Interferometric surface mapping of a spherical proof mass for ultra precise inertial reference sensors.

In the context of our investigations on novel inertial reference sensors for space applications, we have explored a design utilizing an optical readout of a spherical proof mass. This concept enables full drag-free operations, hence reducing proof mass residual acceleration noise to a minimum. The main limitations of this sensor are errors in position determination of the center of mass of the proof mass due to the surface topography and the involved path length changes upon rotation. One solution is to apply a surface map for correction of the measurement data, thus improving the precision of position determination. This article presents the results of our one-dimensional interferometric surface topography measurements of a sphere, achieving uncertainties of ≈10 nm, as a first step to realize a complete surface map. The measurement setup consists of two heterodyne interferometers positioned in an opposing configuration, which measure the surface topography while the sphere is continuously rotated by a rotation stage.

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.

Coupling characterization and noise studies of the optical metrology system onboard the LISA Pathfinder mission.

We describe the first investigations of the complete engineering model of the optical metrology system (OMS), a key subsystem of the LISA Pathfinder science mission to space. The latter itself is a technological precursor mission to LISA, a spaceborne gravitational wave detector. At its core, the OMS consists of four heterodyne Mach-Zehnder interferometers, a highly stable laser with an external modulator, and a phase meter. It is designed to monitor and track the longitudinal motion and attitude of two floating test masses in the optical reference frame with (relative) precision in the picometer and nanorad range, respectively. We analyze sensor signal correlations and determine a physical sensor noise limit. The coupling parameters between motional degrees of freedom and interferometer signals are analytically derived and compared to measurements. We also measure adverse cross-coupling effects originating from system imperfections and limitations and describe algorithmic mitigation techniques to overcome some of them. Their impact on system performance is analyzed within the context of the Pathfinder mission.

Does the timing of tracheal intubation based on neuromuscular monitoring decrease laryngeal injury? A randomized, prospective, controlled trial.

Vocal cord injuries (VCI) and postoperative hoarseness (PH) are common complications after general anesthesia. Poor muscle relaxation at the moment of tracheal intubation may result in VCI. There is a large interindividual variation in neuromuscular depression after administration of neuromuscular blocking drugs. Therefore, the optimal individual timing of tracheal intubation based on neuromuscular monitoring (monitoring) may decrease VCI. In this prospective trial, 60 patients were randomized into 2 groups: Monitoring group: tracheal intubation at maximum block based on monitoring after atracurium 0.5 mg/kg and 2-min group: tracheal intubation 2 min after injection of atracurium 0.5 mg/kg. Intubating conditions were evaluated with the Copenhagen score. VCI were examined by stroboscopy before and 24 and 72 h after surgery. PH was assessed at 24, 48, and 72 h after surgery by a standardized interview. Excellent intubating conditions were significantly increased in the monitoring group compared with the 2-min group: 8 versus 2 patients, respectively (P = 0.036). The incidence of PH between the study groups was comparable: 7 (monitoring) versus 8 patients (2-min) (P = 0.860). Similar findings were observed for VCI: 9 versus 5 patients; respectively (P = 0.268); type of VCI: thickening of the vocal cords: 8 (monitoring) versus 5 (2-min) patients (P = 0.423), hematomas: 2 patients in each group (not significant). The present study demonstrated that neuromuscular monitoring improved endotracheal intubating conditions. However, tracheal intubation at maximum intensity of neuromuscular block was not associated with a decrease in vocal cord injuries.

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.

A phasemeter concept for space applications that integrates an autonomous signal acquisition stage based on the discrete wavelet transform.

We describe a phasemeter designed to autonomously acquire and track a heterodyne signal with low signal-to-noise ratio in a frequency band that spans from 1 MHz to 25 MHz. The background driving some of the design criterions of the phasemeter comes from studies on future space mission concepts such as orbiting gravitational wave observatories and next generation geodesy missions which all rely on tracking phasemeters in order to meet their mission goal. The phasemeter has been implemented within a field programmable gate array trying to minimize the requirement of computational resources and its performance has been tested using signal generators. Laboratory test has shown that the phasemeter is capable of locking to an input signal in less than half a millisecond, while its phase measurement accuracy is in the micro-radian range for measurement frequencies that span from mHz to Hz.