30, 20, and 10 Years Ago in OPN

“A blue semiconductor diode laser has been sought for many years, but the fabrication of p-n junctions in semiconductor materials capable of emitting blue light has proven to be a very difficult technical challenge … Developing compact blue lasers has depended on finding combinations of lasers and nonlinear materials compatible for upconversion and on finding configurations that permit an efficient interaction. So far, the lasers available have been limited to GaAlAs laser diodes at wavelengths in the 700 to 900 nm range and to solid-state lasers that could be pumped by these laser diodes.”