What is a laser and why is the fiber laser the one that melts metal?

laser is a device that emits light with three properties that ordinary light lacks: it is monochromatic (a single color, a single wavelength), it is coherent (all waves are in phase), and it is directional (the beam barely widens as it moves away). Of these three, the only one that truly matters here is the ability to concentrate a huge amount of energy into a tiny point. Everything else follows from that.

A 500W light bulb spreads its energy in all directions and across all colors. A 500W laser focuses it into a circle less than 100 microns in diameter. That's about 6 megawatts per square centimeter; steel melts at that power density in microseconds.

How is that light produced?

The name explains it: Light Amplification by Stimulated Emission of Radiation.

Three things are needed.

An active medium: a material whose atoms can store energy by raising an electron to a higher energy level. A pump source that supplies that energy, usually other laser diodes. And an optical cavity: two mirrors between which the light bounces, passing through the medium many times.

The key phenomenon is stimulated emission. An excited atom, if a photon of just the right energy passes nearby, releases its own identical photon: same direction, same phase, same wavelength. That photon triggers two more, those four, and so on. That's why light is coherent: it doesn't become ordered afterward, it's born already copied.

Why fiber

In an LPBF machine, the active medium is the core of an optical fiber doped with ytterbium, a rare-earth element. The fiber is several meters long and is wound inside the housing. Pump diodes inject light along its entire length.

This geometry—very long and very thin—solves at once the two problems of previous lasers:

• It dissipates heat automatically. The fiber's surface area is enormous compared to its volume, so excess heat dissipates without any sophisticated cooling. A classic solid-state laser has a thick rod that heats up internally and becomes optically distorted.
• The beam is already guided. The fiber itself dictates the beam shape. There's no need to align mirrors: this results in extremely high beam quality that remains stable for years.

Added to this is an electrical efficiency of around 30%, compared to 2-3% for a CO₂ laser, and a lifespan of tens of thousands of hours without optical maintenance. That's why the fiber laser dominated the industry: not because it's more powerful, but because it's the only one that can operate for eight thousand hours a year without any maintenance.

The four numbers that define a laser

Powder bed fusion is almost always performed continuously with modulated power. What is adjusted during the fabrication process is not the presence of the pulse, but rather the instantaneous power and the speed at which the point moves. The combination of these two factors is the linear energy density, and this is the parameter upon which the quality of the part depends.

Why was copper so difficult to obtain?

Here, wavelength ceases to be a catalog data point and becomes the problem.

metal absorbs a fraction of the light that reaches it and reflects the rest, and that fraction depends on the color of the light. At 1080 nm, the near-infrared wavelength of an ytterbium laser, steel absorbs a reasonable amount. Polished copper reflects almost all of it. Moreover, it conducts heat exceptionally well, so what little heat does enter escapes before melting anything.

Two outputs, and both are in use:

• Change the color. Copper absorbs green light, around 515 nm, much better. It's the elegant solution, but it's expensive.
• Change the alloy. GRCop-42 or CuCrZr retain much of the conductivity and are much easier to process than pure copper. This is the path that has brought copper into real-world production.

There's a helpful nuance: as soon as the metal melts, its reflectivity drops. The difficulty lies in getting started, not in maintaining it.

From laser to part: optics

The beam exits the fiber and still has to reach the correct spot in the powder bed, at meters per second and with micron-level precision. That's the job of the galvanometer head: two extremely lightweight mirrors mounted on moving-magnet motors that rotate a few degrees, one for each axis.

Behind it all is a lens that does the hard work. If the beam is deflected by two mirrors and focused by a normal lens, the focus falls on a curved surface: sharp in the center and blurry at the edges. This is solved in two ways. With an F-theta lens, designed so that the focal plane is flat and the shift is proportional to the angle. Or with a motorized beam expander in front of the mirrors, which dynamically corrects the focus depending on where it's pointing.

That assembly is the one that needs to be calibrated, and it's where it's decided whether a part comes out with the dimension in the center of the build plate and out of tolerance in the corner.

Security, in one line

500W beam at 1080nm is invisible and destroys a retina before you can even blink. That's why the chamber of an LPBF machine is a sealed enclosure with interlocks and filtering windows, and why no one opens anything while the laser is emitting. It's not bureaucracy: it's the reason these machines are certified as Class 1 despite containing a Class 4 source.

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How to check and adjust this optical assembly is explained in how to calibrate the laser and its optics. The complete process it's part of is described in what is a metal 3D printer. And which metals absorb light well and which don't is explained in what materials can be printed on metal.

And what power supply does each team use, with its power and optics, in the specifications of ALBA 300 and ALBA 500.