Scientists from Technische Universität Darmstadt and Fraunhofer Institut für Angewandte Festkörperphysik in Freiburg, both in Germany, have determined that the dependence of the intensity noise on optical power in quantum cascade lasers differs from that in edge-emitting laser diodes, vertical-cavity surface-emitting lasers (VCSELs) and lead-salt laser diodes. The finding has implications in spectroscopy and free-space communications applications, for which laser noise is a limiting factor on performance.Since they were first reported in 1994, quantum cascade lasers have been of particular interest for spectroscopy because they emit in the mid-infrared region, where molecular signatures are strong. Electrons in these devices emit photons as they tunnel successively through the barriers between a series of quantum wells. Transition energies are determined by the thickness of the layers in the quantum cascade laser rather than by the material system, easing the development of devices that emit at the desired wavelength.To understand the performance of these lasers, the research team investigated the relative intensity noise of a 25-period GaInAs/AlInAs-on-InP device as a function of emitted optical power, measured at a frequency of 40 MHz. The CW laser produced 4.98-µm radiation in a single longitudinal mode. The results fit a power law, but with a power scaling parameter of approximately two, rather than three as in edge-emitting laser diodes, VCSELs and lead-salt laser diodes.Using a semiclassical noise model, the scientists determined that the different scaling parameter results from the shorter electron lifetime in quantum cascade lasers -- three orders of magnitude shorter than in other semiconductor lasers. They also found that the power scaling parameter is a function of the number of three-level gain stages in the active region of a quantum cascade laser.The results offer interesting perspectives on these devices, said Tobias Gensty of Technische Universität Darmstadt, both theoretical and in terms of applications. The new level scheme represents an interesting quantum optics system, he said, and understanding and controlling noise mechanisms in the lasers will affect their development for sensing and communications.