Until recently, no one has succeeded in coming very close to the ideal laser: theoretically, laser light is known to have only single color, also wavelength or frequency. However, in reality, there is always a specific linewidth.
With a linewidth of just 10 mHz, Researchers from the Physikalisch-Technische Bundesanstalt (PTB) have now developed a laser in collaboration with with US Researchers from JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado, Boulder, thus establishing a new world record. This precision is useful for a wide range of applications that include radioastronomy, precision spectroscopy, optical atomic clocks and also for testing the theory of relativity. The results have been published in the recent issue of "Physical Review Letters".
Lasers were once considered to be a solution without issues – but that is now history. More than 50 years have gone by since the initial technical realization of the laser, and it is just not possible to imagine how one could survive without the laser today. Laser light is used in several applications in information technologies, medicine and industry. Lasers have succeeded in bringing about a real revolution in metrology and several fields of research – or even made a few new fields possible in the first place.
The exceptional coherence of the emitted light is considered to be one of laser's outstanding properties. Researchers consider this to be a measure for the light wave's regular frequency and linewidth. Laser light ideally has just one fixed wavelength (or frequency). In practice, it is possible for the spectrum of most types of lasers to reach from a few kHz to a few MHz in width, which is not fine enough for a number of experiments needing high precision.
Research has this focused on producing even better lasers with a narrower linewidth and higher frequency stability. Within the scope of a practically 10-year-long joint project with the US colleagues from JILA in Boulder, Colorado, a laser has presently been developed at PTB whose linewidth is just 10 mHz (0.01 Hz), thus setting 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.
Thomas Legero, PTB Physicist
Besides discovering the new laser’s very small linewidth, Legero and his colleagues also learnt, by means of measurements, that the frequency of the emitted laser light was increasingly more precise than what had ever been attained before. Even though the light wave oscillates approximately 200 trillion times per second, it is only after 11 seconds that it gets out of sync. By this time, the ideal wave train emitted has already obtained a length of approximately 3.3 million kilometers. This length corresponds to almost 10 times the distance between the Earth and the moon.
Since there was no other comparably precise laser in the world, the Scientists working on this collaboration had to set up two such laser systems straight off. Only by comparing these two lasers was it possible to prove the outstanding properties of the emitted light.
The core piece of each laser is a 21 cm long Fabry-Pérot silicon resonator. The resonator is made up of two highly reflecting mirrors positioned opposite each other and maintained at a fixed distance with the help of a double cone. Similar to an organ pipe, the resonator length establishes the frequency of the wave which starts to oscillate, i.e. the light wave present inside the resonator. Special stabilization electronics guarantee that the light frequency of the laser continuously follows the resonator’s natural frequency. The frequency stability of the laser stability – and thus its linewidth – then relies only on the length stability of the Fabry-Pérot resonator.
The Scientists at PTB isolated the resonator almost perfectly from all environmental influences which could alter its length. Among these influences are pressure and temperature differences, but also external mechanical perturbations because of sound or seismic waves. In doing so, they were able to obtain adequate perfection such that the only influence left was the thermal motion of the atoms in the resonator. This "thermal noise" corresponds to the Brownian motion present in all materials at a finite temperature, and it represents a basic limit to the length stability of a solid. Its extent relies on the materials used to construct the resonator and also on the resonator's temperature.
It is for this reason that the Scientists of this collaboration developed the resonator from single-crystal silicon, which was cooled down to a temperature of -150 °C. The length fluctuations observed only originate from the thermal noise of the dielectric SiO2/Ta2O5 mirror layers since the thermal noise of the silicon body is extremely low. The mirror layers dominate the resonator's length stability even though they are just a few micrometers thick. However, in total, the length of the resonator only fluctuates in the range of 10 attometers. This length matches with no more than a ten-millionth of the diameter of a hydrogen atom. The resulting frequency differences of the laser thus amount to less than 4 × 10–17 of the laser frequency.
Currently, the new lasers are being used both at PTB and at JILA in Boulder in order to further enhance the quality of optical atomic clocks and also to execute new precision measurements on ultracold atoms. The ultrastable light from these lasers is already being distributed at PTB through optical waveguides, and is also used in Braunschweig by the optical clocks.
In the future, it is planned to disseminate this light also within a European network. This plan would allow even more precise comparisons between the optical clocks in Braunschweig and the clocks of our European colleagues in Paris and London.
Thomas Legero, PTB Physicist
In Boulder, a similar plan has been set in order to distribute the laser across a fiber network capable of connecting between JILA and different NIST labs.
The Scientists from this collaboration plan to bring about further optimization possibilities. It is possible to further reduce the disturbing thermal noise by employing novel crystalline mirror layers and lower temperatures. The linewidth could then become smaller than 1 mHz.