The remarkable precision of frequency-comb (FC) lasers is transferred to the extreme ultraviolet (XUV, wavelengths shorter than 100 nm), a frequency region previously not accessible to these devices. A frequency comb at XUV wavelengths near 51 nm is generated by amplification and coherent upconversion of a pair of pulses originating from a near-infrared femtosecond FC laser. The phase coherence of the source in the XUV is demonstrated using helium atoms as a ruler and phase detector. Signals in the form of stable Ramsey-like fringes with high contrast are observed when the FC laser is scanned over P states of helium, from which the absolute transition frequency in the XUV can be extracted. This procedure yields a 4 He ionization energy at h  5 945 204 212ð6Þ MHz, improved by nearly an order of magnitude in accuracy, thus challenging QED calculations of this two-electron system. Mode-locked frequency-comb (FC) lasers [1,2] have revolutionized the field of precision laser spectroscopy. Optical atomic clocks using frequency combs are about to redefine the fundamental standard of frequency and time [3]. FC lasers have also vastly contributed to attosecond science by providing a way to synthesize electric fields at optical frequencies [4], made long distance absolute length measurements possible [5], and have recently been employed to produce ultracold molecules [6]. FC based precision spectroscopy on simple atomic systems has provided one of the most stringent tests of bound state quantum electrodynamics (QED) as well as upper bounds on the drift of fundamental constants [7]. Extending these methods into the extreme ultraviolet (XUV, wavelengths below 100 nm) spectral region is highly desirable since this would, for example, allow novel precision QED tests [8].Currently the wavelength range below 120 nm is essentially inaccessible to precision frequency metrology applications due to a lack of power of single frequency lasers and media for frequency up-conversion. Spectroscopic studies on neutral helium using amplified nanosecond laser pulses [9,10] are notoriously plagued by frequency chirping during amplification and harmonic conversion which limits the accuracy. These kind of transient effects can be avoided if a continuous train of high power laser pulses (produced by a FC) can be coherently up-converted. This would transfer the FC modes, at frequencies f n ¼ f CEO þ nf rep , where f CEO is the carrier-envelope offset frequency, f rep is the repetition frequency of the pulses, and n an integer mode number, to the XUV. Similar to what was shown in the visible [11,12], the up-converted pulse train could be used to directly excite a transition, with each of the up-converted modes acting like a single frequency laser.By amplification of a few pulses from the train, and producing low harmonics in crystals and gasses, sufficient coherence has been demonstrated down to 125 nm to perform spectroscopic experiments [13,14]. To reach wavelengths below 120 nm in the extreme ultraviolet or even x rays, high harmonic generati...

The operation of a frequency comb at extreme ultraviolet (xuv) wavelengths based on pairwise amplification and nonlinear upconversion to the 15th harmonic of pulses from a frequency-comb laser in the near-infrared range is reported. It is experimentally demonstrated that the resulting spectrum at 51 nm is fully phase coherent and can be applied to precision metrology. The pulses are used in a scheme of direct-frequency-comb excitation of helium atoms from the ground state to the 1s4p and 1s5p 1 P 1 states. Laser ionization by auxiliary 1064 nm pulses is used to detect the excited-state population, resulting in a cosine-like signal as a function of the repetition rate of the frequency comb with a modulation contrast of up to 55%. Analysis of the visibility of this comb structure, thereby using the helium atom as a precision phase ruler, yields an estimated timing jitter between the two upconverted-comb laser pulses of 50 attoseconds, which is equivalent to a phase jitter of 0.38 (6) cycles in the xuv at 51 nm. This sets a quantitative figure of merit for the operation of the xuv comb and indicates that extension to even shorter wavelengths should be feasible. The helium metrology investigation results in transition frequencies of 5 740 806 993 (10) and 5 814 248 672 (6) MHz for excitation of the 1s4p and 1s5p 1 P 1 states, respectively. This constitutes an important frequency measurement in the xuv, attaining high accuracy in this windowless part of the electromagnetic spectrum. From the measured transition frequencies an eight-fold-improved 4 He ionization energy of 5 945 204 212 (6) MHz is derived. Also, a new value for the 4 He ground-state Lamb shift is found of 41 247 (6) MHz. This experimental value is in agreement with recent theoretical calculations up to order mα 6 and m 2 /Mα 5 , but with a six-times-higher precision, therewith providing a stringent test of quantum electrodynamics in bound two-electron systems.

