Optimization of coupling interfaces between nanophotonics waveguides and optical fibers
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Wireless, photonics and space engineering (MPWPS), MSc
Publicerad
2025
Författare
Haidar Hussein, Ali
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
The integration of photonic components into compact and efficient circuits necessitates effective coupling mechanisms between optical fibers and on-chip waveguides.
This thesis focuses on optimizing the coupling efficiency between silicon nitride
(Si3N4) waveguides and optical fibers using linear adiabatic edge tapers. Addressing
the significant challenge of minimizing coupling losses, the study explores the interplay between taper geometry, cladding thickness, taper and physical gaps for both
standard single-mode fibers (SMFs) and lensed fibers.
Utilizing both advanced simulation tools—Finite-Difference Eigenmode (FDE), Eigenmode Expansion (EME), and Finite-Difference Time-Domain (FDTD) methods—and
experimental validation, the research systematically investigates the parameters influencing coupling efficiency. FDE simulations provide initial insights into mode
mismatch losses due to mode field diameter (MFD) disparities. EME simulations
are employed to optimize taper lengths, ensuring adiabatic mode transformation
with minimal loss. FDTD simulations offer a comprehensive 3D analysis, capturing detailed electromagnetic interactions at the coupling interface. Experimental
prototypes were fabricated and tested to validate the simulation results and assess
practical fabrication considerations.
The results demonstrate that optimal coupling for SMFs is achieved with a taper
length of approximately 1mm, a facet width between 100nm and 150nm (with 125nm
yielding the lowest loss), and a cladding thickness of 6µm, resulting in coupling losses
around 0.75dB. For lensed fibers, both simulations and experiments indicate that a
taper length of 50µm, a facet width between 230nm and 290nm (with 290nm yielding the losses in the simulations around 0.37 dB per facet and an experimental loss
of around 1.1dB per facet which is possible to reduce even further), and a cladding
thickness of 3µm yield optimal coupling efficiency. The slight discrepancies between
simulated and experimental results are attributed to fabrication imperfections, including sidewall roughness and taper dimension deviations.
The study underscores the critical impact of physical gaps and beam divergence
on coupling efficiency, emphasizing the necessity of precise alignment and minimal
separation in practical applications. Experimental results confirm that minimizing
the taper and physical gaps is essential for lensed fiber coupling, with coupling losses
yielding a constant value for gaps that are larger than 0.5µm.
The thesis identifies limitations related to fabrication imperfections and suggests
avenues for future research, including broadband performance analysis and the incorporation of fabrication-induced variations into simulations. The findings offer
valuable design considerations for enhancing fiber-to-chip coupling in Si3N4 phov
tonic integrated circuits, contributing to the advancement of efficient and scalable
optical communication technologies.
Beskrivning
Ämne/nyckelord
(Silicon Nitride Si3N4) waveguides, Photonic Integrated Circuits, Linear Adiabatic Edge Tapers, Fiber-to-Chip Coupling, Single Mode Fibers, Lensed Fibers, Finite-difference time-domain ,Eigenmode Expansion, Mode Field Diameter