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We present a study of the effect of the active cavity layer thickness variations on the operating characteristics of normally on low voltage high performance asymmetric Fabry–Perot modulators. For a modulator consisting of 25.5 periods of 100 Å GaAs quantum wells confined by 45 Å (GaAs/AlAs) short period superlattices with 5 pairs and 20.5 pairs of top and bottom quarter‐wave stacks, respectively, and assuming only layer thickness variation caused by Ga flux nonuniformity, the shift of the Fabry–Perot mode wavelength with respect to the fractional change of GaAs thickness inside the active cavity is ∼6 times that of the quantum well heavy hole exciton. This affects the relative distance between the wavelengths of the quantum well exciton and the Fabry–Perot resonance, and hence the performance of the modulators. Also, the tolerable percentage change of the Fabry–Perot mode wavelength should be less than 0.13% in order that such modulator arrays have at least 10:1 contrast ratios at a fixed optimum operating wavelength. This defines the epitaxial growth tolerance and precision with which we can obtain a desired operating wavelength, and the uniformity requirement on the two‐dimensional arrays of such kind of modulators.