Optical frequency combs are invaluable tools for precision spectroscopy and many other applications, recognized by the 2005 Nobel Prize for physics. Such combs, typically laser-based, consist of equally-spaced spectral lines forming an extremely precise frequency ruler by periodic emission of pulses.

In high power lasers, free electron lasers, and synchrotron radiation sources the pulse pattern is often interrupted by gaps and reshaped by varying intensity heights in the pulse trains. IBPT researchers have modeled such modifications to reveal their effect on the resulting spectral patterns. In the experiments on electron packages circulating in a synchrotron radiation source, by tuning the number of electrons in each package and the gaps separating them, they were able to increase or decrease the intensity at selected emitted frequencies. By modifying the revolution frequency of the electrons in the ring, continuous frequency coverage can be obtained, dramatically improving the spectral resolution by up to 5 and 7 orders of magnitude compared to recent heterodyne measurement techniques and conventional Fourier transform infrared spectroscopy respectively.

The results open new avenues to optimize and increase the signal-to-noise ratio at specific frequencies, thus enabling novel ultra-high resolution spectroscopy and metrology applications, which with electron-based sources can extend from the terahertz to the X-ray region.

Further Reading
The research was published in Physical Review Letters.

Funding
The work has been supported by German Federal Ministry for Education and Research (BMBF) (Grants No. 05K13VKA and 05K16VKA), Karlsruhe School of Elementary Particle and Astroparticle Physics (KSETA) and Helmholtz International Research School for Teratronics (HIRST). Experiments were performed at the Test Facility ANKA operated by the Institute for Beam Physics and Technology (IBPT).

Reference
J. L. Steinmann, E. Blomley, M. Brosi, E. Bründermann, M. Caselle, J. L. Hesler, N. Hiller, B. Kehrer, Y.-L. Mathis, M.J. Nasse, J. Raasch, M. Schedler, P. Schönfeldt, M. Schuh, M. Schwarz, M. Siegel, N. Smale, M. Weber, A.-S. Müller
Frequency-comb spectrum of periodic-patterned signals
Phys. Rev. Lett. 117(37), 174802 (2016). DOI: 10.1103/PhysRevLett.117.174802.