Photonic integrated circuit implementation of a sub-GHz-selectivity frequency comb filter for optical clock multiplication.

We report a photonic integrated circuit implementation of an optical clock multiplier, or equivalently an optical frequency comb filter. The circuit comprises a novel topology of a ring-resonator-assisted asymmetrical Mach-Zehnder interferometer in a Sagnac loop, providing a reconfigurable comb filter with sub-GHz selectivity and low complexity. A proof-of-concept device is fabricated in a high-index-contrast stoichiometric silicon nitride (Si3N4/SiO2) waveguide, featuring low loss, small size, and large bandwidth. In the experiment, we show a very narrow passband for filters of this kind, i.e. a -3-dB bandwidth of 0.6 GHz and a -20-dB passband of 1.2 GHz at a frequency interval of 12.5 GHz. As an application example, this particular filter shape enables successful demonstrations of five-fold repetition rate multiplication of optical clock signals, i.e. from 2.5 Gpulses/s to 12.5 Gpulses/s and from 10 Gpulses/s to 50 Gpulses/s. This work addresses comb spectrum processing on an integrated platform, pointing towards a device-compact solution for optical clock multipliers (frequency comb filters) which have diverse applications ranging from photonic-based RF spectrum scanners and photonic radars to GHz-granularity WDM switches and LIDARs.

Sub-GHz-resolution C-band Nyquist-filtering interleaver on a high-index-contrast photonic integrated circuit.

Modern optical communications rely on high-resolution, high-bandwidth filtering to maximize the data-carrying capacity of fiber-optic networks. Such filtering typically requires high-speed, power-hungry digital processes in the electrical domain. Passive optical filters currently provide high bandwidths with low power consumption, but at the expense of resolution. Here, we present a passive filter chip that functions as an optical Nyquist-filtering interleaver featuring sub-GHz resolution and a near-rectangular passband with 8% roll-off. This performance is highly promising for high-spectral-efficiency Nyquist wavelength division multiplexed (N-WDM) optical super-channels. The chip provides a simple two-ring-resonator-assisted Mach-Zehnder interferometer, which has a sub-cm2 footprint owing to the high-index-contrast Si3N4/SiO2 waveguide, while manifests low wavelength-dependency enabling C-band (> 4 THz) coverage with more than 160 effective free spectral ranges of 25 GHz. This device is anticipated to be a critical building block for spectrally-efficient, chip-scale transceivers and ROADMs for N-WDM super-channels in next-generation optical communication networks.

Photonic Hilbert transformers based on laterally apodized integrated waveguide Bragg gratings on a SOI wafer.

We experimentally demonstrate high-performance integer and fractional-order photonic Hilbert transformers based on laterally apodized Bragg gratings in a silicon-on-insulator technology platform. The sub-millimeter-long gratings have been fabricated using single-etch electron beam lithography, and the resulting HT devices offer operation bandwidths approaching the THz range, with time-bandwidth products between 10 and 20.

Experimental demonstration of sub-picosecond optical pulse shaping in silicon based on discrete space-to-time mapping.

We experimentally demonstrate on-chip optical pulse shaping based on discrete space-to-time mapping in cascaded co-directional couplers. The demonstrated shapers validate a recent design methodology that exploits the direct relationship between the discrete complex spatial apodization profile of a structure of cascaded couplers and the time-domain impulse response of the device. In this design, the amplitude and phase of the apodization profile can be controlled through the coupling strength of each coupler and the relative time delay between the waveguides connecting consecutive couplers, respectively. This design methodology has been successfully used to demonstrate direct synthesis of high-quality flat-top and phase-coded pulse trains with resolutions down to the sub-picosecond range using passive devices in a silicon-on-insulator platform.

THz-bandwidth photonic Hilbert transformers based on fiber Bragg gratings in transmission.

THz-bandwidth photonic Hilbert transformers (PHTs) are implemented for the first time, to the best of our knowledge, based on fiber Bragg grating (FBG) technology. To increase the practical bandwidth limitation of FBGs (typically <200 GHz), a superstructure based on two superimposed linearly-chirped FBGs operating in transmission has been employed. The use of a transmission FBG involves first a conversion of the non-minimum phase response of the PHT into a minimum-phase response by adding an anticipated instantaneous component to the desired system temporal impulse response. Using this methodology, a 3-THz-bandwidth integer PHT and a fractional (order 0.81) PHT are designed, fabricated, and successfully characterized.

Integrated microwave photonic splitter with reconfigurable amplitude, phase, and delay offsets.

This work presents an integrated microwave photonics splitter with reconfigurable amplitude, phase, and delay offsets. The core components for this function are a dual-parallel Mach-Zehnder modulator, a deinterleaver, and tunable delay lines, all implemented using photonic integrated circuits. Using a demonstrator with an optical free spectral range of 25 GHz, we show experimentally the RF splitting function over two continuous bands, i.e., 0.9-11.6 GHz and 13.4-20 GHz. This result promises a deployable solution for creating wideband, reconfigurable RF splitters in integrated forms.

