Integrable microwave filter based on a photonic crystal delay line

Multimode-enabled silicon photonic delay lines: break the delay-density limit

Integrated optical delay lines have become imperative to meet the growing demand as large aperture antennas and high number of subarrays required for microwave beamforming, high-speed optical communication, and integrated quantum photonics. It is very challenging to achieve large delay ranges, small footprints, and broad bandwidths simultaneously due to the strong trade-off between the propagation loss and the group refractive index of optical waveguides. In this paper, we propose and experimentally demonstrate multimode-enabled silicon photonic delay line for the first time, which breaks the delay-density limit of singlemode waveguide spirals, towards a broadband, mm2-scale, and ultra-large time delay. By demonstrating low-loss-propagation possibilities for different polarizations and modes, we introduce a novel multimode delay unit by integrating the mode (de)multiplexers and the ultralow-loss multimode waveguide spiral supporting the TE0, TE1, and TE2 modes propagating in parallel. The measured propagation losses for the TE0, TE1, and TE2 modes are 0.2 dB/cm, 0.31 dB/cm, and 0.49 dB/cm, respectively. In this way, the highest line delay-density of 376.9 ps/cm and low delay loss of 0.004 dB/ps are achieved. Furthermore, we implement a 7-bit tunable multimode photonic delay line and experimentally demonstrate an ultra-large delay range of 12.7 ns with a delay resolution of 100 ps and within an ultra-compact footprint of 3.85 mm2, enabling a delay density over 3299 ps/mm2, showing the largest delay range and the highest delay density among on-chip delay lines reported to date, to the best of our knowledge.

An Approach to Reduce Tuning Sensitivity in the PIC-Based Optoelectronic Oscillator by Controlling the Phase Shift in Its Feedback Loop

Radio photonic technologies have emerged as a promising solution for addressing microwave frequency synthesis challenges in current and future communication and sensing systems. One particularly effective approach is the optoelectronic oscillator (OEO), a simple and cost-effective electro-optical system. The OEO can generate microwave signals with low phase noise and high oscillation frequencies, often outperforming traditional electrical methods. However, a notable disadvantage of the OEO compared to conventional signal generation methods is its significant frequency tuning step. This paper presents a novel approach for continuously controlling the output frequency of an optoelectronic oscillator (OEO) based on integrated photonics. This is achieved by tuning an integrated optical delay line within a feedback loop. The analytical model developed in this study calculates the OEO's output frequency while accounting for nonlinear errors, enabling the consideration of various control schemes. Specifically, this study examines delay lines based on the Mach-Zehnder interferometer and microring resonators, which can be controlled by either the thermo-optic or electro-optic effect. To evaluate the model, we conducted numerical simulations using Ansys Lumerical software. The OEO that utilized an MRR-based electro-optical delay line demonstrated a tuning sensitivity of 174.5 MHz/V. The calculated frequency tuning sensitivity was as low as 6.98 kHz when utilizing the precision digital-to-analog converter with a minimum output voltage step of 40 μV. The proposed approach to controlling the frequency of the OEO can be implemented using discrete optical components; however, this approach restricts the minimum frequency tuning sensitivity. It provides an additional degree of freedom for frequency tuning within the OEO's operating range, which is ultimately limited by the amplitude-frequency characteristic of the notch filter. Thus, the proposed approach opens up new opportunities for increasing the accuracy and flexibility in generating microwave signals, which can be significant for various communications and radio engineering applications.

All optical tunable RF filter using elemental antimony

In the past decade, the proliferation of modern telecommunication technologies, including 5G, and the widespread adoption of the Internet-of-things (IoT) have led to an unprecedented surge in data generation and transmission. This surge has created an escalating demand for advanced signal processing capabilities. Microwave photonic (MWP) processors offer a promising solution to satisfy this unprecedented demand for data processing by capitalising on the high bandwidth and low latency achievable by optical systems. In this work, we introduce an integrated MWP processing unit for all-optical RF filtering using elemental antimony. We exploit the crystallisation dynamics of antimony to demonstrate a photonic leaky integrator, which is configured to operate as a first-order low-pass filter with a bandwidth of 300 kHz and ultra-compact footprint of 16 × 16 μm2. We experimentally demonstrate the implementation of such a filter as an envelope detector to demodulate an amplitude-modulated signal. Finally, a discussion on achieving bandwidth tunability is presented.

Continuously tunable silicon waveguide optical switched delay line based on grating-assisted contradirectional coupler

The integrated optical delay line plays a crucial role in microwave photonic chips. Continuous tunability is a growing trend in filtering and beamforming techniques of microwave photonics. Based on the silicon platform, we present and experimentally demonstrate an integrated continuously optical tunable delay line (OTDL) chip, which contains a 4-bit optical switched delay line (OSDL) and a thermally tunable delay line based on grating-assisted Contradirectional coupler (CDC). The OSDL can achieve stepwise optical delays, while the CDC is introduced to improve delay tuning resolution within one step delay of the OSDL. The combination of the two modules can realize tuning delays from 0 to 160 ps. Additionally, it is easy to increase the maximum delay by cascading more optical switches. The experimental results demonstrate that the proposed OTDL shows outstanding performance and good expansibility.

