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Looking for the perfect match

Photonic-crystal fibers superimpose light beams that are spectrally vastly separated.

PTB News 2.2021
Especially interesting for

applications in the field of photonics

developers of optical measurement technologies

How can you superimpose light beams in such a way that they propagate spatially in the same direction as perfectly as possible while maintaining important properties such as polarization and spatial coherence? In a systematic investigation of several commercially available optical waveguides, auxiliary means that help master this demanding challenge has been found. This method enables technologies such as multiple-wavelength interferometry to work over several kilometers.

View of a microstructured fiber in an electron microscope (Photo credit: C. Markos, https://doi.org.10.1038/srep31711)

A world without procedures allowing large distances to be measured with great accuracy would be unimaginable today. Whether we are thinking of GPS technology for navigation, Very Long Baseline Interferometry in astronomy or satellite laser ranging for geodesy – all of these procedures need reference distances of up to a few thousand meters so that we can study and optimize their properties. For reference measurements, PTB has been developing an interferometer within the scope of the European EMPIR project titled “GeoMetre”. Beams with wavelengths of 532 nm and 1064 nm have to overlap as perfectly as possible for this interferometer. To this end, these beams have, to date, been superimposed by means of semi-transparent mirrors in free space. The stability of this elaborate adjustment, however, is limited due to the mechanical components used.

A more robust solution was therefore sought for the envisaged use in the field. Light waveguides were an obvious alternative for the superimposition. For interferometry, however, parameters such as beam homogeneity, polarization and the spatial coherence of all the beams must be maintained. Several waveguide architecture models were therefore compared with regard to these properties within the scope of a systematic study. This study has shown that microstructured photonic-crystal fibers are a nearly perfect solution. They can ensure single-mode, polarization-maintaining transmission that preserves spatial coherence over a wide spectral range (e.g. from 400 nm to 1200 nm). The mechanical strains that are typical of the application do not pose any problem either.

The only reason why this is not quite the “perfect match” is the strongly wavelength-dependent numerical aperture ratio: Different wavelengths are emitted with different beam diameters. This can, however, be taken into account when defining the lens system. Thus, photonic-crystal fiber seems perfectly suited for high-quality aligned beam superimposition, both for the above-mentioned application and for other applications such as spectroscopy.


Florian Pollinger
Department 5.4 Interferometry on Material Measures
Phone: +49 531 592-5420
Opens local program for sending emailflorian.pollinger(at)ptb.de

Scientific publication

Y. Liu, A. Röse, G. Prellinger, P. Köchert, J. Zhu, F. Pollinger: Combining harmonic laser beams by fiber components for refractivity-compensating two-color interferometry. J. Lightwave Technology 38, 1945–1952 (2020)