Efficient spot-size transformation using spatially separated tapered InP/InGaAsP twin waveguides

A tapered twin waveguide structure for low-loss chip-fiber coupling with a large spatial separation between the waveguides was fabricated, providing better optical properties than conventional taper structures together with relaxed fabrication tolerances. The mode transformation losses and coupling losses for different taper geometries were investigated theoretically and experimentally. For a double taper structure with a length of 280 μm for a single taper, the total measured insertion loss between two single-mode fibers at a wavelength of 1550 nm using fiber matched refractive index liquid, however, without any antireflection coating, was 3.2 dB