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We demonstrate a broadband and continuously tunable 14 optical beamforming network (OBFN), based on the hybrid integration of indium phosphide (InP) components in the silicon nitride (Si3N4) platform. The photonic integrated circuit (PIC) comprises a hybrid InP-Si3N4 external cavity laser, a pair of InP phase modulators, a Si3N4 optical single-sideband full carrier (SSBFC) filter followed by four tunable optical true time delay lines (OTTDLs), and four InP photodetectors. Each OTTDL consists of eight cascaded thermo-optical micro-ring resonators (MRRs) that impose tunable true time delay on the propagating optical signals. The OBFN-PIC is designed to facilitate the steering of a microwave signal with carrier frequency up to 40 GHz over a continuous set of beam angles. We evaluate the performance of the OBFN-PIC to handle and process microwave signals, measuring the link gain, the noise figure (NF), and the spurious-free dynamic range (SFDR) parameters. Moreover, we assess its beamforming capabilities assuming that the OBFN-PIC is part of a wireless system operating in the downlink direction and feeds a multi-element antenna array. Using microwave signals at 5 and 10 GHz with quadrature amplitude modulation (QAM) formats at 500 Mbaud, we evaluate the performance of the OBFN-PIC under various configurations. We show that error-free performance can be achieved at both operating frequencies and for all the investigated beam angles ranging from 45 to 135, thus validating its potential for high-quality beamforming performance.

During the past years, incorporating ptical Circuit Switches (OCS) in high-bandwidth optical interconnects has outlined the critical challenges of achieving ultra-low fiber-to-fiber losses (FtF) and constantly decreasing costs for Photonic Integrated Circuits (PICs). This work aims to simultaneously satisfy both the low-loss and low-cost requirements by bringing two of the most successful example-technologies in the history of optics, i.e. EDFAs and ROADMs to a common Si3N4 platform. In particular, the proof-of-concept operation of a lossless four-port Silicon Photonic (SiPho) ROADM is experimentally presented for the first time based on two PIC prototypes on a Si3N4 platform, including a monolithic-integrated 5.9cm-long spiral Al2O3:Er3+ Erbium Doped Waveguide Amplifier (EDWA) with 15 dB signal enhancement capabilities and a lattice MZI-interleaver ROAM layout with 100 GHz channel spacing. Considering an ultra-low 2.55 dB FtF loss of the ROADM along with 0.5 dB loss for each of the two coupling-interfaces between the Si3N4 and Al2O3:Er3+ waveguide layers, a cumulative loss of 3.55 dB is obtained, which can be compensated by the 3.6 dB net gain provided by the EDWA to four incoming WDM signals of -1.7 dBm/channel. Lossless wavelength-routing operation is validated at up to 240 Gb/s WDM (460Gb/s) data traffic, while the cascadability of the proposed device is benchmarked in a realistic two-stage optical bus topology with 10 km single mode fiber that selectively routes 425Gb/s WDM data channels to any of its eight Drop output ports. This work forms the first demonstration of lossless ROADM operation exclusively on SiPho technology, highlighting a promising roadmap for large scale SiPho switching matrices and more complex PICs co-integrated with EDWAs.

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.

A reconfigurable photonic integrated transmitter, enabling dynamic resource allocation in the metro-access network, is proposed. The device consists of a multicarrier sliceable bandwidth variable transmitter (MC-SBVT) realized in indium phosphide and a silicon-nitride-based optical cross-connect (OXC). The proposed architecture delivers full flexibility in terms of the choice of data format/bandwidth, channel spacing, and wavelength assignment. The functional design of the MC-SBVT and OXC as well as their practical realization are discussed. Preliminary characterization results of the photonic-integrated-circuit-based MC-SBVT, demonstrating the reconfigurability of the device, are also presented.

