Browsing by Author "Abdou Ahmed, Marwan"

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Open Access
Analysis of laser-induced damage threshold of circular grating waveguide structures exposed to sub-picosecond laser radiation centered at a wavelength of 1030 nm
(2025) Didychenko, Denys; Li, Fangfang; Savchenko, Anton; Zideluns, Janis; Stehlik, Marek; Gallais, Laurent; Lumeau, Julien; Lemarchand, Fabien; Karvinen, Petri; Kuittinen, Markku; Pruss, Christof; Jamon, Damien; Graf, Thomas; Abdou Ahmed, Marwan
The laser-induced damage threshold of a grating waveguide output coupler (GWOC) exposed to laser radiation at a wavelength of 1030 nm and with a pulse duration of 500 fs was investigated. The GWOC is a combination of a sub-wavelength circular grating and a partial reflector based on a Nb2O5 and SiO2 multilayer sequence. It was designed to be used as an output coupler of a thin-disk laser cavity for the generation of beams with radial polarization. The results revealed a laser-induced damage threshold (LIDT) fluence of 0.36 J/cm² for single-pulse tests and 0.26 J/cm² for multiple-pulse conditions with up to 1000 shots. These threshold values are comparable to those of an unstructured output coupler with Nb2O5 and SiO2 coating layers, highlighting the minor influence of the grating on the LIDT.
Open Access
Bending of Lloyd’s mirror to eliminate the period chirp in the fabrication of diffraction gratings
(2024) Bienert, Florian; Röcker, Christoph; Graf, Thomas; Abdou Ahmed, Marwan
Open Access
Design of convex-shaped transparent heat spreaders for symmetrical cooling of a thin-disk laser crystal by mechanical pressing
(2024) Wolter, Jan-Hinnerk; Voss, Andreas; Graf, Thomas; Abdou Ahmed, Marwan
We present the design for a double-sided cooling of a thin-disk laser crystal using transparent, convex-shaped heat spreaders. The thermal contact between laser crystal and heat spreaders is established without bonding by solely applying an axial compressive force. A model of the contact behavior taking into account the deformation of the surfaces of the heat spreaders due to bending under mechanical loading is presented. The scope and accuracy of the modeling is verified by numerical simulations using the finite-element method. Based on the modeling, an exemplary design of the heat spreader which has been used for the first experimental demonstration of this concept is carried out. Based on the model, an almost ideal scalability of the heat transfer coefficient is predicted.
Open Access
Design, fabrication, and characterization of dual-wavelength inhibited-coupling guiding hollow-core fibers
(2025) Chen, Bowen; Kühlthau, Tim; Kleem, Götz; Graf, Thomas; Abdou Ahmed, Marwan
The present paper reports on the design, fabrication, and characterization of an 8-tube inhibited-coupling guiding hollow-core photonic crystal fiber (IC-HCPCF) capable of guiding both the beam emitted from an Yb:YAG laser at the fundamental wavelength of λ=1030nmand its second harmonic at λ=515nm. By controlling the strut thickness of the glass capillaries to approximately 362nm, the transmission of laser radiation at both wavelengths was possible with low losses. Optimizing the outer diameter of the glass capillaries mitigates the bending-induced increase of the confinement loss at the wavelength of 515nmwithout compromising the optical performance of the fiber at the wavelength of 1030nm. Experimental results confirm the near to diffraction-limited beam quality M2<1.15of the laser beams exiting the fiber at both operational wavelengths. Operating in the first transmission band at the wavelength of 1030nm, the calculated chromatic dispersion is 1.02ps/(nm∙km), despite a diameter of the hollow core of 40μm. At the wavelength of 515nmthis value amounts to 0.62ps/(nm∙km). The measured losses are 27.5±0.3dB/kmat the wavelength of 515nmand 25.7±0.7dB/kmat the wavelength of 1030nm, which is comparable to the loss of state-of-the-art IC-HCPCFs with tubular cladding structures. The measured bending-induced increase of the confinement losses confirms the potential of the proposed approach for flexible, low-loss guiding of ultrashort laser pulses at the two wavelengths using a single fiber. This gained flexibility can significantly enhance the options for wavelength selection in laser material processing applications.
