Converging Disciplines
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.
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Add CommentBefore the blogs were canceled here on Sci Am I had a blog under the username "PHAYEZ" which dealt with, among other things, time and measurement. I put forward that time only exists as a feature of three dimensional existence. Time is the "velocity/distance of 3D matter relative to the velocity/distance of other 3D matter". Outside of three dimensions "TIME" as such does not exist and infinity is equal to zero "time". Also outside of three dimensions measurements cannot be made of anything which makes mathematics, with all due respect, irrelevant since the language of mathematics has, as a syntax, products of measurement. The existence and non-existence of time is a spatial relationship oxymoron which is difficult to grasp when you are an entity who is wholly dependent on being made up of atoms which are moving through space.
Reply | Report Abuse | Link to thisOne final comment, if I may, on the speed limit of light and the fact that even information cannot exceed the speed of light. At the speed of light time is zero, in other words for the light there is no time that passes so that regardless of where it arrives, it arrives instantaneously. Nothing can move faster than instantaneously, even information...
Pierre
username: PHAYEZ (Edmonton,Alberta,Canada)
Phayez,
Reply | Report Abuse | Link to thisI would have been interested to read your blog, as you theory is an inverse of that I conceived, namely that particulate was formed in a one dimensional state, that being time and only thereafter is space formed when particulate combines. I nearly agree with the speed of light. Einstein's relativity is based on the speed of light but it does not hold in my reality as we do not see light at all, but feel time "ripples" in spacetime. The speed of light is the speed of the photon, what we see is the ripple of time in the fabric of space. If you can, read my long winded website www.realityofphysics.com for a different view! regards