Nonreciprocal Inter-band Brillouin Modulation (1808.09865v1)

Abstract: Achieving nonreciprocal light propagation in photonic circuits is essential to control signal crosstalk and optical back-scatter. However, realizing high-fidelity nonreciprocity in low-loss integrated photonic systems remains challenging. In this paper, we experimentally demonstrate a device concept based on nonlocal acousto-optic light scattering to produce nonreciprocal single-sideband modulation and mode conversion in an integrated silicon photonic platform. In this process, a traveling-wave acoustic phonon driven via optical forces in a silicon waveguide is used to modulate light in a spatially separate waveguide through a linear inter-band Brillouin scattering process. We demonstrate up to 38 dB of nonreciprocity with 37 dB of single-sideband suppression. In contrast to prior Brillouin- and optomechanics-based schemes for nonreciprocity, the bandwidth of this scattering process is set through optical phase-matching, not acoustic or optical resonances. As a result, record-large bandwidths in excess of 125 GHz are realized, with potential for significant further improvement through optical dispersion engineering. Tunability of the nonreciprocal modulator operation wavelength over a 35 nm bandwidth is demonstrated by varying the optical pump wavelength. Such traveling-wave acousto-optic modulators provide a promising path toward the realization of broadband, low-loss isolators and circulators in integrated photonic circuits.

Nonreciprocal Inter-band Brillouin Modulation: An Overview

The paper "Nonreciprocal Inter-band Brillouin Modulation" presents an innovative approach for achieving nonreciprocal light propagation within integrated silicon photonic circuits, addressing longstanding challenges in the field. This work is critical for improving signal routing and protection against optical back-scatter in photonic systems.

Device Concept and Experimental Validation

The authors introduce a device based on nonlocal acousto-optic light scattering to enable nonreciprocal single-sideband modulation and mode conversion. A traveling-wave acoustic phonon, driven by optical forces in a silicon waveguide, modulates light in a spatially distinct optical waveguide through a linear inter-band Brillouin scattering process. The experimental setup demonstrates nonreciprocity up to 38 dB with single-sideband suppression of 37 dB.

Significantly, unlike traditional Brillouin- and optomechanical approaches, the bandwidth in this scheme is determined through optical phase-matching rather than relying on resonant optical or acoustic modes. This leads to bandwidths exceeding 125 GHz, with potential expansion through optical dispersion engineering. Additionally, tunability over a 35 nm wavelength bandwidth is achieved by altering the optical pump wavelength, further proving the flexibility of this system.

Implications for Integrated Photonic Circulators and Isolators

The device provides a compelling path toward developing broadband, low-loss isolators, and circulators integral to complex photonic circuits. Compared to existing Faraday isolators, which are challenging to miniaturize and integrate due to the lossy nature of magneto-optic materials, this approach offers a CMOS-compatible solution with significantly reduced intrinsic optical losses.

The implementation also aligns well with recent strategies emphasizing driven photonic transitions for nonreciprocal modulation, allowing high linearity and dynamic reconfiguration. However, achieving efficient inter-band coupling remains a key challenge for practical applications, with current modulation efficiency observed around 1%. Enhancements could be explored through free-carrier extraction or electromechanical phonon transduction for unity efficiency.

Future Prospects

These findings pave the way for broad application possibilities such as on-chip broadband isolators and frequency tunable modulators. Extending the optical bandwidth through dispersion engineering could further advance integrated photonic technology, potentially supporting bandwidths between 10-100 nm.

In summary, this paper delivers a significant advancement in nonreciprocal photonic modulation, presenting valuable insights and directions for future research and application in nanophotonic systems.