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We show, for the first time, stimulated Brillouin scattering in tellurite covered silicon nitride waveguides. In the standard geometry the measured peak gain coefficient is 4.5 m−1W−1. We further enhance the Brillouin gain by applying cladding engineering, reaching a peak gain coefficient of 8.5 m−1W−1

We investigate a numerical model to simulate the optical frequency comb generation in Kerr and Brillouin active resonators with gain parameters in the same order of magnitude. We show that either a backward or forward comb can be generated directly on a Brillouin laser or a cascaded Brillouin lasing mode respectively for a multi-layered silicon nitride waveguide structure.

Integrated microwave photonics (MWP) is a fast growing area where high frequency microwave signals are processed in the optical domain, merging key advantages of both microwave photonics and photonic integrated circuits (PICs) technologies including low-loss, reconfigurability, advanced functionality, enhanced stability, and reduced footprint. Plenty of functionalities have been demonstrated in integrated MWP, especially based on spectral shaping technique, where the phase and amplitude of the optical spectrum is precisely tailored by PICs. However, on-chip linearization is lagging behind and has not been investigated deeply. It is crucial and urgent to study on-chip linearization methods, which will lead to advanced integrated MWP systems with large spurious-free dynamic range (SFDR). In this paper, we present two novel techniques for on-chip linearization of microwave photonic links. The first technique is based on line-by-line complex spectral shaping using a series of ring resonators. The second technique relies on spatial separation to achieve parallel spectral shaping in two complementary spatial channels. Both methods are demonstrated in low-loss programmable silicon nitride circuits that can already host a number of advanced functionalities. Our results point to the great potential of integrating advanced functionalities and linearization in the same integrated platform.

Coherent optomechanical interaction between acoustic and optical waves known as stimulated Brillouin scattering (SBS) can enable ultra-high resolution signal processing and narrow linewidth lasers important for next generation wireless communications, precision sensing, and quantum information processing. While SBS has recently been studied extensively in integrated waveguides, many implementations rely on complicated fabrication schemes, using suspended waveguides, or non-standard materials such as As$_2$S$_3$. The absence of SBS in standard and mature fabrication platforms prevents large-scale circuit integration and severely limits the potential of this technology. Notably, SBS in standard silicon nitride integration platform is currently rendered out of reach due to the lack of acoustic guiding and the infinitesimal photo-elastic response of the material. In this paper, we experimentally demonstrate advanced control of backward SBS in multilayer silicon nitride waveguides. By optimizing the separation between two silicon nitride layers, we unlock gigahertz acoustic waveguiding in this platform for the first time, leading up to 15 $\times$ higher SBS gain coefficient than previously possible in silicon nitride waveguides. Using the same principle, we experimentally demonstrate on-demand inhibition of SBS by preventing acoustic guiding in the waveguide platform. We utilize the enhanced SBS gain to demonstrate a microwave photonic notch filter with high rejection (30 dB). We accomplish this in a low-loss, standard, and versatile silicon nitride integration platform without the need of suspending the SBS-active waveguide or hybrid integration with other materials., Comment: 21 pages, 5 figures

Fully integrated supercontinuum sources on-chip are critical to enabling applications such as portable and mechanically stable medical imaging devices, chemical sensing, and light detection and ranging. However, the low efficiency of current supercontinuum generation schemes prevents full on-chip integration. Herein, a scheme where the input energy requirements for integrated supercontinuum generation are drastically lowered by orders of magnitude is presented, for bandwidth generation of the order of 500–1000 nm. Through sign-alternating dispersion in a CMOS-compatible silicon nitride waveguide, an efficiency enhancement by factors reaching 2800 is achieved. It is shown that the pulse energy requirement for large-bandwidth supercontinuum generation at high spectral power (e.g., 1/e level) is lowered from nanojoules to 6 picojoules. The lowered pulse energy requirements enable that chip-integrated laser sources, such as mode-locked heterogeneously or hybrid-integrated diode lasers, can be used as a pump source, enabling fully integrated on-chip high-bandwidth supercontinuum sources.

We highlight a recent trend in photonics where integrated photonic chips are designed and operated as programmable devices like in the case of integrated electronics. We discuss the potential application of these programmable photonic circuits in signal processing.

