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Introduction

Vibrations

Processing

SB Transmission

Exposure

Recording

SB Reflection

MB Transmission

MB Reflection

Lighting

Hardcopy

Resources

Creating a Multi-Beam Transmission Hologram

Figure 20a shows the optical arrangement for a multi-beam transmission hologram with a single object beam. It is called a multi-beam hologram because a beamsplitter splits the laser's beam into two separate beams: a reference beam and an object beam.

multi-beam transmission setup image
Figure 20a: Multi-beam transmission hologram setup with a single object beam.

Multi-beam setups have numerous advantages over single-beam setups:

I will first discuss a multi-beam transmission hologram setup where the object scene is illuminated using one object beam. This will be followed by the same setup where the object scene is illuminated using two object beams. Referencing Figure 20a, I will discuss:

Paths Traveled by the Laser's Beams

The output beam of the laser should be 9 inches above the table as well as the centers of all the mirrors, beamsplitter(s), parabolic mirror, diverging lenses, plate holder, and object scene. The laser's beam polarization should be oriented horizontally since the reference beam will be incident on the plate from the side. We have already assumed horizontal polarization when we set up the single-beam transmission hologram arrangement.

The beam from the laser travels to directional mirror M1 which directs the beam to the beamsplitter BS. Be sure to use the retro-reflection technique whenever you're placing a mirror or beamsplitter into the setup. Also make sure the beam is hitting the optical component at its center.

The beamsplitter splits the laser beam into two beams. The beam transmitted through the beamsplitter will be the reference beam R and the beam reflected from the beamsplitter will be the object beam O. When placing the beamsplitter on the table, make sure the reflective coated side of the beamsplitter faces the incidence beam from the laser and reflects the object beam at a 90° angle from the reference beam passing through the beamsplitter.

Let's look at the path of the reference beam first. The reference beam R continues to mirror M2 which directs the beam through diverging lens DL1. Diverging lens DL1 diverges the beam to the parabolic mirror PM which then directs a collimated beam to mirror M3 which in turn directs the collimated beam to the plate holder PH at an incident angle of 56 degrees. For the sake of clarity, only the center-axis part of the beam is shown, not the actual diverging or collimated beams. A collimated beam is parallel light rays, not diverging or converging light rays. M3 is a 5 inch x 7 inch x 1/4 inch thick mirror. Leave DL1 out of the setup for now.

At the same time, the object beam O travels from the beamsplitter BS to mirror M4 where it is directed to mirrors M5 through M7. Mirror M7 directs the beam to the object scene OS. Diverging lens DL2 produces a diverging beam that uniformly illuminates the object scene OS. The object scene OS then reflects laser light directly to the plate holder PH. M7 should be at least a 5 inch x 7 inch x 1/4 inch thick mirror. Leave DL2 out of the setup for now.

Note: Any time you are using a mirror to direct a diverging or parallel beam, the mirror must always be larger than the diameter of the beam. Otherwise, the mirror size becomes the limiting size of the beam further down the beam's path.

Beamsplitter's Function

The function of the beamsplitter is not only to split the laser's beam into two beams but
also to adjust the beam intensities of the reference and object beams at the plate holder
so that the intensity ratio of the reference beam to the object beam is in the range of 2 to 1 (2/1) and 1.5 to 1 (1.5/1).

If you're using a circular variable gradient beamsplitter, adjusting the beam ratios is straight forward, which I'll be discussing shortly. If you're using the 50/50 fixed beam ratio beamsplitter, you may need to use the extra glass plate technique I discussed in the section Beamsplitter under Building a Holography System.

Parabolic Mirror and Diverging Lens Relationship

The parabolic mirror PM and diverging lens DL1 have a special relationship. It is necessary to create a collimated beam of light reflected from the parabolic mirror to mirror M3 and finally to the plate. This is very important to the size of the pseudoscopic real image which will be used as the object scene in the final multi-beam white light reflection hologram. If the beam is collimated, the magnification of the real and virtual images will be 1x (the same size). If the beam is not collimated, the real image will be magnified, too large, and distorted.