Integrated lithium niobate microwave photonic processing engine

Integrated microwave photonics (MWP) is an intriguing technology for the generation, transmission and manipulation of microwave signals in chip-scale optical systems1,2. In particular, ultrafast processing of analogue signals in the optical domain with high fidelity and low latency could enable a variety of applications such as MWP filters3-5, microwave signal processing6-9 and image recognition10,11. An ideal integrated MWP processing platform should have both an efficient and high-speed electro-optic modulation block to faithfully perform microwave-optic conversion at low power and also a low-loss functional photonic network to implement various signal-processing tasks. Moreover, large-scale, low-cost manufacturability is required to monolithically integrate the two building blocks on the same chip. Here we demonstrate such an integrated MWP processing engine based on a 4 inch wafer-scale thin-film lithium niobate platform. It can perform multipurpose tasks with processing bandwidths of up to 67 GHz at complementary metal-oxide-semiconductor (CMOS)-compatible voltages. We achieve ultrafast analogue computation, namely temporal integration and differentiation, at sampling rates of up to 256 giga samples per second, and deploy these functions to showcase three proof-of-concept applications: solving ordinary differential equations, generating ultra-wideband signals and detecting edges in images. We further leverage the image edge detector to realize a photonic-assisted image segmentation model that can effectively outline the boundaries of melanoma lesion in medical diagnostic images. Our ultrafast lithium niobate MWP engine could provide compact, low-latency and cost-effective solutions for future wireless communications, high-resolution radar and photonic artificial intelligence.

Integrated microwave photonic notch filter using a heterogeneously integrated Brillouin and active-silicon photonic circuit

Microwave photonics (MWP) has unlocked a new paradigm for Radio Frequency (RF) signal processing by harnessing the inherent broadband and tunable nature of photonic components. Despite numerous efforts made to implement integrated MWP filters, a key RF processing functionality, it remains a long-standing challenge to achieve a fully integrated photonic circuit that can merge the megahertz-level spectral resolution required for RF applications with key electro-optic components. Here, we overcome this challenge by introducing a compact 5 mm × 5 mm chip-scale MWP filter with active E-O components, demonstrating 37 MHz spectral resolution. We achieved this device by heterogeneously integrating chalcogenide waveguides, which provide Brillouin gain, in a complementary metal-oxide-semiconductor (CMOS) foundry-manufactured silicon photonic chip containing integrated modulators and photodetectors. This work paves the way towards a new generation of compact, high-resolution RF photonic filters with wideband frequency tunability demanded by future applications, such as air and spaceborne RF communication payloads.

Robust reconfigurable radiofrequency photonic filters based on a single silicon in-phase/quadrature modulator

Combining integrated photonics and radiofrequency (RF) signals in the optical domain can help overcome the limitations of traditional RF systems. However, it is challenging to achieve environmentally insensitive filtering in wireless communications using integration schemes. In this report, the performance of robust RF filters based on a single silicon in-phase/quadrature modulator with significantly improved temperature and optical carrier wavelength sensitivities, which were suppressed by more than three orders of magnitude compared with those of silicon resonators, was experimentally evaluated. Upconversion and the processing of signals were simultaneously realized on the modulator by setting the relative phases of the arms and the bias voltages. Moreover, the filters can be reconfigured as low-pass, high-pass, band-pass, or band-stop filters. From 25 to 75 °C, the center frequency variation was within 0.2 GHz. From 1500 to 1600 nm, the center frequency variation was within 2 GHz. The proposed scheme allows for filtering and reconfiguration without the use of optical processing modules such as resonators or delay lines, thus providing a novel approach to signal processing and a new robust filter for scenarios with dynamic environments.

Continuously tunable photonic true-time-delay device for millimeter-wave beamforming

We present a novel CMOS compatible plasma dispersion modulation scheme for slow wave photonic true-time-delay structure harnessing the frozen mode to enable applications in millimeter-wave (mmWave) beamforming. Leveraging the Soref-Bennett model for the electro-refractive effect in silicon plasma dispersion, continuous tunability of approximately 6.8 ps/V with a peak delay of approximately 11.4 ps is achieved for a low threshold voltage of 0.9 V. This plasma dispersion will enable fast and sophisticated modulation and beamforming in 5G mmWave and 6G terahertz communications.

