Drawing of a solid-core photonic crystal (or microstructured) fiber with an outer diameter of (drawn to scale). The inset shows a scanning electron micrograph of the cleaved fiber core and surrounding air channels (scale bar: ).
PCF tip resulting from the tube etching method under experimental conditions similar to those described in Ref. 28. (a) Side view showing the recessed primary tip. The fingerlike geometry of the outer cladding is due to the transitioning of the hexagonal symmetry of the microstructured cladding to the original cylindrical symmetry of the outer cladding (scale bar: ). (b) View down into the well enclosed by the outer cladding. The central tip is comprised of the core region but recessed compared to the outer cladding. The secondary tips are formed from the remains of the microstructured cladding region (scale bar: ).
Mechanism of tip formation based on sealed-tube etching of PCFs. (a) Schematic of the tip formation process (not to scale): The PCF is sealed and immersed into the etching solution; HF starts to diffuse through the acrylic polymer coating toward the outer cladding of the pure silica fiber. The outer cladding is etched away, and breakthrough to the outermost layer of air channels occurs. Etchant is drawn into these channels by capillary forces, leading to vertical as well as lateral etching action until the next layer of air channels is reached. Finally, the core is attained and thinned out, leaving a tapered core tip behind. (b) Optical microscopy images at different stages of the tip formation process, roughly corresponding to the events depicted in (a).
PCF tip resulting from sealed-tube etching (80 min at ). (a) Side view showing the primary tip extending beyond the rim of the outer cladding (scale bar: ). (b) Magnified view of the PCF tip at an angle of . The protruding tip is formed from the core region which is confirmed by its hexagonal structure which results from the geometry of the microstructured cladding. The secondary tips are formed from the remnants of the air channels making up the microstructured cladding (scale bar: ).
Optical image ( magnification) of a PCF tip with 514.5 nm laser light coupled into the PCF fiber. It can be seen that the transmitted light is concentrated at the end of the protruding central tip; a longer PCF tip was chosen for clarity.
Composite image of the axial transmission from a PCF tip as seen on a CCD camera, overlaid with a 1:1 model of the PCF fiber cross section (line art); scale bar: . Note that the outer cladding region is almost completely etched away and extends only a few microns beyond the microstructured cladding region, as indicated by the dashed line. The transmitted light is clearly confined to the core region, as is also evident from the horizontal line scan through the intensity maximum (full width at half maximum: ).
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