Active fiber-based retroreflector providing phase-retracing anti-parallel laser beams for precision spectroscopy.

We present an active fiber-based retroreflector providing high quality phase-retracing anti-parallel Gaussian laser beams for precision spectroscopy of Doppler sensitive transitions. Our design is well-suited for a number of applications where implementing optical cavities is technically challenging and corner cubes fail to match the demanded requirements, most importantly retracing wavefronts and preservation of the laser polarization. To illustrate the performance of the system, we use it for spectroscopy of the 2S-4P transition in atomic hydrogen and demonstrate an average suppression of the first order Doppler shift to 4 parts in 106 of the full collinear shift. This high degree of cancellation combined with our cryogenic source of hydrogen atoms in the metastable 2S state is sufficient to enable determinations of the Rydberg constant and the proton charge radius with competitive uncertainties. Advantages over the usual Doppler cancellation based on corner cube type retroreflectors are discussed as well as an alternative method using a high finesse cavity.

High-power multi-megahertz source of waveform-stabilized few-cycle light.

Waveform-stabilized laser pulses have revolutionized the exploration of the electronic structure and dynamics of matter by serving as the technological basis for frequency-comb and attosecond spectroscopy. Their primary sources, mode-locked titanium-doped sapphire lasers and erbium/ytterbium-doped fibre lasers, deliver pulses with several nanojoules energy, which is insufficient for many important applications. Here we present the waveform-stabilized light source that is scalable to microjoule energy levels at the full (megahertz) repetition rate of the laser oscillator. A diode-pumped Kerr-lens-mode-locked Yb:YAG thin-disk laser combined with extracavity pulse compression yields waveform-stabilized few-cycle pulses (7.7 fs, 2.2 cycles) with a pulse energy of 0.15 µJ and an average power of 6 W. The demonstrated concept is scalable to pulse energies of several microjoules and near-gigawatt peak powers. The generation of attosecond pulses at the full repetition rate of the oscillator comes into reach. The presented system could serve as a primary source for frequency combs in the mid infrared and vacuum UV with unprecedented high power levels.

A simplified scheme for generating narrow-band mid-ultraviolet laser radiation.

We report on the development and characterization of continuous, narrow-band, and tunable laser systems that use direct second-harmonic generation from blue and green diode lasers with an output power level of up to 11.1 mW in the mid-ultraviolet. One of our laser systems was tuned to the mercury 6(1)S0 → 6(3)P1 intercombination line at 253.7 nm. We could perform Doppler-free saturation spectroscopy on this line and were able to lock our laser to the transition frequency on long time scales.

Optical-frequency transfer over a single-span 1840 km fiber link.

To compare the increasing number of optical frequency standards, highly stable optical signals have to be transferred over continental distances. We demonstrate optical-frequency transfer over a 1840-km underground optical fiber link using a single-span stabilization. The low inherent noise introduced by the fiber allows us to reach short term instabilities expressed as the modified Allan deviation of 2×10(-15) for a gate time τ of 1 s reaching 4×10(-19) in just 100 s. We find no systematic offset between the sent and transferred frequencies within the statistical uncertainty of about 3×10(-19). The spectral noise distribution of our fiber link at low Fourier frequencies leads to a τ(-2) slope in the modified Allan deviation, which is also derived theoretically.

Nonlinear amplification of side-modes in frequency combs.

We investigate how suppressed modes in frequency combs are modified upon frequency doubling and self-phase modulation. We find, both experimentally and by using a simplified model, that these side-modes are amplified relative to the principal comb modes. Whereas frequency doubling increases their relative strength by 6 dB, the growth due to self-phase modulation can be much stronger and generally increases with nonlinear propagation length. Upper limits for this effect are derived in this work. This behavior has implications for high-precision calibration of spectrographs with frequency combs used for example in astronomy. For this application, Fabry-Pérot filter cavities are used to increase the mode spacing to exceed the resolution of the spectrograph. Frequency conversion and/or spectral broadening after non-perfect filtering reamplify the suppressed modes, which can lead to calibration errors.

A 920-kilometer optical fiber link for frequency metrology at the 19th decimal place.

