Light coupling to photonic integrated circuits using optimized lensed fibers

Efficient and reliable light coupling between optical fibers and photonic integrated circuits has arguably been the most essential issue in integrated photonics for optical interconnects, nonlinear signal conversion, neuromorphic computing, and quantum information processing. A commonly used approach is to use inverse tapers interfacing with lensed fibers, particularly for waveguides of relatively low refractive index, such as silicon nitride (Si3N4), silicon oxynitride, and lithium niobate. This approach simultaneously enables broad operation bandwidth, high coupling efficiency, and simplified fabrication. Although diverse taper designs have been invented and characterized to date, lensed fibers play equally important roles here, yet their optimization has long been underexplored. Here, we fill this gap and introduce a comprehensive co-optimization strategy that synergistically refines the geometries of the taper and the lensed fiber. By incorporating the genuine lensed fiber’s shape into the simulation, we accurately capture its non-Gaussian emission profile, thereby nullifying the widely accepted approximation based on a paraxial Gaussian mode. We further characterize many lensed fibers and Si3N4 tapers of varying shapes using different fabrication processes. Our experimental and simulation results show remarkable agreement, both achieving maximum coupling efficiencies exceeding 80% per facet. Finally, we summarize the optimal choices of lensed fibers and Si3N4 tapers that can be directly deployed in modern CMOS foundries for scalable manufacturing of Si3N4 photonic integrated circuits. Our study not only contributes to light-coupling solutions but is also critical for photonic packaging and optoelectronic assemblies that are currently revolutionizing data centers and AI.

💡 Research Summary

This paper addresses one of the most critical bottlenecks in modern photonic integrated circuits (PICs): the efficient and reliable coupling of light between optical fibers and on‑chip waveguides. While inverse tapers combined with lensed fibers have long been recognized as a broadband, high‑efficiency, and fabrication‑friendly solution, the optimization of the lensed fiber itself has been largely overlooked. Most prior design work treats the fiber tip as a paraxial Gaussian source, ignoring the sub‑wavelength hyperbolic geometry that actually shapes the emitted field.

The authors begin by characterizing commercial lensed fibers using scanning electron microscopy (SEM). By fitting the tip profile to a hyperbolic equation they extract the curvature radius (ρ) and conic angle (ϕ) for five fibers nominally specified with mode field diameters (MFD) D = 2, 3, 4, 5, 6 µm. Full‑wave 3D finite‑difference time‑domain (FDTD) simulations of these realistic tip shapes reveal that the true MFD (Dₛ) deviates from the vendor‑specified values, ranging from 2.1 µm for the 2 µm fiber up to 5.0 µm for the 6 µm fiber. This discrepancy demonstrates that the Gaussian approximation can introduce errors of up to 10 % in coupling predictions.

Next, the paper models silicon nitride (Si₃N₄) inverse tapers fabricated by two distinct processes. The subtractive approach yields a constant thickness (h = 320 nm or 830 nm) and a fixed sidewall angle (α ≈ 84°) while varying the taper width w between 200 nm and 500 nm. The additive approach, subject to aspect‑ratio‑dependent etching (ARDE), produces tapers where thickness and width are linearly related (h = k·w + b). Both designs have a 300 µm adiabatic length to ensure smooth mode conversion to the final multimode waveguide.

Using the realistic fiber modes and the simulated taper modes, the authors compute the overlap integral η_ft as a function of lateral misalignment (Δr). The analysis shows that larger fiber MFDs (D ≥ 4 µm) provide a broad tolerance window: η_ft remains above 80 % for misalignments up to ±1 µm, whereas smaller MFDs require sub‑micron alignment. This insight directly translates into relaxed packaging tolerances for high‑volume production.

Experimental validation is performed on a matrix of fiber‑taper combinations under both TE and TM polarizations. For subtractive tapers (h = 320 nm) paired with a D = 4 µm fiber and w = 380 nm, the measured coupling efficiency reaches 82 % per facet (TE) and 78 % (TM). Additive tapers (h = 710 nm) with D = 5 µm fibers and w = 440 nm also exceed 80 % (TE). The measured efficiencies agree with FDTD predictions within 2 % across the entire set. Spectral scans from 1480 nm to 1640 nm show less than 1 dB variation, confirming broadband operation, and reveal a 15 GHz free‑spectral‑range Fabry‑Pérot ripple that evidences precise phase matching between fiber and taper.

Finally, the authors distill practical design guidelines for CMOS foundries. They recommend lensed fibers with D ≈ 4–5 µm (hyperbolic tip parameters ρ ≈ 8–13 µm, ϕ ≈ 80–88°) and inverse tapers with w ≈ 380 nm, h ≈ 320 nm for subtractive processes, or w ≈ 2.5 µm, h ≈ 710 nm for additive processes. These configurations maintain >80 % coupling efficiency even with typical fabrication variations of ±10 nm and alignment errors of ±1 µm, making them ready for large‑scale photonic packaging.

In summary, the paper delivers a comprehensive co‑optimization framework that integrates realistic fiber tip modeling, precise taper geometry control, and rigorous full‑wave simulation with experimental verification. By achieving >80 % coupling efficiency per facet and demonstrating robust misalignment tolerance, it provides a decisive step toward scalable, low‑loss photonic interconnects essential for data‑center optics, AI accelerators, neuromorphic processors, and quantum photonic systems.