Polarization-independent, thickness-insensitive, and refractive-index-difference-independent broad-band fluorinated polyimide waveguide coupler

A statistically optimized design method is proposed for the construction of polarization-independent, thickness-insensitive, and refractive-index-difference-independent broad-band waveguide couplers. Using this method, a fluorinated polyimide waveguide 3-dB coupler for a 1490-1610-nm operation is designed through optimization of the polarization, variation in thickness of the waveguide core, and variation in refractive-index difference of the waveguide core and cladding. The performance of the design is verified through simulation based on the three-dimensional beam-propagation method (3-D-BPM). The two orthogonal polarizations of the final design have a coupling ratio of (50±2.0)% for a bandwidth of 120 nm, a thickness fluctuation of 7-8 μm, and a refractive-index-difference fluctuation of 0.24-0.30%.