Transportable Laser
The Physikalisch-Technische Bundesanstalt (PTB) has developed a frequency-doubling unit for transportable lasers
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. What initially sounds like a bizarre idea has quite practical effects: Two optical atomic clocks having an extremely small relative measurement uncertainty of 10-18 can measure the difference in height between arbitrary points on the Earth at an accuracy of just one centimeter. This so-called "chronometric leveling" is 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 via glass fibers, for example.
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 their 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 cavity and is then used for reading the clock. The QUEST Institute has developed a design for this so-called frequency-doubling cavity which is based on a monolithic – and therefore highly stable – frame onto which all mirrors and the crystal are mounted. The setup 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. Furthermore, it was shown that the frequency doubling efficiency is not even 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.
In view of these properties, the QUEST Institute has made several of these doubling cavities for different wavelengths (not only for UV) which have become 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. This project was supported by the Deutsche Forschungsgesellschaft (grant CRC 1128 geo-Q, project A03, CRC 1227 DQ-mat, projects B03 and B06) and the Leibniz-Gemeinschaft (SAW-2013-FBH-3).es/PTB
Contact
Prof. Dr. Piet O. Schmidt, QUEST, phone: +49 (0)531 592-4700, e-mail: piet.schmidt(at)ptb.de
Scientific publication
S. Hannig, J. Mielke, J. Fenske, M. Misera, N. Beef, C. Ospelkaus, P.O. Schmidt: A highly stable monolithic enhancement cavity for second harmonic generation in the ultraviolet. Review of Scientific Instruments 89, 013106 (2018)
A “Scilight” of the publication was published by AIP ("A robust frequency doubling cavity makes a transportable laser source for use in a UV optical clock"): http://scitation.aip.org/content/aip/journal/sci/2018/3/10.1063/1.5021479