Optical True Time Delay Research Articles

A microwave photonic radio frequency memory with tunable delay resolution and Doppler frequency shift

Radar target simulation is essential for radar development. In this paper, a microwave photonic radio frequency memory (RFM) is proposed and experimentally verified. The input radio frequency (RF) signals are delayed and stored in a recirculating loop with a 4-bit binary optical true time delay line (OTTDL), which enables the achievement of the tunable delay resolution and multiple replications. The Doppler frequency shift (DFS) of the delayed signals is achieved by a Mach-Zehnder modulator (MZM) and two cascaded acousto-optic modulators (AOMs), which facilitate a wide tuning range, high precision, and a high spurious suppression ratio (SSR). The experimental results demonstrate that the system has 16 distinct delay resolutions ranging from 0.045 us to 7.576 us. Furthermore, 30 circulations are achieved in the test, indicating that the theoretical maximum storage time is approximately 225 us. Additionally, the frequency shift for 1 GHz–18 GHz RF pulse signals can be tuned over a range of 10 Hz to 10 MHz, with a SSR exceeding 40 dB.

Optical TTD compensation network-based phase precoding for THz massive MIMO systems

In sixth generation (6G) communications, terahertz (THz) communication is one of the most important technologies in the future due to its ultra-bandwidth, where hybrid beamforming has been widely used to solve the severe transmission attenuation in the THz band. However, the use of frequency-flat phase shifters in hybrid beamforming leads to the beam split effect. To solve the beam split influence, we propose a novel optical true time delay compensation network (OTTDCN)-based phase precoding structure with low power consumption. In the proposed scheme, the OTTDCN pre-generates multiple beam compensation modes to achieve phase compensation for different frequencies. As a result, the compensated beams can be reoriented toward the target direction at different frequencies. Moreover, a low-complexity beam compensation mode-based hybrid precoding algorithm is proposed, where the selection of the optimal beam compensation modes used for all radio-frequency (RF) chains with finite beam compensation modes is considered. The results show that the OTTDCN-based phase precoding scheme can effectively alleviate the beam split effect with low power consumption and achieve near-optimal performance.

Continuously tunable silicon optical true-time delay lines with a large delay tuning range and a low delay fluctuation.

On-chip switchable optical true-time delay lines (OTTDLs) feature a large group delay tuning range but suffer from a discrete tuning step. OTTDLs with a large delay tuning range and a continuous tuning capability are highly desired. In this paper, we propose and experimentally demonstrate a silicon-based broadband continuously tunable OTTDL comprising a 7-bit delay line and a switch-based continuously tunable delay line. The group delay of the entire OTTDL can be continuously tuned from 0 to 1020.16 ps. A delay error within -1.27 ps to 1.75 ps, and a delay fluctuation of less than 2.69 ps in the frequency range of 2∼25 GHz are obtained. We analyze the causes of the delay fluctuation and its influence on beamforming. Moreover, we also propose a simplified non-invasive calibration method that can significantly reduce the complexity of the delay state calibration and can be easily extended to delay lines with more stages of optical switches. The high performance of our OTTDL chip and the calibration method drive practical applications of integrated OTTDLs.

Cladding modulated silicon waveguide Bragg grating with TM-polarized light for optical true time delay line

An optical true time delay line (OTTDL) is an essential component in optical signal processing. A Bragg grating structure is usually employed in OTTDL design, which makes it possible to achieve a slow-light effect. Here, we present the design and experimental demonstration of a cladding modulated waveguide Bragg grating (CMWBG) that supports transverse magnetic polarization for OTTDL application. In contrast to traditional waveguide Bragg gratings that support transverse electric polarization, the proposed structure has low propagation loss and high fabrication tolerance characteristics. The slow-light effect of the CMWBG was experimentally demonstrated with a high group index value. An averaged maximum group delay value of up to 84.4 ps was achieved at a length of 1 mm CMWBG, and a tuning range of about 81.4 ps was obtained.

