Devices based on quantum dots have often been regarded as mere scientific curiosities. Interesting though the research may be, useful quantum-dot products have been thought of as something for the future.

But researchers working on quantum-dot (QD) lasers around the world have made giant strides in recent years. So much so, in fact, that two years ago a team from the University of New Mexico in Albuquerque, US, set up Zia Laser - the first venture to try manufacturing commercial QD lasers.

Spatial confinement

QD lasers have active regions in which the gain material is confined in all three spatial dimensions. If conventional diode-laser heterostructures are regarded as three-dimensional and a quantum-well (QW) structure is confined to two dimensions, a QD structure is zero-dimensional. Typically, QDs are grown into pyramid shapes using standard epitaxy equipment.

The appeal of QD lasers is that they have characteristics approaching the atomic ideal, meaning that they can offer a lower threshold current for lasing, enhanced spectral purity, and a greater independence from the effects of temperature. And now it seems that QD lasers could at last be on the cusp of a commercial breakthrough.

Zia's view is that QD lasers offer "a clear performance advantage while dramatically lowering the cost per bit at the network level". And Zia's founder, Thomas Brennan, has made it plain that he envisages a bright future for QD lasers - at this year's Optical Fibre Conference (OFC), he said: "The next step in the evolution of the laser diode has begun: it is the quantum-dot laser."

QD devices currently in production at Zia include 1310 nm distributed feedback (DFB) lasers based on a gallium arsenide (GaAs) substrate. These have a 5 mW power output and exhibit an uncooled power stability that is five times better than that of equivalent QW lasers. Also available are broadband 1550 nm tunable gain chips, based on indium phosphide (InP), which have a maximum output power of 40 mW throughout the S, C and L telecommunication bands.

This year's OFC marked the commercial debut of Zia's devices. Samples of the tunable gain chip were delivered to potential customers in April, followed by samples of the uncooled DFB lasers to fibre-optic component manufacturers in July. According to company spokesman Chris Wiggins, Zia's expertise lies with controlling dot size, shape and density within the active region of a quantum well - known as the trademarked "dots-in-a-well" (DWELL) technology.

Judging by Zia's latest move to scale up production, the product sampling drive has been a success. Rather than building its own fabrication facility, the company has teamed up with indium phosphide component manufacturer CyOptics, also based in the US. Zia will use CyOptics's foundry services and collaborate on product development to turn the raw QD devices into fully packaged 1550 nm tunable gain chips and 1310 nm uncooled DFB lasers.

European enterprise

Here in Europe, QD lasers have not yet been commercialized by anyone. However, while Zia was trying to make its big splash at OFC, a European enterprise kicked off with a little less fanfare. BIGBAND is a EUR 2.7 m project to develop QD devices based on InP covering the 1.4-1.65 µm range.

Johann Peter Reithmaier, a researcher based at the University of Würzburg in Germany, is leading the project. He told OLE that the primary goal of BIGBAND was to produce QD-based semiconductor optical amplifiers (SOAs) operating over the entire telecommunications window. SOAs are seen as potential replacements for erbium-doped fibre amplifiers in optical networks. "There is a great opportunity for quantum-dot lasers in semiconductor optical amplifiers," said Reithmaier. "SOAs based on quantum wells are of limited use due to their recombination properties, which means that they cannot operate above 10 GHz."

Existing SOAs have another drawback, in that each device can only operate at a single wavelength, says Reithmaier. "With quantum dots, the gain profile is both spectrally and spatially distributed, meaning that we can build lasers and amplifiers with an ultra-wide gain bandwidth."

Wiggins disagrees with Reithmaier's expectations for QD-based SOAs, however. He told OLE that the most promising initial market for QD lasers will be replacing QW-based source lasers in metropolitan and long-haul applications. "The demand for high-speed and ultra-low-chirp performance in an established laser market outweighs the demand for QD solutions in an emerging SOA market," he commented. "At this point, SOAs are simply a smaller emerging market, with less total potential for adoption of QD technology."

