The magnetic field generated by a divergently and radially magnetized magnet ring is shown in Fig.2(b), while that generated by a convergently and radially magnetized magnet ring is shown in Fig.3(b). The former is magnetized in a divergent magnetic field, while the latter is in a convergent magnetic field. The radial magnetization of magnet rings is shown in Fig.2(a) and Fig.3(a). Radially magnetized magnet rings are introduced to generate magnetic field more efficiently. If only an axially magnetized magnet ring is used, the size will be too big for application. However, a TGG rotator needs far more strength of magnetic field than a RIG FR. Where Br is the residual magnetic field of the material, 2La is the length of the magnet ring, Ra1 and Ra2 are the inner and outer radii, respectively.įig.4 Magnetic field strength along the central axis (generated by different magnets) The RIG FR is usually placed at the central of the magnet ring, where the magnetic field is nearly uniform and saturated. (3) and is simulated as curve (a) in Fig.4. The magnetic field strength at the central axis is given by Eq. It generates a magnetic field as show in Fig.1(b). The magnet ring for a RIG FR is usually axially magnetized, as shown in Fig.1(a). The TGG rotator (including magnet rings and TGG crystal) need elaborate design to obtain the required strength of magnetic field with a minimal magnet. (2), a high strength magnetic field is required, which is to be generated by a big magnet. The Verdet constant of TGG crystal is V=0.002º/(G The thickness of 10~15mm excludes wedge-type structure. Thus we know, for high power applications, TGG is required instead of RIG. The assembly of the device is difficult and the two forward rays can’t be received by the output collimator simultaneously. If the TGG rotator is used in the wedge-type isolator, the lateral offset and walk-off of the rays are too much. Moreover, the thickness of TGG crystal (10~15mm) is much more than that of RIG FR. While for a TGG rotator, the magnet ring needs to be elaborately designed to provide a precise rotatory angle. A RIG FR just needs a small magnet ring providing magnetic field more than saturation (usually 200Oe). The thickness of a 45º FR is about 0.36mm (including the substrate thickness). The film thickness is doubled for a 90º rotator than a 45º rotator. The actual rotatory angle depends on the thickness of the RIG film epitaxially grown on a substrate. (2).įor the RIG FR in telecom applications, the rotatory angle is fixed under a saturated magnet field. Usually, the magnet field is not uniform, thus Eq. The rotatory angle by the TGG crystal depends on Verdet constant, crystal thickness and magnetic field strength, as Eq. Thus TGG (terbium gallium garnet) crystal is used for optical rotation as the substitution due to its endurance to high optical power. The RIG film will be damaged by high power laser. The FR for telecom applications is a thin film of Bi-substituted rare-earth iron garnet single crystals (RIG) grown by liquid phase epitaxy. The reason is that the FR for telecom applications can’t be used anymore. While for some applications under high optical power, BD-type in-line optical isolator is employed instead of wedge-type. For some applications under relatively low optical power, wedge-type in-line optical isolator is employed, just with more consideration on heat dissipation. Depending on the actual applications, different types of optical isolator are employed. Optical isolators are important devices to ensure the reliability of fiber lasers. The research and application of fiber lasers flourish in recent years.
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