Microwave Photonics Applications Research Articles

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.

Low-loss compact chalcogenide microresonators for efficient stimulated Brillouin lasers.

Chalcogenide glasses (ChGs) possess a high elasto-optic coefficient, making them ideal for applications in microwave photonics and narrow-linewidth lasers based on stimulated Brillouin scattering (SBS). However, current As2S3-based integrated devices suffer from poor stability and low laser-induced damage threshold, and planar ChG devices feature limited quality factors. In this Letter, we propose and demonstrate a high-quality integrated GeSbS ChG Brillouin photonic device. By introducing Euler bending structures, we suppress high-order optical modes and reduce propagation losses in a finger-shaped GeSbS microresonator, resulting in a compact footprint of 3.8 mm2 and a high intrinsic quality factor of 5.19 × 106. The combination of GeSbS material's high Brillouin gain and the resonator's high-quality factor enables the generation of stimulated Brillouin lasers with a low threshold of 0.96 mW and a fundamental linewidth of 58 Hz. Moreover, cascaded stimulated Brillouin lasers can be realized up to the seventh order, yielding microwave beat frequencies up to 40 GHz.

GHz-rate 57-fs acousto-optic mode-locking fiber laser based on cascaded all-fiber pulse compression.

We demonstrate a compact ultrafast fiber laser system that can deliver 1.87 GHz pulse train at 1550 nm with a pulse energy of 52 pJ and an ultrashort pulse duration of 57 fs. While an acousto-optic mode-locking fiber laser was used as the seed light source at GHz rate, a stage of Er-doped fiber amplifier boosted the laser power to ∼320 mW, giving a pulse energy of ∼170 pJ. Then, a pulse compression setup was constructed, providing a high compression ratio of ∼10 with a total efficiency of ∼32%. In the cascaded compression configuration, multiple fiber samples with alternately normal and anomalous dispersion were fused together, providing efficient nonlinear spectral broadening while suppressing excessive pulse broadening over propagation. This GHz-rate ultrafast fiber laser, with compact configuration, broad optical spectrum, and high time-resolving ability could be used as the seed light source for constructing high-rate, high-power ultrafast laser systems and may find a few applications in optical measurements and microwave photonics.

Tailorable ITO thin films for tunable microwave photonic applications

Tunability is a fundamental prerequisite for functional devices and forms the backbone of reconfigurable microwave photonic (MWP) signal processors. In this paper, we explore the use of indium tin oxide (ITO) thin films, notable for their combination of optical transparency and electrical conductivity, to provide tunability for integrated MWP devices. We study the impacts of post-thermal annealing on the structural, electrical, and optical properties of ITO films. The annealed ITO microheater maintains a low total insertion loss of just 0.1 dB while facilitating the tunability of the microring across the entire free spectral range (FSR) using less than half the voltage required by its non-annealed counterpart. Furthermore, the post-annealed ITO film exhibits a 30% improvement in response time, enhancing its performance as an active voltage-controlled microheater. Leveraging this advantage, we employed the post-annealed device to demonstrate continuous tunable radio frequency (RF) phase shifts from 0–330° across a frequency range spanning 15 GHz to 40 GHz with only 5.58 mW of power. The flexibility in modifying the ITO thin film properties effectively bridges the gap between achieving low-loss and high-speed thermo-optic based microheaters.

Highly efficient photonic radar by incorporating MDM-WDM and machine learning classifiers under adverse weather conditions.

