This paper presents a newly developed highly accurate interferometric 6° of freedom (DoF) measurement system with Ångström resolution for displacement and μ rad resolution for angle measurement, respectively. The new interferometer is based on a modified homodyne Twyman–Green interferometer concept. It uses a novel signal acquisition and processing approach whereby a spatial interferogram is captured by a CMOS camera and the registered fringe pattern is transformed into its frequency spectrum [1]. The spectral representation of a movement of e.g. a positioning stage is analyzed for its major components: the phase information directly correlates with the displacement of the stage, while a possible rotational motion causes a shift in the frequency spectrum. The developed compact 6 DoF head uses multiple rays in parallel to detect x – y – z displacements and roll-pitch-yaw movements.

Since the turn of the millennium, the development and commercial availability of optical frequency combs has led to a steadily increase of worldwide installed frequency combs and a growing interest in using them for industrial-related metrology applications. Especially, GPS-referenced frequency combs often serve as a “self-calibrating” length standard for laser wavelength calibration in many national metrology institutes with uncertainties better than u = 1 × 10⁻¹¹. In this contribution, the application of a He-Ne laser source permanently disciplined to a GPS-referenced frequency comb for the interferometric measurements in a nanopositioning machine with a measuring volume of 200 mm × 200 mm × 25 mm (NPMM-200) is discussed. For this purpose, the frequency stability of the GPS-referenced comb is characterized by heterodyning with a diode laser referenced to an ultrastable cavity. Based on this comparison, an uncertainty of u = 9.2 × 10⁻¹² (τ = 8 s, k = 2) for the GPS-referenced comb has been obtained. By stabilizing a tunable He-Ne source to a single comb line, the long-term frequency stability of the comb is transferred onto our gas lasers increasing their long-term stability by three orders of magnitude. Second, short-term fluctuations-related length measurement errors were reduced to a value that falls below the nominal resolving capabilities of our interferometers (ΔL/L = 2.9 × 10⁻¹¹). Both measures make the influence of frequency distortions on the interferometric length measurement within the NPMM-200 negligible. Furthermore, this approach establishes a permanent link of interferometric length measurements to an atomic clock.

: We provide a survey on the joint European research project “GeoMetre”, which explores novel technologies and their inclusion to existing surveying strategies to improve the traceability of geodetic reference frames to the SI definition of the metre. This work includes the development of novel distance meters with a range of up to 5 km, the realisation of optical multilateration systems for large structure monitoring at an operation distance of 50 m and beyond, and a novel strategy for GNSS-based distance determination. Different methods for refractivity compensation, based on classical sensors, on dispersion, on spectroscopic thermometry, and on the speed of sound to reduce the meteorological uncertainties in precise distance measurements, are developed further and characterised. These systems are validated at and applied to the novel European standard baseline EURO5000 at the Pieniny Kippen Belt, Poland, which was completely refurbished and intensely studied in this project. We use our novel instruments for a reduced uncertainty of the scale in the surveillance networks solutions for local tie measurements at space-geodetic co-location stations. We also investigate novel approaches like close-range photogrammetry to reference point determination of space-geodetic telescopes. Finally, we also investigate the inclusion of the local gravity field to consider the deviations of the vertical in the data analysis and to reduce the uncertainty of coordinate transformations in this complex problem.

A perfect spatial overlap with multiple beams of different wavelengths is a prerequisite for multi-wavelength interferometry. Beam combination with the help of fibers seems to be an interesting method for this. We investigated three different types of fiber components, multi-mode wavelength division multiplexers (MM-WDMs), single-mode wavelength division multiplexers (SM-WDMs), and endlessly single-mode polarization maintaining photonic crystal fibers (PM-PCFs). All three seem potential candidates for a perfect spatial overlap of laser beams separated by an octave, i.e. fundamental and harmonic beams. We performed an experimental study on the impact of these components on polarization, wavefront, and coherence. These properties are essential for high-accuracy interferometry. As a test system, we used the fundamental and second harmonic beam of a Nd:YAG laser system at 1064 and 532 nm as a popular system in two-color interferometry for intrinsic refractive index compensation. The experimental results show the potential of PM-PCFs for this challenging application.

Two multi-wavelength interferometers for large-scale surveying

Zusammenfassung: Dieser Beitrag beschreibt ein Mess- und Regelungssystem für einachsige Positionieranwendungen mit Stabilitäten im Sub-Nanometerbereich. Das Positioniersystem besteht aus einem luftgelagerten Linearschlitten, welcher mit einer Tauchspule gestellt, dessen Position laser-interferometrisch gemessen und unter Verwendung einer FPGA-gestützten Lageregelung mit hoher Bandbreite geregelt wird. Eine selbstentwickelte Auswerteelektronik zur Phasendemodulation wurde realisiert, um Positionen hochgenau zu detektieren und dem Regelungssystem mit geringer Verzögerung bereit zu stellen. Erste Messungen ergaben Positionsstabilitäten im Bereich von 0.1 nm.