Holograms are a way of creating an image of an object with the usage of a fine pattern that shapes a wavefront of light. How this pattern is created depends on the type of hologram, sometimes it is embossed, but in most cases this pattern comes from interference between two beams of light and is recorded photographically on a fine grain emulsion. The two beams of light that are used are called the reference beam and the object beam. The object beam scatters off of the object being holographed and onto the plate and the reference beam is shined directly onto the plate. There are many different types of holograms, but most can be classified as one of two main groups: transmission, and reflection. The difference between these two types lies both in the way they are made and viewed. The interference pattern in a reflection hologram is created when the two beams come from opposite sides of the plate. the part of this pattern that is recorded is shaped like a series of hyperbolic surfaces that reflect light. This is why when viewing a reflection hologram, you look at it from the same side as the light source (usually white light). Transmission holograms on the other hand behave more like diffraction gratings because their interference pattern consists of a series of lines that bend the light into the original object's wavefront. This wavefront can only be observed from the opposite side of the plate because the light is passing through.

Stability is one of the most important factors in creating a good hologram. If either the object or the plate move even a quarter of a wavelength of light, the interference pattern can be lost, destroying its image. For this reason, great attention was paid to whether the setup was stable or not. To assure the most steady optical table, I used an optical lever (mirror screwed into a breadboard) to measure vibrations. I shined a laser onto the lever, reflected it across the room, and watched it for oscillations while bumping the table. This technique was applied to test numerous materials for their shock absorbing qualities. The materials tested were: 4 large packaging bubbles, sheets of both large and small bubble wrap, sad balls, and a plain old table. It turned out that the table worked the best and that is what was used from that point on.

Another factor dealt with in detail was the intensity of the laser used, and the exposure time. all together I used three different lasers in my experiments. The first was a 2.6 mw diode laser which projected an oval shaped beam when its collimating lens was removed. With a minute long exposure, I made my first hologram. Although this laser was good for reflection holograms, it proved to be almost completely useless in making transmission holograms because it diverged so rapidly. To create a descent transmission image I needed an undiverging beam so that I could split it in two. For this task I used a .7 mw Helieum Neon (HeNe) laser and a beam splitter. The reference beam was spread with two concave lenses directly onto the plate while the object beam was directed onto a concave mirror and from there onto the object. This proved to be a very effective method for exposing transmission holograms with an exposure time of only 45 seconds. This particular HeNe also created much sharper reflection images as well. The final laser I tried was a polarized 7 mw HeNe. Because this laser was much more powerful than the other two, a much shorter exposure time was necessary (4-6 seconds). Although I recorded a transmission hologram or two with this laser, none of them were really clear. Besides the fact that the relative intensities of the two beams were harder to adjust accurately at such a high power, the laser's highly undivergent beam also contributed to the difficulty. On the last day that I was making exposures, I tried making a reflection hologram with this laser and was successful so I know that a quality transmission image isn't impossible, just a little more difficult.

One application of holography I explored was creating a diffraction grating. The most widely accepted method of doing this is to place a white piece of paper and the plate at 90 degrees from each other and shine the laser onto them both at the same time so that it hits them both at 45 degrees. The light bouncing off of the paper is supposed to interfere with the light going through the plate causing a pattern of lines (a diffraction grating). When I tried this the results were less than impressive so I tried another setup in which a single diverging beam passes through the plate, bounces off of a mirror and back through the plate. Although this setup did work a little better, it still wasn't anything to call home about. What I eventually discovered is that the transmission holograms that I had been making actually worked better at diffracting light than any of the gratings that I had made intentionally. An interesting thing about this diffraction was that as I moved the beam across the hologram, the pattern on the wall either shrunk or grew. This means that the spacing between lines was not constant over the hologram.

Other experiments included interferometry. When an object is moved slightly during exposure, fringes appear on its image. The number of fringes on the image corresponds to the number of wavelengths of light the object moved. To test this I screwed a few posts into a micrometer, exposed a transmission image for 30 seconds, turned the knob on the micrometer 25 microns, and exposed for another 30 seconds. When I first viewed this hologram I couldn't see any fringes on the posts and I thought that something had gone wrong, but upon viewing it a little more carefully I noticed that the sticker on the side of the micrometer had a few fringes on it.

In the future if I continue work with holograms I hope to do more interferometry. I would like to create better transmission images (because they are capable of displaying more depth) and study fringes caused by stress. I would also like to try to make a hologram of a vibrating object. Under certain circumstances, a hologram of a vibrating object can display its modes and their amplitudes as well. I am interested in these things as well as creating clearer images so hopefully I will have opportunities to do so in the near future.