2. Laser

Polarisation

One distinctive feature of the HeNe lasers is the polarisation of the laser light. In the images below, the light beam is magnified by a lens and passes through a polarising filter. Its transmitting orientation is indicated by the red arrow. Light that is polarised in this orientation is allowed by the filter to pass through, while light that is polarised vertically is blocked.

Left: The HeNe laser and its beam expanded with a lens. The laser beam passes through a linear polariser with a transmitting orientation indicated by a red arrow attached to the filter holder. The transmitting orientation of the polariser is horizontal in this image. Right: The same polariser with a vertical transmitting orientation blocks the laser beam completely. Hence the laser beam is horizontally polarised.

Have a look at the experiment in a Video (1 MB).

In the previous page, it was made clear that resonance in the laser can be easily disrupted through absorption loss. This is the reason why the ends of the discharge tube should be as permeable as possible. This can be achieved by using a so-called Brewster windows. These are windows tilted at Brewster’s angle which is a 56° incidence angle in air for an air-glass interface.

No reflection takes place when, parallel to the plane of incidence polarised light (horizontally-polarised light in the present case) falls on a window tilted at Brewster's angle. What takes place is a complete transmission which then builds up aresonance with lowest possible losses. Hence, the laser emits horizontally polarised light because the Brewster-window is tilted vertically.

Brewster window of the HeNe laser, indicated by arrows, at one end of the discharge tube.