Christos Tsokos

In this work, we present the first fully packaged $16\times 16$ Benes optical switch based on 3D photonic integration operating in the O-band, and demonstrate, for the first time, its system-level evaluation. The 3D switch employs a dual-layer architecture with 56 thermo-optic $2\times 2$ Mach-Zehnder interferometer-based crossbar units and vertical multimode interferometers enabling the transition between the two parallel waveguiding layers and effectively eliminating in-plane waveguide crossings. The static optical performance of the 3D optical switch prototype was evaluated based on insertion and polarization-dependent losses, crosstalk, power consumption, and switching speed. The 3D switch is fabricated on a polymer-based photonic platform, known as PolyBoard, which is suitable for developing 3D structures. In order to perform a system-level demonstration, the switch was integrated into a 25 Gb/s full-duplex optical link using a commercial Network Interface Card and SFP transceivers. Stable throughput was maintained across back-to-back and extended-reach fiber links up to 10 km, demonstrating the viability of this 3D-integrated switch for scalable, energy-efficient intra-data center interconnect.

This work presents the first experimental system evaluation of a hybrid-integrated 4×4 optical beamforming network (OBFN) based on the Blass Matrix architecture employing modulated microwave signals, evaluating the capability of generation and steering of one (single-beam) and two (multi-beam) independent beams. The OBFN-PIC is realized by combining the silicon nitride photonic integration platform of LioniX Int., known as TriPleX with indium phosphide (InP). Its architecture includes a splitting stage implemented with cascaded tunable Mach-Zehnder Interferometers (MZIs), a modulation stage comprising of InP phase modulators (PMs), a 4x4 Blass Matrix beamforming network, and a filtering and carrier re-insertion (CRI) stage using four asymmetric ring-assisted Mach-Zehnder interferometer (RAMZI) filters. Under the assumption that the OBFN-PIC is integrated into a downlink wireless communication system, feeding a 4-element antenna array, single-beam functionality is validated using both pure sinusoidal and modulated signals at 12 GHz, with QPSK and 16-QAM formats at symbol rates up to 1 Gbaud. The evaluation includes beam steering over a 30°–150° angular range, employing error-vector-magnitude (EVM) and bit-error-ratio (BER) performance metrics at the received signal, that confirm error-free performance. Multi-beam functionality is demonstrated by simultaneously generating two independent beams at either 5 GHz or 12 GHz, as well as under a dual-frequency scenario combining both, with varying modulation parameters. A Gaussian excitation profile is applied when sidelobe suppression is required to reduce inter-beam interference. In all examined scenarios, error-free communication is suggested, verifying the suitability of the proposed photonic beamforming architecture for dynamic and scalable wireless communication systems.

Optoelectronic technology is expected to be the cornerstone of sub-THz communication systems, enabling access to and use of the vast frequency resources found in this portion of the spectrum. In this work we demonstrate a photonics-enabled sub-THz wireless link operating in real-time settings, using a PIN-PD-based THz emitter, and a THz receiver based on an ultra-fast photoconductor. The real-time generation and detection of the information signal is performed by an intermediate frequency (IF) unit based on a commercially available mmWave platform, operating at 1.6 GBaud. The evaluation of our setup takes place on two phases. Firstly, a homodyne scenario is demonstrated, where the same pair of lasers is used at the transmitter and receiver side. Secondly, we demonstrate a heterodyne scheme, employing optical phase locking techniques at the receiver. Error-free operation was achieved in both scenarios at a bit rate of 3.2 Gb/s, over 1 m of free-space with ambient air. The broadband characteristics of our setup were validated, achieving error-free transmission over a 0.22 THz range, spanning from 90 up to 310 GHz. Finally, the stability of our real-time link was successfully demonstrated, showing stable SNR performance at the receiver with adaptive capabilities, over a time period of 5 min and 22 sec.

We demonstrate the generation, of a mmWave signal via the injection of an optical frequency comb (OFC) into an integrated tunable dual distributed Bragg reflector (DBR) laser as well as the fiber transmission and the processing of this signal by an optical beamforming network (OBFN). The dual DBR laser is based on a hybrid indium phosphide (InP) – polymer photonic integrated circuit (PIC). Two different cases have been examined in which the microwave signal is centered around 39 GHz and 60 GHz respectively, carrying quadrature amplitude modulation (QAM) formats at 0.5 Gbaud. In this proof-of-concept scenario, the OBFN consists of two optical paths, where the relative true time delay is induced by an optical delay line (ODL). Extensive comparison between the back-to-back (B2B) case and scenarios with transmission over 25 km of standard single-mode fiber (SSMF) has been made using the error-vector magnitude (EVM) and the bit-error ratio (BER) as evaluation criteria. In all cases, error-free transmission was suggested for all QPSK signals, whereas a worst-case EVM of 11.8% was observed for 16-QAM transmission, successfully showcasing the concept’s potential. The generated microwave signal’s frequency can be set arbitrarily high, provided that high-speed photodetection equipment is available for the detection and down-conversion of the signal. Extension to higher antenna elements (AEs) numbers is straight-forward, relying only on the number of available photodetectors.

