Laser beam shaping with membrane deformable mirrors

Beam shaping is the process of redistributing the irradiance and phase of a beam of optical radiation. The beam shape is defined by the irradiance distribution. In recent years, laser beam shaping has been employed in many fields of scientific, engineering and industrial R&D for many applications, such as material processing, medical applications, lithography, optical data storage and laboratory research. In this thesis, a novel beam shaping technique is presented. Different from geometrical optics and diffractive methods, an imaging-based beam shaping system was built and tested. A laser beam was shaped using geometric masks applied on a spatial light modulator (SLM) at objective plane of a lens system and reconstructed at imaging plane. Geometric masks generated based on real beam profile can modulate both the outline profile and intensity distribution of the input laser. Laser parallel processing technique was also employed in this research to increase the beam shaping efficiency. The results obtained in this thesis demonstrate high accuracy and flexibility of this beam shaping technique, showing many potential applications.

Laser beam shaping offers remarkable possibilities to control and optimise process stability and tailor material properties and structure in laser-based welding and additive manufacturing. However, little is known about the influence of laser beam shaping on the complex melt-pool behaviour, solidified melt-track bead profile and microstructural grain morphology in laser material processing. A simulation-based approach is utilised in the present work to study the effects of laser beam intensity profile and angle of incidence on the melt-pool behaviour in conduction-mode laser melting of stainless steel 316L plates. The present high-fidelity physics-based computational model accounts for crucial physical phenomena in laser material processing such as complex laser–matter interaction, solidification and melting, heat and fluid flow dynamics, and free-surface oscillations. Experiments were carried out using different laser beam shapes and the validity of the numerical predictions is demonstrated. The results indicate that for identical processing parameters, reshaping the laser beam leads to notable changes in the thermal and fluid flow fields in the melt pool, affecting the melt-track bead profile and solidification microstructure. The columnar-to-equiaxed transition is discussed for different laser-intensity profiles.

Laser beam shaping significantly influences solidification microstructure evolution in directed energy deposition (DED-LB), with distinct effects on grain morphology and crystallographic texture. To enable quantitative prediction and mechanistic understanding of these beam shaping effects on solidification microstructure evolution in both welding and metal additive manufacturing, an optimized thermal-fluid – microstructure coupling framework was developed. The integrated model incorporates novel features, including spatiotemporal optimization, efficient thermal-to-microstructural data interpolation (158x faster), CPU-parallelized grain growth algorithms (3.17x speedup), and adaptive time-step size calculation. Single-track experiments and corresponding simulations were performed for both laser-induced melting and laser-based directed energy deposition (DED-LB) using uniform circular and uniform square laser beam intensity profiles. The resulting crystallographic texture and grain morphology were quantitatively characterized through cross-sectional analysis, pole figures, and statistical distributions. Excellent agreement was achieved between experiments and simulations, with texture index deviations below 10.8% and accurate reproduction of grain size distributions demonstrating the model’s fidelity. For the chosen process parameters, the two beam shapes have measurable but limited influence on texture development, with variations ranging from -5.3% to +5.1%. However, beam shape had a much stronger impact on grain morphology than on texture: circular beams refined the bulk grain-area and aspect-ratio distributions relative to square beams, while square beams yielded smaller mean grain areas, highlighting the need for distribution-level metrics beyond simple averages. By linking these morphological trends to beam-shape-dependent variations, the presented framework serves as a predictive tool for microstructure-aware process optimization in laser-based additive manufacturing. • Laser beam shaping shows a limited effect on crystallographic texture (±5% variation), but a significant impact on grain morphology • Circular laser beam intensity profiles produce finer and more equiaxed grains than square profiles when grain‐area and aspect‐ratio distributions are considered in both laser melting and DED-LB • Model predictions achieve excellent agreement with experiments, with texture index deviations below 10.8% • Computationally optimized thermal-fluid – microstructure framework with CPU parallelization reduces calculation time by orders of magnitude • Validated framework enables efficient exploration of process-structure relationships for tailored AM microstructures

A two deformable mirror concept for correcting scintillation effects in laser beam projection through the turbulent atmosphere is presented. This system uses a deformable mirror and a Fourier transforming mirror to adjust the amplitude of the wave front in the telescope pupil, similar to kinoforms used in laser beam shaping. A second deformable mirror is used to correct the phase of the wave front before it leaves the aperture. The phase applied to the deformable mirror used for controlling the beam amplitude is obtained using a technique based on the Fienup phase retrieval algorithm. Simulations of propagation through a single turbulent layer sufficiently distant from the beacon observation and laser beam transmission aperture to cause scintillation shows that, for an ideal deformable mirror system, this field conjugation approach improves the on-axis field amplitude by a factor of approximately 1.4 to 1.5 compared to a conventional phase-only correction system.

