Form-birefringence in waveguide devices

摘要

Integrated optic devices for polarization control (e.g., splitters, combiners, rotators) are required in applications such as polarization diversity receivers for coherent communications, integrated optical control of phased array antenna, and fiber optic sensors. The main requirements for device performance are high throughput (～90%) and low crosstalk (～1%). Desirable features include small size (～5 mm), passive operation, and implementation with a fabrication technology that allows integration with other waveguide elements. One can, of course, fabricate such devices in naturally birefringent waveguide materials to obtain the desired polarization sensitivity. A number of uniaxial and biaxial crystals are used in optics, but only LiNbO_3 is currently used in integrated optical devices, although some work has been done to make thin films of SbSI. Alternatively, for isotropic materials with a low refractive index, such as glass, one can make use of the birefringence induced by the differing boundary conditions of the waveguide structure. (This is sometimes referred to as modal birefringence.) However, for isotropic materials with a high refractive index, such as GaAs, the modal birefringence alone yields only small differences in refractive indices for opposite polarization states. Therefore, long device structures are required and/or fabrication tolerances become prohibitively small.