# Why all laser photons are in the same direction?

The lasing process begins when just one photon is accidently emitted. This explains why the photons on the line between the two mirrors are in phase. But what about the neighboring atoms or even distant one not on that line. How are they excited by that initial photon (by its wavefunction? WF) and why will they go perpendicularly to the mirrors. I expect they will go radially because the WF is spherical. And how does a plane wave build? The Huygens principle says that a spherical wave builds also spherical wave.

That is a good question, that can bring you much closer to understanding wave propagation.

First: waves - light waves - are not rays. They are always spread out. If you squeeze them down to a small beam in one place, they will immediately spread out over a wide angle downstream. This is directly related to Huygens Principle.

Second: in a laser, light travels back and forth between the two mirrors many, many times-- so it travels a long distance. The farther a light wave travels, the more it will spread (unless a lens or curved mirror refocuses it). So, typically one or both laser mirrors is curved to keep the beam confined. But what that means is that the light wave - the beam- occupies a volume inside the lasing medium. Every atom in that volume is influenced by the beam, so stimulated emission from all the atoms in the volume ends up in phase. See this.

• it all starts with a lonely single photon. If it were a wave made of many photons your answer would be suitable. But it is surely just one photon! Its impulse is fixed. So it can generate photons in one ray even after the mirrors are curved and it can traverse the different heights and widths of the laser medium and all generated photons would exit as a ray in a very small (even mathematical) point. Nov 16 '18 at 17:55
• I think that a photon somehow can generates new photons not in the same ray but very close to it parallel to the direction. But this way it acts not like a point source but as an extended object or it somehow changes its path slightly (Heisenberg) though keeping the impulse straight foreward. But then I can not understand how is the constancy of speed maintained. Nov 16 '18 at 17:55
• A photon, until it is absorbed, does not have a definite location. It is not a lonely single photon. In fact, its "probability density" corresponds to the whole wave, and the whole wave fills the laser cavity. You are "sort of" right that each photon can generate new identical photons. In fact, an incident photon encountering an excited atom (an atom ready to emit a photon) stimulates the atom to emit a photon that is precisely in step with the incident photon. See [en.wikipedia.org/wiki/Stimulated_emission]. But it's not as if a tiny "photon particle" does it. Nov 16 '18 at 19:03
• @McGrew thank you for your considerations. I see you grasp the concern I have about mechanism of building a plane wave of many photons with a width and height starting from a point. I don't think you deny the ignition of the process is from one photon and the pulse comes out of the laser before any other spontaneous photon is released (two photons birth can not be synchronized). Then as it is registered (absorbed) in a location (point) it is released at a point too. Now if it stimulates a new photon the only O´ Nov 17 '18 at 15:01
• I understand your reasoning, but several assumptions are incorrect. Why do you say that two photons "cannot be synchronized at birth"? No photon in a coherent laser beam can be traced to a single emission event at a point. There are even situations where two lasers are phase locked and it is impossible to know which laser emitted any particular photon. The wavefunction of each photon definitely fills the laser cavity (or in the two-laser situation, both cavities!) Bottom line: photons are not locatable; only photon detection events are licstable. It is tricky but true. Nov 17 '18 at 15:16

As mentioned in S. McGrew's answer, the light waves do spread out. Using a low-quality laser, you can notice that the beam spreads out and becomes less intense when projected on a surface that is farther away. Waves emitted from a single source and in a single direction always radiate outwards in a cone; most lasers are just so focused that it is difficult to notice. Even with the wave-particle duality of light, we know that lasers will behave this way because we can replicate Young's double-slit experiment and create an interference pattern, wherein the waves emitted by the laser interfere with each other after being passed through two parallel slits. here is a resource which proves this.