Spectroscopic ellipsometry and s-polarized reflection have been studied on resonant-cavity enhanced PIN photodiode structures based on MOVPE material systems (AI,Ga)InAs/InP with a bulk InAs absorber for 1300 nm and (Al,Ga)As/GaAs using MQW GaAs-exciton absorption for the 860 nm wavelength region. The optical constants of(AI,Ga)InAs determined for different compositions were used for design and realization of the resonator structure. In the case of(Al,Ga)As MQW exciton absorption the interaction between the exciton and resonator modes influences strongly the tuning of absorption resonance by means of the quantum confined Stark effect.