Photoreflectance (PR) and photoreflectance excitation (PRE) techniques are employed to investigate the optical properties of the GaAs_1-xBi_x(x = 0%–3.7%)/GaAs heterostructure grown by molecular beam epitaxy. The relations between broadenings and intensities of E₀ and E₀ + Δ_SO transitions in PR as a function of Bi content are disclosed. Both PR and PRE experimental techniques independently show that the oscillator strength of the E₀ + Δ_SO transition becomes larger compared to that of the fundamental gap as the Bi content increases. Such results suggest that the incorporation of Bi into GaAs leads to an increase of the effective masses for the spin-orbit split-off band.