Hydrophobia...

No, don't worry, I haven't developed an intense fear of water. But the edges of my plates have. I've used Rain-Clear on them. It's originally used to make car windshields water-repellent to improve your visibility during rain. Instead of clinging to the surface, drops just run off the glass.

I coated one side of the plates completely and only a small border around the edges on the other side. Now during pouring the gelatin only sticks to the uncoated region, and the edges dry more evenly, resulting in very uniform coatings.

WARNING: It looks like this stuff really forms a chemical bond to the glass. It's probably a silane compound with a very hydrophobic group at the end. Even after cleaning many times, it's still on the plates. A good thing if you want to reuse the plates, but make sure you don't get the tiniest drop of the stuff on the region you want coated.

Holomania...

This weekend I was struck by holomania. It's a well known side effect of inhaling the sweet vapors of iso-propanol for a prolonged time.. ;-)

So I made a couple of plates.. Well, OK, a bit more than a couple... :-)


After exposing and developing nearly all of them, these two looked best:



Very satisfied once again!

First holograms!

After having ordered a whole bunch of chemicals and other supplies my first tests were made. Despite its bad mode pattern I used the violet laser. For small pathlength differences (i.e shallow reflection holograms), it worked OK. Even though you can see that I need to improve on processing techniques, there's a hologram and it's quite bright. Thanks for all your support Ahmet, my first DCG!

Coating plates.

DCG plates are available ready-made, but because of their price, most people make their own. I tried several coating methods, but I liked veil coating best. Basically, veil coating is done by holding the (very well cleaned) plate at an angle and pouring a heated solution of dichromate gelatin over it in a special way. While pouring from the top left corner of the plate you slowly move to right top corner of the plate, making sure the "curtain" of gelatin keeps following the spot from where you pour. After gaining a little practice the method is fast and yields good results. The plates I made looked like this:

They don't have dichromate in them yet, but just a blue dye to check the thickness of the emulsion.

Modes and laser diodes

While in the past mainly gas lasers were used in holography, nowadays laser diodes have become a very attractive alternative. Not all laser diodes however are suitable for use in holography. The problem with many laser diodes is that the band of wavelengths that can be amplified in the semiconductor is quite broad. Often this results a laser that emits a lot of regularly spaced wavelengths (corresponding to the modes of the laser cavity). This is very detrimental for making holograms. Especially with larger differences in path length between reference and object beams, the many tiny wavelength differences start to add up. If they sum to about half the average wavelength, constructive interference for one wavelength will become destructive interference for the other. The result is that in the end, no diffraction pattern is recorded for this path difference. In practice, holograms that are made with a laser like this will appear 'breadsliced'.

To test the lasers that I already own, I made a high resolution spectrograph that would be able to resolve the extremely small differences in wavelengths emitted by the laser. Below are some pictures and a video.

I'll start off with a picture of my violet laser, at a very low current. At this current the laser isn't yet lasing. What you see is just the spontaneous emission leaking out of the cavity modes.


When the current is increased, the laser starts to lase. The pattern of modes that lase is quite irregular and depends strongly on the current flowing through the laser. In the next picture the current was set at a value that resulted in operation that resembled 'single mode' the most:

As you can see, one mode is lasing brightly, but there are some parasitic modes on the right. At even higher current, things get worse:

Now you see lasing in a lot of modes at the same time. Clearly not the kind of operation you want for holography.

The next picture is my green laser module. Its mode pattern is highly irregular. At first sight, it looks like there are just three modes lasing. But if you look closer, you'll see that the mode spacing is way smaller than the distance between those three modes. So between the three modes that are lasing, there are many modes that aren't lasing. In the video (see below) you can see this more clearly.
The cause for this is probably the fact that this is a DPSS laser. It consists of a diode pump laser that lasers at around 808 nm. This laser pumps another laser that normally lases at around 1064 nm. But the mirrors in this second laser are designed to keep most of this light inside the cavity instead of letting a portion escape like in a normal laser. Instead there's a crystal inside the laser cavity that doubles the frequency of a tiny portion of the light that passes through it. This light *does* escape the cavity and shows up as 532 nm. I assume that because of the many interacting cavities inside this laser, its mode structure is so irregular.

Last but not least, my red laser. You can see a couple of modes, but clearly one is way brighter than the others. This is the sole mode that's lasing in this laser. It's so bright that it gets diffracted a bit by the diaphragm blades. This shows up as the six rays that extend from the spot. The band on the left is probably an artifact.


As you can see, my red laser is very suitable for holography, it emits almost all of its light in one mode. The green and violet lasers aren't well suited for making holograms. One can make holograms with them, but only of things that can be put very close to the plate like coins.

I also made some videos of the mode changes (see below). In the first video you can see a phenomenon known as mode hopping. One moment the laser is lasing at one wavelength, and the other it flips to another one. In the video I'm tuning the current up and down to induce the effect.

As you can see in the other video of the green laser, it switches between two completely different mode structures. Again, probably due to it being a DPSS laser.

Holography!

A while ago I picked up an old hobby: holography. I've been interested in it since I was a kid and I made a couple of holograms with a homebuilt sand-table at the time I was in high-school. But after I left school and went to university, limited space and lots of other things to worry about kept me from pursuing it further.

But the desire to make holograms never went and especially making holograms on dichromate gelatin (DCG) was something I kept dreaming about. After having met someone on a laser related forum who offered to help me start up I decided this was the time to finally give it a go.

DCG is a very efficient recording material and can yield holograms that are almost as bright as the original under the same lighting conditions. It's not easy to use however. In fact, in the Holography Handbook by Fred Unterseher there's the following recommendation:

"We do not recommend making DCG holograms yourself unless you have an extreme sense of patience, are a little bit crazy, or... preferably both!"

That sure sounds like the description of the author of a certain website about small things! :-)