Displacement Measurement

What is interferometry and what are the types of interferometer

What is interferometry

Interferometry is the measurement of displacement by using laser wavelength. It is the characteristic of non-contact measurement and represents the feedback system for high-precision motion control applications. Because of its accuracy and precise resolution makes it applicable in wafer steppers, flat panel inspection, and laser micro-matching. Interferometry is the process in which waves are superimposed in order to take information about the waves mostly the waves are electromagnetic waves.

What is the use of interferometry

Interferometers are used in industry for the measurement of small displacements. The need for interferometers arose because of the need for high precision calibration in semiconductor and automotive industry. Interferometry is an important investigative technique in the field of fiber optics, metrology, chemistry, quantum mechanics, nuclear physics.

Schematics of interferometer

Light rays from the lens are collimated by lens 1 and split into two streams, one stream to mirror 1 which reflects a parallel beam of light rays to the test section and after that, it is projected on a screen. The second stream passes through the splitter1 projected on mirror 2 which reflects the light to splitter 2 and reunites with the first beam coming from mirror M1 and then moves to the screen. In the absence of flow both the beams give the same pattern of illumination on the screen, but when flow is on various shades of pattern are made up on the screen

Why do astronomers use interferometry

Astronomers use interferometers to combine signals from telescope so they work in the same way as larger and much more powerful instruments that can penetrate deeper into space some of these interferometer works with light waves others uses radio waves.

Types of interferometers

What is Laser interferometer

It is an optoelectronic instrument which measures distance in terms of the wavelength of the light by examining the phase relation between a reference beam and a laser beam reflected from a target object. The laser produces collimated light rays of single frequency present with phase coherence, a laser beam with an optical arrangement produces the reference beam one part of the reference beam is transmitted to the target and another part is sent to the interferometer. The phase difference between the reference beam from the source and the reflected beam from the target is equal to the extra length traversed by the beam and the digitized information from the difference between the two signals provides the distance information. Laser interferometer consists of a stabilized light source an interferometer optic which is a combination of the polarizing beam splitter and a retroreflector, a moving retroreflector or environmental sensor and finally the display or detection electronics.

What are the uses of a laser interferometer

It is used for precision motion measurement and for checking the linearity of precision machine tool slides and perpendicularity of machine tool structures mainly during the installation of machine tools. Laser interferometers are used in many automotive industries, it serves as a tracing reference for standard calibration for various kinds of length measuring equipment like step gauge, scale, angle and coordinates measuring instruments.

What are the types of a laser interferometer

There are two types of laser interferometer they are homodyne and heterodyne a homodyne interferometer uses a single frequency laser source, whereas a heterodyne interferometer uses a laser source with two close frequencies.

Homodyne interferometer

Homodyne interferometer is commonly used as a position sensor in FT spectrometers, the moving mirror’s relative displacement can be determined by counting the sinusoidal pulses. The speed information can be obtained by measuring its frequency, in order to determine the direction of travel additional optics is needed.

Heterodyne interferometer

Heterodyne interferometer measures mirror displacement by measuring the phase change due to the Doppler effect, it is similar to the way a police radar measures the speed of a car in order to do this the radiation temporal frequency needs to be made much smaller and this is done by using a source which contains two frequency components located close to each other, creating a beat phenomenon whose amplitude variation can be made much slower than the radiation frequency itself.

Newton interferometer

The original and simplest interferometer is newton interferometer it is commonly known as the “test plate method” of testing optical surfaces, where the test and reference surface are in contact. The fringes are formed by the narrow air gap between the test and reference surface, test plates have been used by opticians for at least a century and are widely used today. One of the main disadvantages of newton interferometer is the possibility of scratching surfaces when they are in contact. Test plates can only be used to check the quality of individual surfaces. It is not possible to check the total system quality of a telescope with a newton interferometer.

Tymann green interferometer

Tymann green is one of the most modern interferometers, monochromatic light is collimated by a lens and is split into two by a beam splitter to form a reference and test beams. The beams reflect off the master and test flats, to be recombined by the beam splitter the fringes are viewed at the focus of the imaging lens. Tymann green interferometer is capable of producing fringes without the need of having a contact at the reference and test surfaces. Non-contact testing without laser light is possible because of the optical path length or distance the light travel, the test and reference arms of the interferometer are made identical so the test and the reference surfaces are effectively in contact and it will allow forming fringes. This balancing of light path permitted the tymann-green interferometer to test the complete telescope.

What are the applications of an interferometer

Interferometers are widely used in all kind of scientific and engineering applications, for making accurate measurements. By scanning interferometers over objects, we can make very precise maps of the surfaces. Since the wavelength of visible lights are in the hundreds of nanometers so interferometers can be used to measure lengths which are smaller than a human hair.


post-graduate in Electronics & communication.

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