Modulated transmission and reflectance measurements have been performed on a number of MOCVD grown structures of the typep+-x-n+wherexis either an undoped layer of GaAs, a GaAs doping superlattice, or a compensated layer of GaAs. The structures have been characterised byI/V,C/Vand photoluminescence measurements. For the doping superlattices, with individual layer thicknesses of between 18 and 70 nm and doping levels of 1018/cm3, the superposition of the externally applied field on the large built-in field allows very high internal fields to be applied over a limited region of the sample. In this way, fields of up to ∽500 KV/cm have been applied. For photon energies close to the bandgap of GaAs, the modulation properties of the structures are consistent with a Franz-Keldysh mechanism up to the highest internal electric fields, with no evidence for the anomalous behaviour reported elsewhere. At longer photon wavelengths, a dispersionless modulation in optical transmission is observed whose origin is related to contributions from the linear and quadratic electro-optic effects. In this regime, an anomalous dependence on optical polarisation is observed.