With Wavelength Combs to Picometer Resolution

Posted on 2006-07-01. Permanent link: http://www.rp-photonics.com/spotlight_2006_07_01.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: T. R. Schibli, K. Minoshima, Y. Bitou, F.-L. Hong, H. Inaba, A. Onae, and H. Matsumoto, "Displacement metrology with sub-pm resolution in air based on a fs-comb wavelength synthesizer", Opt. Express 14 (13), 5984 (2006)

Optical frequency combs as generated from carefully stabilized mode-locked laser sources represent equidistant set of very well-defined optical frequencies. However, the corresponding wavelengths in air are much less well defined, as the refractive index of air undergoes changes due to varying pressure and humidity. As wavelength stability rather than frequency stability is the actually required feature for some displacement measurement schemes (e.g. based on Fabry-Pérot interferometers), researchers have turned a frequency comb into a wavelength comb, where all spectral components have a quite precisely constant wavelength in air. This allowed them to make extremely precise displacement measurements without using an air-tight enclosure of their setup. For example, they can monitor extremely tiny drifts of a cavity length on the scale of some tens of picometers, or to measure the tiny nonlinear error of an ultraprecise nanopositioning translation stage of just a few a picometers over a translation range of 12 μm.

It is apparent that frequency combs from mode-locked lasers generate a wide range of opportunities in metrology – not only in frequency metrology, but also in other fields. Companies in the field of laser metrology are well advised to take notice of these developments.

See also: frequency combs, optical metrology, distance measurements with lasers