Research Review: Dots Bolster Their Telecom Credentials
Quantum dots enable long-wavelength telecom lasers to combine sufficient output with incredibly low noise figures
ENGINEERS from National Research Council Canada claim to have fabricated the first quantum dot lasers operating at wavelengths longer than 1.5 μm that exceed the 10 mW output power requirement for optical communications.
The team’s single-mode laser produces a continuous-wave output of 18.5 mW at 1.52 μm. Although the emission needs to be increased to 1.55 μm before this class of laser can serve telecom networks, leadauthor Zhenguo Lu says that this next step is fairly simple. “The grating period is easily changed to obtain operation at 1.55 μm," says Lu, “and we have demonstrated quantum dot operation over the C- and Lbands through controlling dot size."
Theoretical benefits associated with switching the active region in a laser from quantum wells to quantum dots include a narrower linewidth and superior temperature performance. The team’s device fulfils the first of these promises – the linewidth is less than 150 kHz, compared to 2-20 MHz for commercial, distributed feedback (DFB) lasers with quantum-well active regions. However, the quantum dot laser falls short on the second promise. “As of yet, InP-based quantum dot lasers have not shown the dramatic improvements in the characteristic temperature predicted for dot-based lasers," says Lu. “They show values similar or slightly better than quantum well-based devices."
A cross-sectional scanning electron microscope image of the quantumdot, distributed feedback laser structure showing the five-layer quantum-dot core and the floating grating. Distances (1) and (2) are 117 nm and 119 nm, respectively
Fiber-pigtailed packaged, quantumdot, distributed feedback laser
Laser fabrication begins with the growth by chemical beam epitaxy of epistructures featuring InAs quantum dots on InP substrates. The undoped active region contains five stacked layers of dots with a density of 4 x 1010 cm-2, sandwiched between 30 nm-thick InGaAsP barriers. After growth of the active core, the wafer is removed to define a grating with a HeCd laser and subsequent chemical etching. Following the formation of this grating with a 236 nm period, MOCVD is employed to add a p-type contact. A single lateral mode, ridge-waveguide laser is formed from the epiwafer with a cavity length of 1 mm and a stripe width of 3 μm.
This device has a threshold current of 48 mA and produces 18.5 mW when driven at 200 mA. Relative intensity noise (RIN) for this laser is –154 to –162 dB/Hz. According to Lu, this compares favourably with both commercial quantum well DFB lasers, which have a RIN of typically –130 to 150 dB/Hz, and commercial quantumdot- based lasers that are usually specified to have RIN values below –130 dB/Hz.
“The potential is there for high-volume commercial laser fabrication," says Lu, who explains that the quantum dot lasers were made in a commercial foundry using a standard commercial process. “The only difference was the use of a quantum dot core rather than a quantum well, but that makes no difference for the processing." Although chemical beam epitaxy is not widely used in foundries, MOCVD has been used by some groups to make quantum-dot lasers on InP.
Z. Lu et al. Electron. Lett 47 818 (2011)