Photonic Radio Frequency Orbital Angular Momentum Generation and Beam Steering

Orbital angular momentum (OAM) is a fundamental property of electromagnetic waves and is inherently carried by all frequencies of waves. In particular, OAM applications at radio frequencies (RFs) have attracted much attention due to their unique functionalities in communications. Enabled by the present wireless communication system, employing a circular antenna array (CAA) is a highly compatible and cost-effective approach for generating RF-OAM beams. In this paper, recent progress on RF-OAM communication systems with CAAs from the transceiver to the controller is summarized. A new scheme for RF-OAM generation and beam steering systems is proposed based on dual-wavelength control using CAA with the energy-conserved feature, where an integrated chip incorporating an arrayed waveguide grating (AWG) structure is introduced. Either microwave photonic phase shift or optical true time delay (OTTD) can be operated by this chip, benefiting from the phase amplification concept and the design of delay channels. Utilizing their combination, RF-OAM beams with arbitrary OAM states can be achieved for multiplexing and steered simultaneously. Furthermore, RF-OAM beam steering can be achieved without distortion in the desired transmission direction. This photonics-assisted method with an integrated chip is a potential strategy for improving the signal quality and reducing the system complexity of RF-OAM transmission systems.

Large Scale Optical Assisted Mm-Wave Beam-Hopping System for Multi-Hop Bent-Pipe LEO Satellite Networks

By introducing the arrayed waveguide router (AWGR) optical true time-delay (OTTD) architecture in bent-pipe satellite optical inter-link to optically assist the RF phased array in Low-earth orbit satellites will extend the multi-hop bent-pipe satellite beam-hopping protocol proposed in DVB-S2X. It solves the challenge of beam steering with the support of precise, broadband, and wide-range scanning. This architecture utilizes a subarray to combine the advantages of AWGR and a high-precision RF phase shifter to realize the beam pointing without an oblique view. Unlike the traditional digital and analog phased array architecture, the introduction of OTTD can solve the problem of beam squint and also ensure the high-precision scanning of the beam.

Time Jitter Analysis of an Optical Signal Based on Gated On-Off Optical Sampling and Dual-Dirac Modeling

A time jitter analysis method for an optical signal based on gated on-off optical sampling and dual-Dirac modeling is proposed and demonstrated experimentally. The optical signal under test is firstly sampled by an optical sampling pulse train generated through the gating on-off modulation of a Mach–Zehnder modulator (MZM). The sampled pulse is then broadened using optical true-time delay and electrical low-pass filtering to reduce its bandwidth to match the sample rate of a low-speed electrical analog-to-digital converter (ADC), which is used to quantify the sampled pulse. An eye diagram is obtained from the quantified data and used to plot a time jitter histogram. Finally, the dual-Dirac model is introduced to analyze the time jitter histogram to obtain the total jitter (TJ), including the deterministic jitter (DJ) and random jitter (RJ). In the experiment, a 19.05 ps TJ, including a 13.20 ps DJ and a 5.85 ps RJ, is measured for a 2.5 GHz optical signal using the proposed time jitter analysis method. The results agree well with those measured with a commercial real-time oscilloscope.

Variable optical true-time delay line breaking bandwidth-delay constraints.

Continuously variable true-time optical delay lines are typically subject to a constraint of the bandwidth-delay product, limiting their use in several applications. In this Letter, we propose an integrated topology that breaks the bandwidth-delay product limit. The device is based on multiple Mach-Zehnder Interferometers (MZIs) arranged in parallel, providing easier control and a larger bandwidth compared to ring resonator-based solutions. The functionality of this architecture is demonstrated with a 4-stage delay line by performing measurements in both the time and frequency domains. The delay line introduces a delay of 90 ps over a bandwidth of more than 22 GHz with a negligible group delay distortion, operates on a wavelength range of about 60 nm, and is scalable to a higher number of MZI stages.

High accurate self-calibration of photonic integrated circuit using lossless thermo-optic phase shifters.

The accurate calibration of large-scale switch networks is critical for integrated photonics, in which the integrated optical true time delay chip is typical. In this work, a novel self-calibration method without extra testing ports is proposed by introducing lossless thermo-optic phase shifters instead to calibrate the network. As a demonstration, a 5-bit delay line based on silicon nitride is fabricated and calibrated. The extinction ratio of all the switches is greater than 30.9 dB at the cross and bar states. Using this method, the 5-bit optical delay line which can be tuned in a range of 118.53 ps and reach a low delay time deviation less than ±0.4 ps.

