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 receiver. By contrast we use an LED having maximum spectral width consistent with available emitters and with the dispersion limitation, if any. (In the fiberoptic version, dispersion limits the product of length by 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 perturbations along the way have no effect. A determined eavesdropper could find the key by tapping the link and autocorrelating the intensity noise. So, for hard-core security, we take further measures. The transmitter scrambles the carrier lightwave just ahead of the phase modulator so that the carrier is practically incoherent with the phase reference in the transmission line that crosses unsecured territory. Inside the receiver, a matched scrambler acts on the phase-reference wave rendering them coherent again. A closed-loop tracking system in the receiver keeps its scrambler in phase lock with the transmitter's. Moreover, following an initial lock-on sequence between receiver and transmitter, the master 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 make the search time arbitrarily long (e.g., years) by choosing the complexity 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 scrambler consists of recirculating loops on directional couplers, each very long compared to a coherence length. This system uses variable delay lines, whose lengths are the scramble parameters. We put one delay line 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 dispersive device made with diffraction gratings and a mirror.