The key advantages of lasers over other light sources are:
  • it emits a very narrow range of frequencies (small linewidth), which for many purposes can be considered a single frequency
    It does however have a small range of frequencies. The smaller the range, the better the laser quality.
  • it can be efficient collimated, while still emitting a lot of output power: Section "Why can't you collimate incoherent light as well as a laser?"
  • can be phase and polarization coherent, though it is not always the case? TODO.
Video 1. How Lasers Work by Scientized (2017) Source.
An extremely good overview of how lasers work. Clearly explains the electron/photon exchange processes involved, notably spontaneous emission.
Talks about the importance of the metastable state to achieve population inversion.
Also briefly explains the imperfections that lead to the slightly imperfect non punctual spectrum seen in a real laser.
Video 1. Principles of the Optical Maser by Bell Labs. Source. Date: 1963.
The type of laser described at: Video "How Lasers Work by Scientized (2017)", notably Mentioned at: That point also mentions that 4-level lasers also exist and are more efficient. TODO dominance? Alternatives?
Video 1. Three-level laser system by Dr. Nissar Ahmad (2021) Source.
You could put an LED in a cavity with a thin long hole but then, most rays, which are not aligned with the hole, will just bounce inside forever producing heat.
So you would have a very hot device, and very little efficiency on the light output. This heat might also behave like a black-body radiation source, so you would not have a single frequency.
The beauty of lasers is the laser cavity (two parallel mirrors around the medium) selects parallel motion preferentially, see e.g.: from Video "How Lasers Work by Scientized (2017)"
Sample usages:
Video 1. Optical Tweezers Experiment by Alexis Bishop. Source. Setup on a optical table. He drags a 1 micron ball of polystyrene immersed in water around with the laser. You look through the microscope and move the stage. Brownian motion is also clearly visible when the laster is not holding the ball.