Dear Investors:Diana Hayden Investor Relations
Please find attached an article from Lightwave on Singe-Fiber transceivers, written by our CTO, Mark
Heimbuch of MRV Communications. To view the article directly, please click on the link below -
registration is free. Search MRV.
lw.pennwellnet.com
Regards,
Diana Hayden
Investor Relations
MRV Communications, Inc.
Lightwave Magazine - March 2000
The future of optically multiplexed SF transceivers
Mark E. Heimbuch - MRV Communications
The best-known optically multiplexed transceiver is the single-fiber
(SF) transceiver. The SF transceiver has begun to appear as a
commodity from many suppliers, but its current use is in niche
applications. The benefits of an SF transceiver are obvious: It allows
a transceiver to use one fiber instead of two. This simple fact makes
the SF transceiver attractive for businesses that are renting fiber,
trying to conserve their available dark fiber, or running at maximum
capacity and would need to pull new fiber (or upgrade their
equipment). These applications will not disappear in the future and
neither will the SF transceiver. Whether optically multiplexed
transceivers remain a niche component, or become a commonly
used standard will depend on future applications and future
developments.
One future use for optically multiplexed transceivers is to extend the
reach of higher-speed data links to transmission distances that equal
today's slower-speed transceivers. This "distance data-rate" product
is a very real limitation for fast data links. With optical multiplexing, a
two-fiber transceiver can be built that is effectively made up of
multiple transceivers. Using multiple slower transceivers to do the
work of one faster transceiver can result in greater distance data
rates. This next-generation transceiver could be referred to as a
"2xTransceiver," "4xTransceiver," or "NxTransceiver," depending on
the number of optically multiplexed channels.
Before delving into this application, we need to explore the current SF
transceiver in more detail.
A SF transceiver uses the same electronics as a standard dual SC
transceiver. The key component that enables SF operation is the
internal optical multiplexing. For short distances, the optical
multiplexing can be full duplex using a single wavelength (transceiver
pair (a) in the Figure). Although this method is the least expensive,
reflection sensitivity makes it inferior for long transmission distances.
The "standard" SF transceiver consists of two wavelengths
(transceiver pair (b), consisting of transceivers B and C, in the Figure)
and is a more robust design that is better suited for long
transmission distances. The disadvantage of the standard SF
transceiver is that a different transceiver is required on each side of
the link.
Click here to enlarge image
There are two primary methods for packaging the optical components
of a SF transceiver. The first is to use bulk optics similar to current
laser packaging. The other method is to use an integrated package
that consists of a laser and receiver, with a waveguide layer to couple
these elements together. The advantage of the integrated package
has yet to be confirmed, but lower assembly cost and a smaller
package size are expected to be the primary benefits.
The optical components necessary for a standard SF transceiver
make it more expensive than the current low-cost Fabry-Perot (FP)
transceivers. The standard SF transceiver requires one 1.3-micron FP
laser and one 1.5-micron distributed feedback (DFB) laser to be used
on one link instead of two 1.3-micron FP lasers. Traditionally, the
cost of a DFB has been significantly higher than the cost of the
1.3-micron FP. This is due to three factors: higher DFB
manufacturing costs, lower volumes, and the requirement for
additional expensive optical components in the package.
However, the cost of the 1.5-micron DFB element can be reduced for
short-distance applications, and with high volumes, the cost can
begin to approach a 1.3-micron FP device. For higher-speed future
applications such as 10 Gbits/sec, the industry will need to move to
DFB devices in order to maintain a 2-km solution, so the issue of
additional cost will be eliminated.
For businesses that are renting fiber or do not have fiber to spare,
any additional cost for a standard SF transceiver will be less than
purchasing a faster switch and much less than installing or renting
more fiber. For businesses that are not fiber limited, it is not cost
effective to use SF transceivers, but it might be prudent to do so in
order to reserve dark fiber for future installations. If a business finds
that next-generation transceivers will not reach their required
distances, then more careful allocation of the fiber becomes
necessary.
This effect of reduced distance has already been observed for Gigabit
Ethernet transmission over multimode (MM) fiber. Currently, the IEEE
Standards Committee 802.3 High Speed Study Group is working to
establish the standards for 10-Gigabit Ethernet. The preliminary
distance specifications are 100 and 300 m on MM fibers and 2, 10,
and 40 km on singlemode (SM) fibers. The physical layer
components used to achieve these distances are still being
discussed but optically multiplexed transceivers are one of the
primary components being considered.
Why is the distance an issue for next-generation transceivers? It is a
basic limitation of fiber that with each increase in speed, a decrease
in transmission distance is realized. This is due to dispersion
limitations and/or the requirements for higher powers at the receiver.
Dispersion limitations are the dominant effect and for components
that are dispersion limited (such as the current low-cost
transceivers), a factor of four increase in speed will result in a factor
of four decrease in the transmission distance. The best new MM fiber
is limited to 2 km at 1.25 Gbits/sec and an uncooled 1310-nm FP
laser operated at 1.25 Gbits/sec over SM fiber is limited to around 10
km (depending on the ambient temperature range). At 10 Gbits/sec,
these existing low-cost components will have very limited distances:
around 250 m for MM fiber and 1500 m for SM fiber. Therefore,
industry will have to accept new solutions to address the required
distances.
