What is the meaning of phase locked?

[cks]

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?

 

[vk6kro]

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.