A range of possibilities

A consortium led by Qinetiq has received £1m from the DTI to develop a laser that could lead to a wide range of commercial applications in a variety of fields, including healthcare and communications.

Mid-infrared lasers are much sought after because the wavelengths at which they operate – in the range of 3-5 microns – are longer than those of conventional lasers.

Qinetiq’s Tim Ashley, the programme leader, said mid-infrared lasers would be particularly useful in detecting certain species of pollutants and environmental gases. As each chemical is detected using different laser wavelengths, a new range of infrared spectrum opens up another group of chemicals that can be detected.

BP, which is part of the consortium, plans to test the the new lasers for remote gas sensing in the petro-chemical industry.

They could also be useful in the area of free-space optical (FSO) communications, in which data is transmitted using a beam of infrared or visible light instead of using fibre-optics. The advantage of FSO is that lasers can distribute high data rates in built-up, urban environments without the associated cost of copper or fibre connections, said Ashley.

A fundamental limitation of FSO is that atmospheric conditions – particularly smog or fog – interfere with the light and stop the system working. The technique is already under investigation at shorter, 1-2 micron wavelengths. Lasers that operate in the mid-infrared are less easily absorbed by atmospheric conditions, meaning that a mid-infrared laser would be operational more of the time than a shorter wavelength laser.

However, the reason these lasers have not been used before is that unlike more conventional lasers, there are many processes that occur in the mid-infrared range that make the lasers inefficient.

‘At those wavelengths the electrons often recombine in the form of heat rather than light, which wastes energy,’ said Ashley. ‘This problem can only be tempered if the instrument is super-cooled to around -200oC, which makes using mid-IR lasers usually very impractical.’

Instead of using expensive mechanical cooling systems to operate the lasers, Qinetiq aims to develop processes that will allow the lasers to operate at normal temperatures, changing the mid-infrared laser from a niche product into a usable technology.

One technique that will be investigated will be to redesign the semiconductor.

Semiconductor manufacture begins by using a substrate upon which different layers are grown until a few microns of material has been built up that can be controlled down to an atomic scale. By altering the way the layers are built up the electronic properties of the semi-conductors can be changed.

For example, Ashley’s team will investigate ‘strained-layer’ engineering whereby a layer with a differing molecular structure is forced to adapt to the host material beneath it.

Researchers at LancasterUniversity have also developed a technology known as a ‘whispering gallery mode’ resonator, which could also improve the laser’s efficiency. To stimulate emissions in conventional lasers the light needs to bounce back and forth inside the cavity using mirrors. The whispering gallery technology sends the light around the perimeter of the cavity and Ashley said this could potentially improve the efficiency of the lasers in the future.

The third main application area for the lasers is in healthcare. One member of the consortium, Anasys, will use the prototype laser in nanoscale photo-thermal imaging to build up 3D images of cellular structures.

‘To do this type of imaging Anasys needs a really good, powerful, compact, mid-infrared range laser,’ said Ashley. ‘This could help in cancer diagnostics by making it quicker and more cost-effective. There is also the possibility that these lasers could have real applications in surgery, although that is likely some time in the future’

According to Ashley, early prototypes that can operate at slightly higher temperatures will be ready within 18 months and the final prototype could be ready for testing in two to three years.