Can two lasers produce light which is coherent. Are the light have same frequency and wavelength
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5$\begingroup$ Possible duplicate of Is it possible to observe interference from 2 independent optical lasers? $\endgroup$– John RennieCommented Mar 12, 2019 at 16:57
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1 Answer
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Yes, they can, but not if they run freely and independently: to have two light beams that are coherent with each other, one of the two lasers (the slave laser) should be locked to the other one (the master laser), that is, there should be a control or a forcing mechanism that makes the phase of the slave laser follow that of the master one.
There are several techniques that allow to lock one laser to another (e.g. phase lock techniques, injection locking etc.)
answered Mar 12, 2019 at 15:59
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2$\begingroup$ While the answer is correct, it should also be noted why locking is critical and what its effect is. Photons don't interfere with each other. Each photon interferes only with itself. So when two lasers are locked, each photon they emit is emitted by both lasers at the same time. And if you install a detector next to each laser to check, which laser emits which photon, then you'd see that each photon is emitted only by one laser, but the coherence is gone (same as in the double slit experiment). $\endgroup$ Commented Mar 12, 2019 at 18:19