Cascaded superluminal propagation via Brillouin lasing resonance in optical fibers

Abstract

All-optical group velocity manipulation of light, so-called slow and fast light, has drawn extensive attention due to its potential applications in the fields of optical signal processing, light storage, light-matter interaction enhancement, hypersensitive sensing and temporal cloak. Tremendous approaches of slow and fast light generation have been widely demonstrated by the creation of the sharp spectral resonances in the past decade. Specifically, superluminal propagation (larger-than-light velocity c ) through the optical media has been proposed and validated in anomalous dispersion media. One effective approach is employing stimulated Brillouin scattering to harness the narrowband spectral resonance and thus generate slow and fast light in the optical fiber. In particular, Brillouin lasing oscillation has been experimentally demonstrated to generate fast and superluminal propagation in ten-meter optical fibers. Afterwards, the propagating distance over hundreds of meters has been experimentally demonstrated by the suppression of multiple-longitudinal-mode operation. However, the delay-bandwidth product or the fractional advancement is still fundamentally limited in conventional fast and slow light system. For example, the maximum advancement as well as the fractional temporal advancement are practically restricted due to the Brillouin gain saturation and multiple longitudinal mode lasing operation under high pump power in the Brillouin lasing resonance-based fast and superluminal system. In this paper, we experimentally validate the extension of time advancement of superluminal propagation based on Brillouin lasing oscillation for the first time. By employing two stages of single-frequency Brillouin lasing fiber ring resonators, Stokes lasing resonance was effectively created in each fiber ring cavity to deliver Brillouin-induced loss resonance at the center frequency of pump, which essentially leads to fast light and superluminal propagation of the pump light. Consequently, Gaussian signal pulses with the pulse width of 500 ns experiences cascaded Brillouin-induced fast light and even negative group-velocity superluminal propagation. The temporal advancement was basically extended to 365.8 ns which is twice than that of single stage fast light and superluminal system. Correspondingly, the fractional advancement was increased from 0.36 to 0.73. The maximum group velocity was 6.3 times larger than the light speed c in vacuum while negative-group- velocity propagation can be found in the range of −9.9 to −0.2 by increasing the total input power from 593.2 to 812.0 mW. Furthermore, the group index can be manipulated in the range of 1.5 to −4.0 by simply varying the total input pump power. It indicates superluminal signals can be cascaded to essentially extend the propagation distance and enlarge the temporal advancement. The scheme exhibits robustness in cascading Brillouin spectral loss resonance for temporal advancement enhancement as well as propagating distance extension. More importantly, the cascading design could basically eliminate the fundamental limitation introduced by Brillouin gain saturation and multiple longitudinal lasing mode under high power operation in single stage Brillouin fast and superluminal system. Further optimization could also be implemented by utilizing the nonstandard specialty fibers as Brillouin gain medium to achieve an efficient Brillouin lasing resonance for an enhanced advancement efficiency. Additionally, the combination with the cascading technique and long-cavity single-frequency Brillouin lasing resonance could enable the feasibility of distance-limitless and large-advancement superluminal propagation, paving a way for promising applications in sensing and temporal cloak.