Optical clocks show unprecedented accuracy, surpassing that of previously available clock systems by more than one order of magnitude. Precise intercomparisons will enable a variety of experiments, including tests of fundamental quantum physics and cosmology and applications in geodesy and navigation. Well-established, satellite-based techniques for microwave dissemination are not adequate to compare optical clocks. Here, we present phase-stabilized distribution of an optical frequency over 920 kilometers of telecommunication fiber. We used two antiparallel fiber links to determine their fractional frequency instability (modified Allan deviation) to 5 × 10(-15) in a 1-second integration time, reaching 10(-18) in less than 1000 seconds. For long integration times τ, the deviation from the expected frequency value has been constrained to within 4 × 10(-19). The link may serve as part of a Europe-wide optical frequency dissemination network.

14 GHz visible supercontinuum generation: calibration sources for astronomical spectrographs.

We report the use of a specially designed tapered photonic crystal fiber to produce a broadband optical spectrum covering the visible spectral range. The pump source is a frequency doubled Yb fiber laser operating at a repetition rate of 14 GHz and emitting sub-5 pJ pulses. We experimentally determine the optimum core diameter and achieve a 235 nm broad spectrum. Numerical simulations are used to identify the underlying mechanisms and explain spectral features. The high repetition rate makes this system a promising candidate for precision calibration of astronomical spectrographs.

Injection locking of a trapped-ion phonon laser.

We report on injection locking of optically excited mechanical oscillations of a single, trapped ion. The injection locking dynamics are studied by analyzing the oscillator spectrum with a spatially selective Fourier transform technique and the oscillator phase with stroboscopic imaging. In both cases we find excellent agreement with theory inside and outside the locking range. We attain injection locking with forces as low as 5(1)×10{-24} N so this system appears promising for the detection of ultraweak oscillating forces.

A deep-UV optical frequency comb at 205 nm.

By frequency quadrupling a picosecond pulse train from a Ti:sapphire laser at 820 nm we generate a frequency comb at 205 nm with nearly bandwidth-limited pulses. The nonlinear frequency conversion is accomplished by two successive frequency doubling stages that take place in resonant cavities that are matched to the pulse repetition rate of 82 MHz. This allows for an overall efficiency of 4.5 % and produces an output power of up to 70 mW for a few minutes and 25 mW with continuous operation for hours. Such a deep UV frequency comb may be employed for direct frequency comb spectroscopy in cases where it is less efficient to convert to these short wavelengths with continuous wave lasers.

Frequency metrology on single trapped ions in the weak binding limit: the 3s(1/2)-3p(3/2) transition in 24Mg+.

We demonstrate a method for precision spectroscopy on trapped ions in the limit of unresolved motional sidebands. By sympathetic cooling of a chain of crystallized ions, we suppress adverse temperature variations induced by the spectroscopy laser that usually lead to a distorted line profile and obtain a Voigt profile with negligible distortions. We applied the method to measure the absolute frequency of the astrophysically relevant D2 transition in single 24Mg+ ions and find 1 072 082 934.33(16) MHz, a nearly 400-fold improvement over previous results. Further, we find the excited state lifetime to be 3.84(10) ns.

High harmonic frequency combs for high resolution spectroscopy.

We generated a series of harmonics in a xenon gas jet inside a cavity seeded by pulses from a Ti:sapphire mode-locked laser with a repetition rate of 10.8 MHz. Harmonics up to 19th order at 43 nm were observed with plateau harmonics at the microW power level. An elaborate dispersion compensation scheme and the use of a moderate repetition rate allowed for this significant improvement in output power of the plateau harmonics of 4 orders of magnitude over previous results. With this power level and repetition rate, high-resolution spectroscopy in the extreme ultraviolet region becomes conceivable. An interesting target would be the 1S-2S transition in hydrogenlike He+ at 60 nm.

Non-collinear high harmonic generation: a promising outcoupling method for cavity-assisted XUV generation.

We present first experimental results of our investigation of non-collinear high harmonic generation (NCHHG) with a chirped pulse amplification system. Collimated high harmonic radiation of higher than 9th order is observed along the bisector of two fundamental beams crossing in a xenon gas target. The obtained results show that cavity-assisted non-collinear high harmonic generation is a promising candidate for efficient generation and outcoupling of extreme ultraviolet (XUV) radiation.

