Glass sensors 1,000x smaller than sand, 3D-printed on optical fiber

Marking a significant advancement in communications, Swedish researchers 3D-printed silica glass micro-optics on the tips of optic fibers, which have surfaces as small as the cross-section of a human hair.

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

According to the team at KTH Royal Institute of Technology in Stockholm, the 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 you could fit 1,000 of them on the surface of a grain of sand, which is about the size of sensors being used today,” said Po-Han Huang, the study’s co-author, in a statement.

Next-gen optical fiber tips

In recent decades, integrating functional materials and structures on optical fiber tips has opened up numerous applications in 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, offering a small footprint, low insertion loss, and compatibility with standard optoelectronic components.

However, researchers highlight that fiber tips’ small, delicate nature poses challenges for standard microfabrication processes designed for planar substrates.

Researchers claim that their approach also solves long-standing issues with the silica glass structure of optical fiber tips. These tips frequently call for high-temperature treatments that jeopardize the integrity of temperature-sensitive fiber coatings.

Unlike other approaches, the process starts with a non-carbon-containing basic material. This implies that the glass structure can be made transparent without requiring high temperatures to remove carbon.

Glass printing enhances photonics

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

In the second step, a 40 percent hydrogen silsesquioxane (HSQ) solution in toluene is drop-casted onto the fiber tip, forming a dome-shaped layer about 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, aiding alignment. Finally, in the fourth step, a femtosecond laser with a 1040 nm wavelength and less than 400 fs pulse width is used for direct laser writing (DLW).

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

Results show that the work solves the problem of high-temperature requirements in 3D direct laser writing glass methods, allowing the creation of glass structures on optical fiber tips without damaging temperature-sensitive coatings.

“We demonstrated a glass refractive index sensor integrated onto 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, the study‘s lead author.

Additionally, 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) demonstrated that light polarization and beam steering can be manipulated, which is useful for fiber-to-chip coupling and integrated quantum photonic circuits.

Researchers claim that photonics can reach new heights with the capacity to 3D print any kind of glass structure directly on the fiber tip.

“By 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, the study’s co-author.

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

Abstract

Integration of functional materials and structures on the tips 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 holds promise for fabricating advanced micro-optical structures on fiber tips. To date, material selection has been limited to organic polymer-based photoresists because existing methods for 3D direct laser writing of inorganic materials involve high-temperature processing that is not compatible with optical fibers. However, organic polymers do not feature stability and transparency comparable to those of inorganic glasses. Herein, we demonstrate 3D direct laser writing of inorganic glass with a subwavelength resolution on optical fiber tips. We show two distinct 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 fields such as fiber sensing, optical microelectromechanical systems (MEMS), and quantum photonics.