Every laser, all types
Stimulated emission
First principles
Cutting mechanisms
LASER stands for Light Amplification by Stimulated Emission of Radiation — the name is the instruction manual. A pump pushes energy into a gain medium, lifting its electrons to a higher energy level. A passing photon knocks one back down, and the atom releases a second photon identical to the first. Two mirrors bounce that light back and forth through the medium so it multiplies on every pass; one mirror leaks a little out, and that’s your beam. Because every photon is a copy, laser light is one wavelength, in step, and travelling one direction — which is what lets a lens squeeze it into a spot small enough to vaporize metal.
1 · Ordinary light is a crowd. Laser light is a formation.
A shop light throws out photons of many wavelengths, in every direction, with no relationship to each other. You can make it brighter, but you can’t make it useful for cutting, because you can’t gather it into one place.
Laser light differs in three ways, and all three matter:
| Property | What it means | Why you care |
|---|---|---|
| Monochromatic | Essentially one wavelength, not a mix. | Wavelength decides what a material absorbs. One wavelength means predictable, repeatable behavior on that material. |
| Coherent | The waves march in step with each other. | They reinforce instead of cancelling — the energy stays concentrated instead of smearing out. |
| Collimated | It travels as a tight column instead of spreading. | It survives the trip from the source to the cutting head, and a lens can bring it to a very small spot. |
Those three together are the whole game. A 100 W laser concentrates its power into a spot a fraction of a millimeter across; a 100 W light bulb spreads the same power over a room. Same watts, completely different power density — and power density is what cuts.
2 · Stimulated emission is the trick.
Electrons in an atom sit at fixed energy levels — they can’t be halfway between. Add energy and an electron jumps up a level. Let it fall back and it releases that energy as a photon of light. So far, that’s just a glowing filament.
The laser’s trick, which Einstein predicted in 1917 and nobody built for 43 years, is that a falling electron can be triggered. When a photon of exactly the right energy passes an already-excited atom, the atom drops and emits a second photon that is a duplicate of the trigger — same wavelength, same direction, same phase. One photon in, two identical photons out. Those two trigger two more. That’s the amplification in the acronym.
Population inversion — the condition that makes it work
Normally most atoms sit at the low energy level, so an incoming photon is more likely to be absorbed than to trigger a release, and the cascade dies. The pump has to push more than half the atoms up to the excited state — a population inversion, an unnatural condition that has to be actively maintained. This is why a laser needs continuous power, and why “the pump” is a real component and not a footnote.
3 · The three parts every laser has.
From a $200 pointer to a fiber cutter, it’s the same architecture:
| Part | Job | What it looks like in practice |
|---|---|---|
| Pump source | Puts energy in and holds the population inversion. | An electrical discharge through gas (CO2), a bank of diodes (fiber), or a flash lamp (older solid-state). |
| Gain medium | Converts that energy into identical photons. | CO2 gas mixture, a doped crystal, or a doped optical fiber. This part names the laser. |
| Resonator | Bounces the light through the medium so it multiplies, then releases a fraction. | A fully reflective mirror at one end, a partially reflective “output coupler” at the other. |
Add the working hardware around that core — beam-delivery optics, a focusing lens, motion control, cooling, and the electronics that run it — and you have a machine.
4 · The gain medium decides everything you care about.
This is the paragraph worth remembering. The gain medium sets the wavelength, and wavelength decides which materials absorb the beam — which is the same thing as deciding what the machine can do.
| Gain medium | Wavelength | Absorbed well by | Boss line |
|---|---|---|---|
| CO2 gas | 10.6 µm (far infrared) | Organics — wood, acrylic, leather, paper, fabric, glass surfaces | LS · HP · EVO |
| Ytterbium-doped fiber | ~1.06 µm (near infrared) | Metals — steel, stainless, aluminum, brass, copper | FC (cutting) · FM (marking) |
| Frequency-tripled solid-state | ~355 nm (ultraviolet) | Nearly everything, with very little heat — plastics, glass, coatings | UV |
This is why “which laser is better” is the wrong question. A CO2 laser engraves wood beautifully and barely marks bare aluminum; a fiber laser cuts steel all day and sets wood on fire without cutting it cleanly. Neither is better — they’re absorbed differently, and absorption is physics, not brand.
Where this goes next
The wavelength-versus-material relationship is the single most useful idea in this whole library. It’s worked through properly in the types of lasers that matter, and applied to real cutting in how laser cutting works.
5 · Continuous or pulsed — the other big fork.
A laser can run its beam two ways. Continuous wave (CW) means the beam stays on, delivering steady power — the mode most cutting runs in. Pulsed means the energy is stored and released in bursts, so the peak power of each burst is far higher than the average, while the material gets time to cool between them. That’s what makes fine marking and heat-sensitive work possible.
Same three parts either way; different behavior at the material. Both get their own page: continuous-wave lasers and pulsed lasers.
6 · A short, honest history.
Einstein described stimulated emission in 1917. The first working maser — the same idea at microwave frequencies — ran in 1954. Theodore Maiman built the first laser in 1960, using a ruby crystal as the gain medium, and commercial lasers appeared the following year. The technology was famously called “a solution looking for a problem” in its early days.
It found plenty. The relevant chapter for this library is industrial: gas lasers made cutting and engraving practical, then fiber lasers — a much more efficient way to make near-infrared light — moved metal cutting from exotic to ordinary.
One safety point, because it follows directly from the physics
Everything above is a description of concentrated, collimated energy. The same properties that let a lens focus a beam onto steel let your eye’s lens focus a stray reflection onto your retina — which is why an enclosed machine with working interlocks matters, and why you never defeat them. Near-infrared fiber wavelengths are especially deceptive: they’re invisible, and the blink reflex can’t protect you from what you can’t see.
7 · FAQs
Frequently asked
How a Laser Actually WorksQWhat does LASER stand for?
Light Amplification by Stimulated Emission of Radiation. Every word is load-bearing: light gets amplified, the amplification happens by stimulated emission, and light is a form of radiation. It began as an acronym and became an ordinary word.
QHow does a laser actually work?
A pump source lifts electrons in the gain medium to a higher energy level. A passing photon triggers one to fall back, and the atom emits a second identical photon. Mirrors bounce that light back through the medium so it multiplies on every pass, and one mirror lets a fraction escape. That escaping fraction is the beam.
QWhat are the three parts of a laser?
A pump source (energy in), a gain medium (converts energy to light), and a resonator (two mirrors that build the beam). The gain medium matters most in practice — it sets the wavelength, and wavelength decides which materials the machine can work on.
QWhy can a laser cut when a brighter lamp can’t?
Power density. A lamp spreads its watts over a room; a laser concentrates the same watts into a spot a fraction of a millimeter across. Monochromatic, coherent, collimated light can be focused that tightly — ordinary light cannot, at any brightness.
QWho invented the laser?
Theodore Maiman built the first working laser in 1960 using a ruby crystal. It rested on Einstein’s 1917 prediction of stimulated emission and on the maser, demonstrated in 1954.
8 · Related in this series.
Part of Laser Fundamentals:
- The types of lasers that matter — what changes when the gain medium changes
- What is a CO2 laser? — the gas laser behind most engraving
- Continuous-wave lasers — the steady-beam mode most cutting uses
- The parameters you actually set — from physics to the control panel
- How laser cutting works — what happens where the beam meets material
Keep exploring
One idea leads to the next.
The fundamentals make every later decision easier. Follow the rest of the series, or talk it through with someone who runs these systems.