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Unwanted high-power and frequency-agile radio-frequency (RF) signals cause saturation and nonlinear distortions to sensitive RF receivers. RF notch filters with reconfigurable responses over multiple frequency bands are highly sought after to prevent these detrimental effects. Microwave photonic (MWP) notch filters have shown increased frequency agility compared to their electronic counterparts. However, demonstrated filter schemes focus only on single notch responses, leaving them unable to simultaneously attenuate multiple interferers over a wide frequency range. In this work, we demonstrate a high-performance chip-based MWP notch filter with three independent notches widely tunable over 20 GHz. The filter exhibits low RF passband losses of 8 dB and peak notch depth greater than 40 dB with 500 MHz spectral resolution. Specifically, each notch is formed through cascaded optical processing from on-chip, low-loss Si3N4 micro-resonators and Brillouin gain on an As2S3 photonic chip. We use this filter to demonstrate high-performance analog RF filtering by substantially attenuating multiple interferers, and show effective image rejection during RF downconversion in a communications receiver to enable a large reduction of error vector magnitude (EVM) from 60% to 15%. Finally, we provide performance analysis and design perspectives for future photonic integration of the proposed filter subsystem.

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. Although there has been progress in achieving optical isolation on-chip, integrated broadband nonreciprocal 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 nonreciprocal delay scheme based on the unidirectional 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 counterpropagating optical signals. Furthermore, we achieve a bandwidth more than an order of magnitude broader than the intrinsic optoacoustic linewidth, linear operation for a wide range of signal powers, and importantly, show that this scheme is wavelength preserving and avoids complicated multimode structures.

Microwave photonic phase shifters based on stimulated Brillouin scattering (SBS) offer tunable and broadband, optically controllable phase shifts. However, achieving a 360° phase shift requires a large amount of SBS gain which often exceeds the available gain and power handling capability of an integrated waveguide. A Radio Frequency (RF) interference technique has recently been utilized in an integrated silicon platform, which uses forward Brillouin scattering in a suspended waveguide to compensate for the lack of available Brillouin gain in standard silicon on insulator platforms. This interference scheme amplifies the phase shift at the expense of link performance. Here, we demonstrate and analytically model a 360° ultra-broadband phase shifter using backward SBS in both fiber and on-chip by combining SBS and RF interference. The phase enhancement scheme greatly reduces the required Brillouin gain and thus the required optical power. Additionally, the backward architecture reduces filter requirements as the residual pump reflections are simpler to remove compared to the pump in the forward Brillouin scattering case, where the pump co-propagates with the signal. The model provides a deeper insight into the properties of the interferometric phase enhancement scheme and predicts the potential trade-offs of an optimized system, showing reduced link loss at higher levels of Brillouin gain. The model also predicts the sensitivity to variations of the interferometric components. Using this technique, we have demonstrated a broadband phase shift over an ultra-broad bandwidth of 0.1 – 65 GHz, limited only by the bandwidth of the available components. Also, we demonstrate a phase enhancement factor of 10 over a bandwidth of 18 GHz in an integrated chalcogenide waveguide.

We significantly narrow the effective linewidth in a coherent optical link from 75 kHz to ~ 2 kHz using SBS-based optical carrier recovery. Experiment results show that the phase recovery window length can be enlarged by over 100x with less than 0.5 dB Q2 penalty.

We demonstrate SBS-based optical carrier recovery scheme with full EDFA-band coverage and 20 dB of gain. This approach shows nearly an order of magnitude improvement in the operation range of the conventional SBS filter. © 2020 The Author(s)

We experimentally demonstrate a chip-based continuously tunable microwave photonic true-time delay (TTD) using Brillouin scattering and phase amplification enhancement, achieving delays of 4-ns over a bandwidth of 150-MHz utilizing approximately 10dB of on-off gain. © 2020 The Author(s)

Coherent measurements of an optical tone amplified by stimulated Brillouin scattering (SBS) show significant noise when the SBS gain is misaligned, which can be reduced (> 100x) by amplitude clamping through optical injection locking.

