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The CREMA collaboration is pursuing a measurement of the ground-state hyperfine splitting (HFS) in muonic hydrogen ($\mu$p) with 1 ppm accuracy by means of pulsed laser spectroscopy to determine the two-photon-exchange contribution with $2\times10^{-4}$ relative accuracy. In the proposed experiment, the $\mu$p atom undergoes a laser excitation from the singlet hyperfine state to the triplet hyperfine state, {then} is quenched back to the singlet state by an inelastic collision with a H$_2$ molecule. The resulting increase of kinetic energy after the collisional deexcitation is used as a signature of a successful laser transition between hyperfine states. In this paper, we calculate the combined probability that a $\mu$p atom initially in the singlet hyperfine state undergoes a laser excitation to the triplet state followed by a collisional-induced deexcitation back to the singlet state. This combined probability has been computed using the optical Bloch equations including the inelastic and elastic collisions. Omitting the decoherence effects caused by {the laser bandwidth and }collisions would overestimate the transition probability by more than a factor of two in the experimental conditions. Moreover, we also account for Doppler effects and provide the matrix element, the saturation fluence, the elastic and inelastic collision rates for the singlet and triplet states, and the resonance linewidth. This calculation thus quantifies one of the key unknowns of the HFS experiment, leading to a precise definition of the requirements for the laser system and to an optimization of the hydrogen gas target where $\mu$p is formed and the laser spectroscopy will occur., Comment: 21 pages, 4 figures

Precision spectroscopy of light muonic atoms provides unique information about the atomic and nuclear structure of these systems and thus represents a way to access fundamental interactions, properties and constants. One application comprises the determination of absolute nuclear charge radii with unprecedented accuracy from measurements of the 2S - 2P Lamb shift. Here, we review recent results of nuclear charge radii extracted from muonic hydrogen and helium spectroscopy and present experiment proposals to access light muonic atoms with Z ≥ 3. In addition, our approaches towards a precise measurement of the Zemach radii in muonic hydrogen (μp) and helium (μ 3He+) are discussed. These results will provide new tests of bound-state quantum-electrodynamics in hydrogen-like systems and can be used as benchmarks for nuclear structure theories., Journal of Physics: Conference Series, 1138 (1), ISSN:1742-6588, ISSN:1742-6596

Recent development of thin-disk multipass amplifiers has resulted in several breakthroughs in average power and pulse energy of both ps- and fs-amplifiers [1-3]. These results confirm the suitability of this laser architecture for amplification of ultrafast pulses with high energy and at high average powers thanks to low nonlinearities in the thin-disk and to an efficient cooling scheme. However, as peak powers are reaching or exceeding the GW, even nonlinearities induced by propagation in air become relevant in terms of spectral broadening due to self-phase modulation (SPM).

We review the status of the proton charge radius puzzle. Emphasis is given to the various experiments initiated to resolve the conflict between the muonic hydrogen results and the results from scattering and regular hydrogen spectroscopy., EPJ Web of Conferences, 113, ISSN:2100-014X, ISSN:2101-6275

Modal content in a 50 µm core diameter photonic bandgap Bragg fiber is studied with respect to the launching condition at a wavelength around 1860 nm by spatially and spectrally resolved imaging.

We report on what is, to the best of our knowledge, the highest power reached with an Yb:Lu2O3 thin-disk laser in the continuous wave (CW) regime reported to date. A maximum output power of 670 W was demonstrated with a V-shaped resonator design. This result was achieved by pumping this laser active material at its zero-phonon absorption line at 976 nm. The potential of Yb:Lu2O3 as well as its limitations are discussed on the basis of several experimental investigations including thermal analysis and laser performance characterization for different doping concentrations.

We present a new cavity design for controlling the spatial quality of the beam delivered by a fibre laser including a multicore fibre. A quasi Gaussian profile is produced at the 7-core fibre output involving self-imaging effects in the multicore fibre and spatial filtering. Numerical and experimental investigations analyse the spatial and spectral characteristics of the multicore fibre laser.

The performance of a 665-nm GaInP disk laser operated continuous-wave at 15°C both in-well-pumped at 640 nm and barrier pumped at 532 nm is reported. The efficiency with respect to the absorbed power was enhanced by 3.5 times when using a 640-nm pump instead of a 532-nm pump. In-well pumping which is based on the absorption of the pump photons within the quantum-well heterostructures of the gain region instead of short-wavelength absorption in the barrier and spacer regions reduces the quantum defect between pump and laser photon and hence the heat generation. A slope efficiency of 60% with respect to the absorbed pump power was obtained by in-well pumping at 15°C. Continuous-wave laser operation was further demonstrated at heat sink temperatures of up to 55°C. Both the measurement of photoluminescence and COMSOL simulation show that the overall heat load in the in-well pumped laser is smaller than in the barrier-pumped laser. These results demonstrate the potential of optical in-well pumping for the operation of red AlGaInP disk lasers if combined with means for efficient pump-light absorption.

