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In-Situ 3D Nano-Printing of Freeform Coupling Elements for Hybrid Photonic Integration
P. -I. Dietrich, M. Blaicher, I. Reuter, M. Billah, T. Hoose, A. Hofmann, C. Caer, R. Dangel, B. Offrein, U. Troppenz, W. Freude, C. Koos
TL;DR
Printed micro-optics face stringent challenges in coupling and alignment. The paper uses in-situ printed beam-shaping elements for fiber-optic assemblies, achieving low coupling losses while relaxing alignment requirements, with incomplete TIR remaining a limitation.
Problem
Printed micro-optics face stringent challenges, including the need to relax alignment tolerances in optical assemblies.
Method
The LIMOS approach fabricates freeform beam-shaping elements at optical facets for a wide range of fiber-optic assemblies, including beam-expanding multi-lens systems.
Results
Coupling losses reach down to 0.6 dB, while beam expansion supports ± 5.5 µm tolerances compatible with high-throughput passive assembly.
Takeaways & Limitations
The approach supports fiber-optic assemblies with relaxed alignment requirements and coupling loss of 1.0 dB at the optimum fiber position.
Takeaways & Limitations
Large laser-beam divergence causes incomplete total internal reflection at the mirror surface, contributing to loss.
Abstract
from arXiv · showhide
Hybrid photonic integration exploits complementary strengths of different material platforms, thereby offering superior performance and design flexibility in comparison to monolithic approaches. This applies in particular to multi-chip concepts, where components can be individually optimized and tested on separate dies before integration into more complex systems. The assembly of such systems, however, still represents a major challenge, requiring complex and expensive processes for high-precision alignment as well as careful adaptation of optical mode profiles. Here we show that these challenges can be overcome by in-situ nano-printing of freeform beam-shaping elements to facets of optical components. The approach is applicable to a wide variety of devices and assembly concepts and allows adaptation of vastly dissimilar mode profiles while considerably relaxing alignment tolerances to the extent that scalable, cost-effective passive assembly techniques can be used. We experimentally prove the viability of the concept by fabricating and testing a selection of beam-shaping elements at chip and fiber facets, achieving coupling efficiencies of up to 88 % between an InP laser and an optical fiber. We also demonstrate printed freeform mirrors for simultaneously adapting beam shape and propagation direction, and we explore multi-lens systems for beam expansion. The concept paves the way to automated fabrication of photonic multi-chip assemblies with unprecedented performance and versatility.
Concept, design, and fabrication
The paper develops in-situ 3D-printed freeform beam-shaping elements as adaptable interfaces for hybrid photonic assemblies. The toolbox includes lenses, mirrors, beam expanders, and multi-lens systems designed to match mode profiles and relax alignment requirements.
- Design principle: Freeform structures provide design freedom for matching the vastly different mode-field profiles of hybrid photonic components.The approach targets chip-chip and fiber-chip coupling across dissimilar device platforms.
- Design toolbox: Printed curved mirrors adapt beam propagation directions, enabling interfaces such as edge-emitting lasers to surface grating couplers.The mirrors use precisely defined freeform surfaces for beam deflection.
- Alignment: Beam expansion transforms small facet modes into larger collimated beams, enabling alignment tolerances of ± 5 µm or above for passive assembly.This supports high-throughput alignment techniques in multi-chip systems.
- Design toolbox: The demonstrated toolbox includes single-surface lenses, total-internal-reflection mirrors, beam expanders, and nine-surface multi-lens optics.These elements can be printed on laser or single-mode-fiber facets and designed for assembly-tolerance relaxation.
- Performance and implementation: Printed components withstand powers up to 3 W and maintain constant coupling efficiency over an optical bandwidth exceeding 100 nm.Low-index cladding reduces reflection and protects the printed optical surfaces, although it also reduces individual-interface refractive power.
Experimental verification and discussion
Experiments validate facet-printed lenses, mirrors, and multi-lens expanders for efficient coupling, beam redirection, and relaxed alignment. The demonstrations span laser–fiber, fiber–fiber, and surface-emitting-device interfaces.
- Beam-shaping toolbox: Facet-attached lenses adapt mode fields for edge-coupled interfaces, while freeform mirrors combine mode-profile adaptation with propagation-direction control.The beam-shaping toolbox also includes beam expanders for relaxing assembly tolerances.
