Patent ID: 7499177

Claim:
An apparatus for locating the position of an event, comprising: a light source; a waveguide for receiving light from the light source 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 being capable of having the counter-propagating optical signals or some characteristic of the signals modified or affected by an external parameter caused by or indicative of the event to provide modified counter-propagating optical signals which continue to propagate along the waveguide; detector means for detecting the modified counter-propagating optical signals affected by the parameter and for determining the time difference between the receipt of the modified counter-propagating optical signals in order to determine the location of the event; and a controller for controlling polarisation states of the counter-propagating optical signals so that the signals are amplitude and phase matched, wherein the waveguide comprises a first arm for receiving the counter-propagating signals, and a second arm for receiving the counter-propagating signals, the first and second arms forming a Mach Zehnder interferometer, and the first arm being of different length than the second arm so that the first and second arms have a length mismatch, the controller further comprising a dither signal producing element to wavelength dither the light from the light source.