In this paper we present the characterization results of an optical coherent quadrature phase shift keying (QPSK) receiver with an application-specific integrated circuit (ASIC) based backend unit. Additionally we investigate the performance of an integrated optical coherent receiver frontend (ICR) and compare it against a fiber-pigtailed 90deg optical hybrid. After characterization of the CMOS ASIC, we measure and analyze single polarization QPSK transmission at 2.5 Gbaud with different optical receiver frontend setups. Also we measure the polarization-multiplexed QPSK transmission and discuss the possible reasons for reduced sensitivity compared to the single-polarization transmission.