Polarization Express

Electronic Musician discusses photonics, which carries and processes information using light instead of electric current.
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I've said it before, and I'll say it again: any advancement in computer technology — be it increased processing speed, data throughput, or storage capacity — has the potential to impact the tools used by electronic musicians and is therefore fair game for this column. So I always keep my eye out for interesting research in any of those areas.

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FIG. 1: MIT''s “optics on a chip” prototype splits light of any polarization into its -horizontally and vertically polarized components, rotates the polarity of one component, processes both components identically, rotates one component back, and recombines the two components into an output signal with the same polarization as the input signal.

One technology that promises to greatly increase processing speed and data throughput is photonics, which uses light (photons) instead of current (electrons) to carry and process information. Because photons are much smaller and faster than electrons, photonic devices have the potential to surpass conventional electronics dramatically in those areas.

Recent advancements have brought photonics closer to commercial reality. For example, the incompatibilities between optical and electronic materials have been bridged by bonding the two together. In addition, silicon has been coaxed into manipulating light, leading to silicon light modulators.

Among the remaining obstacles that researchers face is the difficulty of mass-producing photonic devices. For one thing, microscopic structures designed to move light around on a chip must be manufactured to extremely precise tolerances; even the slightest imperfection can lead to significant light loss. In addition, these structures are very sensitive to the polarization of light that enters them. (Polarization refers to the direction of a photon's vibration, which is always perpendicular to the direction of the photon's travel. A photon's polarization can be vertical, horizontal, or anything in between.) Even if photonic structures could be manufactured with atomic-level precision, some devices would still be sensitive to polarization.

Up till now, this sensitivity has required photonic devices to be assembled carefully by hand so that the polarization of light entering them can be precisely controlled. Recent developments at the Massachusetts Institute of Technology (www.mit.edu), however, could put an end to this arduous approach. Led by professors Erich Ippen, Franz Kaertner, and Henry Smith, the MIT team has successfully demonstrated a photonic device on a silicon chip that can accommodate light of any polarization.

As illustrated in Fig. 1, the MIT chip accepts arbitrarily polarized light (say, from a fiber-optic cable) and splits it into horizontally and vertically polarized components. The polarization of one component is rotated 90 degrees so that both share the same polarization. The two components then pass through identical structures that process them in some way, after which the polarization of one component is rotated 90 degrees and the components are recombined to form an output signal with the same polarization as the original input signal.

This ingenious approach to overcoming polarization sensitivity still requires some delicate machinations. For example, the polarization splitter and rotators manipulate light in an intricate three-dimensional structure consisting of just two layers of silicon. In addition, the photonic processors (in this case, microring add-drop filters that extract one wavelength of light out of many in a multiplexed data stream) must offer nearly identical responses for the system to operate correctly.

For electronic musicians, photonics could facilitate many more audio and video channels flowing through a system than today's technology allows. The technology could also be used to perform more-extensive signal processing on all of those channels, opening heretofore unimagined creative possibilities. The MIT team's work brings us one step closer to realizing those possibilities.