Polarization-independent "figure-eight" birefringent Sagnac variable comb-filter/attenuator

We propose and theoretically analyze a multifunctional integrated optical device based on figure-eight shaped birefringent Sagnac loops. Our analysis shows that the propose device exhibits many unique features which allows it to operate as a tunable high-Q comb filter with a good channel isolation. In addition, the intensity transmission of the filter can be varied from 0% to 100%. It is also shown that the different combinations of the polarization rotations in the loops allow tunability in 1) comb filter channel spacing; 2) transmission attenuation; 3) dc offset; 4) filter modulation depth; and 5) channel isolation. The device can be easily implemented based on optical fibers or integrated waveguide structures.