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We have used a Ti:Sapphire-based fs-comb generator to measure the frequencies of several optical frequency standards available at PTB. The frequency values for HeNe- and frequency-doubled Nd:Yag lasers stabilized to iodine hyperfine transitions as well as the frequencies of the /sup 1/S/sub 0/-/sup 3/P/sub 1/ transition of /sup 40/Ca at 657.5 nm (456 THz) and the /sup 2/S/sub 1/2/-/sup 2/D/sub 3/2/ transition of /sup 171/Yb/sup +/ at 435.5 nm (688 THz) are presented. Comparing our results with previously published data for the frequency variation of transitions of the Hg/sup +/ ion and H atom, we have derived a new upper limit of the variation of the fine structure constant /spl alpha/.

We describe experimental investigations of an optical frequency standard based on a single laser-cooled /sup 171/Yb/sup +/ ion confined in a radiofrequency Paul trap. The electric quadrupole transition from the /sup 2/S/sub 1/2 /(F=0) ground state to the /sup 2/D/sub 3/2/(F=2) state at 435.5 nm is used as the reference transition. The transition was resolved with a Fourier-limited width of approximately 30 Hz. In order to investigate the instability and accuracy of a future frequency standard based on the reference transition, a first experiment was done comparing two laser frequencies stabilized individually to two separate ions in different traps. As a result we found a relative frequency instability /spl sigma//sub y/(640 s)=1.5/spl middot/10/sup -15/ for the difference of the two laser frequencies. We also derived a first estimate /spl Theta/(/sup 2/D/sub 3/2/)=(3.9/spl plusmn/1.9)ea/sub 0//sup 2/ for the electric quadrupole moment of the /sup 2/D/sub 3/2/ state.

We describe experimental investigations on an optical frequency standard based on a single laser cooled 171 Yb + ion confined in a radiofrequency Paul trap. The 435.5 nm electric-quadrupole transition from the 2 S 1/2 (F=0) groudn state to the 2 D 3/2 (F=2) state is probed using a diode laser system whose short-term frequency stability is derived from an environmentally isolated ULE cavity. The transition is resolved with a Foruier-limited full halfwidth of approximately 30 Hz. The absolute optical transition frequency is determined using a femtosecond frequency comb generator and a Cs fountain clock as the frequency reference. The measured frequency value is 688 358 979 321 Hz at an ambient blackbody radiation field of 300 K. The 1σ fractional uncertainty of this frequency measurement is 1×10 -14 .

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 vi = mi×frep+vceo. 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 mi

We developed a novel technique for frequency measurement and synthesis, based on the operation of a femtosecond comb generator as transfer oscillator. The technique can be used to measure frequency ratios of any optical signals throughout the visible and near-infrared part of the spectrum. Relative uncertainties of $10^{-18}$ for averaging times of 100 s are possible. Using a Nd:YAG laser in combination with a nonlinear crystal we measured the frequency ratio of the second harmonic $\nu_{SH}$ at 532 nm to the fundamental $\nu_0$ at 1064 nm, $\nu_{SH}/\nu_0 = 2.000 000 000 000 000 001 \times (1 \pm 7 \times 10^{-19})$., Comment: 4 pages, 4 figures

For nonlinear optical resonators, it has been predicted that the interaction of transverse optical modes of the radiation field can lead to symmetry-breaking and Turing instabilities and to the formation of stationary transverse patterns /1, 2/. It can be concluded from these studies and also from one very early work /3/ that lasers operating in a state of “cooperative frequency locking” /2/ of transverse resonator modes may show bistability or multistability in the sense that stable oscillation is possible with a discrete set of different transverse field patterns.

We have measured the frequency of the $6s^2S_{1/2} - 5d^2D_{3/2}$ electric-quadrupole transition of $^{171}$Yb$^+$ with a relative uncertainty of $1\times 10^{-14}$, $\nu_{Yb}$ = 688 358 979 309 312 Hz $\pm$ 6 Hz. A femtosecond frequency comb generator was used to phase-coherently link the optical frequency derived from a single trapped ion to a cesium fountain controlled hydrogen maser. This measurement is one of the most accurate measurements of optical frequencies ever reported, and it represents a contribution to the development of optical clocks based on an $^{171}$Yb$^+$ ion standard.

It is well known that the output field of lasers oscillating in a superposition of higher-order transverse modes of the optical resonator typically is characterized by a complex spatiotemporal structure /1/. The “cooperative frequency locking” /2/ of near-degenerate transverse resonator modes is accompanied by the formation of stationary transverse patterns of the optical field.