We present a theory and conceptual examples for fibre-optic deformation sensing based on phase changes of transmitted light. As a first result, we establish an exact relation between observable phase changes and the deformation tensor along the fibre. It introduces the delay-line method for measuring phase noise and explains its advantages and. Abstract Optical communication systems have evolved over the years from simple intensity modulation and direct detection systems to those involving modulation of amplitude, phase, polarization and transverse modal pro-file.