True Time Delay Line Research Articles

A structure optimization for integrated binary reconfigurable true time delay lines

The structure optimization for a binary reconfigurable true time delay lines (RTTDL) based on the Mach–Zehnder interferometer (MZI) switches is proposed. Firstly, the number of the variable optical attenuator (VOA), which is generally used to make up for the imperfectness of the MZI switch and improves the RTTDL ’s flatness of broadband amplitude response and the delay accuracy, is minimized. Meanwhile, by adopting a pair of photoelectric detectors (PD) between every two MZI switches for power monitoring, the initialization time can be greatly reduced owing to the realization of the parallel initialization. On the other hand, with the help of the PDs, a continuously adjustable delay scheme is put forward by adjusting the allocation ratio of the last MZI switch. The proof-of-principle calculations are carried out to verify the optimization. The results show that the proposed optimization not only reduces the power consumption, the optical link loss and the integration volume caused by the VOAs, but also solves the problem that the binary RTTDL has only discrete delay states.

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.

위성탑재용 광대역 영상레이다의 빔 분산 및 고도 변화에 의한 거리 방향 모호성 성능 개선 방안

In a wideband radar, a true time delay line (TTDL) is used to compensate for the propagation delay that has a different value according to the operational frequency. This study analyzes the antenna pattern characteristics of wideband synthetic aperture radar (SAR) that does not require any TTDL. In addition, it defines an antenna pattern to compensate for beam dispersive effects and a method to improve the range ambiguity to signal ratio (RAR). Considering the altitude change that occurs in a SAR satellite, this paper proposes a method to optimize the operational parameters and consequently enhance the RAR performance. The proposed method uses altitude-dependent equations to calculate the operational parameters; hence it can be easily applied to the real operation of a SAR satellite. The RAR analysis for operational altitude variation of 35 km shows that an improvement of approximately 2.1-dB, 0.9-dB, and 2.1-dB is achieved at the stripmap, wide-swath, and high-resolution modes respectively. The RAR is improved to comply with the requirement in the stripmap mode, whereas the wide-swath and high-resolution modes have a sufficient margin.

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.

Panda-type few-mode fiber-enabled microwave photonic filter with a reconfigurable finite impulse response.

Leveraging various physical dimensions of optical signals allows significant scaling of the transmission capacity for optical communications. Here we experimentally demonstrate a reconfigurable microwave photonic filter (MPF) enabled by both mode and polarization division multiplexing arising in a Panda-type few-mode fiber. The LP01x, LP01y, LP11x, and LP11y modes are used to realize optical true-time delay lines of the MPF, thus achieving a four-tap finite impulse response (FIR) with a free spectral range of around 2.22 GHz. Due to the employed delay between two orthogonal polarizations, the number of taps arising in its FIR can be doubled. Moreover, the reconfigurable FIR of the MPF can be achieved by loading the corresponding phase pattern onto a spatial light modulator. Finally, the low-pass FIR can be switched to the bandpass FIR due to the introduction of polarization modulation, indicating the flexible manipulation of the FIR for the multi-dimensional division multiplexing enabled MPF.

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

An optical true time delay line (OTTDL) is a fundamental building block for signal processing applications in microwave photonics and optical communications. Here, we experimentally demonstrate an index-variable OTTDL based on an array of 40 subwavelength grating (SWG) waveguides in silicon-on-insulator. Each SWG waveguide in the array is 34 mm long and arranged in a serpentine manner; the average incremental delay between waveguides is about 4.7 ps, and the total delay between the first and last waveguides is approximately 181.9 ps. The waveguide array occupies a chip area of ∼6.5mm×8.7mm=56.55mm2. The proposed OTTDLs bring potential advantages in terms of compactness as well as operation versatility to a variety of microwave signal processing applications.