Two-photon frequency comb spectroscopy of the 6s-8s transition in cesium.

We report a new absolute frequency measurement of the Cs 6s-8s two-photon transition measured using frequency comb spectroscopy. The fractional frequency uncertainty is 5x10(-11), a factor of 6 better than previous results. The comb is derived from a stabilized picosecond laser and referenced to an octave-spanning femtosecond frequency comb. The relative merits of picosecond-based frequency combs are discussed, and it is shown that the AC Stark shift of the transition is determined by the average rather than the much larger peak intensity.

Precision spectroscopy of hydrogen and femtosecond laser frequency combs.

Precision spectroscopy of the simple hydrogen atom has inspired dramatic advances in optical frequency metrology: femtosecond laser optical frequency comb synthesizers have revolutionized the precise measurement of optical frequencies, and they provide a reliable clock mechanism for optical atomic clocks. Precision spectroscopy of the hydrogen 1S-2S two-photon resonance has reached an accuracy of 1.4 parts in 10(14), and considerable future improvements are envisioned. Such laboratory experiments are setting new limits for possible slow variations of the fine structure constant alpha and the magnetic moment of the caesium nucleus mu(Cs) in units of the Bohr magneton mu(B).

New limits on the drift of fundamental constants from laboratory measurements.

We have remeasured the absolute 1S-2S transition frequency nu(H) in atomic hydrogen. A comparison with the result of the previous measurement performed in 1999 sets a limit of (-29+/-57) Hz for the drift of nu(H) with respect to the ground state hyperfine splitting nu(Cs) in 133Cs. Combining this result with the recently published optical transition frequency in 199Hg+ against nu(Cs) and a microwave 87Rb and 133Cs clock comparison, we deduce separate limits on alpha/alpha=(-0.9+/-2.9) x 10(-15) yr(-1) and the fractional time variation of the ratio of Rb and Cs nuclear magnetic moments mu(Rb)/mu(Cs) equal to (-0.5+/-1.7) x 10(-15) yr(-1). The latter provides information on the temporal behavior of the constant of strong interaction.

Observation of light-phase-sensitive photoemission from a metal.

We demonstrate that multiphoton-induced photoelectron emission from a gold surface caused by low-energy (unamplified) 4-fs, 750-nm laser pulses is sensitive to the timing of electric field oscillations with respect to the pulse peak. This observation confirms recent theoretical predictions and opens the door to measuring the absolute value of the carrier-envelope phase difference of few-cycle light pulses with a solid-state detector.

Attosecond control of electronic processes by intense light fields.

The amplitude and frequency of laser light can be routinely measured and controlled on a femtosecond (10(-15) s) timescale. However, in pulses comprising just a few wave cycles, the amplitude envelope and carrier frequency are not sufficient to characterize and control laser radiation, because evolution of the light field is also influenced by a shift of the carrier wave with respect to the pulse peak. This so-called carrier-envelope phase has been predicted and observed to affect strong-field phenomena, but random shot-to-shot shifts have prevented the reproducible guiding of atomic processes using the electric field of light. Here we report the generation of intense, few-cycle laser pulses with a stable carrier envelope phase that permit the triggering and steering of microscopic motion with an ultimate precision limited only by quantum mechanical uncertainty. Using these reproducible light waveforms, we create light-induced atomic currents in ionized matter; the motion of the electronic wave packets can be controlled on timescales shorter than 250 attoseconds (250 x 10(-18) s). This enables us to control the attosecond temporal structure of coherent soft X-ray emission produced by the atomic currents--these X-ray photons provide a sensitive and intuitive tool for determining the carrier-envelope phase.

Optical frequency metrology.

Extremely narrow optical resonances in cold atoms or single trapped ions can be measured with high resolution. A laser locked to such a narrow optical resonance could serve as a highly stable oscillator for an all-optical atomic clock. However, until recently there was no reliable clockwork mechanism that could count optical frequencies of hundreds of terahertz. Techniques using femtosecond-laser frequency combs, developed within the past few years, have solved this problem. The ability to count optical oscillations of more than 1015 cycles per second facilitates high-precision optical spectroscopy, and has led to the construction of an all-optical atomic clock that is expected eventually to outperform today's state-of-the-art caesium clocks.