Optical isolator
An optical isolator is an optical component that only allows light of a certain polarization direction to pass in one direction, but deflects light of any polarization in the opposite direction and, if necessary, absorbs it. It thus acts as a “valve” or, in the opposite direction, as an “isolator” and is therefore sometimes referred to as an “optical diode” in analogy to the electrical diode .
Optical isolators are often used in laser technology in order to avoid undesired back reflections from an optical apparatus into the laser , which can influence the laser power through feedback (up to the interruption of the light emission) or damage the laser. They are also used for the directional decoupling of optical amplifiers .
Optical isolators based on the Faraday effect consist of an optical Faraday rotator between two polarization filters rotated by 45 ° and are also called Faraday isolators .
functionality
Optical isolators use the Faraday effect . An optically inactive material becomes optically active when an external magnetic field is applied and rotates the polarization of the light. In contrast to other polarization-rotating materials, in which the light is always rotated in the same direction independent of direction, the direction of rotation of the Faraday rotator depends on the angle α between the direction of propagation of the light and that of the magnetic field. Light incident in the opposite direction to the magnetic field lines (α = 180 °) is rotated in exactly the opposite direction to light incident in the direction of the magnetic field lines (α = 0 °).
This effect is used in the following way: A magnetic field is applied to a suitable transparent material parallel to the optical axis and the strength of the magnetic field is selected so that the polarization of the light is rotated exactly by 45 °. If you place polarization filters at both ends of the magnetized material , which are rotated by 45 ° to each other, then the light coming from one direction is rotated by 45 ° so that it can pass the rear polarization filter (the analyzer) unhindered . However, light coming from the opposite direction is rotated so that it now strikes the front polarization filter perpendicularly. It is therefore not transmitted, but (in the case of polarization prism cubes) is reflected to the side.
Since the Faraday rotation of light is strongly dependent on the wavelength, optical isolators only work perfectly at a certain wavelength; at all other wavelengths, light is also transmitted in the opposite direction and part of the light in the forward direction is filtered out by the analyzer.
execution
Important properties of the optical medium in Faraday isolators are a high Verdet constant , i.e. strong rotation of the plane of polarization, low absorption of light, and low optical non-linearity and high damage threshold at high laser powers .
In the visible light and near infrared range, terbium- doped glass or terbium gallium garnet (TGG) is used, and yttrium iron garnet (YIG) for wavelengths above 1100 nm . The necessary magnetic fields are generated with strong permanent magnets , e.g. B. neodymium-iron-boron magnets produced; this allows the length of the optical medium to be kept relatively short (one to a few centimeters).
Manufacturers usually offer the Faraday rotators with permanent magnets and the required polarizers separately.
Related applications and components
For modulation purposes, among other things, the magnetic field can in principle also be generated with a current-carrying coil , but the modulation speeds are low and the heating is a disadvantage.
Acousto-optical modulators , Kerr cells or, in particular, Pockels cells are therefore used for rapid modulation of laser beams or for Q-switching . The latter are also based on the rotation of the polarization direction of light.
Insulators are also available for microwaves ; these work with magnetized ferrite components in hollow or waveguides .
Optical isolators are also of crucial importance in laser material processing. You will e.g. B. used in powerful diode lasers in the kilowatt range to protect the laser diode from the radiation reflected back on the surface of the metal to be processed.
literature
- Volkmar Brückner: Elements of optical networks: Basics and practice of optical data transmission, information and communication technology . 2nd Edition. Springer, 2011, ISBN 978-3-8348-1034-2 .
Individual evidence
- ↑ Christian Stegmann: Investigation of the influence of back-reflected light on the radiation properties of a diode laser . Cologne 2013 (thesis, RFH Cologne).