A new direction for gas spectroscopy

Physicists at NIST-Boulder have carried out a powerful new spectroscopic study of a gas sample using optical frequency combs.

The work, showing a new direction for future spectroscopy, 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 10¹⁴ Hz).

The 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.

In the process, a pulsed laser emits light at multiple frequencies, providing a frequency spectrum that resembles a comb. Light occurs at regularly spaced frequencies, covering the infrared part of the light spectrum.

The challenge with frequency comb lasers is to figure out which of the more than 100,000 comb lines experience absorption and which do not. To solve this, the NIST researchers added a second frequency-comb, resulting in a 'beat-frequency' pulse which can be measured with conventional electronics. This pulse allows the scientists to observe the absorption and phase shift experienced by each individual comb line.

This work represents by far the largest number of individually observed frequency comb teeth. Scientists studied the effect of the gas absorption on 155,000 comb lines, spanning a wavelength range of 125 nm. The scientists claim the precision of 1 Hz for spectral lines is comparable to tens of MHz precision with other spectroscopic techniques.