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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.

The Year 2022 marks 100 years since Leon Brillouin predicted and theoretically described the interaction of optical waves with acoustic waves in a medium. Accordingly, this resonant multi-wave interaction is referred to as Brillouin scattering. Today, Brillouin scattering has found a multitude of applications, ranging from microscopy of biological tissue, remote sensing over many kilometers, and signal processing in compact photonic integrated circuits smaller than the size of a thumbnail. What allows Brillouin scattering to be harnessed over such different length scales and research domains are its unique underlying properties, namely, its narrow linewidth in the MHz range, a frequency shift in the GHz range, large frequency selective gain or loss, frequency tunability, and optical reconfigurability. Brillouin scattering is also a ubiquitous effect that can be observed in many different media, such as freely propagating in gases and liquids, as well as over long lengths of low-loss optical glass fibers or short semiconductor waveguides. A recent trend of Brillouin research focuses on micro-structured waveguides and integrated photonic platforms. The reduction in the size of waveguides allows tailoring the overlap between the optical and acoustic waves and promises many novel applications in a compact footprint. In this review article, we give an overview of the evolution and development of the field of Brillouin scattering over the last one hundred years toward current lines of active research. We provide the reader with a perspective of recent trends and challenges that demand further research efforts and give an outlook toward the future of this exciting and diverse research field.

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.

Non-reciprocal optical components such as isolators and circulators are crucial for preventing catastrophic backreflection and controlling optical cross talk in photonic systems. While non-reciprocal devices based on Brillouin intermodal transitions have been experimentally demonstrated in chip-scale platforms, harnessing such interactions has required a suspended waveguide structure, which is challenging to fabricate and is potentially less robust than a non-suspended structure, thereby limiting the design flexibility. In this paper, we numerically investigate the performance of a Brillouin-based isolation scheme in which a dual-pump-driven optoacoustic interaction is used to excite confined acoustic waves in a traditional ridge waveguide. We find that acoustic confinement, and therefore the amount of Brillouin-driven mode conversion, can be enhanced by selecting an appropriate optical mode pair and waveguide geometry of two arsenic-based chalcogenide platforms. Further, we optimize the isolator design in its entirety, including the input couplers, mode filters, the Brillouin-active waveguide as well as the device fabrication tolerances. We predict such a device can achieve 30 dB isolation over a 38 nm bandwidth when 500 mW pump power is used; in the presence of a ± 10 n m fabrication-induced width error, such isolation can be maintained over a 5–10 nm bandwidth.

Microwave photonics offers a promising solution for frequency converting microwave signals, however, demonstrations so far have either been bulky fibre implementations or lacked rejection of interfering image signals. Here, we demonstrate the first microwave photonic mixer with image rejection of broadband signals utilising chip-based stimulated Brillouin scattering and interferometry. We demonstrate frequency down-conversion of carrier frequencies ranging from 10 GHz-16 GHz, ultra-high image rejection for a single tone of up to 70 dB, and 100 MHz and 400 MHz wide analogue signals with 28.5 dB and 16 dB image rejection, respectively. Furthermore, we down-convert 200 Mb/s quadrature-phase-shift keying signals with an error vector magnitude as low as -9.6 dB when simultaneously present interfering image signals are suppressed by the mixer.

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.

In this Letter, we demonstrate a ${{\rm Si}_3}{{\rm N}_4}$Si

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.

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 (Si

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.