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Optical Table





Table Mounts

Optic Mounts

Plate Holder



Mirrors & Lenses

Directional Mirrors

The directional mirror shown in Figure 9 is a front-surface enhanced aluminized mirror that is used to direct the reference and object beams to various locations on the optical table. Because of the small 0.08 inch diameter of the laser beam, I purchase small mirrors about 1 inch x 1 inch x 1/8 inches thick in size to direct the beam around the table. In all multi-beam setups you'll need larger 4 inch x 5 inch mirrors that are placed in the beam(s) beyond the diverging lenses.

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                                   Figure 9: Directional mirror in optical mount.

Diverging Lenses

The function of a diverging lens, shown in Figures 10 and 11, is to spread (diverge) the small diameter of the laser beam into a wider diameter beam so that the photographic plate and object scene are uniformly illuminated. A diverging lens can be a simple plano-concave lens as shown in Figure 10, or it can be a more complex optical component such as a microscope objective as shown in Figure 11 and mounted in the optic mount shown in Figure 12. Or the microscope objective can be used in conjunction with an optical device called a spatial filter shown in Figure 13.

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                    Figure 10: Double-concave diverging lens in an optic mount.

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                    Figure 11: Microscope objectives with different magnifications.

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           Figure 12: Microscope objective mounted in optic mount.

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                                                       Figure 13: Spatial filter.

Through the years, I have found that microscope objectives work much better in obtaining uniform illumination of the photographic plate over simple plano-concave lenses. Simple lenses are not manufactured with the very short focal length and lens diameters needed to achieve uniform coverage of the photographic plate. The smallest useable simple plano-concave lens available today (year 2020) is a 6mm (0.24 inch) diameter lens with a minus 6 mm (-0.24 inch) focal length whereas a 40x objective has a 4.35 mm (0.17 inch) focal length, a 60x objective has a 3.21 mm (0.126 inch) focal length, and a 100x objective has a 1.88 mm (0.074 inch) focal length. Objective lenses are also encased in a metal structure and are easily mounted as shown in Figures 12 and 13.

A simple plano-concave lens will work fine as your first diverging lens. Lenses of this type can be difficult to use if their diameters are less than 9 mm, though I have succeeded with a 6 mm diameter. These lenses should be kept clean, scratch free, and handled only by their edges. All the optical components (beamsplitter, directional mirrors, diverging lenses, and parabolic mirror) must be clean and free from scratches to produce an artifact-free, high-quality hologram. All of the optics shown here can be gently cleaned with cotton swabs and acetone to remove dust and fingerprints. Photographic lens paper can also be used. The best way to clean your optics is to wrap the cotton tip of a swab with a couple layers of photographic lens paper and hold the lens paper in place on the swab shaft with a twisty. Then dip the lens paper covered cotton tip into the acetone, shake off the excess acetone (away from other objects), and gently clean the optic stroking in one direction. Constructing the mounts for a lens, microscope objective, or spatial filter is discussed in the sub-section Optic Mounts.

If you can afford a spatial filter or two, get them. Spatial filters have three functions. The first is to diverge the laser beam as a simple plano-concave lens or microscope objective does. The second is to eliminate the recording of scattered noise (artifact noise) on the hologram plate caused by dust and scratches on the optics. The last function is to partially eliminate internal noise created in the laser cavity that travels along with the beam. A spatial filter cannot totally eliminate internal laser cavity noise. Later, I will show you how to use a black cardboard mask to completely block this noise whether you're using a simple lens, a microscope objective, or a spatial filter. Using a spatial filter will provide the ultimate in a totally clean holographic image.

A spatial filter is comprised of three micrometers ( X, Y, Z ), a microscope objective, and a pinhole as shown in Figure 13. The X and Y micrometers move the pinhole to center on the microscope objective's central light axis, and the Z micrometer moves the microscope objective forward and backward so that the narrowest point of the objective's focal point is located at the center of the pinhole. When both these conditions are achieved, you have a very clean beam at the plate for the reference beam. Having a clean beam for the reference beam is more important than having a clean beam for the object scene, but you should strive for a clean object beam also. Placing a piece of white cardboard in the plate holder, Figure 14 shows what the beam may look like without a spatial filter and its pinhole. Figure 15 shows what the beam would look like using a spatial filter and its pinhole or having a clean lens to start with.

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            Figure 14: Laser beam without spatial filter and pinhole.

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               Figure 15: Laser beam with spatial filter and pinhole.

Spatial filters are expensive. You need not consider these for your first holograms, either single-beam or multi-beam. With your table size being 4 feet x 6 feet, the distance between your diverging lens and the plate holder plus object scene in either the single-beam transmission setup or the single-beam reflection setup will need to be at least 60 inches (152.4 cm). I highly recommend a 40x microscope objective as your first diverging lens which will give you good uniform coverage for both setups. A 6mm (0.24 inch) diameter and -6 mm (-0.24 inch) focal length plano-concave lens will also give you fairly uniform coverage but not a good as the 40x objective. I will cover lenses and objectives in more depth in the section Creating Transmission & Reflection Holograms were I present an equation you can use to determine how far away a diverging lens with a certain focal length needs to be from a certain size photographic plate to achieve uniform illumination of the plate area. There is also a table that compares the focal lengths of various microscope objectives and simple plano-concave lenses.

Parabolic Mirror

The function of the parabolic mirror, shown in Figure 16, is to collimate (make parallel or flat) the diverging reference beam's wave front used in the multi-beam transmission hologram so that the holographic image has a magnification of 1x. This magnification factor is very important because we will use a transmission hologram's real image to create a reflection display hologram that can be viewed with white light instead of laser light and any magnification in transmission hologram's real image will distort the reflection display hologram's image. I will be covering this in more detail in the section on Creating Transmission & Reflection Holograms.

Let me take a moment here to further clarify the term collimate. Since you have to diverge the reference beam with a diverging lens to make sure your plate is uniformly illuminated, this diverging beam has a curvature to it (called the wave front of the beam). This curvature introduces magnification into the holographic image unless it's not curved, but flat. The parabolic mirror flattens the wave front and eliminates the curvature, and in turn, eliminates any magnification so that the recorded holographic image size is the same size as the original object scene.

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                            Figure 16: Parabolic telescope mirror in its mount.

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                               Figure 17: Parabolic telescope mirror mount.

The parabolic mirror should have a diameter of 6 inches and a focal length of 24 inches. This diameter will adequately cover a 4 inch x 5 inch plate and the focal length will be adequate for a 4 feet x 6 feet table.

If you can't afford this mirror, there is another method for eliminating magnification. In this method, you want to place the diverging lens in the reference beam at a distance from the plate holder at least 10 times the diagonal size of the plate. This will essentially give you a flat wave front at the plate. For example, if you're using a 4 inch x 5 inch plate, its diagonal distance is 6.4 inches. Using the equation in Figure 68, a 9 mm diameter plano-concave lens with a 0.35 inch focal length would need to be at least 123.5 inches from the plate and a 40x objective with a 0.17 inch focal length would need to be at least 60 inches from the plate for both of them to have a flat wavefront.

This will require one additional larger directional mirror, 4 inch x 5 inch, for the 40x objective to reach a length of at least 60 inches and two additional larger mirrors for the -9 mm lens to reach a length of at least 123.5 inches since in both cases your beam will be diverging. When we get into the actual optical setups, I will discuss both methods of collimating and the additional directional mirrors.