Optical DPSK Demodulator, also known as Delay Line Interferometer (DLI), converts phase modulation to amplitude modulation over the entire C+L band in support of data transmission rates of 2.5, 10 or 40 Gb/s. The DPSK Demodulator is designed for phase modulated optical communication systems utilized in commercial, defense and space exploration markets. The DPSK device plays a key role in improving signal quality and performance to meet the expanding demand for higher data rates and more complex transmission formats within current and next generation systems without major capital expenditure.