Various apparatus and methods for reducing inter-core crosstalk in a multicore optical fiber are disclosed. A high quality silicon carbide single crystal with large diameter excellent in dielectric breakdown property, heat resistance, radiation resistance and the like, suitable for electronic and optical devices and the like, and showing no contamination of polycrystals and polymorphs, no defect of micropipes and the like can be produced efficiently without cracking and the like. The voltage sensor and gate controller are high speed circuits, and therefore can reduce the duration of the dead time.