Lasers can be linearly or randomly polarized. The resonator of a laser creates standing waves of constant linear polarization. If special precautions are not taken, alternate longitudinal modes will have orthogonal polarization (Figure 4.6). Therefore, a laser is never really like a non-polarized thermal light source, it may be randomly polarized, i. e. it may issue a combination of orthogonally polarized radiation which, moreover, can vary over time. Introducing a Brewster angle window as a polarizing element within the laser cavity eliminates a state of polarization and produces a linearly polarized beam.

However, random polarization can be a problem if the beam interacts with polarizing elements or reflective surfaces and is detected by photo­electric sensors. The temporal fluctuation of polarization, polarization noise, gives rise to fluctuations in intensity and causes noise in the optical detector. In these applications, the use of a linearly polarized laser is recommended.

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