delay line interferometer (DLI)

The detection of complex modulated light signals cannot be done by simple photodetectors like photodiodes or phototransistors, because in this kind of interferometry the components detect only the light intensity, but not the phase angle of the light modulation.

Therefore, for the detection of modulated light signals modulated in quadrature amplitude modulation or in phase shift ke ying, differential phase detection techniques are used, which first convert the phase modulation into intensity information. This conversion can be performed by a delay line interferometer (DLI). Delay line interferometers provide phase-dependent intensity signals that are converted into voltages by photodetectors.