I’m going to start a series of posts chronicling my attempt to build a laser…mostly from scratch.
I’ve always been a little obsessed with lasers. Even my senior engineering project in college used one. For the past few years I’ve committed to building a new portable laser each year, which each successive version double the power of the previous one. But these “lasers” aren’t much on the engineering front: you buy a laser diode, an off-the-shelf driver board, and a few optics and bam, you have something that can start a campfire. And will blind you instantly unless you are very very careful.
But the beam quality of these things is horrible. The laser diodes are designed to spray out a huge amount of light, but not in a terribly well-behaved fashion. Even with a bunch of corrective optics the beam is not great. Also, this isn’t really “building a laser”. All the really cool laser-y stuff happens inside the laser diode.
So I want to really build my own. There are tons of different types of lasers you can build in your garage of varying levels of difficulty and power but I’ve settled on a powerful DPSS (diode pumped solid state laser) that will output a lovely green beam. Reasons:
DPSS lasers don’t need high voltage power supplies, vacuum pumps or gas supplies like gas lasers do.
These are very common in industrial settings so optics and the necessary crystals shouldn’t be too hard to obtain.
The beam quality should be quite good.
I want a visible beam. Infrared is easier but way less satisfying. And I might add more dangerous because you have no indication it’s on.
It needs to have a reasonably high power level. Otherwise it’s just an expensive cat toy.
So begins the saga. And quite a saga it will be. I’ll need to learn power electronics, optics, laser physics and build lots of custom electronics and machine custom parts to build this. Hey, it’s a pandemic. What else am I going to do?
Principle
Ok, I’m not going to go into how a laser works (you can look that up on Wikipedia) but I will describe how the laser I want to build will work. The basis for this laser will be a crystal of Nd:YVO4 (Neodymium Doped Yttrium Orthvanadate). When this crystal is hit with light of a certain color, it will emit its own light in a different color. But it does so in such a way as to make it possible to build a laser from it. The color of light is measured in nanometers: blue is around 400nm and red is around 600nm. Nd:YVO4 is excited when hit by light at 808nm, which is invisible to us (although it will show as a dim pink to most digital cameras). It produces its own light through spontaneous emission at 1064nm, which is also invisible. That light is amplified as it bounces back and forth between two mirrors. The light hitting the excited crystal atoms induces those atoms to emit their 1064nm photon, and on and on. One of these mirrors is very partially transparent and the resulting laser beam emerges from it.
But I said I wanted this to be visible. That’s where another magic crystal comes into play. LiB3O5 (Lithium triborate, or LBO) is a crystal with “non-linear” characteristics. When it is hit by light at 1064nm it absorbs two 1064nm photons and emits one 532nm photon. This is a nice bright green. And there’s some quantum mechanics going on there that I may never understand.
Not That Simple
Ok, so the principle isn’t super difficult. Buy some fancy crystals and mirrors, supply the right light source and boom, instant laser. Well, not quite:
The 808nm light source has to be really bright. Like another laser. I will be driving this laser from another laser. But lasers drift in frequency with temperature, so to keep the source at 808nm I need to carefully control the temperature of this “pump” laser.
Each stage of this induces losses because the conversion isn’t perfect. So to get a high power output, my 808nm pump laser actually has to supply a huge amount of light. I’m choosing dual 40w pump modules, for 80w of incoming laser light. A 40w pump module requires over 50 amps of current.
The Nd:YVO4 crystal needs to stay at the right temperature, so it needs to be cooled too. Also, Nd:YVO4 is hard to cool because the laser action happens in the very center of the crystal and it has very poor heat transfer. If it’s not kept at the right temperature it won’t be efficient. If it gets too hot it can crack or the surface of the crystal can warp (called thermal lensing) which messes up the rest of the optics.
The LBO crystal also needs to be kept at the right temperature to work. It also needs to be positioned at a precise angle and the beam needs to be polarized for the LBO to be efficient.
You want to filter out the infrared so it never leaves the cavity. At high powers the only really good way to do this is to have mirrors that either reflect or transmit light depending on frequency. So you want an output mirror that is 100% reflective at 808nm and 1064nm but 100% transparent at 532nm. That’s a pretty complicated piece of glass.
Good luck aligning all these optics with light that you can’t see and will blind you instantly.
So hey, fun project!