Open Access
Detrimental effects of period-chirped gratings in pulse compressors
(2023) Bienert, Florian; Röcker, Christoph; Dietrich, Tom; Graf, Thomas; Abdou Ahmed, Marwan
Open Access
Efficient delivery of sub-ps laser pulses with pulse energies exceeding 100 µJ at average powers of up to 95 W through a hollow-core fiber
(2025) Kühlthau, Tim; Chen, Bowen; Graf, Thomas; Abdou Ahmed, Marwan
We report on the efficient delivery of sub-picosecond laser pulses with high peak and high average powers through a home-made 7.5 m long tubular inhibited-coupling guiding hollow-core photonic-crystal fiber (IC-HCPCF). The experiments were performed using an ultrafast laser generating pulses with durations between 430 fs and 560 fs at a central wavelength of 1030 nm. Pulses with an energy of either 263 µJ or 150 µJ were coupled into the fiber at average powers of either 48 W or 95 W, respectively. In both cases, the transmittance through the fiber was measured to exceed 85% without detectable distortion of the temporal pulse shapes or damage to the fiber.
Open Access
First thin-disk oscillator with ceramic Yb:LuScO3 in comparison to the operation with ceramic Yb:Lu2O3
(2024) Esser, Stefan; Jing, Wei; Xu, Xiaodong; Graf, Thomas; Abdou Ahmed, Marwan
We report on the characterization and first laser operation of ceramic Yb:LuScO3 in a thin-disk oscillator. The optical performance achieved with a ceramic Yb:LuScO3 disk is compared to the one obtained with an existing ceramic Yb:Lu2O3 disk for reference. The characterization covers the measurement of the fluorescence spectra, the fluorescence lifetimes, and nomarsky imaging. The investigation on the laser operation covers the measurement of resonator losses, output powers, and thermal behavior during continuous-wave operation in a multimode thin-disk oscillator. An average output power of 149 W and a slope efficiency of 51.8% were achieved with the ceramic Yb:LuScO3 disk which reached a maximum surface temperature of about 150 °C. At the same temperature level, a disk made of the already established ceramic Yb:Lu2O3 delivered 957 W of output power with a slope efficiency of 75.7%.
Open Access
General mathematical model for the period chirp in interference lithography
(2023) Bienert, Florian; Graf, Thomas; Abdou Ahmed, Marwan
Open Access
Generation of a radially polarized beam in a polycrystalline ceramic Yb:Lu2O3 thin-disk laser
(2023) Didychenko, Denys; Esser, Stefan; Beirow, Frieder; Savchenko, Anton; Pruss, Christof; Graf, Thomas; Abdou Ahmed, Marwan
AbstractWe report on the generation of a continuous-wave (CW) radially polarized beam with an Yb:Lu2O3 polycrystalline ceramic disk in a thin-disk laser (TDL) oscillator. A circular grating-waveguide mirror (CGWM) with a high polarization discrimination given by a reflectivity difference between the two orthogonal polarization states of 44.6% was used as a polarization-selective cavity end-mirror. An output power of 175 W was achieved with an optical efficiency of 39.6%. A high degree of radial polarization of 96.2% and a beam propagation factor of M2hor. = 2.05 and M2ver. = 2.32 were measured at the maximum output power.
Open Access
High-power thin-disk lasers emitting beams with axially-symmetric polarizations
(2021) Abdou Ahmed, Marwan; Beirow, Frieder; Loescher, André; Dietrich, Tom; Bashir, Danish; Didychenko, Denys; Savchenko, Anton; Pruss, Christof; Fetisova, Marina; Li, Fangfang; Karvinen, Petri; Kuittinen, Markku; Graf, Thomas
We present the intracavity generation of beams with radial polarization at an average output power of 750 W and an optical efficiency of 43% from a continuous wave thin-disk laser. Circular grating waveguide output couplers (GWOC) were used to select the radial polarization. The sensitivity of the polarizing function of the GWOC with regards to the fabrication tolerances is also analysed in details with a particular emphasis on the effect of the duty cycle and the geometrical profile of the gratings. Furthermore, we present the conversion of femtosecond laser pulses from linear to azimuthal polarization using a nanograting-based polarization converter. Azimuthally polarized beams with an average power of up to 850 W, a pulse duration of 400 fs and a pulse repetition rate of 1 MHz were generated in this way with a conversion efficiency of >90%.
Open Access
High-power, high-brightness solid-state laser architectures and their characteristics
(2022) Brauch, Uwe; Röcker, Christoph; Graf, Thomas; Abdou Ahmed, Marwan
The development of high-power diode lasers enabled new solid-state laser concepts such as thin-disk, fiber, and Innoslab lasers based on trivalent ytterbium as the laser-active ion, which resulted in a tremendous increase in the efficiency and beam quality of cw lasers compared to previously used lamp-pumped rod or slab lasers and the realization of ultrafast lasers with several 100 W or even kilowatts of average power. In addition to their beneficial thermo-optical properties, these architectures offer characteristic benefits making them especially suitable to obtain dedicated laser properties. This review article comprises milestone developments, characteristic challenges, and benefits, and summarizes the state of the art of high-power solid-state lasers with the focus on ultrafast lasers.