One solution is to run parallel fiber. However, parallel fiber is not a
long-term solution, as it will quickly consume existing dark fiber or
will require new installations. For SM fiber, the more likely solution is
to use higher-cost components. Directly modulated DFB lasers are
limited to around 20 km at 10 Gbits/sec due to dispersion for
1.5-micron lasers and attenuation for 1.3-micron lasers. Note that
"long-haul" applications of 100 km at 10 Gbits/sec utilize cooled
lasers with integrated electro-absorption (EA) modulators and fiber
amplifiers. The integrated laser/modulator is a possibility for
data-communications transceivers; however, the device cost, cooler
cost, cooler size, cooler power consumption, and cooler heat
dissipation make it unattractive.
For increased transmission distances on both MM and SM fiber at 10
Gbits/sec and higher, there are two primary competing technologies.
One technology is optically multiplexing slower transceivers and the
other is modulating the laser with similar methods used in the
modem industry. By using different electrical-modulation techniques,
one source running at a lower bandwidth can transmit the longer
distances while still achieving the desired high bit rates. This can be
achieved by using multilevel detection (such as PAM5) and/or
phase-shift-keying. For data-communications transceivers, these
electrical-modulation techniques are not commercially in use.
Currently, the IEEE 803.2 High Speed Study Group is considering
both multilevel modulation and wide wavelength-division multiplexing
(WWDM) as standards for 10-Gigabit Ethernet transceivers.
The advantage for using different modulation techniques is that the
long-term pricing points will be lower than the WDM techniques. The
advantage of the WDM technique is that it is currently available and
can be used in parallel with future modulation techniques to enable
solutions for the next several generations of transceiver speeds. The
disadvantage of the higher cost (around 2 to 3 times) associated with
optically multiplexed devices will be offset by the fact that these
devices will enable distances otherwise unattainable. Rather than
1.3-micron FPs providing the short haul (0 to 10 km) and 1.55-micron
DFBs providing the long haul (10 to 50 km), one DFB will provide the
short haul and multiple DFBs will provide the long haul.
Multiple versions of optically multiplexed transceivers can be built
using the current SF transceiver. Using two standard SF transceivers
over two fibers can result in an equivalent 2xTransceiver (transceiver
configuration (c) in the Figure). Note that for channel 1 of transceiver
1 to "talk" to channel 1 of transceiver 2 requires different transceivers
at each side of the link. Using four transceivers and only 1.3-micron
and 1.5-micron components, a low-cost, short-distance
4xTransceiver (transceiver pair (d) in the figure) can be made. This
configuration is limited to applications where the data on each
channel are independent, since the two fibers can have different
lengths, resulting in an uncontrollable amount of skew (or delay)
between the channels.
For deserialized data transmitted over parallel wavelength channels,
(such as a 10-Gigabit Ethernet transmitted by using four 3.125
Gbaud transceivers) skew between channels becomes a critical
parameter. In order to control the skew between channels, all four TX
signals must be sent over the same fiber. This type of operation can
be achieved by using four different wavelengths (transceiver pair (e) in
the Figure). Although this device is a significant deviation from the
standard SF transceiver, this four-color 4xTransceiver shares many
conceptual and technical similarities.
Selection of the four wavelengths for the 4xTransceiver depends
primarily on two criteria. The first criterion is temperature
performance. The laser must operate uncooled and the passive
insertion loss and crosstalk must be tightly controlled from 0 to 70ø
C. The second criterion is cost. The active and passive components
must be high yield, low-cost technologies. Four-color 4xTransceivers
that meet these criteria are becoming commercially available using
wide (or coarse) WDM technology, which consists of 20-nm channel
spacing.
While this technology allows for control of the skew, it does not
eliminate the skew. The overall system design must allow for a
maximum tolerable skew in the transmission. Once the maximum
skew is fixed, the skew-limited maximum transmission distance is
determined from both the channel wavelength separation and the
absolute wavelength. For example, 5-nsec maximum skew allows for
up to 40-km transmission at channel wavelengths of 1.28, 1.30, 1.32,
and 1.34 microns. On the other hand, at 1.5 microns a channel
separation of 20 nm and a 5.8-nsec skew result in 10-km
transmission lengths.
The emerging 4xTransceivers could prove to be the primary entry
point for standardization of WWDM in the data com marketplace.
One hurtle to overcome is the perception that since dense WDM
(DWDM) exists, it is only a matter of time before it is used
throughout the data communications industry. While DWDM does
hold the potential of over 100 channels as compared to the four to 16
channels used in a WWDM system, the key limitation is the eventual
price point of the DWDM technology. DWDM has inherently
higher-cost passive components-and also higher-cost active
components.
While data communications applications for DWDM do exist, the
high-volume, low-cost transceiver does not fit well with DWDM
technology. The SF transceiver, on the other hand, is an excellent,
cost-effective, WDM solution for businesses with fiber-poor
environments.
Optically multiplexed transceivers using WWDM are also a practical
solution for high distance-data rate applications like 10-Gigabit
Ethernet over 20 to 50 km on SM fiber (or over 0.3 to 1 km on MM
fiber). Additionally, WWDM can be extended to 8, 12, or 16
channels, allowing for future 4xTransceivers to operate on the same
fiber as existing 4xTransceivers.
Mark E. Heimbuch received a Ph.D. in electrical engineering from
the University of California at Santa Barbara in 1997, under the
direction of Prof. L.A. Coldren. He joined MRV Communications
(Chatsworth, CA) in 1997 and is currently the director of
semiconductor device R&D. |
