Polar Faraday Analyze Incident Rotator Reflected Backward Beam FIGURE 57.2 Schematic of an optical isolator. The polarization directions of forward and backward beams are shown below the schematic TABLE 57. 1 Characteristics of YIG and BIG Faraday Rotators Verdet constant(min/cm-Gauss) 1300mm 10.54-806 1550mm aturated magnetooptic rotation(degree/mm) 1300mm 20.6 1550mm 93.8 Thickness for 45-degree rotation (mm) 1300mm 1550nm <0.1 Typical reverse isolation(dB) 30-35 Required magnetic field(Gauss) Magnetically tunable a variable b Some big not tunable Source: D K. wilson, Optical isolators adapt to commu needs, Laser Focus World, p. 175, April 1991. @Penn Well Publishing Company With permission. The major characteristics of an optical isolator include isolation level, insertion loss, temperature dependence, nd size of the device. These characteristics are mainly determined by the material used in the rotator Rotating materials generally fall into three categories: the paramagnetic(such as terbium-doped borosilicate glass), the diamagnetic(such as zinc selenide), and the ferromagnetic(such as rare-earth garnets ). The first two kinds have small Verdet constants and mostly work in the visible or shorter optical wavelength range. Isolators for use with the InGaAsP semiconductor diode lasers(o=1100-1600 nm), which serve as the essential light source in optical communication, utilize the third kind, especially the yttrium-iron-garnet(YIG) crystal. A nev available ferromagnetic crystal, epitaxially grown bismuth-substituted yttrium-iron-garnet(BIG), has an order of-magnitude stronger Faraday rotation than pure YIG, and its magnetic saturation occurs at a smaller field [Matsuda et al, 1987]. The typical parameters with YIG and BIG are shown in Table 57. 1. As the major user For the purpose of integrating the optical isolator component into the same substrate with the semiconductor laser to facilitate monolithic fabrication, integrated waveguide optical isolators become one of the most exciting areas for research and development. In a waveguide isolator, the rotation of the polarization is accomplished a planar or channel waveguide. The waveguide is usually made of a magnetooptic thin film, such as YIG or BIG film, liquid phase epitaxially grown on a substrate, typically gadolinium-gallium-garnet( GGG)crystal Among the many approaches in achieving the polarization rotation, such as the 45-degree rotation type or the© 2000 by CRC Press LLC The major characteristics of an optical isolator include isolation level, insertion loss, temperature dependence, and size of the device. These characteristics are mainly determined by the material used in the rotator. Rotating materials generally fall into three categories: the paramagnetics (such as terbium-doped borosilicate glass), the diamagnetic (such as zinc selenide), and the ferromagnetic (such as rare-earth garnets). The first two kinds have small Verdet constants and mostly work in the visible or shorter optical wavelength range. Isolators for use with the InGaAsP semiconductor diode lasers (l0 = 1100–1600 nm), which serve as the essential light source in optical communication, utilize the third kind, especially the yttrium-iron-garnet (YIG) crystal. A newly available ferromagnetic crystal, epitaxially grown bismuth-substituted yttrium-iron-garnet (BIG), has an order￾of-magnitude stronger Faraday rotation than pure YIG, and its magnetic saturation occurs at a smaller field [Matsuda et al., 1987]. The typical parameters with YIG and BIG are shown in Table 57.1. As the major user of optical isolators, fiber optic communication systems require different input-output packaging for the isola￾tors. Table 57.2 lists the characteristics of the isolators according to specific applications [Wilson, 1991]. For the purpose of integrating the optical isolator component into the same substrate with the semiconductor laser to facilitate monolithic fabrication, integrated waveguide optical isolators become one of the most exciting areas for research and development. In a waveguide isolator, the rotation of the polarization is accomplished in a planar or channel waveguide. The waveguide is usually made of a magnetooptic thin film, such as YIG or BIG film, liquid phase epitaxially grown on a substrate, typically gadolinium-gallium-garnet (GGG) crystals. Among the many approaches in achieving the polarization rotation, such as the 45-degree rotation type or the FIGURE 57.2 Schematic of an optical isolator. The polarization directions of forward and backward beams are shown below the schematic. TABLE 57.1 Characteristics of YIG and BIG Faraday Rotators YIG BIG Verdet constant (min/cm-Gauss) 1300 nm 10.5a –806 1550 nm 9.2 –600 Saturated magnetooptic rotation (degree/mm) 1300 nm 20.6 –136.4 1550 nm 18.5 –93.8 Thickness for 45-degree rotation (mm) 1300 nm 2.14 0.33 1550 nm 2.43 0.48 Typical insertion loss (dB) >0.4 <0.1 Typical reverse isolation (dB) 30–35 40 Required magnetic field (Gauss) >1600 120 Magnetically tunable No Yesb a Variable. b Some BIG not tunable. Source: D.K.Wilson,“Optical isolators adapt to communication needs,” Laser Focus World, p. 175, April 1991. ©PennWell Publishing Company. With permission