[Paper Review] Crosstalk-free multi-wavelength coherent light storage via Brillouin interaction

Birgit Stiller, Moritz Merklein+5 more|arXiv (Cornell University)|Mar 23, 2018

Mechanical and Optical Resonators24 references3 citations

TL;DR

This paper demonstrates crosstalk-free, coherent multi-wavelength light storage in a Brillouin-based system by leveraging accumulated phase mismatch over the interaction length, enabling simultaneous storage of optical pulses at 25–100 GHz spacing with negligible crosstalk even when acoustic phonons overlap in frequency. The method preserves coherence and allows selective retrieval per wavelength channel, enabling dense wavelength multiplexing in optical memory and signal processing applications.

ABSTRACT

Stimulated Brillouin scattering drives a coherent interaction between optical signals and acoustic phonons and this effect can be used for storing optical information in acoustic waves. An important consideration arises when multiple optical frequencies are simultaneously employed in the Brillouin process: in this case the acoustic phonons that are addressed by each optical wavelength can be separated by frequencies far smaller than the acoustic phonon linewidth, potentially leading to crosstalk between the optical modes. Here we extend the concept of Brillouin-based light storage to multiple wavelength channels. We experimentally and theoretically show that the accumulated phase mismatch over the length of the spatially extended phonons allows each optical wavelength channel to address a distinct phonon mode, ensuring negligible crosstalk, even if the phonons overlap in frequency. Moreover, we demonstrate that the strict phase matching condition enables the preservation of the coherence of the opto-acoustic transfer at closely spaced multiple acoustic frequencies. This particular phase-mismatch for broad-bandwidth pulses has far-reaching implications allowing dense wavelength multiplexing in Brillouin-based light storage, multi-frequency Brillouin sensing, multi-wavelength Brillouin lasers, parallel microwave processing and quantum photon-phonon interactions.

Motivation & Objective

• To address crosstalk in multi-wavelength Brillouin-based light storage where closely spaced optical frequencies generate overlapping acoustic phonons.
• To demonstrate that phase mismatch over the pulse interaction length can suppress crosstalk between distinct optical wavelength channels.
• To validate experimentally and theoretically that coherent information can be stored and retrieved independently in separate phonon modes despite spectral and spatial overlap.
• To enable dense wavelength multiplexing in Brillouin memory systems without signal degradation or interference between channels.

Proposed method

• Utilizes stimulated Brillouin scattering (SBS) to coherently transfer optical pulses to traveling acoustic phonons in a waveguide.
• Employs a phase-matching condition that accumulates over the spatial length of the pulse, suppressing coupling to mismatched phonon modes.
• Uses optical pulses with controlled bandwidth (500 MHz to 10 GHz) and variable channel spacing (25–100 GHz) to probe crosstalk behavior.
• Employs Gaussian and rectangular pulse shapes to analyze crosstalk dependence on pulse temporal profile.
• Conducts experimental validation using dual-wavelength input pulses and measures retrieval fidelity and crosstalk levels.
• Develops a theoretical model based on three-wave interaction dynamics to explain the suppression of crosstalk via phase accumulation.

Experimental results

Research questions

• RQ1 Can multiple optical wavelength channels be coherently stored in a Brillouin-based system without crosstalk, even when their corresponding acoustic phonons overlap in frequency?
• RQ2 How does the accumulated phase mismatch over the interaction length affect crosstalk between optical modes and mismatched phonon modes?
• RQ3 To what extent does pulse bandwidth and shape influence crosstalk in multi-wavelength Brillouin storage?
• RQ4 Can coherence of the stored optical information be preserved when multiple wavelength channels are processed in parallel?
• RQ5 Do phonon modes remain distinct entities when spectrally and spatially overlapping, or is there effective coupling between them?

Key findings

• Crosstalk between optical wavelength channels is negligible even when acoustic phonons are separated by only 1 MHz, despite overlapping linewidths.
• For 10 GHz optical pulses, channel spacing as low as 20 GHz is sufficient to maintain low crosstalk, with crosstalk decreasing rapidly as spacing increases.
• Crosstalk drops significantly for pulse bandwidths of 1 GHz or less, allowing channel spacing as close as 1–2 GHz with minimal interference.
• Coherence of the stored optical signals is preserved across all wavelength channels, with no measurable degradation in retrieval fidelity.
• Theoretical modeling confirms that phase accumulation along the interaction length suppresses coupling to mismatched phonon modes, explaining the observed crosstalk suppression.
• Phonon modes remain distinct even when spectrally and spatially overlapping, indicating they are fundamental, non-degenerate entities in the waveguide system.