Light Confinement in a 3D Space

The emerging services such as data center cloud inter­connection services, ultra-bandwidth video services, and 5G mobile services stimulate the fast development of photonic integrated circuits (PIC), which can meet the increasing demand of communi­cation systems for internet. However, PICs today are largely perceived as planar structures, able to guide light in a single plane. This planarity arises because of the traditional top-down fabri­cation processes. Multiphoton lithography is a new and promising 3D printing technology that allows for 3D objects to be fabricated more easily, compared to the fabri­cation of 3D objects in conventional cleanroom type fabrication methods used in electronics and opto­electronics.

Example of a high-resolution 3D waveguide to transcend the restrictions of light confinement in a single plane. (Source: SUTD)

With this technique, there is no longer a restriction of the top-down exposure for the realization of PICs as it unlocks the functions availed by the third dimension. Leveraging concepts of additive manufacturing, 3D multi-photon litho­graphy involves the use of a femtosecond light source to initiate two-photon polymerization when focused onto a specific location in material. This technique was used to realize the high-resolution 3D photonic structures. Researchers at the Singapore University of Technology and Design (SUTD) have demons­trated high-resolution 3D waveguides which transcend the restrictions of light confinement in a single plane. Gao Hongwei and colleagues at the photonics devices and systems group demons­trated high-resolution 3D wave­guides which guide light in a spiral and air-bridge confi­guration.

Alongside these novel devices, they also demons­trated very low loss 3D waveguide couplers with 1.6 dB fiber-waveguide coupling losses and 3 dB bandwidth exceeding 60 nanometers. This is in contrast with the current industry standards which require very labor intensive packaging for losses of around 1 dB. The research team demons­trated their losses to be low without requiring any post processing or post-fabri­cation packaging. The high-resolution fabrication also resulted in ring resonators with sub-micron feature sizes. “The fabri­cated photonic devices are an innovative advancement in the domain of photonic integrated circuits. Importantly, we were also able to demonstrate error-free 30 Gb/s NRZ and 56 Gb/s PAM4 data transmission through these waveguides. This is important because these high-speed testing formats and rates are in alignment with those used in commercial direct-detection transceiver products today,” explained group leader Dawn Tan.

Indeed, the team managed to derive only small power penalties of 0.7 dB for NRZ and 1.5 dB for PAM4 from the photonic devices. These results success­fully demonstrate high speed, error-free optical trans­mission through the 3D fabricated waveguides. This also showcases the devices’ suitability as low-loss wave­guides and optical interconnects. “Importantly, the 3D quality of these waveguides allows us to exceed the limitations of traditional planar structures. In this way, it is possible to achieve far higher density PICs. The high resolution, sub-micron feature sizes are also promising, especially to achieve advanced functions such as spectral filtering, resonator structures and meta­surfaces,” said Gao.

“This work demonstrates the potential of additive manu­facturing in making advanced photonic devices with superior 3D designs in high reso­lution,” added Low Hong Yee from SUTD. In the future, the capa­bility to realize high resolution 3D photonic structures may create even more advance­ments in both form and function in photonics, including advanced optical signal processing, imaging techniques and spectro­scopic systems. (Source: SUTD)

Reference: G. Hongwei et al.: High-Resolution 3D Printed Photonic Waveguide Devices, Adv. Opt. Mat., online 26 November 2020; DOI: 10.1002/adom.202070071

Link: Photonics Devices and Systems Group, Singapore University of Technology and Design SUTD, Singapore, Singapore

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