A sensor (10) and corresponding method for sensing variations in a parameter employ an optical device (12) defining two optical paths (14, 16) differentially affected by a variation in the parameter so as to change the differential phase between the two paths. This differential phase is monitored by a spectral interrogation arrangement (18) including a radiation input device (20) for delivering to the optical device (12) incident radiation at a plurality of wavelengths, and a reading arrangement (22) for measuring the interference-modulated optical output. The optical device (12) is configured so that the two optical paths have differing dispersion properties such that a difference between the phase accumulated by light propagating along the optical paths as a function of wavelength exhibits a maximum or minimum at some wavelength designated λcritical. The plurality of wavelengths employed by the spectral interrogation arrangement span a range of wavelengths including, or adjacent to, λcritical.