As an active device, deformable mirror has been successfully used in adaptive optical systems to compensate phase aberrations and improve beam quality. However, it is often limited to correcting the phase aberrations for a flat reference wavefront. In this paper we present a new adaptive beam intensity profiles control technique based on the combination of a 19-element deformable mirror and a global genetic algorithm. This algorithm can adaptively adjust the voltages of the 19 actuators on the deformable mirror to reduce the difference between the target beam shape and the actual beam shape. The numerical simulations results show that within the mechanical constraints of the deformable mirror, this technique can create the given beam intensity profiles in a focal plane.

The uniformity of the intensity and the phase of laser beams is crucial to high-performance atom interferometers. Inhomogeneities in the laser intensity profile cause contrast reductions and systematic effects in interferometers operated with atom sources at micro-Kelvin temperatures and detrimental diffraction phase shifts in interferometers using large momentum transfer beam splitters. We report on the implementation of a so-called top-hat laser beam in a long-interrogation-time cold-atom interferometer to overcome the issue of inhomogeneous laser intensity encountered when using Gaussian laser beams. We characterize the intensity and relative phase profiles of the top-hat beam and demonstrate its gain in atom-optic efficiency over a Gaussian beam, in agreement with numerical simulations. We discuss the application of top-hat beams to improve the performance of different architectures of atom interferometers.

Tissue imaging using matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a well-established technique that, in recent years, has seen wider adoption and novel application. Applications such imaging mass spectrometry (IMS) and biotyping are beginning to gain greater exposure and use; however, with limitations in optimization methods, producing the best result often relies on the ability to customize the physical characteristics of the instrumentation, a task that is challenging for most mass spectrometry laboratories. With this in mind, we have described the effect of making simple adjustments to the laser optics at the final collimating lens area, to adjust the laser beam size and shape in order to allow greater customization of the instrument for improving techniques such as IMS. We have therefore been able to demonstrate that improvements can be made without requiring the help of an electrical engineer or external funding in a way that only costs a small amount of time. Graphical Abstract ᅟ.

A portable fiber laser thermography system with beam homogenizing for CFRP inspection

A theoretical investigation is presented on the generation of terahertz (THz) radiation through the nonlinear interaction of a cubic frequency chirped Hermite–Cosh–Gaussian (HChG) laser beam with an underdense plasma. The structured intensity profile of the HChG beam, characterized by its mode index, beam width parameter, decentered parameter, and Cosh envelope, enables enhanced spatial confinement and field gradients, which are very significant for efficient THz generation. The incorporation of a cubic frequency chirp modulates the temporal phase of the laser pulse, thereby increasing the interaction duration between the laser and plasma electrons. This leads to a stronger ponderomotive force, which drives a transient nonlinear current density at the beat frequency, resulting in the emission of THz radiation. Analytical expressions for the nonlinear current and THz field amplitude are derived, and parametric analysis reveals that the THz output is highly sensitive to the chirp coefficient, laser intensity, mode index, and plasma density profile. We found that higher-order spatial modes and carefully adjusted beam positioning (decentering) further enhance THz output. The best results were achieved at a lower chirp value ( b = 0.00099), which outperformed traditional linear and quadratic chirping. This study demonstrates that cubic chirping significantly enhances the THz conversion efficiency compared to linear and quadratic chirps, offering a tunable and high-intensity THz source for applications in spectroscopy, imaging, wireless communication, ultrafast material characterization, and medical diagnostics.

A shape of a wave-front that is transformed by an optical element (or system) is changed depending on physical and geometric properties of such a system. This fact can be used in many areas of science and technology. For example, various applications can be found in adaptive optics where active optical elements are used for wave-front correction. Our work presents a detailed analysis of aberration properties of a simple deformable planar mirror from both the aspect of modeling the shape of reflecting surface of the mirror and the mechanisms that enable to reach this desired shape for correction of spherical aberration. Finally, a simple experimental device for practical realization of the optical system for partial wave-front correction is proposed.

