Bragg gratings are key to encryption

09 Nov 2005

US researchers unveil a new way of keeping optical data networks safe from prying eyes.

An encryption scheme based on cleverly designed Bragg grating “keys” could provide a simple alternative to quantum cryptography for protecting the security of optical data communications.

That’s the claim being made by scientists from the University of Arizona, US who presented their new encryption scheme at the Optical Society of America's annual meeting last month.

Their communications system comprises a custom designed transmitter and a receiver, each of which contains super-structured Bragg gratings (SSBGs) that act as a key for encoding and decoding the transmission.

Unlike conventional SSBGs which have a regular index modulation to generate a strong reflection at one particular wavelength the Arizona’s SSBGs have a mathematically-generated pseudo-random modulation. As a result, an optical data stream that is sent onto the encoding SSBG module in the transmitter gets returned as a signal that is scrambled and resembles noise. The receiver simply contains a SSBG module that is a conjugate of the version in the transmitter and thus decodes the encrypted signal.

In fact, the system is slightly more complex as the key actually consists of two SSBGs -- one to encode the databit 1s and one to encode the databit 0s. A matching conjugate pair is found in the receiver. The transmitter also employs an additional masking process (made of two further SSBGs) to help disguise the data.

Jose Castro and his colleagues have not yet built a prototype of their idea but say that mathematical simulations show that the scheme works well with a 2 Gbit/s data stream and that any eavesdropper would receive noisy patterns that are extremely difficult to decipher.

The team also says that using ion-exchange or silica-on-silicon fabrication technologies, would allow several SSBG to be made on a compact chip, thus allowing the encryption key to be easily exchanged.

Author

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.