The Sharpest Laser in the World

One of the two silicon resonators to create the sharpest laser pulses so far. (Source: PTB)

Theore­tically, laser light has only one single color. In reality, however, there is always a certain line­width. With a linewidth of only 10 mHz, the laser that the researchers from the Physi­kalisch-Tech­nische Bundes­anstalt PTB have now deve­loped together with US researchers from JILA, a joint institute of the National Institute of Standards and Tech­nology and the Uni­versity of Colorado Boulder, has established a new world record. This precision is useful for various appli­cations such as optical atomic clocks, precision spec­troscopy, radio­astronomy and for testing the theory of rela­tivity.

Lasers have brought about a real revolution in many fields of research and in metro­logy or have even made some new fields possible in the first place. One of a laser’s out­standing properties is the excellent coherence of the emitted light. Ideally, laser light has only one fixed wave­length. In practice, the spectrum of most types of lasers can, however, reach from a few kHz to a few MHz in width, which is not good enough for numerous experi­ments requiring high pre­cision.

Research has there­fore focused on developing ever better lasers with greater frequency stability and a narrower line­width. Within the scope of a nearly 10-year-long joint project with the US colleagues from JILA in Boulder, Colorado, a laser has now been developed at PTB whose linewidth is only 10 mHz, hereby 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”, PTB physicist Thomas Legero explains.

In addition to the new laser’s extremely small line­width, Legero and his colleagues found out by means of measure­ments that the emitted laser light’s frequency was more precise than what had ever been achieved before. Although the light wave oscillates approx. 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 approx. 3.3 million kilo­meters. 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 of the lasers is a 21-cm long Fabry-Pérot silicon reso­nator. The resonator consists of two highly reflecting mirrors which 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 which begins to oscil­late. Special stabilization elec­tronics ensure that the light frequency of the laser con­stantly follows the natural frequency of the reso­nator. The laser’s frequency sta­bility then depends only on the length sta­bility of the Fabry-Pérot reso­nator.

The scientists had to isolate the reso­nator nearly perfectly from all environ­mental influences which might change its length. Among these influences are temperature and pressure variations, but also external mechanical pertur­bations due to seismic waves or sound. They have attained such per­fection in doing so that the only influence left was the thermal motion of the atoms in the resonator. This thermal noise corresponds to the Brownian motion in all materials at a finite temperature, and it represents a funda­mental limit to the length sta­bility of a solid. Its extent depends on the materials used to build the resonator as well as on the reso­nator’s tempera­ture.

For this reason, the scientists of this colla­boration manu­factured the resonator from single-crystal silicon which was cooled down to a temperature of -150 °C. 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. 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 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.

“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”, Legero says. In Boulder, a similar plan is in place to distri­bute the laser across a fiber network that connects between JILA and various NIST labs. The scientists from this colla­boration see further optimi­zation possi­bilities. With novel crystal­line mirror layers and lower tempera­tures, the disturbing thermal noise can be further reduced. The line­width could then even become smaller than 1 mHz. (Source: PTB)

Reference: D. G. Matei et al.: 1.5 µm Lasers with sub-10-mHz Linewidth, Phys. Rev. Lett. 118, 2632202 (2017); DOI: 10.1103/PhysRevLett.118.263202

Links: Quantum Optics and Unit of Length, Physikalisch-Technische Bundesanstalt, Braunschweig, GermanyJILA, National Institute of Standards NIST, Boulder, USA

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