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Development of 1.3 µm InAs/GaAs quantum dot densities (Jth) at room temperature <30 Acm{-2}

This result is directly related to D22, but also includes (and supercedes) D1. USFD, in very close collaboration with BHM, have developed state-of-the-art 1.3µm QD lasers with record low threshold current densities compared to the published literature.

Nature of the result
1.3µm is the minimum dispersion wavelength of standard optical fibres, the type used widely by TDM and CWDM systems. Current laser products are based on InP substrates, which are more fragile, more expensive to fabricate and generally smaller than their GaAs counterparts. In addition carrier confinement in current InP based QW systems is poor, resulting in high laser temperature sensitivity. The growth of InGaAs QWs emitting above -1.1µm on GaAs substrates is prevented by strain effects. Self-assembled QDs enable this strain limitation to be overcome, providing a cost effective GaAs based system. Using QDs, projected yields and costs can be significantly improved by moving to a GaAs based platform.

The ultra-low threshold current densities predicted for QD lasers have taken some time to materialise in practise, primarily due to the extremely high demands placed upon the optical quality of the QDs. The result achieved on this programme, which followed extensive growth optimisation, represents current state of the art. Most significantly a lower threshold current density has been achieved compared to any QW based device. A low threshold current is important in that the current used in reaching threshold is essentially wasted, and merely serves to heat the chip.

Potential applications of the result
Direct modulation QD lasers for uncooled 1.3µm data transmission at 10GBs-1.

End-users of the result
Telecommunications components manufacturers, telecommunications systems

Main innovative features/benefits
Fibre-optic wavelength laser based on a GaAs substrate. High temperature operation possible with low threshold current density.

Analysis of market or application sectors
The main market for 1.3µm lasers is data transmission in optical fibres. Since new 1.3µm systems will run at 10Gbit/s, suitable lasers will need to exhibit 3dB bandwidths of -12GHz (to allow for error correction data etc). Direct modulation of the laser is desirable for short haul applications since a separate modulator would dramatically increase chip real estate or necessitate costly coupling optics. There are currently established 1.3µm lasers based on InGaAsP QWs grown on InP substrates.

A potential market is provided by the fact that these established devices are not suitable for high temperature operation without a costly and power hungry thermo-electric cooler. Additionally, QD lasers are predicted to suffer less than QW lasers from optical feedback effects, and so may not require the same level of optical isolation.

Potential barriers
There is currently a very limited body of results obtained from 1.3µm single frequency lasers based on QDs. In particular much more work is needed on the dynamic properties of QD lasers to ensure that suitable link reach at 10GBs-1 can be achieved.

A more serious barrier to the eventual uptake of QD based 1.3µm lasers may be that the limitations of the incumbent technology are presently being addressed by several device manufacturers via the development of the AlInGaAs material system on InP and GaInAs(Sb)N on GaAs. It is not yet clear which technology (QDs, AlInGaAs-InP and GaInAs(Sb)N-GaAs) will be the most successful.

Informations connexes

Reported by

University of Sheffield
Department of Physics and Astronomy
S3 7RH. Sheffield
United Kingdom