Spherical concave micro-mirror fabricated using gray-tone optical lithography for vertical coupling

Area of Science:

• Optical Engineering
• Nanotechnology
• Photonics

Background:

• Low vertical coupling loss is critical for efficient optical interconnects.
• Fabricating micro-scale optical components requires precise alignment and surface control.
• Rectangular optical waveguides (ROWs) are used in various photonic integrated circuits.

Purpose of the Study:

• To design and fabricate a spherical concave micro-mirror at the end of a ROW.
• To minimize vertical coupling loss using optimized micro-mirror structures.
• To investigate the impact of surface roughness and alignment tolerances on coupling loss.

Main Methods:

• Gray-tone optical lithography and overlay alignment techniques were employed.
• Ray-tracing simulations were used to optimize micro-mirror structures.
• Fabricated micro-mirror dimensions and surface roughness were measured.
• Vertical coupling loss and alignment tolerances were experimentally determined.

Main Results:

• The minimal vertical coupling loss was simulated to be 1.02 dB for a 20 μm × 20 μm ROW core.
• Surface roughness below 106 nm is required to maintain coupling loss under 1.5 dB.
• Fabricated micro-mirrors exhibited a radius of 263.3 μm and surface roughness of 29.19 nm.
• Measured vertical coupling loss was 1.39 dB, with 1-dB tolerances of ±6.9 μm (x), ±6.3 μm (y), and 46.2 μm (z).

Conclusions:

• The fabricated spherical concave micro-mirror effectively reduces vertical coupling loss in ROWs.
• The optimized design and fabrication process demonstrate high precision and performance.
• The results provide valuable insights for the design of efficient optical coupling components.