A new tune for lasers
Long in the research phase, quantum-dot (QD) lasers are beginning to approach commercialization. New Mexico start-up Zia Laser (Albuquerque, NM) is hoping to take advantage of developments in the field to provide widely tunable sources for telecommunications starting later this year.
Zia's device is a distributed feedback laser. Molecular beam epitaxy is used to grow the QDs through the Stranski-Krastanov method, a self-assembly process. Initial layers are grown lattice-matched to the substrate, which is gallium arsenide (GaAs) for the 1300-nm emitters and indium phosphide (InP) for devices in the 1550 window. Engineers then deposit monolayers of indium arsenide (InAs), with a much larger lattice, which leads to surface tension. Tension created by the lattice mismatch forces the InAs to form quantum dots, which measure 20- to 40-nm wide and 7- to 10-nm tall. Only the quantum dots, which make up 10% to 20% of the quantum-well (QW) plane, are active.
Unlike QW lasers, which have a continual energy spectrum, QD structures have an energy gap between the lowest state that lases and the next state (see oemagazine, January 2002, page 18). It takes significant thermal energy to get to the next level, says Victor Klimov of Los Alamos National Laboratory (Los Alamos, NM), which is exploring colloidal chemical synthesis to produce the dots. Because of this, QD lasers emit a narrower spectral band, which translates into a higher differential gain.
So-called quantum dots are actually more like pyramids 70 to 90 Å high and 200 Å along the base. (Zia Laser)
"The smaller the dot size, the wider the energy gap and the larger the energy of the quanta, which is emitted by the dot," Klimov says. "It's a very simple way to tune." In reality, the devices come out with dots of different sizes in the shape of pyramids or discs (see figures). Zia would use its chips in an external cavity laser and would tune the output by tilting a grating to provide the desired wavelength.
"What you have now is single-band tunable laser products, and what we want to do is cover all wavelengths at once," says chief technology officer Luke Lester. The company's laser can tune from 1480 nm to 1620 nm with output of 10 mW. Zia is also developing lasers for the 1300-nm window for use in metropolitan area networks, but they would not be tunable, because metro networks are not currently using much dense wavelength-division multiplexing.
The company says its laser has a threshold current of 10 A/cm2. The gain spectrum of a QD laser is symmetrical, producing a device with less chirp and feedback sensitivity than a QW laser. "A quantum-dot laser looks more like having a gas laser in a semiconductor," Lester says. The characteristics make the design simpler, removing the need for such add-ons as external modulators, electrical isolators, and thermoelectric coolers.
He believes that in three to five years, quantum-dot lasers could widely replace quantum-well designs, just as those lasers replaced earlier double heterostructure designs. "The goal is to have a device that's tunable in two bands or more at $1,000," compared with $5,000 for current devices, Lester says. Right now, though, Zia is still in the process of proving that its laser can last 5000 hours at 85°C.
Other groups in Japan, Russia, and Germany are also working on applying quantum dots to telecom applications. Fujitsu (Tokyo, Japan), for instance, is developing 40-Gb/s optical amplifiers using the technology. Klimov says it seems likely QD lasers could replace QW devices, providing that they overcome some hurdles. QD lasers have lower output right now than QW lasers, and stacking more QD structures on top of one another to increase power leads to problems.
"There are several steps in technology that need to be done to show they are better than quantum-well lasers," Klimov says, although he concedes that from a physics standpoint they clearly are superior. "They may eventually take over. The question is: when?"
