This work presents the use of longitudinal refractive index modulation (apodization) in photosensitive glass for improved sidelobe suppression in volume holographic optical elements. We develop a numerical model for both uniform and apodized volume holograms based on rigorous coupled-wave analysis. We validate the model by comparison with a transmissive 1.55-μm uniform volume grating in photothermorefractive glass. We then apply our numerical model to calculate the spectral response of apodized gratings. The numerical results demonstrate that apodization of the refractive index modulation envelope improves spectral selectivity and reduces the firstorder side-lobe peaks by up to 33 dB. We suggest a method for creating apodization in volume holograms with approximately Gaussian spatial refractive index profile. We Also computed the group delay for a beam with finite transverse width. The results show the delay between the paths increases as the diffraction angles increase.