Question about increasing laser power

[Josiah]

TL;DR

Would this work?

Hi, Im not sure if this is the right forum to post this question. If you had a laser, then reflected it back on its self, and then reflected it back again, would this enable a super high powered laser? Because it would be continuously reflecting back on it self at the speed of light.

 

[.Scott]

There are two kinds of lasers, continuous wave (CW) and pulse. Since you're thinking of CW, that is what I will address.

Most CW lasers work by reflecting light back and forth between two mirrors. A certain amount of energy is pumped into that system and that results in a monochromatic beam that (within some degree of precision) is in-phase with itself. One of the two mirrors will be only partially silvered (or in certain cases, missing) and so a portion of the laser beam crosses to the exterior and is available for use.

If both mirrors are 100% reflective, then you might think that the more energy you pump in, the more that would accumulate in the laser in the form of a steadily power-increasing laser beam. To some extent, this is true. But what will happen is that the elements responsible for pumping energy into the beam (the ionized gas, crystal, or semiconductor) will reach a limit where they are absorbing as much energy as they are pumping into the laser beam.

At that point, the laser beam will remain at that power limit (or possibly begin reducing) as the equipment uses the remaining energy to produce heat.

 

[FactChecker]

I believe that is how they work, up to a certain power (20W?). Trying to do that too much starts to melt parts. I am just a casual amateur, but here is an interesting YouTube video: I Built an Insane 260W LASER That Cuts Metal !
Of course, the military ones probably are a different thing entirely.

ADDED: A lot of the video comments say how crazy reckless it was. I'll leave this stuff to the experts and the nuts, and just enjoy the YouTube videos.

 

[Roberto Pavani]

up to a certain power (20W?)

The answers above are correct. Just adding a safety note: any laser above ~500 mW (Class 4) requires proper eye protection, not just from the direct beam, but from diffuse reflections off walls, dust scattering, and
secondary reflections from ordinary surfaces. At 260W, scattered light alone can cause permanent eye damage.

 

[DaveE]

TL;DR: Would this work?

Hi, Im not sure if this is the right forum to post this question. If you had a laser, then reflected it back on its self, and then reflected it back again, would this enable a super high powered laser? Because it would be continuously reflecting back on it self at the speed of light.

Virtually all lasers use this, or similar, techniques internally to function optimally. You won't make their design better by adding your own. In practice, retroreflections can cause significant problems in performance and reliability for lasers.

 

[Roberto Pavani]

TL;DR: Would this work?

Hi, Im not sure if this is the right forum to post this question. If you had a laser, then reflected it back on its self, and then reflected it back again, would this enable a super high powered laser? Because it would be continuously reflecting back on it self at the speed of light.

What you're describing is essentially a passive Fabry-Pérot cavity. This is actually a well-known setup — the intracavity field can be enhanced (roughly ~100× with good mirrors), but losses always set a steady-state
limit. You don't get infinite accumulation.

In practice, what works is using the cavity with a gain medium (i.e., a laser), where the feedback is actively maintained. Here's a thesis that explains how laser cavities and optical feedback work in detail: Laser feedback interferometry with semiconductor lasers

Keep in mind that even achieving a modest cavity enhancement requires lab-grade alignment (sub-micron precision), that is very hard to obtain on home made project.

 

[DaveE]

Keep in mind that even achieving a modest cavity enhancement requires lab-grade alignment (sub-micron precision), that is very hard to obtain on home made project.

Home built confocal (and similar) resonators aren't THAT finicky. Many people have built low performance, low reliability, and often higher cost lasers at home. It's OK as a maker hobby, but not the way to actually get a good laser. But resonator alignment isn't really very hard. Even I could align the mirrors on big commercial ion lasers with virtually no talent or training.

A true Fabry Perot resonator with (separate) flat mirrors really doesn't make a great laser*, as you implied, it's too finicky, even after alignment. Also, coupling a back reflection into a cavity with proper wavefront matching is just too hard and pointless IMO.

* But often quartz xtals are used as an intracavity BPF, they are built permanently aligned.

 

[Josiah]

There are two kinds of lasers, continuous wave (CW) and pulse. Since you're thinking of CW, that is what I will address.

Most CW lasers work by reflecting light back and forth between two mirrors. A certain amount of energy is pumped into that system and that results in a monochromatic beam that (within some degree of precision) is in-phase with itself. One of the two mirrors will be only partially silvered (or in certain cases, missing) and so a portion of the laser beam crosses to the exterior and is available for use.

If both mirrors are 100% reflective, then you might think that the more energy you pump in, the more that would accumulate in the laser in the form of a steadily power-increasing laser beam. To some extent, this is true. But what will happen is that the elements responsible for pumping energy into the beam (the ionized gas, crystal, or semiconductor) will reach a limit where they are absorbing as much energy as they are pumping into the laser beam.

At that point, the laser beam will remain at that power limit (or possibly begin reducing) as the equipment uses the remaining energy to produce heat.

I don't understand, if the mirrors are 100 percent reflective, then why would they absorb the energy?

 

[PeterDonis]

if the mirrors are 100 percent reflective, then why would they absorb the energy?

He didn't say the mirrors, he said "the ionized gas, crystal, or semiconductor" that's pumping energy into the beam.

No real mirror is 100 percent reflective, but that just adds additional losses on top of the ones @.Scott was referring to.

 

[Josiah]

I believe that is how they work, up to a certain power (20W?). Trying to do that too much starts to melt parts. I am just a casual amateur, but here is an interesting YouTube video: I Built an Insane 260W LASER That Cuts Metal !
Of course, the military ones probably are a different thing entirely.

ADDED: A lot of the video comments say how crazy reckless it was. I'll leave this stuff to the experts and the nuts, and just enjoy the YouTube videos.

What if the parts didn't melt, how much power could it produce?

 

[berkeman]

What if the parts didn't melt, how much power could it produce?

1.21 ggigawatts.

 

[.Scott]

What if the parts didn't melt, how much power could it produce?

If I catch the spirit of you question, you will be gratified to discover that one can design a laser that will continue to operate as it is being vaporized. Project Excalibur created such a design. Had it been developed, it would have been most emphatically a pulse laser.