Microwave Photonics Research Articles

Guest Editorial Guest Editorial for the Special Issue on Microwave Photonics

Guest Editorial Guest Editorial for the Special Issue on Microwave Photonics

Tunable Microwave Photonic Filter With Ultra-Narrow Passband and High Out-of-Band Rejection

Tunable Microwave Photonic Filter With Ultra-Narrow Passband and High Out-of-Band Rejection

Microwave Photonic Adaptive Complex Wideband Self-Interference Cancellation Enabled by Digital Pre-Modeling Search and Two-Tap Delay Adjustment

Microwave Photonic Adaptive Complex Wideband Self-Interference Cancellation Enabled by Digital Pre-Modeling Search and Two-Tap Delay Adjustment

Integrated Pockels Modulators on Silicon Photonics Platform

AbstractElectro‐optic (EO) modulators are essential components in various fields, including optical communication, free‐space communication, microwave photonics, sensing, and light detection and ranging. The EO modulation enables the fast conversion of electric signals into optical signals, facilitating the precise manipulation of light. With advancements in fabrication processing techniques, next‐generation integrated EO modulators have demonstrated substantial improvements in modulation efficiency, bandwidth, and footprint. Here, the latest research progress in integrated EO modulation, focusing on the principle of the Pockels effect, key modulation metrics, novel EO thin‐film material platforms, and innovative device architectures is overviewed. Finally, it is evaluated different schemes and provide perspectives on future trends in developing integrated EO modulators, highlighting both the advantages and challenges of integrated EO modulation, including waveguide and electrode engineering, integrated methods, and other applications for large‐scale photonic integrated circuits.

Fivefold increased power handling in waveguide UTC-PDs using dual-injection shared CPW PD-arrays

This work presents two circuit-based solutions to enhance the power handling capabilities of waveguide-integrated uni-travelling carrier photodetectors (WG-UTC-PDs). Compared to a baseline WG-UTC-PD, these solutions achieve a fivefold increase in photocurrent before thermal breakdown. First, dual-injection improves the optical power distribution within a baseline WG-UTC-PD, raising the photocurrent threshold before thermal breakdown. Second, an array of four optically parallel WG-UTC-PDs, electrically connected to a single coplanar waveguide (CPW) line, further increases the maximum photocurrent by distributing the input optical power across multiple PDs. The design omits a termination resistor, as the arrays do not rely on a traveling wave configuration, maximizing photocurrent without a 50% reduction of bandwidth. Both 4-PD single-injection and dual-injection arrays were designed, fabricated, and characterized. Compared to a baseline UTC-PD, with a maximum photocurrent of 1.8 mA at a 3 dB bandwidth of 55 GHz, the 4-PD single-injection circuit achieved 5.1 mA at 43 GHz. The dual-injection array further increased the photocurrent to 9.2 mA at a bandwidth of 35 GHz. Electrical reflection measurements confirmed the negative effects of CPW losses on RF performance. These power handling improvements enable compact, high-power integrated solutions for microwave photonics.

High-Linearity Dual-Parallel Mach–Zehnder Modulators in Thin-Film Lithium Niobate

Microwave photonic (MWP) systems are inseparable from conversions of microwave electrical signals into optical signals, and their performances highly depend on the linearity of electro-optic modulators. Thin-film lithium niobate (TFLN) is expected to be an ideal platform for future microwave photonic systems due to its compact size, low optical loss, linear electro-optic effect, and high bandwidth. In this paper, we propose a TFLN modulator with a low voltage–length product (VπL) of 1.97 V·cm and an ultra-high-linearity carrier-to-distortion ratio (CDR) of 112.33 dB, using a dual-parallel Mach–Zehnder interferometer configuration. It provides an effective approach to fully suppress the third-order intermodulation distortions (IMD3), leading to 76 dB improvement over a single Mach–Zehnder modulator (MZM) in TFLN. The proposed TFLN modulator would enable a wide variety of applications in integrated MWP systems with large-scale integration, low power consumption, low optical loss, and high bandwidth.

Modeling and application of a bidirectional Mach–Zehnder electro-optic modulator with enhanced high-frequency modulation efficiency

We present a simple way to enhance bidirectional broadband modulation in a traveling wave Mach–Zehnder electro-optic modulator (EOM) by removing matched impedance and effecting a reflection of the microwave signal. A model was devised for determining the modulation efficiency (ME) in the presence of microwave reflection, without access to the physical parameters of the EOM. By using the model, the simulated MEs matched well with the measured ones, which verified the correctness of the model and demonstrated that the internal parameters of the modulator could be inferred from the model. Compared with the traditional unidirectional EOM, the measured ME of the bidirectional modulator for the optical signal that counter-propagates with the incident microwave signal was improved by 10 times at least. When the velocities of microwave and optical signals were matched, the MEs at the frequency higher than 3.3 GHz were further improved. Our model was also used to derive the optical interference signal utilizing bidirectional modulation. It was found that the signal intensity was enhanced significantly, and further improved after introducing the velocity match. This work can be expected to open up many microwave photonics applications where bidirectional optical modulation is utilized in novel sensing and metrology.