Dispersion-Diversity Multicore Fiber Signal Processing

Beyond playing a primary role in high-capacity communication networks, multicore optical fibers can bring many advantages to optical and microwave signal processing, as not only space but also chromatic dispersion are introduced as new degrees of freedom. The key lies in developing radically new multicore fibers where the refractive index profile of each individual core is tailored properly to provide parallel dispersion-diversity signal processing with application in a variety of scenarios such as parallel channel equalization, analogue-to-digital conversion, optical computing, pulse generation and shaping, multiparameter fiber sensing, medical imaging, optical coherence tomography, broadband measurement instrumentation, and next-generation fiber-wireless communications. Here, we experimentally prove, for the first time to our knowledge, reconfigurable two-dimensional dispersion-managed signal processing performed by a novel dispersion-diversity heterogeneous multicore fiber. The fiber comprises seven different trench-assisted cores featuring a different refractive index profile in terms of both radial geometry and core dopant concentration. As a representative application case, we demonstrate reconfigurable microwave signal filtering with increased compactness as well as performance flexibility and versatility as compared to previous technologies.

Low loss silicon nitride 1×4 microwave photonic beamforming chip

In this paper, based on the low loss double strip silicon nitride platform, we designed and fabricated an ultra-low loss 1×4 microwave photonic beamforming chip, which contains a 1×4 beam splitter and four 5-bit optical delay lines. Each optical delay line can achieve 32 delay states varying from 0 ps to about 130 ps, which can support 21 different beamforming angles covers from -56.42° to 56.68° for 10 GHz RF signal. A low on-chip insertion loss of about 4 dB is achieved for each 5-bit optical delay line. Furthermore, a very low loss delay ratio of about 0.0016 dB/ps is achieved and a recorded low loss fluctuation of about 0.3 dB is obtained during the 32 states delay switching. In addition, the switching speed and driving power consumptions of the proposed beamforming chip were investigated. The proposed beamforming chip could have great potential in optical controlled phased antenna arrays systems.

Microcomb-driven silicon photonic systems

Microcombs have sparked a surge of applications over the past decade, ranging from optical communications to metrology1-4. Despite their diverse deployment, most microcomb-based systems rely on a large amount of bulky elements and equipment to fulfil their desired functions, which is complicated, expensive and power consuming. By contrast, foundry-based silicon photonics (SiPh) has had remarkable success in providing versatile functionality in a scalable and low-cost manner5-7, but its available chip-based light sources lack the capacity for parallelization, which limits the scope of SiPh applications. Here we combine these two technologies by using a power-efficient and operationally simple aluminium-gallium-arsenide-on-insulator microcomb source to drive complementary metal-oxide-semiconductor SiPh engines. We present two important chip-scale photonic systems for optical data transmission and microwave photonics, respectively. A microcomb-based integrated photonic data link is demonstrated, based on a pulse-amplitude four-level modulation scheme with a two-terabit-per-second aggregate rate, and a highly reconfigurable microwave photonic filter with a high level of integration is constructed using a time-stretch approach. Such synergy of a microcomb and SiPh integrated components is an essential step towards the next generation of fully integrated photonic systems.

Tunable plasmonically induced transparency with giant group delay in gain-assisted graphene metamaterials

We propose a graphene metamaterial consisting of several layers of longitudinally separated graphene nanoribbon array embedded into gain-assisted medium, demonstrating electromagnetically induced transparency-like spectra. Combined with finite-difference time-domain simulations, the transfer matrix method and temporal coupled-mode theory are adopted to quantitatively describe its transmission characteristics. These transmission characteristics can be tuned by altering the gain level in medium layer and the Fermi energy level in graphene. Additionally, it is the incorporation between gain medium and graphene nanoribbons with optimized geometrical parameters and Fermi energy level that the destructive interference between high order graphene plasmonic modes can be obtained, suggesting drastic phase transition with giant group delay and ultra-high group index up to 180 ps and 10⁴, respectively. Our results can achieve efficient slow light effects for better optical buffers and other nonlinear applications.

Reconfigurable microwave photonic filter based on a quantum dash mode-locked laser

We demonstrate a reconfigurable microwave photonic (MWP) filter using a quantum dash (QDash) mode-locked laser (MLL) that can generate an optical frequency comb (OFC) with ∼50 comb lines and a free spectral range of 25 GHz. Thanks to the large number of comb lines, the MWP filter responses can be easily programmed by tailoring the OFC spectrum. We implement MWP filter responses with Gaussian, sinc, flat-top, and multiple peaks, as well as demonstrate that tuning of the central frequency. We achieve a minimum 3 dB bandwidth of ∼100 MHz for a sinc-shaped MWP filter, while the maximum out-of-band rejection can be up to ∼30 dB with Gaussian apodization. Our results show that the QDash-MLL is a promising OFC source for developing integrated and reconfigurable MWP filters.

SNAP microwave optical filters

If the originally flat bottom of a wide quantum well with multiple eigenstates is periodically modulated, its eigenvalues rearrange into denser groups separated by wider gaps. We show that this effect, if implemented in an elongated bottle microresonator [also called a surface nanoscale axial photonics (SNAP) microresonator] allows us to design microwave photonic tunable filters with an outstanding performance.

