Principle of laser light

Why can a laser light put infinitely far away from the focus remains unchanged? :Ordinary light sources are emitting light in all directions. To let the emitted light propagate in one direction, you need to give the light source equipped with a certain light gathering device, such as car headlights and searchlights are installed with the role of light gathering reflector, so that the radiated light is pooled together to shoot in one direction. Laser emitted by the laser, inherently is shot in one direction, the dispersion of the beam is extremely small, about only 0.001 radians, close to parallel The convex lens of the laser lamp with a convex side or two convex? The lens is convex on one side or both sides.