"The electromagnetic field of laser is confined to a straight line" is not exact, but it should be Gaussian light speed, which is less divergent than ordinary light speed. This is the theoretical calculation result of coherent light+optical cavity model, which is consistent with the experiment. It is difficult to explain without simple mathematics, so we can only try to explain that coherent light is less divergent than natural light, because the electromagnetic fields propagating to the space far away from the center of light speed cancel each other because of their coherence, so they are concentrated, while natural light is divergent because of its irrelevance.