We demonstrate a tunable on-chip all-optical polarisation controller based on Polarisation Pulling via Stimulated Brillouin Scattering, achieving near-complete rotations between orthogonal linear polarisation states. © 2020 The Author(s)

We controlled the phase matching for four-wave mixing (FWM) in a chalcogenide waveguide by Brillouin-induced phase shift. Over 2n phase shift was realized and a 6-dB improvement in FWM conversion efficiency was obtained.

In this paper, we demonstrate a self-homodyne coherent system with a significantly narrowed effective linewidth using optical carrier recovery based on stimulated Brillouin scattering (SBS), employing only coarse path length matching. The effective linewidth of the SBS-based receiver system is reduced from 75 kHz to less than 2 kHz, which is estimated by Lorentzian fitting of power spectra, and confirmed by simulation results of the tolerance window length for phase noise compensation (PNC) with different linewidth. Both experimental and numerical studies on the tracking requirements on PNC algorithms confirm effective linewidth reduction to this level, and show a 32x relaxation of the phase recovery tracking window length. This highlights the potential to significantly reduce the computational complexity of PNC even in coarsely optimized SBS-based self-homodyne coherent systems, providing an alternative to using demanding ultra-low linewidth lasers.

Storing and delaying optical information is of crucial importance for optical communications, signal processing and in the quantum domain. A plethora of techniques for optical storage have been shown which aim to maintain as much as possible the advantages of optics, such as e.g. the coherence, the bandwidth and capacity. In particular the interaction of optical waves with mechanical motion offers a solution to buffer optical information [1–4]. It was shown that one can use acoustic waves to store and delay optical signals coherently via the effect of stimulated Brillouin scattering (SBS) [3,4]. This allows for simultaneous storage at multiple wavelengths [5], cascaded storage at different spatial positions [6], and nonreciprocal storage with a GHz bandwidth [7]. However, the delay time of this technique is so far limited to the acoustic lifetime of about 10 ns.

Detection and frequency estimation of radio frequency (RF) signals are critical in modern RF systems, including wireless communication and radar. Photonic techniques have made huge progress in solving the problem imposed by the fundamental trade-off between detection range and accuracy. However, neither fiber-based nor integrated photonic RF signal detection and frequency estimation systems have achieved wide range and low error with high sensitivity simultaneously in a single system. In this paper, we demonstrate the first Brillouin opto-electronic oscillator (B-OEO) based on on-chip stimulated Brillouin scattering (SBS) to achieve RF signal detection. The broad tunability and narrowband amplification of on-chip SBS allow for the wide-range and high-accuracy detection. Feeding the unknown RF signal into the B-OEO cavity amplifies the signal which is matched with the oscillation mode to detect low-power RF signals. We are able to detect RF signals from 1.5 to 40 GHz with power levels as low as -67 dBm and a frequency accuracy of ± 3.4 MHz. This result paves the way to compact, fully integrated RF detection and channelization.

© 2019 Author(s). Stimulated Brillouin scattering drives a coherent interaction between optical signals and acoustic phonons and 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 cross talk 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 cross talk and preserving the coherence, even if the phonons overlap in frequency. This phase-mismatch for broad-bandwidth pulses has far-reaching implications allowing dense wavelength multiplexing in Brillouin-based light storage, multifrequency Brillouin sensing and lasing, parallel microwave processing, and quantum photon-phonon interactions.

Stimulated Brillouin scattering has great potential for wide-wavelength-range optical carrier recovery, as it can act as a parametrically defined narrowband gain filter. However, due to the dispersion of the Brillouin frequency shift, prior demonstrations have been limited in wavelength range. Here, we demonstrate that frequency modulating the pump light for a gain filter based on stimulated Brillouin scattering enables optical carrier recovery for a broad range of input wavelengths. We demonstrate highly selective ( bandwidth) amplification for optical carriers over an 18 nm wide wavelength range in the optical communications C-band, an improvement over using an unmodulated pump. Measurements of the noise properties of these spectrally broadened gain filters, in both amplitude and phase, indicate the noise performance and SNR are maintained over a wide wavelength range. Our technique provides a potential solution for highly selective, wavelength agnostic optical carrier recovery.

