arXiv subjects

Khu Vu

Publications and source records attributed to Khu Vu.

11 recordsLinked to original sources

arXiv2023

Brillouin light storage for 100 pulse widths

Signal processing based on stimulated Brillouin scattering (SBS) is limited by the narrow linewidth of the optoacoustic response, which confines many Brillouin applications to continuous wave signals or optical pulses longer than several nanoseconds. In this work, we experimentally demonstrate Brillouin interactions at the 150 ps time scale and a delay for a record 15 ns which corresponds to a delay of 100 pulse widths. This breakthrough experimental result was enabled by the high local gain of the chalcogenide waveguides as the optoacoustic interaction length reduces with pulse width. We successfully transfer 150ps-long pulses to traveling acoustic waves within a Brillouin-based memory setup. The information encoded in the optical pulses is stored for 15 ns in the acoustic field. We show the retrieval of eight amplitude levels, multiple consecutive pulses and low distortion in pulse shape. The extension of Brillouin-based storage to the ultra-short pulse regime is an important step for the realisation of practical Brillouin-based delay lines and other optical processing applications.

physics.optics↗

arXiv2020

On-chip broadband non-reciprocal light storage

Breaking the symmetry between forward and backward propagating optical modes is of fundamental scientific interest and enables crucial functionalities, such as isolators, circulators, and duplex communication systems. Whereas there has been progress in achieving optical isolation on-chip, integrated broadband non-reciprocal signal processing functionalities that enable transmitting and receiving via the same low-loss planar waveguide, without altering the frequency or mode of the signal, remain elusive. Here, we demonstrate a non-reciprocal delay scheme based on the uni-directional transfer of optical data pulses to acoustic waves in a chip-based integration platform. We experimentally demonstrate that this scheme is not impacted by simultaneously counter-propagating optical signals. Furthermore, we achieve a bandwidth more than an order of magnitude broader than the intrinsic opto-acoustic linewidth, linear operation for a wide range of signal powers, and importantly, show that this scheme is wavelength preserving and avoids complicated multi-mode structures..

physics.optics↗

arXiv2019

Coherently refreshed acoustic phonons for extended light storage

Acoustic waves can serve as memory for optical information, however, acoustic phonons in the GHz regime decay on the nanosecond timescale. Usually this is dominated by intrinsic acoustic loss due to inelastic scattering of the acoustic waves and thermal phonons. Here we show a way to counteract the intrinsic acoustic decay of the phonons in a waveguide by resonantly reinforcing the acoustic wave via synchronized optical pulses. This scheme overcomes the previous constraints of phonon-based optical signal processing for light storage and memory. We experimentally demonstrate on-chip storage up to 40 ns, four times the intrinsic acoustic lifetime in the waveguide. We confirm the coherence of the scheme by detecting the phase of the delayed optical signal after 40 ns using homodyne detection. Through theoretical considerations we anticipate that this concept allows for storage times up to microseconds within realistic experimental limitations while maintaining a GHz bandwidth of the optical signal. The refreshed phonon-based light storage removes the usual bandwidth-delay product limitations of e.g. slow-light schemes.

physics.optics↗

arXiv2018

Chip-based Brillouin processing for carrier recovery in coherent optical communications

Modern fiber-optic coherent communications employ advanced spectrally-efficient modulation formats that require sophisticated narrow linewidth local oscillators (LOs) and complex digital signal processing (DSP). Here, we establish a novel approach to carrier recovery harnessing large-gain stimulated Brillouin scattering (SBS) on a photonic chip for up to 116.82 Gbit/sec self-coherent optical signals, eliminating the need for a separate LO. In contrast to SBS processing on-fiber, our solution provides phase and polarization stability while the narrow SBS linewidth allows for a record-breaking small guardband of ~265 MHz, resulting in higher spectral-efficiency than benchmark self-coherent schemes. This approach reveals comparable performance to state-of-the-art coherent optical receivers without requiring advanced DSP. Our demonstration develops a low-noise and frequency-preserving filter that synchronously regenerates a low-power narrowband optical tone that could relax the requirements on very-high-order modulation signaling and be useful in long-baseline interferometry for precision optical timing or reconstructing a reference tone for quantum-state measurements.

physics.app-ph↗

arXiv2018

Crosstalk-free multi-wavelength coherent light storage via Brillouin interaction

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.