Integrated mode-locked lasers are useful tools in microwave photonic applications as a local oscillator. In particular, hybrid integrated lasers could easily be integrated with passive processing circuits. In this Letter, we report on the self-mode-locking of a hybrid integrated laser comprising two indium phosphide gain sections and a silicon nitride feedback circuit that filters light using two ring resonators. The hybrid laser is shown to mode-lock and to have a mostly frequency-modulated field in the cavity using a stepped-heterodyne optical complex spectrum analysis. A mostly frequency modulated field output is good for high powers per line due to a more continuous emission, compared with mode-locked lasers using a saturable absorber; additionally, the filter limits the bandwidth of the comb, condensing the power to the fewer comb lines.

We demonstrated, for the first time, an integrated microwave photonic (MWP) circuit exhibiting simultaneous tunable notch filtering and linearization, leading to a high performance filter with enhanced linearity and spurious free dynamic range (SFDR). We unlock this unique feature by manipulating the phase and amplitude of multi-order RF modulation sidebands using a versatile MWP spectral shaping circuit. Through a series of experiments, we show a linearized notch filter that exhibits a stopband rejection of 53 dB, more than 32 dB reduction of third-order intermodulation distortion in the passband, and an enhanced SFDR of 119 dB⋅Hz2/3. Our demonstration breaks the conventional and fragmented approach of functionality and linearization in integrated circuit and will potentially stimulate the implementation of integrated MWP system with advanced functionalities and high performance in the future.

Integrated microwave photonic (MWP) signal processor relies on the integration of multiple optical circuits, advanced functionalities, and high-performance for radio frequency (RF) applications. In this paper, we experimentally investigated a new kind of an on-chip MWP signal processor in silicon nitride, containing of a Modulation Transformer (MT) and a Double-Injection Ring Resonator (DIRR). This optical circuit enables the synthesis of reconfigurable spectral shaping for advanced functionality unmatched by conventional MWP circuits.

For microwave photonics (MWP), integrated mode-locked lasers can find applications as a local oscillator. Using hybrid intregrated lasers, it is possible to directly integrate such a light source with a signal processing circuit. In this paper we show the mode-locking of a hybrid integrated laser designed for continuous wave operation. Due to the filter present in the laser cavity, the laser outputs narrow combs preserving high power per line, which is a key requirement in many MWP-related applications. The filter also serves as a tunable delay, which can be used to tune the line spacing of the optical frequency comb. We show a 573 MHz tuning range of the fundamental beat note of the optical frequency comb, between 1.9 and 2.5 GHz. This wide tuning range, adds flexibility to the modelocked laser without needing a driving radio frequency source to modulate the optics as local oscillator.

There are a host of applications in communications, sensing, and science, in which analogue signal transmission is preferred over today’s dominant digital transmission. In some of these applications, the advantage is in lower cost, while in others, it lies in superior performance. However, especially for longer analogue photonics links (up to 10 s of km), the performance is strongly limited by the impairments arising from using standard single-mode fibres (SSMF). Firstly, the three key metrics of analogue links (loss, noise figure, and dynamic range) tend to improve with received power, but this is limited by stimulated Brillouin scattering in SSMF. Further degradation is due to the chromatic dispersion of SSMF, which induces radio-frequency (RF) signal fading, increases even-order distortions, and causes phase-to-intensity-noise conversion. Further distortions still, are caused by the Kerr nonlinearity of SSMF. We propose to address all of these shortcomings by replacing SSMFs with hollow-core optical fibres, which have simultaneously six times lower chromatic dispersion and several orders of magnitude lower nonlinearity (Brillouin, Kerr). We demonstrate the advantages in this application using a 7.7 km long hollow-core fibre sample, significantly surpassing the performance of an SSMF link in virtually every metric, including 15 dB higher link gain and 6 dB lower noise figure.

We investigated stimulated Brillouin scattering (SBS) in two different multilayer silicon nitride waveguides. We report the first observation of SBS in multilayer silicon nitride waveguides. The gain coefficient measured is 0.16m−1W−1 in one of our samples, but SBS gain is absent in the other sample. We developed a simulation method, which shows how the bigger intermediate layer of 500 nm results in high gain, and the smaller layer of 100 nm results in lower gain. We can use these simulations to optimize our waveguide design. This will allow us to functionalize SBS in silicon nitride waveguides.

