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How to Specify Optical Modules
by Contributed Article on August 18, 2015
Optical modules are key components in networking equipment, and
specifying the right modules can heavily influence overall system
performance. Here, Erin Byrne of TE offers tips on key considerations.
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Optical modules are key components in networking equipment, and specifying the
right modules can heavily influence overall system performance. There are several
issues to consider when choosing an optical module.
To start, the supplier and the customer should agree on a set of specifications that
can be tested and verified. Surprisingly, system designers sometimes want to buy
technology in a format for which performance can’t be verified. For example, an
equipment maker may inquire about the performance of an optical engine or
subassembly, but such a subassembly should have features that allow adequate
optical testing to measure and meet performance specifications. Typically, fullspec performance is only fully measured at the module level.
For optical modules, the most important design considerations are density and
form factor. You can buy transceivers that plug into the faceplate, or you can buy
embedded, mid-board optical modules. You may want to choose a mid-board
module if you want more density at the faceplate or for greater electrical
performance because you’re able to put the module closer to the IC on the circuit
board and minimize electrical losses.
Standard choices determine bit-rate options, and the choices range from the small
form-factor pluggable (SFP) module at 1Gb/s up to the quad small-form-factor
pluggable 28 (QSFP28) module at 100Gb/s. Some parallel optical modules have
incoming signal rates of 25Gb/s, and there are mid-board modules that use 12 lanes
of 25Gb/s to deliver 300Gb/s. You can also choose the quad small-form-factor
pluggable plus (QSFP+) module with four channels of 10 Gigabits each, or the
small-form-factor pluggable plus (SFP+) module as a single 10Gb/s lane.
Another consideration is how far you want the optical signal to travel. This leads to
a decision between an Active Optical Cable (AOC) and a transceiver. An AOC is a
single unit that consists of two transceivers and a piece of optical fiber that joins
them. With a transceiver, you take a passive fiber cable and connect it to the
transceiver. For distances less than 20 to 30 meters, an AOC is probably the
less expensive choice. If you want the signal to go more than 30 meters, you’d
more likely use the transceiver with a passive fiber cable.
If you want to be able to source multiple vendors, you probably need transceivers
that comply with interoperability standards, such as the IEEE 802.3ba Ethernet
standard for 40Gb/s interfaces. AOCs need to meet only electrical standards
because the optical signals are self-contained. Thus, AOCs offer more flexibility in
terms of technology and they often can be a better value than transceivers.
Heat transfer and power consumption are two other considerations. Every optical
module generates heat, but some modules run considerably cooler than others.
Engineers need to assess how much power is being consumed and how much heat
is being generated, as well as whether the system has the capability to remove that
heat. With cooler optical modules, the equipment saves direct power but also can
have a substantial impact on reducing air conditioning costs for the data center.
Finally, designers shouldn’t ignore the electrical connector in an optical solution.
An optical module takes an electrical signal and converts it to optical for transport
around the board or between the customer’s racks. Designers should consider the
availability and suitability of the electrical connection as part of the total channel
solution. You should think about how much room the electrical component is going
to occupy, as well as the quality of the interface in terms of signal integrity.
By considering form factor, density, reach, bit rate, standards compliance, heat
transfer, and electrical performance, designers can properly evaluate optical
modules and specify the right one for the job.
Erin Byrne serves as director, optics product development engineering for TE
Connectivity in Harrisburg, Penn., where she leads a global team of engineering
professionals developing high-speed optical interconnects for data center
applications. Prior to joining TE, Byrne was involved in commercializing leadingedge optical components for the telecommunications, defense/security, and oil/gas
industries. She began her career at AT&T Bell Labs and holds a Ph.D. in inorganic
chemistry from Cornell University
Embedded Optical Engines Find Their
Niche
by Robert Hult on August 18, 2015
Demand for higher data rates, panel density, and practical channel lengths
with acceptable signal integrity have put pressure on copper connectivity to
deliver the most practical solution. Embedded optical transceiver
technology emerges to address this need.
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Copper circuits still rule the high-speed electronic world, but optical alternatives
continue to chip away at that dominance in niche applications. While the technical
advantages of optical interconnects have been recognized for years, adoption in
volume-production products has been hampered by a combination of increased
cost, perceived lack of durability, increased power consumption, and reluctance to
change from a well known technology. Demand for higher data rates, panel
density, and practical channel lengths with acceptable signal integrity has put
pressure on copper connectivity to deliver the most practical solution.
An emerging technology designed to address this problem is the mid-board or
embedded optical transceiver. Rather than converting high-speed electrical signals
at the I/O panel using pluggable interfaces such as SFP or QSFP, a mid-board
transceiver moves the electro-optical conversion process very close to the signal
source such as a processor, ASIC, or serdes chip. Taking high-speed signals off of
the PCB offers several advantages:
PCB design becomes less complex as hand routing is often required to
isolate high-speed lines on the board. Board costs may be reduced with
lower layer counts and use of less costly laminates.
