This Laser Turns Infrared into BLUE

today we're going to be talking about everyone's favorite topic lasers specifically we're going to be talking about how you can turn one color of laser into another now before you get your hopes up this only works with really powerful lasers and is really inefficient in finicky it's also very sensitive to alignment and material quality as you'll see later the laser we're going to be looking at today is one I got online for pretty cheap it's what's called DPSS or a diode pump solid-state laser these units pop up online occasionally as decommissioned medical equipment first a big thanks to styro pyro for telling me about these cool devices also if you haven't checked out his channel yet you really should he does all kinds of cool laser stuff and following his tutorial i learned a lot about how these lasers work link in the description well I bought the device for the laser itself the rest of the machine is full of cool optical phenomena originally this was something called a Raymond spectroscope when it was still in operation and as far as I can tell it still has all the important pieces so after this video I'll continue to tinker with it and see if I can bring the spectroscopy function back online if I can I'll make a video on Raymond spectroscopy and why it's cool but today we'll just be focusing on the laser itself so let's jump right in when you open up the laser you can see the whole optical train but at a glance it can be hard to know what's going on so we'll start at the back and work forwards the first piece is a two and a half watt infrared laser diode that emits light at 808 nanometers hence why the laser is called diode pumped the main energy source is this laser diode rather than a flash lamp when the light leads the diode it's coherent meaning that all the photons are kind of lined up but they're not in a tight beam and the shape isn't ideal so there's a set of prisms and lenses that squish and shape the beam into a nice thin beam line the idea is that you want to pack as much of the laser power into as small of a space as possible with the beam cleaned up and focused it's sent into a small laser crystal the crystal itself is a synthetic material called the atrium aluminum garnet that has been doped with a small amount of neodymium atoms when the 808 nanometer light hits the crystal it bumps electrons orbiting the neodymium atoms up to a higher energy state this makes everything unstable in the atom wants to get rid of the energy as fast as possible to do this they first drop to a slightly lower energy level without releasing radiation mostly by passing it off by special vibrations known as phonons and collisions to other atoms left to their own devices the atoms would release the next bit of energy a spontaneous emission of photons in a random direction and drop to that lower energy State but if a majority of the atoms are bumped up to a higher energy state something called population inversion happens because there's very few electrons now occupying the lower energy state all it takes is a photon passing by to make the electrons suddenly drop back to that lower energy state as stimulated emission when this happens the photons that are emitted are emitted in the same direction as the same wavelength and with the same phase as the passing photon once the electrons are at the low energy state they again release the last little bit of energy without releasing radiation and return to the ground or lowest energy State all of this is known as a four level system the thing is that second energy drop where photons are emitted is variable so different wavelengths of light can come out for this laser they specifically want the 946 nanometer light so to choose which wavelength is used in the system they put special anti reflective and highly reflective coatings on the crystal this traps the kind of light they want in this area here forcing it to bounce back and forth this is known as an optical cavity all of the other light is allowed to leak out or is filtered out of the final beam because stimulated emission makes photons of the same wavelength come out by concentrating the kind of light you want in the cavity you in turn generate more of the same photons as they bounce through the pumped laser crystal now while technically we're working with a different wavelength of laser now we haven't really changed the color we've just kind of made new light to actually change the color we need another special kind of crystal this time the crystal is made of lithium try borate and is what's called a nonlinear crystal this means a few different things first of all the refractive index of the crystal is not the same at different light intensity levels which is kind of weird further because the crystal structure is asymmetrical it opens itself up to a range of optical phenomena now I'll be honest I'm still new to this so I can't give you a good explanation for why a crystal being asymmetrical opens it up to new optical properties but this is found throughout nature all sorts of crystals with a symmetric crystal structure can have weird properties to that end lots of different crystals can be used in this cavity with the same results the one you choose is usually based on the amount of laser power you'll be using as the crystals do saturate past a point and stop working properly low energy systems like this use lithium try borate but higher power systems use the much more expensive beta barium borate when I was first planning this video I had intended to make a crystal of my own for this because monopotassium phosphate is a cheap readily available nonlinear crystal and easy to grow but until I actually got my hands on this laser I didn't realize just how finicky this process is well I still want to make my own crystal and a laser like this from scratch it'll take a while to get working as I still have a lot to learn when the 946 nanometer light is put through the crystal something really weird happens to of the infrared photons actually merge and form a single photon of higher energy and shorter wavelength as the pump beam bounces back and forth in the cavity lots of these higher energy photons are produced and released out both ends this process is called second harmonic generation or frequency doubling and was one of the first nonlinear optical processes to be discovered it's actually a special case of a different effect called frequency summation but it was discovered first because it only requires a single laser beam frequency summing is where you if you take two laser beams of sufficient intensity perfectly match their phase and shoot them through nonlinear crystal the photons can merge into some higher energy photon that is the sum of the two input photons this is obviously a simplification but I think it gets the idea across the reverse of this process is also possible if instead of using a low energy laser light infrared laser use a high-energy laser like a UV laser you can actually have one high-energy photon split into two lower energy ones we'll be exploring this in the future both because it's cool but also because those two photons that come out are actually quantum entangled but back to frequency doubling in the case of this laser the photons being merged are 946 nanometers so when they combine the wavelength is halved remember that smaller wavelengths means higher energies so as a result you end up with a laser that emits 473 nanometer light which happens to be a lovely shade of blue thanks to my camera being able to see a wider range of frequencies than our eyes can you can kind of see this process happening when you look right at the crystal you see a sort of reddish purple haze on either end this is stray infrared light that gets reflected outwards here I've unscrewed the crystal so I can show you how sensitive this setup is to alignment and also the moment when the effect starts to happen this method of converting one color of light to another especially using infrared light as the pump is actually found all over laser optics if you've ever had a green laser you'll have seen a similar system but instead of using the 946 nanometer part of the laser crystals emission they use the much more common 1064 nanometer emission when that's fed into a nonlinear crystal 532 nanometer light comes out which happens to be green so those are diode pumped solid-state lasers a lot of weird and cool physics packed into a tiny box the thing that amazes me is how much math goes into describing this thing working but when you look at it it's just a couple of lenses and some special rocks placed in just the right spot that's what I love about science using materials arranged in just the right way you can make the pieces of the universe do all sorts of weird things if you like lasers and optics and want some sweet science team merch of your own be sure to check out this cool poster I've got in my store speaking of which if you haven't already be sure to check out some of the other cool designs while you're there I'll be making this video in the Wakefield accelerator video into posters and t-shirts next so be sure to check back for those designs I should have the first of them ready by next week's video and with that I'll wrap up this video if you enjoyed be sure to subscribe and click the bell icon to get notifications when I post new videos if you want more content be sure to find me on Instagram I post there regularly and I post lots of stuff there that could be years before it makes it into videos as always a big thank you to my patrons who helped make these videos possible that's all for now and I'll see you next time

Channel: The Thought Emporium

Views: 66,129

Keywords: laser, blue, green, second harmonic, second hamonic generation, shg, frequency doubling, solid state, diode pumped, diode, ir, 808, 946, 1064, 432, 473, raman, spectroscope, spectrum, spectroscopy, nm, spdc, quantum, light, coherant, yag, neodymium, nd:yag, nd, nd yag, population inversion, stimulated emission, spontaneous emission, emission, phonon, photon, lens, prism, refractive index, crystal structure, lbo, lithium triborate, lithium triboride, bbo, beta barium borate, nonlinear, nonlinear crystal, mpk

Id: sF2RDJISnAs

Length: 8min 9sec (489 seconds)

Published: Mon Mar 05 2018