LEDs technology
rhombus4
OCT application. Companies such as Exalos, Superlum,
and Inphenix have emerged as suppliers of sources for Figure 1:
the emerging OCT supply chain. Yet these products are Optical
generally based on InP or GaAs-based quantum well Cohernence
technology, and realising a significantly wide broadband Tomography
output from this class of device is very challenging. Schematic
However following the lead from researchers in EPFL,
who were later to found Exalos, we became interested in
exploiting the properties of quantum dots for such
applications and have been working on these devices
within the European FP6 project ‘Nano-UB sources’
together with OCT technology partners.
Indium Gallium Arsenide quantum dots (QD’s) have many
useful properties for this application. They are naturally
broadband, exhibiting a statistical variation in size and
shape within a typical ensemble. In addition, under
increasing carrier injection the QD ground state can
saturate in favour of emission from a first, or subsequent
excited states. Combining just the ground and first excited
state (GS and ES) emissions can give a broadband
output of typically around 100nm.
But there are two complicating factors. Firstly efforts have
to be introduced to reduce the dip between emission
states otherwise a multiple peaked image results in quantum well can offer 50-100cm
-1
. Despite this, it is
multiple or ‘ghost’ OCT images. This is done by relatively easy to obtain low threshold and high power
introducing a variation into some of the quantum dot output from QD lasers because the losses can also be
layers in the structure to progressively shift (or ‘chirp’) the very low.
emission. In the MBE growth that is carried out at the
University of Sheffield, this is achieved by varying a However the same is not true of superluminescent diodes
structural parameter, usually the position of the QD within which operate on single-pass amplification of
a surrounding quantum well. spontaneous emission, rather than multi-pass feedback to
induce stimulated emission. A quantum dot
The second problem with quantum dot structures is the superluminscent diode is therefore a long device (typically
relatively low gain. Gain values of 20-40cm
-1
are typical of 4-6mm) in comparison to a laser diode (1-2mm) and it
multi-quantum dot active layers whilst a single InGaAs/InP needs to operate under higher injection and therefore
higher thermal loading.
Project partners Alcatel-Thales III-V lab and Optocap Ltd
have fabricated and packaged superluminescent diodes
targeting two wavelengths, one an established market
segment around 1300nm and one at the shorter
wavelength of 1050nm which is more suitable for
ophthalmology.
Output powers (ex-facet) of up to 160mW and
bandwidths of 115nm have been observed, although it is
hard to obtain high power and broad bandwidth at the
same time. For the more commercially viable 1050nm
devices, a more typical result would be the measurement
of 130mW ex-facet and 65mW ex-fibre packaged from a
device exhibiting ~70nm bandwidth. Such a device was
exhibited by Alcatel-Thales III-V lab at the Munich CLEO-
Europe meeting in 2009. The QD technology is very
Figure 2: Spectral evolution of superluminescent diode competitive against any other alternatives at this less
output as a function of injection current developed wavelength.
November / December 2009 www.compoundsemiconductor.net 35
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