What are the effects of a beamsplitter on the beam itself (if any)? I am currently studying the textbook Laser Systems Engineering by Keith Kasunic, and I came across the following diagram:



I understand what a beamsplitter does, but what effect does this splitting of the beam have on the beam itself (if any)? Are any of the properties of the beam, either the split part going to the photodiode, or the part that continues through to the collimating lens, altered in any way (compared to if there was no beamsplitter between them)? I have never read anything that would suggest that anything is altered by beam splitting, but I am curious, since I don't think I've ever read it stated explicitly that nothing is altered.
 A: You describe particular application, but ask about the affect of the beamsplitter in general.   I'll talk first about the general situation because I'm guessing that that is what you are really interested in.
It depends on the type of beamsplitter being used.  All thin-film beamsplitters (the cube beamsplitter you show in your sketch appears to be a cube beamsplitter) affect the polarization of the beam.  Some reflect s-polarized and transmit p-polarized.  Some reflect s- and p- equally, so the net affect is reduction of intensity.  Some do something in between, and the affect on polarization is unknown.    Thin metal film beamsplitters (like a "two-way mirror") will affect the phase between s and p leading to a change of (or creating) ellipticity.  Some beamsplitters comprise an array of reflecting dots, so that light that hits the dot is reflected, and other light passes right through. Additionally, the efficacy of a beamsplitter depends on wavelength so the spectrum of the light is changed. And finally, if the light is not perfectly collimated (and no beam is), and the application is imaging, geometrical aberrations are introduced.
In laser interferometry the affect on spectrum is unimportant. Also, in that application, one would use the type that affects s and p equally.
A: The laser light that goes through the beamsplitter (BS) is reduced in its power: only part of the light is passing through the BS, while the rest is reflected and wasted – it does not hit the photodiode. The light picked up by the photodiode is the light that goes through all optical elements, is then reflected off the disk, and goes back through all elements. Hence, it is a measure of the reflected light. Thus it is used to read-out the disc.
