Doubling Frequency of Transportable Lasers

The laser setups of the optical atomic clock being developed at the QUEST Institute of PTB. (Source: PTB)

The Physi­kalisch-Technische Bundes­anstalt PTB is known for providing time e.g. for radio-controlled clocks. For this purpose, it operates some of the best cesium atomic clocks in the world. At the same time, PTB is already deve­loping various atomic clocks of the next gene­ration. These clocks are no longer based on a micro­wave transition in cesium, but they rather operate with other atoms that are excited using optical frequen­cies. Some of these new clocks can even be transported to other locations. At its QUEST Institute, PTB is currently developing a trans­portable optical aluminum clock in order to measure physical phenomena outside a laboratory. A prerequisite for this is that the required lasers are able to endure trans­portation to other locations. PTB physicists have therefore developed a frequency-doubling unit that will even continue to operate when it has been shaken at three times the Earth’s gravi­tational accele­ration.

It was Einstein who found out that two clocks that are located at two different positions in the gravi­tational field of the Earth tick at different speeds. This has quite practical effects: Two optical atomic clocks having an extremely small relative measure­ment uncer­tainty can measure the dif­ference in height between arbitrary points on the Earth at an accuracy of just one centi­meter. This chrono­metric level­ling represents an important appli­cation of clocks in geodesy. One of the prere­quisites for this is that the optical frequen­cies of the two clocks can be compared e.g. via glass fibers. PTB is currently developing several different types of atomic clocks that can each be transported in a trailer or in a container. Their operation outside a protected labora­tory, however, involves many challenges: The ambient tempera­ture, for example, is much less stable. Furthermore, signi­ficant shocks may occur during transpor­tation. This is why optical structures that have worked perfectly well in the labora­tory may initially be unusable at the desti­nation. They must pain­stakingly be readjusted – which leads to a loss of valuable research time.

This last-mentioned problem concerns in particular the transpor­table aluminum clock that is being developed at the QUEST Institute. This clock requires, among other things, two UV lasers at 267 nm. For this wave­length, it is not possible to simply buy a laser diode. Instead, a long-wave infrared laser must be frequency-doubled twice in succes­sion. During this process, the light is coupled into a closed ring of four mirrors so that a high optical power is circu­lating within the ring. A non-linear crystal placed in this ring transforms the circu­lating light into light of half the wave­length. Due to the dichroic coating of the mirror, it passes out of the reso­nator and is then used for reading the clock. The QUEST Insti­tute has developed a design for this frequency-doubling cavity which is based on a mono­lithic and therefore highly stable frame onto which all mirrors and the crystal are mounted. The set-up is sealed to be gas-tight to the outside in order to protect the crystal, which is highly sensitive even to the slightest conta­minations.

The developers of the cavity were able to demon­strate on a prototype that it also doubles the laser light while it is exposed to acce­lerations of 1 g. Furthermoe, it was shown that the frequency doubling efficiency is not impaired after being subjected to acce­lerations of up to 3 g for 30 minutes. This corresponds to five times the value stated in Standard ISO 13355:2016 about road trans­portation on trucks. The cavity is, however, not only mechani­cally robust, but it is just as efficient as comparable systems that have been developed by research groups of other institutes. Moreover, 130 hours of uninter­rupted continuous operation was demon­strated. The QUEST Institute has made several of these doubling cavities for different wave­lengths which became integral components of various quantum-optical experi­ments, with the aim of providing these experi­ments reliably with laser light. Moreover, a German opto­mechanics company has licensed the design in order to use it as a basis for a commercial product. (Source: PTB)

Reference: S. Hannig et al.: A highly stable monolithic enhancement cavity for second harmonic generation in the ultraviolet, Rev. Sci. Instr. 89, 013106 (2018); DOI: 10.1063/1.5005515

Link: Inst. for Experimental Quantum Metrology QUEST, Physikalisch-Technische Bundesanstalt PTB, Braunschweig

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