High-power Thin-disk Laser Research Articles

Determining thermomechanical response of thin disk lasers with physics-informed neural networks

Acquiring structural behavior related to thermal effects is essential to the design of high-power thin disk lasers. However, quantifying the thermomechanical response of a thin disk laser crystal (TDLC) is challenging since these variables of interest are always highly tensorial. Herein, we developed a method to directly quantify the thermo-induced structural behavior of a TDLC from scalar temperature by a physics-informed neural network (PINN). The PINN is established to approximate the unknown states (thermal displacement, strain, and stress fields) using the loss functions encoded with sparse temperature and thermal mechanics equations. Steady-state and transient results of thermo-induced structural responses are precisely obtained from the temperature field by training PINN. In addition, transfer learning is demonstrated to enhance the computation efficiency of PINN for TDLCs with various materials and geometric properties. Implanting PINNs into thin disk lasers paves the way for precise quantification of highly tensorial thermal effects of lasers in-situ using easily accessible scalars.

Influence of disk aberrations on high-power thin-disk laser cavities.

We present a systematic study on the influence of thin-disk aberrations on the performance of thin-disk laser oscillators. To evaluate these effects, we have developed a spatially resolved numerical model supporting arbitrary phase profiles on the intracavity components that estimates the intracavity beam shape and the output power of thin-disk laser oscillators. By combining this model with the experimentally determined phase profile of the thin-disk (measured with interferometry), we can predict the operation mode of high-power thin-disk lasers, including mode degradation, higher-order mode coupling, and stability zone shrinking, all of which are in good agreement with experiment. Our results show that one of the main mechanisms limiting the performance is the small deviation of the disk's phase profile from perfect radial symmetry. This result is an important step to scaling modelocked thin-disk oscillators to the kW-level and will be important in the design of future active multi-pass cavity arrangements.

High-power thin-disk lasers emitting beams with axially-symmetric polarizations

Abstract 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%.

Silicate bonding of sapphire to SESAMs: adjustable thermal lensing for high-power lasers.

Silicate bonding is a flexible bonding method that enables room-temperature bonding of many types of materials with only moderate flatness constraints. It is a promising approach for bonding components in high power laser systems, since it results in a thin and low-absorption interface layer between the bonded materials. Here we demonstrate for the first time silicate bonding of a sapphire window to a SEmiconductor Saturable Absorber Mirror (SESAM) and use the composite structure to mode-lock a high-power thin-disk laser. We characterize the fabricated devices both theoretically and experimentally and show how the thermally induced lens of the composite structure can be tuned both in magnitude and sign via the thickness of the sapphire window. We demonstrate mode-locking of a high-power thin-disk laser oscillator with these devices. The altered thermal lens allows us to increase the output power to 233 W, a 70-W-improvement compared to the results achieved with a state-of-the-art SESAM in the same cavity.

Closed-loop controlled compensation of thermal lensing in high-power thin-disk lasers using spherically deformable mirrors

We present the closed-loop controlled compensation of the thermally induced lens in a high-power thin-disk oscillator to obtain fundamental-mode operation over a wide range of pump powers. The radius of the oscillating beam was measured in real time at the position of the thin-disk crystal. This signal was used by the control system to adapt the curvature of a spherically deformable mirror to maintain a constant radius of the oscillating laser beam.

Investigating the dynamic behavior of thermal distortions of the wavefront in a high-power thin-disk laser using the moiré technique.

We measure wavefront (WF) distortions in a high-power thin-disk laser induced simultaneously by the gas-lens/wedge and disk front-surface deformation using a two-channel moiré-based WF sensor. Thermal lensing and tilting effects are characterized versus time, their pressure-dependent fluctuations are quantified, and finally the corresponding pure disk front-surface effects are estimated for zero pressure. A divergent probe beam with a WF mean curvature similar to the curvature of the disk is reflected off the disk front surface. The temporal evolution of the WF at laser start-up is characterized using the WF sensor. A camera records both temporal evolution of the moiré fringe patterns and the intensity profile of the laser beam to have simultaneously both phase and intensity profiles of the laser beam. Successive WF data quantify the temporal evolutions of the dioptric power and induced wedges in two directions and their fluctuations during laser operation. We investigate the effect of air pressure on the produced gas-lens/wedge. The method is not sensitive to translational vibrations and is very low cost with adjustable sensitivity.

Simple highly efficient pumping configuration in high-power thin-disk laser

A simple high-power thin-disk pumping configuration using a radiation combination of four commercially available laser–diode stacks is introduced. Two setup modifications are presented to compensate the nonsuitable shape of the pumping spot arising from low beam quality in our combination method. The effects of setup modifications on pumping spot shapes are confirmed by ray tracing simulations using Trace-pro™ software. All setups are arranged in the laboratory, and the experimental measurements show pumping spots improvements on the disk due to modifications in agreement with simulation results. Output power measurements show that by adapting the pumping spot size to the disk cooling capacity the modified setups can deliver higher output powers and efficiencies. Furthermore, the modifications reduce the laser threshold and improve output laser beam quality. Hence, the modifications make the simple four laser–diodes beam combination applicable for thin-disk laser pumping.

