[Paper Review] Free-form micro-optics enabling ultra-broadband low-loss fiber-to-chip coupling

Shaoliang Yu, Luigi Ranno+6 more|arXiv (Cornell University)|Dec 29, 2021

Photonic and Optical Devices4 citations

TL;DR

This paper presents a free-form reflective micro-optics approach for ultra-broadband, low-loss fiber-to-chip coupling, achieving 0.5 dB loss at 1550 nm and a record 300 nm 1-dB bandwidth spanning O to U bands. The method uses a simplified two-simulation optimization framework to design high-performance reflective micro-optics, enabling high alignment tolerance and solder reflow compatibility for scalable photonic packaging.

ABSTRACT

Efficient fiber-to-chip coupling has been a major hurdle to cost-effective packaging and scalable interconnections of photonic integrated circuits. Conventional photonic packaging methods relying on edge or grating coupling are constrained by high insertion losses, limited bandwidth density, narrow band operation, and sensitivity to misalignment. Here we present a new fiber-to-chip coupling scheme based on free-form reflective micro-optics. A design approach which simplifies the high-dimensional free-form optimization problem to as few as two full-wave simulations is implemented to empower computationally efficient design of high-performance free-form reflectors while capitalizing on the expanded geometric degrees of freedom. We demonstrated fiber array coupling to waveguides taped out through a standard foundry shuttle run and backend integrated with 3-D printed micro-optics. A low coupling loss down to 0.5 dB was experimentally measured at 1550 nm wavelength with a record 1-dB bandwidth of 300 nm spanning O to U bands. The coupling scheme further affords large alignment tolerance, high bandwidth density and solder reflow compatibility, qualifying it as a promising optical packaging solution for applications such as wavelength division multiplexing communications, broadband spectroscopic sensing, and nonlinear optical signal processing.

Motivation & Objective

• To overcome the limitations of conventional edge and grating coupling in photonic integrated circuits, such as high insertion loss and narrow bandwidth.
• To develop a scalable, low-loss fiber-to-chip coupling solution compatible with standard foundry processes and high-volume packaging.
• To enable ultra-broadband operation across O to U bands while maintaining low loss and high alignment tolerance.
• To demonstrate a computationally efficient design methodology for complex free-form reflective micro-optics.

Proposed method

• A two-simulation optimization framework is used to simplify the high-dimensional free-form design problem, reducing computational cost while maintaining performance.
• Free-form reflective micro-optics are fabricated via 3D printing and integrated with waveguides in a backend process after standard foundry fabrication.
• The design leverages expanded geometric degrees of freedom to shape wavefronts for optimal mode matching between fiber and chip waveguides.
• The method enables precise control of beam divergence and phase profile to minimize coupling loss across a broad spectrum.
• The fabricated system uses a fiber array coupled to a waveguide array via 3D-printed micro-optics, validated through experimental testing.

Experimental results

Research questions

• RQ1 Can free-form reflective micro-optics achieve ultra-broadband, low-loss fiber-to-chip coupling beyond the limitations of conventional edge and grating couplers?
• RQ2 How can the high-dimensional design space of free-form optics be efficiently optimized without requiring extensive full-wave simulations?
• RQ3 What is the achievable bandwidth and loss performance of a fiber-to-chip coupling scheme using 3D-printed free-form micro-optics?
• RQ4 To what extent does the proposed coupling scheme tolerate alignment errors and survive reflow soldering processes?

Key findings

• A coupling loss as low as 0.5 dB was experimentally measured at 1550 nm, demonstrating high efficiency in fiber-to-chip coupling.
• The system achieved a record 1-dB bandwidth of 300 nm, spanning the O-band (1260–1360 nm) to the U-band (1570–1625 nm).
• The coupling scheme exhibits large alignment tolerance, making it suitable for high-volume, low-cost packaging.
• The design is compatible with solder reflow processes, enabling integration with standard electronic packaging flows.