To achieve this collimated beam, the focal point of the diverging lens DL1 should be placed at the focal point of the parabolic mirror (the focal point of the parabolic mirror is written on the edge or back of the mirror). A 10x or 20x microscope objective or a plano- or double-concave lens with a -15 mm or -9 mm focal length, respectively, in combination with a 6 inch diameter telescope parabolic mirror with a 24 inch focal length, will provide an excellent collimated reflected beam from the parabolic mirror. The beam will have a diameter of 6 inches and will have uniform intensity across its diameter. The plate will have the same uniform intensity across its diagonal which is 6.4 inches. The beam does not cover the corners of the plate completely--only 1/4 inch at each corner is lost which is not significant.

There is another method for producing a collimated beam without using a parabolic mirror. With 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. 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 means that you would need to replace mirror M2 and the parabolic mirror PM with 5 inch x 7 inch x 1/4 inch mirrors and probably place DL1 somewhere between the beamsplitter BS and mirror M2. Two 5 inch x 7 inch x 1/4 inch mirrors are a lot less expensive than a 6 inch diameter parabolic mirror.

Note: If you're using a parabolic mirror, the angle between the incident and reflected beam at the parabolic mirror PM in Figure 20a should be as small as possible when reflecting the beam to mirror M3 (mirror M3 should not block the light from mirror M2). This will minimize aberrations in the reflected beam from the parabolic mirror, and thus minimize aberrations in the real image. This, in turn, will allow the holographic image in the final white light reflection display hologram to be projected further out in front of the plate without causing distortions in the image.

Object Beam Components

Now we will look at the optics in the object beam. Notice in Figure 20a that the object beam has a total of 4 directional mirrors M4, M5, M6, M7 in its path between the beamsplitter and the object scene. This configuration is set up so that the object's beam path length can be changed conveniently, using mirror M5, to match the reference beam's path length. In a multi-beam transmission setup, always try to adjust the object beam path length to match the reference beam path length, not the other way around.

The bottom line is that the distance of the reference beam between the beamsplitter and the plate holder's central point, and the distance of the object beam between the beamsplitter and the plate holder's central point should always be equal, to within 1/2 inch. The closer their path lengths are equal, the brighter the image. Measuring beam path lengths will be discussed a little further on.

The diverging lens in the object beam should have a focal length that will create a diverging beam that uniformly illuminates the object scene. You can try a 10x or 20x microscope objective or a plano- or double-concave lens with a -15 mm or -9 mm focal length. Mirror M7 should be a 5 inch x 7 inch x 1/4 inch thick mirror so that the diverging lens can be placed between mirrors M6 and M7 to achieve a larger diameter beam at the object scene. Again, you can use the divergence equation in Figure 16f to calculate the lens distance from the object scene. You can experiment with illuminating the object scene now, but when you measure path lengths, leave DL2 out of the setup.

The object scene should be centered when looking through the plate holder. It should face the plate holder squarely. The plate holder itself should be perpendicular to the table surface. The front of the object scene should be a predetermined minimum distance from the plate holder in order to allow the recording reference beam a clear path to the plate at a 56 degree angle without projecting any part of the object scene as a shadow on the recording plate. The graph in Figure 20b relates the reference beam angle to the minimum distance that the front of the object scene can be from the plate. Using this graph and an incident angle of 56 degrees, the closes you should have the object scene to the recording plate is 5 inches. This will give the widest possible viewing angle when viewing the image in the white light reflection display hologram discussed later on.

In addition, because you will be using the projected real image of your finished multi-beam transmission hologram as the object scene in your multi-beam reflection hologram setup, this minimum distance becomes important once again. With your recording reference beam being collimated, and since you will use this collimated reference beam to reconstruction and project the real image from your multi-beam transmission hologram, then the projected real image will be the same distance from the plate that the original object scene was from the plate when it was recorded. This will allow the reconstructing reference beam to pass through the transmission hologram and not pass through any part of the projected real image. This is important because you will be placing a second recording plate in the middle of the projected real image when recording your multi-beam reflection hologram and you don't want the reconstructing beam, that passes through the transmission hologram, to illuminate the second plate in any way. I'll cover this again when you setup your multi-beam reflection hologram.

relationship reference beam angle to object distance chart
Figure 20b: Relationship of reference beam angle to minimum object distance from plate.

