Optical EngineeringN-bit negative-to-positive programmable photonic differential-group-delay architectures
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Some N-bit programmable photonic time delay structures are described for use in the compensation and the emulation of polarization-mode dispersion. Our key idea is to have one switchable differential group delay (DGD) module, which specifically contains the zero DGD and the minimum DGD resolution step of τ, connected to the rest of the variable photonic DGD bit modules that offer DGD values from +2τ to +2N–2τ. Our proposed variable photonic DGD architectures use 2×2 polarization-maintaining optical switches or 90-deg polarization rotators to choose the desired DGDs. When a set of all variable photonic DGD bit modules is attached to the polarization rotator, 2N DGD values ranging from –(2N–1–1)τ to (2N–1)τ are obtained. Our simulation study points out that compact programmable photonic DGD modules can be achieved by using materials with strong birefringence and our reflective configuration. In addition, materials with low wavelength sensitivity and weak birefringence are preferred for use in broadband wavelength and low-polarization-dependence systems, respectively. Mirror-based programmable photonic DGD structures are also introduced.