A ground loop occurs when a system of electronic devices has two or more fully independent paths to earth ground. Ground currents flow through this loop, inducing voltages into the system. This can be a serious problem when one or more of the paths goes through the control electronics, which can interfere with proper operation of the system and potentially damage your laser.
In a laser system, a common scenario for a ground loop occurs when the anode or cathode of the laser is connected to the case of the laser package, which is then connected to earth ground through the laser mounting system. If the laser driver is also connected to earth ground, a ground loop is formed through the laser and control electronics.