Locating disturbances in optical fibres

A method of determining a position of a disturbance to an optical fiber assembly that is optically coupled to a first monitoring unit and to a second monitoring unit, the method comprising:

• receiving, at the first monitoring unit, a first digital optical signal carried by the optical fiber assembly;
• monitoring a predetermined parameter of the first digital optical signal received at the first monitoring unit;
• receiving, at the second monitoring unit, a second digital optical signal carried by the optical fiber assembly;
• monitoring the predetermined parameter of the second digital optical signal received at the second monitoring unit;
• determining, using a first clock, first data representative of a first time, being a time at which a change occurs in the predetermined parameter of the first digital optical signal received at the first monitoring unit, the change arising from a disturbance to the optical fiber assembly;
• determining, using a second clock, second data representative of a second time, being a time at which a change occurs in the predetermined parameter of the second digital optical signal received at the second monitoring unit, the change arising from the disturbance;
• and
• processing the first data and the second data to determine a position of the disturbance.

A monitoring unit for use in determining a position of a disturbance to an optical fiber assembly, the monitoring unit comprising a light detector, a clock and an apparatus configured to monitor a predetermined parameter of digital optical signals received at the light detector and to generate analog or digital electrical signals representative of the predetermined parameter, the monitoring unit being configured to:

• receive a digital optical signal from an optical fiber assembly;
• monitor the predetermined parameter of the digital optical signal received at the light detector;
• and
• determine, using the clock, data representative of a time at which a change occurs in the predetermined parameter of the digital optical signal received at the light detector, the change arising from a disturbance to the optical fiber assembly and the data being suitable for processing to determine a position of the disturbance.