REO Ohmeda Raman Particle Counter

[SoulFeast]

So after mild demand, here I am posting about this fascinating laser. Also, some mad props to Dr. Goldwasser in here, a lot of this information is sourced from his tests.

If there is desire to see my other multilines, or the tunable argon, I can certainly bring them over as well. Enjoy!

So this unit popped up on fleebay for about $125 give or take. Looking a little familiar, it activated my brain a little. I did some looking through the pictures on the auction, and the description, and nothing looked bad. So I forwarded it to Sam, and he confirmed it's uniqueness.

We pulled the trigger, I had it sent to him and some tests were done.

First of all, here's far more info than you ever wanted on this unit. The reason this outputs in more than the standard 632.8nm, by my understanding is this. The mirrors, and bore diameter, plain and simple. The coating they used on these is nuts. It's a very broadband coating, that not only allows the reflectance of many lines to occur, but also, it is assumed that the internal OC is about 99%. This means that a lot more light gets reflected within, causing the internal gain to reach tremendous numbers, around 10W of internal gain. This means that with all that extra power, and the broadband mirrors, we have a lot of light breaking through the barrier. Thus, we get 6-7 lines.

Now, some information from Sam on the use of this device.

There is an additional mirror beyond the OC (inside in the area between the two red dots next to the red sticker at the left) which forms an external resonant cavity with the (internal) OC mirror. The external HR mirror is actually coated on the end of a transparent crystal about 1 cm in length, mounted by a pair of electrodes attached to opposite sides which most likely is piezoelectrically active and probably changes length when a voltage is applied to it. A photodiode is mounted beyond the crystal (far left in photo). The signal from the photodiode shows resonance effects at several relatively low frequencies (two dominant ones are around 175 and 350 kHz). The waste beam from the HeNe laser HR mirror can actually be seen to flicker and become much lower in power at the resonance points. The crystal and photodiode may be used to dither the output so that the effects of the inherent laser noise are eliminated. I doubt its supposed to be a very high frequency because the wires to the electrodes are not shielded. It might also be used in a feedback loop at low frequencies.

And now to the laser itself. After receiving it, and powering on...nothing. Just a pink glow.

OH NO! No power! What's happening?! Turns out this one was a soft seal and hadn't been run in ages. This meant it had to be run through the gauntlet to try to get some power back. He ran it for 24/7 for several days:

May 6: 0 uW dim pink/blue
May 7 PM: 19 uW 633 nm dim pink
May 8 AM: 40 uW 633 nm brighter pink
PM: 95 uW 633 nm brighter pink
PM: 110 uW 633 nm brighter pink (Adjusted external mirror.)
May 9 AM: 140 uW 633 nm
PM: 180 uW 633 nm almost normal
May10 AM: 200 uW 633 nm
PM: 235 uW 633 nm
May11 AM: 280 uW 633+640 nm
PM: 313 uW " "
May12 AM: 385 uW " "
PM: 448 uW " "
May13 AM: 578 uW 612,629,633,635,640,650 nm

*May 16 2013*
It's up to almost 1.2 mW, 5-1/2 lines (640 nm is very weak). But the
really neat thing is that the intensity of the other lines doesn't
differ by that much (probably less than a factor of 2). So, one would expect
633 nm to be far brighter but it isn't. Rather, the 650 nm Raman line -
which isn't a fundamental lasing line at all - is similar or may even
be brighter than 633 nm.

Finally, we're up to par with what we're looking for here. 1.2mW and 5-6 lines later, we're lasing a wonderful laser.
612, 629, 633, 635, FAINT 640, and 650nm

Though the last hurdle is still one to not underestimate. Seeing them all!
Via regular means, the 629, 633, and 635nm lines all blend to one due to their proximity. I have pictures from Sam depicting it well through his tests, though I am unable to replicate them with my current setup. Jealousy.

