A method of measuring a selected physical parameter at a location within a
region of interest comprises the steps of: launching optical pulses at a plurality of preselected interrogation wavelengths into an
optical fiber (1) deployed along the
region of interest, reflectors (20, 21, 2n) being arrayed along the
optical fiber (1) to form an array (9) of sensor elements, the
optical path length between the said reflectors (2) being dependent upon the selected parameter; detecting the returned optical interference
signal for each of the preselected wavelengths; determining from the optical interference
signal the absolute
optical path length (L) between two reflectors (2) at the said location; and determining from the absolute
optical path length (L) the value of the selected parameter at the said location; wherein the step of determining the absolute
optical path length (L) comprises carrying out a process in which the
phase difference between the interference signals for a pair of the preselected wavelengths is estimated using an estimated value for the
optical path length (L), the estimated
phase difference is used to estimate the phase at each of those wavelengths, and the phase thus obtained is used to revise the estimated value for the
optical path length (L), the process being repeated for some or all remaining
wavelength pairs in sequence, on the basis of the optical
path length (L) estimated for the immediately preceding pair in the sequence, thereby to progressively revise the optical
path length (L) until it is know to a desired level of accuracy.