Theoretical investigation of the NOLM with highly twisted fibre and a λ/4 birefringence bias

We investigate theoretically the operation of a novel, power-symmetric nonlinear optical loop mirror (NOLM) scheme, which relies on nonlinear polarisation rotation. The device is made of a symmetrical coupler, highly twisted fibre and a quarter wave plate (QWP) located asymmetrically in the loop, which can be rotated in a plane perpendicular to the fibre. Using a weak nonlinearity approach, the nonlinear NOLM transmission is accurately determined through a simple matrix description. For substantial twist, a simple though general analytical expression of the NOLM power transmission is found. This transmission strongly depends on the QWP angle, but also on the polarisation state at the NOLM input. This analysis allows to infer interesting properties of the switching characteristic, in particular its flexibility and, in some cases, its stability against small changes of input polarisation. The QWP angle and the input polarisation state are the control parameters through which the transmission can be adjusted in a versatile way, making the device attractive for various applications. Although the polarisation obtained at the NOLM output is power-dependent in general, we show that, for particular configurations, the output polarisation is constant with power. This is particularly interesting in the frame of polarisation-sensitive applications. We believe that the proposed model will be very useful to optimise the design and control of the proposed NOLM structure, which has the potential to play an important role in future ultrafast optical communication systems.