Glass sensors 1000x smaller than sand, 3D printed on optical fiber

In a sign of significant progress in communications, Swedish researchers have 3D printed silica glass micro-optics on the ends of optical fibers, which have surfaces as small as the cross-section of a human hair.

More sensitive external sensors for the environment and healthcare are among the innovations that could be made possible by integrating silica glass optical devices with optical fibers.

According to the team at the KTH Royal Institute of Technology in Stockholm, this approach combines the superior material properties of glass with the plug-and-play nature of optical fibers. It enables promising applications in fiber sensing, optical microelectromechanical systems (MEMS) and quantum photonics.

“These structures are so small that a thousand of them can fit on the surface of a grain of sand, which is about the size of sensors used today,” said Po-Han Huang, co-author of the study, in a declaration.

Next generation fiber optic tips

Over the past decades, the integration of functional materials and structures onto optical fiber tips has yielded numerous applications in the fields of sensing, imaging and optical trapping.

The light-coupled platform of optical fiber tips enables interaction between the guided light and the device on the tip, providing a small footprint, low insertion loss and compatibility with standard optoelectronic components.

However, researchers emphasize that the small, delicate nature of fiber tips poses challenges for standard microfabrication processes designed for planar substrates.

Researchers claim their approach also solves long-standing problems with the silica glass structure of optical fiber tips. These tips often require high temperature treatments that compromise the integrity of temperature-sensitive fiber coatings.

Unlike other approaches, the process starts with a non-carbon feedstock. This means that the glass structure can be made transparent without the need for high temperatures to remove carbon.

The team demonstrates how silica glass microstructures can be printed on an optical fiber.
The team demonstrates how silica glass microstructures can be printed on an optical fiber.

Glass printing improves photonics

The team’s 3D printing of inorganic glass structures onto optical fiber dots involves four steps. First, a single-mode optical fiber is cut to the desired length and slit at both ends. The fiber is then threaded through a custom aluminum holder and mounted on a motorized stage.

In the second step, a solution of 40 percent hydrogen silsesquioxane (HSQ) in toluene is poured onto the fiber tip, forming a dome-shaped layer approximately 100 μm thick. The HSQ solution is dried, leaving a hard layer on the fiber tip.

In the third step, 650 nm laser light is injected to illuminate the fiber core, which promotes alignment. Finally, in the fourth step, a femtosecond laser with a wavelength of 1040 nm and a pulse width of less than 400 fs is used for direct laser writing (DLW).

The laser selectively cures the HSQ, removes the uncured HSQ and leaves a 3D printed silica glass structure on the fiber tip.

The results show that the work solves the problem of high temperature requirements in 3D direct laser writing methods for glass, allowing glass structures to be created on optical fiber tips without damaging temperature-sensitive coatings.

“We demonstrated a glass refractive index sensor integrated into the fiber tip that allowed us to measure the concentration of organic solvents. This measurement is challenging for polymer-based sensors due to the corrosiveness of the solvents,” said Lee-Lun Lai, lead author of the study.

The process used by researchers to 3D print silica glass micro-optics onto the ends of optical fibers.
The process used by researchers to 3D print silica glass micro-optics onto the ends of optical fibers.

In addition, the refractive indices of acetone and methanol mixtures at near-infrared wavelengths were measured for the first time. A fiber-tip polarization beam splitter (PBS) has demonstrated that light polarization and beam steering can be manipulated, useful for fiber-to-chip coupling and integrated quantum photonic circuits.

Researchers claim that photonics could reach new heights with the ability to 3D print any form of glass structure directly onto the fiber tip.

“Bridging the gap between 3D printing and photonics, the implications of this research are far-reaching, with potential applications in microfluidic devices, MEMS accelerometers and fiber-integrated quantum emitters,” said Po-Han Huang, co-author of the study. .

The details of the team’s research have been published in the journal ACS Nano.

Abstract

Integration of functional materials and structures on the ends of optical fibers has enabled various applications in micro-optics, such as sensing, imaging and optical trapping. Direct laser writing is a 3D printing technology that shows promise for fabricating advanced micro-optical structures on fiber tips. To date, the choice of materials has been limited to organic polymer-based photoresists, as existing methods for direct 3D laser writing of inorganic materials involve high-temperature processing that is incompatible with optical fibers. However, organic polymers do not have stability and transparency comparable to those of inorganic glasses. Herein, we demonstrate 3D direct laser writing of inorganic glass with subwavelength resolution at optical fiber points. We demonstrate two different printing modes that enable the printing of solid silica glass structures (“Uniform Mode”) and self-organized subwavelength gratings (“Nanograting Mode”), respectively. We illustrate the utility of our approach by printing two functional devices: (1) a refractive index sensor that can measure the indices of binary mixtures of acetone and methanol at near-infrared wavelengths and (2) a compact polarization beam splitter for polarization control and beam steering in an all -in-fiber system. By combining the superior material properties of glass with the plug-and-play nature of optical fibers, this approach enables promising applications in areas such as fiber sensing, optical microelectromechanical systems (MEMS) and quantum photonics.

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Jijo Malayil Jijo is an automotive and business journalist based in India. Armed with a BA in History (Honors) from St. Stephen’s College, University of Delhi, and a PG degree in Journalism from the Indian Institute of Mass Communication, Delhi, he has worked for news agencies, national newspapers and motoring magazines. In his spare time he enjoys going off-road, engaging in political discussions, traveling and teaching languages.

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