Optical cavity reaching 10-16 frequency stability with compact optical circulator based in-coupling optics.

Future space missions will benefit from highly stable and compact optical frequency references. While many promising technologies are currently under investigation, optical cavities are a well-suited technique for applications in which relative references are required. To improve the frequency stability of optical cavities, a key step in combining high performance with compactness and robustness is the further development of in-coupling optics. Here, we present our work of using a fiber-coupled circulator based in-coupling for a high-finesse optical cavity. Implementing the new, to the best of our knowledge, in-coupling board to an extensively characterized crossed cavity set-up allows us to identify possible differences to the commonly used free-beam technique. With a frequency stability of 5.5×10-16 H z -1/2 at 1 Hz and with only a slight degradation in frequency stability below the mHz range, no circulator-caused instabilities were observed.

Simultaneous laser frequency stabilization to an optical cavity and an iodine frequency reference.

In this Letter, we demonstrate a method to combine a molecular iodine absolute frequency reference with a high-finesse optical cavity in a single laser to take advantage of the frequency stability properties of both systems at different time scales. The result is a laser exhibiting the long-term and short-term stability levels of the iodine frequency reference and optical cavity, respectively. The method uses frequency offset side-band locking and an acousto-optical modulator driven ac-coupled servo-loop to correct the iodine's short-term frequency fluctuations. Experimental results show cavity-limited stability at 1 Hz of 10-151/Hz and iodine stability below 10 mHz of 10-131/Hz. In terms of the Allan deviation, this corresponds to stability levels close to the 10-15 at 1 s and 10-14 for observation times >100s.

Architecture and performance analysis of an optical metrology terminal for satellite-to-satellite laser ranging.

Interferometric laser ranging is an enabling technology for high-precision satellite-to-satellite tracking within the context of Earth observation, gravitational wave detection, or formation flying. In orbit, the measurement system is affected by environmental influences, particularly satellite attitude jitter and temperature fluctuations, imposing an instrument design with a high level of thermal stability and insensitivity to rotations around the spacecraft center of mass. The new design concept presented here combines different approaches for dynamic heterodyne laser ranging and features the inherent beam-tracking capabilities of a retroreflector in a mono-axial configuration. It allows for a continuously adjustable distance between the optical bench and the location of its fiducial point, facilitating future inter-satellite tracking with nanometer accuracy, e.g., the next-generation gravity mission.

Long-term stable optical cavity for special relativity tests in space: erratum.

We incorrectly cited a maximum acceleration sensitivity of the rigidly-mounted cavity of 2.5 × 10-10 1/(m s-2). The correct coupling factor is a factor of 100 smaller: 2.5 × 10-12 1/(m s-2).

Long-term stable optical cavity for special relativity tests in space.

BOOST (BOOst Symmetry Test) is a proposed space mission to search for Lorentz invariance violations and aims to improve the Kennedy-Thorndike parameter constraint by two orders of magnitude. The mission consists of comparing two optical frequency references of different nature, an optical cavity and a hyperfine transition in molecular iodine, in a low Earth orbit. Naturally, the stability of the frequency references at the orbit period of 5400 s (f=0.18 mHz) is essential for the mission success. Here we present our experimental efforts to achieve the required fractional frequency stability of 7.4×10-14 Hz -1/2 at 0.18 mHz (in units of the square root of the power spectral density), using a high-finesse optical cavity. We have demonstrated a frequency stability of (9±3)×10-14 Hz -1/2 at 0.18 mHz, which corresponds to an Allan deviation of 10-14 at 5400 s. A thorough noise source breakdown is presented, which allows us to identify the critical aspects to consider for a future space-qualified optical cavity for BOOST. The major noise contributor at sub-milli-Hertz frequency was related to intensity fluctuations, followed by thermal noise and beam pointing. Other noise sources had a negligible effect on the frequency stability, including temperature fluctuations, which were strongly attenuated by a five-layer thermal shield.

Development of a compact optical absolute frequency reference for space with 10-15 instability.

