Introduction

In this section, laboratory exercises will be arranged so that we start with simple procedures using simple equipment. The low cost of equipment makes it possible for many of the following exercises to be performed as “homework.”

All the experiments require the use of a laser and chemicals for processing the holograms. Section I, “Diode laser holography,” uses a class IIIa diode laser without the collimating lens, rendering it useless as a “pointer.” This allows the work to be performed by students at home. When higher-power lasers are used, in section II, “Advanced holography,” supervision by a qualified instructor is required.

Always observe all safety rules concerning lasers and chemicals.

Generally, the developer and bleach solutions are mixed by the instructor. Since the photochemistry of holography is an ongoing research, it will change as improvements are found. Thus, no particular regime is discussed here. Use according to the detailed instructions that accompany each processing kit provided by the manufacturer.

Similarly, the holographic plate or film used will change with time. Use them in accordance with the instructions of the supplier. The exposure time and the appropriate processing scheme are also provided.

Equipment and facilities

Holograms are made in darkened areas free from drafts, vibration, and noise. Because of the relatively low sensitivity of the recording material, sufficient light is allowed so that one can see comfortably after dark adaptation. To achieve this, use a 25-watt green light bulb in a lamp. Place the lamp under the table, cover it with aluminum foil to adjust the light, and direct it toward the floor. Do not allow direct light to shine on the holography system or on the developing station.

If the room has windows, cover them with black plastic sheets. Enough light can leak through to allow minimum vision after dark adaptation. In case of doubt, leave a holographic plate on a table and expose it to the ambient light for ten minutes. Develop it. If it turns dark, there is too much light.

Flowing tap water is desirable but not necessary. A large tray of clean water can be used to rinse the developed hologram. White trays are desirable because they allow continual inspection. An alternative is to use glass trays resting on white paper.

Make sure all fire codes are observed.

I. Diode laser holography

Certain class IIIa diode lasers sold as “pointers” are found to have high-frequency stability and thus long coherence length after an initial warm-up of a few minutes. With the collimating lens removed, the laser light spreads directly from the laser with a highly eccentric elliptical profile. Since the beam does not encounter other optical elements, it is completely “clean.” This allows it to be used to make many types of holograms without additional optical components.

These experiments are to be performed on top of a sturdy lab table or kitchen counter or on the floor. Support a thick 50-cm ´ 100-cm wooden board (or optical table) on top of four “lazy balls” (rubber balls that don’t bounce). Put washers under each ball so that they will not roll.

A. Reflection hologram

Equipment requirement: Darkened room with green safe-light, sturdy table or counter, optical table supported by “lazy balls,” mounted diode laser system, object on platform with three-point support, shutter, processing trays with chemicals, and holographic plates.

Figure 10-15—shown earlier—indicates the setup for making a “one-beam reflection hologram,” sometimes called a Lippmann (Nobel Prize in physics, 1908) or Denisyuk hologram.

Procedure

1. Choose a solid object that looks bright when illuminated with laser light and whose size is not bigger than the hologram to be made. Mount (hot glue) it on a small platform made of wood or sheet metal (15 cm ´ 15 cm) with three round-head short screws underneath (to prevent rocking). Mount the laser on a stand about 25 cm high and direct the light down at 45° at the object, with the light spreading horizontally. The distance between the laser and the object is about 40 cm. Now turn on the safe light and turn off the room light.

2. After the laser has been warmed up for at least five minutes, block the light from reaching the object using a self-standing black cardboard. (We will call this the shutter.)

3. Lean a holoplate directly on the object, with the sticky side touching it. Wait at least 10 seconds.

4. Lift the shutter, but still blocking the light, for 2 seconds, to allow any vibration to subside. Then lift the shutter away completely to allow the light to pass through the holoplate. The exposure is usually about 5 seconds. (Consult the instructions that accompany the plates.) Then block the light again.

5. Develop the hologram according to instructions from the manufacturer.

After the hologram is dried, view it with a spot light such as a pen light, projector, or direct sunlight. Optional: Spray paint the sticky side (emulsion side) with a flat (or “antique”) black paint to provide a darker background and greatly improve the visibility of the image.

B. Transmission holograms

1. Without a mirror

Equipment requirement: Same as for the “reflection hologram” in section A. above. In addition, a stand-alone plate holder is needed. Make one exactly the same way as the object platform described above. Instead of the object, install two long (12 cm) screws on top with a separation less than the width of the holoplate to be used. Paint the screws a diffused black color.

