Why do CD players use prisms to separate laser beams?

[Cheman]

CD players and prisms...

We use prisms in a CD player in order to separate the incident ray of the laser for the reflected ray. But why do we need to do this? Couldn't we just place the sensor next to the laser? After all it reflects at a slight angle.

Thanks. :-)

 

[Cheman]

If you do not understand which laser I am referring to then please take a look at this link

http://hyperphysics.phy-astr.gsu.edu/hbase/audio/cdplay.html

Thanks.

 

[Integral]

I would bet that the signal received by the pickup is pretty weak, note that it is first scattered by the disk then reflected back to the sensor (BTW, it would have the same number of reflection as you are proposing). I will bet alignment of that sensor is critical for maximum signal strength, therefore you cannot just willynilly position it hoping to catch some stray off axis rays. In addition this design also allows separation of the two circuits associated with the transmission and reception of the laser signal.

Often, what seems simple to unknowing observers gets quite complicated in the actual manufacture process.

 

[Cheman]

Cool. :-) That makes sense - are there any other reasons why you want a prism to direct the reflected ray and "seperate" it from the incident ray?

I looked in my textbook and it says "its not much help getting the incident ray and reflected ray mixed up ... [ therefore use prism]" - but to me this doesn't make sense. :-S After all, if the did get "mixed up" and overlap, the rays are heading in different directions anyway - one towards detector and one effectively away from if we place sensor and emmitter side by side. Therefore they wouldn't superpose and interfer at the detector.

Thanks. :-)

 

[Integral]

There WILL be mixing of the reference beam and the reflected beam. The interference due to "mixing" of the beams is essential to the operation. The pits will cause a change in optical path length of the reflected beam this change will cause the interference fringes to shift. It is the shift in interference fringes which the final detector "sees".

Essentially the arrangement of prisms creates a simple interferometer, thus is a very sensitive instrument for measuring changes in length. The measured length change is the depth of a pit.