physics.optics↗

arXiv2018

On-chip correlation-based Brillouin sensing: design, experiment and simulation

Wavelength-scale SBS waveguides are enabling novel on-chip functionalities. The micro- and nano-scale SBS structures and the complexity of the SBS waveguides require a characterization technique to monitor the local geometry-dependent SBS responses along the waveguide. In this work, we experimentally demonstrate detection of longitudinal features down to 200$μ$m on a silicon-chalcogenide waveguide using the Brillouin optical correlation domain analysis (BOCDA) technique. We provide simulation and analysis on how multiple acoustic and optical modes and geometrical variations influence the Brillouin spectrum.

physics.app-ph↗

arXiv2018

Brillouin spectroscopy of a hybrid silicon-chalcogenide waveguide with geometrical variations

Recent advances in design and fabrication of photonic-phononic waveguides have enabled stimulated Brillouin scattering (SBS) in silicon-based platforms, such as under-etched silicon waveguides and hybrid waveguides. Due to the sophisticated design and more importantly high sensitivity of the Brillouin resonances to geometrical variations in micro- and nano-scale structures, it is necessary to have access to the localized opto-acoustic response along those waveguides to monitor their uniformity and maximize their interaction strength. In this work, we design and fabricate photonic-phononic waveguides with a deliberate width variation on a hybrid silicon-chalcogenide photonic chip and confirm the effect of the geometrical variation on the localized Brillouin response using a distributed Brillouin measurement.

physics.optics↗

arXiv2018

On-chip multi-stage optical delay based on cascaded Brillouin light storage

Storing and delaying optical signals plays a crucial role in data centers, phased array antennas, communication and future computing architectures. Here, we show a delay scheme based on cascaded Brillouin light storage, that achieves multi-stage delay at arbitrary positions within a photonic integrated circuit. Importantly these multiple resonant transfers between the optical and acoustic domain are controlled solely via external optical control pulses, allowing cascading of the delay without the need of aligning multiple structural resonances along the optical circuit.

physics.optics↗

arXiv2017

Highly localized Brillouin scattering response in a photonic integrated circuit

The interaction of optical and acoustic waves via stimulated Brillouin scattering (SBS) has recently reached on-chip platforms, which has opened new fields of applications ranging from integrated microwave photonics and on-chip narrow-linewidth lasers, to phonon-based optical delay and signal processing schemes. Since SBS is an effect that scales exponentially with interaction length, on-chip implementation on a short length scale is challenging, requiring carefully designed waveguides with optimized opto-acoustic overlap. In this work, we use the principle of Brillouin optical correlation domain analysis (BOCDA) to locally measure the SBS spectrum with high spatial resolution of 800 μm and perform a distributed measurement of the Brillouin spectrum along a spiral waveguide in a photonic integrated circuit (PIC). This approach gives access to local opto-acoustic properties of the waveguides, including the Brillouin frequency shift (BFS) and linewidth, essential information for the further development of high quality photonic-phononic waveguides for SBS applications.

physics.optics↗

arXiv2017

A chip-integrated coherent photonic-phononic memory

Controlling and manipulating quanta of coherent acoustic vibrations - phonons - in integrated circuits has recently drawn a lot of attention, since phonons can function as unique links between radiofrequency and optical signals, allow access to quantum regimes and offer advanced signal processing capabilities. Recent approaches based on optomechanical resonators have achieved impressive quality factors allowing for storage of optical signals. However, so far these techniques have been limited in bandwidth and are incompatible with multi-wavelength operation. In this work, we experimentally demonstrate a coherent buffer in an integrated planar optical waveguide by transferring the optical information coherently to an acoustic hypersound wave. Optical information is extracted using the reverse process. These hypersound phonons have similar wavelengths as the optical photons but travel at 5-orders of magnitude lower velocity. We demonstrate the storage of phase and amplitude of optical information with GHz-bandwidth and show operation at separate wavelengths with negligible cross-talk.

physics.optics↗

arXiv2015

The observation of photon echoes from evanescently coupled rare-earth ions in a planar waveguide

We report the measurement of the inhomogeneous linewidth, homogeneous linewidth and spin state lifetime of Pr3+ ions in a novel waveguide architecture. The TeO2 slab waveguide deposited on a bulk Pr3+:Y2SiO5 crystal allows the 3H4 - 1D2 transition of Pr3+ ions to be probed by the optical evanescent field that extends into the substrate. The 2 GHz inhomogeneous linewidth, the optical coherence time of 70 +- 5 us, and the spin state lifetime of 9.8 +- 0.3 s indicate that the properties of ions interacting with the waveguide mode are consistent with those of bulk ions. This result establishes the foundation for large, integrated and high performance rare-earth-ion quantum systems based on a waveguide platform.

quant-ph↗