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May5-09, 05:29 AM
Sci Advisor
P: 4,016
It isn't possible for two signals that have different frequencies to have the same phase. They might be in phase for an instant, but because of the different frequencies, they cannot stay in phase.

However, you might divide them or mix them to a common frequency to produce square waves that are in phase, out of phase by a fixed amount, or at least near the same frequency.

These can be compared to produce an error signal which can then be used to pull one of the frequencies relative to the other so that the phase relationship is maintained.
This is called a phase locked loop.

This is commonly used to produce a range of stable frequencies from one stable, but expensive, reference frequency like a crystal oscillator.

Not sure how you would do this with lasers, but I guess the principle is the same.