This paper presents a beyond 5G fronthaul network with dynamic beamforming and -steering. The proposed fronthaul solution deploys optical beamforming (OBF) by combining space division multiplexing (SDM), analogue radio-over-fiber (ARoF), and the novel optical beam forming network (OBFN) technologies. From the service management and orchestration (MANO) point of view, the proposed fronthaul solution also deploys an advanced software defined networking (SDN) and Network Function Virtualization (NFV) control and orchestration architecture developed with the goal to optimally manage and reconfigure the physical layer resources (i.e., optical and radio) at the central office and cell sites (i.e., pool of baseband units (BBUs), remote radio heads (RRHs), ARoF transceivers and OBFNs). The proposed beyond 5G fronthaul architecture is primarily oriented to deploy massive machine-type communication (mMTC) services with high-bandwidth requirements, such as for industry 4.0. In this paper we experimentally validate the novel OBFN system, and the dynamic SDN/NFV MANO of the transport connectivity and network services for optical beamforming. The obtained experimental results show that the overall delay for the provisioning and removal of an OBF service, considering the contribution of the involved optical and radio systems and the SDN/NFV MANO layer, is 134s and 18s respectively. The reconfiguration of the OBF service to add or remove a beam can be performed in the range of 65–87s.

We experimentally demonstrate a new kind of integrated Si 3 N 4 microwave photonic (MWP) signal processor that combines a programmable modulation transformer (MT) with a versatile double-injection ring resonator (DI-RR). With this circuit we show an array of MWP filtering functions unmatched by traditional MWP circuits.

A hybrid integrated InP-Si3N4 dual tunable laser module is deployed as a highly flexible source for converged optical/mm-wave fronthaul. Experimental results show the wavelength flexible delivery of 5G signals over analog radio-over-fiber, incorporating wireless transmission at 60 GHz, with received EVMs as low as 5%.

This paper describes the detailed characterization of a novel InP-Si

The development of large-scale optical quantum information processing circuits ground on the stability and reconfigurability enabled by integrated photonics. We demonstrate a reconfigurable 8x8 integrated linear optical network based on silicon nitride waveguides for quantum information processing. Our processor implements a novel optical architecture enabling any arbitrary linear transformation and constitutes the largest programmable circuit reported so far on this platform. We validate a variety of photonic quantum information processing primitives, in the form of Hong-Ou-Mandel interference, bosonic coalescence/anticoalescence and high-dimensional single-photon quantum gates. We achieve fidelities that clearly demonstrate the promising future for large-scale photonic quantum information processing using low-loss silicon nitride., Comment: Added supplementary materials, extended introduction, new figures, results unchanged

We demonstrate a fully tunable microwave photonic bandpass filter fabricated with Si3N4/SiO2 waveguides on a chip. The filter is based on coupled resonator optical waveguide architecture comprising a chain of eight coupled ring resonators, each with an optical roundtrip length of 21 cm (free spectral range of 1.4 GHz), with an adjustable resonance frequency, and with adjustable power coupling. The passband shows a 3-dB bandwidth of 72 MHz with at least 51-dB out-of-band rejection and features simultaneously a flat-top, a shape factor of 1.9, and a group delay variation smaller than 2 ns.

Hybrid integration of semiconductor optical amplifiers with frequency-selective feedback circuits, implemented using low-loss Si3N4 waveguides, enables robust chip-sized lasers with outstanding properties. Deploying ring resonators as a tunable feedback filter provides single-mode operation over a wide wavelength range. Moreover, these rings resonantly enhance the cavity length, which results in ultra-narrow intrinsic linewidth, as low as 40 Hz. Here, we present an overview on state-of-the-art developments regarding these lasers. We compare linewidth and tuning results for different feedback circuit configurations. Finally, we report on the first demonstration of a hybrid-integrated semiconductor laser that operates in the visible wavelength range.

The first demonstration of a lossless four-port silicon photonic ROADM-node based on a monolithic-integrated spiral Al2O3:Er3+ Erbium Doped Waveguide Amplifier and MZI-interleaver layout on a Si3N4 platform is presented, routing a 4×50Gb/s WDM data-traffic capacity.

