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arXiv:2410.05156 · APL Photonics

Brillouin-based storage of QPSK signals with fully tunable phase retrieval

Olivia Saffer, Jesús Humberto Marines Cabello, Steven Becker, Andreas Geilen, Birgit Stiller

Published Jun 1, 2025• 6 citations
arXivHTMLPDF

Abstract

Photonic memory is an important building block to delay, route, and buffer optical information, for instance, in optical interconnects or for recurrent optical signal processing. Photonic–phononic memory based on stimulated Brillouin–Mandelstam scattering (SBS) has been demonstrated as a coherent optical storage approach with broad bandwidth, frequency selectivity, and intrinsic nonreciprocity. Here, we experimentally demonstrate the storage of quadrature-phase encoded data at room temperature and at cryogenic temperatures. We store and retrieve the 2-bit states {00, 01, 10, 11} encoded as optical pulses with the phases {0,π2,π,3π2}—a quadrature phase shift keying signal. The 2-bit signals are retrieved from the acoustic domain with a global phase rotation of π, which is inherent in the process due to SBS. We also demonstrate full phase control over the retrieved data based on two different handles: by detuning slightly from the SBS resonance or by changing the storage time in the memory scheme, we can cover the full range [0, 2π). At a cryogenic temperature of 3.9 K, we have increased readout efficiency as well as gained access to longer storage times, which results in a detectable signal at 140 ns. All in all, the work sets the cornerstone for optoacoustic memory schemes with phase-encoded data.

