Adam Raptakis

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.

We demonstrate a compact heterodyne Laser Doppler Vibrometer (LDV) based on the realization of optical frequency shift in the silicon nitride photonic integration platform (TriPleX). We theoretically study, and experimentally evaluate two different photonic integrated optical frequency shifters (OFSs), utilizing serrodyne and single-sideband (SSB) modulation. Both OFSs employ stress-optic modulators (SOMs) based on the non-resonant piezoelectrical actuation of lead zirconate titanate (PZT) thin-films, deposited on top of the silicon nitride waveguides with a wafer-scale process. To improve the modulation bandwidth of the SOMs we investigate a novel configuration of the electrodes used for the actuation, where both electrodes are placed on top of the PZT layer. Using this top-top electrode configuration we report frequency shift of 100 kHz and 2.5 MHz, and suppression ratio of the unwanted sidebands of 22.1 dB and 39 dB, using the serrodyne and the SSB OFS, respectively. The best performing SOM structure induces 0.25π peak-to-peak sinusoidal phase-shift with 156 mW power dissipation at 2.5 MHz. We use the SSB-OFS in our compact LDV system to demonstrate vibration measurements in the kHz regime. The system comprises a dual-polarization coherent detector built in the PolyBoard platform, utilizing hybrid integration of InP photodiodes (PDs). High quality LDV performance with measurement of vibration frequencies up to several hundreds of kHz and displacement resolution of 10 pm are supported with our system.

We present a real-world Gigabit Passive Optical Network (GPON)-based Fiber to the Home (FTTH) network serving up to 32 users, installed in COSMOTE laboratories in Athens. The deployment relies on the use of fiber optics and GPON telecom equipment which are identical to those found in the currently deployed FTTH infrastructure. We describe real-life network designs that correspond to specific installations considered in Athens urban metropolitan area. We propose a practical upstream Quantum Access Network (QAN) over the installed FTTH network, relied on a centralized quantum receiver (Bob) station located at the operator's Central Office, which exploits the Time-Division Multiplexing (TDM) framing technique to receive single-photons from multiple independent quantum transmitters (Alice) located at users premises. This integration scheme is characterized by performing a series of noise measurements at single-photon level to evaluate the potential of the coexistence of classical/quantum links on the same fiber cores. The noise count rates are found to follow the Raman noise profile for specified values of effective Raman cross-sections of downstream and upstream GPON data channels. Subsequently, the experimental results are exploited to evaluate the performance of the widely-used Coherent One Way (COW) Quantum Key Distribution (QKD) protocol. The presented simulation results for the quantum layer suggest the feasibility of our integration scheme for short feeder fiber segments. Finally, the dependance of the Secure Key Rate (SKR) on the network capacity is also discussed.

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.