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Free Space Zero Skew Optical Routing Disclosure Number: IPCOM000103589D
Original Publication Date: 1993-Jan-01
Included in the Prior Art Database: 2005-Mar-18
Document File: 4 page(s) / 178K

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Huang, K: AUTHOR [+1]


Disclosed are zero skew techniques for optimizing the timing performance of synchronous free-space digital optical systems or free-space optically interconnected electronic systems.

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Free Space Zero Skew Optical Routing

       Disclosed are zero skew techniques for optimizing the
timing performance of synchronous free-space digital optical systems
or free-space optically interconnected  electronic systems.

      A digital optical system [1] usually consists of an (or
multiple) optical gate array(s) where optical gates are connected by
an optical (or multiple) interconnection unit(s) (e.g., hologram(s)),
as shown in Fig. 1.  The optical gate array can be replaced by
electronic gate array with optical I/O (e.g., attached with laser
array and detector array).  The interconnection hologram is closely
attached to (or imaged from) the output side of an optical gate
array.  To reduce the system's cycle time and increase the timing
performance, the clock skew has to be minimized.  This is
particularly important in a high-performance digital system.
However, in prior art [1,2] the output of an optical gate is usually
connected to other gates with different propagation paths whose
distance differences can introduce significant skews (Fig. 2(A)).
The zero skew techniques for digital optical systems is investigated
here.  A recursive bottom-up process is introduced.  Here only one
recursive step is described.  Repeating the process in a bottom-up
fashion will construct a complete zero-skew clock tree.

      First, techniques for every clock distribution subtree to
achieve zero skew will be discussed.  This means the signal delays
from the root of the subtree to its leaf nodes are equal.  In digital
optical systems, the delay is proportional to the propagation
distance.  The zero skew criterion is simply to equalize the
propagation distance from a source to each end terminal.  However,
the prior art as illustrated in Fig. 2(A) do not satisfy this
criterion.  Two zero-skew techniques are presented and illustrated in
Figs. 2(B) and 2(C), respectively.

      For simple illustration, consider the example shown in Fig.
2(B): an optical source A wants to distribute its output signal into
two receiving ends B and C.  If the routing will only go through the
subhologram H as the prior art, the propagation distance d(A-H-C)
will be much greater than the distance d(A-H-B) and introduce a
significant skew.  One way to solve this problem is to elongate the
propagation distance from the source A to the receiver B by adding
multiple reflections between the plane of optical gate array (source)
and the plane of interconnection hologram.  To achieve such function,
the following procedure will be performed:
  1.  Perform total reflection coding to the output plane of the
source substrate (area with no source output) and the input plane of
the interconnection hologram substrate (area with no subhologram
fabricated).  Therefore, an optical beam can be reflected back and
forth between these two planes to adjust its traveling distance
before the beam outputs to its receiving end by passing through a