Photonic radar, a cornerstone in the innovative applications of microwave photonics, emerges as a pivotal technology for future Intelligent Transportation Systems (ITS). Offering enhanced accuracy and reliability, it stands at the forefront of target detection and recognition across varying weather conditions. Recent advancements have concentrated on augmenting radar performance through high-speed, wide-band signal processing-a direct benefit of modern photonics' attributes such as EMI immunity, minimal transmission loss, and wide bandwidth. Our work introduces a cutting-edge photonic radar system that employs Frequency Modulated Continuous Wave (FMCW) signals, synergized with Mode Division and Wavelength Division Multiplexing (MDM-WDM). This fusion not only enhances target detection and recognition capabilities across diverse weather scenarios, including various intensities of fog and solar scintillations, but also demonstrates substantial resilience against solar noise. Furthermore, we have integrated machine learning techniques, including Decision Tree, Extremely Randomized Trees (ERT), and Random Forest classifiers, to substantially enhance target recognition accuracy. The results are telling: an accuracy of 91.51%, high sensitivity (91.47%), specificity (97.17%), and an F1 Score of 91.46%. These metrics underscore the efficacy of our approach in refining ITS radar systems, illustrating how advancements in microwave photonics can revolutionize traditional methodologies and systems.

Turn-key Kerr soliton generation and tunable microwave synthesizer in dual-mode Si3N4 microresonators.

This study investigates the thermal compensation mechanism in dual-mode Si3N4 microresonators that demonstrates the ease of generation of single-solitons with nearly octave-wide spectral bandwidth. The deterministic creation of soliton frequency combs is achieved by merely switching the wavelength of a tunable laser or a semiconductor diode laser in a single step. The pump frequency detuning range that can sustain the soliton state is 30 gigahertz (GHz), which is approximately 100 times the resonance linewidth. Interestingly, these dual-mode resonators also support the coexistence of primary combs and solitons, enabling their utilization as functional microwave synthesizers. Furthermore, these resonators readily facilitate the generation of diverse multi-solitons and soliton crystals. This work presents a simplified system to access high-performance and versatile Kerr solitons, with wide-ranging applications in optical metrology, microwave photonics, and LiDAR.

Multi-format all-optical modulating of microsphere resonator operating in O-band

We propose and demonstrate an all-optical tunable silica microsphere resonator based on a dual-laser pumping scheme. The resonance tunable microsphere resonator can act as a modulator and operates in the O-band. We discussed the relationship between the extinction ratio and pump power in different coupling state, and realized an extinction ratio over 15 dB. Also, we measured the rise and fall time of the modulated signal, which is below 1 μs, and discussed its relationship with the modulation frequency. In addition, based on this scheme, a multi-format modulation is realized for the first time with signals shaping in triangular, sinusoidal and square wave under different duty cycles. Our work reveals the detail of all-optical tunning of the resonance of the silica microresonator and opens up possible applications in optical communication, microwave photonics and quantum photonics.

Optomechanical ring resonator for efficient microwave-optical frequency conversion

Phonons traveling in solid-state devices are emerging as a universal excitation for coupling different physical systems. Phonons at microwave frequencies have a similar wavelength to optical photons in solids, enabling optomechanical microwave-optical transduction of classical and quantum signals. It becomes conceivable to build optomechanical integrated circuits (OMIC) that guide both photons and phonons and interconnect photonic and phononic devices. Here, we demonstrate an OMIC including an optomechanical ring resonator (OMR), where co-resonant infrared photons and GHz phonons induce significantly enhanced interconversion. The platform is hybrid, using wide bandgap semiconductor gallium phosphide (GaP) for waveguiding and piezoelectric zinc oxide (ZnO) for phonon generation. The OMR features photonic and phononic quality factors of >1 × 105 and 3.2 × 103, respectively. The optomechanical interconversion between photonic modes achieved an internal conversion efficiency {eta }_{i}=(2.1pm 0.1)% and a total device efficiency {eta }_{{tot}}=0.57{times 10}^{-6} at a low acoustic pump power of 1.6 mW. The efficient conversion in OMICs enables microwave-optical transduction for quantum information and microwave photonics applications.

Microwave Photonic Filters and Applications

Microwave photonics is a promising and rapidly developing interdisciplinary field. It combines microwave and photonic techniques to generate, transmit, process, and manipulate microwave signals by using the advantages of broadband, high frequency, and low loss provided by photonics. As an important branch of microwave photonics, the microwave photonic filter (MPF) can overcome the limitations set by traditional electronic technology and can realize advanced signal processing in modern communication systems due to its higher performance, selectivity, and flexibility. This review provides a comprehensive overview of MPFs, including fundamental principles, typical structures, and key applications. Additionally, the microwave photonic integration is a very important tendence because of its advantages of small size, light weight, low power consumption, and low cost. The recent advances in integrated MPF are also reviewed.

