Abstract

Stimulated by the development of optical frequency comb generators based on Kerr-lens modelocked femtosecond lasers [1,2], the absolute frequencies of several narrow transition lines in cold atoms or single stored ions, such as H, Ca, Yb⁺ were recently measured by phase- coherently linking optical signals to a cesium-clock controlled hydrogen maser [3-6]. This phase-coherent link is established via a frequency-comb generated by a femtosecond laser. In the frequency domain the femtosecond laser’s pulse train corresponds to a comb spectrum. This comb is spectrally broadened to extend over the whole visible wavelength region and into the near infrared. In our experiment this was achieved in a micro- structure/photonic crystal fiber from Lucent Technologies. This fiber has core diameter of 1.7 μm and its GVD ist zero at 780 nm [7], which is the center wavelength of the spectrum emitted by our Ti:Sa-laser similar to that described in ref. [8]. Its spectrum of some 170 nm width around 780 nm is broadened to extend from 500 nm to about 1100 nm, by passing approximately 30 mW through a 10 cm piece of the micro-structure fiber. During the propagation in the fiber, the comb nature of the spectrum is not altered, only its spectral width is extended. The result is an evenly spaced comb spectrum over almost the whole visible wavelength range and into the near infrared, where the individual comb mode m, has the frequency v_i = m_i×f_rep+v_ceo. The last term, the so called carrier envelope offset frequency is due to different group and phase velocity delay in the laser cavity. It results in a collective shift of all comb modes with respect to the frequency origin. Any external optical frequency Δi is measured by detecting a beat note A; with a suitable comb mode with the order number m_i:

© 2002 Optical Society of America