A detection system for detecting a property of an optical sensing fiber as a function of position along the optical sensing fiber, the detection system comprises - a light source configured to produce a light pulse comprising a plurality of laser modes simultaneously at mutually different wavelengths, wherein the light source comprises a mode locked laser with modes spaced apart by a frequency distance Df; - an optical sensing fiber; - a coupler having a coupler input coupled to the light source, a coupler input-output coupled to the optical sensing fiber and a coupler output; - a splitter with a splitter input, coupled to the coupler output, and a first and second splitter output - a multi-way coupler, configured to produce combinations of light from a first and second input terminal of the multi-way coupler with respective different relative phase at a first, second and third output terminal, the first and second input terminal being coupled to the first and second output of the splitter, via a first and second optical path of different optical lengths respectively, a difference between optical lengths in the first and second optical paths being substantially an integer multiple of the speed of light divided by the frequency distance Df; - a first, second and third photo-detector (18a-c), coupled to the first, second and third output terminal of the multi-way coupler respectively; - a data processing system coupled to receive output signals from the first, second and third photo-detector (18a-c), the data processing system being configured to compute, from the output signals, relative phase values of light from the first and second paths for a series of time points with time delays from a time of transmission of the pulse that are smaller than a time needed for the light pulse to travel from the light source to an end of the optical sensing fiber and back to the first, second and third optical detectors (18a-c). - a light source configured to produce a light pulse comprising a plurality of laser modes simultaneously at mutually different wavelengths, wherein the light source comprises a mode locked laser with modes spaced apart by a frequency distance Df; - an optical sensing fiber; - a coupler having a coupler input coupled to the light source, a coupler input-output coupled to the optical sensing fiber and a coupler output; - a splitter with a splitter input, coupled to the coupler output, and a first and second splitter output - a multi-way coupler, configured to produce combinations of light from a first and second input terminal of the multi-way coupler with respective different relative phase at a first, second and third output terminal, the first and second input terminal being coupled to the first and second output of the splitter, via a first and second optical path of different optical lengths respectively, a difference between optical lengths in the first and second optical paths being substantially an integer multiple of the speed of light divided by the frequency distance Df; - a first, second and third photo-detector (18a-c), coupled to the first, second and third output terminal of the multi-way coupler respectively; - a data processing system coupled to receive output signals from the first, second and third photo-detector (18a-c), the data processing system being configured to compute, from the output signals, relative phase values of light from the first and second paths for a series of time points with time delays from a time of transmission of the pulse that are smaller than a time needed for the light pulse to travel from the light source to an end of the optical sensing fiber and back to the first, second and third optical detectors (18a-c).