We have investigated optical transitions and atomically controlled interface structure in modulated period (GaP)m/(AlP)n (m, n number of monolayers) superlattices (SLs) using low-temperature photoluminescence (PL) and Raman scattering (RS) techniques. The modulated superlattices (GaP)m1(AlP)n1(GaP)m2(AlP)n2 were grown on Si-doped GaAs (0 0 1) substrate by gas source molecular beam epitaxy (m=m1+m2,n=n1+n2,m1>m2,n1>n2), where total number of periods of GaP and AlP are constants, m=13 and n=7. By modulating the internal structure of the superlattice period strong enhancement in PL intensity was observed. In the modulated SLs, with reducing GaP layer (m1) thickness, the PL peak shows blue shift and splitting accompanied with a large intensity enhancement. We attribute enhanced strength of the optical transition to the electronic transition resulting from the disordered period in the superlattice structures. We have evaluated the Γ–X mixing factor as a function of layer thickness, from the relative oscillator strengths of the quasi-direct transitions in the modulated SLs to that of normal SL using first-order perturbation theory. Using confined optical phonons as a probe in the Raman scattering measurements, we have investigated interface structure of these modulated SLs. The confined vibrations are sensitive to the layer thickness and the presence of the atomic scale roughness at the interface.