Introduction to Holography

The difference between a photograph and a hologram is immediately evident: a photograph records the intensity of light, the dark and bright areas of a scene, while a hologram preserves not only intensity but also depth information. That is, a hologram displays the parallax that is evident in viewing real life objects. A hologram accomplished this by reproducing the actual complex wavefront leaving an object, including phase information as well as intensity.

Since recording media such as photographic film respond to light energy, it is necessary to convert phase information to intensity information so that it can be registered on the film. Recall that constructive interference creates a large intensity when the waves are in phase and destructive interference occurs when waves are out of phase. Interference patterns, then, can be used to encode information about the phase relationships of different parts of a wave front.

Creating a hologram: Interference

A qualitative understanding of the creation of a hologram can be obtained by studying Moire´ patterns, named for the swirling patterns of light and dark on the variety of silk of the same name. Consider, for example, two overlapping plane waves. Each wave can be represented by parallel lines; the constant line separation is the spacing between wave crests, that is, the wavelength. Where the two waves overlap, a moire pattern results (Figure 1) The bright bands represent constructive interference, where the

 

Figure 1 - Moire patterns from "interfering" parallel lines

crests from the two waves coincide. The dark bands represent destructive interference, where the crests of one wave meet the troughs of the other. In the case of two plane waves, the distance between the dark (or bright) bands depends upon the angle at which the waves meet. (Figure 2)

 

 Figure 2- Fringe spacing depends upon the angle between the direction of travel of the wavefronts

 Suppose that the object to be holographed is a point source of light, such as the head of a small pin. In that case, the waves leaving the object form concentric spheres. If this object beam interferes with plane waves of the same wavelength (a so-called reference beam), Moire patterns again result (Figure 3). If a film plate is positioned as shown in Figure 3, it will be exposed wherever there are interference maxima. When developed, the plate will contain a record of the interference pattern- a hologram of the point source. The pattern, called a Gabor zone plate, will consist of concentric rings, or zones, whose transmittance (optical density) is a function of distance from the center of the pattern.

 

 Figure 3 -Interference of expanding waves from a point source and a plane reference wave. A zone plate forms on the film

 

If the point source is replaced by an extended object, each point on the object forms its own set of Gabor zones. Thus the hologram is a complex set of overlapping interference fringes which preserves both the phase and intensity information of the original wavefront.

Viewing a hologram: Diffraction

Once the film plate is developed, the wavefront from the point source object may be reconstructed by illuminating the film with the original reference beam (the plane wave). Passing through the hologram, the reference beam is diffracted so that, to an observer looking back through the film plate, the light appears to come from the position of the original object. (Figure 7.4) That is, for the object holographed in Figure 7.3 (a point source) the original spherical wavefront has been reconstructed. A virtual image of the point object forms at the position of the original object. Note that a real image is also formed, however, this is not the image that is usually viewed.

 Figure 4 - Hologram reconstruction. The duplicate reference beam strikes the developed film plate from the left.

 

Figure 3 illustrates the concept of creating a hologram, however there is a practical limitation to placing the object and reference beam along the same line, as shown. The difficulty is shown in Figure 4, where the light forming the real image is directed forward along with the light forming the virtual image. It would be difficult to see the virtual image over the light producing the real image. As will be shown later, practical holography separates the real and virtual images by having the object and reference beams make different angles with the film plate.

 

Types of holograms

The hologram described above is a transmission hologram. It is usually viewed with the same type of laser that was used to create it, and the virtual image is seen by looking back through the hologram toward the reference beam. The image can be quite sharp and, if created with a laser of sufficient intensity and coherence length, can encompass a room.

Reflection holograms are viewed with the light source on the same side as the viewer, and the light reflected from the hologram surface forms the image. Unlike transmission holograms, in the construction of a reflection hologram the object is placed on the opposite side of the film plate from the reference beam. Thus, interference fringes are formed parallel to the film emulsion, not perpendicular as for the transmission hologram. Figure 5 illustrates the difference between fringes forming parallel and perpendicular to the film emulsion layer.

In a typical emulsion which is 10 wavelengths thick, 20 such fringe "layers" form. When illuminated by a white light source, only one wavelength interferes constructively when reflected from the 20 "mirrors". Thus, the reflection hologram can be viewed in white light.

 

Figure 5 The fringes can be parallel or perpendicular to the film emulsion

 Embossed holgrams have become familiar on credit cards as well as stamps and other mass produced applications. In this case, the original hologram is recorded on photoresist, so that the developed hologram consists of grooves rather than intensity variations. A metal layer is deposited on the primary hologram, and peeled off to make a press used to emboss the hologram onto a surface.

Since the reconstruction of a holographic image depends on the relative positions of object and reference beam, it is possible to record more than one scene on the same hologram by altering the position of the plate between exposures. Such multiple channel holograms are being explored for optical data storage.

Computer generated holograms are created by calculating the holographic patterns by computer. This technique is of growing importance in the area of holographic optical elements (HOE) used to control light in many applications.