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.

Terahertz-bandwidth photonic fractional Hilbert transformer based on a phase-shifted waveguide Bragg grating on silicon.

We propose and experimentally demonstrate the first THz bandwidth on-chip photonic fractional Hilbert transformer. The reported design uses a novel approach, based on a uniform and nonapodized single phase-shifted integrated waveguide Bragg grating on silicon, where the fractional order P can be engineered by simply varying the effective index modulation δn. Experimental results for P=1.5 show very low processing error for a broad range of pulse bandwidths between 77 GHz and 2.07 THz, corresponding to a time-bandwidth product as high as 27.

Integrated waveguide Bragg gratings for microwave photonics signal processing.

Integrated Microwave photonics (IMWP) signal processing using Photonic Integrated Circuits (PICs) has attracted a great deal of attention in recent years as an enabling technology for a number of functionalities not attainable by purely microwave solutions. In this context, integrated waveguide Bragg grating (WBG) devices constitute a particularly attractive approach thanks to their compactness and flexibility in producing arbitrarily defined amplitude and phase responses, by directly acting on coupling coefficient and perturbations of the grating profile. In this article, we review recent advances in the field of integrated WBGs applied to MWP, analyzing the advantages leveraged by an integrated realization. We provide a perspective on the exciting possibilities offered by the silicon photonics platform in the field of MWP, potentially enabling integration of highly-complex active and passive functionalities with high yield on a single chip, with a particular focus on the use of WBGs as basic building blocks for linear filtering operations. We demonstrate the versatility of WBG-based devices by proposing and experimentally demonstrating a novel, continuously-tunable, integrated true-time-delay (TTD) line based on a very simple dual phase-shifted WBG (DPS-WBG).

Silicon nitride microwave photonic circuits.

We present an overview of several microwave photonic processing functionalities based on combinations of Mach-Zehnder and ring resonator filters using the high index contrast silicon nitride (TriPleX™) waveguide technology. All functionalities are built using the same basic building blocks, namely straight waveguides, phase tuning elements and directional couplers. We recall previously shown measurements on high spurious free dynamic range microwave photonic (MWP) link, ultra-wideband pulse generation, instantaneous frequency measurements, Hilbert transformers, microwave polarization networks and demonstrate new measurements and functionalities on a 16 channel optical beamforming network and modulation format transformer as well as an outlook on future microwave photonic platform integration, which will lead to a significantly reduced footprint and thereby enables the path to commercially viable MWP systems.

Millimeter-wave to near-terahertz sensors based on reversible insulator-to-metal transition in VO2.

In the quest for low power bio-inspired spiking sensors, functional oxides like vanadium dioxide are expected to enable future energy efficient sensing. Here, we report uncooled millimeter-wave spiking detectors based on the sensitivity of insulator-to-metal transition threshold voltage to the incident wave. The detection concept is demonstrated through actuation of biased VO2 switches encapsulated in a pair of coupled antennas by interrupting coplanar waveguides for broadband measurements, on silicon substrates. Ultimately, we propose an electromagnetic-wave-sensitive voltage-controlled spike generator based on VO2 switches in an astable spiking circuit. The fabricated sensors show responsivities of around 66.3 MHz.W-1 at 1 µW, with a low noise equivalent power of 5 nW.Hz-0.5 at room temperature, for a footprint of 2.5 × 10-5 mm2. The responsivity in static characterizations is 76 kV.W-1. Based on experimental statistical data measured on robust fabricated devices, we discuss stochastic behavior and noise limits of VO2 -based spiking sensors applicable for wave power sensing in mm-wave and sub-terahertz range.

System integration and radiation pattern measurements of a phased array antenna employing an integrated photonic beamformer for radio astronomy applications.

In this paper we describe the system integration and the experimental demonstration of a photonically beamformed four-element receiving array antenna for radio astronomy applications. To our knowledge, the work described here is the first demonstration of the squint-free, continuously tunable beamsteering capability offered by an integrated photonic beamformer based on optical ring resonator true-time-delay units, with measured radiation patterns. The integrated beamformer is realized in a low loss, complementary metal-oxide-semiconductor (CMOS) compatible optical waveguide technology. The measurements show a wideband, continuous beamsteering operation over a steering angle of 23.5 degrees and an instantaneous bandwidth of 500 MHz limited only by the measurement setup.

Impulse radio ultrawideband pulse shaper based on a programmable photonic chip frequency discriminator.

We report and experimentally demonstrate the generation of impulse radio ultrawideband (UWB) pulses using a photonic chip frequency discriminator. The discriminator consists of three add-drop optical ring resonators (ORRs) which are fully programmable using thermo-optical tuning. This discriminator chip in combination with a phase modulator forms a temporal differentiator where phase modulation is converted to intensity modulation (PM-IM conversion). By means of tailoring the discriminator response using either the individual or the cascade of drop and through responses of the ORRs, first-order or second-order temporal differentiations are obtained. Using this principle, the generation of UWB monocycle, doublet and modified doublet pulses are demonstrated. The use of this CMOS-compatible discriminator is promising for the realization of a compact and low cost UWB transmitter.

