Brewster plates (original) (raw)
Definition: coplanar plates inserted into beams at Brewster's angle
Alternative term: Brewster-angled plate
Categories: 
general optics, 
photonic devices
Related: Brewster's angledepolarization losspolarization of lightetalonsConflicting Definitions of s and p Polarization
Page views in 12 months: 723
DOI: 10.61835/dee Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
Content quality and neutrality are maintained according to our editorial policy.
📦 For purchasing Brewster plates, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.
Contents
What are Brewster Plates?
Figure 1: A Brewster plate, inserted into a laser beam at Brewster's angle.
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 _{\textrm{B}}} = \arctan \frac{{{n_{{\textrm{plate}}}}}}{{{n_{{\textrm{air}}}}}} \approx \arctan {n_{{\textrm{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.
A Brewster plate causes a parallel shift of the beam position, which is proportional to its thickness.
Brewster Plates in Lasers
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.
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.
Effect of Depolarization
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.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is a Brewster plate?
A Brewster plate is an optical component with parallel surfaces, inserted into a light beam at Brewster's angle. This setup ensures minimum reflection losses for light with p-polarization.
What is the main application of Brewster plates in lasers?
Brewster plates are often used inside laser resonators to introduce polarization-dependent losses. This forces the laser to emit light with a stable linear polarization, as only p-polarized light experiences low loss.
What is Brewster's angle?
Brewster's angle is the specific angle of incidence where p-polarized light is transmitted through a dielectric surface without any reflection. For a plate in air, it is calculated as ($\theta_B \approx \arctan n$), where ($n$) is the refractive index of the plate material.
Why can a Brewster plate decrease laser efficiency?
If other components in the laser cavity, for example a thermally stressed gain medium, cause depolarization, some of the p-polarized light is converted to s-polarization. This s-polarized light is then strongly reflected by the Brewster plate, resulting in a significant power loss and lower efficiency.
Does a Brewster plate affect the beam path?
Yes, a Brewster plate causes a parallel shift of the beam position. The amount of this shift is proportional to the thickness of the plate.
Suppliers
Questions and Comments from Users
Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.
Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.
By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.