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Hologram Laser Questions

bostjan

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It's been a long time since I posted, but I have a question that you can probably answer easily.

I've been trying to get into holography and I've been struggling with a few things.

I started out obtaining a 200 mW 650 nm diode laser module, a fairly beefy aluminum heat sink, some holographic film, and a little bit of hardware to keep things from moving. I was able to get a pretty grainy-looking exposure of items 1-2 mm away from the film, but any farther and there was no detail.

I then contacted Litholo (a company that used to sell kits to beginner holography enthusiasts), and they put together one of their discontinued kits for me with a 5 mW 638 nm diode, a clip-on heat sink (very small) and some laser cut acrylic fixtures to clip the laser in place and lean the film against. It didn't *seem* sturdy to me, but I was able to get somewhat better results with their setup than with mine; however, the exposure time is super long - they recommend 10-15 minutes, but I've tried 25-30 minutes before I got any promising results. Still, though, the depth of field is super shallow- maybe 2-2.5 cm, and you really only see good detail the first 3 or 4 mm. I tried their hardware with my other lasers and I can only capture fairly faint images. I even said to forget about vibration, and tried an exposure with a high powered (>500 mW) fan-cooled laser, just to see if I could make out any detail, and, shockingly, even with the vibration of the fan (the vibration is said to spoil the fine structure of the interference pattern recorded on the film that makes the hologram), I'm getting about the same results as my 200 mW 650 nm.

I've checked these lasers with a polarized filter and examined the projected spot to verify that the lasers I'm using are TEM00, and, I can't really tell. The seller told me that the laser was TEM00, but my very crude interferometer ($20 amazon beam splitter and a front-surface mirror I scavenged from the trash in a lab I used to work in) didn't produce a very clear interference pattern.

I would simply do a longer exposure with the laser Litiholo supplied to me, but a) I'm worried about cooking the laser and b) I really want to understand what's going on, physically, that is preventing me from using whatever single mode laser I want.

I've reached out to Litiholo for help, but all they told me was that they select their lasers specifically for making holograms. I don't think I'm doing anything that would negatively affect their business by trying to upgrade the kit that I already bought from them.

I truly don't believe that my problem has anything to do with mechanical stability. Either the wavelength of the laser is unstable (which I think may be, but I don't know how to prevent this or even quantify it).

I'd appreciate any advice.
 



Anthony P

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Use interferometer to determine coherence length of laser. It must be at least several inches or more. Even the slightest vibration during exposure will ruin hologram. Exposure time should be seconds, not minutes. Even at 5mW 2-3 seconds should be enough. Are you using glass plates? Are you trying for reflection or transmission type?
See post #11,

Osram PL530 - Tiny 530nm OPSL​

 

bostjan

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Dec 29, 2011
Messages
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Points
8
Use interferometer to determine coherence length of laser. It must be at least several inches or more. Even the slightest vibration during exposure will ruin hologram. Exposure time should be seconds, not minutes. Even at 5mW 2-3 seconds should be enough. Are you using glass plates? Are you trying for reflection or transmission type?
See post #11,

Osram PL530 - Tiny 530nm OPSL​

Thanks so much for the response.

Yes, I've mostly worked with glass plates. I've tried one without, just to see what would happen and it didn't turn out. I'm mostly trying to get a good reflection hologram. These are mostly polymer films that don't require a separate fixing step. I do have access to some other films, but I thought I'd work my way up to that, since I have no experience with the hobby up until last week.

I imagine that the blurriness of the interference pattern I see has something to do with a shorter coherence length. What is the procedure to measure it?

Would a longer coherence length also yield a deeper field of view? Or, perhaps once the laser reflects off of the object (subject), coherence really doesn't matter anymore?
 

WizardG

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"Would a longer coherence length also yield a deeper field of view?"

Yes, It sounds like this may your issue. A spatial filter might help.
 

bostjan

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"Would a longer coherence length also yield a deeper field of view?"

Yes, It sounds like this may your issue. A spatial filter might help.
Ok, thanks.

Where could I obtain a spatial filter of acceptable quality for a reasonable price?

Also, could you suggest a laser that has a long coherence length, preferably at or near 638 nm for good sensitivity and visibility?
 

WizardG

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Ok, thanks.

Where could I obtain a spatial filter of acceptable quality for a reasonable price?

Also, could you suggest a laser that has a long coherence length, preferably at or near 638 nm for good sensitivity and visibility?
To the first question, I'm not sure. You could try making one.

