Light is transverse EM wave, normally the direction of the electric field of light beam is randomly distributed, no direction is dominant, we say such light is not polarized. If the light electric field is not evenly distributed, some directions dominate, we say the light is polarized. The plane of the polarized light electric field is called the plane of polarization.

Present laser systems generate polarized light. It is realized by using fold mirrors inside the cavity, the fold mirrors select the direction of the resonant light, since only light with the preferred direction can build up in the cavity, the output laser is highly polarized. If the light is not properly polarized, the machining quality may change with time because laser polarization plane changes with time and the plane direction affects the machining quality. Polarized beam have a directional effect in machining due to reflectivity effect, hence in material machining, lasers are usually designed first to generate highly polarized beam, then they are adjusted to the required direction changes using depolarizers. The beam out of a depolarizer is "circularly polarized" beam. To summarize, we can say, to avoid the negative effects of polarization, we first let the laser generate highly polarized light, then use depolarizers to change the beam into the form we desired.

Figure 2.17: reflectivity of steel to polarized 1.06 micro Laser light radiation