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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys,
It's mainly dreamware for me now - but I'm just too curious how-it-works and what-it takes.
I've seen many americans here play with the famous SSY1, an incredibly cute little laser that could pop holes in black plastic and even ionize air when focused at a short distance. Some people tended to torment it horribly, blow it's Q switch and ultimately it's flashtube to bits...
But since it's not available to us europeans, I'm wondering how hard would it be to construct such a thing out of raw bits such as rods, flashlamps, mirrors and Q switches? Glancing at SAM's laser FAQ, it doesn't even seem like an overly difficult task. But really - anyone attempted such a feat and succeded? It would be great to have a laser that can fire few hundred pulses per second and produce things like this:
Most rods around are nd:yag, but I've seen some really gigantic ones of nd:glass. What are their pros/cons? Since krypton flash lamps are hard to get, would nd:glass work better with xenon ones (kilojoules of them can be had cheaply) than nd:yag?
Also, how hard would it be to create a suitable laser cavity? As far as I understood, the biggest problem is in alignment of the mirrors, but is that it? Could it be no more than a metal tube with some mirrors on their ends?
Finally, if one wants really high peak powers to ionize air, a Q switch is required... yet they seem like an expensive part that can easily get blown up.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
Don't bother with Nd:Glass. It's pumping efficiency is much lower than Nd:YAG.
To pump the rod, you have 3 options. 1, a spiral flash lamp. 2, 6 regular flash lamps in a circle around the rod, must be in series. 3, A polished copper oval flash cavity with one lamp.
Putting a q switch and mirrors on it will be a moderately difficult task, and you'll have to likely machine holders for the optics, and build the thing on a metal plate. Otherwise things will get knocked around.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi grenadier - have you built something like that before?
Interestingly, for some reason I thought nd:glass to be more efficient than nd:yag (especially with xenon lamps).
I imagined that a flash cavity could be made from a shiny nickel plated brass tube, and flash lamps placed all around the rod.
For a start, mirrors could be simply glued to some metal blocks and aligned by varying the torque to some bolts. Why is this mirror alignment always considered such a hard task with lasers in general?
The biggest problem I can think of is avoiding specks of dust or dirt getting onto mirrors, rod and into optical cavity - they could absorb light and cause damage.
I'm really unfamiliar with how Q switches work. Why do they get blown so easily in SSY1 when it's pushed too far anyway?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Proud Mary wrote ...
Krypton is used because its output spectrum matches the absorption spectrum of Nd:YAG
Hi - I am aware of that, but I never found information about how much worse xenon lamps are in terms of efficiency. Krypton ones are specialized for lasers and very hard to get.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
There's a comparative spectral output graph of some of the noble gases here:
And this might help you, if you haven't seen it already:
Effective Pumping Scheme for Nd:YAG Lasers Maik Frede Martina Brendel, Carsten Fallnich, Ralf Wilhelm, Ivo Zawischa, Prof. Herbert Welling Laser Zentrum Hannover Aug. 2001
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Brain dump. Hope it helps:
Mirror alignment has to be very precise to allow multiple reflections to remain within he resonator. Some resonator designs are harder to align than others. A Plane/Plane resonator is particularly hard to align. Some result in focal spots within the resonator (which you probably want to avoid). Some resonators use a larger volume of your active lasing medium than others, which is good. The confocal resonator is a common choice.
As I remember, Nd:glass is capable of producing more powerful pulses that Nd:YAG. I think this is due to it having a wider fluorescence bandwidth or lifetime or both or something. Nd:glass is what they use in the giant fusion experiment types of laser I think. Glass has a lower thermal conductivity than YAG so total energy throughput is more limited.
You need to be able to get the light efficiently through the rod ends. Some rods incorporate mirrors in the rod ends, but not usually. The rod ends are often antireflection coated, or may be cut at the materials Brewster angle so that at one polarisation you get near perfect transmission of light.
I think Xenon lamps are still good enough to get decent results, as I remember. The 'pumping cavity' can either be elliptical to focus flash tube energy directly into the rod, or diffuse to bathe the rod. People get away with just placing the laser rod and tube next to each other and wrapping them with aluminium foil I believe!
UV from the flash tube can solarise the laser rod. This can be ameliorated by a cerium doped tubed surrounding the laser rod.
A Q switch delays lasing until the rod is fully charged up (think of it like a capacitor for light) and then suddenly allows light to pass along the resonator producing a 'Giant Pulse'. This will 'test' any optical coatings in the laser resonator to their limits.
I've never built one myself, but read/thought about it a lot some years ago. Good luck
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Don't bother with Nd:Glass. It's pumping efficiency is much lower than Nd:YAG.
As I remember, Nd:glass is capable of producing more powerful pulses that Nd:YAG
So which of these is true? Or the the higher pulse power means lower energy efficiency?
So it seems that xenon lamps are less than half as efficient than krypton's. Atleast for nd:yag rods.
I have no intention nor finances for building anything like this in near future, unless someone donates me or cheaply sells the parts, it's just a fantasy.
Mirror alignment has to be very precise to allow multiple reflections to remain within he resonator. Some resonator designs are harder to align than others. A Plane/Plane resonator is particularly hard to align. Some result in focal spots within the resonator (which you probably want to avoid). Some resonators use a larger volume of your active lasing medium than others, which is good. The confocal resonator is a common choice.
What is a plane/plane and a confocal resonators after all? :P I never thought that a 'resonator' is anything more complex than a pair of mirrors and a rod in the middle, and that I just have to set the angle of the mirrors precisely.
It's intuitive that larger rods are better, but what if it results only in a part of the rod being illuminated? I guess that would be undesirable. I'm sorry I'm asking questions that look noob to you laser guys, but I just got tired of searching and not having them answered.
PS. also, one more thing about Q switches, apart from tipycal pockels cells and passive crytals, I've heard of mechanical Q switches - like a shutter or spinning wheel with holes. How is that even possible, considering the switching needs to be done in matter of few ns? Or it doesn't?
Krypton is used because its output spectrum matches the absorption spectrum of Nd:YAG
For CW pumping krypton is used because the emission lines match the nd:yag's absorption, but at higher power (pulsed with flash lamps) the black body emissions dominate the output (the emission is determined by the plasma temperature, the hotter it is the more is shifted to UV) so xenon is used because it is more efficient.
Making a decent laser is not really difficult. while I did play with ready made pump chambers I also experimented with just wrapping a nd:yag rod in aluminum foil next to the flash lamp. it works but it's not too great.
By far the most tricky part is aligning the mirrors. expect to spend up to a couple of hours walking the mirrors attempting to get laser out. kinetic mounts are a must and whatever they are bolted to it has to be very rigid.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Resonator types Neodymium Doped Lasers
Light that pumps a region of the rod outside of the resonating region is lost. Neodymium doped lasers are '4-Level Lasers' so you can get away without pumping the ends of the rod as they won't absorb at the lasing frequency. A '3-Level Laser' like Ruby needs to be pumped along it's entire length as unpumped end regions will absorb at the lasing wavelength.
The best place for any amateur laser enthusiast is Sams Laser faq. It will feed you fantasies until your eyes bleed. Start here
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Homemade flash-lamp pumped lasers
