External-cavity designs satisfy strigent demands

Tunable narrow-linewidth lasers first became commercially available around 20 years ago with the development of etalon-narrowed CW dye lasers from manufacturers such as Coherent and Spectra-Physics. By 1977, high-end research systems could produce linewidths as narrow as 1 MHz under basic microprocessor control. Evolution of these lasers continued with the introduction of higher-gain ring lasers and eventually the replacement of the flowing-dye gain medium with solid-state crystals, especially Ti:sapphire. At the same time, the ion lasers used for pumping became more reliable, with longer lifetimes and improved ease of use.