Power consumption
The electrical power consumption should
be minimized, but a reasonable value would be
comparable to
contemporary circuit
designs for
electrical off-chip
interconnects. State-of-the-art low-power
electrical interconnect drivers and receivers consume about 7 - 10 mW per Gbps
for
an
6.5-m
link
through
lossy
RG58
cable
including the transmitter and receiver circuitry, as well as the
power expended to charge and discharge the wire itself, a resistive-capacitive load [2].
If we plan to design an optical interconnect to have the same power budget and suppose we allocate
10
% of
this
power
budget
for
the
optoelectronic
device
itself
(leaving 90 % for the transmitter driver and receiver circuits), a 10-Gbps link should use about 7 – 10
mW of electrical power
for
the
transmitter device. Note that the performance of an optical interconnect will not be significantly
degraded by increasing the link distance (within reason), whereas the performance of an electrical interconnect
will be somewhat worse. In other
words, the electrical interconnect in [2] would likely consume more power
(than the quoted
7 – 10 mW/Gbps) for a link twice
as long as the
6.5-m link tested, but the power consumption of
a replacement optical interconnect would not increase as much.
It is difficult to make a completely
fair comparison of total
power consumption because the transmitter
and receiver circuits will be optimized for the electrical or
optical interconnect, respectively. The transmitter driver in [2], for example, contains a component that performs equalization for the features
of the RG58 electrical
channel. This component would
be absent in an optical interconnect
transmitter driver (saving
power), though other components would
likely be added to drive
the optoelectronic transmitter
device (costing additional power).
Since most optoelectronic devices can be modeled as a capacitive load C on the
CMOS driver, the power consumption per device should be
P = 1 CV 2
f , where V is the
2
voltage level
and f is the data transfer rate in bits per second. Thus,
using the CMOS
standards of 1-V swing at a rate of 10 Gbps,
it is trivial to calculate
that the capacitance of a single optoelectronic device must be 1
pF,
in order to achieve a power consumption of 5 mW.
Optical requirements
An optoelectronic transmitter must be compatible with the optical system that
will carry the signals from
one chip to another. Many application-specific decisions must be made
regarding,
for
example, free-space propagation or optical-fiber guided waves,
single-channel data transfer
or WDM. Regardless of these
unknown parameters of the optical system,
it remains possible
to lay out
a series of
requirements for
the optoelectronic
transmitters.
Operating wavelength band
First
of all, the operating wavelength range must be chosen, which determines the material system(s) in which the device can be fabricated. Typically, optical
interconnects utilize a wavelength range centered around either 850 nm
or 1550 nm. AlGaAs/GaAs
semiconductor lasers can be fabricated for operation around 850 nm, while InGaAsP/InP
is the materials system of
choice for the C-band around 1550 nm. The semiconductor
devices are usually more advanced for 850 nm
operation, but 1550 nm is the center of the C-band in telecommunications
because it is the wavelength for which the loss
of optical fiber is minimized. Long-haul telecommunications signals
are between 1535 nm and
1565 nm, so any device that might be used for long-haul
should operate around these
same
wavelengths. The ability to operate in
this well-developed wavelength
range is a highly desirable feature, but not necessarily required.
In fact, this thesis will investigate
an 850-nm-wavelength range
AlGaAs/GaAs modulator in Chapter 4
and a 1550-nm- wavelength range
InGaAsP/InP modulator
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