The photoreflectance (PR) spectroscopy of &dgr;‐doped or similar structured sample has been observed to exhibit many Franz–Keldysh oscillations (FKOs). The beats are shown in the FKOs and they are attributed to the different frequencies of the FKOs of the transitions from the heavy and light holes. We have applied the fast Fourier transform on the PR spectra to separate the contributions from the heavy and light holes. The electric fields in the undoped layer can be calculated from the frequencies of heavy and light holes, respectively. The result of the heavy hole agrees well with that from the conventional FKOs fittings if the reduced mass=0.055m0is used in the conventional fittings. ©1995 American Institute of Physics.