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September 11, 2000
When manufacturers found a low-cost method of manufacturing 850-nanometer-wavelength vertical cavity surface emitting lasers (VCSELs), the devices ended up being so cheap that they replaced all other transmitters in short-haul data networks within a couple of years.
History could be about to repeat itself. Startup Bandwidth9 Inc. has hit on a way of using a simple manufacturing process to make tunable 1550nm VCSELs. And although its lasers aren’t particularly high power, their potentially low price could make a big difference to the economics of building metro networks.
VCSELs (pronounced “vixels”) lend themselves to low-cost manufacture because they squirt light out of their tops, rather than their sides, as other semiconductor lasers do (see Tune In!). As a result, VCSELs can be tested while they’re still on the wafer. And that means that dud devices can be discarded before they're put through the packaging process -- which represents a significant proportion of manufacturing costs.
The savings are maximized if VCSELs are made using a “one-step epitaxial growth process," according to Bandwidth9, which says that it’s managed to use this process to make 1550nm VCSELs for the first time.
This breakthrough eliminates the need to bond a high quality mirror to the top of the laser cavity -- a fiddly manual task. Instead, a “metamorphic layer” is automatically deposited on the substrate. “It’s just like amorphous silicon sticks to glass in solar cells,” says Wupen Yuen, director of precision epitaxy at Bandwidth9. Yuen declines to give further details, noting that others have been struggling for 20 years to achieve the same thing.
Bandwidth9 has made its laser tunable by using MEMS (micro-electro-mechanical systems) technology to move the top reflective surface up and down by minute amounts -- to lengthen and shorten the cavity. It says this gives a wide tuning range -- 40 nanometers -- covering the full L or C bands defined by the International Telecommunication Union (ITU).
MEMS tuning only requires a single voltage input, unlike competing technologies that require up to four currents to do the same thing, and that delivers another big plus, according to Bandwidth9. It avoids the laborious manual process of “characterising” each individual laser -- fiddling around with different combinations of voltages to set predefined wavelengths (see Intune Technologies Ltd.).
There’s more. Bandwidth9’s laser can also be directly modulated (made to create the pulses of light needed to carry digital transmissions), eliminating the need for a separate modulator.
Bandwidth9 intends to go further on the integration front. It recently acquired Verifiber Technologies Inc. with a view to making complete transmission modules incorporating its lasers. Verifiber already has integration expertise, from developing optical subsystems such as EDFAs (erbium doped fiber amplifiers) and optical line monitoring equipment.
The big snag with VCSELs is that they only support short-distance transmissions. Bandwidth9 declines to give the power of its laser. All that Yuen will say is that the performance is "adequate." In a recent experiment, however, it was used to send 2.5-Gbit/s signals down 50 kilometers of singlemode fiber -- enough reach for most metro applications.
Other vendors are developing more powerful tunable VCSELs, either by boosting the amount of electric current they use or by pumping them with light from a secondary laser. This second approach has been adopted by Coretek Inc., the startup acquired by Nortel Networks Corp. (NYSE/TSE: NT) earlier this year (see Nortel Gambles $1.43 Billion On Tunable Lasers).
Coretek expects to have sample products to show customers by the end of 2000. That's about two months before Bandwidth9 plans to have samples ready.
Other startups -- including Nova Crystals Inc., Novalux Inc. and Cielo Communications Inc. -- are known to be developing tunable VCSELs and could burst on to the scene at any time (see Nova Crystals Demos High-Power VCSEL,Novalux Promises Cheaper Lasers, and Cielo Shows Laser with Sandia Labs).
-- Pauline Rigby, special to Light Reading http://www.lightreading.com
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