Klaus Döringshoff

High performance iodine frequency reference for tests of the LISA laser system

Forthcoming space missions like the Laser Interferometer Space Antenna (LISA) or the Space-Time Asymmetry Research (STAR) project call for optical frequency references with high frequency stability better than 10-14 at averaging times longer than 1000 s. Since Nd:YAG lasers are planned to be used on these missions, new interest has arisen in the frequency stabilization of Nd:YAG lasers to hyperfine transitions in molecular iodine. Iodine stabilized lasers offer an absolute optical frequency reference with high frequency stability and low sensitivity to temperature fluctuations and magnetic fields in relative simple setups. Here we present our iodine frequency standard using modulation transfer spectroscopy with a multi-pass iodine cell showing a frequency stability of 1·10-14 at 1 s averaging time.

Testing Lorentz Invariance by Comparing Light Propagation in Vacuum and Matter

We present a Michelson-Morley type experiment for testing the isotropy of the speed of light in vacuum and matter. The experiment compares the resonance frequency of a monolithic optical sapphire resonator with the resonance frequency of an orthogonal evacuated optical cavity made of fused silica while the whole setup is rotated on an air bearing turntable once every 45 s. Preliminary results yield an upper limit for the anisotropy of the speed of light in matter (sapphire) of \Delta c/c < 4x10^(-15), limited by the frequency stability of the sapphire resonator operated at room temperature. Work to increase the measurement sensitivity by more than one order of magnitude by cooling down the sapphire resonator to liquid helium temperatures (LHe) is currently under way.

Iodine based optical frequency reference with 10 −15 stability

We present different setups for frequency stabilization of Nd:YAG lasers to absorption lines in molecular iodine featuring frequency stabilities of 3 · 10−15 at averaging times between 100 s and 1000 s. First results of a semi-monolithic setup, developed aiming at a space qualifiable iodine frequency reference, are shown.

An ultra-stable optical frequency reference for space applications

A variety of future space missions require ultra-stable optical frequency references. Setups based on Doppler-free spectroscopy of molecular iodine offer frequency stabilities in the 10−15 domain at longer integration times and have the potential to be realized space compatible on a relatively short time scale. We present a compact optical frequency reference using modulation-transfer spectroscopy of molecular iodine near 532 nm. Using a specific assembly-integration technology, this setup takes into account space mission related criteria such as compactness, robustness, MAIVT and environmental influences. With this setup, a frequency stability of 3 · 10−15 at integration times between 100 s and 10.000 s was demonstrated in a first measurement.

High-Performance Optical Frequency References for Space

A variety of future space missions rely on the availability of high-performance optical clocks with applications in fundamental physics, geoscience, Earth observation and navigation and ranging. Examples are the gravitational wave detector eLISA (evolved Laser Interferometer Space Antenna), the Earth gravity mission NGGM (Next Generation Gravity Mission) and missions, dedicated to tests of Special Relativity, e.g. by performing a Kennedy- Thorndike experiment testing the boost dependence of the speed of light. In this context we developed optical frequency references based on Doppler-free spectroscopy of molecular iodine; compactness and mechanical and thermal stability are main design criteria. With a setup on engineering model (EM) level we demonstrated a frequency stability of about 210-14 at an integration time of 1 s and below 610-15 at integration times between 100s and 1000s, determined from a beat-note measurement with a cavity stabilized laser where a linear drift was removed from the data. A cavity-based frequency reference with focus on improved long-term frequency stability is currently under development. A specific sixfold thermal shield design based on analytical methods and numerical calculations is presented.

Highly stable piezoelectrically tunable optical cavities

We have implemented highly stable and tunable frequency references using optical high finesse cavities which incorporate a piezo actuator. As piezo material we used ceramic PZT, crystalline quartz, or PZN-PT single crystals. Lasers locked to these cavities show a relative frequency stability better than

mSTAR: Testing special relativity in space using high performance optical frequency references

1\times 10^{-14},

A space-based optical Kennedy-Thorndike experiment testing special relativity

which is most likely not limited by the piezo actuators. The piezo cavities can be electrically tuned over more than one free spectral range (>1.5 GHz) with only a minor decrease in frequency stability. Furthermore, we present a novel cavity design, where the piezo actuator is prestressed between the cavity spacer components. This design features a hermetically sealable intra cavity volume suitable for, e.g., cavity enhanced spectroscopy.