On-chip non-reciprocal light storage

Abstract

Storing light as coherent sound waves is an intriguing and powerful concept for delaying optical signals. It was shown that optical data pulses can be transferred to acoustic waves in optical fiber [1] and, more recently, in integrated circuits [2]. The coupling between the optical and acoustic waves has to fulfill a strict phase matching condition, which was exploited in silica fiber-tip resonators to realize non-reciprocal signal processing [3, 4]. Here, we demonstrate for the first time non-reciprocal light storage based on stimulated Brillouin scattering in an integrated photonic circuit. A data pulse is stored in one direction but is transmitted unaffected in the opposite direction. Our planar photonic circuit allows the operation of several wavelength channels, enabling information storage in one wavelength channel without impeding on the behavior of the other channel. The ultra-high Brillouin gain allows efficient conversion between photons and phonons without relying on a resonator configuration, allowing broad bandwidth and operation at arbitrary wavelengths. The optical data pulses are transferred to the acoustic domain via counter-propagating optical write pulses, offset in frequency to the data pulses by the Brillouin frequency shift of the waveguide. A second optical pulse (read pulse) following the write pulse retrieves the optical data pulse by transferring it back from the acoustic to the optical domain. The underlying phase-matching condition is only fulfilled for counter-propagating optical data and write/read pulses and therefore allows non-reciprocal light storage. Fig. 1 a) shows the storage of two channels at different wavelengths simultaneously. In Fig. 1b) the propagation direction of the optical data stream is reversed, hence the phase-matching condition between the data and the write/read pulses is not fulfilled and the data pulse is transmitted. The slight change in the pulse shape can be accounted to other optical nonlinear effects present in the waveguide. The unique phase matching condition between counter-propagating optical waves and travelling acoustic waves allows furthermore storing data pulses at a second channel, separated in wavelength, simultaneously without detrimental distortions.