Double security level in optical communication systems using combined modulation formats

A new optical communication system exhibiting a double security level is pr esented information coding is based on combined modulation formats of two d ata streams over the same optical carrier. Incoherent and coherent optical encryption algorithms are presented and are theoretically analyzed. The opt ical encryption method is transparent to data bit rates. Furthermore, it re quires "on-line" data decoding with no possibility of recording and further processing The incoherent scheme is very practical and can be implemented in existing optical communications systems using currently available techno logy. Computer experiments of the incoherent method indicate less than 25 d B of signal to noise ratio required ed for 10(-9) bit error rate and inelig ible information decoding rime of more than 10(8) years.