A filter-based method of demodulating differentially encoded phase shift keyed (DPSK) optical signals, such as commonly used binary-DPSK (DBDPSK) and quadrature DPSK (DQPSK) signals, that can achieve optimal receiver sensitivity is described. This approach, which combines filtering and differential phase comparison, can reduce the complexity and cost of DPSK receivers by obviating delay-line interferometer-based demodulation. This can improve receiver stability and reduce size, weight, and power, while maintaining the ability to achieve optimal communications performance.