Measuring Path Lengths

With all of your optical components, plate holder, and object scene in place (minus the diverging lenses) and the reference beam incident on the center of the white screen in the plate holder and the object beam is incident on the center of the object scene, you are ready to measure the path lengths of the reference and object beams. Using a tape measure, start by measuring the reference beam from the beamsplitter to mirror M2. Place one end of the tape measure above the beamsplitter where you see the laser beam leaving the beamsplitter and measure the distance to the laser beam dot on M2 from above the dot. Do not touch any of the optical surfaces with the tape measure. Try to be as accurate as possible, but don't lose sleep over it! If you're within 1/8 inch, you're doing great!

Note: Two people doing the measurements helps tremendously, especially for long distances. If you're by yourself, you can lay the approximate tape length out on the table surface, go to the beamsplitter, look down on the top of the beamsplitter and move the front end of the tape under the beam dot. Then go over to mirror M2 and again, looking down on the top of the mirror, move the tape under the dot and read the distance. Write all the distances in the reference beam on a piece of paper in the order you measured.

Continue measuring the distance from M2 to PM, from PM to M3, and then from M3 to the center of PH. Add all these measurements together to arrive at the total path length of the reference beam. Do the same with the object beam starting at the beamsplitter. Be sure to measure to the center of the object scene and from the center of the object scene to the center of the plate holder. If the sums of both beams are not within 1/2 inch of each other, move mirror M5 to adjust the object beam until its total path length is equal to the reference beam total path length.

Re-checking Polarization

We will continue to assume that the laser's output polarization is horizontal with the laser being in an upright position as mentioned in Creating a Single-Beam Transmission Hologram section. If you have set up the optical arrangement properly by following Figure 20a and used the retro-reflection technique so that all the beams are at the same height above the table, you will find that the polarization orientation of the reference beam at the plate holder and the polarization orientation of the object beam at the object scene are both horizontal. Here is how you test this:

Inserting the Diverging Lenses

Now that you have checked the path lengths of both beams and their polarization, place diverging lens DL1 in the reference beam at the focal length of the parabolic mirror and check for uniform lighting across the plate holder with the white screen in place. If the illumination of the screen is off center, you can adjust DL1 right or left, and/or up or down to center the illumination.

Next, place the diverging lens DL2 in the object beam between mirrors M6 and M7 to uniformly illuminate the object scene. Again, by moving DL2 right or left, and/or up or down, you can adjust the centering of the light on the object scene. Re-check that the reference beam is still centered since you just added weight to the table top with DL2's table mount.

Measuring and Adjusting the Beam Intensity Ratio

You are just about ready to make your exposure. You need only take a beam intensity ratio reading with the VOM and solar cell, mask certain parts of the system to keep extraneous light from hitting the plate area, and take an exposure reading.

Block the object beam at the beamsplitter and measure the reference beam intensity at the plate holder with the VOM and solar cell and write down the voltage reading. The white screen should be removed and the solar cell should be placed at the center of the plate holder, straddling it, and fully facing the incident, diverged reference beam. To get the true voltage reading, you need to take the cosine of 56 and multiple that number (0.5592) by the voltage reading.

Next, unblock the object beam, block the reference beam, and measure the reflected light from the object scene at the plate holder. Write down the voltage reading. Again, the solar cell should be centered in the plate holder where the plate will sit and fully facing the object scene. No cosine function is used here since the object scene and the plate holder are parallel to each other.

The reference beam true voltage reading should be 1.5 to 2 times greater than the object beam voltage reading. If this is not the case, adjust your circular beamsplitter or use the glass plate technique for your 50/50 fixed beam ratio beamsplitter, to get this ratio. The intensity of the reference beam must always be greater than the intensity of any reflected point from the object scene to the plate.

Masking Extraneous Laser Light

Once the beam ratios are set, you need to do some masking to keep extraneous laser light from hitting the recording plate shown in Figure 20a. Cut a piece of black mounting board 4 inches wide and 12 inches tall. Without touching DL1's mount, place a 4 inch side of the mask on the table surface close to the back side of DL1's mount and lean the mask towards DL1 with the 4 inch width of the mask centered on the lens. This should be done on M2's side of DL1.

