26 May 2010
Could quantum dot lasers be the ideal optical source for future high-speed data communications?
Fujitsu, Fujitsu Laboratories and the University of Tokyo claim to have developed the first quantum-dot laser capable of 25 Gbps data transmission. The hope is that this new technology will be employed in next-generation high-speed data communication systems, which aim to achieve data transmission speeds of 100 Gbps, 10 times faster than what is currently used.
In order to keep pace with data transmission volumes that continue to grow larger each year, optical networks need to run faster and expand to handle higher volumes of data. Currently, 10 Gbps transmissions are the norm. However, in view of the fact that transmission speeds are continually becoming faster, 100 Gbps Ethernet is being promoted as an international standard for next-generation high-speed data communications. In turn, there is a need for laser sources that can work at these higher speeds, while only consuming low power.
Quantum well lasers have conventionally been used as optical sources for data communications. The problem facing this technology is its sensitivity to higher temperatures and the corresponding dramatic increase in power consumption.
Although offering stable operation over a range of temperatures and low power consumption, until now, the quantum dot lasers developed by Fujitsu Laboratories and the University of Tokyo had been limited to handling data transfer speeds of up to 10 Gbps.
To increase the speed of the quantum dot lasers it was necessary to increase the laser's optical gain, a move which required an increase in the number of underlying quantum dots. New quantum dot fabrication technology was developed and applied, resulting in quantum dot lasers capable of operating at 25 Gbps.
The quantum dots were fabricated using molecular beams in a high vacuum chamber to embed In and As in GaAs. Because the atomic distance in InAs is greater than in GaAs, strain was induced in the InAs crystallized on the GaAs substrate. The strain energy can be relaxed by forming 3D crystals, in which each 3D nanocrystal functions as a single quantum dot.
By optimizing the growth conditions in which the 3D crystals are formed, a technology has been developed to double the in-plane density of the quantum dots to 6 × 1010 per 1 cm2. Technology was also developed that allows eight layers – instead of five layers – to be stacked while preserving the same high density in-plane.
This new technology has made it possible to employ temperature-stable, low-power-consuming quantum dot laser sources in a range of next-generation, high-speed communications applications. It also obviates the need for expensive packages with embedded temperature controllers, thereby helping to lower costs.
Fujitsu Laboratories and the University of Tokyo plan to refine this technology to increase its transmission distance and to confirm reliability and add that QD Laser, Inc, is also considering commercializing the result.
The researchers presented details of their development at the recent Conference on Lasers and Electro-Optics and the Quantum Electronics and Laser Science Conference (CLEO/QELC 2010).