The generation of broadband microwave frequency comb from a femtosecond pulse train by direct photodetection opens the possibility for high-accuracy length measurements of long distances. We demonstrate a relatively simple realization of this measurement principle: an electronic distance measurement system based on a time-of-flight approach, driven by a femtosecond fibre laser source as a modulator. By the evaluation of the phase shifts of two distinct comb frequencies, a coarse and a fine measurement of the absolute distance can be performed. The range of the measurement system is demonstrated up to a length of 100 m. The experimental comparison of the femtosecond laser system with a conventional reference counting interferometer shows a precision better than ±10 μm at 100 m, corresponding to a relative measurement uncertainty of 1 × 10 −7 L. The limiting factors for the measurement uncertainty of the system are theoretically investigated and shown to be of the same order of magnitude.

The calibration and verification of high-precision electronic distance meters (EDMs) requires well-characterized and calibrated geodetic baselines. As the length measurements are performed typically over several hundred metres in air, a thorough understanding of the environmental conditions is necessary. In the course of a major refurbishment, the 600 m baseline of the Physikalisch-Technische Bundesanstalt at Braunschweig, Germany, was equipped with a dense environmental sensor network. This paper presents the characterization of this novel reference baseline, including the calibration of the inter-pillar distances, and identifies the major sources of uncertainty for such a length standard. A preliminary expanded standard uncertainty (k = 2) of is deduced for single-slope distance comparisons on the baseline. In the course of a full calibration, the additive constant cEDM of an EDM can currently be determined with an expanded uncertainty of U(cEDM)k = 2 = 6.1 × 10−5 m, and its scale correction sEDM with an expanded uncertainty of U(sEDM)k = 2 = 8.2 × 10−7. As an example, a femtosecond laser-based distance measurement over 600 m on this baseline is presented.

Humidity is one of the key atmospheric parameters influencing the refractive index of air. Consequently, humidity influences all length measurements whose scale is derived from the speed of light. In this work, we present two laser spectrometers developed for determining the average humidity of air over a long measurement path where local variations may be difficult to measure using conventional sensors. Further, these laser-based systems allow, in principle, good spatial overlap with the beams used in dimensional measurements. The developed systems were compared to each other and to traceable reference sensors during a 65 h measurement campaign. The performance of the systems was investigated under three different conditions: steady state, humidity transient and temperature transient. Both systems were separately tested in outdoor environment at distances up to several hundreds of metres. The measurement results demonstrate that the systems are able to measure the relative humidity below the 4% uncertainty level both in indoor and in outdoor environment.

A new and potentially cost efficient kind of vibration-tolerant surface measurement interferometer based on the Fizeau-principle is demonstrated. The crucial novelty of this approach is the combination of two optoelectronic sensors: an image sensor with high spatial resolution and an arrangement of photodiodes with high temporal resolution. The photodiodes continuously measure the random-phase-shifts caused by environmental vibrations in three noncollinear points of the test surface. The high spatial resolution sensor takes several "frozen" images of the test surface by using short exposure times. Under the assumption of rigid body movement the continuously measured phase shifts of the three surface points enable the calculation of a virtual plane that is representative for the position and orientation of the whole test surface. For this purpose a new random-phase-shift algorithm had to be developed. The whole system was tested on an optical table without vibration isolation under the influence of random vibrations. The analysis of the root-mean-square (RMS) over ten different measurements shows a measurement repeatability of about 0.004 wave (approximately 2.5 nm for 632.8 nm laser wavelength).

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