Optical cryptography under PPM-PAM modulation based in short optical pulses in an acoustic-optic tunable filter (AOTF)

A new optical encryption method using an acousto-optic tunable filter (AOTF) operating simultaneously with pulse position modulation (PPM) and pulse amplitude modulation (PAM) is proposed in this paper. This device operates with ultrashort soliton light pulses (2ps), which are modulated in agreement with the technique of pulse modulation just mentioned. In this way any pulse carries naturally two bits of information but when the encryption proposed here, is applied, each encrypted pulse, carries four bits of information. We have analyzed the sixteen output possible situations, modulating the input pulses polarized in TE and TM modes allowing a variation in the PPM and PAM parameters for each modulation. We have defined all the possibility pairs of PPM-PAM parameters where the encrypting procedure is possible.