Procedure

1. Set up the system as shown in Figure 10-19. The diode laser is mounted 5 cm above the optical table with the beam spreading horizontally. One side of the beam illuminates the object or objects, and the other side serves as reference beam.
   

   Figure 10-19  The simplest configuration for making a transmission hologram

2. Block the beam with the shutter, turn off the room light, and, on the stand-alone plate holder, lean a holoplate vertically against the black screws with the sticky side facing the object(s). Wait 10 seconds.

3. Lift the shutter and expose for about 30 seconds. Note: If there is a draft across your system, the long exposure time of 30 seconds requires you to put a large box over the entire system during the exposure.

4. Develop and dry as before.

5. This hologram must be viewed with laser light. To do so, lean the finished hologram back on the black screws the same way as during exposure. Cover or remove the objects and look through the hologram toward the location of the objects. A virtual image can be seen as if the object is still there.

6. To observe the real image:
   • Relocate the finished hologram in the position where it was exposed.

   • Remove the object and, in its place, position a vertical white screen (cardboard) facing the hologram.

   • Darken the room and direct a collimated laser beam through the center of the hologram in a direction that is 180° from the original reference beam, i.e., back toward the location of the diode laser used for making the hologram.

     All light paths are now reversed and a two-dimensional image is projected onto the screen. Move the laser beam to different locations of the hologram and observe the changing perspectives of the image.

2. Transmission hologram with one mirror

Note that the objects in the foregoing transmission hologram are always illuminated from the side. To get more frontal lighting, we need to add one flat, front-surface mirror.

Equipment requirement: Same as above, plus one front-surface mirror. Mount this mirror in a vertical position by hot-gluing the back side (the glass side) to a block of metal or wood, with the bottom edge raised to the same height as the bases for the object and the plate holder.

Procedure

The procedure is the same as for section B.1 (transmission holograms without a mirror), but using the setup as shown in Figure 10-20. Note that a baffle in the form of a black cardboard is needed to block the laser light from exposing the holoplate from the back side. To project the real image onto a screen, set the finished hologram back onto the plate holder and direct a laser pointer backward through the hologram, onto the mirror so that the beam hits the diode laser. On a screen located at the position of the object, the real image is formed.

Figure 10-20  With the addition of one front-surface mirror, the hologram can be made facing the objects and the objects are better illuminated.

3. Two-channel transmission hologram

Use the same procedure as for the “transmission hologram with one mirror,” except now make the exposure time one-half as long. Pick up the holoplate and rotate it upside down (with emulsion or sticky side still facing the object) and place it back on the plate holder. Replace Object 1 with Object 2. Expose again one-half the full time. Process as before.

In viewing the virtual or real image, notice that, depending on which side is up, a correspondingly different image is observed.

4. Diffraction grating

Set up the experiment according to the layout shown in Figure 10-21. The plane of the mirror should make a 90° angle with the plane of the holoplate. Notice that no object is used in this experiment.

Figure 10-21  Making a hologram of light reflected from a mirror results in a high-dispersion diffraction grating.

Here the “object” is part of the beam from the laser that is redirected by the mirror. This configuration is the same as shown in Figure 10-6, in which both the reference and object beams are point sources.

Use the same procedure as before but with a shorter exposure time (approximately 75% as much) and process similarly.

Project the real image of the finished hologram onto a screen at the location of the mirror. Is there any difference between a real and a virtual image in this case?

If you see more than one spot, the extra ones are caused by internal reflections inside the holoplate. To avoid these reflections, an anti-halation backing of the holoplate is required. This is done by painting on the glass side of the holoplate, before exposure, with a washable blue or black water-color paint. This absorbs the light as it arrives at the back surface during exposure.

This hologram is in effect a high-dispersion diffraction grating. Shine a beam of white light from a slide projector through the hologram and observe the spectrum on a screen. Use other sources such a mercury and sodium lights from the street during the night.

5. Additional project

Design a system using this holographic grating to observe the Fraunhoffer lines from the sun. It will show which elements exist in the sun!

II. Advanced holography

The preceding experiments are fundamental. They use simple equipment to demonstrate the major principles of holography. However, they have many limitations. For example:

• The intensity ratio between the reference and object beam cannot be controlled. Thus the quality of the hologram cannot be optimized.

