INTERFEROMETRYBy

Mr.H.P.VaradeAsst.Professor,

Sanjivani CoE,Kopargaon,Dist-Ahmednagar

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INTERFEROMETRY

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Principle of Interference

If two rays of same wavelength meet at some point, mutual interference occurs & natural interference depends on Phase of two waves at their meeting point.

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•If two rays are in same phase, then resulting intensity will be the sum of two intensity.

•If two rays are out of phase, then resulting intensity will be the difference of two intensity.

•If two rays having same amplitude are out of phase, then resultant will be zero & result will be Dark spot.

•If two rays having same amplitude are in same phase, then resultant will be twice & result will be Bright spot.

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Types of Interferometers

1. Michelson Interferometer2. Fabry-Perot Interferometer3. Fringe counting Interferometer4. NPL Flatness Interferometer5. Pitter-NPL Interferometer6. Zeiss gauge block Interferometer7. Multiple beam Interferometer8. Laser Interferometer

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NPL Flatness Interferometer

The NPL flatness interferometer is used for checking flatness between gauge surfaces.

(a) Equal fringes on parallel

(b) Unequal fringes due to flatness error

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•The light from a mercury vapour lamp is condensed and passed through a green filter, resulting in a green monochromatic light source. •The light will now pass through a pinhole, giving an intense point source of monochromatic light. •Therefore, the collimating lens projects a parallel beam of light onto the face of the gauge to be tested via an optical flat. •This results in the formation of interference fringes. •The light beam, which carries an image of the fringes, is reflected back and directed by 90° using a glass plate reflector. •In Fig. (a), the fringes are parallel and equal in number on the two surfaces. Obviously, the two surfaces are parallel, which means that the gauge surface is perfectly flat. •On the other hand, in Fig. (b), the number of fringes is unequal and, since the base plate surface is ensured to be perfectly flat, the work piece surface has a flatness error.

Working

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Pitter–NPL Gauge Interferometer

•This interferometer is used for determining actual lengths of slip gauges.

•Illuminator provides a concentrated light source

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•Light from a monochromatic source (the preferred light source is a cadmium lamp) is condensed by a condensing lens and focused onto an illuminating aperture. •This provides a concentrated light source. Thus, a parallel beam of light falls on a constant deviation prism. •This prism splits the incident light into light rays of different wavelengths and hence different colors. The user can select a desired color by varying the angle of the reflecting faces of the prism relative to the plane of the base plate. •The prism turns the light by 90° and directs it onto the optical flat. •The optical flat can be positioned at a desired angle by means of a simple arrangement. The slip gauge that is to be checked is kept right below the optical flat on top of the highly flat surface of the base plate. •The lower portion of the optical flat is coated with a film of aluminum, which transmits and reflects equal proportions of the incident light.• The light is reflected from three surfaces, namely the surface of the optical flat, the upper surface of the slip gauge, and the surface of the base plate. •Light rays reflected from all the three surfaces pass through the optical system again; however, the axis is slightly deviated due to the inclination of the optical flat. This slightly shifted light is captured by another prism, so that the fringe pattern can be observed and recorded by the user.

Working:-

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Field of view of fringe pattern

It can be seen that the two sets of fringes are displaced by an amount a with respect to each other. The value of a varies depending on the color of the incident light. The displacement a is expressed as a fraction of the fringe spacing b, which is as follows: f = a/b

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Laser Interferometers

•Laser interferometers can be used for measurements of small diameters as well as large displacements. MQC : UNIT-2

•Laser light first falls on the semi-reflector P, is partially reflected by 90° and falls on the other reflector S. •A portion of light passes through P and strikes the corner cube. Light is turned through 180° by the corner cube and recombines at the semi-reflector S. •If the difference between these two paths of light (PQRS − PS) is an odd number of half wavelengths, then interference will occur at S and the diode output will be at a minimum. On the other hand, if the path difference is an even number of half wavelengths, then the photodiodes will register maximum output.

Each time, the moving slide is displaced by a quarter wavelength, the path difference (i.e., PQRS − PS) becomes half a wavelength and the output from the photodiode also changes from maximum to minimum or vice versa.

Working:-

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