In optical fiber telecommunications, there is much current work on the use of orbital angular momentum (OAM) modes for increasing channel capacity. Here we study the properties of a helically twisted photonic crystal fiber (PCF) that preserves the chirality of OAM modes of the same order, i.e., it inhibits scattering between an order +1 mode to an order -1 mode. This is achieved by thermally inducing a helical twist in a PCF with a novel three-bladed Y-shaped core. The effect is seen for twist periods of a few millimeters or less. We develop a novel scalar theory to analyze the properties of the twisted fiber, based on a helicoidal extension to Bloch wave theory. It yields results that are in excellent agreement with full finite element simulations. Since twisted PCFs with complex core structures can be produced in long lengths from a fiber drawing tower, they are of potential interest for increasing channel capacity in optical telecommunications, but the result is also of interest to the photonic crystal community, where a new kind of guided helical Bloch mode is sure to excite interest, and among the spin-orbit coupling community. (C) 2014 Optical Society of Americ

Abstract:In this paper we show that, in spectral regions where there are no orbital cladding resonances to cause transmission loss, the core mode of a continuously twisted photonic crystal fiber exhibits optical activity, and that magnitude of the associated circular birefringence increases linearly with twist rate and is highly reproducible. A theoretical model based on symmetry properties and perturbation theory is developed and used to show that both spin and orbital angular momentum play a role in this effect. An unexpected finding is that the degenerate LC and RC polarized modes of the untwisted PCF are not 100% circularly polarized, but contain a small amount of orbital angular momentum caused by the interaction between the core mode and the hollow channels.

Continuously twisted solid-core photonic crystal fiber (PCF) exhibits pure circular birefringence (optical activity), making it ideal for current sensors based on the Faraday effect. By numerical analysis, we identify the PCF geometry for which the circular birefringence (which scales linearly with twist rate) is a maximum. For silica-air PCF, this occurs at a shape parameter (diameter-to-spacing ratio of the hollow channels) of 0.37 and a scale parameter (spacing-to-wavelength) of 1.51. This result is confirmed experimentally by testing a range of different structures. To demonstrate the effectiveness of twisted PCF as a current sensor, a length of fiber is placed on the axis of a 7.6 cm long solenoid, and the Faraday rotation is measured at different values of dc current. The system is then used to chart the wavelength dependence of the Verdet constant.

The propagation of light guided in optical fibers is affected in different ways by bending or twisting. Here we treat the polarization properties of twisted six-fold symmetric photonic crystal fibers. Using a coordinate frame that follows the twisting structure, we show that the governing equation for the fiber modes resembles the Pauli equation for electrons in weak magnetic fields. This implies an index splitting between left and right circularly polarized modes, which are degenerate in the untwisted fiber. We develop a theoretical model, based on perturbation theory and symmetry properties, to predict the observable circular birefringence (i.e., optical activity) associated with this splitting. Our overall conclusion is that optical activity requires the rotational symmetry to be broken so as to allow coupling between different total angular momentum states.