We report for the first time, to the best of our knowledge, an innovative design concept for intracavity pulse stretching in a regenerative amplifier, employing a single “grating-mirror” based on a leaky-mode grating-waveguide design. The very compact and flexible layout allows for femtosecond pulses to be in principle easily stretched up to nanosecond durations. The design has been tested in a diode-pumped Yb:CALGO regenerative amplifier followed by a standard transmission grating compressor. Sub-200-fs pulses (stretched pulses ≈110 ps) with 205-μJ energy at 20-kHz repetition rate have been demonstrated. In order to prove the robustness and potential for energy scaling of leaky-mode grating-waveguide intracavity stretcher, we generated stretched pulses with energies of up to ≈700 μJ (400-ps long) at a lower repetition rate of 10 kHz. A simple model is proposed for the study of the cavity in presence of induced spatial chirp.

The fundamental parameters of a laser beam, such as the exact position and size of the focus or the beam quality factor M² yield vital information both for laser developers and end-users. However, each of these parameters can significantly change on a short time scale due to thermally induced effects in the processing optics or in the laser source itself, leading to process instabilities and non-reproducible results. In order to monitor the transient behavior of these effects, we have developed a camera-based measurement system, which enables full laser beam characterization in online. A novel monolithic beam splitter has been designed which generates a 2D array of images on a single camera chip, each of which corresponds to an intensity cross section of the beam along the propagation axis separated by a well-defined spacing. Thus, using the full area of the camera chip, a large number of measurement planes is achieved, leading to a measurement range sufficient for a full beam characterization conforming to ISO 11146 for a broad range of beam parameters of the incoming beam. The exact beam diameters in each plane are derived by calculation of the 2nd order intensity moments of the individual intensity slices. The processing time needed to carry out both the background filtering and the image processing operations for the full analysis of a single camera image is in the range of a few milliseconds. Hence, the measurement frequency of our system is mainly limited by the frame-rate of the camera.

Laser processing of carbon fiber reinforce plastic (CFRP) is a very promising method to solve a lot of the challenges for large-volume production of lightweight constructions in automotive and airplane industries. However, the laser process is actual limited by two main issues. First the quality might be reduced due to thermal damage and second the high process energy needed for sublimation of the carbon fibers requires laser sources with high average power for productive processing. To achieve thermal damage of the CFRP of less than 10μm intensities above 10 8 W/cm² are needed. To reach these high intensities in the processing area ultra-short pulse laser systems are favored. Unfortunately the average power of commercially available laser systems is up to now in the range of several tens to a few hundred Watt. To sublimate the carbon fibers a large volume specific enthalpy of 85 J/mm³ is necessary. This means for example that cutting of 2 mm thick material with a kerf width of 0.2 mm with industry-typical 100 mm/sec requires several kilowatts of average power. At the IFSW a thin-disk multipass amplifier yielding a maximum average output power of 1100 W (300 kHz, 8 ps, 3.7 mJ) allowed for the first time to process CFRP at this average power and pulse energy level with picosecond pulse duration. With this unique laser system cutting of CFRP with a thickness of 2 mm an effective average cutting speed of 150 mm/sec with a thermal damage below 10μm was demonstrated.

We demonstrated for the first time, to the best of our knowledge a new intracavity pulse stretching design, employing a single grating-mirror based on a leaky-mode grating-waveguide design. The extremely compact and flexible layout allows for femtosecond pulses to be easily stretched up to nanosecond durations. The stretcher was implemented in a diode-pumped Yb:CALGO regenerative amplifier followed by a standard transmission grating compressor. We demonstrated sub-200 fs long pulses (stretched pulses ≈ 110 ps) with a maximum energy of 205 μJ at 20 kHz repetition rate. As a proof of the robustness and potential energy scaling of leaky-mode grating-waveguide intracavity stretcher, energies up to 700 μJ and 400 ps long pulses before compression at a lower repetition rate of 10 kHz, have been achieved. A simple model is proposed to investigate the cavity behavior in presence of induced spatial chirp.

