All the papers I have read regarding coherent polarization beam combination split laser beams and then coherently combine the components. But is it possible to coherently combine two different lasers with the same frequency and orthogonal polarizations?
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1$\begingroup$ Yes, but you need to synchronize them, first, which can be achieved optically (involving a beam splitter somewhere along the way) or electronically, which requires a rather delicate setup. $\endgroup$– CuriousOneCommented Oct 25, 2015 at 19:19
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$\begingroup$ @CuriousOne That would need a little expansion to be a pretty good answer. $\endgroup$– dmckee --- ex-moderator kittenCommented Oct 25, 2015 at 19:32
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$\begingroup$ @dmckee: I think it would need a lot of expansion... if nobody else wants to post an answer I'll think about it. $\endgroup$– CuriousOneCommented Oct 25, 2015 at 19:55
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$\begingroup$ By synchronizing them do you mean combine them? $\endgroup$– Clement DeckerCommented Oct 25, 2015 at 20:20
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$\begingroup$ What do you mean by synchronizing them? $\endgroup$– Clement DeckerCommented Oct 25, 2015 at 20:21
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2 Answers
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Your comment explains better what you want
By coherently combine, I mean to combine to different laser sources to produce a beam of greater intensity
even if I wouldn't use the word "coherently". You can combine, or superpose, as many beams as you want with any polarization just using beam splitters (better non-polarizing). Contrary to polarizers, they split and recombine any polarization. But check the specs because you'll never have exactly 50-50% for s and p polarization components.
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Simply getting higher intensity by combining two beams of orthogonal polarization is easy: a polarizing beamsplitter will do it. Of course the resulting beam will be unpolarized.
In order to combine beams from two lasers coherently, it is necessary to make the two lasers mutually coherent: to phase-lock them. This isn't very difficult. You need a fast photodetector, an amplifier, and a piezo-driven mirror (or other frequency-adjusting method) in one of the lasers. Small fractions of the two beams are split off and both used to illuminate the fast photodetector. This produces a "beat frequency" output that goes through the amplifier and is used to drive the piezo mirror to the point where the beat frequency is zero. That means the lasers are phase-locked -- and they are now mutually coherent. Rotate the polarization of one of the laser beams so the two polarizations are mutually orthogonal, and then combine them using a polarizing beamsplitter. If you need to control the polarization of the combined beam, put a phase shifter in one of the beams upstream from the beam splitter and adjust it to get the desired linear or circular polarization, then rotate the polarization downstream as needed.
answered Jan 18, 2020 at 14:40