J. Reichert

Accurate measurement of large optical frequency differences with a mode-locked laser

We have used the comb of optical frequencies emitted by a mode-locked laser as a ruler to measure differences of as much as 20 THz between laser frequencies. This is to our knowledge the largest gap measured with a frequency comb, with high potential for further improvements. To check the accuracy of this approach we show that the modes are distributed uniformly in frequency space within the experimental limit of 3.0 parts in 10(17) . By comparison with an optical frequency comb generator we have verified that the mode separation equals the pulse repetition rate within the experimental limit of 6.0 parts in 10(16).

Measuring the frequency of light with mode-locked lasers

Abstract We have stabilized the modes of a comb of optical frequencies emitted by a mode-locked femtosecond-laser and used it as a ruler to measure differences of up to 45.2 THz between laser frequencies in a new type of frequency chain. Directly converting optical to radio frequencies, we have used it for an absolute frequency measurement of the 1S–2S transition in the hydrogen atom. Here, an intuitive model of the combs properties is given and essential techniques for its stabilization and efficient detection of beat signals are presented.

Frequency Comparison and Absolute Frequency Measurement of I2-stabilized Lasers at 532 nm

We present a frequency comparison and an absolute frequency measurement of two independent I2-stabilized frequency-doubled Nd:YAG lasers at 532 nm, one set up at the Institute of Laser Physics, No vosibirsk, Russia, the other at the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. The absolute frequency of the I2-stabilized lasers was determined using a CH4-stabilized He-Ne laser as a reference. This laser had been calibrated prior to the measurement by an atomic cesium fountain clock. The frequency chain linking phase-coherently the two frequencies made use of the frequency comb of a Kerr-lens mode-locked Ti:sapphire femtosecond laser where the comb mode separation was controlled by a local cesium atomic clock. A new value for the R(56)32-0:a10 component, recommended by the Comite International des Poids et Mesures (CIPM) for the realization of the metre [1], was obtained with reduced uncertainty. Absolute frequencies of the R(56)32-0 and P(54)32-0 iodine absorp tion lines together with the hyperfine line separations were measured.

Accuracy of optical frequency comb generators and optical frequency interval divider chains

We compared two methods for measuring large optical frequency differences: an optical frequency comb generator, which creates a large number of sidebands from a single-mode laser through electro-optic modulation, and an optical frequency interval divider chain, which divides a frequency gap successively by two until it becomes accessible to a radio-frequency counter. By locking two diode lasers to two modulation sidebands of a comb generator, ~1 THz apart, and measuring this interval with a chain of four phase-locked interval dividers, we demonstrate for the first time to our knowledge the accuracy of both techniques within an experimental limit of 6.8 x 10(-15).

Absolute frequency measurement of the 115In+ 5S2 1S0-5S5p 3P0 transition

Abstract We have measured the absolute frequency of the 115 In+ 5s 2 1 S0–5s5p 3 P0 clock transition at 236.5 nm with an accuracy of 3.3 parts in 1011. For this measurement, a frequency synthesis chain was used which links the indium clock transition to a methane-stabilized He–Ne laser at 3.39 μm and a Nd:YAG laser at 1064 nm whose second harmonic was locked to a hyperfine component in molecular iodine. A frequency gap in the chain of 1.43 THz at 850 nm was bridged with the help of an optical frequency comb generator. The frequency of the 115 In+ clock transition was determined to 1 267 402 452 914 (41) kHz, where the accuracy is limited by the uncertainty of the iodine reference. This measurement represents an improvement of more than three orders of magnitude in accuracy compared to previous measurements of the line.

A New Type of Frequency Chain and Its Application to Fundamental Frequency Metrology

A suitable femtosecond (fs) laser system can provide a broad band comb of stable optical frequencies and thus can serve as an rf/optical coherent link. In this way we have performed a direct comparison of the 1S—2S transition in atomic hydrogen at 121 nm with a cesium fountain clock, built at the LPTF/Paris, to reach an accuracy of 1.9×10-14. The same comb-line counting technique was exploited to determine and recalibrate several important optical frequency standards. In particular, the improved measurement of the Cesium D1 line is necessary for a more precise determination of the fine structure constant. In addition, several of the best-known optical frequency standards have been recalibrated via the fs method. By creating an octave-spanning frequency comb a single-laser frequency chain has been realized and tested.

Broadening of femtosecond frequency combs and compact optical to radio frequency conversion

Summary form only given. The periodic pulse train emitted by a modelocked femtosecond laser can be viewed in the frequency domain as a comb of equidistant modes spaced by the pulse repetition rate. This broad frequency comb can be used like a ruler to phase coherently measure large differences between laser frequencies. With our Kerr-lens mode-locked Ti:sapphire laser (pulselength 73 fs, Coherent Inc., model Mira 900) we have shown that the modes of the femtosecond frequency comb are distributed uniformly in frequency space within the experimental limit.

High Resolution Spectroscopy of Atomic Hydrogen

We report on an absolute frequency measurement of the 1S-2S two-photon transition frequency in atomic hydrogen. In our experiment, we have directly linked the transition frequency to a cesium atomic clock. A careful analysis of the spectroscopic line-shape by means of a theoretical model allowed the determination of the line center to about 1/100 of the linewidth, yielding a value for the 1S-2S transition frequency of 2 466 061 413 187 103(46) Hz.

Optical frequency metrology and its contribution to the determination of fundamental constants

Optical frequency metrology plays an important role for the determination of the Rydberg constant and offers an alternative way to determine the fine structure constant α. We have developed a new technique for measuring optical frequencies using femtosecond light pulses culminating in the single laser optical frequency synthesizer. This new technique greatly simplifies the task of measuring optical frequencies.

Chronometry: Effect of the 1999 solar eclipse on atomic clocks

Solar eclipses have been reported to have a strange influence on the behaviour of atomic clocks and pendulums, which has been attributed to some unknown feature of gravity. Here we correct this idea after being unable to detect any anomalous changes in the relative rates of three types of atomic clock, based on the ground-state hyperfine transitions of hydrogen, rubidium and caesium, during the solar eclipse of 11 August 1999 over central Europe.