Lasers are valued for the excellent coherence of emitted light, a measure for the light wave's regular frequency and linewidth. Ideally, laser light has only one fixed wavelength (or frequency). In practice, the spectrum of most types of lasers can, however, reach from a few kilohertz to a few megahertz in width, which is not good enough for numerous experiments requiring high precision.
Efforts have been devoted to developing better lasers with greater frequency stability and a narrower linewidth. Researchers from the U.S. and Germany report the realization of a laser whose linewidth is only 10 megahertz (0.01 hertz), establishing a new world record.
"The smaller the linewidth of the laser, the more accurate the measurement of the atom's frequency in an optical clock. This new laser will enable us to decisively improve the quality of our clocks," explains physicist Thomas Legero, a physicist with Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany.
The emitted laser light's frequency was demonstrated to be more precise than what had been achieved previously. Although the light wave oscillates approximately 200 trillion times per second, it only gets out of sync after 11 seconds. By then, the perfect wave train emitted has already attained a length of about 3.3 million kilometers, a length corresponding to nearly ten times the distance between the Earth and the moon.
The scientists had to set up two such laser systems so that a comparison could confirm the outstanding properties of the emitted light.
The core piece of each laser is a 21-cm long Fabry-Pérot silicon resonator. The resonator consists of two highly reflecting mirrors that are located opposite each other and are kept at a fixed distance by means of a double cone. Similar to an organ pipe, the resonator length determines the frequency of the wave that begins to oscillate. Stabilization electronics ensure that the light frequency of the laser constantly follows the natural frequency of the resonator. The laser’s frequency stability — and thus its linewidth — then depends only on the length stability of the Fabry-Pérot resonator.
The scientists at PTB isolated the resonator from all environmental influences, such as temperature and pressure variations, which might change its length. A solution was needed to manage thermal noise corresponding to the Brownian motion in all materials at a finite temperature.
The resonator was manufactured from a single crystal of silicon that was cooled down to a temperature of negative 150 degrees Celsius. The thermal noise of the silicon body is so low that the length fluctuations observed only originate from the thermal noise of the dielectric SiO2/Ta2O5 mirror layers. Although the mirror layers are only a few micrometers thick, they dominate the resonator's length stability. In total, the resonator length, however, only fluctuates in the range of 10 attometers, and the resulting frequency variations of the laser therefore amount to less than 4 × 10–17 of the laser frequency.
The new lasers are now being used both at PTB and at JILA in Boulder, CO, to further improve the quality of optical atomic clocks and to carry out new precision measurements on ultracold atoms. At PTB, the ultrastable light from these lasers is already being distributed via optical waveguides and is then used by the optical clocks in Braunschweig.