Like the transistor switches in conventional electronic chips, light-beam switches would be the basic component of photonic computers. Lipson's group has made switches in which light is passed in a straight line through a cavity with reflectors at each end, causing the light to bounce back and forth many times before passing through. The refractive index of the cavity is varied by applying an electric field; because of the repeated reflections, the light remains in the waveguide long enough to be affected by this small change. Lipson is working on devices in which the same effect is induced directly by another beam of light. Connecting photonic chips to optical fibers can be a challenge because the fiber is usually much larger than the waveguide, like trying to connect a garden hose to a hypodermic needle. Most researchers have used waveguides that taper from large to small, but the tapers typically have to be very long and THE FUTURE IS FIBER-OPTICAL TECHNOLOGIES thus introduce losses. Lipson's group, however, has made waveguides that narrow almost to a point. When light passes through the point, the waveform is deformed as if it were passing through a lens, spreading out to match the larger fiber. Conversely, the "lens" collects light from the fiber and focuses it into the waveguide. Lipson calls this coupling device "optical solder." Accord- ing to experiments at Cornell, the device could couple 200-nm waveguides to 5-um fibers with 95 percent efficiency. It can also be used to couple waveguides of different dimensions. 4.4 WAVE POLARIZATION A new and novel way of communicating over fiber optics is being developed by physicists supported by the Office of Naval Research. Rather than using the amplitude and frequency of electromagnetic waves, they are using the polarization of the wave to carry the signal. Such a method offers a novel and elegant method of securing communications over fiber-optic lines. Electromagnetic waves, like light and radio waves, have amplitude (wave height), frequency (how often the wave