The last few years have seen major progress in harnessing on-chip photon-phonon interactions, leading to a wide range of demonstrations of new functionalities. Utilizing not only the optical response of a nonlinear waveguide - but also acoustic resonances - enables the realization of microwave devices with unprecedented performance, otherwise hard to achieve in all-optical processing schemes or electronically. Here, we overview on-chip stimulated Brillouin scattering (SBS) with special emphasis on microwave sources and microwave signal processing schemes. We review the different material platforms and structures for on-chip SBS, ranging from chalcogenide rib waveguides to hybrid silicon/silicon-nitride structures, high-Q photonic-phononic silica microresonators, and suspended silicon nanowires. We show that the paradigm shift in SBS research - from long length of fibers to chip-scale devices - is now moving toward fully integrated photonic-phononic CMOS chips.

True-time delays are important building blocks in modern radio frequency systems that can be implemented using integrated microwave photonics, enabling higher carrier frequencies, improved bandwidths, and a reduction in size, weight, and power. Stimulated Brillouin scattering (SBS) offers optically-induced continuously tunable delays and is thus ideal for applications that require programmable reconfiguration but previous approaches have been limited by large SBS gain requirements. Here, we overcome this limitation by using radio-frequency interferometry to enhance the Brillouin-induced delay applied to the optical sidebands that carry RF signals, while controlling the phase of the optical carrier with integrated silicon nitride microring resonators. We report a delay tunability over 600 ps exploiting an enhancement factor of 30, over a bandwidth of 1 GHz using less than 1 dB of Brillouin gain utilizing a photonic chip architecture based on Brillouin scattering and microring resonators.

In this Letter, we report a chip-based photonic radio-frequency (RF) mixer with a maximum conversion gain of − 9 d B and image rejection ratio of 50 dB for 3.2 GHz to 13.2 GHz RF frequency range. This is achieved by the combined use of optical carrier suppression modulation and on-chip stimulated Brillouin scattering. These results will stimulate future implementations of integrated photonic RF mixers in complicated electromagnetic environments.

Acoustic waves can serve as memory for optical information; however, propagating acoustic phonons in the gigahertz (GHz) regime decay on the nanosecond time scale. 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. We experimentally demonstrate coherent on-chip storage in amplitude and phase up to 40 ns, 4 times the intrinsic acoustic lifetime in the waveguide. 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.

In this Letter, we demonstrate a S i 3 N 4 -chip-based photonic approach to generate versatile radio frequency (RF) waveforms with a large tuning range of repetition rates. The amplitude and phase of the RF-phase-modulated signal are spectrally manipulated to synthesize Fourier coefficients of the desired RF waveforms by controlling the resonance conditions and frequencies of S i 3 N 4 optical ring resonators. Full-duty-cycle triangular, square, and sawtooth waveforms with widely tunable repetition rates from 1 to 13 GHz were experimentally generated.

Phase shifters are an important building block of modern RF communications and RADAR systems, with an ever increasing demand to improve the operating bandwidth and to reduce power consumption and footprint. Microwave photonic phase shifters based on stimulated Brillion scattering (SBS) offer tunable and broadband phase shifts, however, their performance has been constrained by the requirement of high Brillouin gain to achieve 360° of phase shift. Here, we demonstrate a 360° broadband phase shifter by combining SBS and RF interference which greatly reduces the required Brillouin gain and thus the required optical power. Using this technique, we are able to enhance the phase shift by more than an order of magnitude, showing bandwidth tunability from 0.1-65 GHz while also being optimized to minimize amplitude 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., 5 pages, 4 figures

Using stimulated Brillouin scattering, we experimentally demonstrate a coherent storage concept that is suitable for multiple optical wavelengths and storage of 200ps-long optical pulses for over 70 pulse widths.

We store optical data in acoustic waves on a planar waveguide based on stimulated Brillouin scattering (SBS) and demonstrate multiple conversions of information between the optical and acoustic domain. We show the coherent acousto-optic transfer by retrieving multiple amplitude and phase levels with GHz bandwidth and demonstrate multi-wavelength operation.

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 five orders of magnitude lower velocity. We demonstrate the storage of phase and amplitude of optical information with gigahertz bandwidth and show operation at separate wavelengths with negligible cross-talk., Optical storage implementations based on optomechanical resonator are limited to one wavelength. Here, exploiting stimulated Brillouin scattering, the authors demonstrate a coherent optical memory based on a planar integrated waveguide, which can operate at different wavelengths without cross-talk.

