For a short tube one mode oscillates and the coherence length is too long to measure directly. Once many modes are allowed the frequency stability, higher power and concentration of the energy in the central cavity modes makes long cavity HeNe lasers prferable for hologralyy ever without an etalon.
If Bob wants to trade me a SP 127 doing 45 mW for a MG LHP 121 doing 3.5 mW I am game; he can use that. Good luck. Hologram of a die, a thimble...lasers like JDS 1145P, MG LHP 925, SP 127, Siemens...work very well for holography; far better than most diodes do. I suppose I am also wrong there...
Long Red Lasers
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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.
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.
Long Red Lasers
Silly and modest me! I forgot about some research that we had done at Lake Forest College in the early ‘90’s.
Dr. Zhimin Qu was a Visiting Scholar from China, and devised the ping-pong coherence length hologram set up described in the paper. Most of the theory is his and TJ’s, who wrote most of the paper together. I did a bit of the gruntwork, along with Qiang Feng, who was a student there. using a scanning Fabry-Perot to look at the lines, shooting some coherence length holos.
Check it out, it is imminently readable, especially in the experimental verification or otherwise of theory.
http://nlutie.com/ewesly/coherence.pdf
Dr. Zhimin Qu was a Visiting Scholar from China, and devised the ping-pong coherence length hologram set up described in the paper. Most of the theory is his and TJ’s, who wrote most of the paper together. I did a bit of the gruntwork, along with Qiang Feng, who was a student there. using a scanning Fabry-Perot to look at the lines, shooting some coherence length holos.
Check it out, it is imminently readable, especially in the experimental verification or otherwise of theory.
http://nlutie.com/ewesly/coherence.pdf
"We're the flowers in the dustbin" Sex Pistols
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Joe Farina
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Don Gillespie told me the best way to measure coherence length was to make a hologram of a yardstick.
Long Red Lasers
OK I think I have this figured out. In general I am wrong that coherence length increases with cavity length however long HeNe lasers are still great for holography and here is why:
Start with a short polarized tube like an old Perkin Elmer 5800 or Spectra Physics 119. The cavity is so short only one longitudinal mode can lase. The coherence length is then too long to measure directly however the output power will be under 500 microwatts (really more like 300 microwatts maximum). No good.
Now stretch the tube and allow two modes. The coherence length will be much shorter as the bandwidth will be on the order of the mode spacing but this can be a fraction of a GHZ so the CL will be quite long. However with an unsterilized cavity the power fluctuations will be large and the frequency stability rather poor; also the power is still wimpy...a milliwatt or at most two. No good.
Now stretch the tube some more and allow several modes to oscillate. These will now fully span the Neon line bandwidth of around 1 GHz. The CL will be shorter but now you start to have some power, say 5 mW and you can start to make holograms of small objects. The longer cavity starts to make the central modes have more power. Useable for holography but exposures will be long and vibrations problematic.
For longer lasers the bandwidth and hence the coherence length only diminishes slightly as many of the additional modes are on the wings of the Neon gain curve and thus have little power. Most of the power (80%) is in the four or five central modes. However now you have some real power...up to about 50 mW is possible for a 1 meter long laser. Power and frequency stability are now excellent and the coherence length can be on the order of half the mirror spacing. This is the best HeNe laser for holography unless you can find one with an etalon in the cavity like the old Spectra Physics 125/125A had as an option. Even with that in place and adjusted one would see a weak second mode but the useable CL was very long.
So although the coherence length does in general decrease as the laser tube gets longer the other factors outweigh that alone and long cavity HeNe lasers are excellent for holography.
So we have many nice lasers to choose from…who wants one?
Thanks to everyone who wrote in. Maybe I can convince Sam to post some actual spectrum measurements.
Phil (813) 974-2378 or pbergero@ usf<dot>edu
Start with a short polarized tube like an old Perkin Elmer 5800 or Spectra Physics 119. The cavity is so short only one longitudinal mode can lase. The coherence length is then too long to measure directly however the output power will be under 500 microwatts (really more like 300 microwatts maximum). No good.
Now stretch the tube and allow two modes. The coherence length will be much shorter as the bandwidth will be on the order of the mode spacing but this can be a fraction of a GHZ so the CL will be quite long. However with an unsterilized cavity the power fluctuations will be large and the frequency stability rather poor; also the power is still wimpy...a milliwatt or at most two. No good.
Now stretch the tube some more and allow several modes to oscillate. These will now fully span the Neon line bandwidth of around 1 GHz. The CL will be shorter but now you start to have some power, say 5 mW and you can start to make holograms of small objects. The longer cavity starts to make the central modes have more power. Useable for holography but exposures will be long and vibrations problematic.
For longer lasers the bandwidth and hence the coherence length only diminishes slightly as many of the additional modes are on the wings of the Neon gain curve and thus have little power. Most of the power (80%) is in the four or five central modes. However now you have some real power...up to about 50 mW is possible for a 1 meter long laser. Power and frequency stability are now excellent and the coherence length can be on the order of half the mirror spacing. This is the best HeNe laser for holography unless you can find one with an etalon in the cavity like the old Spectra Physics 125/125A had as an option. Even with that in place and adjusted one would see a weak second mode but the useable CL was very long.