2-D beamforming system based on photonic lantern and few-mode fiber Bragg grating

In this paper, a 2-D beamforming system based on the photonic lantern (PL) and few-mode fiber Bragg grating (FM-FBG) with a 3 × 3 antenna array is proposed. The FM-FBG is produced by the femtosecond laser point-by-point method, which is flexible and simple; moreover, the 2-D time delay regulation can be realized with different positions of FM-FBG by a single wavelength and single-stage optical true time delay (OTTD) modules to complete the 2-D beam scanning, which reduces the system cost, complexity and simplifies the system structure. The effects of different types of PLs on the system are investigated with the results that the channel amplitude dither of the 2-D beamforming system using non-mode-selective PLs is less than 1.5 dB, and the main lobe beamwidth and sidelobe level are smaller than those of the mode-selective PL system. In addition, the effect of the polarization state of optical on the gain of the system is studied, and the experiment results show that the overall gain of the system is improved by 2.27 dB when we adjust the optical polarization state to the optimum. Under this best-adjusted polarization state, we construct a 2-D beamforming system based on the non-mode-selective PL with time delay steps of (12 ps, 10 ps), and the elevation and azimuth angles are achieved by switching the operating state of the optical switch for the scan ranges of 0°–60° and 0°–35°, respectively.

Experimental demonstration of dispersion-diversity multicore fiber optical beamforming.

Phased array antenna (PAA) beam-steering in high radiofrequency bands will be key for upcoming 5G & Beyond wireless communications systems. As a compact and efficient solution to provide tunable beam steering simultaneously to parallel antenna distribution and connectivity, we experimentally demonstrate, for the first time to our knowledge, tunable optical beamforming implemented on a dispersion-diversity multicore optical fiber. The uniqueness of this fiber lies in the fact that each one of its seven step-index trench-assisted cores has been tailored to provide the required chromatic dispersion to enable tunable optical true-time delay line operation. Continuous 1D beam-steering was demonstrated by measuring the radiating pattern of an in-house fabricated 8-element PAA in an anechoic chamber at the radiofrequency of 26 GHz, sweeping the beam-pointing angle from -43° up to 40° by varying the operating optical wavelength from 1542.50 up to 1548.28 nm. Next-generation fiber-wireless communications systems will benefit from the demonstrated dispersion-diversity MCF optical beamforming in terms of flexibility, versatility, capacity, and connectivity along with reduced size, weight, and power consumption.

Microwave Photonic Phase-/Delay-Tunable Mixer Based On OSSB-PolM With Ultralow Mixing Spurs

In this paper, a microwave photonic (MWP) phase-/delay-tunable mixer with ultralow mixing spurs based on optical single-sideband polarization modulation (OSSB-PolM) is proposed and theoretically investigated which is verified by simulations. In the proposed structure, by passing the OSSB-PolM signal through a polarization controller (PC) and an optical polarizer (Pol.), a carrier-suppressed optical single-sideband (CS-OSSB) modulated signal is generated. In the up-converter mode, optical carrier, intermediate frequency (IF) even- and (4n+3) odd-order sidebands, n is an integer, local oscillator (LO) even- and (4n+1) odd-order sidebands are suppressed without using any optical/electrical filter. Also, in the down-converter mode, optical carrier, radio frequency (RF) and LO even- and (4n+3) odd-order sidebands are suppressed. By beating sidebands of the pure CS-OSSB signal in a photodetector, RF up-converted (or IF down-converted) signal is generated with ultralow mixing spurs. Simulation results show that the sideband suppression ratio (SSR) for both cases of the up-/down-converted signals are >45 dB in the electrical domain. Effects of the phase and power imbalances of the practical non-ideal RF hybrid couplers on the performance of the proposed MWP mixer are investigated. It is shown that with ±3 degrees phase and ±1.5 dB power imbalances, the SSR decreases to 25.6 dB and 15.6 dB, respectively. Closed-form expression for the conversion efficiency is derived which is verified by simulations. Besides, simulations show that the spurious-free dynamic range (SFDR) of the proposed MWP mixer is 117.67 dB.Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> (with −160 dBm/Hz noise floor). In addition, optical frequency conversion and optical phase-shifting functions are combined and a full-range phase-tunable MWP mixer is proposed and investigated. Besides, optical frequency conversion function and optical true-time delay (TTD) are combined and a delay-tunable MWP mixer is introduced and investigated. It can find many applications in MWP systems, such as in MWP TTD beamformers for broadband squint-less array antennas.

