We present a four-channel wavelength division demultiplexing detector using the principle of quantum confined Stark effect. This device is based on a ridge waveguide GaAs–AlGaAs single quantum well graded index separate confinement heterostructure. Four detectors are fabricated sequentially along the wave guide and their band gaps are tuned to progressively smaller values by applying progressively larger reverse bias voltages. Thus each detector responds preferably to one of the four input wavelengths. For transverse electric polarization, better than −10 dB crosstalk was achieved with a 14 nm wavelength separation. When operated as a three-channel device, better than −15 dB crosstalk was achieved with a 18 nm wavelength separation. For transverse magnetic polarization, better than −10 dB crosstalk was achieved with a 16 nm wavelength separation. We also present a theoretical study that leads to the optimization of the device.