Spatial division multiplexing technologies provided a significant increase of the throughput of fiber links involved in optical telecommunications. Two main approaches were proposed based on multicore (MCFs) and multimode fibers (MMFs). These types of optical fibers are currently used in a wide variety of technical and scientific applications, including biomedical imaging, fiber lasers and high-power beam delivery. MMFs have large single core supporting multiple optical modes, while MCFs consist of a set of small cores (generally, single-mode) located under a common cladding.

Recently we have demonstrated the possibility of effective energy concentration with significant light pulse amplification and temporal pulse compression in a certain core after the propagation of high-power pulses along a MCF [1], [2]. Such nonlinear phenomenon can be utilized for the development new classes on high-power fiber lasers and optical switches. First experiments demonstrating the nonlinear compression of optical pulses in MCFs have recently been carried out; on the other hand, the theory of the propagation of soliton solutions was generalized to these fibers [3], [4], [5], [6], [7].

In addition, MMFs and MCFs have great potential for increasing, via spatial division multiplexing, the information transmission capacity of fiber optic links over long distances. Due to a possibility to enlarge the bandwidth proportionally to the number of modes (cores) these fibers provide a possibility to reach record performance of transmission lines [8], [9], [10] and get hundredfold capacity increase relative to conventional single-mode fiber systems [11], [12].

An interesting phenomenon which is associated with multicore fibers is the existence of soliton-like high-energy pulses localized both in time and space, called in the literature light bullets. They can be described as spatio-temporal soliton solutions of a suitable system of coupled nonlinear Schrödinger equations (CNLSEs), so that light bullets spread along the MCF without distortion of their shape. They are formed due to the mutual compensation of different effects such as diffraction, anomalous dispersion, and nonlinearity. The existence of light bullets was demonstrated in experiments [13], [14], [15].

The concept of Parity-Time (PT) symmetry was introduced in works [16], [17], [18] in the context of quantum operators. It declares that there are physical systems which are described by Hamiltonians symmetric with respect to the so-called PT transformations, defined by the parity operator P as Pψ(r,t)=ψ(−r,t), and the time-reversal operator T, as Tψ(r,t)=ψ∗(r,−t), where ψ(r,t) is a wave function in quantum mechanics. In optics, the propagation of light can often be described in terms of an evolution equation with an effective Hamiltonian [19]. Thus, waveguiding systems combining gain and loss regions are described by non-Hermitian Hamiltonians, however if these Hamiltonians are PT-invariant, such systems can still have an entirely real spectrum. This property leads to the fact that the amplitudes of optical eigenmodes are preserved, which means an effective compensation of loss and gain effects.

Another important feature of PT-symmetric systems is the presence of an exceptional point in a parameter plane, where eigenvalues and eigenvectors of the system merge. This point determines so-called PT-symmetry breaking threshold, above which the spectrum ceases to be entirely real (broken symmetry). In the vicinity of this point, light propagation in the system is highly sensitive to small perturbations, hence the waveguide can be employed as a sensor.

In optics, the property of PT-symmetry can be obtained by means of a properly designed complex potential, V(x)=V(−x)∗, where the real part of the potential corresponds to the refractive index, while the imaginary part describes gain and loss regions, respectively [20], [21]. These requirements are met by alternating waveguides with loss and gain at the periphery, as well as with a lossless (or conservative) central waveguide. PT-symmetric multicore structures were previously studied in several works [22], [23], [24], [25]. However, to the best of our knowledge, a MCF configuration including a central lossless core and twisting peripheral cores with loss and gain, was not yet investigated. As we shall see, such type of structure provides additional capabilities for light control in multicore fibers. Specifically, in this work we study spatiotemporal beam propagation in a twisted MCF with PT-symmetry and a centrally symmetric type of placement of the cores (see Fig. 1).