interferometer and interferometry

Interferometer

  • Interferometer is an optical device used for measuring flatness of a surface and also for determining the thickness of slip gauges.
  • It is an extension of the application of optical flat.
  • Interferometers are based upon the principle of “ interference “ and employ wavelength of light as their measuring units.
  • The interferometers make use of beam divider that splits an incoming ray into two parts.
  • These two parts of the ray travel along different paths until they are recombined, usually in the same beam divider.
  • An interferometer is an optical device which utilizes the effect of interference. Typically, it starts with some input beam, splits it into two separate beams with some kind of beam splitter (a partially transmissive mirror),
  • possibly exposes some of these beams to some external influences (e.g. some length changes or refractive index changes in a transparent medium), and recombines the beams on another beam splitter. The power or the spatial shape of the resulting beam can then be used for a measurement

Interferometry

History:

  • The use of interferometric techniques in metrology systems from a suggestion of the French physicist BABINET in 1929 that light waves can be used as a natural standard of length.

Introduction:

  • Interferometry is the technique of superimposing (interfering) two or more light waves, to detect differences between them.
  • Interferometry is applied in a wide variety of fields, including astronomy, fibre optics, optical metrology, oceanography, seismology, quantum mechanics and plasma physics.
  • Wave length of light as a standard of measurement; posses the advantage of being accurate and a very small unit of measure,
  • The wave nature of light is ever present but not apparent under ordinary conditions,
  • Only when the light waves interact with each other the wave effect is visible and thus made use for measurement purposes
  • This phenomenon of interference of light is called “interference”.
  • When the light is made to interfere it produce a pattern of dark bands, which correspond to very accurate scale of divisions.
  • The value of this scale is exactly one half of the wave length of the light used.
  • Because this length is so constant, it is used to be considered as international standard of length.
  • Use of interferometric technique enables the size of slip gauges and end bars to be determined directly in terms of the wave length of light sources.
  • Interferometric ally calibrated reference grade slip gauges forms the basis of controlling the size of subsidiary length standards, such as inspection and workshop grade slip gauges, setting masters etc., used in manufacturing industries

Light waves -shape



Light behaves as wave



Wave form of light




Light wave



  • The Light is a form of energy.
  • It is propagated by electromagnetic waves which may be represented by a sine wave as shown above

Some properties of light wave

  • High point of the wave is called crest
  • Low point is called trough.
  • The distance between two consecutive crests or two consecutive troughs is called wavelength λ.
  • It’s wave length has a precise value.
  • It has an accuracy of about one part in one hundred million.

Principle of interference

  • When two waves of the same wavelength meet at some point, a manual interference occurs .
  • The nature of interference will depends upon the phases of two waves at their meeting point.
  • Wave length may be in the order of 0.0005mm

Principle of interference- Two rays in phase



Principle of interference- Two rays out of phase


Situations of light intensity can be seen in this diagram



Method of producing interference pattern



Principle of interferometry



Types of Interferometers

  • Various types of interferometers used in metrology are as follows.
  • Michelson interferometer
  • N.P.L. Flatness Interferometer
  • Laser Interferometer etc.

Michelson Interferometer





N.P.L. Flatness interferometer



  • This Instrument is used to check the flatness of flat surfaces.




N.P.L Gauge Interferometer



Laser Interferometer



Fringe Patterns :



  • A,B- When gauge face is taper-this type of fringe pattern can be seen
  • C,D,E- When the gauge surface convex or concave this type of pattern cane be seen

Fringe pattern in concave and convex surfaces



Fringe pattern in curved ,concave and convex surfaces



Different Fringe patterns


  • a, b –Flat surfaces
  • c- Spherical and Concave
  • d- Curved more in one direction
  • e- Smooth cylindrical surface
  • f- ridge or valley in the middle

Principle of working of Interferometer

  • This instrument makes use of light wave interference
  • principles and provides a quantitative measurements
  • Interferometry works because two light waves with the same frequency that have the same phase will add to each other while two waves that have opposite phase will subtract.
  • Typically, in an interferometer, a wave is split into two (or more) coherent parts, which travel different paths, and the parts are then combined to create interference.
  • When the paths differ by an even number of half-wavelengths, the superposed waves are in phase and interfere constructively, increasing the amplitude of the output wave.
  • When they differ by an odd number of half-wavelengths, the combined waves are 180° out of phase and interfere destructively, decreasing the amplitude of the output.
  • Thus anything that changes the phase of one of the beams by only 180°, shifts the interference from a maximum to a minimum.
  • This makes interferometers sensitive measuring instruments for anything that changes the phase of a wave, such as path length or refractive index.




working

  • Monochromatic light from a source is passes through a collimating lens so that the rays are parallel
  • The light rays fall at an angle of 45o on the partly silvered surface of the mirror A.
  • Few rays from B passes through the mirror A towards the eye., and few rays from the work piece and table surfaces are also reflected towards the eye.
  • If the rays from mirror B, the work surface and table surface are in equal in length, the light rays will reinforce each other.
  • As a result of this the work surface and table surface appears to be ordinarily illuminated,
  • If the rays reflected from work surface differs in length ( one half of wave length) from that of the light reflected from the mirror B., the work surface ( also the table surface ) appears to be dark.
  • This darkness is caused by the two reunited light waves canceling each other by light-wave interference.
  • By using interferometer a comparison is made between the height of the work piece and desired height above the table, within the range of a half wave length.

Advantage of interferometer:

  • Interferometer is faster and easier than that of optical flats, and is considered as most accurate measuring instrument,

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