Silica Fiber Poling Technology** By Wei Xu,* Paul Blazkiewicz, and Simon Fleming 1. Introduction Silica fiber is the backbone of the modern optical telecom- munications networks and the rapidly expanding Internet. Sil- ica fiber components and silica planar waveguide circuits, such as Bragg and long periodic gratings, couplers, optical splitters, optical filters, optical switches and routers, are the building blocks of dense wavelength±division±multiplexing (WDM) optical communication systems. However, most silica fiber and planar waveguide components are limited to be passive devices because silica intrinsically does not have a second- order nonlinearity (SON) or linear electro-optic (LEO) coef- ficient. Silica glass is amorphous with macroscopic inversion symmetry and thus displays zero second-order nonlinearity, which forbids second-order nonlinear processes and electro- optic modulation. Silica fiber poling technology is the most promising method of inducing useable second-order nonlinear and LEO coeffi- cients in silica fiber. [1±3] Poling is the process in which a large external field is applied to the material simultaneously with suitable perturbations, such as heat, carbon dioxide laser irra- diation, and ultraviolet/vacuum ultraviolet (UV/VUV) laser irradiation. Once the SON or LEO coefficient is induced into silica fibers and waveguides, many new active fiber and wave- guide components can be produced. These components include electrically tunable fiber Bragg gratings and long peri- od gratings, electrically tunable WDM filters, electrically tun- able couplers and splitters, electro-optic modulators and switches, polarimetric intensity modulators, frequency con- verters, parametric oscillators, etc. Potential devices for spec- tral inversion, all optical switching through cascading of non- linearities, or up- and down-conversion can also be realized. This paper reports three commonly used poling techniques: thermal poling, CO 2 laser-assisted poling, and UV poling. Firstly, the mechanisms for thermal poling of silica fiber are presented in detail. Next, in contrast to thermal poling, the characteristics of CO 2 laser-assisted poling of silica fiber are discussed. Finally, UV poling of silica fiber is described, and its current status is also addressed. The silica fiber used in the poling experiment has two holes in the fiber cladding. The two holes are along either side of the fiber core, as shown in Figure 1. Fine metal wires are threaded into the holes in opposite directions through the two side-polished slots so that the external voltage/electrical field can be applied to the fiber through the wires. The outside diameter of the twin-hole fiber is typically 250 lm. The diameter of the holes is about 90 lm. The separation between the two holes is about 18 lm. The fiber core is usually elliptical and has the size of 6 lm4 lm. The core is deliberately positioned closer to one hole than the other, it is positioned about 2 lm from one hole and 10 lm from the other. The core is doped with germanium with con- centration of ~3 mol-%. The dielectric constant of fused silica 1014 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2001 0935-9648/01/1207-1014 $ 17.50+.50/0 Adv. Mater. 2001, 13, No. 12±13, July 4 RESEARCH NEWS Silica fiber poling technology is used to induce a second-order nonlinearity or linear electro-optic coefficient into silica fiber, an amorphous centrosym- metric material without intrinsic second-order nonlinearity. This paper reports on three poling techniques used for silica fiber: thermal poling, CO 2 laser-assisted poling, and ultraviolet (UV) poling. The characteristics, mecha- nisms, and latest research results for each poling technique are addressed. ± [*] Dr. W. Xu, Dr. P. Blazkiewicz, Dr. S. Fleming Australian Photonics Cooperative Research Centre Optical Fiber Technology Centre The University of Sydney 206 National Innovation Centre Australian Technology Park Eveleigh, NSW 1430 (Australia) E-mail: w.xu@oftc.usyd.edu.au [**] Financial support from both the Australian Government through the Aus- tralian Photonics Co-operative Research Centre (APCRC) and the Minis- try of International Trade and Industry (MITI) of Japan through the New Energy and Industrial Technology Development Organization (NEDO) are gratefully acknowledged. Fig. 1. Twin-hole silica fiber with two wires in the holes (not drawn to scale).