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I have two laser beams with same polarization running parallel to each other. Will they interfere? If yes, then what are the conditions (perpendicular distance etc) and how can I observe the interference pattern?

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I'll assume that both lasers are of the same type, i.e that both are generated from the same physical mechanism. All lasers have some tuning range over which their wavelengths can vary. So, even though both are very monochromatic (single frequency) they will, in general, not have the exact same frequency. A HeNe laser, for example, has a tuning range of $1\ \text{pm}$ or $\sim1\ \text{GHz}$. Which frequency actually lases depends on many things, but it can be tuned by changing the temperature of the laser or by adjusting the resonator length inside of the cavity.

So, what happens when you overlap two different lasers is that you will get an interference pattern between the two beams. The interference pattern will oscillate between bright and dark at the difference frequency of the two lasers (the beat note). Since your eye can only see frequencies up to $\sim30\ \text{Hz}$ you will not generally be able to see the interference pattern. If you use a lens to focus the two beams onto a fast photodiode, you can see this beat note on an oscilloscope or spectrum analyzer though.

If your lasers are adjustable, then you can use this beat note between the two lasers to phase lock them to each other. Doing so requires some knowledge of electronics and control theory, but it will lock the frequency of the two lasers together so that as one laser drifts the other follows identically. If you phase lock the two lasers, then you will be able to see the interference pattern with your eye.

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  • $\begingroup$ Will the coherence length of the lasers have any effect on the (time-dependent) nature of the interference pattern? $\endgroup$ – NeutronStar Apr 3 '14 at 22:49
  • $\begingroup$ I was assuming infinite coherence length in my answer. I guess it would only be true if you were less than the coherence length away from the lasers, beyond that you would not get an interference pattern. $\endgroup$ – Chris Mueller Apr 4 '14 at 2:41
  • $\begingroup$ Thanks a lot. Forgive my naivety but the essence is that if you have two lasers from the same source and required electronics one can observe interference pattern around two beams. Or it will only be in between the gap? I thought due to dual nature interference pattern will form around two beams though it will be prominent between two beams. $\endgroup$ – user43794 Apr 4 '14 at 5:14
  • $\begingroup$ @user43794 "if you have two lasers from the same source and required electronics one can observe interference pattern around two beams." That is correct. Unfortunately, I don't understand your last two sentences. What gap? $\endgroup$ – Chris Mueller Apr 4 '14 at 12:52

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