Nonlinear switching and optical limiting in a double-loop fibre Sagnac filter

Abstract

We investigated all-optical nonlinear switching and optical limiting properties of double-loop fibre Sagnac interferometers with symmetric power splitting. We show that significant nonlinear switching and optical power limiting can occur in this device due to nonlinear polarization rotation in the polarization maintaining (PM) fibres forming the Sagnac loops. This device can be considered as a lossless system when used as an optical limiter, since the excess input power, instead of being absorbed, would be reflected back and could be used for other purposes. For the same reason, this device would have higher damage threshold than the absorption-based optical limiters. Numerical simulations were conducted to explore the characteristics of this device. The switching and saturation power depends on the total length of fibres in the loop and the useful bandwidth depends on the polarization conditions and the birefringence of the PM fibres in the loop. We also investigated the intensity-dependent filtering property of this device and its relationship to the device bandwidth when used as either a nonlinear optical switch or an optical limiter.