Long Red Lasers
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.
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
It’s a refreshing to think about something as basic as this. I do like the +- business, as it describes the tolerance on either side of the zero path length distance. But it still gives the same size of an object that could be holographed in its entirety.
"We're the flowers in the dustbin" Sex Pistols
Long Red Lasers
OK this is the for sale page and this thread was supposed to be for selling long red lasers for holography so...how about a 40+ mW Melles Griot LHP 928? Nice TEM 00 beam, about 20 cm minimum coherence length, over $4K new. $1,500 plus actual shipping billed after the fact. I would rather not ship overseas as the box would be huge and the shipping very high. Phil (813) 974-2378 
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Dinesh
Long Red Lasers
It's a touch more complicated than simply the length of the laser. You need to take into account the Doppler and/or Lorentzian broadening (due to possible collisions, though this is low in a gas laser) which creates a gain curve whose width is 2(df), where df is the bandwidth of the laser. However, the gain curve forms an envelope for the longitudinal modes, which are separated by c/2l, these modes may be likened to delta functions within the gain curve. Thus if only one mode occurs, then the laser is operating in single mode (only one delta within the gain curve). This will occur if
c/(2L) ~ df
where L is the length of the laser.
Thus, assume that the bandwidth of a HeNe is 1.5 GHz. Then
L = c/(2df) = 3x10^(8)/(2x1.5x10^(9)) = 3x10^8/3x10^9 = 0.1m
and so single mode will exist if the laser is 10 cm or less.
However, as the length of the laser is increased, then the number of modes within the gain curve increases. The number of modes may be determined by dividing the width of the gain curve by the mode separation. Hence:
N = 2df/(c/(2L))
Thus, if the same HeNe with a bandwidth of 1.5 GHz is now 15cm long (L=0.15m), the number of modes are:
N = {(2x1.5x3 x(10^9)}/{3x10^8)} = 0.3x10^9/10^8 = 3
The coherence length can be approximated as
l(c) = (2L)/N (N>=2)
Thus the coherence length is proportional to the length and inversely proportional to the number of modes. So, insofar as the shorter laser promotes single mode, the shorter laser will have a longer coherence length. However, as the laser length grows, more modes fall with the gain curve and the coherence length decreases and is now proportional to the laser length
c/(2L) ~ df
where L is the length of the laser.
Thus, assume that the bandwidth of a HeNe is 1.5 GHz. Then
L = c/(2df) = 3x10^(8)/(2x1.5x10^(9)) = 3x10^8/3x10^9 = 0.1m
and so single mode will exist if the laser is 10 cm or less.
However, as the length of the laser is increased, then the number of modes within the gain curve increases. The number of modes may be determined by dividing the width of the gain curve by the mode separation. Hence:
N = 2df/(c/(2L))
Thus, if the same HeNe with a bandwidth of 1.5 GHz is now 15cm long (L=0.15m), the number of modes are:
N = {(2x1.5x3 x(10^9)}/{3x10^8)} = 0.3x10^9/10^8 = 3
The coherence length can be approximated as
l(c) = (2L)/N (N>=2)
Thus the coherence length is proportional to the length and inversely proportional to the number of modes. So, insofar as the shorter laser promotes single mode, the shorter laser will have a longer coherence length. However, as the laser length grows, more modes fall with the gain curve and the coherence length decreases and is now proportional to the laser length
Long Red Lasers
Hi Dinesh,
OK so for say an LHP 928 doing 40 mW TEM 00 with a 1 meter cavity and say 10 modes with any real power 2L/n would be 20 cm again. It appears we need to put a few lasers on SFPI's, count modes and do the calculations; then graph coherence length as a function of tube length?
In the limit this has to approach C divided by bandwidth. So short tubes with two modes a few hundred MHz apart would be good except 2L is short and so they can only approach L right? The length of the laser seems to limit coherence length and if you can reduce the number of modes (with an etalon) this helps.
OK...dam...who wants to BUY a laser? I know...buy a 2 mW, 5 mW, 10 mW, 20 mW and a 40 mW and do the tests. We have the lasers.
Phil, pbrgero@usf<dot>edu
OK so for say an LHP 928 doing 40 mW TEM 00 with a 1 meter cavity and say 10 modes with any real power 2L/n would be 20 cm again. It appears we need to put a few lasers on SFPI's, count modes and do the calculations; then graph coherence length as a function of tube length?
In the limit this has to approach C divided by bandwidth. So short tubes with two modes a few hundred MHz apart would be good except 2L is short and so they can only approach L right? The length of the laser seems to limit coherence length and if you can reduce the number of modes (with an etalon) this helps. OK...dam...who wants to BUY a laser? I know...buy a 2 mW, 5 mW, 10 mW, 20 mW and a 40 mW and do the tests. We have the lasers.
Phil, pbrgero@usf<dot>edu
Long Red Lasers
I have a nice, like new melles Griot LHP-928 polarized red HeNe tested and stable at 43+mW to offer at $1,750 with a 90 day warranty. new these are $5,622 with a long lead time ARO. Send me a message to reserve the laser for you, Best, Phil and Sam
PS 15-20 cm ciherence length.....
PS 15-20 cm ciherence length.....