Electro-Optics - Applied Optics Fundamentals and Device Applications - page 51

f max = Hz No. Modes Wavelength Length

25 MHz 30 1 µm 4.98 × 10 3 m

100 MHz 30 1 µm 1.25 × 10 3 m

1.6 GHz 30 1 µm 77.9 m

6.4 GHz 30 1 µm 19.5 m

n _ ⏐ ⎡

⎢

2

⎤
⎥

n
nk

|

Δ T =

2

( l + M +1)

2 max

max

c

2

1
T

f −

max

Δ

FIGURE 2.31
Quadratically graded index multimode fiber dispersion characteristics.

25 MHz lines. The results of Figure 2.31 and the values quoted indicate that
multimode graded-index fiber would be adequate to transmit over distances
of approximately 20 m. With the use of single-mode fibers, the pulse spread-
ing is due to group velocity dispersion. This dispersion, in turn, is depen-
dent on the spectral width of the source.

2.6.3 Broadcast Interconnects

The second technique for interconnecting various parts of high-speed inte-
grated circuits involves the broadcast transmission of the optical signal in
free space. Since substrates are planar, it can be envisioned that interconnec-
tion paths in free space must be perpendicular to the substrate plane to take
advantage of the inherent three-dimensionality of free-space interconnects.

2.6.4 Free-Space Interconnects

With the free-space interconnect, the signal transmitted by the source is col-
limated by a lens or guided by a lens system. Detectors integrated into chips
on the board receive the optical energy and convert it to electrical energy. The
detectors can be made to receive the information with identical delays, due
to the particular location of the source with respect to the focal point of the
lens or mask. The system is highly inefficient, however, because only a small
portion of the optical energy falls on the photosensitive areas of the chip.
This method may therefore require additional amplification of the detected
signal. This leads to increased power needs for boards, so that with many
interconnects, this can be prohibitive in terms of heat dissipation and power
requirements. Moreover, optical energy falling on areas of the chip where it
is not wanted may induce stray electronic signals; therefore, it is necessary
that opaque dielectric blockers be used. Openings in this layer would allow
the detectors to receive the optical power.