EXPERIMENT 9. Holography. Laser Safety. Introduction NEVER LOOK DIRECTLY INTO THE UNEXPANDED LASER BEAM OR ITS REFLECTION FROM A MIRROR SURFACE

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1 EXPERIMENT 9 Holography Laser Safety NEVER LOOK DIRECTLY INTO THE UNEXPANDED LASER BEAM OR ITS REFLECTION FROM A MIRROR SURFACE The Helium-Neon lasers which are used with the Optel Modern Optics Educational system are of low power rating (typically 0.5mW) but the narrow beam of light is still of very high intensity. If a laser beam enters the eye, the eye lens focusses the radiation onto the retina and in so doing increases the power or energy density by a factor of It is well to bear in mind that a 1mW laser is the maximum power which will ensure a class 2 classification - as defined by the British Standards Institute - that is the blink response of the eye provides an adequate safeguard against direct viewing. When a laser beam is made to diverge by a high power lens, the intensity is reduced to a safe level for direct viewing of interference fringes, holograms, etc. Introduction Holography is the method of recording an image of an object by using the total information content of light scattered from the object. The aspect of total information gives rise to the word describing the process, derived from the Greek words holos and grapho, meaning whole and to write. Since light may be described as a wave motion, there are several basic parameters which are used in its description. These are, (a) wavelength, (b) amplitude, (c) phase, (d) polarisation and (e) velocity. For the purposes of this discussion we need only concern ourselves with (b) and (c) since these are the parameters which are recorded in the holographic process. In the ordinary photographic process of recording an image, only the intensity distribution of the light waves is recorded on the photographic emulsion. In holography the phase information is preserved by the interference between the light waves coming from the object being viewed and another set of waves referred to later as the reference light beam. 9-1

2 Creation of a Hologram Since no two separate light sources maintain coherence for periods long enough to record an interference pattern, it is necessary to devise methods of providing coherent sources. One method of achieving this is to use a pair of mirrors and a single light source, S, as shown in Fig The two apparent sources A and B are the images of S as formed by the plane mirrors M 1 and M 2 and, provided that S is capable of emitting wave trains which do not change their regularity too rapidly, (i.e. S is a coherent source) an interference pattern will be formed in the region where the reflected waves overlap. If the position or orientation of M 1 or M 2 is changed this will change the position of A or B respectively and another interference pattern will result. An interesting development of this phenomenon is when M 2, say, is replaced by two or more small mirrors, each having its own position and orientation. Now several interference patterns exist together, being formed by interaction of the light waves from each of the small mirrors and those from M 1 (and also between each of the small mirrors among themselves). As can be imagined, a very complicated interference, or standing wave pattern is now created in the region occupied by the photographic plate CD in Fig If the collection of small mirrors is now replaced by a single body of definite shape such as, for instance, a cube as shown in Fig. 9.2 then every part of the cube facing towards CD will act as a tiny reflector of light waves and consequently each will set up interference patterns as described above. The contrast, spacing and alignment of any particular set of fringes due to a small area on the cube, which we can now call the object, will be governed by the brightness, position and orientation of the small area. It should be remembered that each small area of the object causes an interference pattern all over the photographic plate in Fig. 9.2 and so each part of the photographic plate records an interference pattern from each small area of the object. If the photographic plate is exposed for a suitable period long enough to record the interference patterns then, when developed, the plate is known as a hologram. It is customary to call the light from the cube the object beam and the light from point A the reference beam. Reconstruction of a Hologram Diffraction occurs with all wave-like phenomena. For example, sound waves can still be heard, although a large object such as a wall or a house may lie between the observer and the source of the sound. The amount of deflection of a wave depends on the relative dimensions of the wavelength and any obstacles or gaps involved, long wavelengths are clearly affected by large apertures whereas short wavelengths require very small apertures to show measurable effects. This is the case when we consider light waves whose wavelengths lie between 0.4 and 0.7 microns, (1 micron = 10 6 metre). It is found that the spacing of the interference fringe patterns in a hologram are of the correct size to give diffraction effects. As described in the previous section, each small element of area of an object causes its own interference pattern and in the reconstruction process the light beam is diffracted in various directions by the separate patterns. The resulting wave structure is identical to that which was created by the object itself in the formation process, hence, if one looks at a hologram illuminated by the reference wave only (now called the reconstructing wave) in the direction of the object which is now covered or removed, one sees a reconstruction of the original scene in complete 3-dimensional detail. The in- depth effect can be experienced by moving the head from side to side or up and down whereupon parallax effects show up. The 9-2

3 C M2 D S B M1 A Figure 9.1: Creation of interference patterns from a single source. C D S M1 A Figure 9.2: Formation of a hologram. 9-3

