Excitation of modes in twisted single-ring PCF by prism-grating-coupling

Summary form only given. Single-ring hollow core photonic crystal fibre (SR-PCF) has created much interest over the past several years because of the simplicity of the structure, relatively low transmission losses and broad guidance bands. SR-PCF consists of a ring of thin-walled capillaries (inner diameter d) arranged symmetrically around a central hollow core (diameter D). It has been shown that if d/D = 0.682, the LPci-like core mode is strongly confined by anti-resonant reflection at the capillary ring, whereas higher order modes (HOMs), such as the LP<;sub>11<;/sub>-like mode, are resonant with the capillaries and leak strongly away [1]. This condition for HOM suppression can be tuned by spinning the fibre preform during drawing, so as to produce a helical structure. This extends the length of the spiralling capillaries, slowing down the axial speed of the light and permitting HOM suppression at values of d/D <; 0.682 [2].Here we report a curious effect seen when using prism-assisted side-coupling [3] to excite individual HOM core modes in a twisted SR-PCF with a strongly sub-optimal value of d/D = 0.375 (see Fig. 1(a)). The technique allows a precise axial wavevector to be set by adjusting the incident angle of a laser beam on the prism face. When a mode is excited, its phase index nm can be calculated and its modal field pattern directly imaged at the fibre endface. In an untwisted SR-PCF, the modes are very much as expected, mirroring the modes seen in a hollow capillary, at least for lower order HOMs. In the twisted SR-PCF in Fig. 1(a), however, we observe that the same LP11-like mode is excited at a series of tightly-spaced angles, translating into some 15 distinct modal indices spaced by a constant value of ~4.5X10-4 (D = 40 μm, d = 15 μm, capillary wall thickness 600 nm, wavelength λ = 1.299 μm and twist rate α ~ 349 rad·m-1). At first glance this suggests that many more modes exist in the twisted than in the untwisted SR-PCF, a result that seems rather curious.