Recent Advancements in Frequency Combs

What is a Frequency Comb?

A frequency comb is a light source whose spectrum consists of a series of discrete, equally spaced elements. Frequency combs can be generated by a number of mechanisms, including amplitude modulation (AM) of a continuous wave laser or stabilization of the pulse train generated by a mode locked laser.

Frequency Combs in the News

The devices are useful and are getting more accurate. Here are a couple of interesting articles I've come across recently on Frequency Combs:

Rapid Alignment: A frequency comb can align an ensemble of molecules 150 million times per second.

Molecules in a gas are aligned in random directions, but intense lasers can be used to deliver a torque to the molecules and align them towards the laser polarization. Preparing an aligned ensemble is important for studying chemical reactivity, imaging the molecule's structure, or using the molecules to efficiently convert the laser frequency to short UV wavelengths. Now Craig Benko from the University of Colorado, Boulder, and colleagues have developed a technique that can align molecules at a rate one hundred thousand times larger than previously possible. In the authors' scheme, a pulse of light aligns a molecule by giving it a "kick" in a certain direction. This occurs through a Raman process, in which light scatters off a molecule leaving it in a rotationally excited state. The molecule then rotates in the direction it was kicked. After a certain delay, all the molecules will point in the same direction.

Article Continues Here

Terahertz Combs Get Fine Teeth

Frequency combs-light sources whose spectra are made of a series of discreet, equally spaced frequencies-can be used as rulers that measure the light emitted by atoms or molecules with extraordinarily high precision. Most frequency combs work in the visible or infrared, but terahertz combs would allow more precise measurements of rotational and vibrational resonances of molecules and materials. A team led by Geoffrey Blake at the California Institute of Technology, Pasadena, has now demonstrated a terahertz comb that features greater bandwidth and better frequency precision than current technologies.

The most common way to make a frequency comb is through so called "mode-locked" lasers. Such lasers emit a train of short pulses, whose spectrum is a frequency comb. The authors started with an infrared mode-locked laser and used it to excite currents in an "antenna," which emitted lower frequency terahertz pulses. A second infrared laser detected the electric field of the terahertz pulses by "sensing" how they modified the index of refraction of a crystal in which the two beams co-propagated. Although this approach is not new, the authors found new ways to stabilize the frequencies of the two lasers and minimize noise. As a result, they were able to achieve, over a spectral range extending up to 2.4 terahertz, a frequency precision of a few parts per billion-over two orders of magnitude better than existing schemes for this spectral region.

Article Continues Here