If you had a coherent laser beam that was split between reference and sample beams and the sample beam passes through the emission from a point source, there should be some disturbance right?

I know the point sources is emitting at a number of wavelengths, one of which would be that of the incoming laser beam. Is it that interaction would be so faint that you would never see the difference.

Is there a way for a coherent beam to interact with incoherent sources?

  • $\begingroup$ What do you mean by "pass through a light source"? Most light sources won't allow an incoming beam to pass through without absorbing them. A gas discharge tube is one possible exception. $\endgroup$ – The Photon Dec 11 '17 at 18:43
  • $\begingroup$ @The Photon - Pass through was a poor word choice. I guess incident path with a point source emitting incoherent light over a broader spectrum. Does that make sense? $\endgroup$ – TheCodeNovice Dec 11 '17 at 18:51
  • $\begingroup$ @Code, I edited the question; did I express your meaning correctly? $\endgroup$ – The Photon Dec 11 '17 at 18:53
  • $\begingroup$ @ThePhoton Yes that captured what I am after. Thank you! $\endgroup$ – TheCodeNovice Dec 11 '17 at 20:37

Generally two light beams crossing each other do not interact, regardless of whether the sources are coherent or incoherent. So even if the 2nd source were a laser (a coherent source), you would not see an interaction between the two beams.

If the 2nd source were a laser that was phase locked to the first source (meaning, the two sources are not just coherent, but mutually coherent), it would produce an interference pattern where the two beams crossed.

Otherwise, the main way you might produce an interaction between the two beams would be if they crossed in a nonlinear medium. In principle, any medium (like air) is nonlinear, but the nonlinearities are often so small that it is impractical to produce a nonlinear effect. If we want to demonstrate nonlinearity we use some special material like lithium niobate.

Generally even in a "strongly" nonlinear material producing interactions of this kind is fairly difficult and requires high power sources, careful choice of the wavelengths of the two sources, and careful alignment of the beams, to observe a strong effect.

There is one case I know of where nonlinearity of "ordinary" material plays a role: In wavelength-division multiplexed fiber optic systems, the nonlinearity of the glass fiber limits the number of WDM channels that can be effectively carried on a single fiber.

  • $\begingroup$ Ok, so if I wanted to interact with a coherent light source I have to absorb it? $\endgroup$ – TheCodeNovice Dec 11 '17 at 23:21
  • $\begingroup$ Or do some careful design with a nonlinear optical material. $\endgroup$ – The Photon Dec 11 '17 at 23:23

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