For more than eighty years, infrared spectroscopy research and applications have played an increasingly important role in science and technology. Infrared laser spectroscopy has yielded many important results using a variety of tunable laser-based sources, particularly lead salt diodes, color center lasers, difference frequency generation, optical parametric oscillators, and sidebands on fixed frequency gas lasers. Quantum cascade lasers (QCLs) are a much more recent tunable laser source of infrared light. The first QCL was invented and demonstrated  at Bell Labs in 1994; QCLs developed rapidly; for reviews of this earlier work see Refs. ,  and the June 2002 issue of IEEE Journal of Quantum Electronics.
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