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Introduction

Overview

Optical Table

Environment

Laser

Beamsplitter

Mirrors/Lenses

Table Mounts

Optic Mounts

Plate Holder

Objects/Scenes

Hardcopy

Resources

Mirrors & Lenses

Directional Mirrors

The directional mirror shown in Figure 5 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. In some setups, though, you'll need larger mirrors that are placed in the beam(s) beyond the diverging lens.

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Figure 5: Directional mirror in optical mount.

Diverging Lenses

The function of a diverging lens, shown in Figure 6a, 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 double-concave lens as shown in Figure 6a, or it can be a more complex optical component such as a microscope objective as shown in Figure 6b and mounted in the optic mount shown in Figure 6c. Or the microscope objective can be used in conjunction with an optical device called a spatial filter shown in Figure 6d.

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Figure 6a: Double-concave diverging lens in an optic mount.

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Figure 6b: Microscope objectives with different magnifications.

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Figure 6c: Microscope objective mounted in optic mount.

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Figure 6d: Spatial filter.

A simple double-concave lens will work fine as your first diverging lenses and they are the least expensive. Lenses of this type can be difficult to use if their diameters are less than -9 mm, though not impossible. This lens should be kept clean, scratch free, and handled only by its 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 section under 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 double-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 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 pretty much irrelevant of scratches, dust, or fingerprints.

A spatial filter is comprised of three micrometers ( X, Y, Z ), a microscope objective, and a pinhole as shown in Figure 6d. 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 6e shows what the beam may look like without a spatial filter and its pinhole. Figure 6f 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 6e: Laser beam without spatial filter and pinhole.

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Figure 6f: 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. When you are setting up an optical setup, you can start with a -15 mm focal length, simple, double-concave lens or an approximately equivalent 10x microscope objective for your diverging lenses. Depending on your optical setup, down the road you may want to have handy a range of microscope objectives (5x, 10x, 20x, 40x) or an equivalent range of double-concave lenses (-25 mm, -15 mm, -9 mm, -4.5 mm respectively) to allow you more versatility in how wide you can spread the beam over a certain distance. A 40x microscope objective spreads a beam wider over the same distance than a 5x objective. But as I mentioned above, start with a -15 mm focal length, simple, double-concave lens (or 10x microscope objective). Later, I will provide you with a simple equation that will tell you how far a diverging lens, with a certain focal length, needs to be from the object scene and plate holder to provide uniform illumination.

Parabolic Mirror

The function of the parabolic mirror, shown in Figure 7a, 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 7a: Parabolic telescope mirror in its mount.

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Figure 7b: 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 3 foot x 4 foot 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. Therefore, the diverging lens should be placed at least 64 inches from the plate. This will also require one or two additional larger directional mirrors to reach a length of 64 inches since your beam will be diverging. When we get into the actual optical setups, I'll discuss both methods of collimating and the additional directional mirrors.

 

Revised 5/2/2017