Adolf Giesen

Proton Structure from the Measurement of 2S-2P Transition Frequencies of Muonic Hydrogen

Proton Still Too Small Despite a protons tiny size, it is possible to measure its radius based on its charge or magnetization distributions. Traditional measurements of proton radius were based on the scattering between protons and electrons. Recently, a precision measurement of a line in the spectrum of muonium—an atom consisting of a proton and a muon, instead of an electron—revealed a radius inconsistent with that deduced from scattering studies. Antognini et al. (p. 417; see the Perspective by Margolis) examined a different spectral line of muonium, with results less dependent on theoretical analyses, yet still inconsistent with the scattering result; in fact, the discrepancy increased. A precision spectroscopic measurement of the proton radius indicates a growing discrepancy with respect to scattering results. [Also see Perspective by Margolis] Accurate knowledge of the charge and Zemach radii of the proton is essential, not only for understanding its structure but also as input for tests of bound-state quantum electrodynamics and its predictions for the energy levels of hydrogen. These radii may be extracted from the laser spectroscopy of muonic hydrogen (μp, that is, a proton orbited by a muon). We measured the 2S1/2F=0-2P3/2F=1 transition frequency in μp to be 54611.16(1.05) gigahertz (numbers in parentheses indicate one standard deviation of uncertainty) and reevaluated the 2S1/2F=1-2P3/2F=2 transition frequency, yielding 49881.35(65) gigahertz. From the measurements, we determined the Zemach radius, rZ = 1.082(37) femtometers, and the magnetic radius, rM = 0.87(6) femtometer, of the proton. We also extracted the charge radius, rE = 0.84087(39) femtometer, with an order of magnitude more precision than the 2010-CODATA value and at 7σ variance with respect to it, thus reinforcing the proton radius puzzle.

Thin-Disk Yb:YAG Oscillator-Amplifier Laser, ASE, and Effective Yb:YAG Lifetime

We report on a thin-disk Yb:YAG laser made from a Q-switched oscillator and a multipass amplifier delivering pulses of 48 mJ at 1030 nm. The peculiar requirements for this laser are the short delay time (< 500 ns) between electronic trigger and optical output pulse and the time randomness with which these triggers occur (with trigger to next trigger delay ges 1.5 ms). Details concerning the oscillator dynamics (-switching cycle, intensity stabilization), and the peculiar amplifier layout are given. Simulations of the beam propagation in the amplifier based on the Collins integral and the measured aspherical components of the disk reproduce well the measured beam intensity profiles (with higher order intensity moments) and gains. Measurements of the thermal lens and ASE effects of the disk are also presented. A novel method to deduce the effective Yb:YAG upper state lifetime (under real laser operation and including ASE effects) is presented. That knowledge is necessary to determine gain and stored energy in the active medium and to understand the limiting factors for energy scaling of thin-disk lasers.

Power-scalable system of phase-locked single-mode diode lasers

The direct use of diode lasers for high-power applications in material processing is limited to applications with relatively low beam quality and power density requirements. To achieve high beam quality one must use single-mode diode lasers, however with the drawback of relatively low optical output powers from these components. To realize a high-power system while conserving the high beam quality of the individual emitters requires coherent coupling of the emitters. Such a power-scalable system consisting of 19 slave lasers that are injection locked by one master laser has been built and investigated, with low-power diode lasers used for system demonstration. The optical power of the 19 injection-locked lasers is coupled into polarization-maintaining single-mode fibers and geometrically superimposed by a lens array and a focusing lens. The phase of each emitter is controlled by a simple electronic phase-control loop. The coherence of each slave laser is stabilized by computer control of the laser current and guarantees a stable degree of coherence of the whole system of 0.7. An enhancement factor of 13.2 in peak power density compared with that which was achievable with the incoherent superposition of the diode lasers was observed.

Thin-disk laser - Power scaling to the kW regime in fundamental mode operation

A significant reduction of the influence of the thermal lens in thin-disk lasers in high power laser operation mode could be achieved, using dynamically stable resonators. For designing the resonator, investigations of thermally induced phase distortions of thin-disks as well as numerical simulations of the field distribution in the resonator were performed. This characterization was combined with thermo-mechanical computations. On the basis of these studies, about 500 W output power with an averaged M2 = 1.55 could be demonstrated, using one disk. Almost 1 kW output power with good beam quality could be extracted, using two disks. For the purpose of further power scaling in nearly fundamental mode operation, experiments using more than two disks are in preparation.