Potential electromagnetic interference (EMI) issues are reduced as
optic links generate no EMI nor are susceptible to external noise.
Much greater I/O port density offers improved connectivity. Less
front-panel space consumed per I/O connector allows space for more
connectors and cooling vent areas.
Optic links enable longer channels with reduced signal degradation.
It provides higher bandwidth capacity in smaller and lighter fiber
links.
The I/O capacity of these embedded transceivers is impressive. With up to 12 full
duplex channels running at 25Gb/s, system designers are able to provide up to
300Gb/s
connectivity
using
minimal panel space.
Although most current applications
are focused on front-panel I/O,
fibers coming from mid-board
transceivers can also be directed to
a blind-mate optical interface at the
backplane. The new MXC optical
connector, for instance, can contain
up to 64 fibers, each running at
25Gb/s to deliver a total of 800Gb/s
full duplex I/O in a connector
roughly 10mm X 5mm. Mating fiber cables can extend to external equipment or
connect to an optical shuffle to provide linkage among daughtercards in the rack.
Occupying as little as one square inch each, multiple mid-board transceivers can be
tiled on the board to create immense I/O density.
Avago Technologies was one of the first to introduce mid-board optical modules.
They currently offer Micro and Mini-POD transmitter and receiver modules with up
to 12 channels running at 10Gb/s per channel. These modules mate with the Prizm
connector for coupling to 12-fiber ribbon cable. Modules are attached to the PCB
using the FCI MEG-Array connector.
Several leading connector manufacturers, including FCI Electronics, Molex,
Samtec, and TE connectivity, have invested extensive resources to develop midboard optical transceivers. Amphenol TCS is also developing a mid-board optical
transceiver that will be announced within three to six months.
Samtec introduced its Firefly Flyover optical
module several years ago, which features both
a copper and optic interface using a common
PCB header. Its latest product supports 12channel, 28Gb/s performance at lengths to
100 meters.
More recently introduced modules integrate
transmit and receive functions into a single
device.
Molex
Quatro-Scale
mid-board
transceivers feature eight duplex channels
operating at 25Gb/s each. Pigtailed fibers can be terminated at the backplane or
front panel.
TE CoolBit and
transceivers
FCI
LEAP
deliver 12 channels
at
25Gb/s
per
channel
while
occupying a total of
one square inch of
board
space.
A
standard 2X12 MT
interface
provides
optic
connectivity to both modules.
They also feature integrated
heat sink covers and achieve PCB connectivity via BGA/LGA sockets.
A variety of systems have slowly started to take advantage of embedded optical
transceivers, including switches and core routers, primarily in large data centers
and high-performance computing equipment such as supercomputers.
The Arista 7500 is a high-performance data switch that offers an optional line card
that utilizes embedded MiniPod modules linked to 12 MPO ports on the I/O face
plate. The MPO interfaces consume much less space and power than 12 SFP
pluggable connectors.
Embedded computer boards from Pentek utilize Firefly optical modules that
interface with VITA 66 backplane connectors.
Although 25Gb/s links can be economically implemented in copper, designers of
advanced systems want a clear migration path to 50Gb/s and beyond and do not
want severe restrictions on channel length.
Optical transceivers will likely encounter headwinds that will hinder wide
adoption. Current products on the market are proprietary with no announced
second sources. PBC footprints, power consumption, plug-ability, and physical
profiles differ among products.
Relatively high prices will remain an issue. At some point in time, establishment of
multi-source agreements (MSAs) among module manufacturers will alleviate this
problem. While embedded optical transceivers are not likely to enjoy the rapid
acceptance of active optical cables, new applications will become less nicheoriented over time.
The path to broad acceptance of optical connectivity continues to be delayed.
Utilization of PAM 4 signaling in emerging 50Gb applications will take some of the
pressure off copper links, which struggle using NRZ signaling. Mid-board optical
transceivers open the door to extreme I/O data rates and signal density in select
applications today and pave the way to achieving performance objectives in
equipment slated for the next generation and beyond.
Robert Hult
Director of Product Technology at Bishop & Associates Inc.
Robert Hult has been in the connector industry for more than 39 years. Hult began
his career as a sales engineer for Amphenol in Chicago. He joined AMP Inc. in 1972
and served in several management positions through 1996. In 1997, Hult joined
Foxconn as group marketing manager for Intel in Chandler, Arizona, US. Prior to
joining Bishop & Associates, he was the regional application engineering manager
for Tyco Electronics. Hult graduated in 1968 from Bradley University with a
bachelor of science degree in electronics technology and a minor in business. He
can be reached at [email protected].