NOVELTY - The system has a pair of narrow linewidth optical sources (110, 120) e.g. semiconductor-based external cavity lasers, coherently locked with a predefined frequency offset with respect to each other, where the offset is in an order of a Brillouin frequency shift. A balanced heterodyne receiver receives an optical signal (115) generated by coherent mixing of a backscattered probe signal (145) with the shift and a local oscillation signal (125). The receiver produces an output indicative of one or both of a measured temperature and a measured strain.    USE - Integrated fiber-optic sensing system i.e. Brillouin frequency shift-based sensing system, for distributed temperature and strain sensing along length of an infrastructure e.g. oil pipe, oil well and gas distribution line, to be inspected in a commercial application, a defense application and scientific application such as oil or gas flow characterization, acoustic logging, structural integrity monitoring for terrestrial or under-sea installation, subsurface visualization for geothermal energy exploration and seismic monitoring.    ADVANTAGE - The system uses a low cost low-noise radio frequency (RF) electronics for the heterodyne receiver to efficiently detect low level amplitude of the backscattered probe signal with the Brillouin frequency shift. The system uses a spectrum range requiring lower bandwidth for an optical detector while allowing better signal to noise ratio (S/N) and improving detection accuracy, thus simplifying a heterodyne detection circuitry and a low-noise operation in a cost effective manner. The system is stable to operate.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for distributed temperature and strain sensing along length of an infrastructure to be inspected.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a Brillouin frequency shift-based sensing system.    Narrow linewidth optical sources (110, 120)    Optical signal (115)    Local oscillation signal (125)    Optical coupler (130)    Backscattered probe signal (145)