Photonics-based techniques spearhead the generation of high-frequency signals in the millimeter-and Terahertz wave, crucial for ultrabroadband mobile wireless link development. Photonic integration is enabling to provide the photonic approach with added advantages of energy-efficiency, flexibility and scalability, in addition to signal quality. We present an optical heterodyne system based on a novel dual laser module containing two InP-Si3N4 hybrid lasers with intracavity wavelength selective optical filters with output optical power per laser of up to 15 dBm (31 mW), wide wavelength tuning of about 60 nm, and narrow optical linewidth below 100 kHz. To the best of our knowledge, we present for the first time the continuous-wave generation of RF frequencies over a wide tuning range from C-band (4 GHz – 8-GHz) to W-band (75 GHz – 110 GHz) achieving record low RF electrical linewidth around 108 kHz and long-term drift < 12 MHz with two free-running lasers. This is the best beat-note linewidth obtained with such an integrated source in a free-runnning regime and with a wide tuning range ever reported.

Optical heterodyne signal generation is the most flexible photonic microwave generation technique. Photonic integrated solutions have been shown monolithically on InP, and more recently on silicon. We present novel implementations using hybrid integration technology with silicon nitride (with high-Q resonators for extremely low linewidth) and polymer (with low permittivity material for highly efficient radiation).

This paper presents the design and the performance analysis of a photonics-based beamformer for a spaceborne synthetic aperture radar implementing the scan-on-receive functionality. The considered device is a hybrid photonic integrated circuit composed of actives in InP and passives in TriPleX™, realizing the fast beamforming of three receiver beams out of 12 radio-frequency input signals and providing their simultaneous down-conversion to intermediate frequency. The analysis considers as main performance indicators the gain, the noise figure, and the dynamic range of the photonics-based beamformer, and demonstrates the device compliance to the application requirements and its suitability for satellite missions.

We demonstrate a hybrid integrated and widely tunable diode laser with an intrinsic linewidth as narrow as 40 Hz, achieved with a single roundtrip through a low-loss feedback circuit that extends the cavity length to 0.5 meter on a chip. Employing solely dielectrics for single-roundtrip, single-mode resolved feedback filtering enables linewidth narrowing with increasing laser power, without limitations through nonlinear loss. We achieve single-frequency oscillation with up to 23 mW fiber coupled output power, 70-nm wide spectral coverage in the 1.55 μm wavelength range with 3 mW output and obtain more than 60 dB side mode suppression. Such properties and options for further linewidth narrowing render the approach of high interest for direct integration in photonic circuits serving microwave photonics, coherent communications, sensing and metrology with highest resolution.

The compactness and precise waveguide length control of integrated photonics are fueling the integrated microwave photonic approach, integrating key RF functions on a chip. We describe current efforts using hybrid integration to optimize the performance of different photonic components avoiding the compromises on a monolithic approach.

The introduction of millimeter wave (mm-wave) frequency bands for cellular communications with significantly larger bandwidths compared to their sub-6 GHz counterparts, the resulting densification of network deployments and the introduction of antenna arrays with beamforming result in major increases in fronthaul capacity required for 5G networks. As a result, a radical re-design of the radio access network is required since traditional fronthaul technologies are not scaleable. In this article the use of analog radio-over-fiber (ARoF) is proposed and demonstrated as a viable alternative which, combined with space division multiplexing in the optical distribution network as well as photonic integration of the required transceivers, shows a path to a scaleable fronthaul solution for 5G. The trade-off between digitized and analog fronthaul is discussed and the ARoF architecture proposed by blueSPACE is introduced. Two options for the generation of ARoF two-tone signals for mm-wave generation via optical heterodyning are discussed in detail, including designs for the implementation in photonic integrated circuits as well as measurements of their phase noise performance. The proposed photonic integrated circuit designs include the use of both InP and SiN platforms for ARoF signal generation and optical beamforming respectively, proposing a joint design that allows for true multi-beam transmission from a single antenna array. Phase noise measurements based on laboratory implementations of ARoF generation based on a Mach–Zehnder modulator with suppressed carrier and with an optical phase-locked loop are presented and the suitability of these transmitters is evaluated though phase noise simulations. Finally, the viability of the proposed ARoF fronthaul architecture for the transport of high-bandwidth mm-wave 5G signals is proven with the successful implementation of a real-time transmission link based on an ARoF baseband unit with full real-time processing of extended 5G new radio signals with 800 MHz bandwidth, achieving transmission over 10 km of 7-core single-mode multi-core fiber and 9 m mm-wave wireless at 25.5 GHz with bit error rates below the limit for a 7% overhead hard decision forward error correction.

