Abstract
The creation of an effective second order nonlinearity via the process of thermal poling in materials such as glasses, which naturally lack any second order susceptibility, has been known since the early 1990s. In this review, we present a historical overview via an introduction presenting early evidence of second order nonlinear effects in glass to explain the working principles of the thermal poling technique. An overview is then given to the transfer of the technique from bulk materials to optical fibers. Different configurations of poling are presented and compared, namely the conventional anode-cathode set-up, the development of the cathode-less process and most recently, the induction poling technique, which allows for poling fibers without any physical contact between the embedded electrodes and the high voltage supply. 2D-numerical models of the induction poling technique are later presented. An overview is also given of the different solutions for embedding electrodes inside the cladding holes of the fiber. Apart from solid electrodes, the more recent results have been presented about the adoption of liquid electrodes, both metallic and aqueous. For the first time silica optical fibers have been thermally poled using tap water as electrode. Both these two main results, namely induction poling and the liquid electrodes can allow to overcome some of the apparently intrinsic limits shown by the thermal poling technique so far, such as for example the length of the nonlinear devices and the complexity of the geometrical structure of microstructured optical fibres, both solid and PCF. Finally, we review the most recent outcomes and published applications of periodically poled silica fibers from our group, including high harmonic generation and phase sensitive amplification. All these promising results demonstrate that the way towards a full exploitation of the thermal poling technique for all-fiber nonlinear photonics is opening up many new vistas.
Highlights
The concept of “total internal reflection” (TIR) was known since 1854 [1] and the first glass optical fibers were realized in the 1920s [2,3], it was only in the 1970s that optical fibers started to become what they had promised to be in the previous fifty years, namely reliable waveguides to transmit information, thanks mainly to the work of Kapron et al [4], who realized for the first time single mode waveguides characterized by reasonably low intrinsic transmission losses (7 dB/km)
The main motivation behind the idea of exploiting the quadratic nonlinearity inside an optical fiber comes from the purpose of overcoming some of the issues affecting the conventional approach for realizing nonlinear optical devices, based substantially on the standard scheme which considers the interaction between intense light waves and nonlinear crystals (such as for example lithium triborate (LBO), beta-barium borate (BBO) or lithium niobate (LiNbO3))
Among the most recent outcomes produced by the technological platform developed so far by De Lucia and co-workers there is the first demonstration of phase-matched parametric amplification via fourwave mixing (FWM) in an all-fiber setup including a high-power pulsed source, a periodically poled silica fiber (PPSF), and an optical microfiber (OMF)
Summary
The concept of “total internal reflection” (TIR) was known since 1854 [1] and the first glass optical fibers were realized in the 1920s [2,3], it was only in the 1970s that optical fibers started to become what they had promised to be in the previous fifty years, namely reliable waveguides to transmit information, thanks mainly to the work of Kapron et al [4], who realized for the first time single mode waveguides characterized by reasonably low intrinsic transmission losses (7 dB/km). Nowadays it is possible to dope optical fibers with erbium to obtain optical amplifiers, or with ytterbium or neodymium to get fiber lasers, or to integrate Bragg gratings mirrors and filters into them Their enormous global deployment is mainly for passive photon-based data transport rather than as a platform for nonlinear photonic devices (such as for example frequency converted laser sources, beam modulators and switches, optical sensors, etc.). The main reason of this limited use is the lack of intrinsic second order nonlinear properties in centrosymmetric materials, such as silicate glasses, which are exploited to fabricate most of optical fibers. This means that conventional step index silica optical fibers could not be used to generate second order related nonlinear parametric effects [5]. Almost completely overcome by realizing an all-fiber nonlinear device, where the light waves could be delivered with very low losses and generate second order nonlinear processes
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