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, which 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 conventional photography, a lens is used to focus light from a point on a source onto a corresponding point on a photographic plate. This plate then forms an irradiance map of the original object. The amplitude is recorded in terms of the intensity, but the phase information of the wavefront is lost, and hence there is no perception of depth or parallax in the final photograph.

The basic concept of a hologram is shown in Fig. 1 using a simple example. On the top left, light incident on a point object is diffracted into a pattern of concentric diffraction rings of maxima and minima which can be recorded on a transparency. In a second step, light incident on this transparency will be focused onto a point. The same argument extended to two points for the object (middle diagram) shows two concentric rings. If this transparency is then exposed to light, each set of concentric rings will focus the light onto two separate points. Finally in the bottom example, you can see that a cluster of points (which is shown in this example as the letters A B C) produces a superimposed array of concentric rings for each point on the object. Each set of rings will refocus incident light onto a corresponding point and the net result is a reproduction of the object which are the letters A B C. A photographic film can record intensities but not the phase of the light. In the example above, each set of fringes from a single point on the object interferes, but there is no fixed phase relationship between the different sets of rings and hence no interference. To make the different signals be compatible in phase, another wave called the “coherent background” or “reference signal” is added.

Figure 1: Conceptual Hologram

Therefore, holography is a two-step process. In the first part the image is formed on a photographic record called a hologram. In the second part the image is reconstructed from the hologram by illuminating with the same light source.

To make a hologram of a 3D object, the reference beam is added at an angle “off-axis” with the signal beam. The signal beam is scattered off the object and made to interfere with the reference beam at the film to form fringes. Normally these fringes are very closely spaced and cannot be seen with the bare eye. Under a microscope the holographic plate looks like a mesh of cells each containing a series of fringes of various lengths and sizes. Holograms can only be made using light sources that have a special property known as coherence. Some gas discharge tubes have this property but to a very small degree so that they can only be used for holograms of flat surfaces or for making copies of holograms. Laser light possesses this property of coherence to a much greater degree and is thus better suited for making holograms of 3D objects. (Understand this property and give a brief summary in the introduction or theory section of your report. Use reference 1 for help.) In brief, temporal coherence is related to the extent over which all the photons in the light beam are at the same frequency, or equivalently, in terms of wavelength, and that are also in phase. The spread in the wavelength, or the bandwidth, of a helium-neon laser is 632.8 ± 0.002 nm. You can verify (see references 2 and 3 or any optics textbook) to see that this spread can be transformed into a “coherence length” which is related to the wavelength and the spread in the wavelength. Coherence length is given by

Where λ is the wavelength and Δλ is the spectral or band width of the laser.

This coherence length determines the depth perception of the subject while making a hologram. In general the light used for illuminating the object should have a coherence length that exceeds the path differences due to the unevenness of the object. Thus for a solid three dimensional object, laser light is the most suitable.

It is important to understand that unlike a photograph where each point on the film corresponds to a point on the object, light from every object point goes to the entire hologram. Because of this if the hologram is broken; it will still reconstruct the whole scene or object. There could however be a loss of detail observed in terms of depth perception.

In this experiment you will be creating two different types of holograms. The first hologram is called a transmission hologram. In this hologram both the object and reference beam are incident on the film from the same side. The second type is called a reflection hologram. In this type of hologram, the reference beam and the object beam are incident on opposite sides of the film. (Include a ray diagram in your report to better describe the above hologram types.)

Pre-Lab Exercises

1. To expose a hologram we require 6x10¹⁸ Photons/cm² measured at the hologram stage.Calculate the output power (in mW and Photons/cm^2) to have an exposure time of less than 10 minutes. Begin with a derivation of the equation you will be using.
2. Show the coherence length is given by equation 1 and calculate it for this setup. What is the significance and this length and why is it important?
3. Describe the characteristics of objects that would be ideal for holography. Explain your choice.
4. What is (research) the warm up time for the given laser?

Method

Apparatus

1. LED light
2. toy car
3. coin or similar object
4. beam block
5. glass plate
6. white and black cards
7. Lens
8. 2 Litiholo 2.0 “Instant Hologram” film plates
9. Spectra-Physics 106-1 He-Ne Laser
10. Hologram Stages
11. Spacial Filter/Telescope

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.

Figure 2: Optical Track Setup

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?

Transmission Hologram

A transmission hologram is created when the incoming laser illuminates both the object and the film at the same time, from the same side of the film. 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 a provided object 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.

Figure 3: Diagram of Hologram Stage Setup

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.

Reflection Hologram

The second hologram you will be creating is the reflection hologram. This is done by shining the laser light through the film plate with an image flush against the back of the film plate. The result will look as if the image is imprinted onto the plate itself.

Remove the hologram stage from its stand and place an additional beam block in its place. Using the empty stand, mount the secondary stage (Clamp stage), ensuring that it is at the correct height. Insert the glass plate along with the white paper to ensure the laser beam is illuminating the center section of the plate. At this point choose a second object to holograph. This item should be a coin or a medallion that is no smaller than a quarter. Ensure that your coin is shiny (clean it, if necessary) as this type of object will obtain the best results. Cross the clamps and insert the item such that the clamps support it. If the laser was illuminating the center of the plate the item should be fully illuminated as well. If it is not, adjust the height of your item such that it is fully illuminated.

Figure 4: Optical track for second hologram

Insert the shutter and ensure that all of laser light is blocked. Remove the white paper and glass plate (the item will fall out as well) and set them aside.

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.

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.

Carefully insert the film plate into the camp stand with the clear coating facing away from the laser. Carefully place your object between the crossed clamps (as described above) behind the film. The item should be touching the clear coating on the back of the film.

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.

This hologram does not need laser light to be viewed (why? Compare and contrast this hologram, and how it was developed, to the previous one). Carefully remove the item and the film from the clamp stand. Observe the transmission hologram and describe it in detail in your report.

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.

Clean Up

Once you have made both holograms, remove every optical component (except the laser) from the track and place it neatly on the table.

When you receive your grade on this Lab Report you and your partner will also be able to keep the two holograms you made. The one keeping the first (transmission) hologram should also keep the LED light since it is needed to view it. The second hologram does not require additional light to view so the one who takes the reflection hologram does not get an LED.

References

1. G. Saxby, Practical Holography.

2. F. Jenkins and H. White, Fundamentals of Optics.

3. P. Harihayan, Optical Holography.

Appendix 1-Useful Tips for Making Holograms

Vibration is the enemy of holograms. So pick a solid surface on which to expose your hologram. This is a non-issue in the laboratory as the apparatus is set up firmly on a counter. No talking during hologram exposure; do not touch the surface that the hologram is being exposed on and do not move around, tap your shoes or do anything that will create excess vibrations in the vicinity of the hologram.

If at first you don’t succeed, overexpose. The Litiholo 2.0 “Instant Hologram” film is very different than traditional film. If you are unable to get a hologram with a certain exposure time, try increasing the time. The film develops as it is being exposed, so additional laser exposure will only have a minor effect after the film has used up all its exposure capacity.