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.
15mW Diode Test Shot (3/30/99)
Digital photo of test hologram using a 15mW, 635nm diode
The above photo shows a test shot of my 15mW, 635nm diode module (from NVG, see link page). The single-beam shot (of the usual piece of coral) was made on an old batch of AGFA film, since I am out of plates entirely (except for one last 8 x 10 which will be tested later). Not the greatest hologram ever made, but a great start. That's all I ask at first. I used the coral again to compare results to other shots. Interesting enough, the shots made using the PowerGeneral DC regulated power supply failed. NOTHING. After playing around with it most of the night I just switched over to 2 "D" batteries and nailed the first shot. Sam from Bell Labs is going to look into the AC supply at my studio to see if the feed going into the power supply is the source of the problem.
This is what a REAL workbench looks like at four in the morning.
There were a few new "tricks" that I came up with from my notes to improve this series of test shots (compared to the failed 10mW shots). I hope by this posting that it may help you out as well. A very important one centered around the correct collimation of the beam and placement of the pinhole in relationship to the area which would be considered the most spatially coherent spot along the beam path.
The profile of the beam for these higher-powered modules is actually rectangular, not elliptical. With incorrect collimation the width to height ratio climbs as high as 8 to 1 and as high as 40 to 1 in the much higher-powered diodes such as 500mW (this can cause panic when first firing up the diode, believe me). This astigmatism must be corrected optically to achieve a spatially-coherent beam. A spatially-coherent beam is necessary for strong fringe contrast and recording. This is where I think I slipped with the 10mW diode test. Anyway, here is how I worked this out:
With proper collimation however, the height-width ratio is much lower (I managed to achieve one-to-one). You collimate the beam by rotating the lens assembly threaded into the diode module.
Important Note: Collimation is not focusing the beam to a spot on the wall across the room. A collimated beam is a beam which retains the same beam diameter no matter where along the beam path you measure. Taking infinity into consideration (since it's Wednesday, good excuse as any), I don't know if there is, theoretically, a perfectly collimated beam of light -- but you want to get as close as possible. Use as much distance as you can and walk back and forth along the beam path taking measurements and doing fine adjustments on the lens assembly. When you have it as close as you think you're going to get it -- well, then you've got it.
Once collimated as accurately as possible, you must move a white card to find the exact location where the beam becomes square (yes, square). This appears to the unaided eye as a tighly-focused point spot. Could be mistaken for round, but if you place an expanding lens at this area you will expand the beam and see that it is square. If it is not perfectly square, move your card slightly forward or backward along the beam path until it does become square. You'll have to hold your expanding lens in front of the card to see this. You will notice that your height and width ratios begin to reverse depending on which side of this spot you are on (at least mine did). Once you find it, this is the spot where your beam is most spatially coherent -- having corrected the astigmatism as much as possible. Mark this spot and use it as the spot where your objective will focus the beam through the pinhole.
You will find that you now have a beautiful spatially-coherent diverging wavefront of laser light to work with. HOWEVER, do not be surprised that you do not receive the same divergence as you're used to with your HeNe. Using a 40x objective and 10-micron pinhole I needed around six feet of table length to cover a 4 x 5-inch area. One way to get around this (for larger plates and/or shorter distances) is to introduce a very long-throw negative-focal lens just before the objective of the spatial filter. By increasing the diameter of this spot, while at the same time adding as little divergence as possible, you will find that the light exiting your spatial filter diverges more rapidly.
Beam Profiles: (top): 5mW laser pointer
(lower left): 15mW HeNe laser; (lower right): 15mW laser diode
This probably sounds a little confusing to those used to working with HeNe's, but this is new holography and it brings new things to the table. Believe me, once you have it, it's just as easy as anything else in the past. It's just a new way of doing it. Remember taking your first spatial filter out of the box and aligning it? Ha! This is a piece of cake!
Looking down from behind the coral into the incoming beam.
Large burst of light is exiting the spatial filter.
Smaller dot just above burst is the diode aperture in the distance.
Smaller burst lower left of larger burst is a small alien spacecraft hovering over the table.
It's very surprising to see the holography set-up lit up as if a 15mW HeNe were on the table. The 635nm of this diode is so close to the 633nm of the HeNe that it's virtually impossible to tell the difference side-by-side. It actually has the signature "orange" appearance to it, especially when compared to the 650nm which, in comparison, looks more into the "red" part of the spectrum (which it is, of course). After working with a Spectra Physics 124B for many years, I can say that it certainly holds its own -- all in a module the size of your fingernail.
Another photo of the 15mW test shot. Blurred as usual from my cheap camera.
The laser pointers such as the Infiniter200 have shown to give such great results with stability, depth, table set-up and plate size that one can only imagine the possibilities and doors that this now opens. Everyone loves to move on to bigger and better things, but in traditional holography the cost of doing this has made it very prohibitive. Now the promise of moving on to larger formats and more creative ideas can become a reality for many more people than ever before. At $500-plus, it's still a little pricey (the 15mW, 650nm diode module is less at around $185!), but I would suggest you price a 15mW HeNe for comparison. Even at $500, it's still less expensive than a "new" 5mW HeNe (at $800 in one catalog I have on my desk). Also, the $500-plus pricetag can be brought down significantly by purchasing the diode, driver, lens and housing as separate units and putting it together on your own.
Keep in mind that you do not need to be at 635nm to make a hologram. The 15mW, 650nm diode modules will save you money at around $185 -- giving you the same power as the 635nm above but at a slightly longer wavelength. You can even increase your power by moving to 20mW for around $250. All of the holograms made so far in this entire section have been made using 650nm as the wavelength. Also, the BB-Plates and Slavich recording materials have a very nice response spike at 647nm for Krypton lasers, so the less-expensive 650nm fall right into place. Please keep in mind, though, that the 15 and 20mW, 650nm diodes have not been tested and confirmed at this point.
Shot of beam firing out of 15mW diode laser.
Just like in the past, now that I know that this works, it's time to see what can be done with it. Improve the image quality, go for brighter results, larger formats, etc.
If the little 5mW pointer diodes can handle an 8 x 10, I'm sure this 15mW can handle a 30 x 40cm or even 50 x 60cm size. If it turns out to be as stable as the little pointer diodes, we may even see improvements in image quality just by substituting the HeNe with the diode. Remember, these diodes do not have the "drop off" that the HeNe's do. And don't forget Steve Michael's incredible hologram with 7.5 feet of depth (using a $14.99 laser pointer!).
Stay tuned for more details as this moves forward. If you have any questions about this so far, let me know and I'll answer them the best I can.
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Frank DeFreitas Holography