Due to their orders-of-magnitude higher frequencies, optical frequency standards are beginning to outperform the best microwave standards with respect to their stability and accuracy and, hence, offer very promising prospects for novel space-based applications. We report on recent results for optical standards in PTB based on neutral atoms and discuss the suitability of the different approaches for space applications.
The Space Optical Clocks project aims at operating lattice clocks on the ISS for tests of fundamental physics and for providing high-accuracy comparisons of future terrestrial optical clocks. A pre-phase-A study (2007- 10), funded partially by ESA and DLR, included the implementation of several optical lattice clock systems using Strontium and Ytterbium as atomic species and their characterization. Subcomponents of clock demonstrators with the added specification of transportability and using techniques suitable for later space use, such as all-solid-state lasers, low power consumption, and compact dimensions, have been developed and have been validated. This included demonstration of laser-cooling and magneto-optical trapping of Sr atoms in a compact breadboard apparatus and demonstration of a transportable clock laser with 1 Hz linewidth. With two laboratory Sr lattice clock systems a number of fundamental results were obtained, such as observing atomic resonances with linewidths as low as 3 Hz, non-destructive detection of atom excitation, determination of decoherence effects and reaching a frequency instability of 1×10-16.
The ESA mission “Space Optical Clock” project aims at operating an optical lattice clock on the ISS in approximately 2023. The scientific goals of the mission are to perform tests of fundamental physics, to enable space-assisted relativistic geodesy and to intercompare optical clocks on the ground using microwave and optical links. The performance goal of the space clock is less than 1 × 10-17 uncertainty and 1 × 10-15 τ-1/2 instability. Within an EU-FP7-funded project, a strontium optical lattice clock demonstrator has been developed. Goal performances are instability below 1 × 10-15 τ-1/2 and fractional inaccuracy 5 × 10-17. For the design of the clock, techniques and approaches suitable for later space application are used, such as modular design, diode lasers, low power consumption subunits, and compact dimensions. The Sr clock apparatus is fully operational, and the clock transition in 88Sr was observed with linewidth as small as 9 Hz.
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.
Starting in late 2007 and continuing through the present, NFIRE (Near-Field Infrared Experiment), a
Missile Defense Agency (MDA) experimental satellite and TerraSAR-X, a German commercial SAR
satellite have been conducting mutual crosslink experiments utilizing a secondary laser communication
payload built by Tesat-Spacecom. The narrow laser beam-widths and high relative inter-spacecraft
velocities for the two low-earth-orbiting satellites imply strict pointing control and dynamics aboard both
vehicles. The satellites have achieved rapid communication acquisition times and maintained
communication for hundreds of seconds before losing line of sight to the counter satellite due to earth
blockage. Through post-mission analysis and other related telemetry we will show results for pointing
accuracy, disturbance environments and pre-engagement prediction requirements that support successful
and reliable operations.
A laser that is frequency stabilized to the intercombination transition at (lambda) equals 657 nm of laser cooled 40Ca atoms by means of time-domain atom interferences has been developed at PTB. The relative uncertainty of this optical frequency standard is 2.5 X 10-13 and the frequency of the laser has been measured with both, a conventional frequency measurement chain and a femtosecond laser. To reduce the uncertainty of this standard the dependence of its frequency on various parameters was investigated by using phase sensitive and frequency sensitive atom interferometers. Upper bounds were derived for various contributions to the uncertainty as e.g. the influence of gravitational acceleration, collisions of the ballistic atoms, or curvature of the wave fronts of the interrogating laser beams. Further reduction of the uncertainty of the standard is expected from the application of sub-Doppler cooling techniques. A method was devised based on the narrow intercombination transition that enabled us to reduce the velocity spread of the Ca atoms in one dimension close to the recoil limit. The method uses the repeated selection and accumulation of slow atoms from the pre-cooled atomic cloud and the repeated rethermalization of the remaining atoms and results in an increased visibility of the interference structure.
We present an optical frequency standard at 1.54 micrometer based on the saturated absorption of the (nu) 1 + (nu) 3 band of acetylene 13C2H2. An external resonator containing an absorption cell at a pressure of 1 Pa was used to build up the power and to increase the absorption length. A linewidth-narrowed DFB diode laser was stabilized to this resonator using the Pound-Drever-Hall technique. The molecular absorption was detected by wavelength modulation spectroscopy and by cavity-enhanced frequency-modulation spectroscopy, using a modulation frequency equal to the free- spectral range of the resonator. We have achieved a frequency stability of 2 X 10-12 for averaging times of 1000 s, as measured by interferometrically comparing the laser to an iodine-stabilized helium-neon laser.
For intra-cavity high resolution spectroscopy of acetylene (C2H2), an extended cavity diode laser (ECDL) system emitting at 1.55 micrometers has been built using a 600 lines/mm diffraction grating mounted in Littrow configuration. In a single pass absorption cell the linear absorption of a number of rovibrational overtone transitions of C2H2 has been registered. An external FP cavity (finesse of 300, free spectral range FSR equals 520 MHz) was built and utilized for C2H2 intra-cavity spectroscopy as well as for ECDL frequency stabilization. The Pound-Drever-Hall technique was used to lock the diode laser frequency to the external cavity with the servo bandwidth of 50 kHz. The C2H2 intra-cavity linear absorption was recorded in transmission at a pressure of 1.3 Pa. By combining the reflected and transmitted signals the influence of residual frequency fluctuation of the ECDL with respect to the external cavity was reduced and the signal to noise ratio was improved to 34 dB for a single pass linear absorption of 0.2%.