Tunable lasers in one form or another have been a goal of researchers since the invention of the laser. A single source that emits at various wavelengths is inherently more cost-effective than multiple sources that achieve the same result. Dye lasers date back to 1967 and were the first broadly tunable laser systems—their output can be tuned across the spectral region from the near-ultraviolet to the near-infrared; frequency doubling can further extend the tuning range into the ultraviolet. Unfortunately, dye lasers are relatively expensive and complex, not very user friendly, and do not lend themselves to compact turnkey laser systems. While they are still around (such as for medical applications), other types of tunable laser systems, including solid-state configurations, have displaced almost all dye lasers.
Most recently, development requirements of optical communications has spurred the quest for tunability. A tunable diode laser could address several needs, from "sparing," in which a single tunable device can be held in stock as a potential replacement for many fixed-wavelength devices, to real-time network reconfiguration, in which lasers might be remotely programmed to change wavelengths, thereby adding flexible bandwidth to the network. Such diode lasers are starting to emerge from development and were much in evidence at last month's NFOEC conference (Baltimore, MD). Many alternative design approaches have been taken, some of which are discussed this month on page 101.
At the other end of the size scale, and still firmly in the research arena, developments at one of the largest tunable laser systems in the USA—the free-electron laser (FEL) at the Thomas Jefferson National Accelerator Facility (Newport News, VA)—include upgrades aimed at extending FEL operation in the IR and providing kilowatt operation in the visible and UV (see cover and p. 93).
Tunable sources and detectors also are fundamental to spectroscopy. The ability to adjust the wavelength and to produce narrow-bandwidth output enabled development of novel methods to investigate processes within atoms and molecules. Advances include ultrafast spectroscopy and real-time Raman analysis. These and other modern spectroscopic techniques are discussed in the Optoelectronics World supplement, which follows page 130. Elsewhere this month, we include an update on membrane-mirror light-modulator technology (see p. 177), a review of organic light-emitting diodes (see p. 169), and a look at how a power-control device improves the marking ability of scanned carbon dioxide lasers (see p. 117).