The Zia 1550 nm laser is designed to sit at the heart of a tunable external-cavity laser system. The 1310 nm uncooled DFB laser emits 5 mW optical power at 10 Gb/s operation - which is enough to send a signal a distance of 80 km without the need for additional coolers or isolators.

Like Zia, the BIGBAND project also has designs on producing an external-cavity tunable laser. The plan is to build a device that provides modehop-free tuning over 300 nm. But in contrast with Zia, telecommunications will not be the only beneficiary of BIGBAND. An additional goal of the project is to push InP device emission to longer wavelengths for gas-sensing applications.

The Würzburg team has fabricated quantum-dash lasers (slightly elongated QDs) on an InP substrate and according to Reithmaier, it should be possible to get beyond 2 µm. The active material is indium arsenide, and emission wavelengths range from 1.54 to 1.78 µm. The competition at these wavelengths comes from antimonide-based lasers, but being based on InP gives QD lasers an inherent advantage - the devices can be made with standard epitaxy equipment, and it is easier to tune chips in the wafer to emit specific wavelengths.

Another European project, Ultrabright, has also been on the QD trail. Now near completion, 980 nm lasers emitting nearly 4 W per facet have been produced. Reithmaier says that although some high-power QD products for optical pumping applications might result from the project, they would have to show dramatic advantages over rival quantum-well technology, as well as excellent reliability, to become a serious commercial option.

Reithmaier thinks the chances of commercialization are good for QD lasers, because manufacturers would not need to buy new epitaxy equipment to fabricate the devices. However, European firms entering the fray would have to be ready to play catch-up, according to Wiggins: "Zia currently has an 18-month lead on the competition in developing QD lasers."

Competition is inevitable

In spite of the current dearth of market rivals, Wiggins is convinced that things will change. "We expect that over the next 10 years all new telecoms lasers will start to have a QD structure, thus competition is inevitable. We expect this from both established and start-up companies," he said.

Given the state of the optical telecoms market, survival is a more immediate concern for Zia. Owing to its outsourced manufacturing partnership with CyOptics, and its tight rein on staff numbers, Wiggins says that Zia has always maintained a low rate of cash burn. This should keep it afloat until the market returns to normality.

As with other semiconductor laser technology, some QD laser researchers are now looking towards the blue end of the spectrum and exploiting the wide-bandgap properties of gallium nitride (GaN). A Japanese team is leading the way. Yasuhiko Arakawa and colleagues at the University of Tokyo have been modelling the threshold current expected for such devices, and have also fabricated GaN-based QD laser structures.

Using QD structures has an extra benefit for large-bandgap semiconductors, as the threshold current for lasing increases with the bandgap energy. So although the switch-on current for a conventional GaN laser is typically 10 times that for a GaAs device, this is not the case with QD structures. In fact, the threshold current density for QD lasers in either material is approximately 10 A/cm2 - 100 times less than in QW GaN lasers.

So far, the Tokyo team has seen broad, low-intensity emission from QDs centred at 430 nm. Its efforts are now focused on gaining better control of QD size and distribution during the growth stage, and achieving electrically-pumped lasing.

The practical application of atomic physics: how quantum-dot lasers work

Charge carriers in a normal bulk semiconductor hop between the conduction and valence bands when a current flows. Transitions between these bands result in the emission of light. In bulk crystals, the two bands extend over a range of energies because of interactions between closely packed atoms in the crystal lattice. A broad range of wavelengths of light is emitted.

By confining the dimensions of a semiconductor in one, two or three dimensions to form a quantum well, quantum wire or quantum dot (QD) device respectively, charge carriers are similarly confined to the bottom of the conduction band and the top of the valence band. Lasing in devices confined in this way is therefore restricted to a much narrower range of wavelengths than in a conventional semiconductor, approaching the atomic ideal of infinitely narrow linewidths. The emission wavelength is determined by the dot size, which means that by controlling the size distribution, the range of emission wavelengths can be tailored for individual lasers.

Restricting the effective bandgap also enhances the material gain and reduces the influence of temperature on laser performance. A QD laser therefore enables the practical application of atomic physics in semiconductor devices. This leads to low operating currents due to an enhanced gain, very narrow linewidth and minimal adverse effects with increased temperature.