Compositional Design of Faraday Rotator Materials
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Magnet
0o Polarizer
45'
The principal element is a 450 Faraday rotator. Conventionally this must be encased in a
Polarizer
cylindrical magnet to keep it magnetized in the single domain configuration parallel to the propagation direction of the light that is necessary for proper device operation. This Faraday rotator is sandwiched between two polarizers oriented at 450 to one another. For
this polarization dependent design, only light
Magnet Forward Propagation Polarized or Unpolarized
0o
.45°
Backward Propagation Does Not Propagate
polarized at 00 will be passed by the incoming polarizer-this condition is generally satisfied by the semiconductor lasers used in telecommunications. The effectiveness of this
45'
device is only limited by the perfection of the
Polarized or
rotator, butisolation) it can polarizers and extinction the Faradayratios (back typically have
unpoaizd
of greater than 40 dB. A polarization independent device can be made by replacing the polarizers with rutile wedges that split the
Figure 1 Schematic diagram of sttandard magnetooptic isolator with bias mr lagnet. 225
Mat. Res. Soc. Symp. Proc. Vol. 574 01999 Materials Research Society
polarizations so that both polarizations are directed to the outgoing fiber in the forward direction, but are deflected out of the light path in the reverse direction. The bismuth-doped, rare-earth iron garnets (BI:RIG) have become the magnetooptic material of choice for the near infrared wavelengths of telecommunications interest, 1310 and 1550 nm. The liquid phase epitaxial (LPE) growth technology for these materials was initially developed for magnetic bubble memory applications [1] and they present many advantages. * LPE growth on available high quality rare earth gallium garnet substrates produces low defect single crystal films up to 600 gtm in thickness. Thick film growth with the film of comparable thickness to the substrate challenges conventional notions of epitaxy. * Iron garnets can be prepared with high transparency at the wavelengths of interest. * Bi:RIG has high specific Faraday rotations at these wavelengths and can be made into relatively short path length devices. * Bi:RIG films have a growth induced magnetic anisotropy parallel to the (011) growth direction and are therefore readily magnetized perpendicular to the major face as is required for device applications. The flexible chemistry of the garnets can accommodate half the elements in the periodic table. Therefore the optical, magnetic and magnetooptical properties of the crystal can be tailored to meet the needs of device design. However, there are tradeoffs among these properties when they are simultaneously optimized. This paper will discuss the design of these materials and how these tradeoffs may be resolved for improved device performance. OPTICAL ABSORPTION Obviously photonic devices are desired to be highly transparent to reduce the need for periodic amplification of the signal. Therefore low insertion losses (optical absorptions) are required in magnetooptic device
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