NdYag crystal bandwidth ??

[Hemlock_Mike]

What is the acceptable driver bandwidth for the NdYag (1064 nM) crystal in a module? That is 808 nM +/- how much ? Is +/- 3 nM still in the efficient pump range? I see 808 diodes for sale with +/- 10, +/- 3 etc.
10 seems too high but where is the cutoff in the efficiency curve?

Mike

 

[JLSE]

I have used 1watt pumps with crystal sets and have only used diodes indicating +/- 10. I think it is somewhat dependant on how hard you push the diode. If you run a 1watt diode at 800mW, the wavelength would probably be closer to the desired 808nm where as if you run the diode at its full current it increases.

Just a guess, but I wouldnt be too concerned. If the +/- 3nm diode cost similar, id go with it, but if its twice the cost, I would just underpower the +/- 10nm diode instead.

I have got crystal sets to light with 836nm diodes, just for the sake of trying. But the results were exactly what one would expect, much less efficient.

I dont think it changes the output wavelength any, just less power, and maybe some extra heat. I think Pullbangdead would be the best man to answer this one.

 

[Hemlock_Mike]

Thanks -- I was trying to get an idea of the efficiency curve. WOW -- 836 driving a green !!! I didn't think that possible.

Mike

 

[ElektroFreak]

I'm sure FrothyChimp will say something here sooner or later.. Personally, I know that YAG has a much narrower bandwidth than Nd:YVO[sub]4[/sub], so it stands to reason that a diode with as little variation as possible would be ideal... IIRC the peak pump wavelength for YAG varies by +- 1nm depending on the quality of the crystal which makes temperature tuning a bit more challenging when pumping with a diode vs. a flashlamp. I could definitely be off a bit on the latter though..

 

[JLSE]

Thanks -- I was trying to get an idea of the efficiency curve.  WOW -- 836 driving a green !!!  I didn't think that possible.

Mike

LOL, possible yes, with an efficiency of .2% or less  : I was hoping to get at least 5mW from a 550mW pump, but no cigar

But it lit!! I think that counts for something :-/

 

[FrothyChimp]

Yes YAG does not have as wide an absorption spectrum as vanadate. The nice thing about diode pumps, however, is that you can control the wavelength. Generally the wavelength changes about .3nm for each degree C. Keep in mind that it is better to get a hot diode (one that sits above 808nm) than a cold diode as diode lifetime is related to temperature. So if you temperature tune the diode it is better to do it by cooling it. The peak absorption of YAG is 808.5nm with an FWHW bandwidth of 1nm whereas YVO is 810nm with a bandwidth of 8nm FWHW. If you are buying the diode from a manufacturer you can request a hot or cold diode as they know the specifics of each diode from testing. A variance of +/- 10nm is quite a bit and should be avoided unless testing shows the actual output close to 808.5nm. What it is saying is that you could order an 808nm diode and it could be outputting 818nm. Keep in mind that on multiple emitter pumps each emitter can be outputting it's own unique wavelength.

 

[Hemlock_Mike]

Frothy et all --

Thanks for the info.  That's a lot to consider.  I ordered a c-mount 808 from Snoctony tonight.  His specs say +/- 10 but he emailed me that they are actually +/- 3 nm.  I doubt that I could order them in hot and cold  :-?  
I'm rebuilding a module on which I have no specs.  It has a round 1.8 to 2 mm input crystal followed by a doubler crystal on an adjustable mounting.  I've seen every TEM mode possible tonight but low output in green.  I may have toasted my pump when a small wire broke off the driver board.  

Thanks --  Mike

 

[ElektroFreak]

^I don't know what kind of module you're repairing and you may have done this already, but I've found it helpful to mark the position of the original c-mount before you pull it for replacement.. It's saved me a ton of time fiddling just by doing that..... although it sounds like you've got many enjoyable hours of alignments ahead of you

 

[Hemlock_Mike]

It was fiddled with when I got it. All adjustments had been loosened It was putting out TEM 20. I now have it down to a dot but only 10 mW.

The pump diode appears to have no FAC or coupling lenses to the first crystal. I don't know if it's a Y or a V .... I put in a used c-mount which had a tighter beam but it appears more RED than a normal 808 when I shine it on my hand.
Also, my pump now appears trashed. It puts out a lousy beam with 3 bars and low power.

Frothy appears right however. At 1.5 amps, it was running warm and reading 8 mW. I turned it off to cool and dropped it to 1.2 Amps and ---- 8 mW !!! Adding power does not increase power out !! I will wait for the new pump from Snoc.

Mike

 

[ElektroFreak]

It was fiddled with when I got it.  All adjustments had been loosened    It was putting out TEM 20.   I now have it down to a dot but only 10 mW.

[highlight]The pump diode appears to have no FAC or coupling lenses to the first crystal[/highlight].  I don't know if it's a Y or a V ....  I put in a used c-mount which had a tighter beam but it appears more RED than a normal 808 when I shine it on my hand.
Also, my pump now appears trashed.  It puts out a lousy beam with 3 bars and low power.

Frothy appears right however.  At 1.5 amps, it was running warm and reading 8 mW.  I turned it off to cool and dropped it to 1.2 Amps and ---- 8 mW !!!  Adding power does not increase power out !!  I will wait for the new pump from Snoc.

Mike

Regarding the highlight above: Is the diode mounted so it is nearly touching the crystals? If there is no lens, then the diode and crystals must be extremely close together, but not touching.. Also, a FAC lens is quite hard to see.. it looks like a tiny piece of optical fiber attached horizontally across the front facet of the diode.

 

[FrothyChimp]

Mike it is imperative you consider the collimation of the pump beam. The shape and focus of the pump beam in conjunction with the OC determines the active area within the YAG crystal. Proper energy density is imperative without over saturating the crystal. If the pump beam is greater in diameter than the 1064nm beam waist within the resonator (located somewhere in the crystal) set by the shape of the and distance of the OC, that energy is wasted. Stimulated emission will not occur in those areas.

 

[Hemlock_Mike]

Thanks for the continued help guys. This module was disassembled by the previous owner who intended to replace the "green diode" because it had low power. All allignment was tampered with.
The 808 in it showed a wide line output on my hand and indicated no FAC. I asked the seller if a lens / holder was found in front of the laser diode and he said there was none. It appears that there is over 0.100" between the 808 and the Yag Xtal. That's a bunch -- I'm surprised it even lased.
My 2W 808 projected a dot on my hand but I think I toasted part of the emitter when a small driver wire broke. It now outputs a 200 mW ugly little red beam at 1.5 Amps.

I'll keep fussing with it -- your help is appreciated.

Mike