Preliminary Activity

Holography

Preliminary Activity

The 30 mW He-Ne laser on the holography table takes at least a half hour to warm up before the intensity of the undiverged beam is large enough (about 13 mW) for exposing holograms. So, if its not already turned on, do so immediately at the beginning of the period. After 90 minutes the laser power will peak at about 23 mW. But you can begin making holograms before that if you make correspondingly longer exposures. Just before each exposure you will be measuring the reference beam intensity at the film plane with the laser power meter.

Introduction

Holography is a 3-D image-making process. It recreates what light actually does after reflecting from a real object. The first hologram was conceived of and produced in 1948 by Dennis Gabor, a researcher at the Imperial College in London, several years before the invention of the laser. For this he received the Nobel prize in physics in 1971.

There are a number of different types of holograms. In a transmission hologram, light from a laser is spread out by a lens to light up the holographic film or plate. The viewer stands on the side of the film opposite the laser, so that the light is "transmitted" through the hologram, and peers into the "window" to see the 3-D scene.

A reflection hologram, although it must be made with a laser, can be viewed with ordinary white light. The hologram is seen with the light source and the viewer on the same side of the film, the light reflecting from the hologram.

A 360o (or cylindrical) hologram allows one to see all around an object. It is necessary to view it with a diverged monochromatic beam of light, usually coming from a laser.

In a holographic interferogram, the same object is used for two exposures on the same film with the object having experienced a small change between exposures. When viewing the hologram, an interference pattern is observed corresponding to the interference of the light coming from two slightly different objects. So the hologram enables one to detect minute changes in an object.

Viewing Holograms

Before trying your hand at making your own holograms, look at the various holograms that are set up in lab. In your lab report, briefly describe the images you are viewing. The reflection hologram is best viewed with light from a desk lamp. The transmission holograms and the 360o hologram should be placed in a diverged laser beam. Shining a laser on a piece of ground glass or on a plastic screen are two ways of making such a beam. Some of the transmission holograms are single-scene and others are multiplex (two-scene).

Also, look at the 120o rainbow hologram. Turn on the white light bulb inside and make observations while rotating the hologram. Move your head up and down and note the color variation.

Experimental Setup

To make a hologram, a laser beam is split into two beams, one that directly illuminates the film (the reference beam) and another that illuminates the object (the object beam). Since the reference and object beams originate from the same mono-chromatic (single wavelength), coherent (in-phase) source of light, the reference beam interferes with the light scattered from the object. This creates an interference pattern on the film that is captured during exposure. Then, when laser light of the same wavelength passes through the developed film at thesame angle as the reference beam, the light is diffracted and produces a 3D image of the original object. For a reflection hologram, the film itself serves as the beam splitter with the direct beam being the reference beam and the light from the object being the object beam. This experiment is associated with reflection holograms.

Part of this experimental setup involves the laser beam being reflected so as to pass through a camera and then a spatial filter. The camera provides a convenient shutter. Use the bulb setting and the remote shutter release cable when you are setting up your object.

The spatial filter helps eliminate noise from dust on the mirrors and imperfections in the lenses and produce large, clean beams. It contains a micro-scope eyepiece and a 10m pinhole situated at the focal point of the converging lens (when properly adjusted).

All optical components have been carefully aligned. During the experiment, if you do not see the object or reference beams at their usual brightness, or if something gets bumped out of position, seek assistance from the instructor. Do not touch the surfaces of any of the optical components, especially the front-surface mirrors.

Procedure

In your lab report you should include a sketch of the setup for the making of the hologram. Indicate (in a table on your data page) the x-y coordinates for each active optical component. Adjacent screw holes are separated by 1 inch.

Turn out the room lights and close the door to the store room. There is a green lamp on the holography table that may be useful when setting up your objects. Be sure to turn the green lamp off when film is out.

Each person will make one reflection hologram using film and one using a glass plate. For each hologram in this experiment, record the parameters of its production:

(a)laser beam intensity of the reference beam at the film plane (use the laser power meter set on a lower scale - probably 200 W)

(b)exposure time

(c)development time

After development, use the red wax marker to identify each hologram in the upper right hand corner.

Securely mount the object on the surface behind the opening of the film holder. Best results are obtained if the object is fairly close to the film plane and is light-colored. Try using coins or buttons. Orient the film/object holder at about a 10o angle with respect to the incident diverged laser beam. As indicated above, in this geometry, the film itself serves as the beam splitter with the direct beam being the reference beam and the light from the object being the object beam.

Film that fits the film holder has been precut and placed in a black envelope inside a used film can. The film (8E75) is sensitive to red light and is available from Integraf of Lake Forest, Illinois. Similarly, the glass plates will be placed inside a black envelope in a second used film can. Close the camera shutter and cock the camera. Install the film or glass plate in the holder. Put the cardboard box over the target area. Everyone except the person opening the shutter of the camera should move well away from the hologram table. Wait for about 1 minute for vibrations to subside.

Open the shutter of the camera and expose the film for about 0.25 seconds (longer if the laser has been on less than 90 minutes). During the exposure, everyone in the room should freeze to avoid setting up wind currents or vibrations. If the film moves as little as one wavelength with respect to the object during the exposure, the hologram probably will not turn out. Put the exposed film and plates into a third used film can. Take the exposed film and plates into the darkroom.

Dark Room

Chemical Preparation:

Set out four glass trays.
In the first glass tray, mix 75 ml of Stock Solution A with 75 ml of Stock Solution B. This will be the developeror “workingsolution” and should be discarded after use.
Fill the second glass tray about 1/4 full with distilled water, which can be discarded when finished.
Fill the third glass tray about 1/2 full with the Bleach Solution. This can be returnedto the bottle after use.
In the fourth glass tray, mix 10 ml of Post Treatment Solution with 100 ml of water. This mixture can be discarded when finished.

Place the enameled tray in the deep sink so that tap water can continuously be running into it. This allows the holograms to be washed in running water.

Development Process:

Turn out all lights in the dark room except for the green lamp.
Develop the film or glass plate in the working solution developer for 2 minutes with constant agitation by rocking the tray.

3.Rinse by sloshing the hologram in distilled water for 10 seconds. Wash in running tap water for 3 minutes.

Bleach in the Bleach Solution with the hologram held vertically or with the emulsion side down until it is transparent, normally less than 2 minutes. Leave in the bleach for an additional minute. After bleaching the room light can be turned fully on.
Wash in running water for 3 minutes. Inspect the hologram in the wash and sponge off any visible powder-like precipitates before the final treatment under bright lights.
Under bright lights, soak the hologram in the Post Treatment Solution until the hologram turns from pink to a light brown color.
Wash the hologram in water for 3 minutes.
Use a rubber squeegee to squeegee the hologram. Dry the hologram with the hair dryer set on warm. One can also hang the hologram to dry. A reflection hologram must be dry before it can be viewed.

Hand in all holograms made by the group, in an envelope, whether they turned out or not.

Extra credit:

Make a good transmission hologram. See instructor for details.

Holography