We experimentally demonstrate that the coherence of Brillouin-based storage is not affected by simultaneous storage on another frequency channel because of the accumulated phase mismatch over the length of the spatially extended acoustic phonon.

Using photon-phonon interactions, we experimentally demonstrate the storage of 200ps-long optical pulses for over 70 pulse widths. We moreover show that this concept is suitable for the non-reciprocal storage at multiple optical wavelengths.

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., Comment: 5 pages, 5 figures, Birgit Stiller and Moritz Merklein contributed equally to this work

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., 9 pages, 6 figures, Moritz Merklein and Birgit Stiller contributed equally to this work

We store optical data in acoustic waves on a planar waveguide based on stimulated Brillouin scattering (SBS) and demonstrate for the first time that this coherent light storage concept allows for the simultaneous storage at different frequency channels, separated by 100 GHz, without observable cross-talk between the individual channels.

Stimulated Brillouin scattering (SBS) is a phonon-photon interaction in which the energy of an optical pump transfers to a Stokes wave through an acoustic wave. Chalcogenide glass (As 2 S 3 ) photonic integrated circuits (PIC) are among the most efficient platforms for SBS and have been extensively developed for several applications, including microwave photonic filters, lasers and optical memory. In these applications, the acoustic properties of the waveguide are paramount to achieve high performance. A distributed SBS measurement with high spatial resolution reveals information about the local phonon-photon interactions along the waveguide in contrast to traditional techniques, which only capture an integrated SBS response. Among different SBS-based distributed measurement techniques, Brillouin optical correlation domain analysis (BOCDA) has achieved the highest spatial resolution, which is essential for scanning cm-scale PICs. Different implementations of BOCDA use frequency and phase modulation [1] [2] or are based on amplified spontaneous emission (ASE) [3]. The latter requires only a simple ASE source, however the drawback is that this technique typically leads to a low signal-to-noise ratio (SNR). In this work, we use an ASE-based BOCDA system with a lock-in amplifier to achieve higher SNR by rejecting the excess noise from the ASE spectrum. We scan the local SBS response of a chalcogenide PIC with 2.5 mm spatial resolution. This approach provides critical information about acoustic properties of the waveguide.

In order to deliberately control the flow of optically encoded information, optical storage is an indispensable tool, but it must maintain all the advantages of optics, such as i.e. the coherence, the bandwidth and capacity. An elegant approach to this problem is to coherently transfer the information from optical to acoustic waves via an opto-mechanical nonlinearity, such as stimulated Brillouin scattering (SBS). The basic concept was first demonstrated in highly nonlinear fiber by Zhu et al. [1]. Recently, we experimentally demonstrated the coherence of this memory concept by encoding different amplitude and phase levels in an integrated photonic circuit [2] and the compatibility with wavelength division multiplexing [3]. This concept is based on a straight waveguide without the need of a structural resonance as an opto-mechanical resonator or a photonic crystal cavity. Therefore, the memory operation can be arbitrarily positioned on the photonic chip and can even be repeated and cascaded several times, via external control light pulses.

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].

Stimulated Brillouin scattering describes a way to excite coherent acoustic phonons solely by optically forces in integrated waveguides. It was shown recently that one can use these acoustic phonons to store and delay optical signals [1], [2]. The optical information is resonantly transferred to a coherent acoustic phonon and is then transferred back to the optical domain by a delayed optical retrieval pulse. The storage time, however, is so far limited to a few nanoseconds, determined by the lifetime of the acoustic phonon. A delay of several nanoseconds might be enough for delaying some high speed signals, however increasing the delay time beyond the acoustic lifetime would enable new possibilities for phonon-based on-chip optical signal processing.

We demonstrate for the first time the storage of multiple phase and amplitude levels of an optical signal as coherent acoustic phonons. The storage concept is implemented on-chip with a GHz-bandwidth.

We experimentally demonstrate the storage of optical pulses (200ps-long, eight amplitude levels) on a photonic chip for over 70 pulse widths with minimum distortions in pulse shape and width using a Brillouin-based memory.

We experimentally demonstrate multiple conversions of information between the optical and acoustic domain. We show the coherent acousto-optic transfer by retrieving multiple amplitude and phase levels with GHz bandwidth and demonstrate multi-wavelength operation.