Heat generation in quasi-three-level Yb:YAG thin-disk lasers

A theoretical model is established to evaluate heat generation in quasi-three-level Yb:YAG thin-disk lasers. Fluorescence heating fraction, laser heating fraction, amplified spontaneous emission (ASE) heating fraction, and non-radiative relaxation heating fraction are considered in our analysis. The dynamic process and spectral properties of ASE are analyzed by using quasi-three-level system rate equations. A numerical iterative method is used to study the dynamic behavior, the fractional thermal load, proportions of all the heating factors, and the temperature of the thin-disk laser. Dynamic heating process is clearly demonstrated. Our theoretical analysis shows that the ASE effect is critical in the total heat generation and operation of thin-disk lasers, especially for a high-power thin-disk laser.

Deformable mirrors for intra-cavity use in high-power thin-disk lasers.

We present deformable mirrors for the intra-cavity use in high-power thin-disk laser resonators. The refractive power of these mirrors is continuously adaptable from -0.7 m-1 to 0.3 m-1, corresponding to radii of curvature ranging between 2.86 m (convex) and 6.67 m (concave). The optimized shape of the mirror membrane enables a very low peak-to-valley deviation from a paraboloid deformation over a large area. With the optical performance of our mirrors being equal to that of standard HR mirrors, we were able to demonstrate the tuning of the beam quality of a thin-disk laser in a range of M2 = 3 to M2 = 1 during laser operation at output powers as high as 1.1 kW.

High-power Yb:GGG thin-disk laser oscillator: first demonstration and power-scaling prospects.

We present the first demonstration of a thin-disk laser based on the gain material Yb:GGG. This material has many desirable properties for the thin-disk geometry: a high thermal conductivity, which is nearly independent of the doping concentration, a low quantum defect, low-temperature growth, and a broadband absorption spectrum, making it a promising contender to the well-established Yb:YAG for high-power applications. In continuous wave laser operation, we demonstrate output powers above 50 W, which is an order of magnitude higher than previously achieved with this material in the bulk geometry. We compare this performance with an Yb:YAG disk under identical pumping conditions and find comparable output characteristics (with typical optical-to-optical slope efficiencies >66%). Additionally, with the help of finite-element-method simulations, we show the advantageous heat-removal capabilities of Yb:GGG compared to Yb:YAG, resulting in >50% lower thermal lensing for thin Yb:GGG disks compared to Yb:YAG disks. The equivalent optical performance of the two crystals in combination with the easy growth and the significant thermal benefits of Yb:GGG show the large potential of future high-power thin-disk amplifiers and lasers based on this material, both for industrial and scientific applications.

Laser cutting of composites—Two approaches toward an industrial establishment

Carbon fiber reinforced plastics (CFRP) are of high interest as a lightweight material due to their high strength-to-weight ratio. Within industrial fields with a requirement on lightweight design and energy efficiency, the demand of CFRP increases continuously. Up to now, the cutting of CFRP, as a basic production step within the production of composite parts, is mainly performed by conventional cutting techniques such as milling and abrasive water jet cutting. These techniques are known to have drawbacks, e.g., force input, handling of auxiliaries, and tool wear. The laser cutting of CFRP has already shown high potential to be a practical alternative due to the wear-free and contactless processing. However, as a thermal technique, laser cutting has to overcome the challenge of heat input into the material. Two different approaches concerning deterioration free cutting will be examined and compared within this research. For that purpose, two different laser sources were used. Both laser sources have a maximum average laser power of PL = 1500 W, one emitting continuous wave and the other emitting pulsed wave. The first approach investigated is the laser cutting with a single mode fiber laser emitting continuous wave at high beam quality. A small focal diameter was achieved by the optical setup used, enabling narrow cutting kerfs and finely machined cutting contours. The relative movement of laser beam and material was realized by a combination of scanning optic and three precise linear stages. The second laser source is a high power thin-disk laser emitting nanosecond pulses. The optical setup consists of a fiber with a diameter of d = 600 μm and a galvanometer scanner by TRUMPF, called 3D programmable focusing optic (PFO-3D), leading to a focal diameter bigger than that of the single mode fiber laser. The relative movement was realized by the PFO-3D and a six-axis robot system. The required process strategies, differences, benefits, and drawbacks of both setups will be reviewed. The review will be completed by an examination of the complexity of the setups and introduction of efficient processing solutions. Comparative investigations with an identical material for both described setups were performed as a basis for discussion. Differences exist in appearance of the cuts and the choice of best parameters, while comparable qualities and efficiencies can be achieved for both setups.