Improving Low-Dispersion Bandwidth of the Silicon Photonic Crystal Waveguides for Ultrafast Integrated Photonics

We design a novel slow-light silicon photonic crystal waveguide which can operate over an extremely wide flat band for ultrafast integrated nonlinear photonics. By conveniently adjusting the radii and positions of the second air-holes rows, a flat slow-light low-dispersion band of 50 nm is achieved numerically. Such a slow-light photonic crystal waveguide with large flat low-dispersion wideband will pave the way for governing the femtosecond pulses in integrated nonlinear photonic platforms based on CMOS technology.

Broadband continuously tunable microwave photonic delay line based on cascaded silicon microrings

We present a novel broadband continuously tunable microwave photonic delay line consisting of a modulator, a four-stage microring resonator delay line, a tunable optical bandpass filter, and a photodetector. Unlike the traditional microring delay lines working at the on-resonant wavelength, the microring resonators in our chip work at the anti-resonant wavelengths, leading to a large delay bandwidth and a small delay ripple. The experimental results show that relative group delay can be continuously tuned from 0 to 160 ps for microwave frequencies in the range of 0 to 16 GHz. The delay ripple is less than 6.2 ps. These results represent an important step towards the realization of integrated continuously tunable delay lines demanded in broadband microwave phased array antennas.

Reconfigurable radiofrequency filters based on versatile soliton microcombs

The rapidly maturing integrated Kerr microcombs show significant potential for microwave photonics. Yet, state-of-the-art microcomb-based radiofrequency filters have required programmable pulse shapers, which inevitably increase the system cost, footprint, and complexity. Here, by leveraging the smooth spectral envelope of single solitons, we demonstrate microcomb-based radiofrequency filters free from any additional pulse shaping. More importantly, we achieve all-optical reconfiguration of the radiofrequency filters by exploiting the intrinsically rich soliton configurations. Specifically, we harness the perfect soliton crystals to multiply the comb spacing thereby dividing the filter passband frequencies. Also, the versatile spectral interference patterns of two solitons enable wide reconfigurability of filter passband frequencies, according to their relative azimuthal angles within the round-trip. The proposed schemes demand neither an interferometric setup nor another pulse shaper for filter reconfiguration, providing a simplified synthesis of widely reconfigurable microcomb-based radiofrequency filters.

For microcomb-based radiofrequency filters pulse shapers are required, which increase the system cost, footprint, and complexity. Here, the authors bypass this need by exploiting versatile soliton states inherent in microresonator and achieve reconfigurable radiofrequency filters.

Photonic integrated field-programmable disk array signal processor

Thanks to the nature of strong programmability, field-programmable gate arrays (FPGAs) have been playing a significant role in signal processing and control. With the explosive growth in digital data, big data analytics becomes an important emerging field, in which FPGAs are a major player. However, the computational speed and power efficiency provided by FPGAs are limited by electronic clock rates and Ohmic losses. To overcome the limitations, photonics is envisioned as an enabling solution, thanks to its ultrafast and low power consumption feature. In this paper, we propose a scalable photonic field-programmable disk array (FPDA) signal processor. Ultra-compact microdisk resonators are leveraged as a fundamental execution units in the core to route, store and process optical signals. By field-programming the processor, diverse circuit topologies can be realized to perform multiple specific signal processing functions including filtering, temporal differentiation, time delay, beamforming, and spectral shaping.

Positive link gain microwave photonic bandpass filter using Si3N4-ring-enabled sideband filtering and carrier suppression

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.

Large-capacity and low-loss integrated optical buffer

Temporarily storing light occupies a pivotal position in all-optical packet switching network and microwave photonics. An integrated optical buffer with large capacity and low loss is demonstrated on a silica wafer. The optical buffer consists of four silica waveguide delay lines connected by five thermo-optic switches. With different switch combinations applied, related delay lines are selected to realize a different storage time in the buffer, and a storage time up to 100 ns with a 10-ns step size is implemented. By optimizing the fabrication process and introducing the offsets between straight and bending waveguides, the propagation loss as low as ~1.08 dB/m is achieved. This large-capacity and low-loss buffer enables broad applications in optical communications and microwave photonics.

Silica-microsphere-cavity-based microwave photonic notch filter with ultra-narrow bandwidth and high peak rejection

We propose and experimentally demonstrate a microwave photonic (MWP) notch filter based on a silica microsphere cavity. By using a high-Q-factor (∼1e7) cavity with a diameter of 132 um, the filter bandwidth can be easily decreased to 15 MHz in terms of simple fabrication and flexible coupling. Then we use the advanced modulation technique based on a dual parallel Mach-Zehnder modulator to further improve peak rejection (PR). The experimental results show that the MWP notch filter with its PR beyond 55 dB and frequency tunability range over 8 GHz has been achieved in combination with double-sideband modulation. To the best of our knowledge, this is a record for PR and bandwidth considered simultaneously for an MWP filter based on silica microcavities. Thus, the proposed MWP filter will be useful in the fields of microwave photonic signal processing, radar systems, etc.

