For most of the past 40 years, ultrafast-laser researchers—those who focus on making and using the shortest pulses—largely ignored the pulse phase and the theoretical comblike spectrum of an ideal series of pulses. Their experiments typically only depended on the intensity of individual pulses, in which case the phase has no effect. Although the members of the ultrafast community often measured the spectrum of their mode-locked lasers, they rarely did so with sufficient resolution to observe the underlying comb spectrum; instead the lines would blend together and look like a continuous band of frequencies.
High-resolution measurements were the domain of specialists in precision spectroscopy and optical frequency metrology, wherein highly stable CW lasers reigned as the preferred tools. As mentioned earlier, a CW laser sends out a steady stream of light at a precise frequency, and its spectrum looks like one sharp spike. Not many researchers in the metrology community were cognizant of the workings of mode-locked lasers, and those who did know about them were skeptical that such lasers could produce a well-defined comb spectrum in practice. They expected that modest fluctuations in the timing or the phase of the pulses would wash it out.
But a few researchers, most notably Theodor W. Hänsch of the Max Planck Institute for Quantum Optics in Garching, Germany, had faith that mode-locked lasers could one day be a useful tool for high-precision spectroscopy and metrology. In the 1970s, while a faculty member at Stanford University, Hänsch used mode-locked dye lasers (which have a colorful liquid dye as the medium where the laser light is generated) to do a series of measurements that established the basic concept of the comb spectrum and its offset frequency. These seeds then lay dormant for almost 20 years until laser technologies had advanced enough for further progress with combs to be practical.
In the late 1980s Peter Moulton, then at Schwartz Electro-Optics in Concord, Mass., developed titanium-doped sapphire as a laser gain medium with a large bandwidth. Wilson Sibbett of the University of St. Andrews in Scotland pioneered its use in mode-locked lasers in the early 1990s. Within only a few years, titanium- sapphire lasers were routinely generating pulses shorter than 10 femtoseconds, corresponding to only three cycles of light [see “Ultrashort- Pulse Lasers: Big Payoffs in a Flash,” by John-Mark Hopkins and Wilson Sibbett; Scientific American, September 2000].
With these titanium-sapphire lasers available, Hänsch dusted off his 20-year-old idea of optical frequency combs. He performed a series of experiments in the late 1990s that demonstrated the latent potential of mode-locked lasers. In one measurement, he showed that comb lines at opposite ends of the output spectrum are well defined with respect to one another. The comb teeth were revealed to be like the marks engraved on a steel ruler and not like lines drawn along a rubber band. In another experiment, he measured the frequency of an optical transition in cesium atoms (a change in their state that absorbs or emits light at a precise frequency) using a mode-locked laser to span the difference in frequency between two CW lasers. His results inspired a group of us to undertake serious research in this arena.
At JILA, a joint institute between the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, we were in a unique position to take the technological advances in two branches of laser physics and run with them. JILA has a strong tradition in optical frequency metrology and precision spectroscopy, largely built on the ultrastable CW laser technology developed over 40 years by one of us (Hall). In 1997 another one of us (Cundiff) joined JILA, bringing expertise in mode-locked lasers and short-pulse techniques. It took many hallway and lunch table conversations before we surmounted our conceptual divide and decided to join forces, along with a pair of postdoctoral fellows: Scott Diddams, now at NIST, and David Jones, now at the University of British Columbia. The third of us (Ye) joined the fun at JILA in the summer of 1999, just as the revolution began in earnest; he soon led the way to finding applications for the new frequency combs.