Picosecond Regenerative Yb:YAG Thin Disk Amplifier at 200 kHz Repetition Rate and 62 W Output Power

We report on a picosecond regenerative Yb:YAG thin disk amplifier capable of delivering 62 W of average output power with repetition rates up to 200 kHz and nearly diffraction-limited beam quality.

Numerical Modeling of High Power Continuous-Wave Yb:YAG Thin Disk Lasers, Scaling to 14 kW

A numerical model of the thin disk laser, including inversion, absorption, intracavity power density, temperature and ASE is presented. It is combined with FEM analysis to compute deformation, stress and thermal lensing.

Mode Dynamics and Thermal Lens Effects of Thin Disk Lasers

In principle, the thin-disk laser concept opens the possibility to demonstrate high power, high efficiency and good beam quality, simultaneously. For this purpose, a very homogeneous pump power distribution on the disk is necessary as well as very low phase distortions of the disk itself. Spatial mode structure and thermal lens effects in an Yb:YAG thin-disk laser have been investigated as function of the pump power in linear and folded resonators. Whereas thermal lens is shown to be very weak due to the thin disk geometry, a strong correlation of the laser mode with respect to the power density distribution of the pump radiation is exhibited. The experimental results are compared with numerical simulations of the field distribution within the resonator as well as in the far field demonstrating the excellent homogeneity of the disk as laser active medium.

Status of the muonic hydrogen Lamb-shift experiment

The Lamb-shift experiment in muonic hydrogen (μ– p) aims to measure the energy difference between the atomic levels to a precision of 30 ppm. This would allow the r.m.s. proton charge radius rp to be deduced to a precision of 10–3 and open a way to check bound-state quantum electrodynamics (QED) to a level of 10–7. The poor knowledge of the proton charge radius restricts tests of bound-state QED to the precision level of about 6 × 10–6, although the experimental data themselves (Lamb-shift in hydrogen) have reached a precision of  × 10–6. Values for rp not depending on bound-state QED results from electron scattering experiments have a surprisingly large uncertainty of 2%. In our Lamb-shift experiment, low-energy negative muons are stopped in low-density hydrogen gas, where, following the μ– atomic capture and cascade, 1% of the muonic hydrogen atoms form the metastable 2S state with a lifetime of about 1 μs. A laser pulse at λ ≈ 6 μm is used to drive the 2S → 2P transition. Following the laser excitation...

The 2S Lamb shift in muonic hydrogen and the proton rms charge radius

The determination of the proton rms charge radius with an accuracy of 10−3 is the main goal of our experiment, opening the way to check bound‐state QED predictions in hydrogen to a level of 10−7. The principle is to measure the 2S1/2(F = 1) − 2P3/2(F = 2) energy difference in muonic hydrogen (μ−p) by infrared laser spectroscopy to a precision of 30 ppm. Very low‐energy negative muons are stopped in 0.6 mbar of hydrogen gas, where, following the μ− atomic capture and cascade, 1% of the muonic hydrogen atoms form the metastable 2S state with a lifetime of 1.3 μs. A 6 μm laser pulse is used to drive the 2S → 2P transition. When on resonance, the laser induces the transition, and the subsequent muonic deexcitation to the 1S state emits a 1.9 keV x ray which is detected by avalanche photodiodes. The resonance frequency, and hence the Lamb shift and the proton charge radius, is determined by measuring the rate of laser‐induced x rays as a function of the laser wavelength. Some details of the experiment, recent ...

Cr:ZnSe thin disk cw laser

A Thulium fiber laser pumped or InP diode laser stack pumped Cr:ZnSe thin disk cw multimode laser at 2.4 μm with an output power of 5 and 4 W, respectively, and with optical-tooptical efficiencies of 10% will be presented. An experimentally verified and numerically simulated thermal lensing induced and cyclic instability in the laser system will be shown. As a consequence, in order to prevent the lasing conditions in the resonator to be unstable, power scaling of a Cr:ZnSe thin disk laser is possible by enlarging the pump spot and reducing thereby the thermal lensing condition. Therefore, the instability is not initiated. As a conclusion, the investigated instability will show up in any laser active material which has a strong absorption of the pump beam, for instance in transition metal ion laser material systems in connection with any laser concept, like for instance in thin disk, bulk or slab designs.