Low-loss, high-index-contrast Si3N4/SiO2 optical waveguides for optical delay lines in microwave photonics signal processing.

We report the design and characterization of Si3N4/SiO2 optical waveguides which are specifically developed for optical delay lines in microwave photonics (MWP) signal processing applications. The waveguide structure consists of a stack of two Si3N4 stripes and SiO2 as an intermediate layer. Characterization of the waveguide propagation loss was performed in race track-shaped optical ring resonators (ORRs) with a free-spectral range of 20 GHz and a bending radius varied from 50 µm to 125 µm. A waveguide propagation loss as low as 0.095 dB/cm was measured in the ORRs with bend radii ≥ 70 µm. Using the waveguide technology two types of RF-modulated optical sideband filters with high sideband suppression and small transition band consisting of an Mach-Zehnder interferometer and ORRs are also demonstrated. These results demonstrate the potential of the waveguide technology to be applied to construct compact on-chip MWP signal processors.

On-chip CMOS compatible reconfigurable optical delay line with separate carrier tuning for microwave photonic signal processing.

We report, for the first time, an integrated photonic signal processor consisting of a reconfigurable optical delay line (ODL) with a separate carrier tuning (SCT) unit and an optical sideband filter on a single CMOS compatible photonic chip. The processing functionalities are carried out with optical ring resonators as building blocks. We show that the integrated approach together with the use of SCT technique allows the implementation of a wideband, fully-tunable ODL with reduced complexity. To highlight the functionalities of the processor, we demonstrate a reconfigurable microwave photonic filter where the ODL has been configured in a bandwidth over 1 GHz.

Nano-opto-electro-mechanical switches operated at CMOS-level voltages.

Combining reprogrammable optical networks with complementary metal-oxide semiconductor (CMOS) electronics is expected to provide a platform for technological developments in on-chip integrated optoelectronics. We demonstrate how opto-electro-mechanical effects in micrometer-scale hybrid photonic-plasmonic structures enable light switching under CMOS voltages and low optical losses (0.1 decibel). Rapid (for example, tens of nanoseconds) switching is achieved by an electrostatic, nanometer-scale perturbation of a thin, and thus low-mass, gold membrane that forms an air-gap hybrid photonic-plasmonic waveguide. Confinement of the plasmonic portion of the light to the variable-height air gap yields a strong opto-electro-mechanical effect, while photonic confinement of the rest of the light minimizes optical losses. The demonstrated hybrid architecture provides a route to develop applications for CMOS-integrated, reprogrammable optical systems such as optical neural networks for deep learning.

Reconfigurable photonic generation of broadband chirped waveforms using a single CW laser and low-frequency electronics.

Broadband radio-frequency chirped waveforms (RFCWs) with dynamically tunable parameters are of fundamental interest to many practical applications. Recently, photonic-assisted solutions have been demonstrated to overcome the bandwidth and flexibility constraints of electronic RFCW generation techniques. However, state-of-the-art photonic techniques involve broadband mode-locked lasers, complex dual laser systems, or fast electronics, increasing significantly the complexity and cost of the resulting platforms. Here we demonstrate a novel concept for photonic generation of broadband RFCWs using a simple architecture, involving a single CW laser, a recirculating frequency-shifting loop, and standard low-frequency electronics. All the chirp waveform parameters, namely sign and value of the chirp rate, central frequency and bandwidth, duration and repetition rate, are easily reconfigurable. We report the generation of mutually coherent RF chirps, with bandwidth above 28 GHz, and time-bandwidth product exceeding 1000, limited by the available detection bandwidth. The capabilities of this simple platform fulfill the stringent requirements for real-world applications.

Wideband dynamic microwave frequency identification system using a low-power ultracompact silicon photonic chip.

Photonic-based instantaneous frequency measurement (IFM) of unknown microwave signals offers improved flexibility and frequency range as compared with electronic solutions. However, no photonic platform has ever demonstrated the key capability to perform dynamic IFM, as required in real-world applications. In addition, all demonstrations to date employ bulky components or need high optical power for operation. Here we demonstrate an integrated photonic IFM system that can identify frequency-varying signals in a dynamic manner, without any need for fast measurement instrumentation. The system is based on a fully linear, ultracompact system based on a waveguide Bragg grating on silicon, only 65-µm long and operating up to ∼30 GHz with carrier power below 10 mW, significantly outperforming present technologies. These results open a solid path towards identification of dynamically changing signals over tens of GHz bandwidths using a practical, low-cost on-chip implementation for applications from broadband communications to biomedical, astronomy and more.