Electro-optically Q refers to the laser cavity and place a polarizer and a KD*P crystal. Light becomes linearly polarized light after the polarizer, if applied on the KD*P Crystal ?/4 voltage, due to the Pockels effect, make a roundtrip through the Crystal vibration direction of linearly polarized light change of p/2. If no voltage on the KD*P Crystal, a roundtrip through the Crystal vibration direction of linearly polarized light does not change.