Diode Laser Simultaneously Emits at Three Wavelengths

Michael D. Wheeler

Federico Capasso and a team of researchers at Bell Labs have demonstrated a semiconductor laser that simultaneously emits at three wavelengths in the IR. Such a laser may find applications in spectroscopic analysis of trace gases in the atmosphere for environmental monitoring and industrial process control.
For years, researchers have investigated the prospect of creating a diode laser that emits at well-separated wavelengths at the same time.

Superlattice crystals
After attempts to create a multiwavelength diode using the original quantum well laser -- a type of laser he and several colleagues pioneered in the mid-1990s -- Capasso turned his attention to a quantum cascade (QC) laser that incorporates a superlattice architecture using GaInAs/AlInAs(InP) (see Photonics Spectra, June 1997, p. 30). Its primary advantage over its predecessor is that its energy levels are grouped in minibands that can carry large currents, which give rise to high optical powers.
Capasso enlisted the help of Alessandro Tredicucci, who came to Bell Labs to improve the superlattice design. "While I was optimizing the design of the QC laser -- in particular playing with the doping -- we made some devices that showed this kind of multipeak spectrum in the subthreshold spontaneous emission," Tredicucci said.
Modifying the thickness of each superlattice -- from five to 20 atoms -- yielded the multiwavelength laser. When they pumped the device with 4 A of power, they observed lasing at 100 mW of power at 6.6 and 8.0 µm. When the current exceeded 10 A, a third wavelength appeared at 7.3 µm.
"We've always been chasing the idea of simultaneous emissions," Capasso said. The superlattice structure provided "much more latitude" in designing and controlling the wavelength, he said.
Although it could be years before the device is commercialized, Capasso said the laser would work well in systems that detect the presence of atmospheric gases. They often require two lasers -- one to detect a gas and a second as a reference. A second potential application involves monitoring more than one gas at once, as in the combustion process of jet engines.