This paper presents a 100Gbps Quadrature Phase Shift Keying (QPSK) analog coherent optical receiver. The study emphasizes the utilization of optoelectronic monolithic integration technology to realize the proposed optical receiver, which is based on the COSTAS optical phase-locked loop (OPLL) technique. The analysis thoroughly examines the impact of loop filter bandwidth and loop delay on the system's phase locking accuracy. Moreover, it demonstrates an effective solution to address phase locking challenges arising from increased loop delay. When using a laser with a linewidth of 10MHz for the local oscillator, the optical receiver successfully achieves phase locking with a phase error of less than 0.8°. © 2023 IEEE.