Related Experiment Video
Updated: Jun 29, 2026

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies
Published on: December 18, 2015
Phase coherent vacuum-ultraviolet to radio frequency comparison with a mode-locked laser
1Max-Planck-Institut fur Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany.
Abstract:
We demonstrate a versatile new technique that provides a phase coherent link between optical frequencies and the radio frequency domain. The regularly spaced comb of modes of a mode-locked femtosecond laser is used as a precise ruler to measure a large frequency gap between two different multiples (harmonics or subharmonics) of a laser frequency. In this way, we have determined a new value of the hydrogen 1S-2S two-photon resonance, f(1S-2S) = 2 466 061 413 187.29(37) kHz, representing now the most accurate measurement of an optical frequency.
Related Concept Videos
Atomic Nuclei: Larmor Precession Frequency
UV–Vis Spectroscopy: Molecular Electronic Transitions
NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences
Propagation Speed of Electromagnetic Waves
Electromagnetic Waves in Matter
Consider the electromagnetic wave passing through a dielectric medium. In such a case, Maxwell's equations get modified. In Ampere's law, ε0 , the dielectric permittivity of free space is replaced with ε, the permittivity of dielectric. Also, the vacuum permeability μ0 is replaced by the permeability of the medium, μ.
Furthermore, the...
Standing Waves in a Cavity

