Precision spectroscopy at ultraviolet and shorter wavelengths has been hindered by the poor access of narrow-band lasers to that spectral region. We demonstrate high-accuracy quantum interference metrology on atomic transitions with the use of an amplified train of phase-controlled pulses from a femtosecond frequency comb laser. The peak power of these pulses allows for efficient harmonic upconversion, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft x-ray spectral regions. A proof-of-principle experiment was performed on a deep-ultraviolet (2 Â 212.55 nanometers) two-photon transition in krypton; relative to measurement with single nanosecond laser pulses, the accuracy of the absolute transition frequency and isotope shifts was improved by more than an order of magnitude.In recent years, the invention of the femtosecond frequency comb laser (1-3) has brought about a revolution in metrology. A frequency comb acts as a bridge between the radio frequency (RF) domain (typically tens of MHz) and the optical frequency domain (typically hundreds of THz). Thus, in precision spectroscopy, the optical cycles of a continuous wave (CW) ultrastable laser can be phase-locked and counted directly with respect to an absolute frequency standard such as an atomic clock (4, 5). The resultant frequency measurements approach a precision of 1 part in 10 15 in certain cases, potentially enabling the detection of possible drift in the fundamental constants (6, 7), among other quantum mechanical applications.Here, we perform precision metrology without the use of a CW laser. Instead, an atomic transition is excited directly with amplified and frequency-converted pulses from a femtosecond frequency comb laser. As a result of quantum interference effects in the atomic excitation process, we can achieve an accuracy that is about six orders of magnitude higher than the optical bandwidth of the individual laser pulses.The method used is related to Ramsey_s principle of separated oscillatory fields (8), which probes the phase evolution of an atom in spatially separated interaction zones. This technique is widely used in the RF domain for atomic fountain clocks (9). By extension, in the optical domain, excitation can be performed by pulses separated in time (rather than in space) to maintain phase coherence between the excitation contributions. Several experiments have been performed to investigate Ramsey-type quantum interference fringes in the optical domain (10 -14) and phase-stable amplification of single pulses (15). Actual quantitative spectroscopy with phase-coherent oscillator pulses has been limited to a few relative frequency measurements on fine and hyperfine structure of atoms (13,14,16) and relative and absolute measurements on rubidium (17); absolute frequency measurements with amplified pulses have been frustrated by an unknown phase difference between the pulses or by limited resolution.We generate powerful laser pulses with a precise phase relationship by amplifyin...

We report a frequency metrology study on the Mg 3s 2 1 S → 3s4p 1 P transition near 202.5 nm. For this purpose, the fourth harmonic of the output from an injection-seeded Ti: sapphire pulsed laser is employed in a Mg atomic beam experiment with laser-induced fluorescence detection. Absolute frequency calibration with a frequency comb laser is performed on the cw seeding radiation, while the chirp-induced frequency shift between the pulsed output and the seed light is monitored on line. The resulting transition frequency for the main isotope 24 Mg is determined at 49 346.756 809͑35͒ m −1 . This value is three orders of magnitude more precise than the best value in the literature. The line positions of the other isotopes 25 Mg and 26 Mg are also measured at comparable accuracy, giving rise to very exact values for the isotopic shifts. The achieved precision for the transition frequency at the 7 ϫ 10 −10 level makes this second resonance line of Mg I an additional candidate for inclusion in many-multiplet methods, aimed at detecting a possible temporal variation of the fine-structure constant ␣ from comparison with quasar spectra. The isotopic shifts obtained are also important to correct for possible systematic shifts due to evolution of isotopic abundances, which may mimic ␣-variation effects.

Abstract:We demonstrate a noncollinear optical parametric chirped pulse amplifier system that produces 7.6 fs pulses with a peak power of 2 terawatt at 30 Hz repetition rate. Using an ultra-broadband Ti:Sapphire seed oscillator and grating-based stretching and compression combined with an LCD phase-shaper, we amplify a 310 nm wide spectrum with a total gain of 3×10 7 , and compress it within 5% of its Fourier limit. The total integrated parametric fluorescence is kept below 0.2%, leading to a pre-pulse contrast of 2 ×10 −8 on picosecond timescales.

PurposeTo develop and validate a robust and accurate registration pipeline for automatic contour propagation for online adaptive Intensity‐Modulated Proton Therapy (IMPT) of prostate cancer using software and deep learning.MethodsA three‐dimensional (3D) Convolutional Neural Network was trained for automatic bladder segmentation of the computed tomography (CT) scans. The automatic bladder segmentation alongside the computed tomography (CT) scan is jointly optimized to add explicit knowledge about the underlying anatomy to the registration algorithm. We included three datasets from different institutes and CT manufacturers. The first was used for training and testing the ConvNet, where the second and the third were used for evaluation of the proposed pipeline. The system performance was quantified geometrically using the dice similarity coefficient (DSC), the mean surface distance (MSD), and the 95% Hausdorff distance (HD). The propagated contours were validated clinically through generating the associated IMPT plans and compare it with the IMPT plans based on the manual delineations. Propagated contours were considered clinically acceptable if their treatment plans met the dosimetric coverage constraints on the manual contours.ResultsThe bladder segmentation network achieved a DSC of 88% and 82% on the test datasets. The proposed registration pipeline achieved a MSD of 1.29 ± 0.39, 1.48 ± 1.16, and 1.49 ± 0.44 mm for the prostate, seminal vesicles, and lymph nodes, respectively, on the second dataset and a MSD of 2.31 ± 1.92 and 1.76 ± 1.39 mm for the prostate and seminal vesicles on the third dataset. The automatically propagated contours met the dose coverage constraints in 86%, 91%, and 99% of the cases for the prostate, seminal vesicles, and lymph nodes, respectively. A Conservative Success Rate (CSR) of 80% was obtained, compared to 65% when only using intensity‐based registration.ConclusionThe proposed registration pipeline obtained highly promising results for generating treatment plans adapted to the daily anatomy. With 80% of the automatically generated treatment plans directly usable without manual correction, a substantial improvement in system robustness was reached compared to a previous approach. The proposed method therefore facilitates more precise proton therapy of prostate cancer, potentially leading to fewer treatment‐related adverse side effects.

