What makes lasers so special: Lasers vs other light sources.
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.
- youtu.be/_JOchLyNO_w?t=188 says LED is "also monochromatic", but that is not strictly true, it has way way larger frequency band than a laser. Only narrower compared to other sources such as incandescent light bulbs.
- youtu.be/_JOchLyNO_w?t=517 stimulated emission. This is the key to laser formation as it produces coherent photons.
- youtu.be/_JOchLyNO_w?t=581 spontaneous emission happens too fast (100 ns), which is not enough time for stimulated emission to happen. Metastable electrons to the rescue.
- youtu.be/_JOchLyNO_w?t=832 the parallel mirrors select perpendicular photons preferentially
Laser Fundamentals I by Shaoul Ezekiel
. Source. 2008, MIT. Many more great videos in this series.Bibliography:
Spectrum of laser light by Shaoul Ezekiel
. Source. 2008, MIT.Laser linewidth - measurement and explanation by Your Favourite TA
. Source. TODO why it exists:
The key advantages of lasers over other light sources are:
- lasers emit a narrow spectrum
- 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.
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:As such, lasers manage to largely overcome "temperature distribution-like" effects that create wider wave spectrum
- 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?
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)"
Closely related to optical amplifiers.
This is by far the most important type of laser commercially, as it can be made relatively cheaply, and it doesn't break easily as it ends up being a single crystal.
Compare them for example to the earlier gas lasers.
This is the type of laser that you would get in a simple laser pointer.
But the real mega aplications are:
- fiber-optic communication, where laser diodes are one of the most commonly used methods to generate the light that goes in the fiber. This makes laser diodes one of the most important inventions of the 20th centure without doubt.
- optical storage. But as of the 2020s its usefulness was much diminished by a combination of solid-state storage + faster Internet due largely to fiber-optic communication. So it is partly a matter of laser diodes beating laser diodes!
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?
Three-level laser system by Dr. Nissar Ahmad (2021)
Source. Bibliography:
Sample usages:
- quantum computing startup Atom Computing uses them to hold dozens of individual atoms midair separately, to later entangle their nuclei
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.