On-chip silicon photonic integrated frequency-tunable bandpass microwave photonic filter

An on-chip frequency-tunable bandpass microwave photonic filter (MPF) implemented on a silicon photonic platform is reported. The on-chip MPF consists of a high-speed phase modulator (PM), a thermally tunable high-Q micro-disk resonator (MDR), and a high-speed photodetector (PD). The filtering function of the MPF is realized based on phase modulation and phase modulation to intensity modulation conversion, to translate the spectral response of the MDR in the optical domain to the spectral response of the MPF in the microwave domain. The tunability of the MPF is realized by thermally tuning the MDR. The proposed on-chip bandpass MPF is fabricated and characterized. An MPF with a passband of 1.93 GHz and a tunable range from 3 to 10 GHz is demonstrated. The power consumption, the insertion loss, and the bandwidth over the entire tunable range are studied. This successful implementation of an MPF marks a significant step forward in the full integration of microwave photonic systems on a single chip.

Scalable and reconfigurable true time delay line based on an ultra-low-loss silica waveguide

A scalable and reconfigurable on-chip optical true time delay line consisting of Mach-Zehnder interferometer (MZI) switches and delay waveguides is proposed and demonstrated with the ultra-low-loss silica waveguide platform. The MZI switches provide the reconfiguration of the light traveling paths and hence different delays. Our proposed structure can be easily scaled to an M bit delay line with a slight increase in dimensions. The footprint of our fabricated 1 bit delay line is 33  mm×13  mm (length×width) and can provide a delay of 6.0 ps with an insertion loss of 1.2 dB at the operating wavelength of 1550 nm, while the footprint of the 4 bit delay line is 43  mm×14  mm and can provide a discrete delay tuning from 6.0 to 90.2 ps with a delay deviation lower than 0.2 ps and a tuning response time of 0.84 ms. The insertion losses are lower than ∼2.34  dB, and the extinction ratios are greater than 20.42 dB. The average switching power is ∼132.6  mW. Our proposed optical true time delay lines could find applications for optical beamforming in phased array antennas.

Microwave Signal Processing over Multicore Fiber

We review the introduction of the space dimension into fiber-based technologies to implement compact and versatile signal processing solutions for microwave and millimeter wave signals. Built upon multicore fiber links and devices, this approach allows the realization of fiber-distributed signal processing in the context of fiber-wireless communications, providing both radiofrequency access distribution and signal processing in the same fiber medium. We present different space-division multiplexing architectures to implement tunable true time delay lines that can be applied to a variety of microwave photonics functionalities, such as signal filtering, radio beamsteering in phased array antennas or optoelectronic oscillation. In particular, this paper gathers our latest work on the following multicore fiber technologies: dispersion-engineered heterogeneous multicore fiber links for distributed tunable true time delay line operation; multicavity devices built upon the selective inscription of gratings in homogenous multicore fibers for compact true time delay line operation; and multicavity optoelectronic oscillation over both homogeneous and heterogeneous multicore fibers.

Spatial Division Multiplexed Microwave Signal processing by selective grating inscription in homogeneous multicore fibers

The use of Spatial Division Multiplexing for Microwave Photonics signal processing is proposed and experimentally demonstrated, for the first time to our knowledge, based on the selective inscription of Bragg gratings in homogeneous multicore fibers. The fabricated devices behave as sampled true time delay elements for radiofrequency signals offering a wide range of operation possibilities within the same optical fiber. The key to processing flexibility comes from the implementation of novel multi-cavity configurations by inscribing a variety of different fiber Bragg gratings along the different cores of a 7-core fiber. This entails the development of the first fabrication method to inscribe high-quality gratings characterized by arbitrary frequency spectra and located in arbitrary longitudinal positions along the individual cores of a multicore fiber. Our work opens the way towards the development of unique compact fiber-based solutions that enable the implementation of a wide variety of 2D (spatial and wavelength diversity) signal processing functionalities that will be key in future fiber-wireless communications scenarios. We envisage that Microwave Photonics systems and networks will benefit from this technology in terms of compactness, operation versatility and performance stability.

Lossless and high-resolution RF photonic notch filter

A novel technique to create a lossless and tunable RF photonic bandstop filter with an ultra-high suppression is demonstrated using the combination of an overcoupled optical ring resonator and tailored stimulated Brillouin scattering gain. The filter bandwidth narrowing is counterintuitively synthesized from two broad optical resonance responses. Through a precise amplitude and phase tailoring in the optical domain, the RF filter achieves a minimum insertion loss (<0  dB), a high isolation (>50  dB), and a tunable 3 dB bandwidth (60-220 MHz) simultaneously with wide frequency tunability (1-11 GHz). This ultra-low loss RF filter paves the way toward broadband advanced spectrum management with low loss, high selectivity, and improved signal-to-noise ratio.