We demonstrate that the output of a frequency comb laser can be amplified and upconverted to the vacuum ultraviolet ͑vuv͒ in a gaseous medium while its phase coherence is maintained to a high degree ͑Ͻ 1 30 of a vuv cycle͒. The produced vuv pulses are well suited to perform frequency comb spectroscopy with sub-MHz accuracy, which is experimentally verified using a Ramsey-type quantum interference scheme to excite a transition in xenon at 125 nm. The achieved resolution constitutes an improvement of six orders of magnitude compared to previous demonstrations of frequency domain quantum interference in this wavelength range. DOI: 10.1103/PhysRevA.73.061801 PACS number͑s͒: 42.62.Eh, 32.80.Qk, 39.30.ϩw, 42.65.Ky Frequency comb devices based on mode-locked lasers at infrared wavelengths have led to a dramatic progress in fields such as ultrahigh precision frequency metrology ͓1,2͔, optical clocks ͓3,4͔, and ultrafast laser science ͓5͔. For precision spectroscopy, a frequency comb laser generally acts as a phase-coherent link between the radio frequency of an atomic clock and the optical frequency of a stabilized narrow-band continuous wave ͑cw͒ laser used for the actual spectroscopy. However, many atomic transitions ͑in, e.g., helium and hydrogenlike ions͒ that are of interest for testing fundamental theories such as quantum electrodynamics, require excitation with vacuum ultraviolet ͑vuv͒ and extreme ultraviolet ͑xuv͒ radiation. Suitable narrow-band-width cw sources hardly exist at such short wavelengths ͓6͔.New measurement schemes based on the excitation of atomic transitions with phase-locked pulse sequences ͓7-11͔ may provide a solution. No cw laser is required anymore, while frequency upconversion through harmonic generation is facilitated by the high peak intensity of the pulses. Such upconversion has recently been demonstrated with unamplified frequency comb lasers, using a gas jet inside an external enhancement cavity ͓12,13͔. Radiation down to a wavelength of 60 nm at a 112 MHz repetition rate ͓12͔ was generated, albeit at relatively low power per pulse, and phase coherence was only confirmed at 266 nm through comparison with the third harmonic generated in crystals. In general, degradation of the pulse-to-pulse phase stability in the upconversion process is a potential cause for concern, due to the interaction of the required gaseous nonlinear medium with such high intensity laser pulses: the harmonic generation process itself ͓14͔ as well as competing ionization effects ͓15͔ will adversely affect the phase coherence. Previous experiments in the xuv spectral range have qualitatively shown a certain degree of temporal ͓16,17͔ and spatial coherence ͓18͔, by upconverting two replicas of a single amplified pulse and recording the resulting interference patterns on a charge-coupled device ͑CCD͒ camera. Similarly, frequency domain studies of the phase coherence have also been reported, using pulse pairs created with an interferometer ͓19,20͔ or a birefringent crystal ͓21͔. However, in all these experiments th...

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We present a frequency metrology study on the lowest rotational levels of the hydrogen EF 1 ⌺ g + ← X 1 ⌺ g + ͑0,0͒ two-photon transition near 202 nm. For this purpose, the fourth harmonic of an injection-seeded titanium:sapphire pulsed oscillator is employed in a Doppler-free REMPI-detection scheme on a molecular beam of hydrogen. A frequency comb laser is used to perform the absolute frequency calibration on the continuous-wave ͑CW͒ laser that injection-seeds the oscillator. Chirp-induced frequency differences between the output of the pulsed oscillator and the seeding light are monitored on-line, while possible systematic shifts related to the AC-Stark and Doppler effects are addressed in detail. The transition frequencies of the Q͑0͒ to Q͑2͒ lines in H 2 and D 2 , and the Q͑0͒ and Q͑1͒ lines in HD are determined with an absolute accuracy at the 10 −9 level.

Abstract:We demonstrate the generation of 9.8 ± 0.3 fs laser pulses with a peak power exceeding one terawatt at 30 Hz repetition rate, using optical parametric chirped pulse amplification. The amplifier is pumped by 140 mJ, 60 ps pulses at 532 nm, and amplifies seed pulses from a Ti:Sapphire oscillator to 23 mJ/pulse, resulting in 10.5 mJ/pulse after compression while amplified fluorescence is kept below 1%. We employ grating-based stretching and compression in combination with an LCD phase-shaper, allowing compression close to the Fourier limit of 9.3 fs. 1987-1989 (2003). 10. S. Ito, H. Ishikawa, T. Miura, K. Takasago, A. Endo, K. Torizuka, "Seven-terawatt Ti:sapphire laser system operating at 50 Hz with high beam quality for laser Compton femtosecond X-ray generation," Appl. Phys. B 76, 497-503 (2003). 11. C.P.J. Barty, T. Guo, C. Le Blanc, F. Raksi, C. Rose-Petruck, J. Squier, K.R. Wilson, V.V. Yakovlev, K. Yamakawa, "Generation of 18-fs, multiterawatt pulses by regenerative pulse shaping and chirped-pulse amplification," Opt. Lett. 21, 668-670 (1996