High performance micro-fiber coupler-based polarizer and band-rejection filter

Using full vector finite element method and super-mode theory, we analyzed the feasibility to fabricate micro-fiber-coupler-based optical polarizer. Our theoretical analysis showed that there exist a set of optimal pairs of two coupler geometric parameters, i.e. the coupling length and the micro-fiber diameter of the coupler, that can result in high performance polarizers. Experimentally, we fabricated three such coupler-based polarizers using the dual fiber drawing technique and characterized their performance. Our experimental measurement results confirmed our theoretical prediction in several aspects. When the diameter of the coupler-forming micro-fiber is relatively small (~3.5μm), the degree of polarization (DOP) of the fabricated polarizer was found relatively low (~50%) even over some coupling length range. However, when the diameter of the coupler-forming micro-fiber is larger (about 5μm to 9μm), a much higher DOP (>91.4%) and better linear polarization extinction ratio (LPER) of ~60dB could be achieved. The measured geometric parameters of two polarizer samples that showed high polarizing performance agreed very well with our theoretical values. Furthermore, we also demonstrated that such a coupler-based polarizer can be used as an optical filter as well. The filter exhibited an extinction ratio as high as 20dB at the center wavelength and the full width at half maximum (FWHM) was 10nm.