Optical frequency measurement: 40 years of technology revolutions

The past forty years have witnessed spectacular progress in precision measurements, beginning with the first coherent optical source, the HeNe CW laser. Almost immediately, the pioneers of the stable laser epoch introduced optical heterodyne techniques to explore the stability of the laser's optical frequency, expecting the few milliHertz linewidth predicted by the Schawlow-Townes formula for phase diffusion, and instead finding myriad physical processes that broaden and jiggle optical frequencies far beyond that narrow range. After a huge time and learning effort, most of these technical limitations can now be overcome. The paper is intended to offer a bystander/sometimes-participant's view into the advances in laser tools and the exciting scientific/technical worlds that were opened by them. Roughly one "zillion" further clever inventions and techniques have added capability to the laser and have enabled many bursts of productive research. Now, after nearly forty years of incremental progress, the astonishing reality is that within the past one year - absolute optical frequency measurements have suddenly become feasible, simple - even routine. This welcome and unexpected progress is based on the merger of techniques from the ultrastable lasers and the ultrafast lasers research communities. In a very real sense, these research objectives seem to be the exact opposites - the most unchanging versus the most quickly changing.