The micro-pulling-down technique for crystalline fiber growth is employed to grow fibers and thin rods of bismuth germanate, Bi4Ge3O12 (BGO), for use in electrooptic high voltage sensors. The motivation is the growth of fibers that are considerably longer than the typical lengths (100–250？mm) that are achieved by more conventional growth techniques like the Czochralski technique. At a given voltage (several hundred kilovolts in high voltage substation applications) longer sensors result in lower electric field strengths and therefore more compact and simpler electric insulation. BGO samples with lengths up to 850？mm and thicknesses from 300？μm to 3？mm were grown. Particular challenges in the growth of BGO fibers are addressed. The relevant optical properties of the fibers are characterized, and the electrooptic response is investigated at voltages up to . 1. Introduction Voltage and current measurements are two key functions in the control and protection of electric power grids. Traditionally voltage is measured by means of inductive instrument transformers or capacitive voltage dividers. Such transformers have been state of the art for many decades but have also a number of drawbacks. In particular they represent heavy and space consuming equipment (with weights of up to several tons at the highest voltage levels). In recent years optical sensors for high voltage and current have found considerable attention as attractive alternatives. Advantages of optical voltage sensors over conventional voltage transformers include higher fidelity (e.g., due their larger bandwidth and absence of effects like ferroresonances), smaller weight and size, inherent galvanic isolation of secondary electronics from high voltage, and reduced environmental impact (e.g., no risk of oil spills). Furthermore, their output is readily compatible with the modern digital equipment for substation control and protection. Commonly optical voltage sensors make use of the electrooptic effect (Pockels effect) in materials such as bismuth germanium oxide, Bi4Ge3O12 (BGO), or bismuth silicon oxide, Bi4Si3O12 (BSO) [1–5]. Several sensor configurations have been reported. In gas-insulated high voltage switchgear a small electrooptic crystal may be integrated into a capacitive voltage divider where it measures a small fraction of the line voltage [2–4]. In an optical voltage transducer for air-insulated substations reported in [5] the voltage is derived from several local electric field measurements at different positions along a path from ground to high voltage. The crystals are placed
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