Forum rules
Finding new homes for holography-related items is encouraged. Even commercial enterprises with goods to offer are welcome as long as it is in the spirit of members helping members. If a for-sale post reads like an infomercial, though, you have probably crossed the line of acceptability.
I don't see the point in trying to make anyone feel badly here. Look at al the interest this for sale thread generated! I work in Physics...for 26+ years and I know a hell of alot about lasers too. However I also know that on a good day one learns two things and only forgets one of them. I hope everyone reading this has learned something about the coherence length of HeNe lasers as a function of cavity length; I certainly have. My initial thought on this was wrong but it turns out so was everyone else. Ed Wesley's classic paper is an eye opener and now that I have spent two days thinking about this it all does make sense...when you get several modes spanning the whole Neon gain curve making the laser longer does not hurt coherence much if at all; it just makes more power to allow your exposures to be shorter! Cool!The nicer we are all to one another the more will be posted and the more we can all learn from one another. Have a great weekend everyone...play nice.
Now who wants to buy a nice long HeNe laser with 20-30 cm coherence length? Come on! Phil Bergeron, Teaching Lab Manager, USF Physics Department, Tampa
Phil is not a beginner. He is selling lasers, hopefully to holographers (and I hope the best for his effort), and he should know better. It's 100% appropriate for me to offer a correction here.
142laser wrote: ... when you get several modes spanning the whole Neon gain curve making the laser longer does not hurt coherence much if at all; it just makes more power to allow your exposures to be shorter! Cool!
I've so far gathered about five sources of practical experience with the SP-125 that contradicts that statement, and the graph in Ed's paper regarding what happens to coherence length with He-Ne lasers longer than about 40cm. People selling things always "cherry pick" their "evidence" to support the claims they make about what they're selling.
Forget the Spectra Physics 125 Bob; that is a now obsolete large bore dual discharge path laser that is certainly not perfectly TEM 00 mode especially with a good tube and DC/RF. I think the paper's theory is sound or lasers like the SP 127 would not even make holograms. The longer the HeNe laser the shorter the coherence length is only true to the point that the modes span the whole neon gain curve and making TEM 00 lasers longer does not make the coherence length go toward zero. Nobody here came up with the correct answer including you. Neither of us knows everything about lasers; at least I admit that and am open to learning more. Did you see me admit I was wrong about three times so far? You were wrong also…nobody posted that coherence length decreases up to about a 40 cm tube then stays about the same as the cavity is made longer. So we should all try to learn from this including you. Forums exist for the exchange of information and the paper Ed kindly posted a link to was a great source; much better than my hand waving arguments were. The “ cherry picking” comment is ridiculous; Ed posted the paper not me and nobody else posted anything based on science to counter it.
If you still think the coherence length of HeNe’s gets shorter and shorter at the mirror separation gets longer please explain why long lasers have decent coherence length. It would appear the theory in the paper has something to it; we know SP 127’s and LHP 928 lasers are great for holography and the paper Ed posted shows why. It also makes sense hand waving wise…the Neon gain curve has a finite bandwidth and cramming more modes under it does not make it wider; you just get more power. So TJ and Ed's paper says the lasers I am selling are fine for holography and "should have" a 30 cm coherence length; who wants to buy one? If they have 0 coherence length I will give you your money back... I am NOT worried.
Bob if you really think the long lasers I am selling are no good for holography just come out and say so. I am certainly not trying to "scam" anyone here with "cherry picked" information. I bnelieve all of these lasers are well established for use in holography so exactly what is your point? Here is the list...
Sam and I have a good reputation as laser sellers and if anyone is really unhappy with the way the laser they buy works we will replace it or refund their money. Thanks for helping me sell the lasers Bob; this thread is getting many views!
142laser wrote: ... You were wrong also…nobody posted that coherence length decreases up to about a 40 cm tube then stays about the same as the cavity is made longer.
Incorrect. I never said someone POSTED that. It's shown on the graph of Figure 7 in Ed's paper.
You're wrong again in suggesting I disparaged the lasers you're trying to sell, or that I ever suggested lasers as long as the SP-127 have a problem making holograms. Why do you have to make stuff up?
It seems clear that long HeNe lasers with many longitudinal modes retain decent coherence length in the 20-30 cm range. So It is wrong to say the longer the head the shorter the coherence length. I was also wrong to suggest that the opposite is true. It looks like if the coherence length is C divided by bandwidth that a 1 GHz bandwidth has 30 cm coherence length or 1.5 GHz 20 cm...period. So putting mode longitudinal modes under the same gain curve does not in any way broaden it. This all assumes the lasers run polarized and TEM 00; adding any other spatial modes will degrade the coherence length so numbers for a particular laser may be somewhat shorter than calculated. "Your results MAY vary"...
I am not really surprised the SP 125 or 125A coherence length would be shorter that the curve on the paper; I think we measured maybe 15 cm max with one doing 95 mW years ago-dead now. I loved those lasers but they are monsters...
So you finally agree that the SP-125 has a shorter coherence length than the paper and calculations suggest. The presence of a small amount of higher order spatial mode light probably decreases it from the theoretical minimum, but never showed up as a problem on its own. That's the problem with making statements based only on theory. Everyone who's actually USED an SP-125 says it has a shorter coherence length. And I've actually USED the SP-127 and SP-124 to MAKE holograms, and I tell you the former has a noticeably shorter coherence length than the latter and that's consistent with the theory.
142laser wrote:It is wrong to say the longer the head the shorter the coherence length.
No, it's a good generalization. Please have a look at the Facebook Holography page. This question has just been discussed in incredible detail, ad nauseam, and I hope the question has been laid to rest for many years to come.
Amen. I will take a look; I have been too bust taking care of my wife and daughter with flu; I either have good immunity or I am next. Long powerfull HeNe's work well for holography so if anyong wants to buy one at a reasonable cost I am here...
Ed Wesley's classic paper is an eye opener and now that I have spent two days thinking about this it all does make sense...when you get several modes spanning the whole Neon gain curve making the laser longer does not hurt coherence much if at all; it just makes more power to allow your exposures to be shorter! Cool!The nicer we are all to one another the more will be posted and the more we can all learn from one another. Have a great weekend everyone...play nice.