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A low-cost monitoring system for measuring the optical power losses of drop fibres in wavelength division multiplexing (WDM) passive optical networks (PON) is reported. The proposed system is based on radio-frequency intensity modulation and fibre Bragg grating self-referencing measurement techniques using amplitude normalisation. The monitoring unit is built with an optical broadband light source illuminating the PON from the central office for testing purposes, and the signal processing of the measurements is provided by the low-frequency analogue filters and by a lock-in amplifier. The system allows measuring the optical power losses of the PON branches independently, with increased sensitivity in a re-configurable and flexible way. The monitoring technique has been studied and experimentally validated in a coarse WDM-PON prototype, as a first approach to dense WDM-PON applications. The merits of the proposed monitoring system with regards to other WDM-PON monitoring techniques and the scalability of the proposed system to dense WDM-PON architectures are also discussed.