• Document: Optical Isolators 700 Series Faraday Rotator and Accessories POLYTEC GmbH
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Optical Isolators 700 Series Faraday Rotator and Accessories The unique feature of a Faraday rotator is its non-reciprocity, that is, the fact that the "handedness" of the polarization rotation it imparts on a transmitted beam is dependent on the beam's propagation direction. By far the most common application of Faraday rotators is their use in optical isolators, devices used to protect laser sources from harmful back reflections. POLYTEC GmbH Büro Berlin Schwarzschildstraße 1 D - 12489 Berlin GERMANY wl@polytec.de Tel: +49 (30) 63 92 51 40 Fax: +49 (30) 63 92 51 41 www.polytec.de FIGURE 1: A basic optical isolator Conoptics manufactures a family of modular Faraday rotators and interchangeable accessories which can be combined to perform various isolation functions. A basic optical isolator is shown in Figure 1. Light from a presumed well polarized laser source enters an aligned beamsplitting polarizer from the left. It then proceeds through a Faraday rotator which is tuned to 45° for the particular laser wavelength. Only one magnet is shown but, whether a single magnet or a three magnet array is used, its major field component is parallel to the optical propagation direction. Light returning from any downstream reflecting surface undergoes an additional 45° rotation in its backward pass through the glass. It is now polarized orthogonal to the forward beam and is rejected by the polarizer. Faraday Rotators The Faraday rotators included in the 700 Series consist of one or more high flux NIB permanent magnets and a terbium glass element. Tuning is accomplished by moving the glass between regions of high and low field strength. An important feature of the Conoptics design is that the glass does not rotate as it is moved thereby eliminating the possibility of transmitted beam nutation due to residual wedge. Polarization rotation is a function not only of the axial magnetic field strength, but also of the glass length, L, and its Verdet constant, V. More precisely, on axis where z is along the propagation direct. POLYTEC GmbH Büro Berlin Schwarzschildstraße 1 D - 12489 Berlin GERMANY wl@polytec.de Tel: +49 (30) 63 92 51 40 Fax: +49 (30) 63 92 51 41 www.polytec.de The value of V is strongly wavelength dependent and diminishes rapidly as wavelength increases. In order to avoid the need for extending the length of glass elements to achieve 45° rotation at long wavelengths, the local field strength must be increased by using combinations of magnets with opposing fields. The Model 713, 714, and 715, while outwardly quite large, have only a fraction of their lengths filled with glass. The magnet array design is intended to increase axial field strength but also affects the field radial uniformity. Only relatively small volumes along the axis of the magnet array offer sufficiently low gradients. Furthermore, tuning the rotator (changing Zo in equation 1) by definition means moving the glass into a region of lower integrated axial field strength and higher gradients. It is generally recommended, therefore, that for best isolation ratio, a model be chosen in which the intended operating wavelength is near the upper end of the tuning range. A strong absorption peak at approximately 490nm is typical of terbium glass. At wavelengths larger that 500 nm absorption is low and the models listed are capable of operation with more than 6 watts average power. Operation at 488 nm is not recommended but a special unit operable at 458 nm is available. TABLE 1 lists the specifications of standard Conoptics Faraday rotators. The isolation ratio (extinction) is dependent on the type of polarizer(s) used, the operating wavelength in relation to the tuning range, and beam diameter. Polarizer’s Figure 1 shows that it is actually the polarizer that performs the beam separation function in an optical isolator. It is an obvious fact that, while the homogeneity of the Faraday rotator is of critical importance and generally limits performance in single stage devices, the extinction of the isolator can be no better than the extinction of the polarizer. Conoptics offers the Model 720 polarizer in two forms; one containing a low cost, low power polarizer, the other a high performance, high power Glan prism. Extinction as high as -40dB (10 -4) is possible using the 720 Glan with a small beam and a properly tuned Faraday rotator. Extinction with the low power type ranges between -20 and -30dB. Both types are fully rotatable. The polarization of light reflected back towards the laser source may not be the same as that in the forward direction. Any orthogonal polarization component accidentally produced will not be rejected by the polarizer-rotator combination shown in Figure 1. A second polarizer must be added to the assembly on the opposite end of the rotator and aligned at 45° to the original polarizer. This allows the forward beam to proceed with only the additional static loss of the second polari

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