Granulated silica method to produce an “inversed” guiding fibre structure

Blaser-Lopez, Dunia; Pilz, Sönke; Hayati, Mozhgan; Hochstrasser, Martin; Romano, Valerio (6 April 2020). Granulated silica method to produce an “inversed” guiding fibre structure. In: SPIE Photonics Europe, 2020, Online Only 11355 (p. 39). SPIE PHOTONICS 10.1117/12.2556690

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The granulated silica method for preform and fibre production offers a high versatility with respect to material composition and shape [1]. The main advantages of the granulated silica method are: composition flexibility [1], high doping concentration [1], homogeneity [1] and flexibility with respect to the geometry [2,3]. Based on this composition flexibility of the materials used 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, via the granulated silica method. In particular fibres with an inverted refractive index step (npassive>nactive) can be realized.

In general, for active standard step-index fibres, the refractive index of the passive cladding is lower than the refractive index of the active core (npassive,clad<nactive,core). The generated light from the active core is confined and guided in the core due total internal reflection. A local lack of population inversion would lead for actively Ytterbium-doped standard step-index fibres 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 guiding area are physically separated. This can be realized by inverting the refractive index step (npassive>nactive). The generated light from the active area will be refracted to the passive area and can be guided through the passive area by total internal reflection (if the passive area is surrounded by a lower refractive index) with less or no reabsorption. By using the granulated silica method [1], we can influence and tailor the refractive index of both areas in order to realize such an “inversed” guiding fibre structure. For the active area a doping combination of Yb/Al has been used. For the passive area just Al doping with a high content (in order to surpass the refractive index of the active area) has been used.

The realization of standard step-index fibres featuring a high aluminum content have been demonstrated by a) rod-in-tube using single crystal sapphire rods via the molten core method [4] and b) powder-in-tube with high alumina content [5].

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). However, based on this specific structure, light crosses the active core and experiences some reabsorption. Both areas are based on the granulated silica method.
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.

[1] Pilz, S.; Najafi, H.; Ryser, M.; Romano, V.; “Granulated Silica Method for the Fiber Preform Production”, Fibers 2017, 5, 24
[2] Scheuner, J.; Raisin, P.; Pilz, S.; Romano, V.; “Design and realisation of leakage channel fibers by the powder-in-tube method”, Proceedings of SPIE: Micro-structured and specialty optical fibers, 9886, 2016
[3] Di Labio, L.; Lüthy, W.; Romano, V.; Sandoz, F., Feurer, T.; “Broadband emission from a multicore fiber fabricated with granulated oxides”, Applied optics ,47, 10, 1581-1584, 2008
[4] Ma. Z.; Liu, H.; Shang, Y.; Pang, F.; Wang, Z.; Chen Z.; Gong, X.; Wang, T.; “Design and fabrication of sapphire-derived fiber with controllable core diameter”, Proceedings of SPIE: Optical communications and Networks, 11048, 2019
[5] Dragic, P.; Hawkins, T.; Foy, P.; Morris, S.; Ballato, J.; "Sapphire-derived all-glass optical fibres", Nature Photonics, 6, 627-633, 2012

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