Circle the laser's beam dot with a pencil. Don't press too hard or you might touch the lens mount with the mask and move the mount. Remove the mask and with a single-edge razor blade, cut a 1/4 inch square hole in the mask around the pencil mark. This square should be large enough so that the laser beam does not touch the edges of the square as it passes through the hole. Now place the mask back on the table and gently lean the mask against the lens so the laser beam passes through the hole. This will prevent all of the extraneous noise from the laser cavity, that travels along with the beam, from passing through to the plate and will result in a cleaner, noise-free hologram.

Do the exact same thing to diverging lens DL2, placing the mask on M6's side of DL2.

Using five more masking pieces of the same size and without holes, place one mask between mirror M6 and the plate holder, close to M6, so the plate can't see the laser beam reflected off the surface of M6. Place a second mask between the beamsplitter and the plate holder, close to the beamsplitter, so the plate can't see the laser beam passing through, and reflected off, the surfaces of the beamsplitter. Place a third mask between DL2 and the plate holder, close to DL2, so the plate can't see the laser beam passing through DL2. Place a fourth mask between the parabolic mirror PM and the plate holder, close to PM, so the plate can't see the laser beam reflected off the surface of PM. Place the fifth mask between M4 and the plate holder, close to M4, so the plate can't see the laser beam reflected off the surface of M4. These masks can be held in place using large binder clips as shown in Figure 20c. I don't recommend using table mounts to lean the masks against. The weight of an additional mount on the table can through your beam alignments off which means more work realigning the beams.

reconstructing reflection hologram image  masking extraneous light image
Figure 20c: Large binder clips holding masks on table surface.

Now look through the plate holder toward the object scene. You should be able to scan the area around the object scene and see no other light sources. Also look from the object scene side towards and through the plate holder from various angles to see if you can see any reflections off mirrors or other extraneous light reflections. Put the white screen back in the plate holder and check both sides of the card for extraneous light. If both sides of your white screen are not white, you can just flip the screen around depending on which side of the plate holder you're checking. By doing all this masking, only the image of the object scene will be visible in the hologram. When doing this final check for extraneous light, you can splay your fingers and move your hands in front of the screen on both sides and if there is some extraneous light, you can usually see it as the shadow of your fingers pass over the screen. You then get to play detective and figure out where it's coming from. Bottom line: you don't want any laser light hitting the plate except for the laser light being reflected from the object scene and the reference beam.

Exposure, Processing, and Drying

Note: Once you get the correct exposure density using film, finish processing the film and, once it's dry, place it back in the plate holder, block the object beam and object scene, and use the original reference beam to reconstruct the image and view it. Look for any extraneous light showing up around the image area, or on the hologram itself, that was not masked out. Do this before making the final plate hologram.

Once the plate is made and you have an image, rotate the plate 180 degrees horizontally to project a pseudoscopic real image out into space on the opposite side of the plate from the original object scene. If the reference beam was collimated correctly, the real image should be the same size as the original object scene.

Using Two Object Beams

If you want to use two object beams to illuminate more areas of the object scene, you can setup the arrangement shown in Figure 20d. For this arrangement, you will need one additional beamsplitter, one additional diverging lens, and one additional 5 inch x 7 inch x 1/4 inch thick mirror. These additional optics are shown in Figure 20d as BS2, DL3, and M8. Beamsplitter BS2 is aligned with the center of the plate holder and object scene as shown in Figure 20d. This makes it easier to have the path lengths of the two object beams equal. The path lengths of both object beams should be the same and the beamsplitter should be another 50/50 fixed beam ratio beamsplitter.

multi-beam transmission setup with two object beams image
Figure 20d: Multi-beam transmission hologram setup with two object beams.

When measuring the total path length of the object beam to equal the reference beam's total path length, just use one of the paths after beamsplitter BS2 in your calculation. You will also need two additional masks. One between DL3 and the plate holder positioned close to DL3 and one between BS2 and the plate holder positioned close to BS2.

 

Revised 5/2/2017