Brewster windows (original) (raw)

Definition: transparent plates which are oriented at Brewster's angle such that [[parasitic reflections|parasitic reflection]] losses are minimized.

Alternative term: Brewster-angled windows

Category: general optics

• optical elements
  • optical windows
    * Brewster windows

Related: Brewster's angleBrewster platesoptical windowsanti-reflection coatingsgas lasersdepolarization loss

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📦 For purchasing Brewster windows, 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 Windows?

Brewster windows are optical windows oriented towards a light beam with the incidence angle near the Brewster's angle such that p-polarized light experiences very low reflection and thus low transmission losses, while s-polarized light is substantially reflected.

Example: Helium–neon Laser

A typical example is a helium–neon laser with a sealed glass tube and external resonator mirrors as shown in Figure 1, where glass windows separate the laser gas mixture from ambient air. Given the small gain and the small output coupler transmission, the losses at these interfaces need to be far below 1% per pass. This is achieved by using Brewster windows, where the angle of incidence is close to Brewster's angle. In that situation, the reflectivity at the air–glass interfaces becomes very small for p-polarized light, i.e., when the polarization direction (= direction of the electric field) is in the plane of incidence.

Figure 1: Setup of a helium–neon laser. Although two glass windows are within the laser resonator, these induce very little optical loss for p-polarized light, as they are oriented for Brewster-angled incidence.

An uncoated glass plate at normal incidence would normally have a reflectivity of several percent on each side due to Fresnel reflections. With an anti-reflection coating, this could be reduced to e.g. 0.2%. Brewster windows can have at least 10 times lower losses. In addition, any residual reflection will leave the resonator, rather than lead to interference effects (as can occur for windows with normal incidence). Of course, both Brewster windows in a setup as shown above must be oriented accurately such that one can have entirely p-polarized light at both of them.

Effects on Lasers

Polarized Laser Emission

Due to the strong loss difference between p and s polarization, the polarization of laser emission is usually forced to be in the p direction only if a Brewster window or Brewster plate is within the laser resonator. In many lasers, this is the only effect determining the polarization direction.

Note that a Brewster window is not strictly a polarizer since it does not strongly suppress the unwanted polarization direction. Nevertheless, it easily leads to completely polarized emission of a laser.

Depolarization Loss

A potential disadvantage of Brewster windows (or other polarizing optical elements) in a laser resonator is that large depolarization loss can arise if, e.g., thermal effects within a laser crystal affect the polarization. In that case, reflected beams with significant optical power and typically poor beam quality leave the Brewster windows.

Other Uses of Brewster Windows

While the application in gas lasers is particularly common, there are other applications; some examples:

• Gas and vacuum cells are often equipped with Brewster windows.
• Brewster windows are also often used for optical modulators and for laser housings.

Frequently Asked Questions

What is a Brewster window?

A Brewster window is an optical window oriented at Brewster's angle to an incident light beam. This setup ensures very low reflection and thus minimal transmission losses for p-polarized light, while s-polarized light is substantially reflected.

Why are Brewster windows used inside laser cavities?

They are used to minimize intracavity losses, which is crucial in low-gain lasers like helium–neon lasers. By minimizing reflection for one polarization, they allow for efficient laser operation and also enforce a linearly polarized output beam.

How does a Brewster window polarize the light from a laser?

It introduces a significant loss difference between p- and s-polarized light within the laser resonator. The laser will naturally operate in the lowest-loss mode, which is the p-polarization, resulting in a completely polarized output beam.

Are Brewster windows superior to anti-reflection coatings?

For minimizing losses for a single polarization inside a laser, they can be far superior, potentially offering more than ten times lower losses than a typical anti-reflection coating. They also prevent interference effects from residual reflections.

What is a disadvantage of using Brewster windows in a laser?

If other components in the laser cavity, such as a thermally stressed laser crystal, degrade the polarization of the light, the Brewster window will reflect the unwanted polarization component out. This causes a power drop known as depolarization loss.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 22 suppliers for Brewster windows. Among them:

⚙ hardware

EKSMA Optics offers uncoated Brewster windows made from N-BK7 or UV fused silica.

⚙ hardware

Perkins Precision Developments (PPD) manufactures Brewster windows in a variety of shapes and sizes from 2 mm to 8” in diameter and with parallelism errors as low as 1 arcsecond. PPD manufactures precision plano substrates in a wide range of materials including fused silica, fused quartz, infrasil, N-BK7, YAG, silicon, SF11 and other high index glasses.

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