In the optical communication system, light rays emitted from a light source are reflected by the end faces of various optical systems incorporated into the communication system and the reflected light rays are returned towards the light source. The optical isolator is an irreversible optical device that permits the passage of light in one direction but blocks the passage thereof in the opposite direction. An optical isolator permits the transmission of light in one direction while blocking the reverse transmission of that light. An optical isolator is especially useful when used in conjunction with a laser in that it prevents optical feedback into the laser. Optical isolators provide electrical isolation between different portions of an electronic circuit. The electrical isolation provides high voltage protection for the electronic circuit and also reduces the level of noise in the electronic circuit. The optical isolator works by rotating the plane of polarization of polarized light as the polarized light passes through the medium known as an optical element. In most circumstances, an optical isolator will incorporate a structure referred to as a Faraday rotator to manage and control the behavior of light in a desired fashion. A Faraday rotator's operation and use are based upon the Faraday effect that refers to the rotation of the plane of polarization of light propagating through a medium in the presence of an externally applied magnetic field. By providing the ability to transfer light in essentially one direction only, the optical isolator may shield other optical components whose performance suffers from light traveling in a reverse direction. Devices such as optical amplifiers, lasers, and other components suffer performance degradation from light entering in a reverse direction. The optical isolator is used as such an optical device. The optical isolator used in the wavelength division-multiplex transmission system is required to have high light-extinction properties such that a plurality of transmitted light rays which fall within different wavelength ranges and are reflected and returned back towards a light source can effectively be cut off.

The general structure of an optical isolator consists of a Faraday rotator, an input polarizer and an output polarizer. Around the Faraday rotator, a cylindrical permanent magnet such as a rare-earth magnet is disposed and serves as a field-application magnet which generates a magnetic field for magnetizing the magnetic garnet thick film of the Faraday rotator in one direction. An isolator casing made of stainless steel surrounds the permanent magnet. A Faraday rotator comprises a material, typically a crystalline material, which is capable of rotating the plane of polarization of light passing through it in response to the application of an external magnetic field. The direction of rotation of the plane of polarization is dependent on the direction of passage of the light through the Faraday rotator, relative to the direction of the applied external magnetic field. The rotation of the plane of polarization provided by the Faraday rotator in one direction allows light to pass through both polarizers, whereas in the opposite direction the plane of polarization is rotated so that the passage of the light through the isolator is blocked by the polarizer. A polarizer is a device for producing light beam polarized in a specific direction and is used in optical communication, optical sensors, optical interferometers, etc. The input polarizer is typically aligned to a linear polarization angle of input light. The output polarizer is aligned to a non-parallel polarization angle so as to transmit this polarization state at the angle of 45 degree and pass light. The polarizer must be interposed between the input or output side and the Faraday rotator and the relative angles must be determined precisely. Optical isolators are used integrally with a semiconductor laser, it is small in size and can be mounted directly to the surface of the semiconductor laser.