808 to NdYag

Will an 808 nM bar excite an NdYag rod? I think I read
that it works.
HMike

I have a laser which produces 1064nm when the rod is pumped by 808nm and a pair of mirrors on each end of the rod, it is part of a 532nm 2w surgical laser, driven with 20W of IR using a thin little rod. I don't know its output power at 1064nm.

Hemlock_Mike said:

Will an 808 nM bar excite an NdYag rod? I think I read
that it works.
HMike

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Every diode-pumped YAG I've looked at the specs for was pumped by 808nm. I think the issue is that some rods are made for diode pumping, and others for flashlamp pumping. The darker rods I think are for flashlamp pumping with more translucent ones for diode pumping.

Rivem said:

The darker rods I think are for flashlamp pumping with more translucent ones for diode pumping.

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You've got that backwards. A lower intensity pump requires more gain, and therefore higher doping. A flashlamp is much more intense than a laser diode, and therefore requires less doping.

Here's what I have which is being pumped by a 20W 808nm Oclaro:

Cyparagon said:

You've got that backwards. A lower intensity pump requires more gain, and therefore higher doping. A flashlamp is much more intense than a laser diode, and therefore requires less doping.

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Wouldn't the higher dopant level also be required for greater energy density to make use of the more intense pumping? Couldn't the dopant atoms be saturated to their maximum energy state and not take any more energy if there weren't enough? Would they take up the energy before it reached the core?

I feel like I've been told both ways a few times, so I'd really like to know what's what.:thinking:

A rod designed for use with a flashlamp pump will have a heavy concentration of dopant, since the peak power of the flashlamp is enormous. With a CW IR laser coupled to the rod, it is doped quite lightly so that it reaches saturation quickly.

Surplus rods can be judged by their colour. If the rod is almost colourless (In the case of my CW rod most of the colour comes from the AR coating on the rod ends) it is designed for CW use. A rod designed for flashlamp pumping will show a pronounced violet colouring. Note, too, that the dopant concentration can be judged by the colour of the rod when a flash photograph is taken. A rod which appears clear under normal lighting conditions but violet in a flash photo has a relatively low concentration of dopant as opposed to a rod which always appears violet.

All of this from a google search: http://technology.niagarac.on.ca/people/mcsele/lasers/LasersYag.htm

I had assumed trying to absorb all the pump energy at once would damage the YAG. I guess not. It seems instead that high and low dopant will work for either, although efficiency suffers. Thanks, Alaskan. I stand corrected. :shhh:

Alaskan said:

A rod designed for use with a flashlamp pump will have a heavy concentration of dopant, since the peak power of the flashlamp is enormous. With a CW IR laser coupled to the rod, it is doped quite lightly so that it reaches saturation quickly.

Surplus rods can be judged by their colour. If the rod is almost colourless (In the case of my CW rod most of the colour comes from the AR coating on the rod ends) it is designed for CW use. A rod designed for flashlamp pumping will show a pronounced violet colouring. Note, too, that the dopant concentration can be judged by the colour of the rod when a flash photograph is taken. A rod which appears clear under normal lighting conditions but violet in a flash photo has a relatively low concentration of dopant as opposed to a rod which always appears violet.

All of this from a google search: The Professor's Homebuilt Lasers Site - YAG Lasers

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Thanks for the quick research Alaskan.

Cyparagon said:

I had assumed trying to absorb all the pump energy at once would damage the YAG. I guess not. It seems instead that high and low dopant will work for either, although efficiency suffers. Thanks, Alaskan. I stand corrected. :shhh:

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Wouldn't the threshold be higher with the higher level of dopant, so you'd need a higher-energy pump? So, the low dopant would work in a flashlamp situation, but the high dopant wouldn't work in the diode situation?

Edit- Side note: If anybody's got a spare YAG or glass laser rod, I'm interested.

Rivem said:

Wouldn't the threshold be higher with the higher level of dopant, so you'd need a higher-energy pump?

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The doping level "determines the (unbleached) absorption coefficient α for the pump wave (assuming optical pumping) according to α = N σabs. Similarly, the gain coefficient". In other words, higher doping means higher gain and lower threshold, if I'm reading that correctly. (source)

Thanks for all the Info --
I now have to look at my rods.
HMike

PS: I have another FAP coming!!

Cyparagon said:

The doping level "determines the (unbleached) absorption coefficient α for the pump wave (assuming optical pumping) according to α = N σabs. Similarly, the gain coefficient". In other words, higher doping means higher gain and lower threshold, if I'm reading that correctly. (source)

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I don't think that absorptivity and gain are all there is to the lasing threshold. They both are definitely related to the threshold though. The doping concentration page does point out that too high a doping concentration may lead to quenching which may reduce gain or raise the threshold.

The interesting thing about the formulas for gain and absorbtivity was that they were both related to volume as well as the volume concentration of the dopant. Maybe a smaller rod with a higher doping concentration will perform similarly to a normal thickness/length rod with a the same amount of dopant at a lower concentration.

I'd still say Mike will probably have better luck using a more transparent, lower doping concentration rod. They also look like they're better-suited to CW. Seems like the goal of SS lasers' designs is to minimize the dopant for the application anyway.

Hemlock_Mike said:

Thanks for all the Info --
I now have to look at my rods.
HMike

PS: I have another FAP coming!!

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Good luck with the project!

Also, if you need to get rid of any YAG rods, I'm in need of one for a flashlamp-pumped SS laser project.

The rod I'm looking to use is about 3 x 55 mm and appears doped less than some I've used. I just need to make mirror mounts now.
Too many projects for an old fart.
HMike

good luck hemlock. yes, there is a balance between dopant and input power. just be sure to get it relatively even across the rod, and there are no fingerprints/dust, else you'll create hotspots, and the thermal shock will shatter the YAG. its not pretty. more dopant=more absoption requireing higher pump power to saturate the rod and thus lots more heat. but it also means higher energy density. most flashlamp rods I see are nice light pink at about 1% Nd3+ dopant, but I've seen them go higher. most crystals used in pointers are doped with much lower values typically.

Basically too much dopant and you get a high threshold and risk breaking the rod, especially when all the power is concentrated in one spot from side pumping, and too little and you get a weak laser vs your input power. This is especially true of ruby as it often works the same way. most are 0.05% doped, because they already have a high threshold as it is, so few systems are daring enough to go higher, as that much chromium is quite hard to completely saturate with light, as unsaturated parts of the rod reabsorb the laser emission-though this isn't as much of a problem with YAGs, but its still relevant to mention. I've only ever used a bar to pump a small rod thats similar in length to the bar in use, and usually I dont go higher than 15-20A or else the sudden shock will shatter the crystal.

Good Advice --- I can control my PS from 0 to 35 Amps. The diode bar is about 10 mm wide so I'll be a bit concentrated on this rod. The rod is very light purple so I assume light doping.
Thanks for the precaution.
HMike

yeah, I usually just use a lamp, but if you have a yag bar, perhaps 10-20mm ish, then a diode bar is a great way of side pumping it. if CW, just slowly let it warm up over a few seconds and keep the crystal on a TEC or something to manage the temperature. it wont be picky as far as output goes like an NLO crystal, but it does still need to be kept cool and consistant. if pulsing it, then be cautious not to feed too much at a time then it will quickly develop internal stress cracks, or in an extreme case you'll shatter it to pieces outright instantly. perhaps no more than 20W is a good idea? you may have to experiment and find a good balance between output vs input, as obviously past a certain point, you're not going to get any more power either. Thankfully YAGs don't have a lot of re-absorbance problems, and a short florescence time.