Integrated Optics - Applied Optics Fundamentals and Device Applications - page 279

performance. Low reflection terminations (LRT) are manufactured by creat-
ing an asymmetric sphere or fiber loop to prevent reflections from unused
splitter ports in bi-directional systems causing unstable laser performance
or noise.
5.12.3 Passive Optical Interconnects
Passive optical components guide, filter, route, adjust and stabilize opti-
cal signals transmitted through an optical network. Recent technological
advances in the design and production of both passive and active compo-
nents allow communication equipment suppliers to optimize fiber-optic sys-
tems. The ongoing trend is to integrate increasing optical component count
into compact modules and custom solutions.
The concept of flexible optical circuits originated at Bell Labs in the late
1980s [104-106]. Originally named “Optiflex” the concept is now produced
in similar fashion by nearly a dozen companies including FCI, Polyguide
Photonics, Tyco, AIT, Molex, NTT, USConec, and others. Fiber crossovers are
designed to minimize stress in the fibers. Several companies manufacture
multifiber array connectors suitable for use with the flexible optical routing
circuits. These employ precision molded ferrules as well as micromachined
silicon v-grooves for accurate alignment of fibers for optimal coupling con-
figurations. Complex fiber management schemes developed at DuPont and
later licensed to PolyGuide Photonics, aka Optical Crosslinks Inc., provide
the capability to manufacture flexible routing circuits as well as related poly-
mer waveguide based passive components [107].
One key requirement for optical communication systems is the perfect
optical shuffle or optical flex board shown in Figure 5.42. Eight sets of eight-
input fiber arrays are routed to each of eight sets of eight-fiber output arrays.
In other words, one fiber from each of the inputs goes to one fiber of each
of the outputs. In Figure 5.42, 1:8 splitters are used to create the eight sets
of eight-fiber inputs; and an 8 × 1 switch is used to direct each of the output
fibers to a set of single-fiber outputs. While the specific optical wiring may
vary from one system application to another the basic optical shuffle compo-
nent provides the fundamental building block.
This can be accomplished as shown in Figures 5.43 and 5.44, with various
fiber routing and packaging schemes. In Figure 5.43, the fibers are ribbon-
ized into 12 sets of 12-fiber arrays, and one fiber from each of the arrays goes
to one of the output arrays. The fiber crossovers are housed in a “black box”
called the shuffle. A variant of this scheme is shown in Figure 5.44, where
the ribbonized fibers enter a sheet of material which contains the routing
paths for the fibers which are shuffles as before. The ribbonized fibers are
terminated in these examples with optical connectors called the MPO. This
approach will work with the variety of mulitfiber connectors available, for
example, MTP, MPX, MAC, miniMAC, etc., as the fibers are always ribbon-
ized with 250 μm center-to-center spacing. In the assembly process, standard