Two-Dimensional Optical Code-Division Modulation With Quantum-Noise Aided Encryption for Applications in Key Distribution

We propose and demonstrate an advanced optical modulation format that makes use of both spectral and temporal phase encodings (2-D) for applications requiring exceptional security. The method combines modulation techniques used in direct-sequence spread-spectrum coding, spectral-phase encoding, and M-ary phase-shift keying with codes generated using cryptographically secure pseudorandom number generators. The wideband transmission signal is very difficult for an eavesdropper to record or analyze. Signal-to-noise ratio limitations imposed by quantum effects enhance the security further. The properties of the transmitted signal make it especially useful for physics-based key expansion systems. We have successfully used this setup to transmit encrypted 155 Mb/s data over 70 km of fiber with a BER value of 4E-5.