Faraday Effect Devices, Optical Isolators
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enters the rod and, by virtue of the Faraday effect, the plane of polarization rotates as the light propagates. Assuming no deteriorating effects are present, the light emerges from the opposite end of the rod with its plane of polarization rotated by an amount
0 = VHL where 0 is the amount of rotation in minutes, V is the Verdet constant in minutes/Gausscentimeter, H is the magnetic field strength in Gauss, and L is the length of the rod in centimeters. It is important to emphasize the nonreciprocal nature of the Faraday effect. The direction of rotation is dependent only upon the direction of the magnetic field and the sign of the Verdet constant, not on the direction of light propagation. This is exactly opposite to the case of rotation in optically active materials such as crystalline quartz or sugar solutions in which the rotation depends upon the direction of light propagation. FARADAY ROTATING MATERIALS Rotating materials generally fall into three categories: the paramagnetics, the diamagnetics, and the ferromagnetics. Paramagnetics have a negative Verdet constant that varies inversely as the absolute temperature, and varies approximately as the inverse square of the wavelength. Diamagnetics have a positive Verdet constant that is essentially unaffected by temperature and, like paramagnetics, varies approximately as the inverse square of the wavelength. Ferromagnetics have a positive Verdet constant that is affected by temperature according to the specific material. As with paramagnetics and diamagnetics, it varies approximately as the inverse square of the wavelength. It is important to note that extreme deviations from these simple relationships are possible. THE PARAMAGNETICS Among the most commonly available paramagnetic materials is terbium-doped borosilicate glass. Although the base glass is diamagnetic, it is the paramagnetic ion of terbium that causes the Verdet constant to be much higher than in any of the high-index glasses (such as the heavy flints). However, because the Verdet constant of the paramagnetics is inversely proportional to the absolute temperature, relatively low rotations at room or elevated equipment temperatures can be a determining factor in the choice of this material. This, along with thermally induced strain birefringence caused by a high-power laser, can degrade polarization purity, thus reducing isolation. Additionally, the terbium absorption band at 470-490 nm renders this glass useless at the blue line of the argon-ion laser (488 rnm), though not at 500 nm and longer wavelengths. Another significant paramagnetic material is terbium-gallium-garnet (TGG). Its Verdet constant is 50% to 80% greater than the terbium glass described above. The very low absorption of this water-clear crystal makes it an excellent candidate for isolation of wavelengths in the visible and near-IR regions. TGG is one of the most commonly used materials in optical isolators. It is capable of withstanding very high laser power, and is used with high power YAG lasers. THE DIAMAGNETICS Many glasses fal
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