Optical Fabrication: Glassblowing technique creates arrays of axicon lenses

Axicon lenses, which are useful for microfabrication, OCT, and other applications, can now be easily made in large quantities by glassblowing.

Several rotationally symmetric cavities are etched into a silicon wafer for glassblowing of 0.9-mm-diameter (left) and 1.8 mm (right) axicons.
Several rotationally symmetric cavities are etched into a silicon wafer for glassblowing of 0.9-mm-diameter (left) and 1.8 mm (right) axicons.

Axicon lenses, which are conical in shape, are widely used to create so-called nondiffracting Bessel beams, which form a long, line-shaped focal region rather than the usual point focus. The line focus is useful for alignment, microfabrication, and other applications due to its insensitivity to axial motion, but cones are much more difficult than spheres to create in glass. Traditional optical techniques, such as grinding and polishing, or other approaches, such as femtosecond-laser ablation followed by polishing with a CO2 laser, can be used, but are complex and not suited to large-volume production.

Researchers at the FEMTO-ST Institute (Besançon, France) are now using a glassblowing technique they developed to make small axicons.1 In the approach, a glass sheet is overlaid on an array of small cavities on a silicon wafer and heated—the cavities are pressurized to push the glass outward, creating an array of axicons. The inward side of the glass is then polished smooth. The researchers used their approach to create glass axicons with diameters of 0.9 and 1.8 mm and showed that the axicons successfully generated Bessel beams (see figure).

“Our technique has the potential of producing robust miniature axicons in glass at a low cost, which could be used in miniaturized imaging systems for biomedical imaging applications, such as optical coherence tomography, or OCT,” says Nicolas Passilly, one of the researchers. “Wafer-level microfabrication allows the axicons to be integrated into more complex microsystems created also at a wafer-level, leading to a system made of a wafer stack. This type of integration comes with better optical alignments, high-performance vacuum packaging, and much lower costs for the final systems because a large number can be processed simultaneously.”

High-quality optical surface

Micro-glassblowing has been previously used to make microlenses, but it usually involves gas expansion from a single reservoir. The researchers developed an axicon fabrication method that combines gas expansion from multiple reservoirs to produce the optical component’s conical shape. The technique shapes the surface from underneath, leaving a high-quality optical surface, unlike commonly used methods like etching transfer from a 3D mask that engrave the wafer from above.

To shape the axicons properly into cones, the researchers deposited silicon cavities in concentric rings (which look a bit like Fresnel zone plates) that were then sealed with glass under atmospheric pressure. Placing the silicon and glass stack in a furnace caused gas trapped in the cavities to expand, creating ring-shaped bubbles. These bubbles pushed out the glass surface to form the cone shapes. In one example, an axicon 1.8 mm in diameter was formed from five rings of varying diameter and widths of 200, 100, 50, and 25 µm, along with a central circular cavity 200 µm in diameter. The axicon had a sag of 38.78 µm and a wedge angle of 3.83°.

The researchers plan to incorporate these optical components in OCT instruments that they are developing for cancer detection and other medical applications. Nonspherical shapes other than axicons could also be made using the new glassblowing technique, they say.

REFERENCE

1. J. Vincente Carrión et al., Opt. Lett., 44, 13 (Jul. 1, 2019); https://doi.org/10.1364/ol.44.003282.

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