Why do lasers need mirrors at the edge of the cavity? If the avalanche stimulated emissions start from the first absorbtion that occurs in the cavity then why are mirrors needed? I imagine a photon that even without mirrors stimulates extra coherent emissions, provided a continuous pumping energy. I apologize from my inappropriate terminology but I'm not a physics student 
 A: Contrary to popular belief, lasers do not require mirrors nor do they require a cavity. Laser stands for light amplification by stimulated emission of radiation. The only requirement being amplification of light and that stimulated emission is the dominant effect by which it is done.
Take for example EDFA (Erbium doped fibre pumped out of band). It is used as a repeater in long haul telecoms networks to amplify the light signal. No cavity is involved. No mirrors are involved.
A laser with mirrors are also known as losers. I.e. light oscillation by stimulated emission of radiation. Actual they do amplify, it is just the background thermal noise that they are amplifying.
Stimulated emission produces the coherency. As light is produced in reaction to the light that stimulates it, the phase is synchronised. For spontaneous emission it is merely that. Spontaneous. No phase relationship to existing light. No coherency.
A: Tapered amplifiers are another technology which utilizes amplification of the forward propagating mode by stimulated emission. In this technology a semi-conducting gain medium is pumped electronically (current is flowed into the device to excite charge carriers to the excited band) while laser light is shined into the input port of the device. The light propagates though the device increasing in intensity as it propagates along the optical axis until it exits the device, now amplified by multiple orders of magnitude.
A: I know physics but I'm no laser expert, but the mirrors are perfectly parallel so that gets the emissions going in the perpendicular direction. It has to do with probabilities and when the population of photons starts moving in the correct direction then they all start to move that way.  Note that while one mirror is almost 100% reflective the other is designed to be only say 90% reflective allowing 10% of the light to escape as what we see as the beam.
