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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.
Sam and I have several long red HeNe lasers available for adoption. The laser pets come fully housebroken with all their shots and a 90 day warranty. They will not lase on the floor, we promise. We have Siemens, Spectra Physics and Melles Griot heads available with adequate coherence length for holography. The coherence length is proportional to the mirror separation so long cavity lasers (1 meter class) are even better than 20 mW length heads like JDS 1145P and Melles Griot LHP 925's. Prices are in the $1K to $2K range with healthy tubes; 22-39 mW output. Who needs a stable red holography laser?
The useable coherence length of MLM Helium Neon lasers is proportional to the mirror separation and can approach about 1/2 the cavity length. So 1 meter long lasers are better than 1/2 meter long lasers. If you only have a couple modes the power will be very low. Spectra Physics 127 and 125A lasers are great for holography right? Your statement is not correct ; yes there are more longitudinal modes but the energy is concentrated in the few near the peak of the gain curve because the cavity is more frequency selective (long interferometer Vs short one). Look this up on Sam's Laser FAQ.
The coherence length of the SP-127 is much longer than that of the SP-125, and all the He-Ne lasers I've ever worked with that were shorter than the SP-127 have longer coherence length than it does. Coherence length is not directly related to cavity length.
I have a SP-127, and I agree that it's a great laser for holography.
But if the question is the coherence length of short tubes versus long tubes (in general, for HeNe lasers), I would wager that short tubes have longer coherence length. In the early literature, it was a common statement that the mirror separation determines the coherence length, i.e., the longer the mirror separation, the longer the coherence length. But on a practical level, I doubt this. I don't know of anyone who has taken the trouble to actually do these experiments with various lengths of HeNe tubes, except apparently TJ who wrote in "A Study Guide on Holography" (1975):
"If lasers of various lengths are available, a useful and instructive experiment is to measure the relative coherent lengths among them and compare them to the lengths of the laser cavities. It should bear out the fact that the longer the laser, the shorter the coherence length."
Look at how the energy is distributed in the modes as you increase the cavity length of the resonator. if you have a short laser with three modes the two side modes have a great deal of the total energy but with a long cavity and many modes the few modes near the center of the gain curve will have most of the energy. Sure your statement is true with one mode and 500 microwatts...SLM by virtue of the short cavity like a SP 119 but as the cavity gets longer it gets far more frequency selective. If your simple rule (more longitudinal modes means shorter coherence length) were true long HeNe's would not be useable for holography at all but fortunately this is not true.
Joe Farina wrote:I have a SP-127, and I agree that it's a great laser for holography.
But if the question is the coherence length of short tubes versus long tubes (in general, for HeNe lasers), I would wager that short tubes have longer coherence length. In the early literature, it was a common statement that the mirror separation determines the coherence length, i.e., the longer the mirror separation, the longer the coherence length. But on a practical level, I doubt this. I don't know of anyone who has taken the trouble to actually do these experiments with various lengths of HeNe tubes, except apparently TJ who wrote in "A Study Guide on Holography" (1975):
"If lasers of various lengths are available, a useful and instructive experiment is to measure the relative coherent lengths among them and compare them to the lengths of the laser cavities. It should bear out the fact that the longer the laser, the shorter the coherence length."
I did that experiment in the Introduction to Lasers class in the L.E.O.T. program at Vincennes University. Coherence length of a He-Ne is inversely proportional to cavity length.
"In the early literature, it was a common statement that the mirror separation determines the coherence length, i.e., the longer the mirror separation, the longer the coherence length."
This seems to be the case. Long HeNe lasers are always recommended for holography and interferometry for both power and useable coherence length. Of course if power is not important one should use a single longitudinal mode laser like a SP 117A or Coherent 200. However for holography power is important also. I have asked Sam to help me on this issue. If it were true that coherence length got shorter and shorter as the laser resonator got longer and a 12" laser head had say a 10" coherence length then a 40 inch laser would not make holograms! This is not so simple.
Try this. Use a short polarized HeNe and make a Michelson interferometer. Start with equal path lengths and high fringe contrast. Move one mirror beyond where the fringes disappear and keep going. The beams will again become coherent and have decent (but lower) fringe contrast at multiples of the mirror spacing; this is basic wave physics. Now it should be evident that the longer the mirror spacing the longer the range you can more the second mirror within the cavity length spacing and still have good fringe contrast so long lasers are better even if they do have more longitudinal modes!
