Ortwin Hellmig

Space-borne frequency comb metrology

Precision time references in space are of major importance to satellite-based fundamental science, global satellite navigation, earth observation, and satellite formation flying. Here we report on the operation of a compact, rugged, and automated optical frequency comb setup on a sounding rocket in space under microgravity. The experiment compared two clocks, one based on the optical D2 transition in Rb, and another on hyperfine splitting in Cs. This represents the first frequency comb based optical clock operation in space, which is an important milestone for future satellite-based precision metrology. Based on the approach demonstrated here, future space-based precision metrology can be improved by orders of magnitude when referencing to state-of-the-art optical clock transitions.

Ultrastable, Zerodur-based optical benches for quantum gas experiments

Operating ultracold quantum gas experiments outside of a laboratory environment has so far been a challenging goal, largely due to the lack of sufficiently stable optical systems. In order to increase the thermal stability of free-space laser systems, the application of nonstandard materials such as glass ceramics is required. Here, we report on Zerodur-based optical systems which include single-mode fiber couplers consisting of multiple components jointed by light-curing adhesives. The thermal stability is thoroughly investigated, revealing excellent fiber-coupling efficiencies between 0.85 and 0.92 in the temperature range from 17°C to 36°C. In conjunction with successfully performed vibration tests, these findings qualify our highly compact systems for atom interferometry experiments aboard a sounding rocket as well as various other quantum information and sensing applications.

Multicolor diode-pumped upconversion fiber laser

We present a new method to control the power of individual spectral components of a multicolor laser by mirrors with variable air gaps and by a composite resonator configuration. We demonstrate a Pr/Yb-ZBLAN fiber laser with arbitrary spectral composition of three simultaneously emitted components at 492 nm, 520 nm, and 635 nm. With 100 mW pump power at 850 nm launched into the fiber, the total laser output exceeds 10 mW.

A frequency comb and precision spectroscopy experiment in space

A frequency comb, DFB diode laser and rubidium spectroscopy cell have been developed and commissioned on a sounding rocket mission to demonstrate their technological maturity. The first laser spectroscopy experiment on an optical transition in space is performed.

The role of mode match in fiber cavities

We study and realize asymmetric fiber-based cavities with optimized mode match to achieve high reflectivity on resonance. This is especially important for mutually coupling two physical systems via light fields, e.g., in quantum hybrid systems. Our detailed theoretical and experimental analysis reveals that on resonance, the interference effect between the directly reflected non-modematched light and the light leaking back out of the cavity can lead to large unexpected losses due to the mode filtering of the incoupling fiber. Strong restrictions for the cavity design result out of this effect and we show that planar-concave cavities are clearly best suited. We validate our analytical model using numerical calculations and demonstrate an experimental realization of an asymmetric fiber Fabry-Pérot cavity with optimized parameters.

Space-borne Bose–Einstein condensation for precision interferometry

Owing to the low-gravity conditions in space, space-borne laboratories enable experiments with extended free-fall times. Because Bose–Einstein condensates have an extremely low expansion energy, space-borne atom interferometers based on Bose–Einstein condensation have the potential to have much greater sensitivity to inertial forces than do similar ground-based interferometers. On 23 January 2017, as part of the sounding-rocket mission MAIUS-1, we created Bose–Einstein condensates in space and conducted 110 experiments central to matter-wave interferometry, including laser cooling and trapping of atoms in the presence of the large accelerations experienced during launch. Here we report on experiments conducted during the six minutes of in-space flight in which we studied the phase transition from a thermal ensemble to a Bose–Einstein condensate and the collective dynamics of the resulting condensate. Our results provide insights into conducting cold-atom experiments in space, such as precision interferometry, and pave the way to miniaturizing cold-atom and photon-based quantum information concepts for satellite-based implementation. In addition, space-borne Bose–Einstein condensation opens up the possibility of quantum gas experiments in low-gravity conditions1,2.A Bose–Einstein condensate is created in space that has sufficient stability to enable its characteristic dynamics to be studied.

Corrigendum: STE-QUEST—test of the universality of free fall using cold atom interferometry (2014 Class. Quantum Grav. 31 115010)

The theory of general relativity describes macroscopic phenomena driven by the influence of gravity while quantum mechanics brilliantly accounts for microscopic effects. Despite their tremendous individual success, a complete unification of fundamental interactions is missing and remains one of the most challenging and important quests in modern theoretical physics. The spacetime explorer and quantum equivalence principle space test satellite mission, proposed as a medium-size mission within the Cosmic Vision program of the European Space Agency (ESA), aims for testing general relativity with high precision in two experiments by performing a measurement of the gravitational redshift of the Sun and the Moon by comparing terrestrial clocks, and by performing a test of the universality of free fall of matter waves in the gravitational field of Earth comparing the trajectory of two Bose–Einstein condensates of 85Rb and 87Rb. The two ultracold atom clouds are monitored very precisely thanks to techniques of atom interferometry. This allows to reach down to an uncertainty in the Eötvös parameter of at least 2 × 10−15. In this paper, we report about the results of the phase A mission study of the atom interferometer instrument covering the description of the main payload elements, the atomic source concept, and the systematic error sources.

Efficient femtosecond laser written Nd:YAG channel waveguide laser with an output power of more than 1 W

Femtosecond lasers are widely used for three-dimensional volume micro-structuring of dielectric materials. The fabrication of buried channel waveguides inside these materials is interesting especially for the production of passive and active optical devices. Waveguide lasers in glass and crystals were demonstrated in the past [1–4]. The advantages of dielectric waveguide lasers are high optical gain, high damage threshold, and a good overlap between pump mode and laser mode. Neodymium-doped YAG with its high emission cross-section, long fluorescence lifetime, good thermal conductivity, and high mechanical stability is a well suited material for waveguide lasers.

Dual wavelength and switchable laser operation of Pr 3+ :LiYF 4 crystals at 523 nm and 640 nm

Solid state lasers in the visible spectral region have many possible applications like projection technology and spectroscopy [1,2]. In these fields the generation of green light is still a challenge. Recent projection devices generate green laser light by frequency doubling of near-infrared solid state laser radiation. A compact, simple, and efficient laser based projection device is not yet developed. The trivalent Pr3+ ion in the host material LiYF4 (YLF) has several transitions at wavelengths in the visible spectral range, for example at 523 nm (green) or at 640 nm (red). Pr:YLF can be efficiently pumped by GaInN laser diodes at 444 nm (blue) [3].

Femtosecond laser written Nd:YAG channel waveguide laser with a PLD grown Cr4+:YAG thin film absorber

The Pulsed Laser Deposition (PLD) technique has several advantages such as the possibility to produce thin films of any compound with a stoichiometric material transfer from the target to the substrate. Also the high energy of the ablated material is a benefit, leading to crystalline film growth due to the high mobility of the atoms on the substrate surface. In combination with a recently demonstrated efficient Nd:YAG waveguide laser, the PLD technique is suitable for producing passively Q-switched waveguide lasers by depositing thin films of highly doped Cr4+:YAG on one endface of the waveguide [1–2].