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Broadband laser tailored for sensing

17 Jun 2002

A quantum cascade laser that emits at all wavelengths between 6 and 8 µm is described in today's issue of Nature.

The quantum cascade laser (QCL) research team at Bell Labs, US, has made a broadband device that emits over the 6-8 µm range. The source ought to prove useful for multiple-gas sensing as well as terabit data communication and high-precision metrology (Nature 415 883).

Claire Gmachl and colleagues made the device by stacking 36 different active-layer regions on top of each other. Each of these has a slightly different layer structure, and emits at a particular wavelength range. In the stack formation, the light-emitting properties combine to produce broadband laser emission.

"In the new laser, we make use of the wings seen [beside laser peaks] in the optical spectra. By stacking up the active regions, these wings become as useful as the laser peaks," explained Gmachl.

According to her, a mathemetical concept describing the ultra-broadband source was first formulated. The QCL was then grown using molecular beam epitaxy - and to the team's surprise the device worked at the first attempt.

The InGaAs and AlInAs structure was grown on an indium phosphide substrate and the waveguide structure optimized to lase at 8 µm. At cryogenic temperatures (10 K), the device emitted about 1.35 W with a 7 A driving current.

The lasing threshold was seen at 1.07 A for 7.3 µm emission. As this current was increased to 5 A, the wavelength range increased to 6-7.6 µm, and at 8 A the range extends to 8 µm.

At room temperature, the laser threshold was 4.5 A, with emission between 7.1 and 7.7 µm seen under a 7 A driving current. Gmachl told Optics.org that the work on improving room temperature performance is well underway.

The team adds in its paper that the broadband nature of the emission should mean that few-femtosecond pulses could be produced with modelocking. In principle, the design could also be extended to operate over a much wider wavelength range, said Gmachl.

She believes that there will be many applications in gas sensing initially, as the broadband QCL could be used in sensor devices in place of many individual sources: "We believe that this will be a great source for spectroscopy. On one hand, one can easily turn it into an ultra-wide tunable device; on the other, multi-wavelength lasers could be custom-designed to probe certain absorption spectra."

The first applications in trace-gas sensing are expected within a year, and the technology is to be licensed as with the previous QCL devices. "Nevertheless, our goal is to apply the idea to the shorter fibre-optic wavelength (i.e. the 1.3-1.6 µm region)," Gmachl added.

Michael Hatcher is technology editor of Opto & Laser Europe magazine.

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