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I'm currently planning a laser setup in which I want to send a (1mW) laser into the environment using a commercial scanning unit (made of two galvanometers). I would like to measure the reflectivity of the environment by deflecting the reflected laser into a photodiode. The setup will probably look like this:

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I am currently searching for this "one-way mirror" through which the laser can pass through from one direction but is deflected when returning. It's a bit hard to find a supplier if I don't know the name of that thing, but I have the feeling that this should be a rather standard component.

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  • $\begingroup$ Have you tried (optical) Splitter? $\endgroup$
    – Bill N
    Commented Aug 9, 2015 at 0:49
  • $\begingroup$ To leading order optics is time-reversal invariant. There is no such component. Even one-way mirrors don't do what you think: they pass the same fraction of light through in both direction. What makes them work is controlling the light levels on both sides combined with the logarithmic response of your eye. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Aug 9, 2015 at 4:31
  • $\begingroup$ @BillN: Does a beam splitter work like that? The reflected light would go 50/50 to diode and back to the laser, but would the original Laser simply pass through? @ dmckee: I'm not exactly understanding what you want to say. $\endgroup$
    – FooBar
    Commented Aug 9, 2015 at 8:02
  • $\begingroup$ As others say, you're not going to find the a right-angle-changing component that transmits 100% in one direction and reflects 100% in another. $\endgroup$
    – Bill N
    Commented Aug 9, 2015 at 21:59
  • $\begingroup$ If the reflectivity is polarization insensitive you can use a quarter waveplate and a polarizing beam slitter if the laser source is polarized. $\endgroup$
    – Chris Mueller
    Commented Aug 10, 2015 at 19:36

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That component is a "Faraday rotator" or "optical isolator" or "optical diode", but that is almost certainly not what you want. Those are used mainly to prevent back-reflections from entering your laser cavity. The quality of an optical isolator is therefore usually not precise enough for optical measurements, and the beam is not usually deflected at a convenient angle.

What you want is a beam-splitter, as BillN and dmckee pointed out. No, the original laser will not simply pass through; you will lose 50% of it. If you need that missing 50% of the laser power, then use a 2 mW laser instead of 1 mW.

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