Brewster windows
2024-3-3
Definition:
A transparent disk pointed at Brewster's angle.
In some cases when a beam of light passes through some transparent window, the loss in the window must be very small. A typical case is a helium-neon laser sealed by a glass tube and containing an external cavity mirror, as shown in Figure 1, where the glass window isolates the gas mixture in the laser from the outside air. Assuming that the gain is small and the output coupler transmission is also small, the loss per pass through the surface needs to be much less than 1%. This can be achieved using a Brewster window, where the angle of incidence of the beam is Brewster's angle. In this case, the reflectivity of p-polarized light at the air-glass interface is very small, that is, the polarization direction is parallel to the plane of incidence.
Figure 1: Schematic diagram of a helium-neon laser. The glass window is in the resonant cavity, and its direction is adjusted to Brewster's angle, causing little loss to p-polarized light.
Normally incident reflectance onto an uncoated glass disk is typically a few percent. If anti-reflective coating is used, it can be reduced to 0.2%. The loss of the Brewster window is at least 10 times lower than it. Additionally, any residual reflections will leave the cavity without causing interference effects (which would occur with a normal incidence window). Of course, the two Brewster windows in the above setup must have strictly the same orientation.
Since the loss of p-polarized and s-polarized light is very different, the polarization state of laser radiation is usually p-polarized. In many lasers, this is the only effect that determines the polarization direction.
One disadvantage of Brewster windows (or other polarizing components) in laser resonators is that they can cause large depolarization losses when thermal effects in the laser crystal affect the polarization state. In this case, the reflected beam from the Brewster window has high power but poor beam quality.
Brewster windows can also be used for optical modulators and laser shields.
A transparent disk pointed at Brewster's angle.
In some cases when a beam of light passes through some transparent window, the loss in the window must be very small. A typical case is a helium-neon laser sealed by a glass tube and containing an external cavity mirror, as shown in Figure 1, where the glass window isolates the gas mixture in the laser from the outside air. Assuming that the gain is small and the output coupler transmission is also small, the loss per pass through the surface needs to be much less than 1%. This can be achieved using a Brewster window, where the angle of incidence of the beam is Brewster's angle. In this case, the reflectivity of p-polarized light at the air-glass interface is very small, that is, the polarization direction is parallel to the plane of incidence.
Figure 1: Schematic diagram of a helium-neon laser. The glass window is in the resonant cavity, and its direction is adjusted to Brewster's angle, causing little loss to p-polarized light.
Normally incident reflectance onto an uncoated glass disk is typically a few percent. If anti-reflective coating is used, it can be reduced to 0.2%. The loss of the Brewster window is at least 10 times lower than it. Additionally, any residual reflections will leave the cavity without causing interference effects (which would occur with a normal incidence window). Of course, the two Brewster windows in the above setup must have strictly the same orientation.
Since the loss of p-polarized and s-polarized light is very different, the polarization state of laser radiation is usually p-polarized. In many lasers, this is the only effect that determines the polarization direction.
One disadvantage of Brewster windows (or other polarizing components) in laser resonators is that they can cause large depolarization losses when thermal effects in the laser crystal affect the polarization state. In this case, the reflected beam from the Brewster window has high power but poor beam quality.
Brewster windows can also be used for optical modulators and laser shields.
