a light source;

a dither circuit for modulating or dithering the wavelength of the light produced from the 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;

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, wherein a path length mismatch exists between the first and second arms of the Mach Zehnder interferometer;

a detector 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;

polarisation controllers for setting the polarisation states of the counter-propagating optical signals; and

a main controller for controlling the polarisation controllers so that the output signals are phase matched;

the main controller comprising:

(i) a fringe visibility monitor having:

(a) a circuit for determining a maximum and minimum value of each counter-propagating optical signal after the signals have passed along the first and second arms in one direction and recombined and interfered to create fringes, and after the signals have passed along the first and second arms in the opposite direction and recombined and interfered to produce fringes; and

(b) a circuit for determining the difference between the counter-propagating optical signals; and

(ii) a processor for determining fringe visibility of the fringes by the ratio (max−min)/(max+min) wherein max is the maximum value and min is the minimum value, the processor having a memory for storing polarisation states corresponding to each fringe visibility and determining those which provide maximum fringe visibilities, the processor also being for determining phase matching of the counter-propagating output signals by determining at least one pair of the stored polarisation states providing the maximum fringe visibilities and which correspond to a minimum difference between the counter-propagating optical output signals; and

the processor being for setting the polarisation controllers to at least one pair of optimum polarisation states that achieve phase matched output signals.