Beamexpander

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How to design a Gaussian to top hatbeamshaper

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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.

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.

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Gaussianbeam

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.

Flat-topbeam

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.

Pi shaper

Nonetheless, the association between fluorescence and haziness persists, and diamonds in the D to H range with very strong fluorescence often sell for less than diamonds that do not have fluorescence, even though their color and transparency are most likely unaffected by their fluorescence. These diamonds may prove to be a good deal. If you’re not familiar with the GIA Color Scale, learn more with the GIA Diamond Color Chart.

No, fluorescence does not cause haziness in diamonds. Fluorescence can increase the pre-existing haziness of a diamond caused by light-scattering defects, thus reducing the contrast in its face-up pattern, but it does not cause haziness in and of itself. As light-scattering defects are incredibly rare, most consumers don’t need to worry about fluorescence impacting a diamond’s appearance in most lighting situations, even if the diamond has Strong to Very Strong fluorescence. But this does mean that consumers should look at a diamond in person. Any haziness intensified by fluorescence, if present, will be noticeable in (for example) daylight-equivalent lighting.

No. Only about 25% to 35% of diamonds exhibit some degree of fluorescence in reaction to long-wave UV light. More than 95% of these diamonds fluoresce blue. A rare few fluoresce other colors such as yellow or green.

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.

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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Beamshaper

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.

Laguerre Gaussbeam

Some trade professionals think blue fluorescence enhances a diamond’s appearance, especially in diamonds with I to M color grades. Bluish fluorescence can make a faint yellowish diamond appear more colorless in UV light, which is part of natural daylight. As a result, diamonds with color grades I to N with a very strong to medium bluish fluorescence may have a slightly higher per-carat price than diamonds with similar color grades that do not fluoresce.

Laser beamhomogenizer

In order to study the effect of blue fluorescence on diamond appearance, GIA scientists assembled sets of E, G, I, and K color-grade diamonds. The diamonds in each set were as similar as possible except for the intensity of their blue fluorescence. Diamond graders, trained professionals, and average observers viewed the diamonds in controlled conditions to make a judgment about their appearance. It seems that, for the average observer, meant to represent the jewelry-buying public, no systematic effects of fluorescence were detected. In general, viewers perceived that strongly blue fluorescent diamonds had a better color appearance when viewed table-up. Most observers saw no relationship between fluorescence and transparency.

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Diamond fluorescence is a fascinating phenomenon where diamonds glow when exposed to long-wave UV rays. It can sometimes increase or decrease the value of a diamond. It can also affect diamond appearance—or not. How should you take fluorescence into account when choosing the perfect diamond? In this article, we’ll cover:

Fluorescence is not a grading factor like the GIA 4Cs (color, clarity, cut, and carat weight), but it is an identifying characteristic. GIA Diamond Grading Reports and Diamond Dossiers describe a diamond’s fluorescence by its intensity under long-wave UV light (None, Faint, Medium, Strong and Very Strong). If the fluorescence is Medium, Strong, or Very Strong, the color of the fluorescence will be noted.

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Seen here are seven diamonds seen under daylight-equivalent illumination versus long-wave UV illumination. Photo: Kevin Schumacher/GIA

The opposite is true for diamonds with higher color grades. In the trade, diamonds in the D to H color range with bluish fluorescence are often considered less desirable than similar grade diamonds without fluorescence, because some people believe that bluish fluorescence may cause diamonds to have a hazy or oily appearance. In a recent study, GIA observed that blue fluorescence has little to no impact on transparency except in extremely rare cases where a diamond (such as the Portuguese diamond) has a light-scattering defect. These defects cause haziness that can sometimes be intensified by strong fluorescence, thus reducing the contrast in the face-up pattern of the diamond. Fluorescence does not cause haziness by itself, however. Also, this “overblue” hazy effect occurs in fewer than 0.2% of the fluorescent diamonds submitted to GIA.

Schematic of the experimental setup for beam shaping using a combined refractive beamshaper and spatial light modulator. The red lines indicate the ray tracing of the beamline (not toscale), and the blue lines indicate the unwanted higher-order diffraction. (Credit: Berkeley Lab)

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If you are considering a diamond with bluish fluorescence, look at it under different kinds of lighting, including natural daylight and compare it to other diamonds of the same color grade, and see if you notice any difference. If you do notice a difference, see if you like it!

Fluorescence is the glow you sometimes see when an object emits visible light. Some diamonds fluoresce when they are exposed to long-wave ultraviolet (UV) rays from sources like the sun. This can cause them to emit a bluish light or more rarely, a yellow or orangy light. Once the UV light source is removed, the diamond stops fluorescing.

The center diamond is an H color diamond with Strong to Very Strong fluorescence. The two diamonds flanking it on either side are H color diamonds with no fluorescence. There is no visible difference in color or transparency. Courtesy: Jian Xin (Jae) Liao

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

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Diamond fluorescence is neither good nor bad. Some people find fluorescent diamonds beautiful and fascinating—others may not. Opinions range across the spectrum.