Composite acousto-optical modulation

We identify a new class of acousto-optical modulation (AOM) schemes based on composite diffraction by multiple AOMs. We show that by adjusting the amplitudes and phases of weakly-driven daughter AOMs re-imaged by 4-F optics, the composite AOM can be optimized for highly efficient optical diffraction beyond the Bragg condition. Furthermore, by re-imaging counter-propagating sound waves, rapid switching of output orders can be achieved at the driving radio frequency (rf) limit toward GHz level. Here, a proof-of-principle demonstration with a simplest example using two AOMs demonstrates a uniform $>90\%$ diffraction efficiency over a wide rf-tuning bandwidth, with which we route a mode-locked laser pulse train into two paths with $15\sim20$~dB side-pulse suppression. With the unprecedented acoustic optical modulation efficiency and bandwidth combination, we expect the composite technique to help paving a practical pathway toward arbitrary modulation of mode-locked lasers for applications across fields.