Implementation of the concept of neutral gain modules in thin-disk lasers

Experimental results of the implementation of our concept of neutral gain modules (NGM) in high-power thin-disk lasers are presented. To demonstrate power scalability and high brightness, series of NGMs comprising up to four Yb:YAG thin-disk laser modules have been integrated in various ways into stable and unstable resonators. Furthermore, to compensate for the parabolic component of the remaining thermal lens effects in the high-power regime, an intracavity deformable mirror was embedded. On the basis of the underlying concept, in our experiments an output power of nearly 3 kW with a beam propagation factor \(M^2 = 5\) as well as a brightness up to \(B = 21\,{\rm GW}/{\rm cm ^2} \, {\rm sr }\) could be demonstrated. Compensating systematically for the nonparabolic components of the phase aberrations, the concept also enables a further enhancement of the beam quality. The principle is transferable to other laser configurations, e.g., master oscillator power amplifier systems.

Measurement of thermal lensing effects in high power thin disk laser

Two different experimental methods for disk characterization in thin disk lasers are investigated in this paper. First method is based on the stability regions in optical resonator and the second method is based on using an expanded probe beam. Both of the mentioned techniques are used to determine the thermal lensing effects in high power thin disk laser. The results of the experiments are presented and analyzed. Also the advantages and disadvantages of each method are explained.

Frontiers in passively mode-locked high-power thin disk laser oscillators

Semiconductor saturable absorber mirror (SESAM) mode-locked thin disk lasers define the state-of-the-art performance for high average power and high pulse energy femtosecond laser oscillators. To date pulse energies above 30 µJ and average powers above 140 W have been demonstrated. In this paper we review the achievements of mode-locked thin disk lasers in terms of average power and pulse energy. Stable mode locking requires single transverse mode operation even at the highest average power, which is challenging and therefore addressed in more detail. We then summarize our expectations on the main challenges and limitiations for the next generation of mode-locked thin disk laser oscillators with an average power above 500 W and pulse energies in excess of 100 µJ.

Pulse compression of a high-power thin disk laser using rod-type fiber amplifiers

We report on two pulse compressors for a high-power thin disk laser oscillator using rod-type fiber amplifiers. Both systems are seeded by a standard SESAM modelocked thin disk laser that delivers 16 W of average power at a repetition rate of 10.6 MHz with a pulse energy of 1.5 μJ and a pulse duration of 1 ps. We discuss two results with different fiber parameters with different trade-offs in pulse duration, average power, damage and complexity. The first amplifier setup consists of a Yb-doped fiber amplifier with a 2200 μm2 core area and a length of 55 cm, resulting in a compressed average power of 55 W with 98-fs pulses at a repetition rate of 10.6 MHz. The second system uses a shorter 36-cm fiber with a larger core area of 4500 μm2. In a stretcher-free configuration we obtained 34 W of compressed average power and 65-fs pulses. In both cases peak powers of > 30 MW were demonstrated at several μJ pulse energies. The power scaling limitations due to damage and self-focusing are discussed.

大口径薄片激光器中的谐振模式及光束质量诊断

大口径薄片激光器中的谐振模式及光束质量诊断

Efficient pump beam shaping for high-power thin-disk laser systems

We report a beam-shaping technique whereby the output power from a high-power laser-diode stack is efficiently coupled, reconfigured, and transmitted to a thin-disk laser by means of a compact optical fiber bundle. By using this technique, the power density is increased by a factor of 2 when compared to direct coupling with a octagonal fused silica rod while the numerical aperture is kept constant. Transmission efficiency of 80% was measured for the beam shaper without antireflection coating. The top-hat distribution is numerically calculated at the thin-disk laser crystal.

Thermal and laser properties of Yb:LuAG for kW thin disk lasers

Thin disk laser experiments with Yb:LuAG (Yb:Lu(3)Al(5)O(12)) were performed leading to 5 kW of output power and an optical-to-optical efficiency exceeding 60%. Comparative analyses of the laser relevant parameters of Yb:LuAG and Yb:YAG were carried out. While the spectroscopic properties were found to be nearly identical, investigations of the thermal conductivities revealed a 20% higher value for Yb:LuAG at Yb(3+)-doping concentrations of about 10%. Due to the superior thermal conductivity with respect to Yb:YAG, Yb:LuAG offers thus the potential of improved performance in high power thin disk laser applications.

Dependence of the Yb^3+ emission cross section and lifetime on temperature and concentration in yttrium aluminum garnet

Measurements are reported of the spectroscopic properties (absorption and emission spectra, stimulated-emission cross section, and radiative lifetime) of (YbxY1-x)3Al5O12 for nominal x values of 0.025, 0.05, 0.1, 0.2 and 0.3 at temperatures of 15–300 K. The emission cross sections of Yb:YAG with different Yb3+ concentrations were determined by use of the Füchtbauer–Ladenburg formula and the reciprocity method. At low temperatures, the product (στ) of the effective stimulated-emission cross section and the radiative lifetime is greater than at room temperature for all concentrations. Product στ is nearly independent of Yb3+ concentration at a given temperature. These results will aid in the design of high-power thin disk lasers by use of highly doped Yb:YAG.