Performance Comparison of Seed Generation Techniques of Stimulated Brillouin Scattering-Based Microwave Photonics Amplifier and Filter

This work presents a Brillouin amplification performance comparison of seed generation techniques using double-sideband suppressed carrier (DSB-SC) and single-sideband suppressed carrier (SSB-SC) modulations. The SSB-SC is obtained using an optical bandpass filter (OBPF) and in-phase and quadrature Mach-Zehnder modulator (IQ-MZM). All three techniques provide high amplification performance with optical signal-to-noise ratio (OSNR) enhancement of 37.47 dB, 33.14 dB, and 32.67 dB using DSB-SC, SSB-SC/OBPF, and SSB-SC/IQ-MZM, respectively. The best seed generation technique is using the DSB with a signal amplification of 62.47 dB. The technique presents ~4 dB higher OSNR enhancement due to the dual-energy transfer obtained from the beating process of the DSB than SSB. A ~3 dB OSNR reduction is found when pump linewidth (LW) was changed from 1kHz to 50 MHz, which suggests using a low-cost pump source whenever the OSNR reduction is not critical. The work also shows that the three techniques required 10 dBm stimulated Brillouin scattering threshold (SBST) to stimulate the process. An additional analysis of DSB-SC shows that a high-carrier suppression during the seed generation technique using MZMs is insignificant to the amplification performance. The high-carrier suppression produces a high seed signal power that distorts the Brillouin gain spectrum (BGS) and the pump depletion region, hence reducing the Brillouin gain (BG). Since carrier suppression is not a primary consideration, a cost-effective MZM with a modest extinction ratio requirement is allowed. The relaxed requirement of the pump’s linewidth and MZM’s extinction ratio suggest a cost-effective development of the SBS-based optical amplifier with narrow filter bandwidth.

A Cross-Eye Jamming Scheme Based on Microwave Photonics and Its Hardware-in-Loop Performance Investigation

A Cross-Eye Jamming Scheme Based on Microwave Photonics and Its Hardware-in-Loop Performance Investigation

Stimulated Brillouin scattering in silica optical nanofibers

Stimulated Brillouin scattering offers a broad range of applications, including lasers, sensors, and microwave photonics, most of which require strong Brillouin gain within a narrow bandwidth. Here, we experimentally report the first measurement of stimulated Brillouin scattering in silica optical nanofibers from both hybrid and surface acoustic waves. Using a pump–probe technique in the radio frequency domain, we measured a Brillouin gain as high as 15 m−1 W−1 and linewidth to 16 MHz for the L03 hybrid acoustic mode near 9 GHz using a 990-nm diameter nanofiber. This gain is 65 times larger than the highest gain obtained in standard single-mode fibers. In addition, we report a Brillouin gain of up to 5 m−1 W−1 from surface acoustic waves around 5 GHz. We further demonstrate a nanofiber-based Brillouin laser with a threshold of 350 mW. Our results create opportunities for advanced Brillouin-based applications utilizing optical nanofibers.

Quantum Entanglement between Optical and Microwave Photonic Qubits

Entanglement is an extraordinary feature of quantum mechanics. Sources of entangled optical photons were essential to test the foundations of quantum physics through violations of Bell’s inequalities. More recently, entangled many-body states have been realized via strong nonlinear interactions in microwave circuits with superconducting qubits. Here, we demonstrate a chip-scale source of entangled optical and microwave photonic qubits. Our device platform integrates a piezo-optomechanical transducer with a superconducting resonator which is robust under optical illumination. We drive a photon-pair generation process and employ a dual-rail encoding intrinsic to our system to prepare entangled states of microwave and optical photons. We place a lower bound on the fidelity of the entangled state by measuring microwave and optical photons in two orthogonal bases. This entanglement source can directly interface telecom wavelength time-bin qubits and gigahertz frequency superconducting qubits, two well-established platforms for quantum communication and computation, respectively. Published by the American Physical Society 2024

