The granulated silica method for preform and fibre production offers a high versatility with respect to material composition and shape. Based on this composition flexibility along with the possibility to introduce dopants and co-dopants at high concentration levels, the refractive indexes of the passive and active fibre areas can be tailored in a wide range. In particular fibres with an inverted refractive index step (npassive>nactive) can be realized.
A local lack of population inversion in Ytterbium-doped standard step-index fibres leads to reabsorption of the Ytterbium emission. In order to avoid this reabsorption, an “inversed” guiding fibre structure is proposed, where the light generating area and the guiding area are physically separated. This can be realized by inverting the refractive index step (npassive>nactive). By using the granulated silica method, the refractive index of both areas can be influenced and tailored in order to realize such an “inversed” guiding fibre structure.
Within this research, the first results for two different fibre designs featuring the “inverse” guiding fibre structure are presented:
1) Guiding cladding structure:
Al-passively doped cladding and Yb/Al-actively doped core with npassive,clad>nactive,core. The light will be generated in the active core but refracted into the passive cladding and guided by it (due to total internal reflection between the passive cladding and the surrounding air).
2) Guiding core structure:
Al-passively doped core and Yb/Al-actively doped cladding with npassive,core>nactive,clad. The light will be generated in the active cladding but refracted into the passive core and only guided by it (due to total internal reflection between the passive core and active cladding). Here, the active area is based on the granulated silica method while for the passive core a sapphire rod has been used.