Microwave signal filtering is a fundamental and central functionality in radio-frequency (RF) systems. Underpinned by advanced integrated photonics technologies, emerging integrated microwave photonic (IMWP) filter platforms enable reconfigurable and widely tunable RF signal filtering functionalities that were unattainable using conventional electronics while also exhibiting superior features in terms of compactness, light weight, stability, low power consumption, and low latency. This paper presents a comprehensive review of the principles, architectures, and performance of IMWP filters. We highlight recent advances of IMWP filters enabled by on-chip nonlinear optics, RF-interference technology and emerging integration platforms, with an emphasis on the RF performance which is critical for their usability in real-world applications. We conclude with a perspective on future research challenges and new possibilities for IMWP filters.

Microwave photonic (MWP) filters using on-chip stimulated Brillouin scattering offer great potential for next-generation radio frequency (RF) applications due to the unprecedented high spectral resolution, flexible programmability, and ultra-wideband frequency agility. Although impressive functionalities have been reported, limited link performance, due to the amplification noise induced by the Brillouin gain process, hinders the practical deployment in existing RF applications. In this paper, we present the first comprehensive numerical and experimental study of chip-based Brillouin MWP bandpass filters implemented in a range of link configurations. In the experiments, key RF performance figures of merit, including link gain, noise figure, passband resolution, bandwidth tunability, and passband extinction of chip-based Brillouin filters are experimentally investigated and numerically analyzed. This comprehensive study points out a preferable filter scheme with optimized system-level performance. The numerical model developed in this paper allows for extrapolations to further improve the performance. The systematic work establishes a feasible design route for achieving high link-performance chip-based Brillouin microwave photonic bandpass filters.

One of the key functionalities in microwave photonics is to be able to define controllable time delays during the signal processing. Optical fibers are often used to achieve this functionality, especially when a long delay or a widely-tunable delay is needed. However, the stability of this delay in the presence of environmental changes (e.g., temperature) has not, to the best of our knowledge, been reviewed yet. Here, we firstly discuss the impact of temperature-induced variations on the signal propagation time in optical fibers and its implications in microwave photonics. We compare the impact of the thermal sensitivity of various delay lines for applications in which the signal is transported from point A to point B, as well as for applications in which the propagation time through a fiber or the fiber dispersion is used to create a fixed or tunable delay. In the second part of the article we show the impact of fiber thermal sensitivity on a narrow-band microwave photonics filter made of standard single mode fiber (SSMF) and a hollow core fiber (HCF), which has significantly lower thermal sensitivity of propagation time to temperature. The central frequency of the band-pass filter changes almost 16 times more in the filter made of SSMF as compared to that of HCF, dictating very tight (0.05 °C) temperature stabilization for SSMF-based filters. On the basis of our thermal sensitivity analysis we conclude that HCFs are very promising for environmentally stable microwave photonics applications.

Optical modulation plays arguably the utmost important role in microwave photonic (MWP) systems. Precise synthesis of modulated opti- cal spectra dictates virtually all aspects of MWP system quality including loss, noise figure, linearity, and types of functionalities that can be executed. However, for such a critical function, the versatility to generate and transform analog optical modulation is severely lacking, blocking the pathways to truly unique MWP functions including ultra-linear links and low-loss high rejection filters. Here, we demonstrate a versatile radiofrequency (RF) photonic spectral shaper integrated in a silicon photonic circuit. The spectral shaper controls the two modu- lation bands generated from an electro-optic modulation process in their relative amplitude and phase, offering an enhanced versatility for microwave-photonic modulation applications. Using the spectral shaper, we show electrically tailorable modulation transformations. Fur- thermore, we show a series of unprecedented RF filtering experiments through monolithic integration of the spectral shaper with a network of reconfigurable ring resonators.

Integrated microwave photonics (MWP) is a rapidly growing field where high frequency microwave signals are processed in the optical domain, bringing key advantages of both microwave photonics and integrated optics technologies including low-losses, reconfigurability, advanced functionality, enhanced performance, and reduced footprint. The MWP spectrum generated from optical modulation carry utmost importance for the functionalities and performance of integrated MWP systems. Thus, precise phase and amplitude shaping of the MWP spectrum can open avenues to advanced functionalities presently unattainable. In this tutorial, we present a new concept of integrated MWP spectral shaping covering the basic principles, theory, and implementations of MWP spectral shaping for modulation transformation, reconfigurable filters, link linearization, and RF waveform generation.

