The fundamental limitations on the practical realizations of white light cavities

We study the possibility of realizing white light cavities (WLCs) - cavities which essentially resonate over a continuous band and not at discrete frequencies, by utilizing linearly chirped Bragg reflectors (LCBGs) for phase compensation. Analytical and numerical analyses show that this goal cannot be achieved because the reflection of a specific frequency by a LCBG cannot be modeled as occurring at the position where the Bragg condition is satisfied. The accumulated effect of multiple scatterings at different locations along the LCBG produces a positive group delay, preventing the realization of a WLC using this approach. We also present a generic, filter theory based, argument showing that any phase component that exhibit a negative group delay necessarily has a corresponding dip in its amplitude response. The implications of this conclusion on the limitations and design rules of WLC based devices are discussed in details.