- Facet-attached lenses: ±1.9 µm horizontal and vertical, and ±25 µm axial, were the measured 1 dB positioning tolerances for a laser-facet lens.These results were close to simulation and comparable to complex ion-beam-milled structures not suited to industrial production.
- Facet-attached lenses: 0.6 dB loss (η = 88 %) was measured for optimum coupling between an InP laser and a fiber-mounted freeform lens.This exceeded the 80 % efficiency reported for best-in-class lensed fibers.
- Freeform mirrors: 2.9 dB loss (η = 51 %) for the laser-facet mirror was mainly attributed to incomplete total internal reflection caused by the laser beam’s large divergence.The text identifies higher-index single-TIR designs or reflective coatings as possible ways to overcome this limitation.
Summary and outlook
The paper demonstrates a toolbox of printed freeform optical elements that enables efficient, position-tolerant coupling across hybrid photonic assemblies. The experiments include lenses, mirrors, and multi-lens beam expanders, with coupling losses as low as 0.6 dB and alignment tolerances compatible with passive assembly.
- Summary and outlook: Printed freeform lenses, mirrors, and multi-lens beam expanders provide beam shaping for hybrid optical assemblies.The elements are printed directly onto device facets and are intended to relax positioning tolerances sufficiently for passive alignment.
- Summary and outlook: 0.6 dB coupling loss was achieved using single freeform lenses printed onto laser or fiber facets.These experiments demonstrated the viability of facet-attached beam-shaping lenses.
- Summary and outlook: 1.1 dB coupling loss was achieved between VCSELs and single-mode fibers using printed freeform total-internal-reflection mirrors.The mirrors can be attached to either the laser or fiber end face and support compact coupling between surface- and edge-emitting components.
- Summary and outlook: 1.9 dB coupling loss and ±5.5 µm lateral 1 dB alignment tolerance were demonstrated between a pair of single-mode fibers.The measured tolerance is compatible with high-throughput passive assembly techniques.
- Summary and outlook: Printed beam-shaping elements can be transferred to edge-emitting and surface-emitting devices for large-scale passive alignment.The demonstrated components also allow compact combinations of surface- and edge-coupled devices.
- Summary and outlook: 1.0 dB coupling loss was obtained with paired LIMOS beam expanders comprising 18 lens surfaces.The loss was below the minimum theoretically achievable with lenses operated in air, and the authors expect further reduction of reflection loss.
Additional Information
Supplementary Information is available online.
- Additional Information: Supplementary Information is available online.
Competing financial interests
The authors disclose financial and intellectual-property interests related to commercialization of the reported technology.
- Competing financial interests: P.-I. D. and C. K. are co-founders and shareholders of Vanguard Photonics GmbH.The startup company is engaged in exploiting 3D nano-printing for photonic integration and assembly.
- Competing financial interests: The declared interests concern both a startup company and patents related to the reported photonic integration technology.
- Competing financial interests: P.-I. D., M. B., I. R., and C. K. are co-inventors of patents owned by KIT in the publication’s technical field.
Methods
The methods combine physical-optics simulation, two-photon lithographic fabrication, and power-based coupling measurements for printed beam-shaping elements. The study evaluates lenses, mirrors, and beam expanders under specified material, geometry, and alignment assumptions.
- Simulation: Physical-optics simulations optimize freeform components for maximum coupling efficiency in each configuration.The simulations use assumed mode fields and vary surface parameters.
- Simulation: TIR mirror surfaces are represented by polynomial profiles in a coordinate system tilted 45° relative to the optical axis.Optimization assumes ideal specular reflection and varies polynomial parameters.
- Fabrication: All structures are fabricated with a commercial two-photon lithography system using a 40×, numerical-aperture-1.4 objective and IP-Dip photoresist.The dedicated software targets high shape fidelity and automated alignment; the liquid resist has refractive index n = 1.52 and acts as an immersion medium.
- Coupling experiments: Coupling efficiency is determined from integrating-sphere measurements of free-space and fiber-coupled power.Measurements compare bare and lens-equipped devices where applicable, with stated Fresnel-loss corrections.
- Coupling experiments: 0.6 dB is the best achievable coupling loss predicted for the current laser-facet lens devices, while fiber-facet lenses have a 0.2 dB theoretical minimum.The laser-facet estimate includes residual mode mismatch, Fresnel reflection, and post-fabrication laser-power reduction; the fiber-facet limit is set by Fresnel reflection.