We report on a compact and ruggedized setup for laser frequency stabilization employing Doppler-free spectroscopy of molecular iodine near 532 nm. Using a 30 cm long iodine cell in a triple-pass configuration in combination with noise-canceling detection and residual amplitude modulation control, a frequency instability of 6×10-15 at 1 s integration time and a Flicker noise floor below 3×10-15 for integration times between 100 and 1000 s was found. A specific assembly-integration technology was applied for the realization of the spectroscopy setup, ensuring high beam pointing stability and high thermal and mechanical rigidity. The setup was developed with respect to future applications in space, including high-sensitivity interspacecraft interferometry, tests of fundamental physics, and navigation and ranging.

Dilatometer setup for low coefficient of thermal expansion materials measurements in the 140 K-250 K temperature range.

Space applications demand light weight materials with excellent dimensional stability for telescopes, optical benches, optical resonators, etc. Glass-ceramics and composite materials can be tuned to reach very low coefficient of thermal expansion (CTE) at different temperatures. In order to determine such CTEs, very accurate setups are needed. Here we present a dilatometer that is able to measure the CTE of a large variety of materials in the temperature range of 140 K to 250 K. The dilatometer is based on a heterodyne interferometer with nanometer noise levels to measure the expansion of a sample when applying small amplitude controlled temperature signals. In this article, the CTE of a carbon fiber reinforced polymer sample has been determined with an accuracy in the 10-8 K-1 range.

Atom interferometry in space: thermal management and magnetic shielding.

Atom interferometry is an exciting tool to probe fundamental physics. It is considered especially apt to test the universality of free fall by using two different sorts of atoms. The increasing sensitivity required for this kind of experiment sets severe requirements on its environments, instrument control, and systematic effects. This can partially be mitigated by going to space as was proposed, for example, in the Spacetime Explorer and Quantum Equivalence Principle Space Test (STE-QUEST) mission. However, the requirements on the instrument are still very challenging. For example, the specifications of the STE-QUEST mission imply that the Feshbach coils of the atom interferometer are allowed to change their radius only by about 260 nm or 2.6 × 10(-4) % due to thermal expansion although they consume an average power of 22 W. Also Earth's magnetic field has to be suppressed by a factor of 10(5). We show in this article that with the right design such thermal and magnetic requirements can indeed be met and that these are not an impediment for the exciting physics possible with atom interferometers in space.

Interferometric characterization and modeling of pathlength errors resulting from beamwalk across mirror surfaces in LISA.

An alternative payload concept with in-field pointing for the laser interferometer space antenna utilizes an actuated mirror in the telescope for beam tracking to the distant satellite. This actuation generates optical pathlength variations due to the resulting beamwalk over the surface of subsequent optical components, which could possibly have a detrimental influence on the accuracy of the measurement instrument. We have experimentally characterized such pathlength errors caused by a λ/10 mirror surface and used the results to validate a theoretical model. This model is then applied to predict the impact of this effect for the current optical design of the LISA off-axis wide-field telescope.

Ultrastable assembly and integration technology for ground- and space-based optical systems.

Optical metrology systems crucially rely on the dimensional stability of the optical path between their individual optical components. We present in this paper a novel adhesive bonding technology for setup of quasi-monolithic systems and compare selected characteristics to the well-established state-of-the-art technique of hydroxide-catalysis bonding. It is demonstrated that within the measurement resolution of our ultraprecise custom heterodyne interferometer, both techniques achieve an equivalent passive path length and tilt stability for time scales between 0.1 mHz and 1 Hz. Furthermore, the robustness of the adhesive bonds against mechanical and thermal inputs has been tested, making this new bonding technique in particular a potential option for interferometric applications in future space missions. The integration process itself is eased by long time scales for alignment, as well as short curing times.

Offset compensation by use of amplitude-modulated sidebands in optical frequency standards.

We present a general method for continuously measuring and compensating for offsets (due to residual amplitude modulation, parasitic resonances, or electronic offset voltages, for example) in frequency stabilization systems. The spectral power distribution of the oscillator waveform is modified by amplitude-modulated sidebands, and the error signal is corrected to null the induced periodic lock-point shifts. We demonstrate significant improvements to the frequency stability of standards based on cryogenic optical resonators and molecular iodine.