We report a highly localized SBS response with 780 µm spatial resolution in an As2S3 photonic integrated waveguide. This approach is used to characterize the waveguide based on its acousto-optic properties.

Microwave photonic bandpass filters (MPBPFs) are important building blocks in radio-frequency (RF) signal processing systems. However, most of the reported MPBPFs fail to satisfy the stringent real-world performance metrics, particularly low RF insertion loss. In this paper we report a novel MPBPF scheme using two cascaded integrated silicon nitride (Si3N4) ring resonators, achieving a high link gain in the RF filter passband. In this scheme, one ring operates at an optimal over-coupling condition to enable a strong RF passband whilst an auxiliary ring is used to increase the detected RF signal power via tuning the optical carrier-to-sideband ratio. The unique combination of these two techniques enables compact size as well as high RF performance. Compared to previously reported ring-based MPBPFs, this work achieves a record-high RF gain of 1.8 dB in the passband, with a high spectral resolution of 260 MHz. Furthermore, a multi-band MPBPF with optimized RF gain, tunable central frequencies, and frequency spacing tunability is realized using additional ring resonators, highlighting the scalability and flexibility of this chip-based MPBPF scheme.

Integrated silicon microwave photonics offers great potential in microwave phase shifter elements, and promises compact and scalable multi-element chips that are free from electromagnetic interference. Stimulated Brillouin scattering, which was recently demonstrated in silicon, is a particularly powerful approach to induce a phase shift due to its inherent flexibility, offering an optically controllable and selective phase shift. However, to date, only moderate amounts of Brillouin gain has been achieved and theoretically this would restrict the phase shift to a few tens of degrees, significantly less than the required 360 degrees. Here, we overcome this limitation with a phase enhancement method using RF interference, showing a 360 degrees broadband phase shifter based on Brillouin scattering in a suspended silicon waveguide. We achieve a full 360 degrees phase-shift over a bandwidth of 15 GHz using a phase enhancement factor of 25, thereby enabling practical broadband Brillouin phase shifter for beam forming and other applications., Comment: \copyright 2019 Optical Society of America. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved

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$\mu$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., Comment: 8 pages, 6 figures

Brillouin distributed measurement techniques have been extensively developed for structural health monitoring using fibre optic nerve systems. The recent advancement in the spatial resolution capabilities of correlation-based Brillouin distributed technique have reached the sub-mm regime, making this approach a suitable candidate for monitoring and characterizing integrated photonic devices. The small dimension associated with the short length of these devices—on the order of the cm- and mm-scale—requires high sensitivity detection techniques and sub-mm spatial resolution. In this paper, we provide an overview of the different Brillouin sensing techniques in various micro-scale structures such as photonic crystal fibres, microfibres, and on-chip waveguides. We show how Brillouin sensing is capable of detecting fine transverse geometrical features with the sensitivity of a few nm and also extremely small longitudinal features on the order of a few hundreds of μ m . We focus on the technique of Brillouin optical correlation domain analysis (BOCDA), which enables such high spatial resolution for mapping the opto-acoustic responses of micro-scale waveguides.

We demonstrate for the first time the storage of multiple phase and amplitude levels of an optical signal as coherent acoustic phonons. The storage concept is implemented on-chip with a GHz-bandwidth.

We demonstrate an optoelectronic oscillator based on stimulated Brillouin scattering as an active filter on a photonic chip. An ultrahigh Brillouin gain chalcogenide rib waveguide is used to generate low-noise single-mode 10 GHz RF signals.

We demonstrate the first widely tunable chip-based opto-electronic microwave source based on stimulated Brillouin scattering. The output frequency can be tuned over 40 GHz and the phase-noise performance is comparable to high-range RF sources.

We demonstrate the first integrated multi-wavelength photonic-phononic memory. The memory is based on stimulated Brillouin scattering and enables the storage of light pulses as acoustic phonons up to the acoustic lifetime.

Using the principle of light storage in acoustic waves, we demonstrate the direct measurement of the acoustic decay time in an integrated photonic chip and discuss the relation to the respective Brillouin gain spectrum characterization.

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 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. This approach gives access to local opto-acoustic properties of the waveguides, including the Brillouin frequency shift and linewidth, essential information for the further development of high quality photonic-phononic waveguides for SBS applications.