Photonics-assisted high-precision free space RF synchronization technology

In this paper, a high-precision frequency synchronization scheme in free space based on microwave photonic architecture is proposed, which can be used between multi-nodes in relative motion or in a motionless state. The reference frequency is modulated to high frequency through microwave photon up-conversion, and the reference signal is recovered at the receiver through photonics self-mixing technology. The experiment over free space in the laboratory in 4m, shows that the performance of Allen variance is s under the noise floor is s, and it is better than that of the electrical generation method with the Allen variance is s. The impact of system signal-to-noise ratio (SNR) on frequency stability is analyzed, and the experimental results are consistent with the theoretical value. At the same time, a functional experiment based on free space RF synchronization is conducted, compared to the direct synchronization method, the phase degradation of this method is only 0.25 rad, and the degradation of peak side lobe ratio (PLSR) of pulse compression loss is 0.29 dB. The good, coherent result further verifies the correctness of this synchronization method.

Seamlessly merging radar ranging and imaging, wireless communications, and spectrum sensing for 6G empowered by microwave photonics

To facilitate intelligent interconnection among people, machines, and things, the next generation of communication technology must incorporate various sensing functions besides high-speed wireless communications. Integration of radar, wireless communications, and spectrum sensing is being investigated for 6G with increased spectral efficiency, enhanced system integration, and reduced cost. Microwave photonics, a technique that combines microwave engineering and photonic technology is considered an effective solution for implementing the integration and breaking the bottleneck problems of electronic solutions. Here, we show a photonics-assisted joint radar, wireless communications, and spectrum sensing system that enables precise perception of the surrounding physical and electromagnetic environments while maintaining high-speed communication. Communication signals and frequency-sweep signals are merged optically using a shared system architecture and hardware to achieve signal level sharing, ultimately simultaneously achieving high-accuracy radar ranging and imaging with a measurement error within ± 4 cm and an imaging resolution of 25 × 24.7 mm, high-data-rate wireless communications at 2 Gbaud, and wideband spectrum sensing with a frequency measurement error within ±10 MHz in a 6 GHz bandwidth.

Quantum-enhanced learning with a controllable bosonic variational sensor network

Abstract The emergence of quantum sensor networks has presented opportunities for enhancing complex sensing tasks, while simultaneously introducing significant challenges in designing and analyzing quantum sensing protocols due to the intricate nature of entanglement and physical processes. Supervised learning assisted by an entangled sensor network (SLAEN) (Zhuang and Zhang 2019 Phys. Rev. X 9 041023) represents a promising paradigm for automating sensor-network design through variational quantum machine learning. However, the original SLAEN, constrained by the Gaussian nature of quantum circuits, is limited to learning linearly separable data. Leveraging the universal quantum control available in cavity quantum electrodynamics experiments, we propose a generalized SLAEN capable of handling nonlinear data classification tasks. We establish a theoretical framework for physical-layer data classification to underpin our approach. Through training quantum probes and measurements, we uncover a threshold phenomenon in classification error across various tasks—when the energy of probes exceeds a certain threshold, the error drastically diminishes to zero, providing a significant improvement over the Gaussian SLAEN. Despite the non-Gaussian nature of the problem, we offer analytical insights into determining the threshold and residual error in the presence of noise. Our findings carry implications for radio-frequency photonic sensors and microwave dark matter haloscopes.

Suppression of intensity noise and phase noise for thulium-holmium co-doped fiber laser by self-injection locking

The present study proposes and demonstrates a narrow-linewidth thulium-holmium co-doped fiber laser based on self-injection locking. The laser with single-longitudinal-mode operation is realized using a fiber Bragg grating as a wavelength-selection component and a dual-ring compound cavity as a mode-selection filter. The linewidth was compressed by increasing the photon lifetime by utilizing delay fibers of different lengths in the feedback cavity. The results show that the optical signal-to-noise ratio of the output laser is 57.2 dB, and the relative intensity noise is -130.43 dB/Hz with self-injection locking. The intrinsic linewidth is compressed from 39.45 kHz to 3.21 kHz, a reduction of nearly an order of magnitude. The linewidth compression mechanism solves the problems associated with deep linewidth compression in fiber lasers with long ring cavities and fosters the development of practical and reliable all-fiber structures. The laser source is characterized by low cost, high coherence, and low noise, which are highly desirable features in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing.

