Tech Briefs Magazine - May 2021 - PIT-14
TECHNOLOGY LEADERS Test & Measurement Systems
O
Testing 800G Direct Modulated
Optical Signaling
ptical communications systems have been a key enabler
to the buildout of our information infrastructure. Many
data centers used to store and transmit
information have miles of fiber and
thousands of laser/photodetector
receivers to send and receive information across the fiber. There is relentless commercial pressure to increase
capacity and the process to develop
new systems that operate at higher
data ratescontinues. This is not a
process of simply designing systems
that move more information. The cost
of these systems need to drop. Data
centers are sometimes described in
terms of acres and megawatts, indicating that the power required to run the
data center is huge. There is a strong
motivation to find ways to not only
operate at higher capacities, but to do
it while using less energy.
The basic optical communications
system has a laser transmitter that converts electrical data into modulated
light, an optical fiber, and a photodiode receiver to convert the modulated
Figure 1. Typical TDECQ transmitter conformance test on a 100Gbit (PAM4) optical signal.
Figure 2. Stressed Receiver Sensitivity (SRS) impaired optical signal.
14
light back to an electrical signal.
Designing the optical communications
system is complicated by the fact that
in the data center environment, there
is rarely a requirement that the optical
link be manufactured by a single vendor. The transmitter, fiber, and receiver are likely to be produced by three
different companies. This concept,
known as interoperability, gives the
data center designer flexibility, and it
facilitates competition among vendors
leading to more innovation and lower
costs. The downside to this is that
designing the system and specifying
the components within that system
becomes more complex.
A standards organization such as
IEEE 802.3 offers a public forum to
define communications systems. Meetings are open to all and are attended by
data center designers, as well as the
manufacturers of network equipment,
transceivers, and fiber. Since the standard will define performance and how
it is verified, test and measurement
companies also participate. One of the
essential outputs of the standards
group is a set of specifications for the
transmitters and a set of specifications
for the receivers. Again, the two sets
exist to promote interoperability. Most
recently the IEEE 802.3cu working
group has released the 100Gbps per
wavelength draft document, which will
be the key specification for future fiberbased interconnects.
Specifications typically start with the
receiver, where limits to the signal
strength will determine how reliably a
photo detector can convert the optical
signal to electrical data. If the signal
level drops below a recommended sensitivity, the receiver will make too many
mistakes, which are typically regarded as
bit errors. This threshold is known as the
receiver sensitivity limit.
There will usually be an objective for
the distance that the signal must travel, perhaps as short as 100 meters or as
long as 40 kilometers. The attenuation
caused by the fiber is well known, so
Photonics & Imaging Technology, May 2021
Cov
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Tech Briefs Magazine - May 2021
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