Simple and fully CMOS-compatible low-loss fiber coupling structure for a silicon photonics platform

Area of Science:

• Photonics and Optical Engineering
• Materials Science
• Semiconductor Device Fabrication

Background:

• Efficient fiber-to-chip coupling is crucial for integrated photonic circuits.
• Existing coupling structures often suffer from high losses and polarization dependence.
• Complementary metal-oxide semiconductor (CMOS)-compatible fabrication is desired for scalable manufacturing.

Purpose of the Study:

• To design and fabricate a simple, low-loss fiber coupling structure.
• To minimize polarization-dependent losses in fiber-to-chip connections.
• To demonstrate the viability of CMOS-compatible processes for advanced photonic integration.

Main Methods:

• Fabrication of a silicon (Si) inverted-taper waveguide integrated with a silicon nitride (SiN) waveguide.
• Design of a SiN waveguide with dimensions of 435 nm width and 290 nm thickness.
• Characterization of fiber-to-chip coupling losses for quasi-TE and quasi-TM modes at 1550 nm wavelength.
• Measurement of polarization-dependent losses across a 1520-1580 nm wavelength range.

Main Results:

• Achieved low fiber-to-chip coupling losses of 0.25 dB/facet for quasi-TE and 0.51 dB/facet for quasi-TM modes.
• Demonstrated minimal polarization-dependent losses (<0.3 dB for Si-to-SiN transition, <0.5 dB for fiber-to-chip coupling).
• The SiN waveguide effectively expanded the optical field to match a 4.1 µm mode-field diameter fiber.

Conclusions:

• The developed Si/SiN waveguide structure offers a simple and effective solution for low-loss fiber coupling.
• CMOS-compatible fabrication enables scalable production of high-performance photonic integrated circuits.
• The low polarization dependence makes this structure suitable for a wide range of optical communication applications.