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There are two cases I'm asking about The square in the middle is a cube beam splitter in the same orientation for both cases

There are two cases I'm asking about. The square in the middle is a cube beam splitter in the same orientation for both cases. We are looking at the beam splitter from the top. In the first case, the beam comes in from the left and half gets transmitted and half gets reflected downwards.

So, now changing only the beam direction in case 2, with the beam splitter in the same orientation, will the beam split like I've drawn? Does the beam splitter work if the laser comes from opposite directions?

What would happen if the Beam came from the top direction? The bottom?

Does the beam splitter only work one way?

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    $\begingroup$ Case 2 is commonly called "a theoretician's beamsplitter" by experimentalists. $\endgroup$
    – Frédéric Grosshans
    Oct 17, 2012 at 16:19

2 Answers 2

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enter image description here

A beam splitter works like a mirror that transmits part of the light. So there is always part of light that goes directly through without changing the direction. The rest gets reflected from the diagonal, which represents the reflecting surface. Therefore, beam coming from the left will be reflected downwards (as is shown in your case 1) but the beam coming from the right will be reflected upwards (case 2). In case the beam comes from top (or bottom), part is always going through, i.e. down (up) and the other part gets reflected like on a mirror -- to the right (left).

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  • $\begingroup$ Thank you so much. I was finding an answer to this before I purchase a beam splitter. $\endgroup$
    – QEntanglement
    Oct 11, 2012 at 20:57
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    $\begingroup$ @OndřejČernotík, I added a diagram - I hope you don't mind. $\endgroup$
    – nibot
    Oct 12, 2012 at 9:57
  • $\begingroup$ @nibot, not at all, it looks fine. $\endgroup$
    – Ondřej Černotík
    Oct 12, 2012 at 10:08
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there is actually a difference between the two cases other than the diection of the reflected beam. Cube beamslpitters are constructed using two parts glued together using a special cement. The mirror coating is done on one of the two parts (look at the picture from Thorlabs). If this is the case, then you should use such that the beam hits the mirror coating before hitting the cement as marked on the beamsplitter.

enter image description here

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