Blowing Tiny Optical Lenses

Inserting air into hot glass to form a bubble has been used to make glass objects since Roman times. In new work, researchers apply these same glass blowing principles on a micro­scopic scale to make specia­lized miniature cone-shaped lenses known as axicons. Axicons are used to shape laser light in a way that is beneficial for optical drilling, imaging and creating optical traps for mani­pulating particles or cells. These lenses have been known for more than 60 years, but their fabri­cation, especially when small, is not easy.

A new wafer-level technique uses microscopic glass blowing to form microlenses known as axicons. (Source: FEMTO-ST)

“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,” said research team member Nicolas Passilly from FEMTO-ST Institute in France. Now, the researchers describe the new fabrication approach, which is based on the same processes used to make large numbers of photonic and electronic circuits in parallel on semi­conductor wafers. The researchers used their approach to create glass axicons with diameters of 0.9 and 1.8 millimeters and showed that they success­fully generated Bessel beams.

“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,” said Passilly. “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.”

When used with a laser, axicons create a beam of light that begins as a Bessel-like beam – a non-diffracting beam with maximum intensity on its axis – and then turns into a hollow beam further away from the axicon. Bessel-like beams feature a depth of field that can be orders of magnitude larger than that of a beam focused by a tradi­tional rounded lens with a similar diameter. The beam’s high depth of field allows optical drills to reach deeper and creates higher quality OCT images. For optical tweezers, both the Bessel-like and hollow portions of the beam can be used to trap particles or cells.

Techniques tradi­tionally used to make glass axicons can produce only one lens at a time. Although less expensive axicons can be made in polymer, these can’t withstand high temperature processes such as wafer-level fabri­cation or be used in appli­cations that require high levels of light power. “Polymer axicons can’t be used in optical drilling, for example, because the instan­taneous light power is comparable to the power of a nuclear plant but with an extremely short duration,” said Passilly.

Micro glass blowing 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 carry out the new micro glass blowing method, the researchers deposited silicon cavities in concentric rings 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 cone shapes and then the opposite side was polished away to leave only the shaped lenses.

“Although all the processes we used are standard for micro­fabrication, we applied these techniques in non-standard ways to make miniature glass axicons,” said Passilly. “The technique could be applied to create other shapes, even ones without cylin­drical symmetry.” The researchers plan to incor­porate these optical components in OCT devices they are developing for cancer detection and other medical appli­cations. (Source: OSA)

Reference: J. V. Carrión et al.: Wafer-level process would allow easy integration into biomedical imaging instruments, Opt. Lett. 44, 3282 (2019); DOI: 10.1364/OL.44.003282

Link: FEMTO-ST Institute, Université Bourgogne Franche-Comté, Besançon, France

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