Coupled resonators exhibit coherence effects which can be exploited for the delay or advancement of pulses with minimal distortion. The bandwidth and normalized pulse delay are simultaneously enhanced by proper choice of the inter-resonator couplings.
We study pulse propagation in the slow-light regime with positive and negative group delays in auto-regressive moving average (ARMA) filters, including the ring-enhanced Mach-Zehnder interferometer (REMZI) and side-coupled integrated spaced sequence of resonators (SCISSOR) optical architectures. Both architectures are optimized to maximize the delay-bandwidth product for a fixed output pulse-distortion level. In the negative-group-delay regime, absorption is incorporated in the rings. The REMZI architecture has the highest delay-bandwidth product in both the positive and negative-group-delay regimes due to greater efficiency in incorporating filter phase within the passband. We also explore the effects of pulse shape in the lossy regime and the limitations it poses in the amount of pulse advancement that is usable for practical applications.
Induced transparency and absorption effects can be observed in the throughput of an optical microresonator that has two coresonant modes with very different quality factors. There are several different methods for achieving these effects, which enable slow light and fast light, i.e., the delay or advancement of an incident resonant pulse. For example, mode coupling can be employed. This coupling can take place between modes of the same polarization or modes of orthogonal polarization. Another method is based on superposition, when two orthogonally polarized modes are driven simultaneously by linearly polarized input light and throughput of the same polarization as the input is detected. In general, induced transparency is accompanied by pulse delay, whereas induced absorption can be accompanied by pulse advancement or delay. A number of different methods for producing induced transparency or absorption are compared here. Several properties are considered for comparison. One involves the widths of the induced transparency or absorption window and of the corresponding spectral region of steep dispersion. Achievable pulse delays or advancements, along with pulse distortion and delay-bandwidth (or advancement-bandwidth) products, are also compared. Different methods allow for different amounts of control over various system parameters, and these are compared as well. The differences among the several methods are evaluated in consideration of suitability for employment in various applications.
Under the constraint of fixed pulse distortion, we study the bandwidth-delay product and nonlinear phase shift performance of coupled-resonator slow-light waveguides with designs that produce maximally-flat transmission and maximally-flat group delay responses. Even though improvement in bandwidth-delay product can be obtained with increasing number of resonators, the nonlinear response fails to improve beyond a certain number of resonators due to the increased filter bandwidth necessary to maintain a fixed pulse distortion. However, as expected, the nonlinear response improves with resonator finesse and degrades with resonator loss.