Engineering Physics 3(2+1)

LESSON 27. Application of LASER

Types of masers

Atomic beam masers

• Ammonia maser

• Free electron maser

• Hydrogen maser

Gas masers

• Rubidium maser

Solid state masers

• Ruby maser

• Whispering-gallery modes iron-sapphire maser

Holography

• It is a technique which enables three-dimensional images to be made.

• It involves the use of a laser, interference, diffraction, light intensity of recording and suitable illumination of the recording.

• It consists of an apparently random structure of either varying intensity and density or profile.

Difference between Holography and photography

• It is recording of information regarding the light that came from the original scene as scattered in a range of directions rather than from only one direction, as in a photograph.

• A photograph can be recorded using normal light sources like sunlight or electric lighting whereas a laser is required to record a hologram.

• A lens is required in photography to record the image, whereas in holography, the light from the object is scattered directly onto the recording medium.

• A holographic recording requires a second light beam (the reference beam) to be directed onto the recording medium.

• A photograph can be viewed in a wide range of lighting conditions, whereas holograms can only be viewed with very specific forms of illumination.

• When a photograph is cut in half, each piece shows half of the scene. When a hologram is cut in half, the whole scene can still be seen in each piece.

• A photograph is a two-dimensional representation that can only reproduce a rudimentary three-dimensional effect, whereas the reproduced viewing range of a hologram adds many more depth perception cues that were present in the original scene.

• A photograph clearly maps out the light field of the original scene. The developed hologram's surface consists of a very fine, seemingly random pattern, which appears to bear no relationship to the scene it recorded.

Physics of Holography

• For a better understanding of the process of Holography, It is necessary to understand interference and diffraction.

• Interference occurs when one or more wavefronts are superimposed.

• Diffraction occurs whenever a wavefront encounters an object.

• The process of producing a holographic reconstruction is explained below purely in terms of interference and diffraction.

• It is somewhat simplified but is accurate enough to provide an understanding of how the holographic process works.

(i) Recording a hologram

• To make a hologram, the following are required:

• A suitable object or set of objects

• A suitable laser beam

• Part of the laser beam to be directed so that it illuminates the object- the object beam)

• Another part so that it illuminates the recording medium directly- the reference beam, enabling the reference beam and the light which is scattered from the object onto the recording medium to form an interference pattern

• A recording medium which converts this interference pattern into an optical element which modifies either the amplitude or the phase of an incident light beam according to the intensity of the interference pattern.

 (ii) Reconstructing and viewing the holographic image

• When the hologram plate is illuminated by a laser beam identical to the reference beam which was used to record the hologram, an exact reconstruction of the original object wavefront is obtained. Fig.18(1) and (2)

• An imaging system (an eye or a camera) located in the reconstructed beam 'sees' exactly the same scene as it would have done when viewing the original.

• When the lens is moved, the image changes in the same way as it would have done when the object was in place.

• If several objects were present when the hologram was recorded, the reconstructed objects move relative to one another, i.e. exhibit parallax, in the same way as the original objects would have done.

• A holographic image can also be obtained using a different laser beam configuration to the original recording object beam, but the reconstructed image will not match the original exactly.

Applications

• Art

• Data storage

• Dynamic holography

• Hobbyist use

• Holographic interferometry

• Interferometric microscopy

• Sensors or biosensors

• Security

• Other applications