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Surpassing the tuning speed limit of slow-light-based tunable optical delay via four-wave mixing Bragg scattering.

We demonstrate a tunable optical delay that surpasses the tuning speed limit of the conventional slow-light-based optical delay. A novel nonlinear optical coupler, implemented by the four-wave mixing (FWM) Bragg scattering process, is utilized to perform destructive interference of the slow-light delayed signal pulse and a nondelayed reference pulse. As a result, the Brillouin-induced frequency-dependent phase shift, as well as the group delay of the synthesized pulse, is amplified. The group delay amplification factor, determined by the coupling ratio of the nonlinear optical coupler, can be tuned through varying the FWM pump power to provide an ultrafast response. Our experimental result demonstrates that an initial 6.2 ns Brillouin-induced optical delay can be amplified and rapidly tuned within the range of -5.2 to 27.2 ns.