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US scientists make portable ultra-broadband laser

Compact laser could sense chemicals for a variety of applications

Nearly all chemicals, including explosives, industrial, and pollutants, strongly absorb light in the mid-infrared wavelength region (often called the "˜fingerprint region' for chemicals). But mid-infrared lasers that could be used for chemical sensing have limitations for this application:  optically pumped lasers are too large and complex to use out in the field, while compact, lightweight diode laser sources have a limited spectral range.

Manijeh Razeghi and her team at Northwestern University's Center for Quantum Devices have used quantum mechanical design, optical engineering, and materials development to create a custom-tailored, compact laser diode by integrating multiple wavelength emitters into a single device. The research is described online in the August issue of Optics Express journal.

Described in their paper as a heterogeneous quantum cascade laser, it consists of multiple stacks of discrete wavelength quantum cascade stages, emitting in 5.9 to 10.9 µm, wavelength range. The broadband characteristics are demonstrated with a distributed-feedback laser array, emitting at fixed frequencies at room temperature, covering an emission range of ~760 cm−1, which is ~59 percent relative to the centre frequency. 

By appropriate choice of a strained AlInAs/GaInAs material system, quantum cascade stage design and spatial arrangement of stages, the distributed-feedback array has been engineered to exhibit a flat threshold current density across the demonstrated range, according to the researchers.

Capable of emitting broadband wavelengths on demand, the device is smaller than a penny and works at room temperature. It can also emit light at frequencies within +/- 30 percent of the laser central frequency, which has never before been demonstrated in a single-laser diode, say the team.

"When we started, we knew this technology had great potential," said Razeghi, Walter P. Murphy Professor of Electrical Engineering and Computer Science at Northwestern's McCormick School of Engineering. "It has always been my dream to have such broadband sources, but it took a lot of effort and experience to realize a truly useful device. We can access any frequency in the laser's range on demand at room temperature, which is ideal for sensing applications."

The research is supported by National Science Foundation, US Department of Homeland Security, Naval Air Systems Command, and NASA. 

'Ultra-broadband quantum cascade laser, tunable over 760 cm−1, with balanced gain' by N. Bandyopadhyay et al; Optics ExpressVol. 23, issue 16 (2015)

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