Study of the polarization sensitivity of asymmetric directional couplers for optical bandpass filter and the TE/TM mode splitter application

Wavelength selective optical bandpass filter is a key component in the multi-wavelength transmission system. It has been demonstrated that such a filter can be made using two vertically coupled asymmetric waveguide couplers. To obtain a narrow filter bandwidth, a large asymmetry between the two waveguides is required. However, this causes the geometry induced birefringence and results in different coupling wavelength for the TE and TM modes. Due to the random polarization characteristic of the optic fiber, it is required that the bandpass filter at the receiver end be polarization insensitive in a practical communication system. Due to the inherent birefringent property of the optical waveguide, it remains a challenge to obtain a polarization insensitive filter using two coupled asymmetric rectangular waveguides. In the paper, the authors study the dependence of the TE and TM central wavelengths on the device parameters and explore the possibility of achieving a polarization insensitive filter based on the birefringent compensation