semiconductor lasers or quantum cascade (QC) lasers were developed at
in the USA in the eighties, with the help of some key European researchers.
The Europeans eventually opted to come back across the Atlantic and continue
their work on home ground. Under Brite/Euram project UNISEL, they have
demonstrated successful use of a new class of devices for gas sensing
applications in the areas of environmental monitoring and process control.
exploit the quantum behaviour whereby electrons assume a series of discrete
energy levels. When electrical or light energy is applied to these minuscule
devices, they produce powerful emissions at certain specific wavelengths.
By adjusting the thickness and composition of the semiconductor layers
in a microchip it is possible to tune the emission wavelength to a desired
particular interest for industrial purposes are wavelengths in the
mid-infrared range (3-15 µm). Using light sources in this spectral
range, molecular spectroscopy techniques enable various gases to
be detected and identified at concentrations as low as a few parts
per billion. These methods have great potential for monitoring trace
discharges of waste gases, as well as for checking combustion efficiency
and controlling processes in industries such as foods, beverages
date, however, the only commercially available sources for such
applications have been semiconductor lead salt lasers, which function
only at cryogenic temperatures. The original unipolar semiconductor
lasers, manufactured from indium phosphide, suffered from similar
aim of UNISEL was therefore to support the first European research
in this area, demonstrating that other semiconductor materials could
be employed for laser production and seeking to achieve operation
at ambient temperatures. The consortium, led by the Thompson
CSF Central Research Laboratory (now Thales),
brought together a number of the former Bell workers and other researchers.
It included academic participants from the Universities of Paris,
Neuchâtel, and Vienna, and Italy's INFM
(Institute of Material Physics), together with industrial partners
and Orbisphere Laboratoires.
on several fronts
the initial year of the 36-month project, the group was able to
reproduce the QC effect using various materials. In particular,
they focused on gallium arsenide (GaAs), which is a readily available
compound semiconductor already widely used in the electronics industry.
While the optical and electrical characterisation of materials and
devices continued in the second year, the effort shifted progressively
towards the development of unipolar lasers into devices for spectroscopic
major requirements for an infrared technology suitable for gas detection
and trace gas analysis were wavelength stabilisation control and
the elimination of cryogenic cooling. By creating novel designs
and processing architectures, the group succeeded in both respects.
It has produced devices that emit at well-defined wavelengths, and
demonstrated unsurpassed electrical tunability at a fixed temperature
of -10 Cº. Cooling to temperatures in this range can easily
be achieved with thermo-electric (Peltier) coolers. Improvements
in the performance of optically pumped lasers have also led to devices
with a record peak power of more than 15W.
further practical achievement of UNISEL was the establishment of
design rules for device packaging and power supplies, facilitating
exchanges among the partners and producing standard components for
supply to end users.
the second year, industrial partners worked actively, studying and
preparing detection systems that incorporate the unipolar semiconductor
lasers. Two different approaches have been tried: the first of these
is based on conventional absorption spectroscopy, while the second,
more suitable for implementation in low cost systems, uses photoacoustic
techniques. Both are now at the prototype stage, and could reach
commercialisation within one or two years.
well as giving Europe a clear global lead in QC laser, and especially
the GaAs-based QC laser market, UNISEL has produced a number of
other spin-off benefits. Following involvement in the project, EC
scholarship student Hideaki Page has found permanent employment
as an engineer with Thales, while an earlier PhD student has joined
Orbisphere. A new start-up company, Alpes
Laser, formed as an offshoot of the University of Neuchâtel
will become the first volume producer of such devices. Moreover,
their application in environmental monitoring and process quality
improvement will have positive implications for businesses and citizens
throughout the EU.
is a good example of how the 'vertical integration' of basic research
on the materials system, devices and components, and end-user applications
can lead to novel product prototypes.