M. Abdou Ahmed

99% efficiency measured in the -1st order of a resonant grating

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 -1st 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.

A 1-kW radially polarized thin-disk laser

Radially and azimuthally polarized beams have attracted significant attention and increasing interest in various fields of physics and engineering because of their specific and very attractive properties and their promising applications [1, 2]. At high power levels, the most interesting application of such beams is material processing where it was reported that the process efficiency can be significantly enhanced by using radial or azimuthal polarization in comparison to circular or linear polarization [1, 2]. This encouraged several scientific groups to investigate and to develop different approaches to generate such modes [3, 4]. In the present contribution we discuss two techniques which were applied to high power Yb:YAG thin-disk laser systems for the generation of radially polarized beams.

Enhanced efficiency of AlGaInP disk laser by in-well pumping

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.

Microchip-laser polarization control by destructive-interference resonant-grating mirror

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 6.1microJ 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.

Efficient processing of CFRP with a picosecond laser with up to 1.4 kW average power

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 108 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.

Wavelength-selective grating mirrors for high-power thin-disk lasers

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.

Thin-disk multipass amplifier delivering 10 GW of peak power

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).

Single grating mirror intracavity stretcher design for chirped pulse regenerative amplification

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.

Real-time analysis of laser beams by simultaneous imaging on a single camera chip

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.

High-performance intra-cavity polarization- and wavelength-selective grating-mirrors for Yb:YAG thin-disk lasers

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.