• The object is always illuminated with a single point source from a fixed direction, thereby casting shadows that cannot be controlled.

• The location of the image is always behind the plate.

• A laser is needed to observe the image formed by transmission holograms.

To make holograms with none of the above restrictions, the following additional equipment is needed:

• A higher-power laser that operates in a single transverse mode. It’s even better if it operates in a single axial mode. A 10- to 30-milliwatt HeNe laser is recommended. Observe all laser safety rules !!

• A power meter with a sensitivity range of 1 microwatt to 1 watt

• A large (4 feet × 8 feet is typical) isolation table with adjustable front-surface mirrors (4); lens with the largest aperture and shortest positive focal length (1); variable beam splitters (2); spatial filters with 10X objective and 25-micron pinholes (3); 4 × 5-inch ground glass plate (1); plate holder (1); and hardware for supporting all the above components

• Alternative, low-cost system: Build a sandbox (3 feet × 4 feet × 10 inches) with thick plywood and fill it with washed silica sand (the kind sold for sandblasting). Substitute spatial filters with small, double-concave lenses and concave mirrors. Mount (glue) all optical elements on top of 2.5 inch diameter PVC pipes (which allows x-y-z position adjustments when stuck into the sand). Use film instead of plates and sandwich it between large glass plates with strong paper clamps. (Just stick the sandwich into the sand.) Support the sandbox by a dozen “lazy balls” and put the system on a rolling table. The cost of this system is an order of magnitude less than the commercial system and is more mobile and versatile for the purpose of learning.

A. Split-beam transmission hologram

The following symbols are used in the diagrams for the remaining experiments:

Note: A lens followed by a pinhole is a spatial filter.

Figure 10-22 is the layout for making a highest-quality transmission hologram. The procedure is general and is applicable to all of the remaining experiments.

Figure 10-22  A general configuration for making the highest quality transmission holograms

1. Lay out the components as shown in Figure 10-22 except for the two spatial filters L₁, S and L₂, S (or meniscus double-concave lenses).

2. Equalize the beam paths by using a cord. Start from the beam splitter BS, and measure the total distance from BS to M₂ and on to H. Now measure the distance from BS to M₃, on to the center of the object O and on to H. Equalize the paths within 2 or 3 cm by moving mirror M₂.

3. Expand the two beams by positioning the spatial filters (lenses) as shown. The reference beam should cover the holoplate (here substituted for by a white card during alignment). The object beam should light up the object.

4. Baffling: From the position of the holoplate, carefully observe light scattered from anywhere other than from the object and the reference beam. Block all the unwanted light using black cardboard.

5. Measure the intensity ratio of the two beams incident on the holoplate with the power meter by positioning the meter at the location of the holoplate. Block the light from the object beam and read the intensity of the reference; then block the reference beam and read the intensity of the light beam from the object. Adjust the intensity ratio of reference beam to object beam reading to about 4:1 to 8:1 by two methods: (1) use a variable beam splitter; (2) move the beam-expanding lenses appropriately along the beam paths.

6. Determine the exposure time.

7. Now expose and develop the hologram.

If “soft” lighting of the object is desired, a ground glass can be placed between the object and mirror M₃. Carefully baffle any light from the ground glass directly to H.

B. Dual-object beam transmission hologram

Figure 10-23 shows a layout for illuminating the object with two beams. This allows artistic lighting through the addition of a beam splitter, spatial filter, and mirror. Soft lighting with ground glass(es) is also possible.

Figure 10-23  The object is illuminated from two independent beams, allowing more artistic lighting.

The beam path of the added beam (object beam 2) must be equalized, starting from the beam splitter BS₁. Then proceed with instructions given in II.A. above.

C. Focused-image reflection hologram

Figure 10-24 shows a layout for making a reflection hologram in which the image appears in the plane of the hologram. Here the large lens with a short focal length is used to image the object onto the plane of the hologram. The object and the holoplate are located at a distance equal to 2f on the opposite sides of the lens. This allows the image to be the same size as the object.

Figure 10-24  A configuration for making a focused image reflection hologram

During alignment, use a white card to find the real image of the object and place the plate holder there. As is true for making reflection holograms in general, a good beam ratio is 1:1.

Notice that in this experiment we are making a photograph and a hologram at the same time!