A fuel injection system having a fuel flow regulating valve that directs a flow of pressurized fuel to a portion of pressurized fuel to an operable portion of a fuel pump. The polarization converter converts the input light with the given polarization into a linear polarization with a desired angle using two Faraday rotators and a quarter wave plate between them. A controller is configured to receive the signal and indicate a fastening result based on the signal. The capacitor is mounted on the inner surface of the circuit board and has two leads.