As for a laser with good coherence length @ or near 638nm, hard to beat good old fashioned HeNe. You won't have 100s of mW to play with but the beam quality from a good HeNe tube is great for holography.
 

icecruncher

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Spectra Physics 127
40mw HeNe - polarized
Not cheap



Be super careful with shipping, you are dealing with a 4' glass tube that will break if you look at it wrong :(
 

bostjan

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To the first question, I'm not sure. You could try making one.

As for a laser with good coherence length @ or near 638nm, hard to beat good old fashioned HeNe. You won't have 100s of mW to play with but the beam quality from a good HeNe tube is great for holography.
Thank you.

I have a ~1 mW HeNe. It's a little bit old, but it still works. I believe it's fixed focus into a line. I suppose I'll try a lens to spread the beam out over the subject and see how that goes.

Spectra Physics 127.
40mw HeNe
Not cheap

Wow, you're not kidding. I'm doing this as a hobby, so a laser that's >$1k is a non starter. It looks like there are some "as is" (might not work) ones on eBay for a little less, but still a lot pricier than I was expecting for a decent holography laser.

Any idea the coherence length spec?
 

icecruncher

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I really don't know - I'm not into holography, although I would like to be :)
 
Last edited:

WizardG

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Thank you.

I have a ~1 mW HeNe. It's a little bit old, but it still works. I believe it's fixed focus into a line. I suppose I'll try a lens to spread the beam out over the subject and see how that goes.



Wow, you're not kidding. I'm doing this as a hobby, so a laser that's >$1k is a non starter. It looks like there are some "as is" (might not work) ones on eBay for a little less, but still a lot pricier than I was expecting for a decent holography laser.

Any idea the coherence length spec?
Should be in the 30-40 cm range. A spatial filter can improve performance considerably. An SP-127 is the holography laser I learned on.
Ah the nostalgia.......
 

Anthony P

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To measure coherence length set up interferometer as shown in the post I referenced with the two mirrors equal distance from the beam splitter. Move one mirror further and further away from the splitter until you can no longer get a nice interference pattern on your screen. The distance you were able to move the mirror is the coherence length. For most diodes it is 0 and they can not be used for holography. There are some diodes, however, that are well suited. The OPSL in the post/thread is the best holography laser I have ever used.
Also, check out Intergraf for supplies.https://www.integraf.com/
 

icecruncher

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Use interferometer to determine coherence length of laser. It must be at least several inches or more. Even the slightest vibration during exposure will ruin hologram. Exposure time should be seconds, not minutes. Even at 5mW 2-3 seconds should be enough. Are you using glass plates? Are you trying for reflection or transmission type?
See post #11,

Osram PL530 - Tiny 530nm OPSL​

I missed this post the first time through. Much cheaper than a SP127
 

Anthony P

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I would recommend starting with transmission holograms first just to get the hang of it. They are simpler and more forgiving. Reflection holograms are far more demanding. Everything has to be perfect. Also, viewing a reflection hologram can be difficult. It may look like a blank plate until you get lighting and angle just right to view. Sunlight is often the best light source for viewing.
 

bostjan

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Thanks again for your help!

I'll have to sign up for the holowiki forum. I'm sure I'll have a lot more questions.

In the mean time, looking at that Osram 530 nm laser, the specifications look very nice but I have no experience working with (i.e. building) anything other than regular old direct diode lasers. I'm sure there is a builder around here who could help me out, though. Does that device need to be TE cooled? That driver looks really complex compared to what I'm accustomed to doing.

Searching for the part itself keeps leading me here. It appears some people here did a group buy of them a while ago. If anyone has one for sale, please message me.

For the red laser, I've searched all around, but can hardly find any published data on coherence length, frequency band, nor regarding wavelength range. I suppose as cheap as diodes are, the general strategy would be to buy a bunch of different ones and test them to see if they have good properties or not. And, for the price of an SP-127, I could probably get just about a bushel of <100 mW red LD's.

Up until now, I had always assumed that the wavelength output from red diodes was very very narrow by default, and that only blue and violet diodes had broader spectral output. Well, now I know that's not quite the case, at least within the context of holography. I've learned a lot today. Thanks once again for that information.

I'll try transmission holograms and I'll play around with my old HeNe this weekend and report back early next week if I find anything that might interest future people who might stumble on this thread.
 

Anthony P

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The PL530 OPSL is not that hard to power. It does require a heater circuit. Basically, it just needs 2 LM317 circuits. One for CC to the diode and one for CV to the heater. A simple Al heat sink is adequate, no TE. Green sensitive plates are available from Thor and Integraf. I don't remember all the details about AH coating on the plates, but it is something you will need to check into.
One more quick tip, the use of convex mirrors to spread out your laser beam is a preferred method over concave lenses.
 




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