An optical reactor has a reactor enclosure defining a reaction chamber. In the reaction chamber, a light source operating at a very high frequency is arranged to radiate light to a fluid to be treated. A low voltage electrode is arranged to surround the light source. The light source is energized by a driving circuit which is arranged adjacent to the reactor enclosure. The driving circuit has a high voltage output terminal connected to a high voltage input terminal of the light source. The driving circuit has a low voltage output terminal which is connected to the low voltage electrode.