Two low-latency chirped fiber Bragg gratings (CFBGs) were used by researchers at Princeton University (Princeton, NJ) to completely hide a “stealth” signal in an ordinary signal transmitted over a 1550 nm fiber-optic network. The technique relied on optical steganography, rather than optical encryption, to secure the signal (steganography hides a signal by obscuring it).
The public channel was a code-division multiple-access (CDMA) signal consisting of four wavelengths (1550.6, 1551.6, 1552.4, and 1553.4 nm); the stealth channel was created at 1551.5 nm–the same as one of the CDMA wavelengths–but at a power 10 dB lower than the public channel. Before combining the channels, the stealth channel was greatly dispersed by a compact 753 ps/nm CFBG, which spread it out and totally buried it in the public channel, hiding it in both the temporal and spectral domains. A second CFBG with equal but opposite dispersion to the first CFBG then reconstituted the stealth signal at the receiver. The technique induced a mere 1 dB power penalty in the successfully received public channel. For additional security in a real system, steganography could be combined with encryption. Contact Mable Fok at mfok@princeton.edu.