Tunable semiconductor lasers with a narrow linewidth have found a wide range of important applications, reaching from coherent communications, remote optical sensing such as Lidar to arising applications such as microwave photonics (MWP). Especially, integrated microwave photonics [1,2] aims to achieve RF-to-RF signal processing in the optical domain with a small footprint while offering wide bandwidths. Recently, we developed a hybrid integrated photonic platform [3] based on active InP components such as gain sections, modulators, and detectors with passive silicon nitride-based TriPleX enabling low loss optical signal processing due to its low propagation losses (< 0.1 dB/cm). The combination of these two platforms enables fully integrated RF-to-RF analog photonic links (APL). The link gain of an APL quadratically depends on the sensitivity of the modulator, responsivity of the photodetector (PD) as well as the optical power at the receiving PD [2]. Therefore, to achieve an efficient APL with a high link gain an optical input power of typically more than 20 dBm is needed. Furthermore, high optical powers can keep the noise figure at a minimum by making the use of additional amplifiers obsolete. However, so far current hybrid as well as heterogeneously integrated lasers lack sufficient optical power to achieve such efficient APLs. Here, we present, to the best of our knowledge, the first hybrid integrated laser consisting of more than one InP gain sections sharing a common laser mirror. The resulting on-chip power of 20.7 dBm using two gain sections at a pump current of twice 300 mA is the highest optical power of a hybrid integrated laser to date. Furthermore, the laser shows a low intrinsic linewidth of 320 Hz as well as a low relative intensity noise ( The scheme and a photograph of the dual-gain section hybrid integrated laser is shown in Fig.1 (a) and (b), respectively. The laser comprises of two InP reflective semiconductor optical amplifier (RSOA) chips and a low-loss TriPleX frequency selective mirror chip. The frequency selective mirror is based on a well-known principle of Vernier mirror with two micro-resonators of slightly different length and, hence, free-spectral ranges of 208 GHz and 215 GHz. To achieve a higher optical output power the mirror chip is coupled to two individual InP RSOAs with a length of 700 μm instead of one [4]. The coupling losses from the InP RSOAs to TriPleX laser cavity are estimated to be about 1 dB. Each of the RSOAs has an individual electrical connection to apply a pump current. On the TriPleX chip, thermo-optic tuners are used to set the frequency selective mirror, to control the phases of the RSOAs within the cavity, two tunable Mach-Zehnder couplers are used to combine both output coupling of the cavity and the power in the output waveguide. The output waveguides are coupled to a fiber array with standard polarization maintaining fibers with a coupling loss of 0.5 dB.

We present an integrated semiconductor-dielectric hybrid dual-frequency laser operating in the 1.5 μm wavelength range for microwave and terahertz (THz) generation. Generating a microwave beat frequency near 11 GHz, we observe an intrinsic linewidth as low as about 2 kHz. This is realized by hybrid integration of a single diode amplifier based on indium phosphide (InP) with a long, low-loss silicon nitride (Si3N4) feedback circuit to extend the cavity photon lifetime, resulting in a cavity optical roundtrip length of about 30 cm on a chip. Simultaneous lasing at two frequencies is enabled by introducing an external control parameter for balancing the feedback from two tunable, frequency-selective Vernier mirrors on the Si3N4 chip. Each frequency can be tuned with a wavelength coverage of about 80 nm, potentially allowing for the generation of a broad range of frequencies in the microwave range up to the THz range.