Compact real-time RF spectrum analyzer with 16 GHz instantaneous bandwidth based on photonic frequency-shifting loops

Spectral analysis of broadband RF signals in real time is of primary importance for numerous applications. So far, the instantaneous bandwidth of real-time spectrum analyzers based on conventional digital techniques is limited to a few GHz. This limitation is set by the clock jitter of the analog-to-digital converters, and by the processing capabilities in real time of digital electronics. On the contrary, analog architectures based on microwave photonics are not constrained by such limitations, and offer potentially a very high instantaneous bandwidth. However, they generally suffer from inherent limitations, such as large footprint and high complexity. Here, we propose a much simpler architecture of RF spectrum analyzer based on frequency-shifting loops. It utilizes only compact commercial telecom components, a single CW laser, and slow electronic resources (10 MSa/s). The probability of intercept is 100%, the instantaneous bandwidth reaches 16 GHz, and the spectral and temporal resolutions are respectively equal to 160 MHz and 50 µs. Our system is expected to open new avenues in embedded applications of microwave photonics.

Linearization of a Dual-Parallel Mach-Zehnder Modulator Using Optical Carrier Band Processing

The linearization of a microwave photonic link based on a dual-parallel Mach-Zehnder modulator is theoretically described and experimentally demonstrated. Up to four different radio frequency tones are considered in the study, which allow us to provide a complete mathematical description of all third-order distortion terms that arise at the photodetector. Simulations show that a complete linearization is obtained by properly tuning the DC bias voltages and processing the optical carrier band. As a result, a suppression of 17 dB is experimentally obtained for the third-order distortion terms, as well as a SDFR improvement of 3 dB. The proposed linearization method enables the simultaneous modulation of four different signals without the need of additional radio frequency components, which is desirable to its implementation in integrated optics and makes it suitable for several applications in microwave photonics.

Multiresonant metasurfaces for arbitrarily broad bandwidth pulse chirping and dispersion compensation

We show that ultrathin metasurfaces with a specific multiresonant response can enable simultaneously arbitrarily strong and arbitrarily broadband dispersion compensation, pulse (de)chirping, and compression or broadening. This breakthrough overcomes the fundamental limitations of both conventional nonresonant approaches (bulky) and modern singly resonant metasurfaces (narrowband) for quadratic phase manipulations of electromagnetic signals. The required nonuniform trains of resonances in the electric and magnetic sheet conductivities that completely control phase delay, group delay, and chirp are rigorously derived and the limitations imposed by fundamental physical constraints are thoroughly discussed. Subsequently, a practical, truncated approximation by finite sequences of physically realizable linear resonances is constructed and the associated error is quantified. By appropriate spectral ordering of the resonances, operation can be achieved either in transmission or reflection mode, enabling full space coverage. The proposed concept is not limited to dispersion compensation, but introduces a generic and powerful ultrathin platform for the spatiotemporal control of broadband real-world signals with a myriad of applications in modern optics, microwave photonics, radar, and communication systems.

Observation of deterministic double dissipative-Kerr-soliton generation with avoided mode crossing

Dissipative Kerr solitons (DKSs) in microresonators have boosted the development of the chip-scale ultrastable microcomb sources, and thrived in both fundamental physics and a wide range of applications. Among various DKS states, single DKS, double DKS, and a perfect soliton crystal could be identified simply based on the optical spectrum. Especially, the double DKS state, due to its two-pulse-interference nature, has recently found its own application in microwave photonics, such as reconfigurable rf filters. However, the traditional method to generate double DKS usually yields stochastically relative positions of the DKS in the cavity, which limits the versatility of the application. Here a method to deterministically generate double DKSs with fixed relative positions assisted by the dual-pump driven scheme in a 97-GHz ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ microresonator is demonstrated. Via the dual-pump scheme, not only has the single soliton been repeatably generated, double DKS with deterministic relative positions has also been realized through backward tuning. The effects of pump wavelength tuning and pump power on the relative positions are investigated. A direct bridge between the relative positions and the avoided mode crossing induced modulated cw background is established. This work not only provides insight to DKS dynamics in a dual-pump scheme, but also improves the versatility of double-DKS based applications in microwave photonics.

