Laser Emits Broadband Radiation

Richard Gaughan

Think laser, and odds are that you'll think monochromaticity. A quantum cascade laser developed at Lucent Technologies Inc.'s Bell Labs in Murray Hill, N.J., however, threatens to overturn this "narrow" conception.

Quantum cascade lasers are unusual exhibits in the laser menagerie in that their physical construction rather than their material characteristics determines the gain profile. Their emission is the result of electrons transitioning from a higher to a lower energy state within a quantum well, with the width of the well primarily determining the energy levels. Interspersed injector regions cause the electrons to move from well to well and therefore to emit a photon with each transition.

Researchers at Lucent Technologies Inc.'s Bell Labs, including Deborah Sivco (left) and Claire Gmachl -- shown here characterizing an earlier laser -- have developed a quantum cascade laser with an output of 6 to 8 µm. Courtesy of Claire Gmachl, Bell Labs.

Lucent had developed a variety of quantum cascade lasers, with emissions of 3.4 to 17 µm -- each constructed of the same material but with different well thicknesses. The new device, rather than employing identical quantum wells repeated a number of times, features 36 different InGaAs/AlInAs wells in the cascade. The material is transparent to photons throughout the desired wavelength range, and the quantum wells are designed so that the optical gain curve of one well overlaps that of the next.

The net result is a laser with a broad emission profile. In tests, the 12-µm-wide, 2.25-mm-long laser, driven in pulsed mode with a current of 8 A, emitted from 6 to 8 µm. Although individual Fabry-Perot modes were visible in the spectrum, the profile varied smoothly. At a temperature of 10 K, the laser displayed a peak power of 1.3 W; at 300 K, the laser still operated, but the optical output power was reduced by an order of magnitude.

Claire Gmachl, the physicist leading this work, said that the laser's performance indicates how far the understanding of these devices has matured in the past few years, but she hopes to widen its spectral range.

Its first application may be in trace gas sensing. "As the laser is able to operate over a very wide wavelength range at the same time," Gmachl said, "one should also be able to manipulate the laser as to give one wavelength at a time, thus making a very broadly tunable source."

The prospects for custom spectra are also intriguing, she said, enabling the laser to produce one wavelength to manipulate a target while another probes it.