Unprecedented Spectroscopy Using the Best Ever Ruler for Light

Physicists at NIST-Boulder have carried out a powerful new spectroscopic study of a sample of gas using optical frequency combs. The NIST work, which might well change the way spectroscopy is done, is remarkable in that it provides the full spectrum of the gas over a broad spectral region and with frequency accuracy that can reach 1 Hz (for spectral frequencies of the order of 2 x 10^14 Hz). The NIST spectroscopic feat is equivalent to simultaneously sending 155,000 individual single frequency lasers through the sample and measuring the resulting amplitude and phase shift on each individual laser. Moreover, the spectrum is measured rapidly, using a device with no moving mechanical parts.

The invention of the optical frequency comb method was a great step forward in laser science. John Hall (NIST) and Ted Haensch (Max Planck) the Nobel prize in 2005 for their pioneering work in this area. (For a tutorial on frequency combs, see http://www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm) In the comb process, a pulsed laser emits light not merely at a single frequency, but at a series of frequencies. A frequency spectrum of this composite laser output looks like a comb, with light occurring at regularly spaced frequencies, covering the infrared part of the light spectrum. In many ways the frequency comb is an ideal tool for spectroscopy. Its light covers enormous amounts of the optical spectrum and the frequency of each individual comb line can be known to 1-Hz precision. When you pass a frequency comb through a gas cell a given comb line will, like any laser beam, be absorbed when it is resonant with any of the many quantum energy levels of the gas.

The challenge with frequency combs is to figure out which of the more than one-hundred thousand comb lines experience absorption and which do not. To solve this problem NIST researchers take the comb used for spectroscopy and mix it with a second carefully crafted frequency-comb. This ensemble of light pulses results in a “beat-frequency” pulse which can be measured with conventional electronics. From this beat-frequency pulse the absorption and phase shift experienced by each individual comb line can be separately observed. This work represents by far the largest number of frequency comb teeth that have been individually observed.

The present NIST experiment interrogates the effect of the absorption from the gas on 155,000 comb lines, spanning a wavelength range of 125 nm. The NIST precision of 1 Hz for spectral lines is to be compared with tens of MHz precision characterizing other spectroscopic techniques. NIST researchers believe that this new work might change the way people perform spectroscopy. (Coddington{ian@nist.gov, 303-497-4889}, Swann, Newbury, Physical Review Letters, 11 January 2008; PRL editors designate this as a Suggested Article)