We present an extensive analysis of an optical Blass-matrix architecture as a beamforming network with potential for multibeam operation in wireless systems. Its design relies on the use of phase shifters and Mach–Zehnder Interferometers (MZIs) inside an $M\times N$ matrix, and enables the generation of M beams by N -element antenna arrays. We start our analysis from an optical signal with amplitude modulation by discrete microwave tones, and confirm the possibility to translate its optical phase shifts inside the matrix into equivalent phase shifts in the microwave domain. We show this is possible when the input is an optical single-side band signal and the optical carrier is reinserted before photodetection. We extend the conclusions to the case of an optical signal carrying a microwave with quadrature amplitude modulation (QAM) and the case of simultaneous inputs at the M input ports. Based on this analysis, we propose a Blass-matrix configuration algorithm taking into account the properties of the MZIs. Through simulations, we validate the potential for multibeam operation, and evaluate the beamforming performance at 28.5 GHz with respect to the QAM order, symbol rate, and pulse shaping parameters. In all cases with rate up to 3 Gbaud, the bit-error rate remains lower than 10–3, showing that the beam squinting effect, which is present in our design, can be tolerated. Finally, we study the frequency dependence of the beamforming performance due to inevitable asymmetries of the MZIs and length variations of the waveguides, and evaluate the impact of the imperfections in the couplers inside the MZIs and the phase shifters. We show that in all cases the performance degradation is negligible for realistic fabrication and operation conditions.

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.

Integrated optical signal processors have been identified as a powerful engine for optical processing of microwave signals. They enable wideband and stable signal processing operations on miniaturized chips with ultimate control precision. As a promising application, such processors enables photonic implementations of reconfigurable radio frequency (RF) filters with wide design flexibility, large bandwidth, and high-frequency selectivity. This is a key technology for photonic-assisted RF front ends that opens a path to overcoming the bandwidth limitation of current digital electronics. Here, the recent progress of integrated optical signal processors for implementing such RF filters is reviewed. We highlight the use of a low-loss, high-index-contrast stoichiometric silicon nitride waveguide which promises to serve as a practical material platform for realizing high-performance optical signal processors and points toward photonic RF filters with digital signal processing (DSP)-level flexibility, hundreds-GHz bandwidth, MHz-band frequency selectivity, and full system integration on a chip scale., Nanophotonics, 7 (2), ISSN:2192-8614

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

Universal linear optical networks made of on-chip tunable beam splitters and phase shifters form a very promising platform for quantum information processing (QIP). Thanks to their phase stability and reconfigurability, they are robust and enable a variety of quantum information and communication protocols such as quantum teleportation [1], quantum key distribution [2], photonic qubit gate protocols [3] and boson sampling [4]. Two known materials for on-chip platforms are silicon-on-insulator (SOI) and doped silica, where SOI allows for a high component density due to its high index contrast and silica has a low loss.

In this paper a microwave photonic broadband true time delay (TTD) continuously tuneable beamformer module for phased array antenna applications is presented. This microwave photonics beamformer is based on the hybrid integration of Indium Phosphide and Silicon Nitride chips. This paper features results from the latest transmit microwave photonic beamformer based on a completely integrated photonic assembly interface.

The presentation slides for “European Network for High Performance Integrated Microwave Photonics” are available at http://doi.org/10.1117/12.2536107 , under the Supplemental Content tab.

Centralized Radio Access Network (C-RAN) comprises one of the main technologies for high capacity and low latency fronthauling in 5G networks. In this paper, we propose and evaluate an optical fronthaul 5G C-RAN architecture that targets to meet the bandwidth, latency and energy requirements of high traffic hot-spot areas. The proposed architecture employs Intermediate-Frequency-over-Fiber (IFoF) signal generation by Photonic Integrated Circuit (PIC) Wavelength Division Multiplexing (WDM) optical transmitters at the Base-Band Unit (BBU), while a novel design of Reconfigurable Optical Add-Drop Multiplexers (ROADMs) is used at the Remote Radio Head (RRH) site. An aggregate capacity up to 96 Gb/s has been reported by employing two WDM links with 4-band Orthogonal Frequency Division Multiplexing (OFDM) 64-QAM 0.5 Gbaud signals, showing error vector magnitude performance within the acceptable 8% limit. The physical layer evaluation of the proposed fronthaul is also extended with the evaluation of the network throughput and mean packet delay latency, using a Medium Transparent-Medium Access Control (MT-MAC) protocol, which employs gated service indicating latencies below 10ms.