The 1S-2S two-photon transition in atomic hydrogen is resolved to DELTAnu/nu = 1.1 x 10(-11).The transition frequency is compared to a reference frequency derived from an infrared CH4-stabilized helium-neon laser at 88 THz with a phase-locked laser frequency chain. The helium-neon laser is compared to the microwave cesium standard with the help of another phase-locked frequency chain. With our optical frequency measurement, not limited by the I2-stabilized helium-neon laser standard at 633 nm, we found a 1S-2S transition frequency of 2466061413.182(45) MHz with an 18-fold improved accuracy. A value of the Rydberg constant, R(infinity) = 109 737.315 684 6(41) cm-1, is deduced.

We have constructed a frequency synthesis chain in order to compare the 1s2s Hydrogen transition (Lyman-(alpha) , 2466 THz) with the Methane stabilized He-Ne-laser at 88,4 THz. Phaselocks for all transfer oscillators have been established. The 88 THz line serves as a secondary frequency standard, currently operating at an absolute reproducibility of 2 X 10-12 and a stability of 10-13 at 10 - 100 s integration time. We report on the transfer of this precision to the Lyman-(alpha) frequency of the hydrogen atom, which yields an improved value of the Rydberg constant.

We have resolved the 1S-2S-transition of atomic hydrogen with a relative accuracy of DELTAv/v = 2.8 . 10(-11) and compared this transition frequency to a reference that was synthesized by a phasecoherent frequency-chain starting from an infrared CH4-stabilized helium-neon-laser at 88 THz. In this optical frequency measurement, which is not limited by the less precise I2-stabilized helium-neon laser-standard at 633 nm, we could determine the 1S-2S-transition frequency to be 2 466 061 413.182(45) MHz with an 18-fold increased precision of 1.8 . 10(-11). One can derive a new value for the Rydberg constant R(infinity) = 109 737.315 684 1(42) cm-1 where the uncertainty is reduced by a factor of 4.3 compared to previous measurements.

Hydrogen, the simplest of the stable atoms, provides unique opportunities for critical confrontations of spectroscopic experiment and quantum electrodynamic theory. Recent advances in the generation, stabilization, and measurement of optical frequencies are extending radiofrequency resolution and accuracy into the ultraviolet spectral region. Precise measurements and comparisons of the optical frequencies of sharp hydrogen two-photon transitions are yielding new values of fundamental constants, and they are permitting stringent new tests of basic physics laws.

We have achieved cw laser action on the 4S3/2→4I9/2 transition of 1.25 at% Er-ions in a YA1O3 host crystal at room temperature. The observed wavelengths are 1.6632, 16776, 1.7061, and 1.7296 μm. The lasing threshold shows a strong temperature dependence, reducing the threshold by more than 50% when the laser rod is cooled from 300 K to 250 K. The influence of the temperature dependent ion-lattice coupling is effective not only in low lying relaxation processes but also in excited state absorption as indicated by fluorescence spectroscopy.