Optical fiber poling by induction: analysis by 2D numerical modeling

Abstract

Since their first demonstration some 25 years ago, thermally poled silica fibers have been used to realize device functions such as electro-optic modulation, switching, polarization entangled photons and optical frequency conversion with a number of advantages over bulk free-space components. We have recently developed an innovative induction poling technique that could allow for the development of complex microstructured fiber geometries for highly efficient χ (2) based device applications. In order to systematically implement these more advanced poled fiber designs, we report here the development of comprehensive numerical models of the induction poling mechanism itself via 2D simulations of ion migration and space-charge region formation using finite element analysis. © 2016 Optical Society of America OCIS codes : (190.4370) Nonlinear optics, fibers; (230.4320) Nonlinear optical devices; (190.2620) Harmonic generation and mixing; (230.1150) All-optical devices; (000.4430) Numerical approximation and analysis.The development of thermal poling, a technique to generate effective second order nonlinearities in silica optical fibers [1], has found widespread applications in parametric frequency conversion [2], electro-optic modulation, switching [3] and polarization-entangled photon pair generation [4]. During thermal poling, the optical fiber is heated in order to increase the mobility of the impurity charge carriers (typically Na + , Li + , K + ), while a high voltage is applied for a certain time between two electrodes embedded into the fiber [5]. The static electric field due to the application of the high voltage causes the impurity charges to drift from regions at high potential towards regions at lower potential creating a space charge region located near the anode. When the sample is cooled down whilst the voltage is still applied, an electric field is frozen into the depleted region and an effective nonlinear susceptibility ( ) is induced into the sample due to a process of third order nonlinear optical rectification. The early issues mainly related to the high risk of breakdown between the two electrodes (typically separated by a few tens of microns) were addressed by Margulis et al. [6], who demonstrated that it is possible to induce a value of ( ) higher than the one obtained in the conventional case[5] by means of a poling configuration in which the two embedded electrodes are both connected to the same positive potential of the anode. The method for "charging" optical fibers has been recently further developed by De Lucia et al. [7], who discovered that it is possible to create a space charge region using electrostatic induction between an external inductor and the floating electrodes embedded inside a fused silica twin-hole fiber. As this novel technique avoids any physical contact to the internally embedded electrodes, it automatically lifts a number of restrictions on the use of microstructured optical fibers for poling where the multiple contacting of individual electrodes becomes a prohibit...

Thermal Poling of Optical Fibers: A Numerical History

“…In 2016, De Lucia et al published the 2D numerical model of the process of creation of a depletion region inside a twin-hole fiber poled via electrostatic induction [44]. The model was inspired by Camara et al [39], even if it shows some important differences.…”

“…The H3O + ions can consequently neutralize the NBO − centres previously depleted of Na + ions migrated as a result of the application of the external electric field. The two electrodes inserted in the cladding channels of the inductor are both connected to the same electric potential of +5 kV [44]. The Na + ions are considered mobile only in the fiber to be poled, while they are assumed to be motionless inside the inductor.…”

“…The concentration of the Na + ions is 1 ppm before the start of the poling process. The fiber is considered at a temperature of 300 • C and the injection of the H 3 O + is assumed to be inexhaustible [44].…”

Thermal Poling of New Double-Hole Optical Fibers

“…The maximum electric field is recorded after the bias is switched off and temperature is lowered to room temperature [12]. The optical fiber is initially neutral and maintains this neutrality all along the poling process [13].…”

“…The transport of diluted species module mainly sets the concentration, diffusion coefficient and mobility of Na + and H 3 O + in the fiber. The general form boundary PDE module sets the concentration of H 3 O + at the holes boundary ∂c 2 ∂t = σ 2 E [13]. The poling conditions of the two models are the same: assuming that the holes are filled with metal electrodes [16], the poling temperature is 285 • C, the outer surface of the fiber is set to zero potential, and the potential of the two electrodes are Appl.…”