Adding Teeth to Spectroscopic Detection

Lynn M. Savage

With conventional broadband spectroscopy techniques, detecting optical frequencies can be performed to a resolution of ~10 MHz. Now, however, researchers at the National Institute for Standards and Technology (NIST) in Boulder, Colo., have devised a method that can provide a resolution of as low as ~1 Hz.

The investigators took advantage of two recent optical developments: multiheterodyne spectroscopy and optical frequency combs.

Frequency combs result from the broadband output of a femtosecond laser, which acts like a collection of thousands of single-frequency lasers. The very even spacing of the frequencies makes the resulting frequency spectrum appear comblike.

Using an additional laser pulse, called a local oscillator (LO), enables high-resolution multiheterodyne spectroscopy because the LO and signal beams have slightly different frequencies, creating a beat signal that can be plotted as a radio-frequency (RF) comb. Courtesy of Ian Coddington of NIST.

Until recently, however, systems that produce frequency combs have been unstabilized, causing the “teeth” to wander over time and blurring the effect. Following the Nobel Prize-winning work of Theodor W. Hänsch of Max Planck Institut für Quantenoptik in Garching, Germany, and of John L. Hall of NIST, the researchers stabilized the frequencies generated by a pair of custom-built lasers by providing electronic feedback.

“The thing that is exciting about the frequency comb is that there are only two free parameters to stabilize,” said team member Ian Coddington. “If you stabilize just two comb teeth, the other hundred thousand are stabilized for free.”

The group, which also included William C. Swann and Nathan R. Newbury, used the two lasers to demonstrate massively parallel heterodyne spectroscopy. For this, they generated a 100,016-kHz signal comb and a 100,017-kHz probe comb, both stabilized to 1550- and 1535-nm lasers. The slight difference in repetition rates caused the reference and probe signals to create a beat signal, which the researchers observed as a radio-frequency comb with more than 155,000 teeth, each directly related to a known optical frequency.

With a telecom-standard hydrogen cyanide as their sample, the scientists tested the spectroscopy setup by passing the signal comb through the chamber-bound gas and heterodyning it with the probe signal, also known as a local oscillator. To stabilize the signal and probe combs, they attached a generic piezo stack to the fibers in the laser cavities and performed feedback control through the stacks, in part by using an acousto-optical modulator with a 100-kHz feedback bandwidth.

The resulting resolution is limited only by the linewidth of the lasers. According to Coddington, the system can support up to a 1-Hz resolution, although the researchers measured at a spacing of only 100 MHz, which was compatible with the room-temperature Doppler linewidth of the gas. Importantly, the technique captures the phase information along with the absorption spectra of the analyte.

The investigators believe that the heterodyne spectroscopy technique will have applications in a number of areas, including molecular fingerprinting, explosives detection and atmospheric chemistry. Additionally, its ability to capture phase information makes it adaptable to time-domain applications such as lidar-based ranging.

“At this point, there are more options than we know what to do with,” Coddington said.

Physical Review Letters, Jan. 11, 2008, Vol. 100, 013902.