How Lasers Work (in practice) - Smarter Every Day 33

Hey it's me, Destin. Welcome to Smarter Every Day. So I'm in the Netherlands today and I'm hanging out with a buddy of mine that I met through a research project. His name is Johan Kr...

• Reinink.

• That. So, anyway, Johan is a laser expert, and I've worked with him on stuff on the Internet. So we're going to show you how lasers work today.

[Music]

(Destin) So this is an open cavity laser, and can you explain the main components of an open cavity laser?

• Here we have the laser tube, with a helium-neon gas that amplifies the light, and we have at both ends mirrors that are aligned to bounce the light back and forth.

• Should we fire it up?

• Definitely.

[Click]

(Johan) There's a 4 second delay. It's a safety feature.

• Alright. Sweet.

(Johan) Well a helium-neon laser tube is just a glorified neon light. It's just... you have the electrical discharge going through it, like a neon sign. And the special thing is you have here two apertures, and the light can bounce straight through. So at both sides of the laser tube we've placed two high reflecting mirrors, and the light that goes through the laser tube hits the mirror and bounces back in forth through the tube. And each time it passes the tube it gets amplified. You make this mirror slightly transparent - it's 2% transparent - and each time the light passes, 2% moves out and you have, like, a beam going on here.

• Okay, so basically what comes out the other end of the mirror is the imperfection in the mirror. If it was a perfect mirror we wouldn't get any output from the laser, correct?

• Correct.

• But because the mirrors are imperfect we do get a little lasing.

• Yep. In this case, the laser is just specifically made to reflect only 98%, and this is the 2% of the light that comes out.

(Destin) Alright, one thing I've noticed is that inside the cavity the beam is much stronger. You can see the laser hitting particles in the air. But downstream the photons that are leaking out of the mirror... It's not quite as strong. Is that correct?

• Yeah. The light circulating inside the laser is far higher intensity. And if you hold your hand outside of the laser and block the beam, you see that you catch the beam. It looks pretty nice. But if you hold your hand inside the cavity, you'll block the laser from amplifying all the light and the light won't circulate around, and you don't have a laser anymore.

(Destin) So it stops way on the other side. Even though we're stopping it on this side of the tube, we don't get lasing on the other side. I'm waiting on photon torpedo technology myself, so...

• I'm working on it.

• You're working on it? Excellent. But anyway, we want to thank Johan for his time, and uh... can you thank the people who let us in, in Dutch?

• Absolutely.

(Destin) What... did you just call me something?

• Nee... absolutely not.

[Both laugh]

(Destin) Do you wanna get even smarter? Then check out this video by MinutePhysics. Henry explains what's going on inside that helium-neon tube in one of his cool science videos.

[Music]

(Destin) We're going to do Laser Month. Laser Month consists of a lot of videos shot over here in Holland with Johan, Because he is a laser genius. Can I call you a laser genius?

• Definitely.

[Destin laughs] Alright, so now we're going to play laser limbo, ready?

• No.

• Why not?

• It's dangerous.

• Aw, c'mon, laser limbo. Captioning in different languages welcome. Please contact Destin if you can help.