Researchers have created a new method for fabricating the high-precision, ultrathin mirrors needed for high-performance X-Ray telescopes using femtosecond laser pulses. The method could assist advance the capabilities of space-based X-Ray telescopes that are used to observe high-energy cosmic phenomena including the formation of supermassive black holes and new stars.
Researchers developed a new way to use femtosecond laser pulses to fabricate the high-precision ultrathin mirrors required for high-performance x-ray telescopes. Femtosecond laser surface ablation is applied to selectively remove stressed film regions on the mirror substrate, correcting the shape of the mirror’s reflective surface. Image Credit: Heng Zuo, MIT Kavli Institute for Astrophysics and Space Research
Detecting cosmic x-rays is a crucial piece of our exploration of the universe that unveils the high-energy events that permeate our universe but are not observable in other wavebands. The technologies our group developed will help telescopes obtain sharp images of astronomical x-rays that can answer many intriguing science questions.
Heng Zuo, Research Team Leader and Assistant Professor, Department of Mechanical Engineering, University of New Mexico
Thousands of small mirrors make up the X-Ray telescopes, which orbit above the Earth’s atmosphere. Each mirror in an X-Ray telescope must be properly bent and positioned in relation to the others.
The researchers detail how they employed femtosecond laser micromachining to bend these ultrathin mirrors into a precise form and fix mistakes that can occur during the fabrication process in Optica, Optica Publishing Group’s journal for high-impact research.
Zuo added, “It is difficult to make ultra-thin mirrors with an exact shape because the fabrication process tends to severely bend the thin material. Also, telescope mirrors are usually coated to increase reflectivity, and these coatings typically deform the mirrors further. Our techniques can address both challenges.”
Precision Bending
As new mission ideas continue to push the boundaries of X-Ray imaging, it is necessary to develop new techniques for manufacturing ultra-precise and high-performance X-Ray mirrors for telescopes. For instance, the Lynx X-Ray Surveyor idea from NASA will feature the most potent X-Ray optic ever created and demand the production of several ultra-high-resolution mirrors.
To address this need, Zuo’s research team coupled stress-based figure correction, a previously discovered method, with femtosecond laser micromachining. Stress-based figure correction makes use of the bendability of thin mirrors by introducing a deformable film to the mirror substrate to alter its stress states and induce controlled bending.
The process entails eliminating particular areas of a strained coating that has developed on the underside of a flat mirror. The reason the researchers chose femtosecond lasers to do this is that their pulses can build incredibly accurate holes, channels, and markings with little collateral damage.
Additionally, compared to conventional techniques, these lasers’ high repetition rates enable quicker machining speeds and throughput. This could hasten the manufacture of the massive quantity of incredibly thin mirrors essential for the next-generation X-Ray telescopes.
Mapping Stress
The researchers had to pinpoint precisely how laser micromachining altered the surface curvature and stress states of the mirror to use the new approach. After taking measurements of the original mirror form, they developed a map showing the stress adjustment needed to achieve the required shape.
Furthermore, they created a multi-pass correction method that uses a feedback loop to reduce mistakes until a suitable mirror profile is attained repeatedly.
“Our experimental results showed that patterned removal of periodic holes leads to equibiaxial (bowl-shaped) stress states, while fine-pitched oriented removal of periodic troughs generates non-equibiaxial (potato-chip-shaped) stress components. Combining these two features with proper rotation of the trough orientation we can create a variety of stress states that can, in principle, be used to correct for any type of error in the mirrors,” stated Zuo.
In this study, the researchers used regular patterns to show the new technique on flat silicon wafers. The researchers are creating a more complicated optical system for the movement of the substrates in three dimensions to adjust genuine X-Ray astronomy telescope mirrors, which are bent in two directions.
Journal Reference:
Zuo, H., et al. (2022) Femtosecond laser micromachining for stress-based figure correction of thin mirrors. Optics. doi:10.1364/OPTICA.461870