Suitablity of Diode lasers for holo's

On: Tue, Jan 22, 02 10:35:39 PM

Jim McPherson wrote:

Hi, I'm doing experiments with using different diode lasers for holography. So far all of the diodes that I have used have been perfectly (nearly at least) polarized. I just came across a fantasitc diode, however when I put it through the polarizer it never completely dissappears. I'd say it loses 90% of its intensity. Will this laser still work for holography or is total polarization a must? -Jim
RESPONSES

Tom Burgess - Wed, Jan 23, 02 02:07:21 AM

Polarized light is not essential for holography. It can be very useful, though, for reducing unwanted internal reflections in film or glass plates that can cause ugly fringe patterns. (search term: Brewster's angle) If you want closer to 100% polarization you can always just use a polarizing filter. I'm curious as to which laser diode you are referring to, why it is fantastic, and whether you are looking at the bare diode output or using a collimating lens assembly. Also curious as to how well it works for holography. 24.67.253.203

Colin Kaminski - Wed, Jan 23, 02 11:10:17 PM

Jim, Please post your results. I would love to know what diodes are TEM 00 as well as having a long coherence length. Thank you. 64.170.193.81

Jonathan - Fri, Jan 25, 02 01:20:38 PM

Jim, to answer your question "Will this laser still work for holography or is total polarization a must?": From what you describe it sounds like your diode is fairly well linearly polarized, and even if it was completely randomly polarized you could still make a hologram with it, as long as its other properties were suitable (TEMoo, sufficient coherence length, narrow enough bandwidth, etc.). The real question is, how good would the hologram be? If you want to make better holograms it's important to understand polarization. It's a property of all light, which is why polarized sunglasses are popular. But in holography you can take advantage of it in several ways. One way is to reduce unwanted reflections, as Tom pointed out above. The other important point is how it affects the efficiency of the interference process itself. The higher the efficiency the brighter will be the hologram, because the fringes will be higher in contrast. It depends on the relative angle of the polarization vectors of the object and reference beams. The interference process works best when they match, and this can be arranged only if you have a laser that has high linear polarization. The interference process works but only with low efficiency if the laser is not polarized (ie. randomly polarized), because there is still a substantial amount of light with polarization vectors in the same direction. The holograms will never be very bright. The interference process won't work at all (efficiency=0) if the laser is linearly polarized, but you arrange the object and reference beams so that the polarization vectors are at 90 degrees. (Not that hard to do if you work with overhead geometries.) In a similar way, the efficiency of interference is directly affected by the relative angle of incidence of the object and reference beams, parallel being highest and orthogonal being lowest. Again, brightness is controlled by degrees. Hope this helps. I suggest Saxby's book for a fuller explanation. What diodes are you referring to in your post? 209.90.160.55

Jim McPherson - Fri, Jan 25, 02 06:39:44 PM

Thanks for the explanation. The diodes I'm refering to are "silver bullet" laser pointers from ebay. They are 650nm and rated as class IIIa. They cost about $3.00 each. These are NOT the cheaper 5 head pointers that you see so many of on ebay. They have a nice driver inside and are generally of very high quality. When I took them apart I found that the pot on the driver board was positioned right above the lasing threshold, so I gave it a slight turn. BOOM the diode JUMPS in output with just a small turn. So far I've run it a good bit at this higher output and everything seems ok. I'd estimate that I'm getting 7-10mW out of this thing... not bad for $3.00. I haven't had time to do any holography with it yet, but I've got to give it a try... if only it were completely polarized it would be perfect. -Jim 68.49.37.119

Tom B. - Sat, Jan 26, 02 01:36:23 AM

Thanks Jonathan for the useful info - I sure wish Saxby's "Practical Holography" book was still available. Recall seeing a comment here a while ago that a new edition was in the works, but not likely to be out for a long while. Will have to experiment with various objects and coatings to see how practical it will be to maintain polarization of reflected object light to match the reference. It should indeed give better fringe contrast, but I expect it will be hard to do for natural looking objects with diffuse reflectivity that scrambles the polarization. Something I might want to try is to put a large sheet polarizer between the object and plate to reject diffuse reflected light of the "wrong" polarization. In a split beam setup the object beam could be made brighter to make up for the filter loss. I wonder if there would be any net gain in a single-beam reflection setup? Probably not enough to make up for the filter loss, but maybe worth a try. regards, tom 24.67.253.203

Colin - Sat, Jan 26, 02 02:01:21 AM

Would a sheet of polarizer become a HOE in the final hologram? Would it only admit the polarized portion of the replay beam? 64.170.193.81

Tom B. - Sat, Jan 26, 02 02:49:46 PM

Re polarizing HOEs, this question came up under the topic "Polarization" in the archives 1-50. And never got anwered :( I kind of doubt that it would work that way, otherwise a hologram with linearly polarized object and reference beams would replay only in one polarization, which does not seem to be the case. Noticed that Steve Michael expressed the opinion that inserting polarizing filters in holographic setups is a no-no. Not sure what he's talking about when he refers to lines-per-inch with regard to polarizing filters - I thought these filters had no regular line structure at all - that they were just a thin sheet of aligned submicroscopic crystals. 24.67.253.203

Tom B. - Sun, Jan 27, 02 04:44:16 AM

Re the Saxby book, there's a used copy available on Amazon.com for $115. Unfortunately the seller will ship only within the U.S. (I'm in Canada). Saxby has some review comments, mentioning that the 3rd edition should be out towards the end of 2002. Tonight I tried a single beam reflection hologram with a polarizing filter over part of the (relatively flat) object, oriented for maximum transmission. On replay, the area covered by the polarizer was very slightly dimmer than the rest of the object, but otherwise no difference. I was expecting something more dramatic - either much dimmmer or much brighter. Would be interesting to see how a split-beam setup with only a single pass through the polarizer would work. Nothing unusual was seen when viewing the image through a polarizing filter or when using polarized light to reconstruct. regards, tom 24.67.253.203

Colin - Sun, Jan 27, 02 05:01:06 AM

So I would guess it is not that easy to make a HOE of a polarizer. Thank you for sharing. I have been in contact with the publisher for Graham Saxby and he thinks it will be closer to summer 2003. If you need to use a USA address I will gladly reship the book for you. I got the first edition for something like $65 but I was unable to find a copy of the second edition. colinsk@pacbell.net 63.193.192.181

Use this form to add your comments to the current forum entry above
Your Name:
The Text...

in
HTML, or
Plain text
Action or
| Main Index | Reload | Help? |