Pulsed Laser Diodes - thoughts? - Page 1
Forum section: electronics / general

author: axemaster
author_rank: Contributor
timestamp: December 22, 2020, 05:00:17 pm
content: I am a grad student looking into the feasibility of designing a custom laser cutting/ablation system. The idea is to use a very fast pulsed laser (probably <50ns pulse) to vaporize a few nanometers at a time from the surface of a target. The laser spot size would be focused to <30um. Between pulses, the laser would run at low power to perform an interferometry measurement. The duty cycle and average power can be very small.I forsee the following difficulties:1. I want to use a reasonably cheap laser diode. What is the susceptibility for it to be damaged by very brief, high power pulses? How should I find a suitable diode?2. What is the level of difficulty in terms of damaging any lenses or optics in the path of the laser (including a beam splitter for the interferometry).I have plenty of experience designing electronics and plan to build my own control hardware.What do you guys think?

author: Slh
author_rank: Regular Contributor
timestamp: December 22, 2020, 05:28:48 pm
content: Aside from the usual warnings (warning - do not look into laser with remaining eye etc...) A few questions doing to mind.What do you plan to cut? Have you ever worked with lasers or laser cutters before? Have you ever worked with optics? What wavelength do you need? What peak power (light, not electrical) do you need? Do you have access to the appropriate safety gear (only joking, I expect that this will be a class 4 so you want to put it in a sealed box)I've worked on the electronics for class 4 solid state YAG lasers before (but never with the lasers as I like my eyesight). I don't think you can get the pulse width that you're after with a laser diode. You could probably get a few microseconds but you'll struggle to do better than that at significant peak powers. You probably want a q-switched YAG or YILF laser. Budget £5k-100k if you want a new one depending on average and peak power.The sort of problems that you'll come across include ablating the expensive optical coatings on the optics, aligning everything, keeping it aligned when it all heats up, focussing the laser, keeping the laser beam away from your eyes and everyone elses etc. If you did end up with a YAG laser then you'll have fun attenuating the signal for inferometry (or just add a small laser diode for that bit).

author: axemaster
author_rank: Contributor
timestamp: December 22, 2020, 07:20:02 pm
content: I plan to do extremely precise 3D "shaping" of piezo materials such as PZT, with the goal of eventually achieving a resolution down to a few micrometers.I have not worked with laser cutters. I have worked with some very basic optics.I am currently working to research the piezo material parameters to determine exactly how much power is needed. My hope is that by using a very small spot size, the peak power of the laser can be reduced. I also plan to use the very fast pulse to minimize thermal diffusion into the material - the goal is to vaporize the top few nm without heating up the whole material too much.In terms of not damaging the optics - If I position the focusing lens a few mm away from the target, the beam spot size at the surface of the lens would be much larger than at the target. I suspect that this will allow me to avoid burning the lens coating. Obviously I will have to do the math on this though.I'm pretty confident about generating fast pulses, I have achieved sub-nanosecond pulses before with cheap laser diodes (at lower power levels). Why do you expect fast pulsing to be difficult?I do plan to put it in a box and follow all University safety regs. I will consult with the appropriate safety people.

author: james_s
author_rank: Super Contributor
timestamp: December 22, 2020, 07:42:14 pm
content: You can't pulse laser diodes at much higher than continuous rated power the way you can with LEDs. You will experience something called catastrophic optical damage where the energy circulating in the cavity becomes high enough that it damages the polished surfaces forming the mirrors. Essentially what you are trying to do to the material you are pointing the laser at will instead happen to the optics formed by the die of the laser diode itself.

author: jogri
author_rank: Frequent Contributor
timestamp: December 22, 2020, 07:49:39 pm
content: Well, if YAGs work for you why not start with a (relatively cheap) used tattoo removal system? Don't know the exact pulse length (somewhere in the low ns to ps range), but they can easily reach the MW range. Also, they usually include screw-on KTP crystals to get 532nm.But you really, really, really shouldn't work with such lasers if you've never touched one before. They are even more dangerous than HV, one screw-up and you are blind for the rest of your life. I personally wouldn't touch one with a stick and i have messed with invisible lasers in the 100+W range. You really can't overstate how dangerous those things are, if you want to work with any class 4 lasers get yourself help from someone who knows what he's doing.

author: Marco
author_rank: Super Contributor
timestamp: December 22, 2020, 09:53:52 pm
content: Quote from: axemaster on December 22, 2020, 05:00:17 pmLaser diodes do not have significant pulse power. AFAICS the "standard" academia DIY pulsed laser is with pumped double clad fibre, either as a fibre amplifier or with Q-switching.Some cheap fibre pulse lasers have been coming out of china last few years. I think mostly for the market of glue removal beneath glass backs for mobile phones.

author: Slh
author_rank: Regular Contributor
timestamp: December 22, 2020, 11:23:37 pm
content: Maybe I'm missing a trick but my reasoning for not being able to generate the power and short pulses that you want via laser diodes is mostly the issues with getting the electrical power into them.Even if you only need a few kW peak powers (I'd expect 10s of MW) you'll struggle to turn it on and off again in sub 50ns. You'll have to overdrive them with voltage to get them on and off that quick. 10kW at say 10V (assuming a single cheap and short-lived laser diode) will need you to be switching 1000A. You'll need to be playing with GaN to get that much power while switching so quickly. By the time you've got that figured out you may as well have got a YAG laser.

author: dmills
author_rank: Super Contributor
timestamp: December 23, 2020, 02:26:59 pm
content: Laser diodes run stupidly high intracavity power levels, and you will case optical damage to the laser diode itself if you try pulsing a nominally CW diode.There are such things as "Quasi CW" laser diodes, which really means pulsed diode with enough cooling that you can get a reasonable PRF.Then there are explicitly pulsed diodes, these have low gain so you need a lot of current to get to threshold, and thus cannot be run CW, but they do manage reasonable peak power levels. Pulse forming networks are key to getting these to work properly.What you usually want for ablation is a Q Switched laser, but seriously dangerous to vision in many cases. Were I doing it I would probably be reaching for a fibre diode laser from someone like ILP which are available in a Q Switched variant, and are an easy to operate self contained package that usually runs at somewhere in the 1um wavelength region.The laser engineering on its own is a nice grad student project, never mind doing the micromachining once you have the miserable thing. If the machining is the interesting part then I would buy the source in.Do talk to your institutes laser safety people, not only are they generally knowledgeable folks, but they know where the lasers that the university already has are hiding, using something that some previous research used is good for your budget....