A novel device is described which can be used as an optical fibre sensor. The device is formed by leading two of the three output ports of a 3×3 fused optical fibre directional coupler back to two of the three input ports to form a doubly resonant structure. This produces two rings, one of which can be used as the reference arm while the other can be used as the signal arm. The theory and some experimental results with the device are presented.