Preliminary Activity

Holography

Preliminary Activity around an object. It is necessary to view it with a diverged monochromatic beam of light, usually There are two alternative lasers you may use for this coming from a laser. In a holographic interferogram, experiment. The diode laser only needs about 5 the same object is used for two exposures on the same minutes to warm up to full intensity. However, the film with the object having experienced a small 30 mW He-Ne laser on the holography table takes at change between exposures. When viewing the least a half hour to warm up before the intensity of hologram, an interference pattern is observed the undiverged beam is large enough (about 13 mW) corresponding to the interference of the light coming for exposing holograms. So, if you plan to use it and from two slightly different objects. Thus the it is not already turned on, do so immediately at the hologram enables one to detect minute changes in an beginning of the period. After 90 minutes the laser object. power will peak at about 20 mW. But you can begin making holograms before that if you make Viewing Holograms correspondingly longer exposures. Just before each exposure you will be measuring the reference beam Before trying your hand at making your own intensity at the film plane with the laser power meter. 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 Introduction best viewed with light from a desk lamp. The transmission holograms and the 360o hologram Holography is a 3-D image-making process. It should be placed in a diverged laser beam. Shining a recreates what light actually does after reflecting from laser on a piece of ground glass or on a plastic screen a real object. The first hologram was conceived of are two ways of making such a beam. Some of the and produced in 1948 by Dennis Gabor, a researcher transmission holograms are single-scene and others at the Imperial College in London, several years are multiplex (two-scene). before the invention of the laser. For this he received the Nobel Prize in physics in 1971. Experimental Setup

There are a number of different types of holograms. To make a hologram, a laser beam is split into two In a transmission hologram, light from a laser is beams: one that directly illuminates the film (the spread out by a lens to light up the holographic film reference beam) and another that illuminates the or plate. The viewer stands on the side of the film object (the object beam), then reflects to the film. opposite the laser, so that the light is "transmitted" Since the reference and object beams originate from through the hologram, and peers into the "window" to the same mono-chromatic (single wavelength), see the 3-D scene. coherent (consistent phase) source of light, the reference beam interferes with the light scattered A reflection hologram, although it must be made with from the object. This creates an interference pattern a laser, can be viewed with ordinary white light. The on the film that is captured during exposure. Then, hologram is seen with the light source and the viewer when laser light of the same wavelength passes on the same side of the film, with the light reflecting through the developed film at the same angle as the from the hologram. reference beam, the light is diffracted and produces a diffraction pattern that is a 3-D image of the original A 360o (or cylindrical) hologram allows one to see all object. For a reflection hologram, the film itself 11 Holography

1 serves as the beam splitter with the direct beam being the laser to the power supply, and observe the beam. the reference beam and the light from the object being It should spread out horizontally in an elliptical shape the object beam. This experiment explores reflection to fully illuminate the subject region. holograms, though you may attempt a transmission hologram also, if you wish. Once you are satisfied with the illumination of the subject, turn off the main lights, and block the laser Procedure beam with a box, so the subject is no longer illuminated. Remove one piece of film from its In your lab report you should include a sketch of the container in a dark part of the room — keep it away setup for the making of the hologram. This sketch from the laser light. Close the container to protect should be to scale with the scale of the drawing unexposed film. Place the film on top of the subject, indicated clearly. preferably with the sticky emulsion-side down.

Each person will make one reflection hologram using Wait quietly at least 10 seconds after setup to allow flexible film and one using a glass plate. For each the subject and plate to settle. Now slowly lift the hologram in this experiment, record the parameters of box off the table slightly (1 or 2 cm) while still its production, such as: blocking the laser light. Wait at least 5 seconds for table vibrations to subside, then lift the box away (a) laser beam intensity of the reference beam at the slowly (don’t make a breeze). During the exposure, film plane (use the laser power meter set on a everyone in the room should freeze to avoid setting lower scale - probably 200 W) up wind currents or vibrations. If the film moves as (b) distance and angle between the source and the little as one half wavelength with respect to the object film, and between the film and the object during the exposure, the hologram probably will not (c) description of object turn out. Expose the hologram for about 10 seconds (d) exposure time (you may want to try various times to see which work (e) development time(s) best). Bring the box back down to block the laser light. Place the plate in a container marked for After development, use the red wax marker to identify exposed, undeveloped film. each hologram in the upper right hand corner. You may want to expose several plates before Before opening the package containing the developing. If so, be sure to keep them in order in the holography film or plates, turn out the room lights container, so you remember which is which. and close all doors. If you are working in the darkroom, you may leave the safety light on. If you Reflection Hologram with HeNe Laser are working in the lab, you may use the green lamp, but keep it from shining directly or reflecting directly The setup for making holograms with the HeNe laser on the film or plates. is on the holography table. The optical components must be carefully aligned. During the experiment, if Reflection Hologram with Diode Laser you do not see the object or reference beams at their usual brightness, or if something gets bumped out of Unscrew the black collimating lens and spring from position, seek assistance from the instructor. Do not the laser and store in the laser’s container. Clip the touch the surfaces of any of the optical laser with a clothespin and stick the other end of the components, especially the front-surface mirrors. clothespin in a cup of sand to dampen vibrations. Position the cup with the laser about one foot above Securely mount the object on the surface behind the and one foot horizontally from the subject. Connect opening of the film holder. Best results are obtained

2 Holography

2 if the object is fairly close to the film plane and is acceptably. What might one do to compensate to light-colored. Orient the film/object holder at about a produce acceptable holograms? 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.

The camera in the setup provides a convenient shutter. Use the bulb setting and the remote shutter release cable when you are setting up your object. When you are convinced that the subject will be properly illuminated, close the camera shutter and cock the camera. Change the shutter setting from bulb to an appropriate time (try various times, ¼ to ½ second may work well). 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. 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 half wavelength with respect to the object during the exposure, the hologram probably will not turn out. Place the plate in a container marked for exposed, undeveloped film.

Development and Analysis

Keep the exposed plates in the container, and take them to the dark room. Do not open the container with any light except the darkroom safety light. Also be sure the darkroom’s “In Use” light is on, to avoid having anyone open the door while your film is in the open.

Develop the film using the chemicals and instructions available in the dark room.

Analyze your group’s holograms, both those that turned out well and those that didn’t. What parameters seemed to lead to the best holograms? Why? Consider the holograms that did not turn out

33 Holography