U-band multiwavelength Brillouin-Raman random fiber laser with single Brillouin frequency spacing

A single Brillouin frequency interspacing (∼10 GHz) multiwavelength Brillouin-Raman random fiber laser with lasing operation in the U-band is demonstrated through nonlinear interactions of stimulated Raman scattering and stimulated Brillouin scattering as amplification mechanisms. Structurally, dispersion compensation fiber of 11.2 km length was employed as a highly nonlinear gain medium. For a single Brillouin frequency interspacing operation, a quasi-bidirectional double pass distributed Raman amplification was proposed by looping back the forward lasing spectrum together with the residual Raman signal into the laser cavity. The optimized Raman pump power was set to 1020 mW at 1550 nm; whereas Brillouin pump power and wavelength were optimized at 0 dBm and 1640 nm respectively. Under these conditions, 476 channels interspaced by 10 GHz and spectrally flattened within 3 dB peak power variation were generated over ∼40.8 nm spectral bandwidth from 1640.9 to 1681.7 nm, with 16.28 dB average optical signal-to-noise ratio, and −15.25 dBm average Stokes peak power. Meanwhile, total output power of 19.77 dBm was recorded at 20 % optical power conversion efficiency with 0.10 dB power fluctuation during 1 hour of continuous operation at ambient conditions. The proposed U-band Brillouin-Raman random fiber laser has potential applications in optical communication, optical sensing, and microwave photonics.

A High Performance Silicon Nitride Optical Delay Line With Good Expansibility

In this paper, based on the double strip silicon nitride platform, we designed and fabricated a low loss continuously tunable broadband optical delay line, which contains a 5-bit optical switched delay line and a single tunable optical ring resonator working at “off-resonance”. Optical double sideband modulation link measurement results show it can achieve continuous delay tuning of 395.5 ps and in-band delay fluctuations of about 4 ps and 1 ps were obtained in 8 GHz and 5 GHz working bandwidths, respectively. In addition, the proposed optical delay line has a low on-chip insertion loss of 3 dB and a very low optical loss delay ratio of 0.0022 dB/ps. Moreover, it's delay tuning range can be easily increased by cascading more thermo-optic switches. The delay bandwidth can be expanded by adopting the optical single sideband modulation or increasing the FSR of the optical ring resonator. This integrated tunable optical delay line with good overall performances and expand ability shows great potential for microwave photonic applications.

Silicon nitride-based Kerr frequency combs and applications in metrology

Kerr frequency combs have been attracting significant interest due to their rich physics and broad applications in metrology, microwave photonics, and telecommunications. In this review, we first introduce the fundamental physics, master equations, simulation methods, and dynamic process of Kerr frequency combs. We then analyze the most promising material platform for realizing Kerr frequency combs—silicon nitride on insulator (SNOI) in comparison with other material platforms. Moreover, we discuss the fabrication methods, process optimization as well as tuning and measurement schemes of SNOI-based Kerr frequency combs. Furthermore, we highlight several emerging applications of Kerr frequency combs in metrology, including spectroscopy, ranging, and timing. Finally, we summarize this review and envision the future development of chip-scale Kerr frequency combs from the viewpoint of theory, material platforms, and tuning methods.

Tunable kilohertz microwave photonic bandpass filter based on backscattering in a microresonator.