We propose a novel concept for the implementation of 2-dimensional (2D) optical phased arrays (OPAs) with end-fire waveguides as antenna elements (AEs), and we present its theoretical model and experimental proof. The concept is based on the use of 3-dimensional (3D) photonic integrated circuits (PICs) with multiple waveguiding layers on the PolyBoard platform. In their simplest form, the 3D PICs comprise AEs at different layers, vertical and lateral couplers for the distribution of light among the AEs, and phase shifters for the execution of the 2D beam scanning process. Using the field equivalence principle, we model the radiated field from the single-mode waveguide of the platform at 1550 nm, and we find that the expected beam width is 12.7°. We also investigate the perturbation that is induced into propagating fields inside parallel waveguides in proximity, and we conclude that waveguide spacings down to 6 µm can be safely used for development of uniform OPAs in the PolyBoard platform. For OPAs with 6 µm pitch and 4 AEs, we find that the maximum steering angle is 14.0° and the expected angular clearance, wherein the main radiation lobe is higher than any grating lobe by at least 3, 6 and 10 dB is 10.8°, 7.6° and 2.8°, respectively. Based on our simulations, we design and fabricate single- and 2-layer PICs with 1 × 4 and 2 × 4 OPAs. The lateral pitch of the OPAs ranges from 10 down to 6 µm, while the vertical pitch is 7.2 µm. We experimentally characterize these OPAs and validate the potential of the 2-layer PICs for 2D beam scanning on the azimuthal and elevation plane. The beam profiles and the main scanning parameters such as the maximum steering angle and the relative intensity between the main and the grating lobes are found in excellent agreement with our simulations.

We demonstrate a broadband and continuously tunable 1×4 optical beamforming network (OBFN), based on the hybrid integration of indium phosphide (InP) components in the silicon nitride (Si ₃ N ₄ ) platform. The photonic integrated circuit (PIC) comprises a hybrid InP-Si ₃ N ₄ external cavity laser, a pair of InP phase modulators, a Si ₃ N ₄ 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.

We propose a novel concept for the implementation of 2-dimensional (2D) optical phased arrays (OPAs) with end-fire waveguides as antenna elements (AEs), and we present its theoretical model and experimental proof. The concept is based on the use of 3-dimensional (3D) photonic integrated circuits (PICs) with multiple waveguiding layers on the PolyBoard platform. In their simplest form, the 3D PICs comprise AEs at different layers, vertical and lateral couplers for the distribution of light among the AEs, and phase shifters for the execution of the 2D beam scanning process. Using the field equivalence principle, we model the radiated field from the single-mode waveguide of the platform at 1550 nm, and we find that the expected beam width is 12.7 ^o . We also investigate the perturbation that is induced into propagating fields inside parallel waveguides in proximity, and we conclude that waveguide spacings down to 6 µm can be safely used for development of uniform OPAs in the PolyBoard platform. For OPAs with 6 µm pitch and 4 AEs, we find that the maximum steering angle is 14.0 ^o and the expected angular clearance, wherein the main radiation lobe is higher than any grating lobe by at least 3, 6 and 10 dB is 10.8 ^o , 7.6 ^o and 2.8 ^o , respectively. Based on our simulations, we design and fabricate single- and 2-layer PICs with 1×4 and 2×4 OPAs. The lateral pitch of the OPAs ranges from 10 down to 6 µm, while the vertical pitch is 7.2 µm. We experimentally characterize these OPAs and validate the potential of the 2-layer PICs for 2D beam scanning on the azimuthal and elevation plane. The beam profiles and the main scanning parameters such as the maximum steering angle and the relative intensity between the main and the grating lobes are found in excellent agreement with our simulations.

We demonstrate a broadband and continuously tunable 1×4 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. The performance of the OBFN-PIC is experimentally characterized by measuring the link gain, noise figure, and spurious free dynamic range of the microwave photonics links. 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 multielement 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. An error-free performance is achieved for all the experimental cases validating the potential of the proposed OBFN-PIC for high-quality beamforming performance. To our best of knowledge, this is the first thorough performance evaluation of a fully integrated OBFN-PIC.

We propose and experimentally demonstrate a photonic integrated circuit (PIC) that operates as an optical equalizer (OE) with multi-rate and multi-channel capability. The OE has the structure of a 3-tap direct form finite impulse response (FIR) filter and is based on the use of micro-ring resonators (MRRs) for the tuning of its delay lines. The PIC is fabricated on TriPleX platform and has 17 reconfigurable elements in total including nine MRRs, five optical couplers and three standalone phase shifters. Using this OE in an on-off keying system with bandwidth limitations we achieve an eye-diagram opening improvement more than 14 dB working with signals at 4.67 and 5.84 Gbaud both in single- and dual-channel operation. Extension to higher modulation formats is direct. Extension to higher symbol rates is also possible via the use of smaller MRRs.