Open Access
Influence of fabrication deviations on the polarization-maintaining behavior of inhibited-coupling guiding hollow-core fibers
(2024) Chen, Bowen; Kühlthau, Tim; Röhrer, Christian; Graf, Thomas; Abdou Ahmed, Marwan
Numerical simulations were conducted to analyze the influence of the design parameters of tubular inhibited-coupling guiding hollow-core photonic crystal fibers (IC-HCPCFs) on the bending-induced phase shift. The possibility to implement polarization-maintaining (PM) tubular IC-HCPCFs using a low-birefringence approach (with a modal birefringence parameter B<7×10-9) is discussed. Two different tubular IC-HCPCF designs with 7 and 8 glass capillaries are proposed for operation at a wavelength of 1030 nm. The numerical simulation predicts low guiding losses and a PM behavior sustaining a degree of linear polarization (DOLP) larger than 90% after 10 m of fiber with a bend radius larger than 0.2 m. This shows great potential for high-power beam delivery in an industrial environment. The influence that fabrication deviations have on the polarization-maintaining behavior was also investigated and indicates tight fabrication tolerances for both proposed fiber designs. Small deviations from the ideal symmetrical structure can lead to an enhancement of undesired modal birefringence. To ensure a DOLP larger than 90% at the exit of a 10 m-long fiber and for a bending radius > 0.2 m precise control of the drawing parameters during the fiber production is required, which is challenging but is considered to be technically feasible.
Open Access
Intra-cavity wavelength multiplexing of high-brightness thin-disk laser beams
(2022) Boubekraoui, Ayoub; Beirow, Frieder; Graf, Thomas; Abdou Ahmed, Marwan
We report on the first demonstration of an intra-cavity spectral beam combining of two fundamental-mode laser beams generated by a dual Yb:YAG thin-disk resonator. The two thin-disk lasers (TDLs) were operated at the two slightly different wavelengths of 1028 nm and 1032 nm. A resonant diffraction grating waveguide structure was used as common spectral stabilizer and combiner. An average power exceeding 200 W with close to diffraction-limited beam quality (M2<1.3) was obtained with the presented approach.
Open Access
Modelling of natural convection in thin-disk lasers
(2020) Dietrich, Tom; Röcker, Christoph; Graf, Thomas; Abdou Ahmed, Marwan
In this paper, we present a FEM-model that can be used to investigate the effects of thermally induced natural convection at the thin-disk laser crystal. Based on this simulation, we calculated the distribution of the refractive index of the ambient gas for the case of air and helium. By evaluating the optical path difference of a beam at normal incidence, the angular tilt (gas wedge) in the plane of the direction of convection as well as the spherical contribution (gas lens) was calculated for a set of different pump spot geometries and temperatures of the pumped area on the surface of the laser disk. Equations were derived that allow to simply calculate the tilt angle and the focal length of the gas lens for different temperatures of the disk and pump spot diameters for air as ambient medium.
Open Access
Process limits for percussion drilling of stainless steel with ultrashort laser pulses at high average powers
(2021) Brinkmeier, David; Holder, Daniel; Loescher, André; Röcker, Christoph; Förster, Daniel J.; Onuseit, Volkher; Weber, Rudolf; Abdou Ahmed, Marwan; Graf, Thomas
The availability of commercial ultrafast lasers reaching into the kW power level offers promising potential for high-volume manufacturing applications. Exploiting the available average power is challenging due to process limits imposed by particle shielding, ambient atmosphere breakdown, and heat accumulation effects. We experimentally confirm the validity of a simple thermal model, which can be used for the estimation of a critical heat accumulation threshold for percussion drilling of AISI 304 steel. The limits are summarized in a processing map, which provides selection criteria for process parameters and suitable lasers. The results emphasize the need for process parallelization.
