Methods of and systems for optical fiber sensing

An optical fiber sensing system, which comprises:

• a first optical fiber which carries routinely communications traffic signals;
• a second optical fiber which carries routinely communications traffic signals;
• a pair of wavelength division multiplexer splitters which are spaced along said first optical fiber;
• a pair of wavelength division multiplexer splitters which are spaced along said second optical fiber;
• a source of optical power which is adapted to provide signals of a wavelength which differs from the wavelengths of those communication traffic signals which are carried routinely on said first and second optical fibers;
• an optical fiber splitter which is connected to one of the wavelength division multiplexer splitters along said first optical fiber, to one of said wavelength division multiplexers along said second optical fiber, and to said source of optical power, said optical fiber splitter being effective to receive incoming signals from said source to split each incoming signal into two subsignals which are provided as outputs and which have a phase relationship;
• means for connecting the other wavelength division multiplexer splitter along the first optical fiber to the other wavelength division multiplexer along the second optical fiber to provide a loop which includes said optical fiber splitter, said wavelength division multiplexer splitters and portions of said first and second optical fibers, said loop being effective to receive said subsignals of each said signal and to provide a common path for said subsignals to be moved in opposite directions around said loop and return to said optical fiber splitter which is effective to recombine said subsignals;
• and
• a detector connected to said optical fiber splitter for detecting changes in the phase relationship between the two subsignals which are routed around said loop in opposite directions.

A method of sensing disturbances of an optical fiber system, said method comprising the steps of:

• providing a source of optical signals;
• splitting each optical signal into two subsignals;
• directing the two subsignals in opposite directions around a common loop path and returning the two subsignals to entry points of said path;
• recombining the two subsignals after the two subsignals have moved about the common path and returned to the entry points;
• and
• detecting changes in the phase relationships as between successive pairs of the split subsignals to indicate a disturbance of the optical fiber system.

A method of sensing disturbances of an optical fiber system, said method comprising the steps of:

• providing a first optical fiber which carries communications traffic signals;
• providing a second optical fiber which carries communications traffic signals;
• providing a pair of wavelength division multiplexers which are spaced along the first optical fiber;
• providing a pair of wavelength division multiplexers which are spaced along the second optical fiber;
• providing a source of optical signals;
• splitting each optical signal into two subsignals which have a phase relationship;
• establishing a common loop path for the two subsignals which includes that portion of the first optical fiber between the two wavelength division multiplexers which are spaced along the first optical fiber and that portion of the second optical fiber between the two wavelength division multiplexers which are spaced along the second optical fiber;
• directing the two subsignals in opposite directions around the common loop path;
• recombining each set of two subsignals after each set of two subsignals has moved in opposite directions around the common loop path;
• and
• detecting changes in the phase relationship as between successive sets of split subsignals to indicate disturbance of the optical fiber system.