The phase stability of broadband (280 nm bandwidth) terawatt-class parametric amplification was measured, for the first time to our knowledge, with a combination of spatial and spectral interferometry. Measurements at four different wavelengths from 750 to 900 nm were performed in combination with numerical modeling. The phase stability is better than 1/23 rms of an optical cycle for all the measured wavelengths, depending on the phase-matching conditions in the amplifier. © 2007 Optical Society of America OCIS codes: 190.4970, 320.7090, 320.7160, 350.5030. The generation and amplification of phase-controlled few-cycle laser pulses is a necessity for applications such as quantum interference metrology [1], attosecond science [2], and quantum control of, e.g., molecular dynamics [3]. Intense, phase-stable few-cycle laser pulses have been produced by using Ti:sapphire amplifiers and subsequent spectral broadening in filaments. However, filamentation in gas-filled hollow fibers [4], or directly in a gas cell [5], is difficult to scale beyond Ϸ0.2 TW. In parametric amplification phase-stable pulses, albeit at moderate energies of a few hundred microjoules [6][7][8][9], have also been demonstrated. The generation of multimillijoule-level phase-controlled few-cycle pulses with terawatt (TW) intensity has not been demonstrated to date. In this Letter we report what is, to the best of our knowledge, the first measurement of the phase stability of TW-class ultrafast amplification. The amplifier is based on noncollinear optical parametric chirped pulse amplification (NOPCPA) and was described elsewhere in detail [10]. It consists of a double-pass preamplifier and a single-pass power amplifier using BBO crystals. The seed laser is a home-built 6.2 fs frequency comb oscillator, producing phase-locked 5.5 nJ pulses at a 75 MHz repetition rate. The carrier-envelope phase (CEP) stability of the oscillator is 1 / 46 rms of an optical cycle. The 532 nm pump laser provides 170 mJ pulses with a duration of 60 ps and is synchronized to the oscillator laser. The system operates at a repetition rate of 30 Hz and is capable of generating 7.6 fs pulses at 2 TW (15.5 mJ after compression) when the normal full seed energy of 1 nJ per pulse is available.The phase stability of the NOPCPA output is measured with linear interferometry. The advantage of this method over the frequently used f :2f technique [11] is that pulse intensity fluctuations (typically a few percent) do not influence the measurement; also the wavelength dependence can be measured. The system is based on a double interferometer, to be able to correct for optical path fluctuations due to external noise and drift (see Fig. 1). The interferometer path length variations, of the order of a wavelength, are too small to influence the CEP. Changes induced by thermal effects due to the 20 mm of optical material in the NOPCPA path are small and are in addition compensated by a similar amount of material in the reference arm. The interferometer compares interference [12] between...

Abstract:We demonstrate phase stable, mJ-level parametric amplification of pulse pairs originating from a Ti:Sapphire frequency comb laser. The amplifier-induced phase shift between the pulses has been determined interferometrically with an accuracy of ≈ 10 mrad. Typical phase shifts are on the order of 50-200 mrad, depending on the operating conditions. The measured phase-relation can be as stable as 20 mrad rms (1/300 th of an optical cycle). This makes the system suitable for Ramsey spectroscopy at short wavelengths by employing harmonic upconversion of the doublepulses in nonlinear media.

Abstract:We demonstrate phase stable, mJ-level parametric amplification of pulse pairs originating from a Ti:Sapphire frequency comb laser. The amplifier-induced phase shift between the pulses has been determined interferometrically with an accuracy of ≈ 10 mrad. Typical phase shifts are on the order of 50-200 mrad, depending on the operating conditions. The measured phase-relation can be as stable as 20 mrad rms (1/300 th of an optical cycle). This makes the system suitable for Ramsey spectroscopy at short wavelengths by employing harmonic upconversion of the doublepulses in nonlinear media.