A method for locating an event in an optical network, in which the method comprises: receiving from photodetectors, signalling indicative of the length of a first time period, in which the first time period extends between: a first change of state of a first photodetector between a first state in which the first photodetector detects light received through a first optical fibre and a second state in which the first photodetector detects loss of light received through the first optical fibre; and a second change of state of a second photodetector between a third state in which the second photodetector detects light received through a second optical fibre and a fourth state in which the second photodetector detects loss of light received through the second optical fibre; in which the first optical fibre has a first latency and in which the second optical fibre has a second latency; in which the first optical fibre and the second optical fibre extend along a common path through the optical network; in which a first end of the path is defined by the photodetectors and in which a second end of the path is defined by a remote location in the optical network; and in which the method comprises by dividing the length of the first time period by the difference between the first latency and the second latency, deriving an indication of the distance from the photodetectors along the path to the location of the event resulting in the losses of light, wherein the losses of light occur effectively simultaneously in the first and second optical fibres at the location of the event, and wherein the first and second optical fibres share a common path along a protective guide. receiving from photodetectors, signalling indicative of the length of a first time period, in which the first time period extends between: a first change of state of a first photodetector between a first state in which the first photodetector detects light received through a first optical fibre and a second state in which the first photodetector detects loss of light received through the first optical fibre; and a second change of state of a second photodetector between a third state in which the second photodetector detects light received through a second optical fibre and a fourth state in which the second photodetector detects loss of light received through the second optical fibre; a first change of state of a first photodetector between a first state in which the first photodetector detects light received through a first optical fibre and a second state in which the first photodetector detects loss of light received through the first optical fibre; and a second change of state of a second photodetector between a third state in which the second photodetector detects light received through a second optical fibre and a fourth state in which the second photodetector detects loss of light received through the second optical fibre; in which the first optical fibre has a first latency and in which the second optical fibre has a second latency; in which the first optical fibre and the second optical fibre extend along a common path through the optical network; in which a first end of the path is defined by the photodetectors and in which a second end of the path is defined by a remote location in the optical network; and in which the method comprises by dividing the length of the first time period by the difference between the first latency and the second latency, deriving an indication of the distance from the photodetectors along the path to the location of the event resulting in the losses of light, wherein the losses of light occur effectively simultaneously in the first and second optical fibres at the location of the event, and wherein the first and second optical fibres share a common path along a protective guide.