re: subsea and other long distance optical amplification advancements
This article highlights some of the deep sea DWDM amplification advancements [and challenges] that I've referenced here recently, from Lightwave Magazine. The focus is on the work that has taken place at SDLI in this area, and SDL is mentioned throughout.
lw.pennwellnet.com
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"New pump modules will increase submarine systems to 32 channels and beyond"
Article Date: October 01, 1999
By Robert Pease
The collaboration of two companies to produce highly
reliable, grating-stabilized 980-nm pump modules will enable
submarine systems to increase wavelength channel counts
to 32 and beyond. SDL Inc. (San Jose, CA), and Alcatel
Optronics, headquartered in France, initiated an extensive
reliability testing project of such modules in 1998 and
recently announced successful results at the Topical
Meeting on Optical Amplifiers and their Applications in Nara,
Japan.
Alcatel now plans to incorporate SDL's latest 980-nm chips
and fiber Bragg gratings to build high-power, high-reliability,
wavelength-stabilized undersea pump modules.
Until its development partnership with Alcatel, SDL had
never developed a product for undersea applications. The
fundamental difference in qualifying a component for
undersea application lies in the difficulty of replacing faulty
components. In terrestrial systems, faulty components can
be replaced in a few hours. The situation becomes much
grimmer in a submarine application, where it may take days
to repair and resume transmissions. Lost revenues could be
disastrous.
"The main cost involved is not in replacing a failed
component but in the loss of traffic you have on a fiber
that is now dead," says Jo Major, director of
communications, laser and device products at SDL. "A
980-nm pump for submarine use must have an
order-of-magnitude better reliability than required for a
terrestrial application. To prove that very, very low failure
rate with confidence, you have to have extensive
long-term testing of thousands of devices."
An example of the higher testing demands for a submarine
product can be seen in a simplified model. A terrestrial test
that may be successful enough for deployment of 980-nm
pump chips in terrestrial applications might require 100
lasers. By contrast, to qualify for submarine applications,
the same chips would be tested using 1000 or more lasers.
In the initial fabrication of the diode chip, the processes are
identical for both terrestrial and undersea products.
Sampling techniques are used to separate materials suitable
for undersea applications based on both performance and
reliability screening. The wafers suitable for submarine
applications are processed through the line and become
eligible for submarine applications.
Following all manufacturing and testing, fully
specification-compliant product moves on to further
analysis, known as "pedigree review," which eliminates any
diodes with unusual qualities or parameters. If a particular
characteristic is outside the normal parameters, higher or
lower than the norm, the component is rejected.
To date, undersea systems have been deployed with 16
channels, says Major. But by achieving higher power with
the same reliability required for undersea applications, a
new breed of pump modules will now increase the use of
dense wavelength-division multiplexing (DWDM) for
submarine fiber deployment. Power is key to using DWDM
technology under the sea.
"In a simple model for DWDM," says Major, "each 'on' state
requires a certain number of photons so the detector
accurately reads an 'on' state. As the data stream moves
to higher bit rates, more power is required to detect an 'on'
state, because you have less time to deliver those
photons. In addition, each channel requires power. As
channel counts and bit rates go up, the power requirement
for a DWDM system also rises. That forces the amplification
systems to have higher output power.
"So if we want to double the capacity on a network from
eight to 16 or 16 to 32, we have to increase the output
power of each amplifier by a factor of two. In simple terms,
anytime you want more channels or higher bit rates, you
need to get more optical power flowing through the system,
either by using amplifiers with higher output powers or by
spacing the modules closer together in the network."
Increasing the power poses a traditional problem--the
tradeoff of power for reliability. As the output power is
increased in a 980-nm chip, the reliability rate decreases.
Terrestrial customers will normally run at higher power and
slightly lower reliability. The submarine system community,
without the luxury of relatively quick repairs, must turn the
power down in favor of higher reliability.
While the advances offered by the SDL/Alcatel partnership
promise to improve the performance of undersea systems,
even more powerful pump lasers are on the horizon,
according to Major. SDL is on the verge of qualification
testing of its latest pump chip, the SDL 6540, which could
double the reliable operational output power for pump
modules.
SDL's current chip has a "kink" power (the maximum power
for maintaining good optical qualities) of 310 mW. That
equates to an actual operating power between 200 and 250
mW. SDL expects its 6540 chip will increase the optical kink
power to beyond 700 mW, removing a major limitation in the
manufacture of pump modules. The reliability work
accomplished thus far, says Major, suggests that terrestrial
operation could exceed 400 mW. SDL expects to have
enough qualification data on the SDL 6540 by the first
quarter of next year to bring the chip to market.
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