October/November/December 2020 I 47
PART 1 — MMF TECHNOLOGY,
MARKET, AND INDUSTRY TRENDS
VCSEL Light Sources Drive MMF’s Growth
Vertical cavity surface emitting lasers (VCSELs) are
the light source of choice for today’s MMF transmission
over advanced optical multimode OM3, OM4, and OM5
optical fiber. The VCSELs offer several advantages1 over
light-emitting diode (LED) and edge-emitting laser (EEL)
technology typically used in singlemode links, including:
• Low power
• Cost
• Power efficiency
• Manufacturability
• Integration
• Reliability
• Testability
• Custom packaging
Multimode optical fiber, with its larger core size
compared to SMF, can more easily capture light from
a transceiver, thereby decreasing alignment costs. While
the cost of SMF optical sources have declined recently
due to advances in silicon photonics, the less stringent
alignment costs for MMF relative to SMF extend to multimode
connectors. A small 8μm SMF core is also much
more susceptible to dust and contamination than a 50μm
MMF core, making MMF links more robust in enterprise
and data center networks.
For distances of less than 150 m and up to 550 m
in some applications, the lower cost of connectivity,
installation and ongoing maintenance, along with lower
cost of 850 to 940 nanometer (nm) transceiver modules,
make the combination of laser-optimized MMF and
VCSEL-based transceivers the most cost-effective
optical short-reach solution for enterprise and
data center applications.
Ethernet Market and Fibre Channel Market Trends
The Ethernet market continues to grow and move
to higher speeds. For traditional on-premises enterprise
data center and LAN applications, OM3/4/5 fiber and
VCSEL-based optics dominate structured cabling and
point-to-point optical short-reach links. The vast majority
of traditional enterprise links are less than 100 m, while
virtually all traditional enterprise optical data center
links are less than 300 m.
In applications with speeds less than or equal to 10
gigabits per second (Gb/s), twisted-pair copper solutions
have had remarkable success. However, at speeds greater
than and equal to 10 Gb/s, optical solutions including
traditional fiber cable/transceiver links and active optical
cables (AOCs) are typically used. For distances less than
100 to 150 m, MMF is the solution of choice, while SMF
is typically used for longer links. Direct-attach copper
(DAC) cables are often used for reaches less than 5 m.
In 2018, IEEE 802.3 completed IEEE
Std 802.3cdTM-2018, which defined standards for
50GBASE-SR, 100GBASE-SR2 and 200GBASE-SR4
on 50 Gbaud lanes over OM3/OM4/OM5 operating
at 850 nm.
In January 2020, IEEE 802.3 completed IEEE Std
802.3cm™-2020, defining two new standards for 400
Gb/s Ethernet over OM3/OM4/OM5 MMF. Moreover,
400GBASE-SR4.2 is notable, as it is the first multimode
solution that uses wavelength division multiplexing
(WDM) to increase the data rate traveling over a pair
of fibers.
Proprietary and multisource agreement
(MSA) solutions often fill in the gaps for specific applications.
Extended reach (up to 400 m for 40 Gb speeds)
of standards-based solutions are often available for
customers who may have the need to support longer
distances. In 2017, the short wavelength division multiplexing
(SWDM) MSA released specifications for 40 Gb
SWDM4 and 100 Gb SWDM4; commercial products
became available at the end of that year. The 100 Gb
BiDi solutions were introduced in the same time frame,
adding another 100 Gb/s MMF/VSCSEL solution
for duplex links.
On the Road to 400 Gb/s Ethernet
Currently, 100 Gb/s speeds are being used primarily
by cloud data centers and leading-edge enterprise data
centers. A significant portion of these adopters are
utilizing the 100GBASE-SR4 solution, underscoring
the continued cost/power/overall value proposition
of MMF-VCSEL solutions with increasing data rates.