Ultra-broadband near-field Josephson microwave microscopy

Abstract Advanced microwave technologies constitute the foundation of a wide range of modern sciences, including microwave integrated circuits, quantum computing, microwave photonics, spintronics, etc. To facilitate the design of chip-based microwave devices, there is an increasing demand for state-of-the-art microscopic techniques capable of characterizing the near-field microwave distribution and performance. In this work, we integrate Josephson junctions onto a nano-sized quartz tip, forming a highly sensitive microwave mixer on-tip. This allows us to conduct spectroscopic imaging of near-field microwave distributions with high spatial resolution. Leveraging its microwave-sensitive characteristics, our Josephson microscopy achieves a broad detecting bandwidth of up to 200 GHz, as well as remarkable frequency and intensity resolutions. Near-field characterizations of microwave circuits are also conducted to demonstrate the capabilities of Josephson microscopy. Our work emphasizes the benefits of utilizing Josephson microscopy as a real-time, non-destructive technique to advance integrated microwave devices.

All-optical organic photochemical integrated nanophotonic memory: low-loss, continuously tunable, non-volatile

Controlling changes in the optical properties of photonic devices allows photonic integrated circuits (PICs) to perform useful functions, leading to a large breadth of applications in communications, computing, and sensing. Many mechanisms to change optical properties exist, but few allow doing so in a reversible, non-volatile manner. Without such mechanisms, power inefficiencies and use of external memory are inevitable. In this work, we propose and experimentally demonstrate reversible, non-volatile phase actuation of a silicon nitride PIC with thermally stable photochromic organic molecules vapor-deposited within a slot waveguide structure. The use of a high-core-index platform allows the photochemical phase actuation of a planar-resonator-based photonic memory unit, which enables positive and negative signal weighting and permits integrated spectroscopic analysis. We show properties of this all-optical memory for a silicon photonics platform, including low loss in the optical C-band, first-order photokinetics of the photoconversion, bidirectional scalable switching, and continuous tuning. Such features are critical for memories in analog applications such as quantum, microwave, and neuromorphic photonics, where bipolar weights, low loss, and precision are paramount. More generally, this work suggests that back-end-of-line-compatible vapor deposition of organic molecules into silicon photonic circuits is promising to introduce non-silicon-native functionality.

A Microwave Photonic Channelized Receiver Based on Polarization-Division Multiplexing of Optical Signals

Aimed at the problems of optical frequency combs, such as their large number of comb lines, their high flatness, and their lack of ease in generating, as well as the fact that the channelization efficiency of the scheme based on optical frequency combs is low, we proposed a microwave photonic channelization receiver based on signal polarization multiplexing. Using two-line local optical frequency combs with different frequencies to demodulate the RF signal in the orthogonal polarization state, 16 sub-channels with a bandwidth of 1 GHz can be received simultaneously. The experimental results show that the image rejection ratio can reach 28 dB, and the third-order spurious-free dynamic range of the system can reach 96.8 dB·Hz2∕3. This scheme has the advantages of a large number of sub-channels and a high channelization efficiency; it has great application potential in broadband wireless communication, radar, and electronic warfare systems.

Performance Analysis of Microwave Photonic Spectral Filters based on Optical Microcombs

AbstractMicrowave transversal filters, which are implemented based on the transversal filter structure in digital signal processing, offer a high reconfigurability for achieving a variety of signal processing functions without changing hardware. When implemented using microwave photonic (MWP) technologies, also known as MWP transversal filters, they provide competitive advantages over their electrical counterparts, such as large operation bandwidth, strong immunity to electromagnetic interference, and low loss when processing signals at high frequencies. Recent advances in high‐performance optical microcombs provide compact and powerful multi‐wavelength sources for MWP transversal filters that require a larger number of wavelength channels to achieve high performance, allowing for the demonstration of a diverse range of filter functions with improved performance and new features. Here, a comprehensive performance analysis for microcomb‐based MWP spectral filters based on the transversal filter approach is presented. First, the theoretical limitations are investigated in the filter spectral response induced by finite tap numbers. Next, the distortions are analyzed in the filter spectral response resulting from experimental error sources. Finally, the influence of input signal's bandwidth on the filtering errors is assessed. These results provide a valuable guide for the design and optimization of microcomb‐based MWP transversal filters for a variety of applications.

Microwave photonics frequency scanning approach to absolute distance measurement

An absolute distance measurement approach based on microwave photonics is proposed to provide a high-resolution and non-ambiguity range measurement solution by microwave frequency scanning. The microwave interference spectrum containing optical path difference information can be generated through two rounds of intensity modulation. The approach combines the characteristics of easy focusing and collimation of light waves, as well as easy manipulation of microwaves, to achieve accurate absolute distance measurement. The experimental results show that high-precision measurements can be achieved within the frequency scanning range of only 2 GHz. The error of the distance measurement is less than ±0.079 mm. In addition, the effectiveness of the scanning pattern and the data processing algorithm are verified.