An on-chip linearization method for phase modulated microwave photonic link based on integrated ring resonators is proposed. By properly tailoring the phase and amplitude of optical carrier band and second-order sidebands, the third-order intermodulation distortion (IMD3) components can be suppressed. Theoretical analysis are taken and a proof-of-concept experiment is carried out. Experimental results demonstrate that IMD3 is suppressed by 21.7 dB. When the noise of the link is properly optimized, an SFDR of 112.7 dB·Hz2/3 can be achieved. This opens the possibility of integrating linearization into a functional photonic integrated circuit.

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

A silicon nitride chip-based tunable microwave photonic notch filter based on phase-to-intensity modulation conversion with dual-sideband processing is proposed. Furthermore, an integrated optical carrier filtering is added to enhance the noise performance of the filter.

We discuss a new concept in integrated microwave photonics (MWP) aiming at complex phase and amplitude tailoring of RF modulated spectrum for advanced functionalities including modulation transformation and all-optimized MWP filtering.

Integrated microwave photonics (IMWP) is poised to release the bottlenecks in modern wireless communication systems. The manipulation of microwave signals in the optical domain offers key advantages of broad bandwidth, reconfiguration, and fast tuning speeds. However, in current IMWP devices, there are some challenges that need to be overcome. These include the ~1 GHz frequency resolution of the IMWP functionalities, limited by the on-chip photonic functional devices' performance, which has prompted research into on-chip stimulated Brillouin scattering (SBS) to achieve sub-30 MHz signal processing capabilities. Equally important, the performance metrics including the noise figure, dynamic range, and the insertion loss, need to be improved before commercial deployment. While SBS offers significant advantages of high resolution and reconfigurability, there is potential for improved signal-to-noise ratios and, therefore, to obtain a low noise figure. In this letter, we present an overview of recent approaches for achieving high-performance IMWP functionalities, including SBS-induced noise management and the optimized MWP link configurations.

Simultaneously achieving high link performance and advanced microwave photonic processing functionalities in compact integrated circuits is crucial but challenging for the deployment in future and existing practical applications. In this paper, we present the first comprehensive experimental study to optimize the link performance of a Si 3 N 4 -photonic-chip-based microwave photonic notch filter. This systematic study leads to an optimized link performance of the chip-based filter, demonstrating an RF net gain, a sub-20-dB noise figure and an overall third-order spurious-free dynamic range of 115 dB.Hz 2/3 over a frequency range of 0-12 GHz, in conjunction with advanced filtering functionalities with a notch rejection >50 dB. The achieved performance is based on a unique and low-loss chip-based filter scheme which offers the compatibility with existing link performance optimization techniques, while maintaining the advanced notch filter functionality. Numerical calculations are also performed to explore the future feasibility of implementing a fully integrated microwave photonic filter that approaches the same level of link performance demonstrated in this work. This study is expected to provide a feasible design route to approach fully integrated microwave photonic filters with high link performance.

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). Self-coherent optical orthogonal frequency-division multiplexing (self-CO-OFDM) is a modern technology that retrieves the frequency and phase information from the extracted carrier without employing a LO or additional DSP. However, a wide carrier guard is typically required to easily filter out the optical carrier at the receiver, thus discarding many OFDM middle subcarriers that limit the system data rate. Here, we establish an optical technique for carrier recovery, harnessing large-gain stimulated Brillouin scattering (SBS) on a photonic chip for up to 116.82 Gbit·s−1 self-CO-OFDM signals, without requiring a separate LO. The narrow SBS linewidth allows for a record-breaking small carrier guard band of ∼265 MHz in self-CO-OFDM, resulting in higher capacity than benchmark self-coherent multi-carrier schemes. Chip-based SBS-self-coherent technology reveals comparable performance to state-of-the-art coherent optical receivers while relaxing the requirements of the DSP. In contrast to on-fiber SBS processing, our solution provides phase and polarization stability. Our demonstration develops a low-noise and frequency-tracking filter that synchronously regenerates a low-power narrowband optical tone, which could relax the requirements on very-high-order modulation signaling for future communication networks. The proposed hybrid carrier filtering-and-regeneration technique could be useful in long-baseline interferometry for precision optical timing or reconstructing a reference tone for quantum-state measurements.

