My research, which is mainly related to communication, involves the use of optical sources (mainly lasers). However, my background in photonics and optics is not yet solid, so my question might be a very simple one.

In one part of my experiment, I have two laser beams crossing a piece of glass as shown in the figure below. The glass is formed by a liquid crystal material which is in the transparent (mirror like) state.

enter image description here

The two lasers come from different sources, and they might be of the same wavelengths.

My question is, do the lasers interfere at the center of the glass (i.e., at their point of intersection)? If the lasers carry information, what happens to the information? Is the effect a change in phase, attenuation?! Does that have anything to do with the Kerr Effect Kerr effect?

While searching for an answer, I found this website which talks about Phase Conjugation. An Intuitive Explanation of Phase Conjugation. In this website, I found this paragraph:

"In linear optics this interference pattern is a transient phenomenon that has no effect on anything else. However if the crossing of laser beams occurs in the transparent volume of a nonlinear optical medium,..."

So, is the glass in my case considered as a linear optical surface, or it depends on some other properties?


  • The application is very similar to optical switches (e.g., MEMS), except that instead of free space, the glass is always there, so beams must cross it and intersect.
  • The laser beams are output of two different fibers.
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    $\begingroup$ I'm way out of my realm of competence on this, but I have a vague memory that the linearity of the medium depends (at least in part) on the field strength. $\endgroup$ Commented May 15, 2014 at 4:29
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    $\begingroup$ Since glass itself has inversion symmetry (due to being a disordered amorphous material), there are no $\chi^{(2)}$ nonlinearities, so the Pockels effect is forbidden, as are any other two-wave mixing processes. Meanwhile, Kerr effects, which rely on the $\chi^{(3)}$ nonlinearity, should also probably not occur; this is because the DC Kerr effect requires an applied static field (which you don't have), and the AC Kerr effect only occurs at power intensities approaching $1\text{GW/cm}^2$, which is extremely high. $\endgroup$ Commented May 15, 2014 at 4:43
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    $\begingroup$ I'm not sure how phase-conjugated mirrors work, but the link you provided talks about 2, 3, and 4-wave mixing, which only occur at very high powers, so I'd guess that phase-conjugated mirror effects aren't happening. However, the liquid crystal is probably anisotropic, so I don't know whether there might be any nonlinearities associated with that. But in general, if I had to guess, I would guess that nonlinear effects are probably not going to happen unless your lasers are extremely powerful pulsed lasers. $\endgroup$ Commented May 15, 2014 at 4:46
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    $\begingroup$ Thank you all for your useful responses. I am not sure about the intensities that I might need. The application is very similar to optical switches (e.g., MEMS), except that instead of free space, the glass is always there, so beams must cross it and intersect. I will add this part to the question, it might help the reader. $\endgroup$
    – BHamza
    Commented May 15, 2014 at 5:01
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    $\begingroup$ @BHamza, it means if your beam intensity is less than 4 W / mm^2 you very very likely don't have to worry about nonlinearity in the glass. At 40 or 400 W/mm^2 you might have to worry or you might not --- that's outside my expertise. Your diagram doesn't specify beam power or diameter, so it really says nothing about whether you'll have nonlinearities or not. $\endgroup$
    – The Photon
    Commented May 15, 2014 at 15:32

1 Answer 1


If you are not working with very high intensity beams, you are not likely to see nonlinearity effects. However, there are nonlinear effects in glass and they are a limiting factor on the carrying capacity of optical fiber WDM systems, for example. Key words to look at are four-wave mixing, stimulated Brillouin scattering, and cross-phase modulation.

Let's make a quick estimate for the optical fiber case. I know that 1 mW of power in a 9 um core fiber causes no signficant nonline effects. That's about 40 W/mm^2. Whether undesirable nonlinearites are seen at 10x, 100x, or 1000x this level is outside my knowledge.

Furthermore, nonlinear behavior in fibers is occurs when the "beams" are phase-matched and overlapping for many meters, whereas your system likely only has the beams overlap for a few mm. You'd need much higher intensity in your system to see measurable nonlinear effects compared to what's needed in fiber systems.


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