waveguide to thereby provide counter-propagating optical signals in the waveguide, the waveguide (10, 12, 13) being capable of having the counter-propagating optical signals modified by the event to provide modified counter-propagating optical signals which continue to propagate along the waveguide, the waveguide comprising a first arm adapted to receive each of the counter-propagating signals and a second arm adapted to receive each of the counter-propagating signals, the first and second arms (14, 15) forming a Mach Zehnder interferometer; and detector means (40 Det1 50 Det2)adapted to detect the modified counter-propagating optical signals and adapted to determine the time difference between the receipt of the modified counter-propagating optical signals in order to determine the location of the event; characterised in that: there is a controller (20, PCcw43, PCccw44) adapted to control polarisation states of the counter-propagating optical signals so that the signals are amplitude and phase matched, the first arm of the Mach Zehnder is of different length than the second arm so that the first and second arms have a length mismatch, and the controller comprises a dither signal producing element to wavelength dither the light from the light source by an amount which leads to dithering of a phase difference between the Mach Zehnder arms (14, 15) by at least 360° to produce artificial fringes so that with a drifting operating point the Mach Zehnder counter-propagating outputs always display their true fringe visibility whereby the controller acts to maintain both phase matching and fringe visibility. a light source; a waveguide (10, 12, 13) adapted to receive light from the light source from a first end of the waveguide and a second end of the waveguide so that the light is caused to propagate in both directions along the waveguide to thereby provide counter-propagating optical signals in the waveguide, the waveguide (10, 12, 13) being capable of having the counter-propagating optical signals modified by the event to provide modified counter-propagating optical signals which continue to propagate along the waveguide, the waveguide comprising a first arm adapted to receive each of the counter-propagating signals and a second arm adapted to receive each of the counter-propagating signals, the first and second arms (14, 15) forming a Mach Zehnder interferometer; and detector means (40 Det1 50 Det2)adapted to detect the modified counter-propagating optical signals and adapted to determine the time difference between the receipt of the modified counter-propagating optical signals in order to determine the location of the event; there is a controller (20, PCcw43, PCccw44) adapted to control polarisation states of the counter-propagating optical signals so that the signals are amplitude and phase matched, the first arm of the Mach Zehnder is of different length than the second arm so that the first and second arms have a length mismatch, and the controller comprises a dither signal producing element to wavelength dither the light from the light source by an amount which leads to dithering of a phase difference between the Mach Zehnder arms (14, 15) by at least 360° to produce artificial fringes so that with a drifting operating point the Mach Zehnder counter-propagating outputs always display their true fringe visibility whereby the controller acts to maintain both phase matching and fringe visibility.