Abstract: An optocoupler for isolating a high voltage from a low voltage comprising: an elongated light transmissive rod mounted inside a cavity of a housing, the cavity being filled with a fluid surrounding and in contact with an outside wall surface of the rod; a light emitter mounted at one end of the rod, the light emitter receiving a current input and emitting light in response to the input; a light detector mounted at an opposing end of the rod, the light detector having a detection surface receiving light emitted from the emitter and generating a current output signal in response to the received light; the emitter and detector being mounted such that light emitted from the emitter travels through the rod and is directed toward the detector; a portion of the light emitted from the emitter traveling in a path into impingement with the outside wall surface of the rod; the rod having a first refractive index and the fluid having a second refractive index; the outside wall surface of the rod having a selected contour