We present a novel coherent transceiver for optical differential phase-shift keying/differential quadrature phase-shift keying (DPSK/DQPSK) signals based on heterodyne detection and electrical delay interferometer. A simulation framework is provided to predict a theoretical sensitivity level for the reported scheme. High sensitivity of {45.18 dBm is achieved for 2.5-Gb/s return-to-zero (RZ)-DPSK signal, and high sensitivities of {36.83 dBm (I tributary) and {35.90 dBm (Q tributary) are observed for 2.5-GBaud/s RZ-DQPSK signal in back-to-back configuration. Transmission for both signals over 100 km is also investigated. Experimental results are discussed and analyzed.