An overview of the most recent developments and improvements to the low-loss TriPleX Si3N4 waveguide technology is presented in this paper. The TriPleX platform provides a suite of waveguide geometries (box, double stripe, symmetric single stripe, and asymmetric double stripe) that can be combined to design complex functional circuits, but more important are manufactured in a single monolithic process flow to create a compact photonic integrated circuit. All functionalities of the integrated circuit are constructed using standard basic building blocks, namely straight and bent waveguides, splitters/combiners and couplers, spot size converters, and phase tuning elements. The basic functionalities that have been realized are: ring resonators and Mach–Zehnder interferometer filters, tunable delay elements, and waveguide switches. Combination of these basic functionalities evolves into more complex functions such as higher order filters, beamforming networks, and fully programmable architectures. Introduction of the active InP chip platform in a combination with the TriPleX will introduce light generation, modulation, and detection to the low-loss platform. This hybrid integration strategy enables fabrication of tunable lasers, fully integrated filters, and optical beamforming networks.

Manipulating radio frequency (RF) signals in integrated photonic devices has recently emerged as a new paradigm for wireless communications, enabling high-frequency signal processing and broadband frequency agility in miniaturized photonic platforms. These advances are crucial for space and airborne applications which are weight and size sensitive. Recent progress in on-chip stimulated Brillouin scattering (SBS) using high gain has shown great potentials for RF photonic signal processing, owing to its inherent advantages of high resolution, high reconfigurability, and flexible programmability. On-chip SBS serves as a versatile and high-performance toolbox to process and manipulate the amplitude and phase of RF signals in the optical domain. In this paper, we provide a brief overview of recent advances achieved using SBS in planar waveguides, with a focus on applications requiring phase control of RF signals, including phase shifters, delay lines, and separate carrier tuning. We introduce a power-efficient scheme based on the concept of phase amplification to improve Brillouin-based phase processing capability.

Processing of microwave signals using photonics has several key advantages for applications in wireless communications. However, to bring photonic-based microwave signal processing to the mainstream requires a reduction of the form factor. Integration is a route for achieving high-performance, low-cost, and small-footprint microwave photonic devices. A high on-chip stimulated Brillouin scattering (SBS) gain is essential for synthesizing several key functionalities for advanced integrated microwave signal processing. We have optimized our on-chip SBS platform to achieve a record on-chip gain of 52 dB. In this paper, we discuss the implications of this giant gain from the viewpoint of new enabled technologies. The giant gain can be distributed over wide frequencies, which can be exploited for the realization of reconfigurable microwave bandpass, bandstop, and multiband filters. High gain also enables the demonstration of low-threshold on-chip lasers, which can be of relevance for a low-noise radio-frequency signal generation. These wide ranges of functionalities are made possible by the breakthrough on-chip gain makes Brillouin-based microwave photonic signal processing a promising approach for real-world implementation in the near future.

The thirty-two papers in this special issue focus on the field of programmable photonics, one of the fastest growing fields in photonics with applications ranging from optical signal processing and computing to quantum photonics.

Hybrid integrated semiconductor laser sources offering extremely narrow spectral linewidth as well as compatibility for embedding into integrated photonic circuits are of high importance for a wide range of applications. We present an overview on our recently developed hybrid-integrated diode lasers with feedback from low-loss silicon nitride (Si3N4 in SiO2) circuits, to provide sub-100-Hz-level intrinsic linewidths, up to 120 nm spectral coverage around 1.55 um wavelength, and an output power above 100 mW. We show dual-wavelength operation, dual-gain operation, laser frequency comb generation, and present work towards realizing a visible-light hybrid integrated diode laser., 25 pages, 16 figures

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

Signal processing using on-chip nonlinear or linear optical effects has shown tremendous potential for RF photonic applications. Combining nonlinear and linear elements on the same photonic chip can further enable advanced functionality and enhanced system performance in a robust and compact form. However, the integration of nonlinear and linear optical signal processing units remains challenging due to the competing and demanding waveguide requirements, specifically the combination of high optical nonlinearity in single-pass waveguides, which is desirable for broadband signal processing with low linear loss and negligible nonlinear distortions required for linear signal processing. Here, we report the first demonstration of integrating Brillouin-active waveguides and passive ring resonators on the same integrated photonic chip, enabling an integrated microwave photonic notch filter with ultradeep stopband suppressions of >40 dB, a low filter passband loss of