摘要： Single-ring hollow-core photonic crystal fibre (SR-PCF), consisting of a ring of thin-walled glass capillaries surrounding a central hollow core, can offer remarkably low transmission loss [1], and is finding applications in, e.g., wavelength conversion and pulse compression in gases, high-power beam delivery and circular dichroism [2]. As with all microstructured fibres, it is highly desirable to continuously measure the internal structural parameters (e.g. the capillary diameter) during fibre drawing. This would improve the yield of useful fibre lengths, as well as offering better control of structural uniformity along the fibre. Successful tapering of hollow-core fibres also requires a non-destructive method of verifying structural integrity along the taper. We here report for the first time a non-invasive technique for monitoring the capillary diameter in SR-PCF during the draw. The technique relies on excitation of whispering gallery modes (WGMs) in the capillary walls. The frequency spacing between successive WGMs fulfils the well-known condition: (cid:16)(cid:81)(cid:81)1l(cid:14)l(cid:32)cLcap(cid:173)(cid:176)(cid:174)(cid:176)(cid:175)lnl(cid:14)1,m1(cid:14)((cid:81)l(cid:14)1,(cid:16))mnl,m(cid:81)l,m(cid:189)(cid:176)(cid:190)(cid:176)(cid:191)(cid:124)cLncapWGM(cid:81)(1) where c is the speed of light in vacuum, Lcap is the circumference (~2(cid:652)R, where R is the mean radius), l the azimuthal and m the radial mode order, and nl,m is the effective index of the mode. Both approximate analytical solutions to nl,m and full numerical simulations show that, for the SR-PCFs studied, the approximation in Eq.(1) is accurate to within ±0.9% if nWGM is set to 1.484. As illustrated in Fig. 1 (right), the fibre is illuminated transversally by a broadband light source, thereby exciting WGMs inside selected capillaries. By placing a light collection fibre (400 μm core diameter) at a suitable position ~1 cm from the SR-PCF, the WGM spectrum originating from one of the capillaries could be selectively measured. A fast Fourier transform could then be used, together with Eq. (1), to compute the effective capillary diameter 2R in real-time with sub-micron accuracy. The results shown in Fig. 1 (left) were made at a fibre drawing speed of 32.3 m/min and a spectrometer integration time of 70 ms, corresponding to an axial resolution of <4 cm. We also used the technique to measure the diameter of individual capillaries at points along a tapered hollow-core fibre (not shown here).