Summary form only given. Novel amplifier systems and laser oscillators generating short pulses and high pulse energies are required in numerous applications, not least in material processing. Currently there are significant efforts to develop such systems [1]. Among other Ytterbium-doped laser gain materials, Yb:CaF 2 is an interesting candidate for highpower cw and passively mode-locked operation for the generation of short pulses because it combines a very broad emission spectrum with a high undoped thermal conductivity (~ 9.7 W/(mK)) [2]. The proof-of principle in ultrafast laser operation was already shown with a bulk crystal by demonstrating pulses shorter than 100 fs in a SESAM passively mode-locked laser system [3]. In the present contribution we report on the first investigation of a Yb:CaF 2 thin-disk laser as this concept is ideally suited for power scaling of ultrafast lasers [4]. For the experimental investigation, two crystals from the same boule with different thicknesses were compared. The nominal doping concentration was specified by the supplier to be around 4 %. The first and non-wedged crystal has a thickness of 200 μm whereas that of the second and wedged (~0.1 °) crystal is 250μm. The thin-disk crystals were glued on copper heat sinks and water cooled from the backside with nominal operating temperatures between 15 and 18 °C. A thin-disk pumping module with a total of 24 pump light passes through the laser crystal was used for sufficient absorption of the pump radiation. The crystals were pumped either with a fiber-coupled laser diode with up to 40 W of power centered at a wavelength of 976 nm with a spectral bandwidth of about 10 nm or at high-powers or, with a 1.2 kW pump source exhibiting a spectral bandwidth of 4 nm centered at a wavelength of 976 nm. In this latter case, only one half of the pump power was used to avoid the damage of the crystal.Figure 1 shows the thermal behavior of the crystal which was examined using a thermal-imaging camera to measure the temperature at its front surface in fluorescence and laser operation. It is recognizable that the temperature at the front surface of the disk in fluorescence operation is significantly lower than in laser operation because of the smaller quantum defect of the shorter centre wavelength of the fluorescence emission. The fact that the temperature is not exceeding 100 °C at power densities of around 8 kW/cm2 represents a relevant and extremely promising information for the further development of high-power Yb:CaF2 thin-disk lasers. After this confirmation, high-power laser operation was demonstrated in a multimode resonator. As shown in Figure 2 a maximum output power of 250 W with an optical efficiency of 47 % was obtained with the 200 μm thick laser crystal. In a further experiment the wedged 250 μm thick crystal was characterized in the perspectives of modelocked operation. The pump spot was set to 1 mm. In a fundamental mode V-shaped resonator (M2

In this contribution, the authors present, the first laser experiments ever demonstrated with Yb:Gd2O3 and Yb:Y2O3 crystals grown by this new flux method. The demonstrated the first diode-pumped Yb:Gd2O3 laser. Furthermore the new and advantageous flux method allows for the growth of highly Yb-doped sesquioxides which is very promising for the elaboration of cubic Re2O3 dedicated to thin-disk laser applications.

In this contribution, what is believed to be the first demonstration of an Yb:YAl3(BO3)4 (Yb:YAB) thin-disk laser is reported. Due to its broad gain bandwidth of >40 nm (σ- polarization), Yb:YAB offers the potential for generating ultrafast laser pulses with sub-100-fs durations; the thin-disk geometry should allow to scale the average power of such pulses beyond the 100 W range. For these experiments, Yb:YAB boule material with a doping concentration of 12 % was polished to a disk with a diameter of 6.3 mm and a thickness of 250 μm. The thin-disk crystal was glued with its HR coated backside on a copper heat sink. Although the Yb YAB crystal was damaged during the mounting process, an usable optical aperture of the disk of up to 3 mm remained. For this reason pump spot diameters of 2.3 and 2.6 mm were selected for the high power laser experiments. A fiber-coupled laser diode high-power laser operation at a central wavelength of 976 nm emitting up to 1 kW of power was utilized as the pump source. In order to guarantee sufficient pump light absorption, a pump module allowing 24 passes of the pump radiation through the thin-disk was chosen. During laser operation the thin-disk crystal was efficiently cooled by an impingement cooling operated at a water temperature of 15 °C. Prior to the high power experiments a thermal characterization of this thin-disk crystal with a thermal-imaging camera was performed. The temperature of the front side of the crystal was measured both during fluorescence and laser operation. For a pump power density of 2.7 kW/cm2 the maximum surface temperatures was determined to be 71 °C in fluorescence and 56 °C in laser operation. These values confirmed the good suitability of Yb YAB for high-power laser operation in thin-disk configuration.

The present contribution reports on the modelling, fabrication, as well as the intra-cavity characterization of fully dielectric grating-mirrors for Yb:YAG thin-disk lasers. By integrating a low-index leaky-mode waveguide and a sub-wavelength diffraction grating onto an HR mirror, such devices achieve high diffraction efficiencies, in our case >99.8% in the - 1st order. Used as simple one-to-one replacement for laser cavity end mirrors, they allow a highly efficient generation of beams with very narrow spectral bandwidth and high-polarization purity. Since the grating-waveguide-mirrors (GWM) are used in Littrow configuration, they also permit to tune the wavelength of the emitted laser beam. Therefore they can replace a combination of etalons, birefringent filters, Brewster plates and/or thin-film polarizers usually employed for these tasks.

