Eur. Phys. J. Special Topics 225, 2775-2784 (2016)
https://doi.org/10.1140/epjst/e2016-60092-0

Regular Article

Difference-frequency combs in cold atom physics

¹ TOPTICA Photonics AG, Lochhamer Schlag 19, 82166 Graefelfing, Germany
² Joint Quantum Centre (JQC) Durham-Newcastle, Department of Physics, Durham University, Durham DH1 3LE, UK

^a e-mail: thomas.puppe@toptica.com

Received: 29 March 2016
Revised: 11 August 2016
Published online: 19 December 2016

Abstract

Optical frequency combs provide the clockwork to relate optical frequencies to radio frequencies. Hence, combs allow optical frequencies to be measured with respect to a radio frequency where the accuracy is limited only by the reference signal. In order to provide a stable link between the radio and optical frequencies, the two parameters of the frequency comb must be fixed: the carrier envelope offset frequency, f_ceo, and the pulse repetition-rate, f_rep. We have developed the first optical frequency comb based on difference frequency generation (DFG) that eliminates f_ceo by design — specifically tailored for applications in cold atom physics. An f_ceo-free spectrum at 1550 nm is generated from a super continuum spanning more than an optical octave. Established amplification and frequency conversion techniques based on reliable telecom fibre technology allow the generation of multiple wavelength outputs. The DFG comb is a convenient tool to both stabilise laser sources and accurately measure optical frequencies in Rydberg experiments and more generally in quantum optics. In this paper we discuss the frequency comb design, characterization, and optical frequency measurement of Strontium Rydberg states. The DFG technique allows for a compact and robust, passively f_ceo stable frequency comb significantly improving reliability in practical applications.