decolorante wrote:Can you please, if you have some time to spare, explain what characteristics one should look for in a diode laser, to be able to use it for holograms? I mean, why you can't just point a (good quality, not cheap) green diode laser beam in a spreading lens and on an emulsion and expect it to work? On the forum I found many hints to this question, but never seen it actually addressed directly. Does it need to be polarized (and how)? Does it need to be power-regulated? Frequency-regulated? Of constant intensity in all beam diameter? I don't know, things like that.
First, welcome to the forum.
I will now try to give you some answers. Please, laserists out there, correct me if there are inaccuracies.
The main reason green laser pointers, or most common lab style lasers won´t work for holography is that they have a longitudinal multimode behavior, or that they mode hop.
Longitudinal multimode means that there are several laser lines at the same time, closely separated. The green laser light is still green, but it consists of two or more color wavelengths separated with just a few parts of a nanometer. The laser light is then not just one color, but several, and has a broad linewidth. (Mode spectrum, linewidth and coherence length are very closely related.) This also makes the laser having a short coherence length (temporal coherence), making the recorded hologram appear like "sliced bread" with light and dark lines through the image. The coherence length can in this case be of just a few millimeters.
When a laser is said to be in single longitudinal mode it has basically just one color or one dominating mode, and the spectrum or linewidth is very narrow. This gives a long coherence length, up to meters or hundreds of meters. The "sliced bread" slices then are of equal long length and you can make very deep holograms.
Mode hopping is when the laser jumps between different spectrum modes, or between different laser lines. This behavior can also give the "sliced bread" appearance or render a weak or feeble hologram.
A laser can also drift over time so the color wavelength gets longer or shorter destroying the recording of the interference fringes.
There may also be chaotic and noisy behavior.
So, what you need is a laser with a long coherence length (and short linewidth) that are at least as long as the depth you want in your hologram. You also want it to be stable, and with no mode hopping.
In gas lasers, the most common is the red Helium-Neon laser which often has a coherence length of 15-20 cm. Blue and green argon lasers usually have shorter coherence length. Blue Helium-Cadmium lasers also have short coherence length. However, there are filter devices called etalons that can separate and enhance certain laser lines and by this increase the coherence length.
DPSS lasers like green laser pointers or common green or blue lab style lasers usually have a very short coherence length. But there are some DPSS lasers that have a very long coherence length like the green Coherent Compass lasers, the JDSU µGreen lasers and the blue Melles 58 lasers, among a few. These lasers are usually much more expensive than common "light show" lasers.
Low powered diode lasers like red laser pointers can have quite good characteristics with a coherence length of 50 cm up to meters. The Integraf diode is one example. High powered red laser diodes are either longitudinal multimode or are very prone to mode hop or drift. You need extreme temperature and current controls to find and stay in a stable region with these. Violet Blue-Ray diodes are most often multimode and mode hopping.
Here is an excellent page by wler of his longitudinal spectrum measurements of various lasers.
http://pagesperso-orange.fr/redlum.xohp ... ectra.html
One more thing to consider is the polarization of the laser. Hene lasers are often linearly polarized, but can also be randomly polarized. Since the interference pattern is strongest between light of the same polarization there is a better chance to get good fringes with a linearly polarized laser. The polarization vektor is also controllable with certain optics, which sometimes is useful. Diode or DPSS lasers are usually linearly polarized.
You also want the laser to be in transversal single mode (TEM00), which usually means that you have a Gausian distribution of the light when you look at the transversal section of the beam. There are other transversal modes where there are different patterns in the distribution, which means the 'laser spot' will not be just one spot but several spots or lines instead.
Also, the power output of the laser shouldn´t change too much over time.
So, to sum up, for cheap experiments use a cheap red helium-neon laser (preferably linearly polarized) or a low power (5 mW) diode laser. For more elaborate work buy a green Compass or µGreen (10-100 mW) laser or a high powered Hene. Both green and red lasers can be used with Silver-Halide film, and red with Methylene blue DCG and green lasers with Green-sensitive-DCG. For ordinary DCG use a blue DPSS or an etalon enhanced argon laser. For full color holography, you need one of each; blue, green and red.
I hope this made it a bit clearer.
Or, I might go the homebrew DCG route. I have some experience in coating sheets with gum arabic / dichromate emulsion for photography. This also means I already have the potassium dichromate. Gum won't work, I suppose.