Use of a half wave plate plus polarizer to attenuate lasers?

[IMESS]

The combination of a half-wave plate followed by a polarizer is commonly used as a variable attenuator for polarized lasers. The idea is that the beam enters the half-wave plate, and you rotate the half-wave plate leading to a rotation of the polarization direction of the beam. As the beam is then rotated with respect to the direction of the polarizer plate, you get attenuation of the beam. However, as the beam is already polarized before it enters the 1/2 wave plate, wouldn't you be able to achieve the same thing by just leaving out the 1/2 wave plate, and rotating the polarizer?

 

[davenn]

hi there
welcome to PF

You have marked your thread with an A for a post graduate level education
So what has your research led you to discover so far ?Dave

 

[Cthugha]

However, as the beam is already polarized before it enters the 1/2 wave plate, wouldn't you be able to achieve the same thing by just leaving out the 1/2 wave plate, and rotating the polarizer?

In theory: yes.
In practice: Many optical elements such as beamsplitters have properties depending on the polarization of the light beam. Also whatever sample you try to investigate using a laser may show effects depending on the polarization of your light beam. So you want to be able to change the intensity of your laser without varying its polarization at the same time. If you just use a polarizer, you will change laser intensity, biut also rotate the polarization of your laser. Adding the half-wave plate allows you to avoid these problems.