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Brewster Plates

Definition: coplanar plates inserted into beams at Brewster's angle

German: Brewster-Platten

Categories: general opticsgeneral optics, photonic devicesphotonic devices


Cite the article using its DOI: https://doi.org/10.61835/dee

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A Brewster plate is a plate of an optical material with coplanar surfaces, which is inserted into a polarized laser beam under Brewster's angle so that minimum reflection losses occur for p polarization.

Brewster's angle is the angle of incidence (measured against the direction of normal incidence), calculated from the refractive index of the plate according to

$${\theta _{\rm{B}}} = \arctan \frac{{{n_{{\rm{plate}}}}}}{{{n_{{\rm{air}}}}}} \approx \arctan {n_{{\rm{plate}}}}$$

Figure 2 shows how the reflection losses of p-polarized and s-polarized light at each surface of the plate depend on the angle of incidence.

Brewster plate
Figure 1: A Brewster plate, inserted into a laser beam at Brewster's angle.

The reflection loss becomes minimal for p-polarized light.

A Brewster plate causes a parallel shift of the beam position, which is proportional to its thickness.

Brewster plates are often inserted into laser resonators of bulk lasers, often with the purpose of introducing polarization-dependent losses. This can force the laser to emit light with a stable linear polarization, the direction of which corresponds to p polarization at the Brewster plate. In this state of operation, the losses caused by a Brewster plate can be very low, if the polarization state is not rotated or distorted by the other intracavity components.

Fresnel reflectivity
Figure 2: Fresnel reflectivity of a fused-silica surface at 1064 nm versus angle of incidence from air. For p polarization, the reflectivity vanishes at the Brewster's angle of incidence (55.4°), whereas for s polarization the reflectivity increases monotonously with increasing incidence angle.

Note that the resulting reflection losses occur at both surfaces of a Brewster plate.

Depolarization within the resonator (e.g. caused by thermal effects in the gain medium) (→ thermal lensing) can couple some of the power into s polarization, which then experiences high reflection losses at the Brewster plate. Therefore, insertion of a Brewster plate can significantly decrease the laser efficiency even if the losses of the Brewster plate for p polarization are very small. The spatial shape of the reflected light can be complicated and reveal information on the depolarization mechanism.

Apart from polarization-dependent loss, a Brewster plate can exhibit thermal lensing when used in a high-power laser, and its nonlinearity and chromatic dispersion can be relevant in a mode-locked laser.

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