Compact Disc Optics - Why use a linear polariser and a quarter wave plate? I just came across this website about the application of a quarter wave plate. Link: Compact Disc Optics. 
My question is why does the beam need to be linearly and then circularly polarised before sending to the compact disc? And the returned beam undergoes the same before reaching the detector? Is it related to power loss?
 A: What you may not realize from that website is that the same optical element is shown on the left and on the right. Laser emission and detection happen simultaneously, but for clarity they are drawn separately.
See how circular polarization changed after the reflection from the CD. Now, when it comes through the quater-wave plate it has polarization perpendicular to the incoming laser beam and gets reflected towards the detector. Neat!
A: A quarter wave plate together with a polarizing beam splitter is a standard way of building an optical isolator.  In an optical isolator the light traveling in one direction is transmitted but the light travelling in the reverse direction is reflected.  This is important when reading compact discs because the reflected light carries the signal that you want to read.  
As for how it separates the forward travelling and backward travelling light.  The initial pass through the linear polarizer leaves the light linear polarized in one direction.  The first pass through the quarter wave plate converts it to circular polarization, and the reflection off of the CD converts it to the opposite handedness circular polarization.  Finally, the quarter wave plate converts it to the opposite linear polarization from the input beam which is then reflected by the polarizing beamsplitter.  