A wideband 1[formula omitted]4 optical beam-forming chip based on switchable optical delay lines for Ka-band phased array

Comparing with electronic beam forming, integrated optical beam forming is very promising for optical controlled phase array system, because much broader band beam steering can be achieved without the beam squint effect. The single channel optical true time delay lines based on the cascade microring resonators and switched optical delay waveguides have been extensively investigated. The optical beam forming chip based on multi-channel microring resonators have also been demonstrated, however only limited working bandwidth were obtained due to their inherent tradeoff between the delay and bandwidth. The bandwidth limitation can be overcome by using integrated optical beam forming chip composed of switched optical delay waveguides, which has not been demonstrated and evaluated. Here, a 1×4 optical beamforming chip with four integrated switchable delay lines units designed to control the Ka-band phased antenna arrays is proposed and demonstrated experimentally. The maximum delay of 96 ps with a delay increment about 3.02 ps/stage was obtained, which is in accordance with the design well. Based on the measured delay times, the directional diagrams of the optical beamforming network were simulated and beam angle coverage of 90° can be achieved. Furthermore, a magnitude response experiment was carried out to evaluate the performance of the optical beam forming chip, and a remarkable working bandwidth of 29 GHz was achieved.

Performance analysis of reconfigurable multifunctional photonic filters using triple coupled microring resonators

In this paper, we have introduced a new device architecture based on triple coupled microring resonators structure. The analytical model of the device is developed and performance analysis is carried out. The novelty of this device is that it performs as a multifunctional reconfigurable photonic filter in which multiple filtering functions such as notch, bandpass and bandstop responses are achieved by programming the coupling coefficients associated with it using thermal or electro optic tuning. Moreover, it can be used as a tuneable photonic true time delay line. As a bandpass reconfigurable filter, the maximally flat-top rectangular shape like bandpass response of 42 dB suppression ratio, and maximally flat-top bandwidth tunability of 1 GHz to 45 GHz are achieved. The notch filter response has an ultra-high rejection ratio of 148 dB. The flat bottom bandstop response has high suppression ratio of 81.5 dB within 7 GHz bandwidth. The device exhibits minimum group delay distortion over 18 GHz bandwidth, and a minimum of 8 ps and a maximum of 58 ps time delay tunability.

Analysis and Design of On-Chip RF Interconnect Line for Wideband True-Time Delay Line Application

This paper presents the analytical resistance–capacitance–inductance–conductance (RLCG) model of the on-chip interconnect line (IL) based on its structure, and the proposed model can be used to design IL and analyze the delay characteristics. Using electromagnetic (EM) simulation, the relations between the inductance, quality factor and the width, length of IL are obtained, which verifies the proposed RLCG model of IL. The delay model of IL is derived and verified with respect to the effects of the [Formula: see text] and [Formula: see text] by simulation, which can provide the benefit for the true-time delay line (TTDL) design using IL. This work proposes the experiments on the delay characteristics of 3-bit TTDL with IL based on 0.13[Formula: see text][Formula: see text]m SiGe BiCMOS technology. The group delay and transient delay of the TTDL are measured, which exhibits a maximal relative delay of 35 ps with an average of 5 ps delay resolution over a frequency range of 14–34[Formula: see text]GHz. The results are consistent with the delay analysis based on the proposed IL model.

Two-Dimensional Fiber Beamforming System Based on Mode Diversity

In this paper, a novel two-dimensional (2D) beamforming system based on mode diversity and true-time delay (TTD) of optical fiber is proposed. The system is composed of two cascaded TTD units, which are single-mode fiber TTD line array and mode TTD line array of few-mode fiber. For the first time, the mode diversity technology is applied to the 2D beamforming system which can realize beam scanning at a single wavelength and greatly reduces the cost and complexity. A beamforming system with 3 × 3 array antenna is built, and the properties of which are experimentally evaluated under a single wavelength of 1550.52 nm. The azimuth and elevation angles can reach the scanning range of ±60°. The gain performance are characterized, which decreases about 6 dB when the angle changes from (0.11°, -0.11°) to (59.82°, 60.21°) or (-59.25°, -60.02°). The influence of frequency and mode coupling on the system performance are evaluated. The beam direction of the system is independent of microwave signal frequency while the test scope is 8.5 GHz to 11.5 GHz. Mode coupling has no effect on the beam direction of the system.