Together with ongoing experimental efforts to detect gravitational waves, several fronts of theoretical research are presently being pursued, including second-generation detector design, data analysis, and numerical-relativity simulations of sources. This thesis presents a study in each of these topics: i) The noise in the most sensitive frequency bands in second-generation ground-based gravitational-wave interferometers is dominated by the thermal noise of the test masses. One way to reduce test-mass thermal noise is to modify shape of the laser beam so that it better averages over the thermal fluctuations. When edge effects are neglected, the test-mass thermal noise is related to the beam shape by simple scaling laws. This thesis presents a rigorous derivation of these laws, along with estimates of the errors made by neglecting edge effects. ii) An important class of gravitational-wave sources for space-based gravitational-wave interferometers is extreme-mass-ratio inspirals (EMRIs). These are binaries in which an object of a few solar masses spirals into a (typically million-solar-mass) supermassive black hole (or, if any exist, other type of massive body). Ryan (1995) proved that, under certain simplifying assumptions, the spacetime geometry is redundantly encoded in EMRI waves. One of Ryan's assumptions was negligible tidal coupling. After first finding that only the time-varying part of the induced tide is unambiguously defined when the central body is a black hole, this thesis extends Ryan's theorem by showing that both the spacetime geometry and details of the tidal coupling are encoded in EMRI waves. iii) Merging black holes with comparable masses are important sources of gravitational waves for ground-based detectors. The gravitational waves near the time of merger can only be predicted by numerically solving the Einstein equations. Initial data in numerical simulations must contain the desired physical content but also satisfy the Einstein constraint equations. But conventional binary-black-hole initial data has physical flaws: a nonzero orbital eccentricity and an initial, unphysical pulse of spurious gravitational radiation. Using the Caltech-Cornell pseudospectral code, this thesis develops and implements methods to reduce both of these effects.

The use of additively manufactured metal components is rapidly expanding, prompting ongoing research into more advanced and efficient processing strategies. Beam shaping is one of the latest trends in Laser Powder Bed Fusion (LPBF). As a matter of fact, some authors reported the effectiveness of this approach in producing parts with tailored microstructure and properties, while on the other hand, very little has been said about the sustainability implications of using ring-shaped beams in LPBF. This work aims to bridge some of the gaps of knowledge existing in this regard, thus investigating the influence of different beam shapes on energy consumption and process stability, while boosting productivity. Four laser beam shapes were investigated, ranging from the traditional full Gaussian distribution—used as the reference in this study—to a full ring configuration. Nickel alloy 718 was selected as the feedstock material due to its growing adoption in the aerospace sector, where productivity and sustainability are critical factors for additively manufactured components. Mechanical testing was conducted to assess process performance, alongside considerations of sustainability. The energy consumption of the laser system, as well as the building time, has been recorded to properly calculate the specific energy consumption (SEC) and the productivity of the laser system used, selecting the different modalities. Surface roughness, density, microhardness, and microstructure of the printed samples have been chosen as quality indicators of the printed parts. Among the main experimental findings is that the full ring modality improved productivity of the laser system by approximately 46% and reduced its SEC by 16% compared to the conventional full Gaussian beam, despite an approximate 29% increase in surface roughness. Also, the outcomes confirmed the potentiality of beam shaping in tailoring the microstructure of metal alloys parts manufactured by LPBF.

The technology of membrane deformable mirror (DMs) that has the potential to achieve comprehensive wavefront compensation and control in high power laser has been developed rapidly in recent years. Experimental results reveal that strong nonlinearity is induced to the deformation of DMs with respect to the square of input voltage when operating voltage is more than 120V. The nonlinear response and strong coupling effect of control channel in DMs make it difficult to obtain the desired mirror surface shapes. A test bed is built up to measure the deformation of DMs driven by specified voltages. An efficient nonlinear model of deformation with respect to input voltages is presented using a back propagation neural network (BPNN). Deformation due to arbitrary actuator voltages applied to actuators to correct wavefront aberration can be calculated directly with a higher precision using the BPNN model proposed. The residual relative error of the proposed model shows the improvement of accuracy of an order about 5 as compared to that of linear model, and with no significant increase of time consumption. A preliminary open-loop control experiment of laser wavefront compensation is performed to exam the validity of applying the proposed BPNN model in laser wavefront compensation application.

Microstructural control in metal laser powder bed fusion additive manufacturing using laser beam shaping strategy

Laser beam shaping is an important subject for industrial and medical applications since different applications may require different laser intensity distributions. We present an all-fiber beam-shaping device that could transform a Gaussian-shaped laser beam into an intensity uniform beam in the 1.0 microm wavelength region using a long-period grating (LPG). The device can be used to shape the beam of a Yb-doped high-power fiber laser. The mode structure of the LPGs made of HI1060 fiber is analyzed. The design parameters that affect the beam shaping are discussed, and the results of the beam shaping using two cladding modes, LP(03) and LP(04), are presented.