How Brewster Windows Function In Lasers

Posted by Jason Wickersham on

Lasers play an important role in many modern devices, from tiny laser pointers to CD-ROM’s to industrial lasers that can drill precise holes in a sheet of steel. The power of a laser beam depends on the material used to produce it, but its utility derives from its ability to be focused into a narrow beam of monochromatic light, or light that is all the same wavelength. In order to understand the function of a Brewster window, it is necessary to understand how lasers work.

How lasers work

All matter, whether a solid, a liquid or a gas, is composed of atoms. Atoms consist of a nucleus, made up of protons neutrons and a cloud of electrons that circle the nucleus in orbits defined by their various energy levels. Although the power of a laser beam depends on the energy levels of these electrons in the atoms of the material used to produce it, all lasers function on the same basic principle.

A beam of energy, usually light consisting of many different wavelengths, like the beam of a flashlight only more tightly focused, is used to excite the electrons to “jump” to a higher energy level. If enough electrons are excited, the material can reach a state called “inversion.” The excited electrons can then be “stimulated” to drop back to their original state by “emitting” a photon. This photon will exactly match the photon that stimulated it in both wavelength and phase. These photons can then stimulate more photons to be emitted. This repeated process leads to an increase in the light output and is the reason for the name “laser” – Light Amplification by Stimulated Emission of Radiation.

Generally speaking, the photons are collected in a tube with a mirror at either end. As they “bounce” from end to end, the number of photons (and the amount of energy) in the tube increases to very high levels. The mirror at one end of the tube reflects 100% of the photons that impact it. The mirror at the other end, however, transmits a small fraction of the photons – the actual laser beam – while reflecting the rest.

Polarized light

Light is an electromagnetic wave, meaning that it has both an electric component and a magnetic component. For this discussion, we can ignore the magnetic part and just focus on the electric portion.

This wave is much like a wave on the surface of the water with peaks and valleys. The distance between the peaks defines its wavelength. While a water wave oscillates only up and down, the electric portion (called the electric vector) of the light wave can be in any orientation, which may be horizontal, vertical or anywhere in between. Monochromatic light is light of the same wavelength (or color) but this says nothing about the orientation of the electric vector. Polarized light, on the other hand, consists of light waves all traveling in the same orientation and is very useful in a number of applications, such as microscopy, for example.

Polarized light is the principle behind Polarized lenses. When ordinary sunlight is reflected from water or from a solid surface such as asphalt, for example, most of it is polarized in a horizontal plane – meaning that the electric vector is oriented horizontally. Polarized lenses, however, are designed to block horizontally polarized light, thus eliminating annoying glare and making the landscape appear darker than it would without the polarized lenses.

The beam of light produced by a laser, while monochromatic and highly focused, is not necessarily polarized. The function of a Brewster window is to produce a laser beam that is not only monochromatic but polarized as well. Here’s how the process works.

Brewster’s angle

When light encounters a boundary between two different media – such as air and glass, for example – some of it is refracted, or transmitted through the glass, while the rest is reflected. The ratio of transmitted to reflected light is dependent on the angle at which the light strikes the surface of the glass and the orientation of the electric vector.

Brewster’s angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization (called p-polarization) is perfectly transmitted through a transparent dielectric (electrically nonconducting) surface, with no reflection. When unpolarized light, whether monochromatic or consisting of various wavelengths, is incident at this angle, the light that is reflected from the surface is therefore perfectly polarized (s-polarization). The s-polarization has its electric vector perpendicular to the plane of incidence while the p-polarization is parallel. This special angle of incidence is named after the Scottish physicist Sir David Brewster (1781–1868).

Brewster’s angle is proportional to the index of refraction (denoted by “n”) of the medium through which the light is refracted divided by the index of refraction of the medium through which it initially travelled. For a glass medium (n ≈ 1.5) in air (n ≈ 1), Brewster’s angle for visible light is approximately 56°, while for an air-water interface (n ≈ 1.33), it is approximately 53°. Since the refractive index for a given medium changes depending on the wavelength of light, Brewster’s angle will also vary with wavelength which, in the case of monochromatic laser light, is uniform.

By inserting a Brewster window into the path of a laser beam in such a position that the laser light strikes it precisely at Brewster’s angle, the laser beam can be configured to produce exactly the polarized light required for a particular application. Learn more about Esco Optics that manufactures quality Brewster Window, lasers and other optical components. Also find a reference you may like related to this – Laser Optics.

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