Holography Experiment

In order to understand the holographic process you should be familiar with conventional optical diffraction phenomena. In particular, consult optics texts if you are not thoroughly familiar with:

1. diffraction gratings
2. Fresnel zone plates
3. Michelson interferometer.

Consult the Newport manual, references, and instructor regarding the theory of holography and details of the procedure.  In brief: The holographic film is exposed to laser light scattered from an object and simultaneously to a reference beam from the same laser.  The film records the interference pattern of the two waves.  After development, the hologram is replaced in the reference beam.  The wavefront of light scattered by the interference pattern comprising the hologram produces a replica of the wavefront of light previously scattered by the real object.
 

Eye Hazard:

Exposure Time:

Coherence considerations:

Drift of the film, if not adequately clamped down, or of a "soft" object, due to temperature change, air currents, or mechanical relaxation, could wash out the hologram.

The effective coherence length of the laser is   the length of the laser.  This is because it is lasing in two, or maybe three, longitudinal modes at any time.  One can think of the two (say) laser wavelengths as making separate holograms.  If the path lengths of the reference and object beams from the beam splitter differ by an amount equal to the length of the laser tube, then one of these holograms will be shifted by half a fringe with respect to the other, and they will interfere destructively, perhaps giving no net hologram.  Therefore you may get more reliable results if you limit the path difference to a few cm.  (On the other hand, it may work fine with a large path difference.)

In any case, allow the laser to warm up for at least 15 min., because while it is warming up, successive lasing modes are sweeping rapidly across the atomic resonance, so the time-averaged spectral width is characteristic of the natural atomic line width.
 

Processing:

1. Develop 5 min. with continuous agitation (D-19 at 68 degrees F).
2. Stop 30 sec. with agitation (Kodak Indicator Stop Bath).
3. Fix 5 min. with frequent agitation.  Lights can be turned on after 30 sec.
4. Wash 5 min. with running water.
5. Air dry at room temperature.  Be sure water drains from plate holder.

Possible causes of no reconstructed image despite a reasonable exposure include:

1. Vibration or drift of the film or optical components
2. Reference beam much stronger than object beam at film
3. Laser not warmed up, drifting in frequency
4. Excessive difference in path length between reference and object beam
5. Too large an angle between the image and object beams for the resolution of the film (which could be degraded by improper development.)

References:

• G. Saxby, Practical Holography (Prentice-Hall, 1988).  Recommended.
• J. E. Kasper and S. A. Feller, The Complete Book of Holograms (1987).
• P. Hariharan, ??? (cambridge, 1984).
• R. J. Collier, et. al., Optical Holography (Academic Press, 1971).
• H.M. Smith, Principles of Holography (Wiley, 1969).
• E. Camatini, ed., Optical and Acoustic Holography (short articles, see article by Guber, pp. 15-21.)

Possible Experiments:

1. Split Beam Holography.  Make holograms of several different objects.  Possible objects include a block with nails, a gear, a coin, chess pieces, a watch with a lens in front of it, or anything else you think of.
2. Diffraction grating.  Make a holographic diffraction grating by exposing the film to two expanded and collimated beams of nearly equal intensity, intersecting at an angle of about 10 degrees or less (a wedge prism is a convenient way to produce this.)  Measure the grating spacing under a microscope.  Use the grating to diffract the laser beam.  Using the measured spacing, calculate the wavelength of the laser.  (Alternatively, use the laser wavelength to calculate the grating spacing).  How strong is the second order diffraction, relative to first order?
3. Single Beam Holography.  Make a Fresnel zone plate by exposing the film to a reference plane wave with a lens placed in it.  Illuminate the developed hologram with the plane wave and observe focusing.  You will have to think of a clever way to hold the lens without introducing significant perturbations from the lens holder.  A small liquid droplet on a clean glass plate might do it
4. Holographic Interferometry (Double exposure):  Study the stress pattern of an object by making a hologram with the object subjected to differing amounts of stress (possibly a metal rod with a clamp attached).  Expose the film for half of the total exposure time while the object has little or no stress applied.  Then apply stress to the object (tighten the clamp) and expose the film for the rest of the exposure time.  Be careful to not move the subject between exposures? (Why?)
5. Holographic Interferometry (Vibration):  Study the periodic motion of a vibrating membrane by using a function generator to apply a sinusoidal voltage to a speaker.  Since an object in sinusoidal motion spends much of its time near the ends of the motion, you can get an image due to interference of light reflecting off the object while in different positions (the two ends of the motion).  Study the normal modes of a diaphragm at several frequencies.  Study the vibration amplitude resulting from varying the amplitude of the drive signal.
6. Make a simulated "motion picture" hologram by exposing three or four different parts of the film to an object that has been moved between exposures.  You can do this by using some kind of mask to expose the film in separate vertical slits.  Observe the motion by moving your eye horizontally along the developed hologram.  Possible images include an object that is rotated between exposures.
7. Make a reflection hologram (object and reference beam on opposite sides of the film.)  This requires high resolution film (Agfa 8E75) and good mechanical stability, and may require optimum development conditions (the resolution must be better than 1/2 wavelength.)  This hologram can be viewed in white light.
8.  You can find other suggestions in papers in the lab or books on Reserve.