So although the coherence length does in general decrease as the laser tube gets longer the other factors outweigh that alone and long cavity HeNe lasers are excellent for holography.
So we have many nice lasers to choose from…who wants one?
Thanks to everyone who wrote in. Maybe I can convince Sam to post some actual spectrum measurements.
Phil (813) 974-2378 or pbergero@ usf<dot>edu
Long Red Lasers
Check out the section in the paper above about one of the coherence length recording set ups. We used a Scotch-Lite covered piece of angle iron for more stability and thrifty use of light.Joe Farina wrote:Don Gillespie told me the best way to measure coherence length was to make a hologram of a yardstick.
I have slides of the experiments which were part of the presentation of the paper. First I need to find them, then I need to scan them, but I just might get them posted sometime next week.
If the equations in the paper make your head spin, persevere to the end, where TJ gives a wonderful geometric explanation of the phenomenon.
"We're the flowers in the dustbin" Sex Pistols
Long Red Lasers
Great Ed! So the coherence length of HeNe's degrades to about 30 cm and stays there as the laser gets longer; and the longer the laser the shorter the coherence length is NOT TRUE (but we knew that right)! Good...maybe someone will buy some lasers after all. Thank TJ for me please. Best. 
Long Red Lasers
1) The coherence length of a HeNe laser WITH AN ETALON is proportional to the mirror separation
2) The Coherence Length of a HeNe WITHOUT AN ETALON = c/Bandwidth.
Bandwidth is typically 1.7GHz for a HeNe laser and 3.5GHz for an Ar+ laser, hence the coherence length is ca. 17cm for HeNe and ca. 8cm for Ar+, not seriously affected by the cavity length.
The spacing of the longitudinal modes (this is approx. equal to the bandwidth of each line) is (c/2L).
A HeNe with an etalon has only one longitudinal mode, so coherence length = 2L, L is the length of the cavity. (coh. length=c/Bandwidth = c/(c/2L) = 2L).
2) The Coherence Length of a HeNe WITHOUT AN ETALON = c/Bandwidth.
Bandwidth is typically 1.7GHz for a HeNe laser and 3.5GHz for an Ar+ laser, hence the coherence length is ca. 17cm for HeNe and ca. 8cm for Ar+, not seriously affected by the cavity length.
The spacing of the longitudinal modes (this is approx. equal to the bandwidth of each line) is (c/2L).
A HeNe with an etalon has only one longitudinal mode, so coherence length = 2L, L is the length of the cavity. (coh. length=c/Bandwidth = c/(c/2L) = 2L).
Long Red Lasers
Ed, we did the yardstick hologram experiment too at VU. It was one of the first experiments in "Measurements & Holography". I still have the hologram I made and a bunch of the objects placed at intervals along the stick.
Phil, nobody ever said that long lasers wouldn't make nice holograms. The statement "So the coherence length of HeNe's degrades to about 30 cm and stays there as the laser gets longer" is not true. The coherence length of the SP-125s I've used were half of that. The statement "However now you have some real power...up to about 50 mW is possible for a 1 meter long laser. Power and frequency stability are now excellent and the coherence length can be on the order of half the mirror spacing." is also not correct in that there are no He-Ne lasers that long that have a coherence as you say.
Paulos, I don't believe the coherence length of a laser with an etalon in it is limited to twice the cavity length. Those I've used had much, much greater coherence length than that.
Phil, nobody ever said that long lasers wouldn't make nice holograms. The statement "So the coherence length of HeNe's degrades to about 30 cm and stays there as the laser gets longer" is not true. The coherence length of the SP-125s I've used were half of that. The statement "However now you have some real power...up to about 50 mW is possible for a 1 meter long laser. Power and frequency stability are now excellent and the coherence length can be on the order of half the mirror spacing." is also not correct in that there are no He-Ne lasers that long that have a coherence as you say.
Paulos, I don't believe the coherence length of a laser with an etalon in it is limited to twice the cavity length. Those I've used had much, much greater coherence length than that.
Long Red Lasers
Bob look at the graph in the paper Ed posted near the end. The SP 125 is a special case and is not really even pure TEM 00 even when perfectly aligned due to the large bore and alignment may be critical to get rid of undesirable spatial mode contamination. Were you using RF also or just DC? Adding RF excitation for sure promotes multi-mode output; you can see the beam get larger by eye when the RF is activated (I added a switch). The resonator length is what is important -not the plasma tube length (although the tube has to fit). Once the laser resonator gets long enough (with sufficient gain, that is the tube part) to allow several modes to occupy the whole Neon emission bandwidth making the laser longer causes little if any degradation of coherence length. Adding an intracavity aperture to a SP 125 would have helped some but also cut down the power.
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. 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.
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. 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.
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Joe Farina
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I don't think so. If a number of different HeNe's (with different cavity lengths) are available for testing, then I would lay heavy odds that a "useful generalization" could be made by saying that shorter cavities have longer coherence lengths. I have not done this testing myself, so I hope that is clear. But both Bob and Ed have, so I think they should be listened to.142laser wrote:...I was wrong to say the longer the laser the longer the coherence length. It is just as wrong to say the opposite...