Low Loss Fiber-Coupled Volumetric Interconnects Fabricated via Direct Laser Writing

Optics Engineering Electrical Engineering Information Science Semiconductors and Information Devices Physics
low loss fiber-coupled volumetric interconnects direct laser writing broadband polarization insensitive single-mode

Background Introduction

Photon integrated circuits (PICs) are significant for achieving high-speed data transmission. However, traditional photon integrated circuits, which only use a single plane or a limited number of stacked planes, are restricted in optical signal routing. Additionally, coupling losses need to be as low as possible for practical applications. Current photon integrated circuits are primarily built through planar fabrication technologies, including materials such as silicon on insulator (SOI), silicon nitride (SiN), and lithium niobate on insulator (LNOI). However, these methods often face issues such as high coupling losses in optical paths and the complexity of realizing 3D freeform pathways.

To overcome these constraints, the research team proposed a new fabrication method—SCRIBE (Subtractive Refractive Index by Beam Exposure) technology, capable of writing precise 3D gradient refractive index (GRIN) distributions within mesoporous silicon dioxide structures. The authors of this paper aim to use the SCRIBE technology to fabricate low-loss, broad-band, and polarization-insensitive single-mode 3D volumetric interconnect devices for optical fibers, and to realize waveguides in arbitrary 3D paths.

Paper Source

In July 2014, authors Alexander J. Littlefield, Jack Huang, Mason L. Holley, and others jointly published a research paper titled “Low loss fiber-coupled volumetric interconnects fabricated via direct laser writing”. This article was published in Volume 11, Issue 7 of the “Optica” journal and involved research departments such as Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign.

Research Workflow

The research process mainly includes the following steps:

  1. Material Selection and Preparation: Mesoporous silicon dioxide film was chosen as the base material. This material is formed by electrochemically etching a highly doped p-type silicon wafer and oxidizing it for 30 minutes at 900°C.

  2. Optical Device Design and Fabrication: A Nanoscribe Photonic Professional GT 3D printer was used to perform beam exposure within the mesoporous silicon dioxide film, printing waveguides, GRIN optical components, and lenses.

  3. Experimental Method: Positioning calibration and multiple exposure techniques greatly improved fabrication precision. Additionally, automatic alignment technology and the use of fibers with anti-reflective coatings significantly reduced the coupling loss from the fiber to the PIC.

  4. Data Collection and Analysis: For different optical devices, such as micro-ring resonators and Bragg reflectors, key parameters such as quality factor, propagation loss, and coupling loss were measured to assess the performance of optical devices and their improvements.

Detailed Research Results

  1. Waveguide Path Optimization: Optical alignment and multiple exposure techniques significantly reduced splice losses in the waveguide path from an initial 50dB to 2.14dB. On the other hand, by integrating subsurface lenses and GRIN waveguide gradients matching modes, the overall interconnect loss from fiber-to-waveguide-to-fiber was reduced from 50dB to 2.14dB. Excluding the loss of the fiber array, the total loss was only 1.47dB.

  2. Quality Factor and Bending Loss Optimization: The performance of micro-ring resonators under different radius conditions showed a significant increase in the quality factor. For instance, for a micro-ring resonator with a radius of 30 µm, the quality factor was raised from 4,600 to 77,000, with bending loss reduced to 3 dB/cm.

  3. Polarization Rotation and Wavelength Splitting Achievement: The study demonstrated the functionality of polarization rotation and wavelength splitting interconnect devices. Specifically, a 17-layer MacNeille film polarizing beam splitter was fabricated. This novel splitter exhibited good performance across a broad bandwidth of 1450 to 1700 nm, with a splitting ratio of +29.3 dB and −11.7 dB at 1550 nm.

  4. Multi-Channel Array Interconnect Circuit Achievement: Independent seven-channel interconnect circuits were successfully fabricated on a hexagonal array with a 25 µm pitch. The coupling from fiber to chip was good across all channels, with uniform transmission loss per channel and no noticeable crosstalk between channels.

Conclusion and Application Value

This paper fully demonstrates the potential applications of the SCRIBE technology in 3D photon integrated circuits (3D PICs). The main scientific value of this technology is that it enables the interconnection of optical waveguides in arbitrary 3D paths and integrates various micro-optical components, significantly reducing fiber coupling losses. Moreover, this research provides a feasible technological approach for realizing new 3D photonic devices, opening new doors for applications such as high-density optical interconnects and layout transitions in future telecommunications and data centers.

Highlights Summary

  1. Significantly Reduced Coupling Loss: Through the use of subsurface lenses and GRIN waveguide gradient techniques, the coupling loss from fiber to waveguide was reduced to 0.45 dB.

  2. Improved Quality Factor for Waveguides and Micro-Ring Resonators: Multiple exposure techniques and precise positioning calibration greatly improved the performance of waveguides and micro-ring resonators, achieving a quality factor of 77,000 and reducing bending loss to 3 dB/cm.

  3. Polarization Rotation and Wavelength Splitting Functionality: Successfully achieved polarization rotation and wavelength splitting devices, demonstrating that complex optical functions can be integrated with SCRIBE technology.

Future Research Directions

Future research directions may include further integration of III-V and SOI photon circuits through SCRIBE technology, investigating higher refractive index photoresists to reduce bending loss and improve the focusing efficiency of lenses. Additionally, more efficient parallel printing systems could be explored to meet the needs of large-scale production.

This research takes a critical step in the field of fiber-coupled photon integrated circuits, laying a solid foundation for the development of future 3D photonic technologies.