Main Page/PHYS 3220/Holography

Introduction

Holograms are three dimensional images of scenes or objects recorded on two dimensional film or glass plates coated with photographic emulsion. The hologram actually records the interaction of two beams of coherent light, and these are mutually correlated in the form of a microscopic pattern of interference fringes. The first beam of coherent light is called the reference beam and it directly illuminates the film. The second, or object beam travels from the light source to the object, and then is reflected onto the film. The interference pattern at the film is used to store the information of the depth of the recorded object. When properly illuminated, the film or photographic plate produces a three dimensional image which is the hologram.

In this experiment, you will produce different holographic plates using different optical beam configurations to gain a broad understanding for the physics principles behind holography, and also the various techniques employed in producing such holograms.

Method

Apparatus

Setting Up the Optical Track

Using the height of the laser as a guide, insert the spacial filter, lens, hologram stage, and beam block into the optical track stands. It is VERY important to ensure that the height of all of the optics are the same (verify this, explain method). Use the diagram below to help you set up the optical track. Make sure to include an explanation in your report (with any necessary background) of the purpose of every optic used.

Maximizing the Power through the Pin Hole

Once all your optics are in place it is important to maximize the power through your special filter in order to minimize the exposure time. The pinhole and telescope are on a translation stage that can be finely adjusted using the vernier screws attached. Scan back in fourth in the horizontal and vertical direction until the beam is maximized. You may have to adjust the forward motion of the stage for complete maximization (Note that when you do, you will have to readjust the vertical and horizontal screws as well). Use a white piece of paper along with the provided powermeter to help you in your calibration. Remember that we wish to have an exposure time of less than 10 minutes. Record your experimental maximum power. How does this value compare to the power you calculated in your pre-lab exercise? What is the efficiency of the spacial filter?

First (reflection) Hologram

The first hologram will be the reflection hologram. In this method, the incoming laser illuminates both the object and the film at the same time. There is a reflection off of the object which creates an interference pattern and ultimately the hologram. Place a glass setup plate in the groove on the ‘Litiholo’ hologram stage. It will rest along some angle along the black plastic support piece.

Select on object (provided) for your hologram and place it inside the rectangular area etched on the plate holder. Using a white card to see where the expanded laser beam is projected, you should see that the beam is spreading toward the object and the glass plate. Now place the white card together with the glass plate in the groove to see how the laser beam will illuminate the area (Take note of this and describe/image this in your report). The brightest part of the beam should be about halfway up the white card. The larger the bright area of the beam is the better, so if the majority of the white card is bright that is a good sign and will help you achieve a better hologram. It is VERY important that the laser is illuminating both the object and the plate so adjust the height and angle of the hologram stage to ensure this. Once you are sure that the laser is illuminating the stage correctly, remove the white card and the glass plate.

The provided black card will act as a simple shutter to block and unblock the laser light during the hologram exposure. Devise a way to use this card as a shutter and explain your method in your report. Keep in mind that it is important that the stutter can be inserted and removed with almost no perturbation to the rest of the setup. Ensure that when your shutter is in place, no laser light reaches the object of the film. Insert the shutter.

At this point you are almost ready to create your hologram. However, first ensure that your laser has been sufficiently warmed up. If the laser is not sufficiently warmed-up you will not get a good hologram. At this point see appendix 1 for other important information and tips on creating good holograms.

Turn on the provided LED darkroom light and turn off all other lights. A small amount of light coming under a door will probably not harm your hologram but total darkness is best. The LED light will give you ample light to maneuver, and perform the experiment and will not affect the exposure. Explain in your report why this is true.

With all of the lights turned off, open the Litiholo 2.0 “Instant Hologram” film box and remove one film plate. Leave the remaining film plates in the box and close the box. You may notice that the film plate has a thin, clear coversheet on one side; do not remove it. Place the film plate in the groove on the hologram stage with the clear cover sheet facing away from the object. Let it lean on the black support on the stage so that it is stable. If the film plate is not stable or positioned properly your hologram will appear dim or not develop.

Now that everything is in place, you must wait for any remaining small vibrations or movements to die down. We suggest that you wait 3 minutes; do not talk, remain still and make sure you are not touching the table. If there are any loud noises wait for them to die down. All of the aforementioned items can cause vibrations and unwanted perturbations to the hologram. Gently lift the black card so that it no longer blocks the laser beam. Expose the hologram for 10 minutes. Again, do not talk, remain as still as possible and do not touch the table or anything on the table. Note that although the laser’s power should be maximized to obtain an exposure time of 10 minutes, the longer you expose your hologram the better it will come out. Gently replace the black card so that it blocks the laser. At this point you may turn on the lights but your hologram will be best viewed at first in the dark/dim lighting.

Without moving the film plate, remove the object and set it aside. Remove the black card so the laser light now hits the film plate again. Looking THROUGH the film plate toward the area where the object was originally located, you should now see a holographic image sitting in the same location. If necessary, rotate the hologram stage slightly to maximize the hologram’s brightness. You may also use the LED light to view your hologram. You may have to move the LED around to get the best angle. Place your hologram in the provided envelope and label the envelope with your names, student numbers and the date. Also provide an appropriate title. You will submit this along with your report to the TA for grading.