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An in-line in-fibre ring cavity is fabricated by writing two blazed gratings in the one fibre to form a Fabry-Perot cavity. Interference is set up between an injected guided mode and a ring mode formed by light scattered by the gratings from the core mode to a reverse propagating ghost mode and then back into the original core mode again. The ability to measure the external refractive index and the variability of this response with a different cavity length is experimentally demonstrated. Within a given region of interest, a sensitivity to the surrounding refractive index as good as 9.5 x 10(-5) is established.