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How to Specify Optical Modules
by Contributed Article in connectorsupplier.com by Erin Byrne, TE Connectivity
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.
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 OEM
customer
should
agree
on
specifications that can be tested and
verified by both. Surprisingly, OEM 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, full-spec
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 smallform-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, PA., 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 leading-edge 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
Note from PSMC Interconnect TWG: Mid Board Optical Engines (MBOs)are an
elegant solution for today’s need for in-system photonic circuitry, particularly in Big
Data/data center applications. They are compatible with today’s CPUs and ASICs
which do not have photonics built in. Thus, they precede the eventual movement
toward on-chip/in package photonics capability. AOCs are of a similar vein: they
provide optical interconnect at the IO port between systems, reducing cable counts
and power consumption by converting electronic signals and the board level to fiber
optics. Both of these applications have the capability to move to single mode with
wave division multiplexing and expanded beam where and when necessary.
Embedded Optical Engines Find Their Niche
by Robert Hult, Bishop & Associates, connectorsupplier.com, connectorindustry.com
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 highspeed 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 mid-board 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
features
common
channel,
meters.
optical module several years ago, which
both a copper and optic interface using a
PCB header. Its latest product supports 1228Gb/s performance at lengths to 100
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
and FCI
CoolBit
LEAP
transceivers
shown
above
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 niche-oriented
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.
Note from PSMC Interconnect TWG: Most of the applications discussed here involve
board-level optical interconnect. In order to provide this capability, discrete ‘flyover’ optical fiber cables are employed. This provides a flexible solution to optical
interconnect at the board, subsystem, system and intersystem levels. Until
embedded waveguide technology is commercialized. Even then, single-mode glass
optical fiber will provide the greatest, bandwidth. Think of fly-over cables as a
connector-terminated photonic component instead of field terminated cables. They
are available in various lengths to accommodate circuit applications. Their
embedded waveguide cousins will employ polymer waveguide materials, such as
DOW-Corning’s Silicone. In these applications a cost-effective rigid or flexible OPCB
will need a connector to be developed – in concert with an optical via structure built
into these future OPCBs. Free-space optical interconnect may also be employed if
certain issues can be addressed including mechanical packaging, size, power and
cost.