Femtosecond laser micromachining optimises x-ray telescope mirror fabrication

Researchers have developed an ultrafast laser micromachining technique to optimise the fabrication of precise, ultrathin mirrors for space-based x-ray telescopes.

Such telescopes are used to capture high-energy cosmic events involved in forming new stars and supermassive black holes. They contain thousands of thin mirrors that must each have a precisely curved shape and be carefully aligned with respect to all the other mirrors. 

In Optica, researchers from MIT’s Kavli Institute for Astrophysics and Space Research describe how they used femtosecond laser micromachining to bend these ultrathin mirrors into a precise shape, and correct errors that can arise during the fabrication process.

They combine femtosecond laser micromachining with a previously developed technique called stress-based figure correction, which exploits the bendability of thin mirrors by applying a deformable film to the mirror substrate to adjust its stress states and induce controlled bending. The femtosecond laser is used to selectively remove regions of stressed film grown onto the back surface of flat mirrors.  

The new combined technique could help speed up fabrication for the large numbers of ultra-thin mirrors required for next-generation x-ray telescopes.

“It is difficult to make ultra-thin mirrors with an exact shape because the fabrication process tends to severely bend the thin material,” said Heng Zuo, who led the research team. “Also, telescope mirrors are usually coated to increase reflectivity, and these coatings typically deform the mirrors further. Our techniques can address both challenges.”

Mapping stress

In carrying out their work, the researchers first had to determine exactly how laser micromachining changes the mirror’s surface curvature and stress states. They then measured the initial mirror shape and created a map of the stress correction necessary to create the desired shape. They also developed a multi-pass correction scheme that uses a feedback loop to repeatedly reduce errors until an acceptable mirror profile is achieved.

“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,” said Zuo. “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.”

The researchers demonstrated the new technique on flat silicon wafers using regular patterns. To correct real x-ray astronomy telescope mirrors, which are curved in two directions, the researchers are developing a more complex optical setup for 3D movement of the substrates.