Optics 101

Sagnac Interferometer - Definition and Applications

Interferometry is the study of interaction between electromagnetic waves. It is widely used in numerous fields such as fiber optics, astronomy, quantum mechanics, and particle physics. There are two groups of interferometers - common path interferometers and double path interferometers.

In the common path interferometer, two laser beams travel the same path before combining. The sagnac interferometer is a type of common path interferometer. It uses the concept of Sagnac effect to measure rotation using optical interferometry.

French physicist Georges Sagnac discovered the concept of the Sagnac effect in 1913. He provided the first optical experiment that helped understand the state of rotation of a frame of reference, by making measurements within that frame.

The Sagnac interferometer uses counter-propagating beams in a ring path. This is realized by either using multiple mirrors or with an optical fiber. Generally, three or more mirrors are used, so that the counter-propagating light beams follow a closed path such as a triangle or square.

When the interferometer is rotated around an axis that is perpendicular to the drawing plane, this causes a relative phase shift of the counter-propagating beams, which is termed as the Sagnac effect or the Sagnac interference. The shift of the interference fringes is proportional to the platform's angular velocity. The sensitivity for rotations is based on the area covered by the ring, multiplied by the number of round trips.

Sagnac interferometers are used in inertial guidance systems, ring laser or ring laser gyroscope, and other optical systems.



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