This invention relates to optical sensing systems and relates more specifically but not exclusively to reflectometric sensing systems in which light signals are arranged to be transmitted along an optical sensor array as may be provided by an array of optical fibre sensing elements defined by a plurality of partially-reflective discontinuities (e.g. reflective optical fibre and splices) so that small proportions of the light signals propagating along the sensor array are reflected back along the array and interfere with the light reflected from preceding optical sensors of the array. However the same result may be achieved by having partially transmissive couplers for coupling the signal to a return fibre. Changes in length or deformation of optical fibre sensing elements located between discontinuities due, for example, to the impingement thereon of acoustic pressure waves in the case of hydrophones, produce phase modulation of the propagating light signals and such phase modulation detected in the reflected or redirected signals can be utilised for determining the incident acoustic pressure etc.
Some interferometric sensing systems have hitherto utilised wavelength division multiplexing techniques in which light signals of different wavelengths are transmitted along the optical fibre sensor array but such techniques are effectively ruled out for large arrays of interferometric sensors because of the requirement for a large number of coherent light sources of slightly different wavelengths and of associated narrow band optical couplers and other requisite optical components.
Other interferometric sensing systems have utilised time division multiplexing techniques but in this case the number of sensing elements in the optical fibre sensor is limited by the optical loss suffered by light signals as they propaqate along the sensor array and also by the sampling rate of the reflected light signals which is directly related to the number of sensing elements.
The present invention is directed to an optical sensing system which combines the techniques of wavelength division multiplexing and time division muiplexing in order to increase the multiplexing gain and enhance the overall system performance.