SECURE COMMUNICATIONS BY OPTICAL HOMODYNE

A normal optical homodyne link uses a very stable laser emitter in the transmitter, and a similar laser as local oscillator (LO) in the rece iver. By contrast we use an LED having maximum spectral width consiste nt with available emitters and with the dispersion limitation, if any. (In the fiberoptic version, dispersion limits the product of length b y bandwidth.) This bandwidth physically precludes any LO. Instead, the transmitter sends an unmodulated phase-reference lightwave as well as the signal. The signal is digitally phase-modulated 180-degrees with respect to the phase reference, and is offset (by delay line) a secret distance, which serves as the security key. These two lightwaves can share a common channel (fiber or open beam) so that phase perturbation s along the way have no effect. A determined eavesdropper could find t he key by tapping the link and autocorrelating the intensity noise. So , for hard-core security, we take further measures. The transmitter sc rambles the carrier lightwave just ahead of the phase modulator so tha t the carrier is practically incoherent with the phase reference in th e transmission line that crosses unsecured territory. Inside the recei ver, a matched scrambler acts on the phase-reference wave rendering th em coherent again. A closed-loop tracking system in the receiver keeps its scrambler in phase lock with the transmitter's. Moreover, followi ng an initial lock-on sequence between receiver and transmitter, the m aster scrambler in the transmitter randomly varies the parameters that define its scramble function while the slave in the receiver tracks. We call this coherence tracking; it leaves the trapper no alternative but to search the key space of scramble parameters. The designer can m ake the search time arbitrarily long (e.g., years) by choosing the com plexity of the scrambler. The residual vulnerability is exponentially small (with the number of parameters) and quantifiable. We have built and demonstrated an experimental fiberoptic version in which the scram bler consists of recirculating loops on directional couplers, each ver y long compared to a coherence length. This system uses variable delay lines, whose lengths are the scramble parameters. We put one delay li ne in each fiber loop plus one in the direct path, each controlled by a tracking servo loop. For delay lines, we use fibers wound on pulleys with mechanical stretchers. An alternative scramble could be a disper sive device made with diffraction gratings and a mirror.