Holographic interferometry allows the measurement of very small motions through the interference of one hologram with another, or the light from a hologram with the light from original object. If a hologram is made of an object, then the object is moved slightly and a second (double) exposure is made, dark fringes will be seen on the image indicating the displacement. In addition to the double exposure hologram, it is also possible to replace the developed hologram in its original position and view the virtual image superimposed on the object. Any motion or deformation of the object will appear as dark lines in this real time holographic interferometry.

Practical considerations

Creating holograms no longer has the air of mystery it had in the early 70s, when practitioners were mostly scientists in expensive laboratories. In fact, hobby holography has a large following (see reference 1). Nonetheless, holography is not quite the same as photography with a "point and shoot" camera and there are some important considerations for successful outcome.

Laser source

The laser must have an adequate coherence length so that interference is possible. This is important especially for two beam holograms. The path taken by the reference beam must not differ from the object beam path by more than the coherence length of the laser. If the laser is the usual lab HeNe, the coherence length can be taken to be approximately the length of the laser tube.

The output power is not necessarily a limiting factor, except that very low power lasers will require long exposure times. Unlike a photograph where motion causes a blurring of the photo, motion of the holographic film plate can result in no hologram at all. Recall that the plate is recording interference fringes that are separated by less than one micron. If the plate moves during exposure relative to the fringe pattern, bright fringes will cross the position of dark fringes, resulting in uniform illumination- no hologram.

Recording medium

The easiest medium to work with is glass plates with silver halide emulsion. Plates are easier to mount and hold steady than film holograhpic, which is also available. Film plates for holography are not inexpensive, thus planning and care are required. Although the emulsions are similar to those for black and white photography, the grain size is much smaller to allow resolution of fringes spaced by less than the wavelength of light. Cheaper black and white photography film won't work.

Dichromated gel films are used to produce the extremely bright holograms used in hologram stickers and other novelties. These are difficult to work with, requiring an exacting development procedure.

A more expensive, but reusable, medium is thermoplastic holography plates. These can be reused several hundred times before needing to be replaced; however, an inexpensive "instant holography" system costs upwards of $5000 and the film plates are $250-$200 apiece. Once very common for holographic interferometry, the cameras are now being replaced by electronic holography.

Vibration

Since motion of the film plate relative to the object is an issue, care must be taken to reduce vibrations to a minimum, especially with a low laser power requiring a long exposure time. Placing all components on a heavy base (such as a concrete block) which is itself placed on neoprene rubber balls will help lessen vibration. During exposure there should be no moving about, and the shutter, if it is operated by hand, should be moved slowly without touching any of the components.

Ambient light

Commonly available holographic film is red sensitive, so a "safelight" should be green so as not to expose the film. A green night light bulb, placed so that it does not shine directly on the film, is ideal. The room must be darkened, but it does not need to be totally dark.

Developing Chemicals

Developing chemicals (as well as film plates) are readily available (see, for example, Reference 2). Less toxic chemicals have been developed which may be more appropriate in some home lab and classroom settings. The development procedure for the less toxic developers is slightly more complicated however, and requires more time than standard development. Follow all directions on the package inserts, and be sure to dispose of used chemicals according to local regulations.

Subjects for holography (the object)

The best objects are small, stable and light colored, that is, reflective but not shiny. Objects that are too large will produce noisy holograms or, if placed far from the plate to reduce noise, may cause the beam path length difference to exceed the coherence length of the laser. Very light objects are more subject to movement because of air currents. The lighter color Pokemon® figures make excellent subjects for holography!

Laboratory set ups for beginner holography

Single beam reflection hologram

The simplest hologram is made with a single beam, expanded by a short focus converging mirror. (A short focus lens can also be used, but aligning the mirror is simpler.) The object is placed against the emulsion (sticky) side of the plate so that if there is any vibration, plate and object move together. The reference beam strikes the plate from one side, the object beam is scattered toward the plate from the opposite side. This is a good type of hologram for the beginner!

Figure 6 illustrates the geometry of constructing and viewing a single beam reflection hologram.

Creating the single beam reflection hologram

Viewing the single beam reflection hologram (a small source of white light, such as a flashlight bulb, is used)

Figure 6

 

Split beam transmission hologram

A split beam hologram requires a somewhat more stable configuration, since relative motion is possible between two beams and the film plate. The beam is divided by a beam splitter, which may be a variable splitter, allowing control over beam intensities. Each beam is expanded; the reference beam is directed to the film plate, and the object beam illuminates the object. Care must be taken to ensure that the two beam paths are the same within the coherence length of the laser.

Figure 7 illustrates construction and viewing of a transmission hologram.

 

Creating a two beam transmission hologram. The reference beam is directed toward the film plate by the mirror.

Viewing a two transmission hologram. A laser is needed.

 Figure 7

References

1. http://www.holoworld.com. Comprehensive source of information on holograms, including site for kids. Directions for "home made" holograms with laser pointers.

2. http://members.aol.com/integraf/ Integraf. Provider of holography plates and chemicals. Technical assistance available for customers.

3. Unterseher. F.,Ross, F., Kluepfel, B.. Holography Handbook: Making holograms the easy wasy., Ross Books, Berkeley, CA, 1996. Step by step guide to making holograms.