Instantaneous microwave frequency measurement using few-mode fiber-based microwave photonic filters.

Instantaneous frequency measurement (IFM) of microwave signals using photonic methods provides a novel and efficient approach for fast and broadband radio frequency (RF) signal analysis. Here, we propose and experimentally demonstrate a photonic-assisted IFM method utilizing a few-mode fiber-based microwave photonic technique. By offset splicing the few-mode fiber (FMF) with a single mode fiber, both LP01 and LP11 modes can be excited, which is used to develop a microwave photonic filter (MPF). A detailed analysis of the FMF as the true time delay line is presented. An amplitude comparison function (ACF) that is the ratio of frequency response traces of an MPF pair is established, which is used to determine the unknown microwave frequency instantaneously. The proof-of-concept experiment demonstrates a frequency measurement range of 0.5 GHz to 17.5 GHz and a measurement accuracy of ±0.2 GHz in most of the frequency points. The proposed system has the merits of simplicity, cost-effectiveness, compactness and robustness.

Dispersion-tailored few-mode fiber design for tunable microwave photonic signal processing.

We present a novel double-clad step-index few-mode fiber that operates as a five-sampled tunable true-time delay line. The unique feature of this design lies in its particular modal chromatic dispersion behavior, which varies in constant incremental steps among adjacent groups of modes. This property, which to the best of our knowledge has not been reported in any other few-mode fiber to date, is the key to tunable operation of radiofrequency signal processing functionalities implemented in few-mode fibers. The performance of the designed true-time delay line is theoretically evaluated for two different microwave photonics applications, namely tunable signal filtering and optical beamforming networks for phased array antennas. In the 35-nm optical wavelength tuning range of the C-band, the free spectral range of the microwave filter and the beam-pointing angle in the phased array antenna can be continuously tuned from 12.4 up to 57 GHz and 12.6° up to 90°, respectively.

LNOI waveguide grating based true time delay line for tunable bandpass microwave photonic filter

A precisely tunable true time delay line (TTDL) based on lithium niobate on insulator waveguide grating is proposed. Utilizing the electro-optic effect of lithium niobate, active tuning of reflected wavelength and time delay can be realized. The numerical simulation results show that, with the optimized grating structure and flexible electrode distribution, wide tuning range and high tuning accuracy of time delay can be achieved by simply controlling the charging position of the electrodes. This TTDL is applied to a tunable bandpass microwave photonic filter (TBMPF), which using PM-IM conversion to suppressed dc signal to achieve bandpass. In the TBMPF, the TTDL plays the role of introducing the dispersion for PM-IM conversion, time delay between the taps and tuning of central frequency. This TTDL can reduce the system complexity and cost of microwave photonic filter.

Inkjet-Printed Graphene-Based 1 × 2 Phased Array Antenna.

Low-cost and conformal phased array antennas (PAAs) on flexible substrates are of particular interest in many applications. The major deterrents to developing flexible PAA systems are the difficulty in integrating antenna and electronics circuits on the flexible surface, as well as the bendability and oxidation rate of radiating elements and electronics circuits. In this research, graphene ink was developed from graphene flakes and used to inkjet print the radiating element and the active channel of field effect transistors (FETs). Bending and oxidation tests were carried out to validate the application of printed flexible graphene thin films in flexible electronics. An inkjet-printed graphene-based 1 × 2 element phased array antenna was designed and fabricated. Graphene-based field effect transistors were used as switches in the true-time delay line of the phased array antenna. The graphene phased array antenna was 100% inkjet printed on top of a 5 mil flexible Kapton® substrate, at room temperature. Four possible azimuth steering angles were designed for −26.7°, 0°, 13°, and 42.4°. Measured far-field patterns show good agreement with simulation results.