On-chip optical true time delay lines based on subwavelength grating waveguides

On-chip optical true time delay lines based on subwavelength grating waveguides

Optical true time delay pool-based beamforming and limited feedback for reconfigurable intelligent surface-empowered cloud radio access networks

Optical true time delay pool-based beamforming and limited feedback for reconfigurable intelligent surface-empowered cloud radio access networks

True Time Delay Optical Beamforming Network Based on Hybrid Inp-Silicon Nitride Integration

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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) platform. The photonic integrated circuit (PIC) comprises a hybrid InP-Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> external cavity laser, a pair of InP phase modulators, a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> 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.

Optical true time delay pool based hybrid beamformer enabling centralized beamforming control in millimeter-wave C-RAN systems

Optical true time delay pool based hybrid beamformer enabling centralized beamforming control in millimeter-wave C-RAN systems

Large-scale true-time-delay remote beamforming with EO frequency combs and multicore fiber.

Optical true-time-delay (OTTD) beamforming is a promising solution to support the ultra-broadband radio access network. However, large-scale antenna arrays set at remote radio units require the OTTD counterpart to have corresponding larger-scale channel numbers. Here, we demonstrate an OTTD remote beamforming network with a record 287 channel number using electro-optic frequency combs and multicore fiber. Our proposed scheme can generate beams for both one-dimensional and two-dimensional antenna arrays. We highlight that using multicore fiber not only increases the channel numbers but also supports remote beamforming. We estimate the long-term stability of this remote beamforming network, and 1-ps-level relative time delay variation in 2 h is obtained when using multicore fiber. It is one order of magnitude better than using parallel single-mode fibers. Thus, highly stable beamforming is achieved. These results pave the way for the application of OTTD beamforming in 5G and beyond networks.

Reflective coupled microring resonators for reconfigurable photonic systems: Performance analysis

We propose reflective type triple coupled microring resonators (TCMMRs) for reconfigurable photonic based systems. We have developed an analytical model and analyzed its useful performance characteristics. The TCMMRs based device exhibits a wide range of performance characteristics such as multi-band filtering, optical logical operations, and optical true-time delay lines. Reconfigurable multiple resonances and their splitting features associated with the device provide useful filtering performance characteristics such as high out-of-band rejection, flat-top bandwidth, and near to ideal shape factor. The device can perform multiple functions such as bandpass, bandstop, and notch filtering with a good performance. Unlike two input narrowband logical operations demonstrated in the literature, our proposed device can perform as three-input optical logical operator, and it can switch wideband microwave frequency signals optically. Moreover, the device exhibits wideband flat top group delay response. It carries out delay tuning using a single variable coupling coefficient which simplifies the complexity of the tuning encountered in binary tree-structured optical beamforming networks based on conventional microring resonators.

An Up/Downstream Shared Optical Beam Forming Network for Remote Phased Array Antenna

An up/downstream shared optical beam forming network based on wavelength-swept for remote phased array antenna is presented and experimentally demonstrated. A shared optical true time delay (TTD) network minimizes the size and cost, which is made by 16 paths of variable lengths of dispersion compensation fibers splicing with single-mode fiber, and then corporate with optical circulators. The experimental results show that the proposed system provides wide bandwidth from 12 GHz to 17GHz. The main lobe is oriented from -49.3° to 45.9° and the spurious free dynamic range (SFDR) of 104.58 dB·Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> is achieved in this system. Besides, the optical beam forming network is not affected by external temperature interference. This structure could have applications in remote phased array antenna.

Frozen mode in coupled silicon ridge waveguides for optical true time delay applications

We propose a simple photonic waveguide structure that exhibits light propagation modes with vanishing group velocity via mode degeneracy. This enables stationary inflection point dispersion leading to the frozen mode and a true time delay device suitable for ultra-wide-band beamforming for millimeter wave (mmWave) phased arrays. The structure consists of three silicon ridge waveguides in proximity with periodic gaps introduced in the outer waveguides to create a band gap. The structure is complementary metal–oxide–semiconductor compatible with a very small footprint of only about 56 µ m 2 and more resilient to fabrication uncertainties as compared to the previously studied structures. Simulation results show transmission of 70% of the incident wave for the frozen mode at the 1.55 µm (193.6 THz) wavelength through the waveguide. It also enables a delay-bandwidth product of 6.75 along with unprecedented frequency independent bandwidth of about 0.5 THz for RF-mmWave–terahertz beamforming.