4 boundary of the hologram acts to limit the amount of parallax and may be compared to a window through which the object is seen. An understanding of parallax can be obtained by looking at any two objects which are in line with the observer s eye. As the observer moves his head from side to side or up and down, then if the objects are at differing distances they will cease to be in line, however if they are at the same position in space then they will remain in line. The existence of parallax then indicates whether or not a scene has three dimensions. The fact that each part of a hologram records contributions from each part of the object means that the complete scene is reconstructed by each region of the hologram. If a hologram is partially covered or broken up we simply have the effect of reducing the size of the window through which the object is viewed. It should be remembered however that this property of redundancy of information is confined to holograms of the diffuse and three-dimensional types. Diffuse holography is described further on. Properties of Laser Light In the previous section the requirement for a regular wave train was demonstrated and although the principles and the practicability of holography were demonstrated by Gabor using a high pressure mercury lamp as light source, the technique only achieved importance with the development of the laser. The reason for this is that the light waves given out by a laser have the properties of being ; (a) highly monochromatic, (b) highly coherent, (c) highly directional and (d) highly intense. The above properties are provided by the laser by virtue of the physical process which underlies its action. Types of Holograms In the course of the development of holography several types of hologram have emerged as a result of different aplications and requirements. The basic method remains, however, generally unchanged, namely, the superposition of a light wave scattered by an object and another reference wave. The interaction of these two waves is recorded at some location by a suitable detector which is commonly a photographic plate. Hologram Demonstration Before proceeding with the experiments we suggest you view the two holograms included in the M.O.E.S. These have been constructed to demonstrate the true 3-D nature of the image produced by holography. The optimum conditions for viewing the holograms are as shown in Fig To view the virtual images hold the hologram upright with the hologram label facing you. Place it in the expanded beam about 50 cm from the lens, with the holder vertical and at an angle of about 60 o to the beam. Now look perpendicularly through (not at) the hologram. The image should now be visible; if it is not or it it is faint, make slight alterations to the orientations of the hologram and your viewpoint until it becomes visible. The blue slide-holder contains a hologram of the holography classic scene - chessmen. The grey slide-holder contains a hologram of a circuit-board in front of which is mounted a magnifying 9-4

5 Mirror LASER L1 0.5 m 60 Figure 9.3: Configuration for viewing demonstration holograms. lens. By moving your head from side to side and up and down different views can be seen of each hologram. The circuit-board plus lens gives a very striking effect as the viewpoint is altered. Experimental Procedure Production of a Hologram of a Diffusely Illuminated Transparency Lay out the components on the table as in Fig It is recommended that the procedure given below is followed. 1. Place laser and plate-holder in approximately the positions shown. 2. Place the white card in the plateholder. 3. Place the beam-splitter in approximately the position shown and adjust its orientation until one of the reflected beams hits the centre of the intended hologram area, as viewed on the card. (Note : The stronger transmitted beam at the beam- splitter should always be the object beam. This gives the correct beam intensity ratio at the hologram). 4. Place mirror in position and adjust its orientation until the beam reflected from the mirror overlaps the beam from the beam-splitter at the desired point on the card. 5. Place L 1 in position and adjust its orientation and position so that the expanded beam is centred on and fills the hologram area. 9-5

6 6. Place L 2 in position and centre as above. 7. Place object slide in the expanded beam from L 2 such that the diffuse area is completely filled with light. 8. Check that no components have been moved. 9. Remove the card from the plate holder. 10. Place a piece of black card directly in front of the laser so that it shuts off the laser beam. STEPS 11 TO 14 WHICH FOLLOW MUST BE DONE IN TOTAL DARKNESS 11. Remove the photographic plate from the box and place it firmly but gently in the plate holder. (Do not forget to replace the lid on the box if there are other plates in it!) 12. WAIT for at least 1 minute. 13. Standing clear of the table gently lift the black card clear off the table - DO NOT ALLOW THE BEAM TO PASS AS YET - just hold the card clear of the table but blocking the beam for seconds. Now lift the card allowing the beam to pass for 1-2 seconds. (This exposure time will vary with the power of the laser and the geometry of the set up ; the time given is for a 1mW laser. An exposure of 4-6 seconds is recommended for a 0.5mW laser.) 14. Remove the plate from the holder and place it in a light- tight box until developed. 15. Develop the plate - See DEVELOPMENT on page Reconstruction of the Hologram To view the reconstruction replace the beam-splitter in Fig. 9.4 by a mirror as shown in Fig This enables all the available laser light to be utilised in producing the reconstructed image. Replace the developed hologram in the plate holder. Check that the beam being expanded by L 1 is hitting the hologram - if not align the beam in a similar manner to that outlined in notes 3-6. Now cover the object with a piece of black card as shown in Fig. 9.5 and look through the hologram in the direction of the object - a bright reconstruction of the object is visible. If nothing is seen or if the object appears very faint try slight adjustments of the plate-holder to alter the angle at which the beam hits the hologram. Production of a Hologram of a 3-D Scene Three Dimensional Holography The most publicised type of hologram is undoubtedly the three- dimensional type wherein an observer can view different aspects of three-dimensional objects by looking in different directions through a hologram of the object. The two previous experiments made holograms of a diffusely illuminated transparency and although they used all the concepts of holography the threedimensional effect was missing simply because the object in itself was not three-dimensional. 9-6