Silicon integrated microwave photonic beamformer

Optical beam-forming networks (OBFNs) based on optical true-time delay lines (OTTDLs) are well known as the promising candidate for solving the bandwidth limitation of traditional electronic phased array antennas (PAAs) due to beam squinting. Here we report, to the best of our knowledge, the first monolithic 1 × 8 microwave photonic beamformer, based on switchable OTTDLs on the silicon-on-insulator platform. The chip consists of a modulator, an eight-channel OBFN, and eight photodetectors, thus including hundreds of active and passive components in total. It has a wide operating bandwidth from 8 to 18 GHz, which is almost two orders larger than that of electronic PAAs. The beam can be steered to 31 distinguishable angles in the range from − 75.51 ∘ to 75.64°, based on the beam pattern calculation with the measured radiofrequency response. The response time for beam steering is 56 µs. These results represent a significant step toward the realization of integrated microwave photonic beamformers that can satisfy compact-size and low-power consumption requirements for the future radar and wireless communication systems.

Integrated waveguide true time delay beamforming system based on an SOI platform for 28 GHz millimeter-wave communication.

In this paper, we propose an on-chip waveguide beamforming system for a 28 GHz millimeter-wave signal based on silicon-on-insulator (SOI) technology. The system consists of four true time delay (TTD) lines, each of which consists of nine Bragg gratings with different periods. The minimum period of the grating is 312 nm, and the maximum period is 344 nm. By optimizing the position of the Bragg grating in the adjacent TTD lines, a time delay difference can be generated between the adjacent channels. By adjusting the operation wavelength of the optical carrier, the TTD lines can provide 9 different beamforming direction angles from -60∘ to 60° when the direction angular resolution is 15°. This proposed system has a useful application prospect in the phased array antennas of millimeter-wave communication.

True-Time Delay Line based on dispersion-flattened 19-core Photonic Crystal Fiber

True-Time Delay Line based on dispersion-flattened 19-core Photonic Crystal Fiber

True-Time Delay Line Based on Dispersion-Flattened 19-Core Photonic Crystal Fiber

A novel design of a tunable True-Time delay line (TTDL) based on a multicore photonic crystal fiber (PCF) is proposed. It enables simultaneous transport and processing of microwave photonic signals over a broad radiofrequency processing range. Independent group delay behavior in 19 different cores characterized by a constant differential group delay between cores provides TTDL operation on 19 signal samples. The 19-core PCF structure allows tailoring the chromatic dispersion range between 1.5 and 31.2 ps/nm·km, which translates into a very broad microwave signal processing range from a few up to tens of GHz. A near-zero dispersion slope reduces the time delay errors in the TTDL's operation to less than 5% in a 40-nm optical wavelength range, thus ensuring its satisfactory performance. Its performance as a TTDL is evaluated in terms of higher-order dispersion as well as other degradation effects such as crosstalk and nonlinear fiber response. A high index contrast between core and cladding, between 0.3 to 1.5%, enables low intercore crosstalk and confinement losses as well as greater robustness against fiber bends and twists. Fabrication of this type of MCF might be possible although it will prove challenging. This work advances the state-of-the-art of a TTDL based on SDM technology by increasing the number of samples and microwave processing range.

Tunable Microwave Photonic Filter Based on LNOI Polarization Beam Splitter and Waveguide Grating

A tunable microwave photonic filter (MPF) based on a polarization beam splitter (PBS) and waveguide grating true-time delay line (TTDL) is proposed and demonstrated experimentally. Taking advantage of the high birefringence property of lithium niobate (LN), the PBS is achieved by a simple structure in a short length. The TTDL consists of an LNOI waveguide grating and flexible electrodes covering along the grating. Owing to the electro-optic effect of LN, a wide tuning range and high tuning accuracy of delay time can be achieved simply by controlling the charging position of the electrodes. With the proposed PBS and TTDL, the MPF exhibits low drive voltage, low power consumption, and high response rate in a simple construction.

Losses do not prevent wide band metamaterial true time delay line applications, GaAs MMIC experimental verification

Losses are often the impairing feature of metamaterial true time delay line devices. In this study, the authors analytically demonstrate that ohmics losses in this kind of structures as well as impedance mismatch depend on the bandwidth. These results allow to consider wide band application for radar systems. It is verified experimentally that metamaterial delay line devices are fitted for wide band delay line applications for they allow low insertion losses, low return loss and tunable delay.