The development of the thin-disk laser [1] marks a major improvement in terms of power scaling at good beam quality when compared to classical solid-state laser concepts. The highly efficient cooling scheme leads to a significant reduction of thermally induced wave front distortions. The reduced spherical component of the distortions, often referred to as “thermal lensing” [2], enables the realization of brilliant lasers with a broad dynamic stability range. When it comes to fundamental-mode operation, however, the aspherical components of the wave-front distortions introduced by the strong temperature gradient at the edge of the pump spot in the laser crystal lead to diffraction losses and therefore severely limit the power scalability. As stable fundamental-mode operation is an important prerequisite for the efficient generation of ultra-short pulses by mode-locking, these losses are also limiting the scalability of the average power of thin-disk based femtosecond oscillators. This limitation can be overcome by means of intra-cavity deformable mirrors which are used to compensate for the aspherical components of the wavefront distortions [3].

We present Yb:SSO as promising material for femtosecond thin-disk oscillators confirmed by 80 nm of tuning range, 280 W of multimode cw output, and 298 fs mode-locked pulses at 27.8 W of average output power.

We present first experimental investigations on Ytterbium-doped Sc 2 SiO 5 as a promising gain medium for thin-disk lasers. This oxyorthosilicate laser material combines good thermo-mechanical properties and a broad emission bandwidth suitable for high-power cw and mode-locked laser operation. The demonstrated average output powers of up to 75 W with a not yet optimized thin-disk crystal confirm the power capability of this new material and preliminary tests on passive mode-locking indicate the high potential for future ultrafast thin-disk laser oscillators.

International audience; An output coupler comprising a resonant grating submirror monolithically associated with a standard multilayer submirror polarizes the emission of a Nd:YAG microchip laser linearly over its full emission bandwidth by intra-mirror destructive interference for the undesired polarization. A polarization extinction ratio of more than 25 dB is obtained up to 6.1µJ pulse energy. This passively Q-switched laser performance is almost identical to that of a gratingless non-polarized microchip laser. The design and fabrication of the resonant grating mirror are described.

Laser sources with tuneable or stabilized emission wavelength and/or reduced spectral bandwidth have many applications in various fields, like second-harmonic generation [1], spectroscopy, wavelength multiplexing etc. The required wavelength control can be accomplished by several means, e.g. by introducing wavelength-selective elements such as prisms, gratings, etalons, birefringent filters, etc. into the laser resonator. Some of these elements (e.g. conventional gratings and multi-plate birefringent filters) introduce significant additional losses to the resonator. This limits their application to high-power lasers, especially to thin-disk lasers which are, due to the low gain of the disk, very sensitive to intra-cavity losses. Most of these elements (e.g. prisms, birefringent filters, and etalons) are used in transmission, which can lead to a reduced dynamic stability and to beam quality deteriorations due to thermal lensing effects which occur under high power levels.

Starting from a standard single-mode fiber with an effective mode area (A eff ) in the order of 30 µm2, the most obvious way to increase the power-handling capability is to increase the core diameter - and therefore the effective mode area - since the threshold of most nonlinear effects [1] is proportional to A eff . This approach led to the so-called large-mode area (LMA) fibers with an A eff of about 150 – 220 µm2. To preserve the single-mode guiding property, the numerical aperture (NA) of these fibers has to be low, typically between 0.05 and 0.07. Besides the difficulties of the production of very low NA fibers, further increasing the core diameter will result in an excessive bending sensitivity [2].

The multi-core fiber laser is a promising fiber laser concept since it combines good beam quality and a high mode field diameter to reduce nonlinear effects especially for pulsed laser operation. Therefore this concept is a good candidate for high power fiber laser operation with a good beam quality. In the present paper we report on the characterization and the laser operation of a fiber laser with a hexagonal array of 19-cores. The near and the far field intensity distribution of the emitted beam as well as the bending-induced transversal mode selection have been investigated. The obtained experimental and simulation results and show a good agreement.

International audience; A resonant diffraction grating comprising a mirror, a dielectric layer and a high index corrugation at the layer-air interface is shown to exhibit off-Littrow the record diffraction efficiency of 99% in the -1 st reflected order at 1064 nm wavelength thanks to the excitation of a leaky mode of the layer. Such high figure is obtained by a grating 5 to 10 times shallower than in current attempts to realize high efficiency all-dielectric gratings.