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I'm working on phase locked loop.

There are two lasers where I'm working at where the slave laser is supposed to get locked to the master laser with a fixed frequency difference between them. In other words, the master laser has frequency f1 and slave laser has frequency f2 and the difference f2-f1 is fixed by the phase locked loop. The phase between them is supposed to be stable as well.

But, I don't understand is the phase can only be fixed if the two lasers have the same frequency, how can two lasers that don't have the same frequency to have fixed phase relationship????