Fig. 1
From: Statistical parity-time-symmetric lasing in an optical fibre network
A lumped-component model of a PT-symmetric laser cavity. a, A PT-symmetric structure formed of two homogeneous layers of refractive indices n g and n l , (corresponding to optical gain and loss, respectively) and equal thicknesses. PT-symmetry requires \({n_{\rm{g}}} = n_\ell ^*\). As a reference, the gain layer alone (corresponding to the reference cavity in Table 1) after removing the loss layer is shown on the right. b, A discrete model composed of lumped components to replace the continuum model in a: the interfaces are replaced with localised mirrors M1, M2 and M3, with reflectivities R 1, R 2 and R 3, respectively. The distributed gain and loss are replaced with an amplifier (amplification factor G) and an attenuator (attenuation factor \({\cal L}\)), respectively. PT-symmetry requires that R 1=R 3=R and \(G{\cal L} = 1\). The cavity corresponding to the gain layer alone is formed of the side mirrors containing the amplifier. c, Schematic representation of an experimental realisation of the PT-symmetric structured cavity shown on the left in b using single-mode optical fibres. Specially designed FBGs are used as partially reflecting mirrors with reflectivies R, R 2, and R from left to right. Gain is provided by a SOA and attenuation by a VOA. d, Optical setup in c after inserting an additional 1-km-long fibre spool. A polarisation controller (PC) is added to maintain the state of polarisation throughout the cavity
