Well, to be quite honest, phase and frequency (when modulation is referred) are basically the same thing. Just remember that in modulation schemes frequency and phase modulation are just a differentiation away, which means that is the same thing. Now for the distinction it is basically a passive/active issue. A fiber optic gyroscope falls in the passive case. You have two counter propagating fields injected so what you notice is the beating of these two waves thus their relative phase change. In a laser however waves are generated in the device itself so there is no point in talking about phase since the waves already include that in their linewidth manifestation. So the only noticeable effect of the gyroscope is the frequency drift of the laser line. Hope this helps

Thanks a lot

for more information you can see my new post about analysis and full my Tesis about gyroscope, do you make that device ?

thanks a lot . I am planning to make a ring laser gyroscope.