SO! You won't get any beamshots, but you do get to see a beautiful laser

Without further adieu, enjoy!





Here you can see the issues that I am faced with when it comes to getting the 629-635 lines to separate. I am going to invest into some decent focal length lenses, and a refraction grating. Should really help out here.

[Jem]

Brilliant

Great to see this in a thread all of its own. Oh, and don't be fooled into thinking we're all PJ heads over here, it's just that we're not too impressed with laser pointers that's all

Thanks for starting the new thread.

Jem

[The_Doctor]

...don't be fooled into thinking we're all PJ heads over here...

Ok, I'll admit my ignorance here (and testify that Worzel Gummidge is probably wearing my deductive head as I seem to have his...) What's 'PJ'? I know of LPF and already assumed it can't be that, now the inference is kind of opposite. I'm not sure anyone mentioned it last time I was here, so is it new?


To topic.. SoulFeast, I take your point. When Sam said the 650 appeared to match, even exceed the 632.8, I wasn't prepared for a 632.8 measured in microwatts. While I don't expect to find (or afford) a laser this interesting, I might go for a HeNe of exceptionally fine narrow TEM00 beam with good collimation if I can get one in the UK easily. I agree with all who said they are beautiful. The last HeNe I had was multimode, much of it hidden behind an aluminium screen, and it still looked good. I've seen far more elegant tubes in pictures, and I think just for aesthetic reasons alone I might justify getting one. I don't have anything in my home for that reason alone, so a good HeNe will look all the more amazing because of that. A green one would be nice too. But likely expensive.

Before my 'thinking' spiralled out of any kind of intellect, I thought of this:
That laser has multiple lines. As I recall, if you can select one wavelength, energy is 'robbed' from other transitions favouring those that create that wavelength, a bit like selectivity in radios designed to suppress competing signals in a crowded band. Is that happening in that laser, and if so, is a dominant part of the selection, or just sort of incidental? And does it arise purely from careful use of the diffraction grating to angle other wavelengths out of a resonant path?

(Late night, if my post is a tad incoherent, sorry..)

[lasersam]

It's quality, not quantity you're after here. While the 633 nm and 650 nm may be a few hundred uW at the output, they may be several WATTs inside the laser cavity. And 650 nm is a line that's almost non-existent in common HeNes so that's semi-infinitely more power even in the uW level! ;-)

As for common HeNes, there are many to be found at very affordable prices on eBay. Most HeNes have that pure TEM00 low divergence beam one expects from a real laser.

--- sam

[SoulFeast]

The Ohmeda is the only HeNe I've seen with poor divergence. My others are fantastic, with my 543 and 594 being the best of the bunch.

PJ = Projector

[Stiffler]

And here I thought 'PJ' was what I wore to bed. Bad jokes aside, I'm enjoying the thread:-)

[SoulFeast]

Busy researching, but I'll post the other multilines tomorrow.

[chad]

Get some 2d diffraction grating. But that is a unique beast. REO used to be PMS. Right now REO is one of a hand full of coating places that REALLY know what they are talking about. They make some of the finest coatings on the planet. Unfortunately they have attained a status that is totally and completely out of reach for us mere serfs.

But back in the day a very nice gentleman named Ken Sample gave me a couple of my very first lasers. One of which I still have today and it is still making 4.7mw of 632.5.
Jesus! that was 27 years ago.

chad

[SoulFeast]

I have gratings. That very faint one is it going through a grating, then 2 prisms. Now, I could try a grating, then at a distance, use a prism, then a mirror, and another prism. However, I need to get a good FL lens. This thing puts dots about .75in in 15ft.

That's the issue I should have outlined in the beginning. If the spots were small, no problem, but they're so large that they overlap each other like a Mastercard symbol.

[chad]

sounds like it needs an alignment. Take off that danger highvoltage cap and take a couple of pics. It looks like it is hitting the bore. You might have to walk the beam.

Where is mixedgas?
chad