Integrated line-by-line optical pulse shaper for high-fidelity and rapidly reconfigurable RF-filtering

We present a 32 channel indium phosphide integrated pulse shaper with 25 GHz channel spacing, where each channel is equipped with a semiconductor optical amplifier allowing for programmable line-by-line gain control with submicrosecond reconfigurability. We critically test the integrated pulse shaper by using it in comb-based RF-photonic filtering experiments where the precise gain control is leveraged to synthesize high-fidelity RF filters which we reconfigure on a microsecond time scale. Our on-chip pulse shaping demonstration is unmatched in its combination of speed, fidelity, and flexibility, and will likely open new avenues in the field of advanced broadband signal generation and processing.

Subwavelength grating enabled on-chip ultra-compact optical true time delay line

An optical true time delay line (OTTDL) is a basic photonic building block that enables many microwave photonic and optical processing operations. The conventional design for an integrated OTTDL that is based on spatial diversity uses a length-variable waveguide array to create the optical time delays, which can introduce complexities in the integrated circuit design. Here we report the first ever demonstration of an integrated index-variable OTTDL that exploits spatial diversity in an equal length waveguide array. The approach uses subwavelength grating waveguides in silicon-on-insulator (SOI), which enables the realization of OTTDLs having a simple geometry and that occupy a compact chip area. Moreover, compared to conventional wavelength-variable delay lines with a few THz operation bandwidth, our index-variable OTTDL has an extremely broad operation bandwidth practically exceeding several tens of THz, which supports operation for various input optical signals with broad ranges of central wavelength and bandwidth.

Lower bound for the spatial extent of localized modes in photonic-crystal waveguides with small random imperfections

Light localization due to random imperfections in periodic media is paramount in photonics research. The group index is known to be a key parameter for localization near photonic band edges, since small group velocities reinforce light interaction with imperfections. Here, we show that the size of the smallest localized mode that is formed at the band edge of a one-dimensional periodic medium is driven instead by the effective photon mass, i.e. the flatness of the dispersion curve. Our theoretical prediction is supported by numerical simulations, which reveal that photonic-crystal waveguides can exhibit surprisingly small localized modes, much smaller than those observed in Bragg stacks thanks to their larger effective photon mass. This possibility is demonstrated experimentally with a photonic-crystal waveguide fabricated without any intentional disorder, for which near-field measurements allow us to distinctly observe a wavelength-scale localized mode despite the smallness (~1/1000 of a wavelength) of the fabrication imperfections.

Flexible RF filter using a nonuniform SCISSOR

This work presents a flexible radiofrequency (RF) filter using an integrated microwave photonic circuit that comprises a nonuniform side-coupled integrated spaced sequence of resonators (N-SCISSOR). The filter passband can be reconfigured by varying the N-SCISSOR parameters. When employing a dual-parallel Mach-Zechnder modulator, the filter is also able to perform frequency down-conversion. In the experiment, various filter response shapes are shown, ranging from a flat-top band-pass filter to a total opposite high-rejection (>40  dB) notch filter, with a frequency coverage of greater than two octaves. The frequency down-conversion function is also demonstrated.

Ultra High-Speed Radio Frequency Switch Based on Photonics

Microwave switches, or Radio Frequency (RF) switches have been intensively used in microwave systems for signal routing. Compared with the fast development of microwave and wireless systems, RF switches have been underdeveloped particularly in terms of switching speed and operating bandwidth. In this paper, we propose a photonics based RF switch that is capable of switching at tens of picoseconds speed, which is hundreds of times faster than any existing RF switch technologies. The high-speed switching property is achieved with the use of a rapidly tunable microwave photonic filter with tens of gigahertz frequency tuning speed, where the tuning mechanism is based on the ultra-fast electro-optics Pockels effect. The RF switch has a wide operation bandwidth of 12 GHz and can go up to 40 GHz, depending on the bandwidth of the modulator used in the scheme. The proposed RF switch can either work as an ON/OFF switch or a two-channel switch, tens of picoseconds switching speed is experimentally observed for both type of switches.

Passband switchable microwave photonic multiband filter

A reconfigurable microwave photonic (MWP) multiband filter with selectable and switchable passbands is proposed and experimentally demonstrated, with a maximum of 12 simultaneous passbands evenly distributed from 0 to 10 GHz. The scheme is based on the generation of tunable optical comb lines using a two-stage Lyot loop filter, such that various filter tap spacings and spectral combinations are obtained for the configuration of the MWP filter. Through polarization state adjustment inside the Lyot loop filter, an optical frequency comb with 12 different comb spacings is achieved, which corresponds to a MWP filter with 12 selectable passbands. Center frequencies of the filter passbands are switchable, while the number of simultaneous passbands is tunable from 1 to 12. Furthermore, the MWP multiband filter can either work as an all-block, single-band or multiband filter with various passband combinations, which provide exceptional operation flexibility. All the passbands have over 30 dB sidelobe suppression and 3-dB bandwidth of 200 MHz, providing good filter selectivity.