Open Access
Radially polarized passively mode-locked thin-disk laser oscillator emitting sub-picosecond pulses with an average output power exceeding the 100 W level
(2018) Beirow, Frieder; Eckerle, Michael; Dannecker, Benjamin; Dietrich, Tom; Abdou Ahmed, Marwan; Graf, Thomas
We report on a high-power passively mode-locked radially polarized Yb:YAG thin-disk oscillator providing 125 W of average output power. To the best of our knowledge, this is the highest average power ever reported from a mode-locked radially polarized oscillator without subsequent amplification stages. Mode-locking was achieved by implementing a SESAM as the cavity end mirror and the radial polarization of the LG*01 mode was obtained by means of a circular Grating Waveguide Output Coupler. The repetition rate was 78 MHz. A pulse duration of 0.97 ps and a spectral bandwidth of 1.4 nm (FWHM) were measured at the maximum output power. This corresponds to a pulse energy of 1.6 μJ and a pulse peak power of 1.45 MW. A high degree of radial polarization of 97.3 ± 1% and an M2-value of 2.16 which is close to the theoretical value for the LG*01 doughnut mode were measured.
Open Access
Resonant grating for wavelength and polarization selection in high-power lasers emitting in the 2-µm wavelength region
(2025) Bashir, Danish; Mourkioti, Georgia; Boubekraoui, Ayoub; Tomilov, Sergei; Redkin, Mykyta; Saraceno, Clara J.; Graf, Thomas; Mackenzie, Jacob I.; Abdou Ahmed, Marwan
This paper presents the design, fabrication, and implementation of a single-layer resonant-reflection grating-waveguide structure (RR-GWS) optimized for laser operation in the 2 μm wavelength region. The RR-GWS was designed to function effectively across a broad spectral region ranging from 1.9 μm to 2.1 μm. Reflectance measurements confirmed the structure’s wideband performance within this spectral region, with 99% resonant reflectivity. Incorporating the RR-GWS into a Ho: YAG thin-disk laser system, as a cavity folding mirror, it demonstrated very good power-handling capability and efficacy in wavelength and polarization selection. The 2 μm laser system achieved a linearly polarized output power of 36 W, with an optical efficiency of 34.6%. Additionally, the laser demonstrated a spectral bandwidth of < 0.5 nm full width at half maximum, i.e., 12 times narrower than that obtained with an equivalent all-mirror reference cavity.
Open Access
Sapphire-based resonant waveguide-grating mirrors : advancing their intra-cavity power density capability
(2023) Bashir, Danish; Boubekraoui, Ayoub; Mourkioti, Georgia; Li, Fangfang; Karvinen, Petri; Kuittinen, Markku; Mackenzie, Jacob. I.; Graf, Thomas; Abdou Ahmed, Marwan
We report on the design, fabrication, and implementation of a single-layer resonant waveguide-grating (RWG) mirror on a sapphire substrate. Our goal is to enhance these optics capability to withstand high intra-cavity power densities by exploiting the superior thermal properties of sapphire. The RWG was implemented as an intra-cavity folding mirror in an Yb:YAG thin-disk laser to generate linearly polarized and spectrally stabilized radiation. A linearly polarized output power of 191 W with an optical efficiency of 39% was obtained in multi-mode operation. This corresponds to a power density of 52 kW/cm 2 on the RWG, for which the increase of its surface temperature was measured to be 12 K, which resulted in a 46-fold reduction of the surface temperature rise dependence on the intra-cavity power density with respect to what has been reported for a RWG on a fused silica substrate. In near fundamental-mode operation, a linearly polarized emission with an output power of 90 W, an optical efficiency of 30%, and a spectral bandwidth of 28 pm FWHM was obtained.
Open Access
Simple spatially resolved period measurement of chirped pulse compression gratings
(2023) Bienert, Florian; Röcker, Christoph; Graf, Thomas; Abdou Ahmed, Marwan
Open Access
Single-crystal and ceramic Yb:Lu2O3 gain media for thin-disk oscillators
(2023) Esser, Stefan; Xu, Xiaodong; Wang, Jun; Zhang, Jian; Graf, Thomas; Abdou Ahmed, Marwan
We report on the direct comparison of single-crystal and ceramic Yb3+:Lu203 gain media with respect to emission spectra, fluorescence lifetime, depolarization, and laser performance in a continuous-wave thin-disk laser oscillator. The most efficient laser operation was achieved with a single-crystal disk in multimode operation with a slope efficiency of 72.1% and an average output power of 997 W. At the same temperature level, a ceramic disk delivered 861 W with a slope efficiency of 68.6%. In fundamental-mode operation, the highest average power of 360 W and highest optical efficiency of 41.3% were obtained with a ceramic disk. For the single-crystal disk, the fundamental-mode output power was limited to 113 W at an optical efficiency of 29%, potentially due to stress within the crystal.