The National Metrology Institute of South Africa (NMISA) is developing a new optical frequency standard based on the Rubidium two-photon transition in collaboration with the National Institute of Standards (NIS, Egypt) that will use both bulk and fiber optics in the system. This is system is called A-POD; an acronym for a portable photonic oscillator device. Rubidium two-photon standards can yield relatively simple and precise standards that are compatible with standard Ti:Sapphire optical frequency combs, as well as the need for a precise frequency standard in the optical telecommunication domain and for measurement of length with a visible beam. The robustness and transportability of the standard are important considerations for the optical frequency standard. This projects implements a framework for better two-photon standards that can be highly accurate, and possibly compete with much more complex clocks in the metrology environment, and especially so in the smaller national metrology institutes found in the developing world. This paper discusses the design constraints and the development considerations towards the optical setup. The robustness and transportability was greatly improved via the usage of optical fiber in the light source of the system, or even in atom-light interaction region. Of particular importance are the beam parameters inside the atomic interaction area. The extent of Doppler broadening and the intensity dependent line shift have to be optimized within practical extents, where both these aspects are affected by the beam shape and optical geometry. A way to fully treat the optical beam effects together with atomic movement is proposed. Furthermore a method is proposed to do real time compensation of intensity dependent light shift, which could have major applicability to frequency standards in general - the complexity is shifted from physical setups to digital signal processing, which is easily adaptable and stable.
Optical clocks largely rely on interrogation lasers with sub-Hz linewidth and low short term instability. The
laser stability is mostly determined by the properties of the cavities that are used as short term references. With
suitable mounting the influence of vibrations is strongly suppressed and the short term stability is limited by
thermal fluctuations to a fractional instability around 1 • 10-15. Here we give an overview of the present status
of our ultrastable lasers used for optical clocks and present possible ways to further reduce their noise levels and
to transfer their stability to other wavelengths and to remote lasers.
We have characterized the 24Mg optical frequency standard at the Institute of Quantum Optics (IQ), Hanover, using a
clock laser at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, via a noise compensated 73 km fiber
link and present preliminary results for the stability of the Mg standard. The stability of the clock laser (λ = 657 nm) is
transferred with a femtosecond frequency comb to a telecommunication laser at λ = 1542 nm. The signal is then
transmitted from PTB through the fiber link to IQ. A second comb at IQ (the remote end) is used to compare the
transmitted laser frequency with that of the Mg clock laser λ = 914 nm. The frequency ratio of the clock lasers νMg/νCa
shows a relative instability < 10-14 at 1 s. The upper limit for the contribution of the fiber link to the frequency instability
is measured by connecting another optical fiber following the same 73 km route at Hanover computer center. The
comparison performed at PTB between the local and the transmitted signal after a round trip length of 146 km showed a
relative uncertainty below 1 x 10-19 and a short term instability σy(τ)= 3.3 x 10-15 / (τ/s).
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