Integrated remotely tunable optical delay line for millimeter-wave beam steering fabricated in an InP generic foundry

A compact and fabrication-tolerant integrated remotely tunable optical delay line is proposed for millimeter-wave beam steering and is fabricated in an InP generic foundry. The proposed delay line is based on a spectrally cyclic-arrayed waveguide grating feedback loop. Its major features include the tolerant architecture with reduced chip size, and bi-directional operation with simplified remote tuning. Moreover, its cyclic feature guarantees further cascaded operations either for 2D radio beam steering or for high-resolution delay generation. The experimental results show less than 6.5-dB insertion loss of the integrated delay line. Five different delays from 0 to 71.6 ps are generated with less than 0.67-ps delay errors.

Electrically driving bandwidth tunable guided-mode resonance filter based on a twisted nematic liquid crystal polarization rotator

A novel bandwidth-tunable filter is proposed based on nonpolarizing guided-mode resonance effect. The compact, electrically driving bandwidth-tunable optical filter is realized by taking advantage of the effect of bandwidth-to-polarization sensitivity and using a twisted nematic liquid crystal polarization rotator for simple and precise polarization control. The operation principle and the design of the device are presented. The center wavelength is fixed at 623.1 nm with a relatively symmetric line shape. The full-width at half-maximum bandwidth is tuned from 12 to 44.8 nm by controlling the voltage in the polarization rotator.

Design of heterogeneous multicore fibers as sampled true-time delay lines

We present a novel procedure for designing a sampled discrete true-time delay line (TTDL) for Microwave Photonics applications based on a heterogeneous MCF. Both simple step-index (SI) and trench-assisted SI profiles are numerically evaluated in terms of physical dimensions and material dopant concentrations in order to individually tailor the group delay and chromatic dispersion of each core. The proposed TTDL features unique properties beyond the current state of the art in terms of record bandwidth, compactness, flexibility, and versatility.

Reconfigurable radio-frequency arbitrary waveforms synthesized in a silicon photonic chip

Photonic methods of radio-frequency waveform generation and processing can provide performance advantages and flexibility over electronic methods due to the ultrawide bandwidth offered by the optical carriers. However, bulk optics implementations suffer from the lack of integration and slow reconfiguration speed. Here we propose an architecture of integrated photonic radio-frequency generation and processing and implement it on a silicon chip fabricated in a semiconductor manufacturing foundry. Our device can generate programmable radio-frequency bursts or continuous waveforms with only the light source, electrical drives/controls and detectors being off-chip. It modulates an individual pulse in a radio-frequency burst within 4 ns, achieving a reconfiguration speed three orders of magnitude faster than thermal tuning. The on-chip optical delay elements offer an integrated approach to accurately manipulating individual radio-frequency waveform features without constraints set by the speed and timing jitter of electronics, and should find applications ranging from high-speed wireless to defence electronics.

Performing radio-frequency arbitrary waveform generation in the optical domain offers advantages over electronic-based methods but suffers from lack of integration and slow speed. Here, Wang et al. propose a fast-reconfigurable, radio-frequency arbitrary waveform generator fully integrated in a silicon chip.

High-speed tunable microwave photonic notch filter based on phase modulator incorporated Lyot filter

A high-speed tunable microwave photonic notch filter with ultrahigh rejection ratio is presented, which is achieved by semiconductor optical amplifier (SOA)-based single-sideband modulation and optical spectral filtering with a phase modulator-incorporated Lyot (PM-Lyot) filter. By varying the birefringence of the phase modulator through electro-optic effect, electrically tuning of the microwave photonic notch filter is experimentally achieved at tens of gigahertz speed. The use of SOA-polarizer based single-sideband modulation scheme provides good sideband suppression over a wide frequency range, resulting in an ultrahigh rejection ratio of the microwave photonic notch filter. Stable filter spectrum with bandstop rejection ratio over 60 dB is observed over a frequency tuning range from 1.8 to 10 GHz. Compare with standard interferometric notch filter, narrower bandwidth and sharper notch profile are achieved with the unique PM-Lyot filter, resulting in better filter selectivity. Moreover, bandwidth tuning is also achieved through polarization adjustment inside the PM-Lyot filter, that the 10-dB filter bandwidth is tuned from 0.81 to 1.85 GHz.

On-chip programmable ultra-wideband microwave photonic phase shifter and true time delay unit

We proposed and experimentally demonstrated an ultra-broadband on-chip microwave photonic processor that can operate both as RF phase shifter (PS) and true-time-delay (TTD) line, with continuous tuning. The processor is based on a silicon dual-phase-shifted waveguide Bragg grating (DPS-WBG) realized with a CMOS compatible process. We experimentally demonstrated the generation of delay up to 19.4 ps over 10 GHz instantaneous bandwidth and a phase shift of approximately 160° over the bandwidth 22-29 GHz. The available RF measurement setup ultimately limits the phase shifting demonstration as the device is capable of providing up to 300° phase shift for RF frequencies over a record bandwidth approaching 1 THz.

