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).
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 report on the evaluation of an optical lattice clock using fermionic 87Sr. The measured frequency of the
1S0 → 3P0 clock transition is 429 228 004 229 873.7Hz with a fractional acuracy of 2.6 × 10-15. This evaluation
is performed on mF = ±9/2 spin-polarized atoms. This technique also enables to evaluate the value of the
differential Landé factor, 110.6Hz/G. by probing symmetrical σ-transitions.
Special, narrow-linewidth fiber Bragg gratings (FBGs) can serve as wavelength references for both sensor applications and optical telecommunications. With line-widths of a few GHz and contrast exceeding 95%, phase-shifted FBGs offer a good alternative to etalons or gas cells as easy-to-use and cost-effective wavelength references. To enable calibration and to assess the wavelength stability of newly developed FBGs down to parts in 107, quantifying drifts caused e.g. by aging, polarisation dependence, residual sensitivity to temperature or strain, we have developed high resolution measuring methods which are linked to a traceable wavelength. We present results from three complementary methods based on Fourier Transform spectroscopy, tunable laser spectroscopy and a new laser stabilisation technique using polarisation modulation. For the central wavelength of phase-shifted FBGs we achieved an accuracy below 10-7.