OpECS is an optical probe current sensor that enables current measurement by capturing the magnetic field generated around the current through the Faraday effect. When a magnetic field exists at the sensor head, the linearly polarized light is affected by the magnetic field and its oscillation direction changes. The direction of oscillation follows an elliptical orbit according to the direction of the magnetic field, and the change in direction is proportional to the magnitude of the magnetic field. The elliptically polarized light information is sent to the main unit (control unit) at the latter stage, where it is converted from an optical signal to an electrical signal. This signal can be input to an oscilloscope or other device equipped with a 50Ω-terminated BNC connector to measure the current.