OPTICAL INTERCONNECTS: Fiber and free space create circuits

Jan. 1, 2000

Computer makers are constantly looking for ways to increase the amount of data they can process in a given space, and one way to improve bandwidth is the use of optical interconnects.

Neil Savage

Red, green, and blue wavelengths, input from the upper left, lower left, and upper right corners, respectively, show how light that is passed through an image-guiding fiber can be redirected to other fibers by a free-space optical interconnect, pointing the way to a new kind of high-bandwidth circuit.

Computer makers are constantly looking for ways to increase the amount of data they can process in a given space, and one way to improve bandwidth is the use of optical interconnects. One drawback of such free-space optics, however, is that they must be carefully aligned, making them difficult to build into packages that will withstand being shifted around. Guided-wave optics with optical fibers make for more robust packages, but the output of each fiber is independent of all others, so the data streams coming out do not match up.

Two researchers at the NEC Research Institute (Princeton, NJ), Jun Ai and Yao Li, think they can get the best of both systems by combining free-space optics with guided-wave optics, producing a hybrid circuit that has the advantages of both. They married polymer-fiber image guides (PFIGs) to a lens-and-beamsplitter component and came up with a setup they said "mimics that of a parallel electronic data bus, except that the optical version has a much larger modulating bandwidth."¹

Li likens the free-space components to a highway intersection, made from four achromatic argon-coated lenses and a 50/50 beamsplitter cube. The PFIG is a multicore coherent poly(methylmethacrylate) fiber bundle, 2 mm in diameter, containing 3500 fiber pixels. The flexibility and stability of the fibers make the system more reliable, while each free-space intersection acts as a tap, where signals can be added in or taken out. Groups of lasers at the input points can transmit multiple data streams through either time-division or wavelength-division multiplexing, and the signals are put together at the output point.

One limit of the system is loss of power—this four-stage experimental setup had an aggregate loss of 25 dB, of which 12 dB was the fundamental loss due to beamsplitting. The researchers said the use of polarizing beamsplitting components may significantly reduce such losses. Another limit, introduced by the PFIGs, is in resolution as an image passes through subsequent lengths of fiber. A finer fiber array could increase resolution, Li and Ai said. And alignment of the free-space optics remains a concern. The researchers have recently come up with a scheme to integrate the five free-space components into a single beamsplitting ball.