The question of laser light phase has its long history. Correct phase locking is achieved only for the difference frequency, there is no any phase locking of the light waves, of cause. Historically systems with phase locking of the difference frequency are called phase locking systems, but tradition point of view is often not the best one.
The question of the phase measuring of the light always had the most significant status. One of the most important experiments of all times in physics was attempt of Michelson to reveal the influence of the laboratory moving by means of observation of the shifts of the interference bands. As there was no any shift so the absence of the ether was accepted by physic science.
The correct study of the experiment conditions proved, that no shift effect could be revealed at any rate even with assumption of the existence of the ether.
Indeed let us discuss the phase of the light from the monochromatic source at the distance S. Tension of the light field in the source point is
E1=E0 cos (2πνt). (1)
At the distance S the phase is backward upon the value S/C, as the light velocity is С. Hence there
ES=E0 cos[2πν (t-S/C)]. (2)
If the source is moving with the velocity V along the axis X we get the value (S - Vt) instead of S:
ES=E0 cos{2πν [t-(S-Vt)/C]}. (3)
This equation can be transformed to the next one
ES=E0 cos{2πν [(C+V)/C] [t-S/(C+V)]}. (4)
Or with CV = C+V and VV = (C+V)/C we get
ES=E0 cos[2πνV(t - S/CV]. (5)
So the phase velocity of the light depends on the velocity of the source as usual sum of this velocity V with the speed of the light C. This effect is well-known as Doppler wavelength shift. This phase velocity in the reference system concerned with the source is equal to that without the velocity of the light source
CV = C + V - V = C. (6)
We must conclude that the phase velocity in the base of the source is invariant to the velocity of the source relatively the outer medium. Hence the results of the Michelson experiment don't exclude the existence of the ether. Let the receiver moves too with the same velocity V along the axis X. The reverse Doppler shift will occur. In this case we must set into (15) the value S = L + Vt:
EL=E0 cos{2πν [t-(L + Vt - Vt)/C]}. (7)
EL=E0 cos[2πν (t-L/C)]. (8)
So we get the initial equation (12) which don't contain value V. If the source and receiver both move with the equal velocities than the phase of the received light don't depends on this velocity. We've got this result from simple assumption that the velocity of the light C is constant only in the only based system but in any other system it is calculated in accordance with Galilean rule of the velocities summing. In this case the phase velocity is invariant to joint moving of the source and the receiver. So the famous wind of the ether couldn't be discovered in the Michelson experiment.
Phases in the Michelson interferometer are discussed from the new point of view. In this example we can see as it is important to distinguish the phase velocity from the velocity of the wave expansion. It is necessary also to distinguish pure effect from preconceived interpretation, as in the case of the curvature if the light beams in the solar crown.