X. M. Xi

Orbital-angular-momentum-preserving helical Bloch modes in twisted photonic crystal fiber

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.

Measuring mechanical strain and twist using helical photonic crystal fiber

Solid-core photonic crystal fiber (PCF) with a permanent helical twist exhibits dips in its transmission spectrum at certain wavelengths. These are associated with the formation of orbital angular momentum states in the cladding. Here we investigate the tuning of these states with mechanical torque and axial tension. The dip wavelengths are found to scale linearly with both axial strain and mechanical twist rate. Analysis shows that the tension-induced shift in resonance wavelength is determined both by the photoelastic effect and by the change in twist rate, while the torsion-induced wavelength shift depends only on the change in twist rate. Twisted PCF can act as an effective optically monitored torque-tension transducer, twist sensor, or strain gauge.

Topological Zeeman effect and circular birefringence in twisted photonic crystal fibers

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 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.

Current sensing using circularly birefringent twisted solid-core photonic crystal fiber

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.

Enhanced optical activity and circular dichroism in twisted photonic crystal fiber

We demonstrate experimentally and theoretically that the core-guided mode in helically twisted photonic crystal fiber exhibits resonantly enhanced optical activity and circular dichroism in the vicinity of anti-crossings with leaky orbital angular momentum (OAM) modes in the cladding. This arises because the anti-crossings for left and right circularly polarized core modes occur at slightly different wavelengths.

Theory of optical activity in twisted photonic crystal fibers

In optically active materials and devices, the plane of vibration of linearly polarized light is rotated due to different phase indices of left- and right-circularly (LC and RC) polarized light, i.e., there is circular birefringence. Optical fibers with high circular birefringence are able to maintain circular polarization states against external perturbations such as bending and mechanical stress. In this paper, we present theoretical and experimental analysis of optical activity in twisted photonic crystal fibers (PCFs).

Optical Activity Enhanced by Orbital Angular Momentum Resonances in Helically Twisted PCF

We demonstrate that twisted solid-core PCF develops strongly enhanced optical activity and circular dichroism in the vicinity of orbital angular momentum resonances in the cladding. It may be used as a circular polarizer.

Optical Properties of Helical Photonic Crystal Fibre

Recent results on excitation of orbital angular momentum states, tuning of these states with mechanical torque and axial tension and optical activity in continuously twisted helical solid-core photonic crystal fibres, will be reviewed.

Helically Twisted Solid-Core Photonic Crystal Fibres

Recent results on optical activity in twisted six-fold symmetric solid-core PCF are discussed. The associated circular birefringence, which scales linearly with twist rate, arises from a non-resonant geometrical effect that is linked to angular momentum.