7 Beam Splitter Mirror LASER L1 L2 Object Plateholder Figure 9.4: Component lay-out for production of a hologram of a diffusely illuminated transparency. Mirror LASER L1 Object Card Plateholder Figure 9.5: Component lay-out for viewing the virtual image of the hologram constructed of a diffusely illuminated transparency. 9-7

8 Object Laser Beam Reference Mirror Holographic Film Figure 9.6: Three dimensional hologram - single beam method. Hence the next experiment is to form a hologram of a solid three-dimensional object or objects. There are many different geometrical lay-outs for forming 3-D holograms but the simplest is that shown schematically in Fig In this set up the object beam scattered from the object and the object and the reference beam reflected from the mirror are brought together at the plate holder to form the hologram. A more practical arrangement is that shown in Fig. 9.7 and is the one chosen for the next experiment. This set up should give a good reconstruction because it has fairly equal paths for both reference and object beams. It also allows the object to be well illuminated. The components required to produce a 3-D hologram and their relative positions on the table are shown in Fig Until you gain more experience you should set up as similar a layout as to Fig. 9.7 as possible. It is recommended that the procedure outlined below is followed. 1. Place the laser and plate-holder approximately in the positions shown. 2. Place the beam-splitter (B.S.) and mirror M 1 approximately in their positions and adjust B.S. until one of the beams hits L 1 ; then adjust L 1 until the beam hits the hologram area. 3. Position and orient L 1 such that the expanded beam fills the hologram area. 4. Position the object or objects to be holographed in approximately the position shown so that they do not obstruct the beam from L 1 - it is recommended that these objects are of a light colour. 5. Orient M 1 such that the reflected beam hits the centre of the object, or collections of objects to be holographed. 6. Position and orient L 2 such that the object is well illuminated - do not waste light by having the beam larger than the object or scene to be holographed. Proceed to develop the hologram, following the instructions of pg

9 Mirror M1 L2 Object Beam Splitter L1 Laser Plate Holder Figure 9.7: Component layout for production of a hologram of a 3-D scene. Reconstruction of the hologram To view the reconstruction replace the beam-splitter in Fig. 9.7 by a mirror. Now remove the object or objects and look through the hologram in the direction of the object. A bright reconstruction should be visible. If not, rotate the hologram slightly, as before, to increase the brightness of the image. Practical Notes General Points What we do need to produce a hologram? We need an object to be holographed (this is not strictly true as holograms have been produced from computer generated interference patterns of objects that have never existed. However for our purposes we require an object). We require a source of coherent waves. We require a recording medium capable of recording the interference pattern generated by two sets of coherent waves. We also require a means of keeping the interference pattern stationary for the period of the recording. In our case the source of coherent waves will be a laser and the recording medium a high resolution photographic plate or film. High resolution materials are required because the fringe spacing in the interference pattern is of the order of 1 to 10µ, typically. Our means of keeping the interference pattern stationary is to have all the components mounted as stably as possible, by having them all magnetically connected to a rigid vibration isolated table. 9-9

10 Vibration Isolation Although the platform has been carefully designed for vibration isolation and has successfully created holograms close to workshop machinery in operation, for best chances of success, the system should be located as remotely as possible from sources of vibration and shock. It is particularly important to avoid vibration just before and during an exposure. Draughts and movement should also be avoided during exposure. Thermal Conditions A uniform temperature should be maintained in the region of the holographic system. After prolonged handling of any component a few minutes should be allowed to elapse before making an exposure. The movement of your body close to the platform during an exposure should also be avoided - try to stand as still as possible - particularly if the paths of the two beams after leaving the beam splitter are quite long before they recombine at the plate. Beam Ratio Measurement Ideally for holography the ratio of the reference beam to the object beam should be about 1 : 1. However varying regions of object brightness in the case, for instance, of a three-dimensional object may affect the exposure of the photographic plate so that a non-linear region at either the top or bottom end of the Amplitude Transmission versus Exposure curve is involved. This produces losses in the contrast of the fringes in the hologram which in turn degrades the brightness of the reconstruction. Many workers prefer a reference to object beam ratio of about 5 : 1 to 10 : 1 for best holograms, however beam ratios down to 1 : 1 have given good reconstruction for objects which exhibit a uniform brightness across their surface. Development It is recommended that the following development procedure is strictly adhered to if consistently good holograms are to be produced. Temperatures in particular should be kept as close as possible to those recommended. Development must be carried out in the photographic dark-room. 1. Wash plate in water at 25 0 C for 5 minutes. 2. Develop for 5 minutes at 20 0 C. 3. Wash in running water for 5 minutes. 4. Fix for 4 minutes in at 20 0 C. 5. Wash for 5 minutes in running water. 6. Stand the plate on edge on a piece of absorbent paper until dry. If the above development procedure is adhered to and attention is paid to all the previous notes, particularly regarding vibration, then there should be no difficulty in producing good holograms. 9-10

11 References 1. University Physics, Young & Freedman WWW : Quantum holography - Medical applications of holography