The first low voltage, substrate access, reflective InGaAs/InP multi-quantum well modulator exploiting the enhanced performance obtainable through the use of an asymmetric Fabry-Perot cavity around the quantum well absorbing region is reported. This device utilises a low reflectivity AlInGaAs/InP multi-layer mirror (R~35%) on the substrate side of the quantum well region, and a high reflectivity metal mirror (R~95%) on the epitaxial side of the cavity. Devices have been fabricated which exhibit a reflectivity change of >30% and contrast ratio of 3 dB at 5 V bias, with a 1.8 dB insertion loss.