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Design and analysis of a narrowband filter for optical platform

Design and analysis of a narrowband filter for optical platform,10.1109/ICASSP.2011.5946811,Yujia Wang,Andrew Grieco,Boris Slutsky,Bhaskar Rao,Yeshaia

Design and analysis of a narrowband filter for optical platform  
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This paper presents an approach to designing narrowband digital filters that are realizable using optical allpass building blocks. We describe a top-down design method by explicitly examining the derivation of an Infinite Impulse Response (IIR) architecture. Our result demonstrates a design that can achieve a 0.0025π passband edge while providing 60dB stopband attenuation. The design is aimed to reduce filter pole magnitudes, providing tolerance for waveguide losses and fabrication errors. The narrowband filter is based on the foundation of latticed allpass sections, which makes it naturally realizable using basic photonic components. Furthermore, analysis is performed on delay length variations that can result from the fabrication process. as a result of fabrication error and waveguide loss [5]. Our challenge is therefore to derive a filter design method that yields a narrow passband with high stopband attenuation, while maintaining small pole magnitudes. Traditional design approaches typically aim to minimize the approximation error or to achieve maximally flat passband/stopband. The consideration in filter designs for photonic platforms are drastically different in that the pole magnitudes are the critical constraints. We describe an approach to obtain the desired cutoff frequency and attenuation while lowering the pole magnitudes to be suitable for the optical platform. A number of papers have shown that an allpass structure can naturally be realized in a photonic scheme [4, 6]. To be consistent with the fabrication of the narrowband filter, our structure is based on allpass sections throughout the design process. We demonstrate the novelty in combining the design method for lowpass through allpass [7], and technique for linear phase allpass [8] to achieve narrowband filter suitable for photonic filtering. Most studies in digital filter design analyze the performance of the structure under error by examining finite word length effects on the weights [9]. Photonic filters need to be analyzed differently because the tap weights are not controlled by finite memory. Although waveguide losses and errors in fabrication will affect the coefficients, the effects are minimal so long as the pole magnitudes are within bound. Instead, phase sensitivity that results from delay length variations in the form of H(e −jδ z) must be carefully studied. In this paper, we consider the phase error for two different implementations: cascaded direct form and cascaded lattice structures.
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