Optical Time-Frequency Scaling for Signal Processing Applications

摘要

Optical time-frequency processing requires a combination of high-speed, quadratic phase modulators and dispersive delay lines. The latter is typically achieved using optical fibers, but can be compactly implemented and tunable using dispersive filters. Time scaling, either dilation or compression, can be achieved with these building blocks. While basic time scaling followed by direct detection has been demonstrated, we focus on cascading time-scale operations for potential signal processing applications and implementations using integrated-optic platforms. For cascaded operations, both the phase and amplitude of the scaled output must be correct. Time scaling is studied analytically and by simulations. Practical implementation issues are addressed such as the time aperture limits imposed by using sinusoidal phase modulation to approximate the desired quadratic response. The chirp and dispersion relationships are given for "factor of one half and "factor of two" time scaling. The evolution of the signal's time support at intermediate points in the time-scaling operation is shown to be a critical parameter for practical implementations. Two optical time-scaling architectures are studied, and one is clearly better in this respect. Furthermore, a special case arises for a Gaussian input pulse whereby the number of elements needed to realize the time scaling can reduced by a factor of two. Applications for cascaded time scaling operations are discussed, including optical wavelet processing and photonic-assisted analog-to-digital conversion. By using the time-scale operation in the optical domain to mimic the discrete-time downsampling operation, we show that physical scaling of the optical filters between subsequent decomposition levels is not required.