3D Printing of Glass Micro-Optics with Subwavelength Features on Optical Fiber Tips

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.

Keywords: 3D glass; direct laser writing; microstructured fiber; optical fiber sensing; polarization beam splitter.

Figure 1

Printing process and example 3D structures in glass on optical fiber tips. (a) The fabrication process. Step 1: Mounting single-mode optical fiber in a customized fiber holder. Step 2: Drop-casting HSQ solution on the optical fiber tip. Step 3: Evaporating solvent. Injecting a visible laser from the other end of the fiber to illuminate the fiber core for alignment. Step 4: Exposing the HSQ layer with the femtosecond pulsed laser. Uniform Mode and Nanograting Mode can be selected by choice of exposure parameters. (b) A woodpile structure printed using Uniform Mode. The inset shows a close-up of the printed structure: the lateral width of each beam is below 400 nm. (c) Characters “KTH” and three blocks printed using Nanograting Mode. The inset shows that the three segments of the letter “K” are made of Nanogratings with distinct selected orientations.

Figure 2

Characterization of the refractive index of a 3D-printed glass cube. (a) The colored SEM image shows the solid glass cube printed on the fiber tip. The inset is a close-up view of the cube. (b) The experimental setup for the refractive index characterization. (c) The reflection spectrum obtained from the fiber-tip glass cube immersed in a 50 wt % sucrose solution. (d) 2D profile plot shows the profile data measured by the optical profilometer. The plots were captured across the center of the cube, indicated by the dashed lines in (a).

Figure 3

3D-printed refractive index sensor on the optical fiber tip. (a) Colored SEM image of the fiber-tip refractive index sensor. (b) An enlarged view illustrating the working principle of the fiber-tip refractive index sensor. (c) Normalized reflection spectra of the sensor immersed in binary mixtures of acetone and methanol at different molar fractions: from left to right, χ_acetone increases from 0 to 1. (d) Refractive index of the binary mixtures are plotted against χ_acetone. The experimental data are fitted with a third-order polynomial model. Literature data measured at 589 nm are plotted for comparison.^,

Figure 4

3D-printed PBS on the optical fiber tip. (a) The colored SEM image of the fiber-tip PBS. The inset is a close-up view of the PBS, and the arrows indicate its orientation. (b) The experimental setup. (c) Output beam profiles recorded by the NIR camera with different experimental settings. From the first to the third row, the polarization of the input light was set to horizontal, vertical, and diagonal with respect to the orientation of the fiber-tip PBS, respectively. The profiles in the first column were recorded without an analyzing polarizer. The profiles in the second and the third column were recorded with the analyzing polarizer’s transmission axis set horizontal and vertical to the orientation of the fiber-tip PBS, respectively. The color scale bar on the top left indicates the intensity of light.