The verdet constant for most materials is extremely small and is wavelength dependent. The effect is at its strongest in those substances containing paramagnetic ions such as terbium. The highest Verdet constants are in fact found in terbium doped dense flint glasses and better still in crystals of terbium gallium garnet (TGG). Although expensive, this material has significant benefits over glasses and other substrates, notably excellent transparency, high optical quality and very high resistance to laser damage. All of Leysop's Faraday rotators are manufactured from the highest quality crystals of TGG because of this.
Although the Faraday effect is not itself chromatic, the verdet constant itself is quite strongly a function of wavelength. At 632.8nm, the verdet constant for TGG is reported to be -134 radT-1m-1 whereas at 1064nm, it has fallen to -40radT-1m-1. This behaviour means that the devices manufactured with a certain degree of rotation at one wavelength, will produce much less rotation at longer wavelengths. Our rotators and isolators are all user adjustable by varying the degree to which the active TGG rod is inserted into the magnetic field produced by an extremely strong permanent magnet. In this way, the device can be tuned for use with a range of lasers within the design range of the device. Truly broadband sources (such as ultra-short pulse lasers and the tunable vibronic lasers) will not see the same rotation across the whole wavelength band as a result. We do however intend in the near future to launch a range of devices which offer a well balanced performance simultaneously over the wavelength range of operation of the important Ti:sapphire laser. This is achieved by the use of a Faraday rotator with a carefully selected and matched passive optical rotator plate.