To shorten a delay incident to detection of an amplitude modulated optical bit pattern significantly by eliminating the need for optoelectronic conversion in the detection, thereby eliminating the need for converting an optical signal into an equivalent electric signal and the need for optically encoding digital data, using an optical phase modulation technology.

An optical signal processor for optically processing a digitally amplitude modulated optical signal input to determine the presence or absence of a predetermined digital bit pattern within the optical signal is provided. The optical signal processor comprises an optical processing means, including a signal duplicating means operable to derive from the input digital optical signal one or a plurality of duplicate digital optical signals, each of which is a duplicate of the digital bit patterns present within the input optical signal, and a signal duplicating/polarity inverting means operable to derive from the input digital optical signal one or a plurality of polarity inverted duplicate digital optical signals, the polarity of each bit of which is the inverse of that of the signal bit from which it is derived, wherein the optical processing means is operable to derive from the duplicate signals and the polarity inverted duplicate signals an optical detection signal which indicates the presence or the absence of the predetermined amplitude-modulated digital bit pattern within the input digital signal processed.