Scientific Achievement
Researchers from the Berkeley Lab Laser Accelerator (BELLA) Center in the Accelerator Technology & Applied Physics Division at Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated an efficient laser beam shaping technique that transforms a quasi-Gaussian beam with imperfections into a flat-top beam with high uniformity while maintaining a throughput efficiency of 92%. This technique combines a refractive beam shaper and a spatial light modulator (SLM). While refractive beam shapers are commercially available, they only work for perfect input conditions (Gaussian profile, radial symmetry, and unique beam size); real-life lasers often have minor imperfections in beam shape, size, and alignment input. This means a secondary shaping technique is needed to address these input imperfections, which usually lead to excessive energy loss. In this work, the researchers combined a refractive beam shaper with an SLM to achieve an efficient (high-throughput) way to realize shaping for imperfect input Gaussian laser pulses.
Significance and Impact
Shaping of laser pulses that are approximately (but not perfect) Gaussian in transverse shape into flat-top, homogenized beam profiles has applications in laser machining with femtosecond laser and non-linear optics in high-power laser science. Every percent of energy loss that can be avoided is critical, while non-linear processes, such as frequency doubling, laser machining, or amplification crystal pumping, benefit from flat-top beam profiles. The research allows power scalability with higher-energy systems and specialty optics, such as deformable and free-form mirrors, to replace the SLM. The work can improve the precision and efficiency of laser applications, presenting new possibilities for efficient, large-scale, high-quality material processing, discovery, and manufacturing. The basic concept can also be applied to other laser beam shapes, for example, by shaping the non-perfect super-Gaussian profiles typically observed in bulk-crystal laser amplifiers to Gaussian profiles that enable laser-plasma acceleration optimization.
Research Details
Enhanced precision and efficiency for laser applications
Schematic of the experimental setup for beam shaping using a combined refractive beam
shaper and spatial light modulator. The red lines indicate the ray tracing of the beamline (not to
scale), and the blue lines indicate the unwanted higher-order diffraction. (Credit: Berkeley Lab)
Laser applications such as lithography, drilling, and material processing demand high spatial uniformity, which requires a flat-top beam. Currently, commercially available beam-shaping devices are limited to a few applications due to their requirements, such as radial symmetry, perfect Gaussian beam, and particular beam size. To overcome this limitation, the researchers combined a refractive beam shaper with an SLM to deliver the desired profile uniformity while maintaining the system’s high throughput. They leveraged the refractive beam shaper’s ability to approximate a flat-top profile and the SLM’s precise residual intensity correction capability. This has proven its versatility by shaping a circularly asymmetric beam.
Methodology
A quasi-Gaussian beam from a femtosecond laser was routed onto a refractive beam shaper after beam expansion, where “quasi” indicates an asymmetry in the transverse intensity profile and hot spots in the beam. The beam shaper produced an approximate flat-top beam, which could also benefit from additional flattening. The beam was then illuminated onto the SLM to redirect unwanted light. A user-specified intensity target was set on the in-built SLM control software. The SLM applied diffraction gratings and adjusted the phase-amplitude according to the desired attenuation by comparing the real-time camera feed intensity with the target profile. This process was iteratively applied to achieve a minimum error with the resultant intensity profile. An iris blocked the higher-order diffracted intensity. At the same time, a camera captured the desired flat-top beam profile.
Results
Laser beam profiles: (a) input, (b) output, and (c) combined output. (Credit: Berkeley Lab)
Fig. (a) shows the input quasi-Gaussian beam, illuminated onto the beam shaper, while Fig.(b) displays the output without further SLM manipulation. The profile looks flatter but could benefit from further improvement. Fig. (c) shows the achieved flat-top beam captured by the camera when a calculated phase profile pattern was applied to the SLM. This flattop beam has a high uniformity within 90% of the beam area and a system energy throughput efficiency of 92%. This method markedly improved the combined performance of uniformity and efficiency.
Contacts: Deepak Sapkota, Hailang Pan, Jeroen van Tilborg, and Tong Zhou
Researchers: Hailang Pan (Science Undergraduate Laboratory Internship program student), Deepak Sapkota, Aodhan McIlvenny, Anthony Lu, Alex Picksley, Adrian Woodley, Vassilia Zorba, Anthony Gonsalves, Tong Zhou, and Jeroen van Tilborg
Funding: The U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists under the Science Undergraduate Laboratory Internship program.
Publication: Hailang Pan, Deepak Sapkota, Aodhan McIlvenny, Anthony Lu, Alex Picksley, Adrian Woodley, Vassilia Zorba, Anthony Gonsalves, Tong Zhou, and Jeroen van Tilborg. “High-throughput homogenization of a quasi-Gaussian ultrafast laser beam using a combined refractive beam shaper and spatial light modulator,” Opt. Eng. 63(9), 094103, 2024. https://doi.org/10.1117/1.OE.63.9.094103
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