Electro-Optics - Applied Optics Fundamentals and Device Applications - page 52

2.6.5 Holographic Interconnects
Holographic interconnects fall into a class called free-space “focused” inter-
connects, which can also be called “imaging” interconnects. For such inter-
connections, the optical source is actually imaged by an optical element onto
a multitude of detection sites simultaneously. The required optical element
can be realized by means of a hologram that acts as a complex grating and
lens to generate focused grating components at the desired locations. The
efficiency of such a scheme can obviously exceed that of the unfocused case,
provided the holographic optical elements have suitable efficiency. Using
dichromated gelatin as a recoding material, efficiencies close to 100% can be
achieved for a simple grating. The higher the number of focused spots, the
lower the efficiency of available holographic methods. The flexibility of this
method is excellent for any desired configuration of connections.
The chief disadvantage of the focused interconnect technique is the very
high degree of alignment precision that must be established and maintained
to assure that the focused spots are striking the appropriate locations on the
chips. The spots can be intentionally defocused, with a trade between effi-
ciency and alignment tolerance.
With holographic optical elements, a future possibility is the incorporation
of dynamic holographic materials, such as those now being studied for four-
wave mixing optical phase conjunction applications. Other future forms
are banks of holographic mapping elements in conjunction with real-time
masks. Candidates for such interconnections would be matrix-addressed
liquid-crystal devices or the matrix-addressed spatial light modulator [78].
Another approach for a dynamic interconnect is the implementation of an
optical matrix-vector multiplier [79].
2.6.6 Guided Wave versus Broadcast Interconnects
This represents the most critical trade-off decision as it determines the course
of many others. Guided wave interconnects are inherently more efficient than
broadcast interconnects. Since GaAs ICs will operate in the Gb/s range, the
sensitivity of receivers is limited. As a result, the optical losses in the trans-
mission of data from one board to another are limited to −27 dB for a signal-
to-noise ratio of 100 (bit error rate = 1 × 10 −10 ) [80]. Broadcast interconnection
between devices on a board would require the use of large-area detectors
and is not suitable for GaAs ICs. On the contrary, broadcast interconnects
allow a large fanout of data, but guided wave interconnects also meet the
needs of a large fanout with little or no loss occurring during transmission.
Optical fibers are currently the preferred choice. Owing to their flexibility,
fibers can accommodate complicated topologies more readily than planar
waveguides or broadcast methods. As mentioned, multimode fiber is more
tolerant to misalignment than single-mode fibers, and is more tolerant than
all other methods.