A third family of semiconductor diode lasers emit at longer wavelengths than GaAs- or InP-based lasers and have quite distinct applications. They are called lead salt lasers because many (but not all) of the semiconductor compounds contain lead. They emit wavelengths between about 3.3 and 29 micrometers ( ?m), and most of their applications are in infrared spectroscopy.

Jack F.Butler and colleagues at the Massachusetts Institute of Technology (MIT) Lincoln Laboratory made the first lead salt diode laser of lead telluride in 1963 (Butler et al., 1964). That demonstration came only a year after the first diode lasers of any kind were reported, but for many years it stimulated little interest because of its long wavelength. Fiber-optic and military system needs for compact near-infrared sources instead pushed development of III-V diode lasers.

Interest in high-resolution infrared spectroscopy, which developed during the 1970s, has been the major impetus behind lead salt laser development. Most work has focused on spectroscopic applications, including measurement and monitoring of various compounds, but there has been some interest in long-wavelength fiber-optic communications and the testing of infrared detectors, Serious technological limits remain, however, including the need for cryogenic cooling and limited output powers from narrow-line lasers. Performance has been improved by adapting technology originally developed for shorterwavelength diode lasers, and continuing work in that area should pay more dividends.