![]() For visible to YAG laser Isolators, Thorlabs' Faraday Rotator crystal of choice is TGG (terbium-gallium-garnet), which is unsurpassed in terms of optical quality, Verdet constant, and resistance to high laser power. patents, our isolators typically have higher transmission and isolation than other isolators, and are smaller than other units of equivalent aperture. When the light encounters the input birefringent beam displacer, it will be deflected away from the collimating lens and into the walls of the isolator housing, preventing the reverse mode from entering the input fiber. ![]() The faraday rotator is a non-reciprocal rotator, so it will cancel out the rotation introduced by the half wave plate for the reverse mode light. A Faraday rotator and a half-wave plate rotate the polarization of each branch before they encounter a second birefringent crystal aligned to recombine the two beams.īack-reflected light will encounter the second birefringent crystal and be split into two beams with their polarizations aligned with the forward mode light. In a polarization independent fiber isolator, the incoming light is split into two branches by a birefringent crystal (see Figure 3). Light is deflected away from the input path and stopped by the housing. Hence, the light will either be reflected or absorbed.įigure 3. This results in a net rotation of 90° with respect to the input polarizer, and thus, the POP is now perpendicular to the transmission axis of the input polarizer. It then passes through the Faraday rotator rod, and the POP is rotated another 45° in the positive direction. Light traveling backwards through the isolator will first enter the output polarizer, which polarizes the light at 45° with respect to the input polarizer. Therefore, the light leaves the isolator with a POP of 45°. Finally, the light exits through the output polarizer which has its axis at 45°. The Faraday rotator will rotate the plane of polarization (POP) by 45° in the positive direction. Laser light, either polarized or unpolarized, enters the input polarizer and becomes vertically polarized. In this example, we will assume that the input polarizer's axis is vertical (0° in Figure 2). Light propagating in the reverse direction is rejected by the input polarizer. This light's polarization is now perpendicular to the transmission axis of the input polarizer, and as a result, the energy is either reflected or absorbed depending on the type of polarizer.įigure 2. In the reverse direction, the Faraday rotator continues to rotate the light's polarization in the same direction that it did in the forward direction so that the polarization of the light is now rotated 90° with respect to the input signal. The output light is now rotated by 45° with respect to the input signal. The Faraday element rotates the input light's polarization by 45°, after which it exits through another linear polarizer. The input polarizer works as a filter to allow only linearly polarized light into the Faraday rotator. H: the magnetic field strength in Oersted.Īn optical isolator consists of an input polarizer, a Faraday rotator with magnet, and an output polarizer. L: the path length through the optical material in cm. V: the Verdet Constant, a property of the optical material, in minutes/Oersted-cm. Faraday Rotator's Effect on Linearly Polarized Light Faraday Rotation The key specifications of an optical isolator are the insertion loss, the return loss, and the reverse isolation of the device. An optical isolator is a nonreciprocal device that transmits an optical wave in one direction but blocks it in the reverse direction much as the function of a diode in an electric circuit. Optical isolators are needed to avoid such problems. Sometimes the problem is so severe that it renders the entire system useless. A feedback to a photodetector or other parts of an optical system also has many undesirable effects. It can change the laser frequency, increase the laser noise, create fluctuations in the laser intensity, lock the laser to a different mode of operation, or drive the laser into instability, even chaos. A feedback, even at an extremely low level, to a laser usually has a significant effect on the laser characteristics. In an optical system, reflections and backscattering of light often cause serious problems ranging from noise in the photodetectors to instabilities in the light sources. This is continuation from the previous tutorial - magneto-optic Kerr effect.
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