Optical signals of individual wavelengths are logically combined by directing the signals at an incident surface of an integrated series of photodiodes each comprising an undoped light-absorbing semiconductor region which absorbs a high proportion of a respective one of the individual wavelengths and a low proportion of the other individual wavelengths.
Each of the integrated series of photodiodes consists of an undoped light-absorbing semiconductor region contiguous with one component region of a pair of doped semiconductor regions forming a quantum mechanical tunnelling pn junction, the combined thickness of the pair of doped semiconductor regions being of the order of the mean free path length of a charge carrier therein. The undoped light-absorbing region of a succeeding diode in the series is contiguous with the other component region of the pair of doped semiconductor regions of the preceding diode in the series.