What makes lasers so special: Lasers vs other light sources.
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.
Laser linewidth - measurement and explanation by Your Favourite TA
. Source.
The key advantages of lasers over other light sources are:
One cool thing about lasers is that they rely on one specific atomic energy level transition to produce light. This is why they are able to to be so monchromatic. Compare this to:
  • incandescent bulbs: wide black-body radiation spectrum
  • LED: has a wider spectrum fundamentally related to an energy distribution, related: Why aren't LEDs monochromatic
  • TODO think a bit about fluorescent lamps. These also rely on atomic energy transitions, but many of them are present at once, which makes the spectrum very noisy. But would individual lines be very narrow?
As such, lasers manage to largely overcome "temperature distribution-like" effects that create wider wave spectrum
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.
electronics.stackexchange.com/questions/477264/spectrum-of-leds claims cheap LEDs have 20 nm width at 50% from peak, and cheap lasers can be 1 nm or much less
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.: youtu.be/_JOchLyNO_w?t=832 from Video "How Lasers Work by Scientized (2017)"
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 youtu.be/_JOchLyNO_w?t=581. Mentioned at: youtu.be/_JOchLyNO_w?t=759 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)
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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.

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