An optical 90° hybrid based on fiber passive devices is developed to enable optical coherent detection. By introducing the optical phase difference monitoring and control,the hybrid is stabilized over 12 hours without apparent deterioration. With the help of this hybrid,the differential phase shift keying (DPSK) signal is demodulated by mixing signal light with local light,and then,is sampled and digitalized by high-speed Analog-to-digital converters(ADCs). Off-line algorithm is used to estimate the phase error between the signal light and local light and compensate chromatic dispersion. We restore the constellation diagrams of 8 Gb/s DPSK signal transmited over 90 km single mode fiber using FIR filter to compensate the fiber dispersion and get an error free result in our experiment.