On-chip microwave photonic beamformer circuits operating with phase modulation and direct detection

We propose and experimentally demonstrate the working principles of two novel microwave photonic (MWP) beamformer circuits operating with phase modulation (PM) and direct detection (DD). The proposed circuits incorporate two major signal processing functionalities, namely a broadband beamforming network employing ring resonator-based delay lines and an optical sideband manipulator that renders the circuit outputs equivalent to those of intensity-modulated MWP beamformers. These functionalities allow the system to employ low-circuit-complexity modulators and detectors, which brings significant benefits on the system construction cost and operation stability. The functionalities of the proposed MWP beamformer circuits were verified in experimental demonstrations performed on two sample circuits realized in Si(3)N(4)/SiO(2) waveguide technology. The measurements exhibit a 2 × 1 beamforming effect for an instantaneous RF transmission band of 3‒7 GHz, which is, to our best knowledge, the first verification of on-chip MWP beamformer circuits operating with PM and DD.

Frequency agile microwave photonic notch filter with anomalously high stopband rejection

We report a novel class microwave photonic (MWP) notch filter with a very narrow isolation bandwidth (10 MHz), an ultrahigh stopband rejection (>60 dB), a wide frequency tuning (1-30 GHz), and flexible bandwidth reconfigurability (10-65 MHz). This performance is enabled by a new concept of sideband amplitude and phase controls using an electro-optic modulator and an optical filter. This concept enables energy efficient operation in active MWP notch filters, and opens up a pathway toward enabling low-power nanophotonic devices as high-performance RF filters.

Reducing disorder-induced losses for slow light photonic crystal waveguides through Bloch mode engineering

We present theory and measurements of disorder-induced losses for low loss 1.5 mm long slow light photonic crystal waveguides. A recent class of dispersion engineered waveguides increases the bandwidth of slow light and shows lower propagation losses for the same group index. Our theory and experiments explain how Bloch mode engineering can substantially reduce scattering losses for the same slow light group velocity regime.

Si₃N₄ ring resonator-based microwave photonic notch filter with an ultrahigh peak rejection

We report a simple technique in microwave photonic (MWP) signal processing that allows the use of an optical filter with a shallow notch to exhibit a microwave notch filter with anomalously high rejection level. We implement this technique using a low-loss, tunable Si₃N₄ optical ring resonator as the optical filter, and achieved an MWP notch filter with an ultra-high peak rejection > 60 dB, a tunable high resolution bandwidth of 247-840 MHz, and notch frequency tuning of 2-8 GHz. To our knowledge, this is a record combined peak rejection and resolution for an integrated MWP filter.

Integrable high order UWB pulse photonic generator based on cross phase modulation in a SOA-MZI

We propose and experimentally demonstrate a potentially integrable optical scheme to generate high order UWB pulses. The technique is based on exploiting the cross phase modulation generated in an InGaAsP Mach-Zehnder interferometer containing integrated semiconductor optical amplifiers, and is also adaptable to different pulse modulation formats through an optical processing unit which allows to control of the amplitude, polarity and time delay of the generated taps.

Microwave Photonics: current challenges towards widespread application

Microwave Photonics, a symbiotic field of research that brings together the worlds of optics and radio frequency is currently facing several challenges in its transition from a niche to a truly widespread technology essential to support the ever-increasing values for speed, bandwidth, processing capability and dynamic range that will be required in next generation hybrid access networks. We outline these challenges, which are the subject of the contributions to this focus issue.

Optimizing photonic crystal waveguides for on-chip spectroscopic applications

We investigate the applicability of photonic crystal waveguides to high-resolution on-chip spectrometers. We argue that the figure of merit by which their performance should be gauged is not the group index bandwidth product, which photonic crystal waveguides are usually optimized for, but the working finesse, which relates to the maximum number of spectral lines resolvable by a slow-light spectrometer. Through numerical simulation, we show that a properly-optimized photonic crystal waveguide could form the basis of a spectrometer with a spectral resolution of 0.04 nm over a 12.5 nm bandwidth near 1550 nm and with a footprint six times smaller than a conventional spectrometer.

Optimal design of suspended silica on-chip splitter

Photonic splitters and couplers are one of the fundamental elements in integrated optical circuits. As such, over the past decade significant research efforts have been dedicated to the development of low loss, wide bandwidth devices. While silica-based devices have clear advantages in terms of bandwidth, silicon and silicon nitride devices have lead the field in terms of ease of integration. In the present work, we provide design parameters for a novel splitter based on a suspended silica device. Unlike previous coupler devices which have smooth transition regions, the proposed device has a small defect which enables coupling across a large membrane. The designs are based on 3D FDTD models, and incorporate wavelength, refractive index and polarization dependence. The model is experimentally verified at select wavelengths from the visible through the near-IR. For comparison, we have also modeled the splitting ratio for several materials which are commonly used as waveguiding devices.