Long Red Lasers

BobH · Post by **BobH** » Thu Jan 17, 2013 3:51 pm

142laser wrote: ... What you and others said was the longer the HeNe laser the shorter the coherence length. This is only true up to a point as the paper clearly shows...thank God. [/b]I was wrong to say the longer the laser the longer the coherence length. It is just as wrong to say the opposite or nobody could make a hologram with a SP 127 or a MG 928. Both of those lasers are excellent.

The graph in Figure 7 of that paper does not describe what I've seen in practical lasers. According to it, the coherence length of the SP-127 and the SP-125 would be essentially the same at 30cm. That's just not true. According to the graph, coherence length of He-Ne lasers essentially hits the bottom at 30cm with about a 40cm long resonator and stays there. Also not true in my experience.

I had a studio with two 4x8' tables in it. I had an SP-124B with 20mW and a 70cm resonator, and an SP-107 (same as the SP-127) with 35mW and a 90cm resonator. I always used the SP-124B for mastering because the obviously noticeable extra coherence length gave better holograms. Well worth the decrease in available power when using silver halide materials. According to the graph, there should be no difference between the two. There is. But I certainly agree that the lasers you're selling are great for making holograms.

142laser · Post by **142laser** » Thu Jan 17, 2013 3:57 pm

Read the paper Ed posted a link to. This seems to agree with what everyone has observed...a 10 mW HeNe has only slightly more CL than a long head like a SP 127 and only 1/3 the power; no wonder holographers use longer heads like JDS 1145P, Melles Griot LHP 925 and 928, Spectra Physics 127 etc. The longer the head the shorter the coherence length (no etalon) is not true past a certain resonator length.

142laser · Post by **142laser** » Thu Jan 17, 2013 4:05 pm

Hi Bob,
Any one laser can fall off the curve in the graph if the mode is not pure TEM 00 and few real HeNe's are. Perhaps lasers with shorter than expected CL have some non TEM 00 modes lurking due to bore size Vs resonator length and or misalignment. The measured CL of a "real" laser will be equal to or less than the graph predicts; not longer. Long live HeNe lasers!

142laser · Post by **142laser** » Thu Jan 17, 2013 4:19 pm

It is great to see there is real interest in this topic regardless of who is right or who is wrong and the classic paper Ed posted a link to is very good to show why longer heads don't suffer short coherence length. The 1.2 mW short laser in that paper had a 1 meter CL but would be hard to use for holograms of any size...

BobH · Post by **BobH** » Thu Jan 17, 2013 4:25 pm

I currently have SP-125, SP-127 and a shorter polarized He-Ne about 14"long, and could measure them. I don't think you'd accept the results from me though, and so won't waste the time. I know! Let's BOTH set up simple Twyman-Green interferometers and measure the coherence lengths of a few lasers. I'll only take the time it's taken for us to discuss it here. You have a BS cube, a couple of mirrors and a lens don't you?

142laser · Post by **142laser** » Thu Jan 17, 2013 10:38 pm

Hi Bob,
The results of the paper make sense! The graph of cavity length Vs coherence length makes sense too. Once the modes span the whole Neon gain curve increasing length should not degrade coherence length within reason (no 10 meter cavity).
The SP 125 in the paper Ed kindly posted was only outputting 45 mW so maybe the low amplitude modes had really low energy extending the CL beyond what a SP 125 doing 105 mW would have. All these discussions assume TEM 00 mode and that is only approximately true for real lasers. My simple hand waving argument came close but Ed's paper really shows the plateau on the curve at a 40 cm resonator length giving about 30 cm coherence length from then on as the cavity is extended further! Therefore 1 meter HeNe lasers work fine for holography!
For a 1 GHz Ne linewidth C/ bandwidth would be C/1 GHz or 30 cm. Nice round number isn't it? I have also seen 1.5 GHz quoted so that would be 20cm CL. Somewhere in there...This discussion has been great! I think we all learned at least one thing...no?

BobH · Post by **BobH** » Thu Jan 17, 2013 11:56 pm

Ed's paper states they couldn't measure the number of modes in the SP-125 without the etalon, so their calculation of its coherence length is speculation, and I don't buy it. The old Jodon laser they used is essentially the same as the SP-124B and their results with it are in line with what I saw. The SP-127 is between the two. If the SP-125's coherence length is shorter than they say (as I say it is), my experience with the SP-127 makes sense. I guess I'll just have to make the measurements.

Another point is the coherence length of a He-Ne laser with an etalon. Philips (ISDH #1, 1982) and Collier, Burckhardt & Linn (Optical Holography, 1971) both calculate it to be on the order of a km at least. Certainly not 400cm! I'm certain that length wasn't measured (via hologram or otherwise) using the apparatus described in the paper above. John Perry is the only one still using an SP-125 with an etalon (I think). Paula Dawson may be the only one who's made pictorial holograms with path length differences greater than 400cm, so the issue is really academic.

Paulos · Post by **Paulos** » Fri Jan 18, 2013 2:53 am

BobH wrote:Paulos, I don't believe the coherence length of a laser with an etalon in it is limited to twice the cavity length.

Bob,
The equations are from Hariharan's "Optical Holography, Principles, Techniques and Applications".

BobH wrote:Those I've used had much, much greater coherence length than that.

Are you referring to gas lasers? I have a 1 meter long Ar+ with an etalon, but I don't think that it's coherence length is more than 2 meters.
Solid state lasers like Coherent's ring lasers or C315M may have a coherence length of 100 meter or even more, but they don't simply consist of a mirror resonator and an etalon.

142laser · Post by **142laser** » Fri Jan 18, 2013 11:49 am

Bob,
I agree the SP 125 numbers seem somewhat suspect but the laser was only outputting 45 mW with no etalon and 8 mW with an etalon so the gain was proably much lower than normal; this could extend the CL with no etalon. I would like to measure one with a "hot" tube doing 100 to 125 mW. TEM 00 mode purity is also suspect on 125 lasers and that will kill coherence length. Also most SP 125 lasers don't run pure SLM even with the thing in perfect adjustment; there is almost always a small second more and that for sure shortens the CL some. At the time they had no way to put the SFPI scope pictures into the paper but all the lasers were also measured on a spectrum analyzer. I believe the SP 125 had a second mode hence the 4 meter limit. This all makes so much more sense to me now...cool.

jdufrasne · Post by **jdufrasne** » Fri Jan 18, 2013 3:47 pm

BobH wrote:How is correction of an embarrassingly fundamental physics error "elitist"? Maybe I should've asked how the one making the error felt about it? Maybe something about how his incorrect belief about coherence length and laser cavities should be considered as seriously as the facts that science gave us long ago? Perhaps I could offer him a trophy for participating?

The problem here is your use of sarcastic "smileys".
In the good old time of holography forum, everybody knews that lack of knowledges or flagrant mistakes can be due to less educational level. Not everybody has the chance to begin or finish a Master in Physic. Think about that, if you can.
Jeff Blyth, a leading light in chemistry (look at the archives) never point errors with a heavy finger. Dinesh was always there to explain "fundamental physic" with simple words. And was a lot of people like this... Colin, Martin, Jeff, Danny, Michael, Hans, holodisc and a lot of other...

Please, keep your trophy, I don't need it.
All what I can found here as reply to my (future) answers is sarcasm... no, thanks...