Room-temperature quantum-cascade lasers generate 200 mW at 5.2 microns
Modifying the design of a vertical-transition quantum-cascade laser has enabled Jerome Faist and coworkers at Lucent Technologies, Bell Laboratories (Murray Hill, NJ) to develo¥devices capable of pulsed-mode, room-temperature operation, thus demonstrating the first uncooled mid-infrared-emitting laser. In quantum-cascade lasers, electrons tumble between upper and lower energy levels in a quantum well, emitting photons in the process; the thickness of the quantum-well layer determines emission wavelength. The new devices, described in paper CPD9-2 at CLEO `96, consist of indium gallium arsenide/aluminum indium arsenide (InGaAs/AlInAs) heterostructures grown on indium phosphide (InP) substrates. The In¥cladding offers a lower thermal resistance than the AlInAs cladding used on previous designs, allowing increased operating temperature (see Laser Focus World, June 1994, p. 15 ). To increase gain, the design features a three-well active region, low n-type doping, and a "funnel injector"--a narrowed miniband to guide electrons to the upper state of the next period.
At 300 K, the 5.2-µm devices generated peak powers of 200 mW; at 320 K, output power was above 100 mW. Corresponding average powers ranged from 5 to 10 mW. All results were obtained in pulsed mode, with pulse durations of 50 ns at 670 kHz. In continuous wave mode, the devices still require cooling and produced average powers of 2 mW at 140 K. The researchers have also produced room-temperature devices operating at 8.5 µm with peak powers of around 13 mW.