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by fsilva 3981 days ago
FWiW you can have 3 or even more frequencies in a laser beam coherent with each other, i.e. where te phase of the waves is locked together. This allows you to interfere the waveforms of the different lasers in time, as their relative phases are locked. This is very hard to do with continuous wave lasers, but rather routine with short pulsed lasers (e.g. femtosecond).