Moreover, the highly nonlinear fiber (HNLF) has been employed to improve the advancement performance. Owing to the perfect self-adaptation and the linewidth narrowing of the lasing Stokes, anomalous dispersion at the frequency of the pump light can be generated to boost the group velocity of the light. Recently, low-loss superluminal propagation in optical fibers has been successfully demonstrated via Brillouin lasing oscillation. However, the dependence on the well-defined modulation frequency of the light renders the schemes impractical for high modulation frequency requirements.
Superluminal propagation can also be generated in erbium-doped fibers via coherent population oscillation (CPO) and cross gain modulation (XGM). However, the transmission distance of signals is extremely limited. Gain-assisted scheme in the ultra-cold atomic vapor was demonstrated to achieve superluminal propagation. However, owing to operating on strong absorption band, the propagating distance and the time advancement of Brillouin-induced fast light is severely limited due to the operation on the strong absorption band. Stimulate Brillouin scattering (SBS) in optical fibers has become an effective way to realize slow/fast light. Although various approaches have been proposed to study fast light ring laser gyroscope, new-fashioned all-optical systems in developing flexible group index manipulation are still required to pave the way for practical applications. In particular, the rotation sensor based on fast light technology exhibits an enhanced sensitivity and brings benefits to the gravitational frame-dragging effect detection. Slow/fast light plays an important role on optical delay lines, signal processing, fiber-optic communications, light-matter interaction enhancement, microwave photonics filter and temporal cloak. In the past few years, impressive progresses in controlling light speed have been reported by creating sharp spectral resonances in dispersive media.
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