An optical isolator is an optical device that transmits light in the forward direction, but absorbs it in the opposite direction. Optical isolators are used in optical communication lines to protect the resonators of laser transmitters from reflected signals, and also as an input element of optical amplifiers.
Content
Faraday Rotator
The main element of the optical isolator is a Faraday rotator. Faraday rotator is a plate of optically inactive crystal, placed in a constant magnetic field. With the passage of linearly polarized light through such a plate along the magnetic field lines, the Faraday effect is observed: the plane of polarization of the light is rotated by an angle proportional to the intensity of the magnetic field and the thickness of the plate. The direction of rotation depends on the direction of the magnetic field, but does not depend on the direction of light. The proportionality coefficient is the so-called Verde constant , depending on the wavelength and material properties; For example, various garnets , such as terbium — gallium garnets (Tb 3 Ga 5 O 12 ) and yttrium — iron garnets (Y 3 Fe 5 O 12 ) demonstrate high values of the constant Verde in the infrared range.
Principle of action
For an optical isolator, the thickness of the Faraday rotator plate and the magnitude of the magnetic field are chosen so that the rotator rotates the plane of polarization by 45 °. On both sides of the rotator is placed on one polarizer , the axes of which are rotated relative to each other also by 45 °. We take the axis of the first polarizer for 0 °, and the axis of the second (acting as an analyzer) for 45 °. In this case, a beam of light that passes in the forward direction through the first polarizer will be linearly polarized along the 0 ° axis. Then this beam will fall into the Faraday rotator, which rotates the plane of polarization by + 45 °, and the light polarized in this direction will freely pass through the second polarizer. If the light propagates in the opposite direction (from the second polarizer to the first), then after the second polarizer its polarization plane will be inclined by 45 °. Since the direction of rotation does not depend on the direction of light propagation, the Faraday rotator will rotate the plane of polarization by + 45 °. At the first polarizer, the light will fall polarized along the axis of 90 ° and will be absorbed.
See also
- Faraday effect
Literature
- Sklyarov O.K. Fiber-optic networks and communication systems. - SPb. : “Lan”, 2010. - P. 267. - ISBN 978-5-8114-1028-6 .