Cladding shaping of optical fiber preforms via CO2 laser machining (Conference Presentation)

Double-clad silica fibres used in high power lasers typically comprise a core doped with a laser active ion, a silica inner-cladding pump guide and a low refractive index outer polymer coating for protection and low loss pump guidance. For efficient pump absorption in the active-ion doped core, the inner-cladding must be shaped in order to scramble the pump radiation to maintain overlap with the core. This shaping is traditionally undertaken via diamond, or ultrasonic, milling of the fibre preform into an octagon or hexagon, leaving a rough surface which is subsequently fire polished before drawing into a fiber. We report on our developments of an alternative approach for shaping the inner-cladding using a 10.6µm pulsed CO2 laser to machine the fibre preform. This process is shown to allow fabrication of N sided polygon shaped fiber claddings as well as novel cladding structures, which include concave and convex surfaces, as well as core to cladding area ratio adjustment. Processing speed is significantly increased whilst maintaining improved surface qualities that remove the requirement of further fire polishing prior to fibre drawing. We will discuss recent developments of novel cladding geometries such as Reuleaux polygon cladding shapes that allow use of automated x-y profilometers on the draw tower whilst maintaining a shaped cladding for pump scrambling.