We present a double sided, single pass Michelson heterodyne interferometer for dimensional stability measurements. In preliminary measurements, the double deadpath configuration (no sample) showed better than ±1.5 nm (2σ) over 13 hours. A 30 mm stainless gauge block was then measured with a stability of ±1.2 nm (2σ) over 9 hours. The interferometer was then moved to a facility capable of measuring in vacuum. In a pressure sealed environment, but not vacuum, the interferometer stability was better than ±0.6 nm (2σ) over 23 hours. Using a Fourier analysis on this drift measurement, the limiting factor is the slight spatial gradients in the refractive index. With relatively large air paths greater than 400 mm, refractive index fluctuations on the order of parts in 109 are needed to cause this drift.
We report on the development of measurement facilities for the calibration of ultra-precision displacement sensors and for the dimensional stability validation of materials, joint structures and sensors. A Fabry-Pérot interferometer and a double-ended heterodyne interferometer are discussed, both with a dedicated design aiming for sub-nanometer displacement measurement accuracy.
Optical interferometry enables highly accurate non-contact displacement measurement. The optical phase ambiguity needs to be resolved for absolute distance ranging. In controlled laboratory conditions and for short distances it is possible to track a non-interrupted displacement from a reference position to a remote target. With large distances covered in field applications this may not be feasible, e.g. in structure monitoring, large scale industrial manufacturing or aerospace navigation and attitude control. We use an optical frequency comb source to explore absolute distance measurement by means of a combined spectral and multi-wavelength homodyne interferometry. This relaxes the absolute distance ambiguity to a few tens of centimeters, covered by simpler electronic distance meters, while maintaining highly accurate optical phase measuring capability. A virtually imaged phased array spectrometer records a spatially dispersed interferogram in a single exposure and allows for resolving the modes of our near infrared comb source with 1 GHz mode separation. This enables measurements with direct traceability of the atomic clock referenced comb source. We observed agreement within 500 nm in comparison with a commercial displacement interferometer for target distances up to 50 m. Furthermore, we report on current work toward applicability in less controlled conditions. A filter cavity decimates the comb source to an increased mode separation larger than 20 GHz. A simple grating spectrometer then allows to record mode-resolved interferograms.
This paper presents the progress in the development of two Fabry-Pérot filter cavities for repetition rate multiplication of two femtosecond frequency combs. The optical design of both setups consists of mode matching optics and a resonant cavity for the repetition rate multiplication. In one case, the cavity consists of two dielectric mirrors with near-zero group velocity dispersion and in the other of two silver coated mirrors. We demonstrate multiplication of a 1 GHz repetition rate to 10 GHz for a Ti:Sa femtosecond frequency comb with central wavelength around 820 nm and of 250 MHz repetition rate to 1 GHz for a Er-doped fiber femtosecond frequency comb with central wavelength around 1560 nm.
Fabry-Perot displacement interferometry (FPI) offers high sensitivity and resolution with direct traceability to optical
frequency standards. FPI can provide means for demanding calibration tasks in precision engineering and high-tech
systems. We report on our investigation of the measurement methodology applied to highest precision capacitive
displacement sensors. We use a dedicated metrological FPI instrumentation that provides an actuated reference target
with a relatively large traceable displacement stroke. The envisaged sub-nanometer measurement uncertainty seems very
challenging under practical ambient atmospheric conditions and with the necessary sensor mounting components. In
anticipation of these limitations, we propose a new FPI instrumental configuration with a very short cavity and discuss
expected benefits, most importantly the very low sensitivity to air refractive index variations and the versatility for
practical calibration purposes. We aim again for sub-nanometer measurement uncertainty and report on the status of the
experimental set-up for this short cavity FPI.
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