Optical isolator

An optical isolator is an optical device which light of a specific polarization direction to pass through in only one direction, in the opposite direction, however, light of arbitrary polarization to deflect and optionally absorbed. It thus acts as a " valve " or in the opposite direction as the " insulator " and is therefore sometimes referred to, in analogy to electrical diode also known as " optical diode ".

Optical isolators are often used in laser technology to avoid unwanted back reflections from an optical apparatus in the laser, the feedback by the laser power influence ( up to the demolition of the light emission) can damage or the laser. They are also used for the direction- dependent decoupling of optical amplifiers.

Optical isolators according to the Faraday effect, are made of an optical Faraday rotator between two polarization filters rotated by 45 ° and are called Faraday isolator.

Operation

Optical isolators use of the Faraday effect. An optically inactive material is by applying an external magnetic field, is optically active, and rotates the polarization of the light. Unlike other polarization rotating materials, in which the light is independent of the direction is always turned in the same sense of direction, the direction of rotation of the Faraday rotator depends on the angle α between the direction of propagation of the light and magnetic field. Light, the opposite to the magnetic field lines incident ( α = 180 ° ) is rotated exactly opposite to light in the direction of the magnetic field lines ( α = 0 °) incident.

This effect is exploited in the following manner: It creates a suitable transparent material, a magnetic field parallel to the optical axis of and selects the intensity of the magnetic field so that the polarization of light is rotated by exactly 45 °. If they are put in the both ends of the magnetic material of polarization filters to each other are rotated by 45 °, then the light, which comes from one direction, rotated by 45 ° so that it can be ( the analyzer ) to pass unimpeded to the rear polarizer. The light coming from the opposite direction, but is rotated so that it now is perpendicular to the front polarizing filter. It is thus not allowed to pass through but reflects (in the case of polarization prism cubes ) to the side.

Since the Faraday rotation of the light strongly depends on the wavelength, optical isolators work perfectly only at a specific wavelength; at all other wavelengths, light is transmitted in the opposite direction and are filtered out some of the light in the forward direction by the analyzer.

Execution

Important characteristics of the optical medium in Faraday isolators are a high Verdet constant, so strong rotation of the polarization plane of the small light absorption, and low at high laser powers optical non-linearity and a high damage threshold.

In the area of visible light and near infrared terbium - doped glass or terbium gallium garnet is ( TGG ) used at wavelengths greater than 1100 nm and yttrium iron garnet ( YIG). The necessary magnetic fields can be generated with strong permanent magnets, such as neodymium-iron- boron magnets; characterized the length of the optical medium can be kept relatively short (one to a few centimeters ).

Manufacturers usually offer provided with the permanent magnet Faraday rotators and the required polarizers to separate.

Related applications and components

Among other purposes can modulation principle, the magnetic field generated with a current-carrying coil, however, the modulation speed is low, and the heating is disadvantageous.

For fast modulation of laser beams or for Q-switching, therefore, acousto-optic modulators, Kerr cells or particular Pockels cells are used. The latter is based on the rotation of the polarization direction of light.

Faraday rotators and isolators are also available for microwaves, they work with magnetized ferrite components in hollow or waveguides.

• Optical component