We report the observation of Franz-Keldysh oscillations (FKO) with a modulation technique other than electromodulation. Three GaAs samples with well-controlled, uniform built-in electric fields were used for this study. Room-temperature piezoreflectance and photoreflectance measurements were taken under identical experimental conditions. Both types of spectra exhibited distinct FKO and the obtained electric fields are in excellent agreement. While FKO have been seen in electromodulation spectra for both uniform and nonuniform electric fields, we show that a uniform field provides the best condition for the observation of FKO in other modulation methods. © 1991 The American Physical Society.