School of Holography
LASER POINTER / SEMICONDUCTOR LASER HOLOGRAPHY
by Frank DeFreitas Holography Studio
Creative Holography Using
My magical journey of making
Inexpensive Laser Pointers
holograms with a $7.99 laser pointer
and inexpensive laser diodes.
Poor Results on First 10mW Diode Hologram (2/4/99)
Above is the first test shot using a 10mW, 640nm laser diode module. I used a silver pop screen from one of the microphones in the HoloTalk studio. The results are less than expected, to say the least.
The reference beam gave a reading of 40 on the 2 scale of the Science and Mechanics PhotoMeter. This would have put the exposure time between 1/2 sec. (with a 50 reading) and 2/3 sec. (with a 30 reading) for AGFA8E75HD. When your exposure times are 18 seconds (or even 10 or 5) it's not that difficult to use a card for your shutter and have your exposure close to accurate. But at less than a second it's very difficult to do manually -- especially since the slightest hesitation on closing that shutter can mean double, even triple, the exposure in no time at all.
This plate went black the minute it hit the developer. I would say that after 20 seconds or so it achieved 2.0 density. I left the plate in until it was nearly opaque. This is what leads me to believe that overexposure was the problem here. All other parameters (other than exposure time) were carefully controlled.
The Good News
The good news is that the hologram, although very dim, exhibits the correct depth. Brightness can be worked on, but if the image was lost within a few millimeters from the emulsion that would be a serious problem to overcome. So all may not be lost afterall. Time will tell.
Even in the worst-case scenario (that the higher power diode modules do not work), we at least know that the $7.99 laser pointer has the ability to create stunning 8 x 10 single-beam holograms. Taking the power, exposure time and stability needed to achieve this, we may have enough punch with the little laser pointer to attempt split-beam set-ups for 4 x 5's.
The above photo shows the beam profile of the 10mW diode module. It is very rectangular, even with the corrective optics in the module. However, I'm still able to get a somewhat satisfactory clean spread through the spatial filter using my 10x objective and 25-micron pinhole -- although it's not as clean and nice as the pointer.
A slightly rectangluar shape is not bad since the plate itself is either 4 x 5 or 8 x 10. Just orient the spread beam to the orientation of the plate and it actually works out better for overall coverage than a circular beam. However, I'm wondering if this beam profile effects the spatial coherency of the light reaching the plate.
The diode module was powered off of 2 DuraCell AA ULTRA batteries connected directly to the pigtail leads coming out of the module housing.
So that's my report. Back to work I guess, and see if I can get this brightness straightened out. It's almost 5:00 a.m. here, so I MAY get back in the lab for another shot or may hold out till tomorrow night (although it will probably bug me all day!).
UPDATE 9:00 AM
Shot another hologram, this time using the coral as in previous test holograms. I spread the light out to drop the exposure time down to where it was for the other holograms when using the laser pointer (6 seconds) using my 20x objective and 10-micron pinhole. I figured it's easier to nail a 6-second exposure than it is a fraction of a second.
Once again, this hologram exhibited great depth (all the way back), but was very dim.
Cannot figure this out. If it weren't for the hologram being dim, it would be a great hologram overall. It's sharp and deep. Except for the problem with brightness, it looks very much like the typical single-beam holograms done with a low power HeNe. I checked polarization and it's fine.
UPDATE 12:00 Noon
Never one to throw in the towel, I was back at it later this morning. This time I did not pre-soak in TEA and I cut the exposure time in half. Development was perfect at 2 minutes, but once again, another very dim image. Depth all the way back, but dim.
I can't really think of anything left to try. With this last test, there was nothing really left to improve on. It's a shame -- the diode provides a lot of power, probably enough for larger-format holography.
So, for all practical purposes, I'd have to say that we're left with working with the laser pointers at around 5mW. I'd be a little more convinced if NOTHING showed up. But a dim image with great depth is hard to close the book on. I'll have to set up an interferometer to see how strong the fringes are.
Back to "Laser Pointer" holography, I guess.
Update 3:00 P.M.
Still scratching my head on this one. I've been up for quite more than 24 hours. I just got off the phone with the technical representative at NVG (which I purchased the diode module from, see listing on link page). I was very pleased with the time and attention he gave to my concerns -- especially for one single diode -- since they probably sell thousands at a time.
He seems to think there is a problem with the lens in the housing. He quoted a size ratio of 3 to 5 for the output beam after collimation, and he looked at the above photo of the output beam on this page and said something is wrong. As it stands, I would put the the beam size ratio at 4 or 5 to 1 (go up and measure the photo) -- so perhaps this is where our problem lies. Remember the quote from the Graham Saxby book on diodes (practical holography, prentice-hall, page 38) " . . . but the cone of light is elliptical rather than circular . . . if the astigmatism of this beam is corrected by means of aspherical optics, a spatially-coherent beam can be obtained" . So, perhaps we're looking at a problem with the output beam due to the optics. Although I would be under the impression that this would have more of a pronounced effect on depth rather than brightness.
I'll be sending the diode module back for them to check on. At present, I can get it focused down to a nice point, similar to the laser pointer, but the divergence is quick and pronounced (and always stays at the 4 or 5 to 1 ratio no matter what size the dot is focused to). The laser pointer stays nice and tight over the length of the lab. The diode module beam is rather large at the same distance. Let's see if this helps things out.
We're not finished with this yet!!!
Check back for updates.
Frank DeFreitas Holography