Ultrastable lasers

Optical domain oscillators are lasers that deliver an extremely high frequency oscillating electromagnetic field in the range 500-1000 THz (1 THz = 1012 Hz) for optical radiation in the visible domain.

However, the spectral purities of “normal” commercial lasers are absolutely not sufficient to meet the demanding need of frequency metrology. This is why many lasers frequency stabilization techniques have been developed over several decades in order to push the limits of lasers spectral purity and frequency stability. These achievements were motivated by the optical clocks’ development, and also the need to perform very high precision interferometric measurements. We can mention among others, tests of special relativity, identical in their basic principle to those carried out by Albert Michelson and Edouard Morley at the end of the 19th century, or giant optical interferometers for gravitational waves’ detection.

The most commonly employed laser frequency stabilization technique uses a Fabry-Perot optical cavity as a passive reference. The cavity typically used in these applications have extremely narrow optical transmission lines (in the kHz range), owing to their very high finesse (reaching one million or more in the most efficient devices). Note that thanks to the development and application of this technique to precision measurements, one of its inventors, John Hall, won the Nobel Prize in Physics in 2005. The cavities used nowadays for such applications are made of very low coefficient of thermal expansion materials, in order to best reduce the sensitivity to temperature fluctuations. Improvements have also been made to push back the limits imposed by the sensitivity of the cavities to vibrations, leading to original carefully engineered cavity and mounting posts designs. A rather fundamental limitation also arise from thermal noise, which corresponds to the thermal agitiation (at room temperature) of the atoms composing the cavity, which, even though averaged over a very large number of atoms still has a measurable impact on the cavity length and therefore on its resonance frequencies. Operation at cryogenic temperature and/or with materials of very high mechanical quality factors (such as crystals) can significantly improve ultimate performance from that prospect.

It is also possible to consider other techniques for stabilizing the lasers frequency, in which the high finesse Fabry-Perot cavity is substituted by another device: a fibered delay line, a crystal offering spectroscopic properties called “Spectral Hole Burning”, or even a very narrow linewidth optical transitions in atoms on which a lasing effect can be produced (a regime called “bad cavity laser” which maybe, can produce extremely narrom linewidt laser light in the right conditions).

Different Fabry-Pérot cavity geometries for laser stabilization

The signal delivered by a stabilized laser lies in the optical frequency range at a given optical wavelength. One must be able to transfer its spectral purity to other wavelengths, and other frequency domains, such as, in particuler the microwave domain (where many applications are) or the mid-Infrared. The comparison between optical and microwave clocks also requires this optical to microwave connection, which 30 years ago involved a very complex experimental device, but which has become for the last 20 years a simpler operation to carry out, thanks to the development of femtosecond laser-based optical frequency combs. This technique, for which its inventor Theodor Hänsch was awarded the Nobel Prize in Physics in 2005, now makes it possible to directly connect an optical frequency to a microwave one with a relative frequency uncertainty in the 10-19 range or lower. Femtosecond frequency combs are also very effective in transferring the stabilized lasers’ exceptional performances (spectral purity, frequency stability) in the microwave domain where such performances cannot be obtained directly.