DOI
10.1063/5.0241508

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Photonic memory is an important building block to delay, route, and buffer optical information, for instance, in optical interconnects or for recurrent optical signal processing. Photonic–phononic memory based on stimulated Brillouin–Mandelstam scattering (SBS) has been demonstrated as a coherent optical storage approach with broad bandwidth, frequency selectivity, and intrinsic nonreciprocity. Here, we experimentally demonstrate the storage of quadrature-phase encoded data at room temperature and at cryogenic temperatures. We store and retrieve the 2-bit states {00, 01, 10, 11} encoded as optical pulses with the phases {0,π2,π,3π2}—a quadrature phase shift keying signal. The 2-bit signals are retrieved from the acoustic domain with a global phase rotation of π, which is inherent in the process due to SBS. We also demonstrate full phase control over the retrieved data based on two different handles: by detuning slightly from the SBS resonance or by changing the storage time in the memory scheme, we can cover the full range [0, 2π). At a cryogenic temperature of 3.9 K, we have increased readout efficiency as well as gained access to longer storage times, which results in a detectable signal at 140 ns. All in all, the work sets the cornerstone for optoacoustic memory schemes with phase-encoded data.
High Resolution Image Download MS PowerPoint Slide In recent years, remarkable advances in photonic computing have highlighted the need for photonic memory, particularly high-speed and coherent random-access memory. Addressing the ongoing challenge of implementing photonic memories is required to fully harness the potential of photonic computing. A photonic-phononic memory based on stimulated Brillouin scattering is a possible solution, as it coherently transfers optical information into sound waves at high-speed. Such an optoacoustic memory has shown great potential as it fulfills key requirements for high-performance optical random-access memory due to its coherence, on-chip compatibility, frequency selectivity, and high bandwidth. However, the storage time has so far been limited to a few nanoseconds due to the nanosecond decay of the acoustic wave. In this work, we experimentally enhance the intrinsic storage time of an optoacoustic memory by more than 1 order of magnitude and coherently retrieve optical information after a storage time of 123 ns. This is achieved by employing the optoacoustic memory in a highly nonlinear fiber at 4.2 K, increasing the intrinsic phonon lifetime by a factor of 6. We demonstrate the capability of our scheme by measuring the initial and readout optical data pulses with a direct and double homodyne detection scheme. Finally, we analyze the dynamics of the optoacoustic memory at different cryogenic temperatures in the range of 4.2–20 K and compare the findings to continuous wave measurements. The extended storage time is beneficial not only for photonic computing but also for Brillouin applications that require long phonon lifetimes, such as optoacoustic filters, true-time delay networks, and synthesizers in microwave photonics.
We experimentally show an optoacoustic memory based on Brillouin scattering with one order of magnitude higher storage time that retrieves amplitude and phase information after 120ns. We increase the intrinsic phonon lifetime of a highly nonlinear fiber by a factor of six by cooling the fiber down to 4.2K. We demonstrate the performance enhancement of optoacoustic memory by measuring the amplitude and phase information of an initial data pulse and its corresponding retrieved readout pulse using direct and double homodyne detection. Furthermore, we present the influence of different cryogenic temperatures between 4.2K and 20K on the optoacoustic memory and compare the results with continuous-wave measurements. In conclusion, our work can not only accelerate photonic computing but also advance other applications of stimulated Brillouin scattering that require long phonon lifetimes, such as optoacoustic filters in microwave photonics. In addition, the presented long-lasting sound wave optoacoustic memory is compatible with active acoustic refreshment technique potentially leading to all-optical coherent memory beyond 1 μs.
The importance of optical signal processing techniques is growing rapidly in recent years due to the exponentially increasing demand for bandwidth, capacity and power efficiency in communications and computing. However, due to their bosonic nature photons do not interact with each other, unless there is a nonlinear medium mediating the interaction. One of the strongest nonlinear effects is the interaction of light waves, photons, with sound-waves, acoustic phonons, which is known as stimulated Brillouin scattering (SBS). This thesis experimentally investigates SBS in photonic chips. It is shown in this thesis that the fundamental interaction strength between light and sound waves can be tailored by using one-dimensional photonic bandgap structures, completely suppressing the effect or alternatively enhancing the interaction to form phase-locked Brillouin frequency combs. It was shown furthermore that efficiently generating SBS on-chip enables the generation of stable RF signals that are widely tunable in frequency. Finally, it is shown in this thesis that SBS enables the storage of light signals on a chip, one of the holy grails of all-optical signal processing. Delaying optical signals is of key importance in optical networks to enable synchronization, buffering, and rerouting. SBS enables large delays by resonantly transferring an optical signal to an acoustic wave, that travels five orders of magnitude slower and retrieving it after a certain storage time. It is demonstrated in this thesis that a Brillouin-based memory (BBM) technique allows storing amplitude and phase of optical data pulses and operate at multiple wavelengths with minimal cross-talk. Replenishing of the acoustic wave to overcome storage time limitations imposed by the lifetime of the acoustic wave as well as non-reciprocal light storage is also shown.
Efficient and extended light storage mechanisms are pivotal in photonics, particularly in optical communications, microwave photonics, and quantum networks, as they offer a direct route to circumvent electrical conversion losses and surmount bandwidth constraints. Stimulated Brillouin Scattering (SBS) is an established method to store optical information by transferring it to the acoustic domain, but current on-chip SBS efforts have limited bandwidth or storage time due to the phonon lifetime of several nanoseconds. An alternate approach known as quasi-light storage (QLS), which involves the creation of delayed replicas of optical data pulses via SBS in conjunction with a frequency comb, has been proposed to lift the storage time constraint; however, its realization has been confined to lengthy optical fibers, constraining integration with on-chip optical elements and form factors. Here, we present an experimental demonstration of QLS on a photonic chip leveraging the large SBS gain of chalcogenide glass, achieving delays of up to 500 ns for 1 ns long signal pulses, surpassing typical Brillouin storage processes' acoustic lifetime by more than an order of magnitude and waveguide transit time by two orders of magnitude. We experimentally and numerically investigate the dynamics of on-chip QLS and reveal that the interplay between the acoustic wave that stores the optical signal and subsequent optical pump pulses leads to a reshaping of the acoustic field. Our demonstrations illustrate the potential for achieving ultra-long storage times of individual pulses by several hundred pulse widths, marking a significant stride toward advancing the field of all-optical storage and delay mechanisms.
With the recent developments in photonic computing platforms, the need of novel optical memories becomes increasingly important. Optoacoustic memories have shown great potential due to their quick access time, coherent nature, and broad bandwidth. These continuously tunable memories can be controlled entirely optically, but so far their intrinsic storage time is limited by the acoustic life time to approximately 10 ns. In this work, we experimentally demonstrate a cryogenic optoacoustic memory with an intrinsic storage time beyond 135 ns.