The resulting device is generally known as a circulator, taking its name from the path of the optical signal which follows a closed loop or a circle. Isolators can be connected together to form multiport devices where, depending upon their isolation characteristics, an optical signal can leave the device at an end terminal or it can continue to flow towards the next connected isolator. Thus the reflected light, having horizontal SOP will be blocked. Polarizer 1 is designed to pass the light having vertical SOP only. Thus the original vertical SOP becomes horizontal SOP. The faraday rotor again provides orientation by 45°. Now the reflected light with 45°orientation gets passed through the polarizer 2. Thus the light gets propagated in the forward direction without any loss. The polarizer 2 is designed in such a way that it passes the light ray having SOP rotated by 45°. It rotates the polarization by 45° as shown in figure 4.20.
The light is then passed through Faraday rotor. This light allowed to pass through first polarized whose function is to pass the light having only vertical SOP. Consider that, incoming light has vertical polarization (vertical SOP) as shown in figure 4.20. Optical isolator works by rotating the plane of polarization of incoming light ray. It is also possible to develop optical waveguide isolators using either the TE or TM modes for the propagation of an optical signal. In addition, magneto-optic devices can be used to function as isolators. The wavelength blocking feature makes the optical isolator a very attractive device for use with optical amplifiers in order to protect them from backward reflections.
Ideally, an optical isolator should transmit all the signal power in the desired forward direction. An optical isolator is essentially a passive device which allows the flow of optical signal power (for a particular wavelength or a wavelength band) in only one direction preventing reflections in the backward direction.
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