Locating disturbances in optical fibres

A monitoring unit for use in a method of determining a position of a disturbance to an optical fiber assembly, the monitoring unit comprising a light detector, a clock, and state-of-polarization-measuring apparatus configured to measure a state of polarization of a digital optical signal received at the light detector, the monitoring unit being configured to:

• receive a digital optical signal from an optical fiber assembly;
• monitor a state of polarization 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 state of polarization 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, wherein the data encodes a time derivative of the state of polarization for the digital optical signal.

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 state of polarization 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 a state of polarization 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 state of polarization of the first digital optical signal received at the first monitoring unit, the change arising from a disturbance to the optical fiber assembly, wherein the first data encodes a time derivative of the state of polarization for the first digital optical signal;
• determining, using a second clock, second data representative of a second time, being a time at which a change occurs in the state of polarization of the second digital optical signal received at the second monitoring unit, the change arising from the disturbance, wherein the second data encodes a time derivative of the state of polarization for the second digital optical signal;
• and
• processing the first data and the second data to determine a position of the disturbance.

An apparatus for determining a position of a disturbance to an optical fiber assembly, the apparatus comprising:

• a first monitoring unit, comprising a first light detector, a first clock, and first state-of-polarization-measuring apparatus configured to measure a state of polarization of a digital optical signal received at the first light detector;
• a second monitoring unit, comprising a second light detector, a second clock, and second state-of-polarization-measuring apparatus configured to measure a state of polarization of a digital optical signal received at the second light detector;
• and
• a processing system,
• wherein the apparatus is configured to:
• transmit a series of one or more time-stamped synchronization packets, generated using a clock signal provided by the first clock, from the second monitoring unit to the first monitoring unit, embedded within a first digital optical signal;
• receive, at the first monitoring unit, the first digital optical signal from an optical fiber assembly;
• receive, at the second monitoring unit, a second digital optical signal from the optical fiber assembly;
• use the series of one or more time-stamped synchronization packets to synchronize the first clock and the second clock;
• monitor a state of polarization of the first digital optical signal received at the first light detector;
• determine, using the first clock, first data representative of a first time, being a time at which a change occurs in the state of polarization of the first digital optical signal received at the first light detector, the change arising from a disturbance to the optical fiber assembly;
• monitor a state of polarization of the second digital optical signal received at the second light detector;
• determine, using the second clock, second data representative of a second time, being a time at which a change occurs in the state of polarization of the second digital optical signal received at the second light detector, the change arising from the disturbance;
• and
• process the first data and the second data to determine a position of the disturbance.

An apparatus for determining a position of a disturbance to an optical fiber assembly, the apparatus comprising:

• a first monitoring unit, comprising a first light detector, a first clock, and first state-of-polarization-measuring apparatus configured to measure a state of polarization of a digital optical signal received at the first light detector;
• a second monitoring unit, comprising a second light detector, a second clock, and second state-of-polarization-measuring apparatus configured to measure a state of polarization of a digital optical signal received at the second light detector;
• and
• a processing system,
• wherein the apparatus is configured to:
• receive, at the first monitoring unit, a first digital optical signal from an optical fiber assembly;
• receive, at the second monitoring unit, a second digital optical signal from the optical fiber assembly;
• monitor a state of polarization of the first digital optical signal received at the first light detector;
• determine, using the first clock, first data representative of a first time, being a time at which a change occurs in the state of polarization of the first digital optical signal received at the first light detector, the change arising from a disturbance to the optical fiber assembly;
• monitor a state of polarization of the second digital optical signal received at the second light detector;
• determine, using the second clock, second data representative of a second time, being a time at which a change occurs in the state of polarization of the second digital optical signal received at the second light detector, the change arising from the disturbance;
• and
• process the first data and the second data to determine a position of the disturbance, and wherein the apparatus is configured to monitor a further parameter of one or both of the first and second digital optical signals, received at the respective light detectors, in addition to monitoring the state of polarization, wherein the further parameter is amplitude, or phase, or signal-to-noise ratio, or bit error ratio, and wherein the apparatus is configured to process data representative of the further parameter, for one or both of the first and second digital optical signals, when determining the position of the disturbance.