Fig. 1.

Capillary waveguide (CWG): (a) Light escaping the CWG: a closer look to the silica annular part show the speckle diffraction pattern characteristic of multimode waveguides. Scale bar 500 μm; (b) Schematic of the side cross–sectional view of a CWG: the silica annular part of the CWG has a refractive index n₂ higher than the cladding (n₁) and the hollow core (n₃, usually air or water), so light can be guided by total internal reflection. The hollow core adds a degree of freedom to this kind of waveguides respect to common optical fibers. The overall diameter of the CWG is 330 μm. The cladding is 10 μm-thick. The remaining part is composed of the silica part and the hollow core. In this work we used inner diameters of 150 μm (silica part 76.5 μm-thick) and 100 μm (silica part 106.5 μm-thick).