A tunable microwave photonic bandpass filter (MPBPF) with a kilohertz bandwidth based on the backscattering mode of a silica microsphere resonator is proposed and experimentally demonstrated. In this work, an ultrahigh-quality-factor microsphere resonator is used to generate a radio frequency bandpass response with a bandwidth of 600 kHz. Meanwhile, scattering-induced coupling between the clockwise mode and the counterclockwise mode is introduced to reduce the number of resonance modes, and a single backscattering mode which has a high extinction ratio is obtained. Therefore, an MPBPF with a tuning range of 40 GHz and a rejection ratio of 16.9 dB is realized. This MPBPF possesses advantages such as ultranarrow bandwidth, large tuning range, and compactness, and shows great potential for microwave photonic applications.

Reconfigurable unidirectional propagation of electromagnetic waves in photonic crystal waveguides

We demonstrate reconfigurable unidirectional propagation of electromagnetic waves in waveguide channels sandwiched by two dielectric photonic crystal (PC) slabs or magnetic PC (MPC) slabs, where the extrinsic and intrinsic responses emerge in two kinds of systems. Concretely, the unidirectionality of the MPC system originates from the time-reversal symmetry breaking nature of magnetic material, while in the dielectric PC system, the unidirectionality is achieved by active control of two separated line sources. The results indicate that in the MPC based system, not only the amplitude but also the directionality of the guiding electromagnetic waves is flexibly regulated by controlling the separation and phase difference of two active line sources. However, the directionality in the MPC based system is determined by the magnetization due to the intrinsic unidirectionality of the magnetic system, but for the dielectric PC based system, directionality is controlled by active sources. The introduction of a second line source in the MPC based system can result in two remarkably different consequences. On one hand, the directionality can be switched from forward to backward propagation by reversing magnetization. On the other hand, the same operation results in the transition from unidirectional propagation to localization of the electromagnetic waves by dynamically controlling two active sources. The functionality and flexibility in the present systems might find potential applications in microwave photonics and integrated photonics.

Fully Reconfigurable Fano Resonator on a Silicon Photonic Chip

A fully reconfigurable Fano resonator on a silicon photonic chip is reported. The Fano resonator is realized with the use of an add-drop micro-disk resonator (MDR) and a Mach-Zehnder interferometer (MZI). The waveguides at the through and drop ports of the MDR connect to two spiral waveguide arms of the MZI directly. Two independent metallic micro-heaters are incorporated on top of the MDR and the lower arm of the MZI. Thanks to the resonant mode interference between the MDR and the MZI, the Fano resonance occurs. By thermal tuning the resonance coupling between the MDR and the MZI, the generated Fano resonance is fully tunable. In addition to the independent tunable slope rate (SR) and the resonance wavelength, the Fano resonance wavelength can shift while its line shape remains by simultaneously controlling the two micro-heaters. A chip is designed, fabricated and evaluated. The measurement results show that the static Fano resonance has a line shape with an extinction ratio (ER) of 17.1 dB and SR of −13.6 dB/nm. By simultaneously controlling the two micro-heaters, full reconfigurability of the Fano resonance is demonstrated. Thanks to its straightforward configuration, the proposed Fano resonator holds the key advantage in terms of ultra-compactness and fully reconfigurability, which offers a great potential for applications such as on-chip optical switching, modulation and microwave photonics applications.

High-performance modified uni-traveling carrier photodiode integrated on a thin-film lithium niobate platform

Lithium niobate on insulator (LNOI) has become an intriguing platform for integrated photonics for applications in communications, microwave photonics, and computing. Whereas, integrated devices including modulators, resonators, and lasers with high performance have been recently realized on the LNOI platform, high-speed photodetectors, an essential building block in photonic integrated circuits, have not been demonstrated on LNOI yet. Here, we demonstrate for the first time, heterogeneously integrated modified uni-traveling carrier photodiodes on LNOI with a record-high bandwidth of 80 GHz and a responsivity of 0.6 A/W at a 1550-nm wavelength. The photodiodes are based on an n-down InGaAs/InP epitaxial layer structure that was optimized for high carrier transit time-limited bandwidth. Photodiode integration was achieved using a scalable wafer die bonding approach that is fully compatible with the LNOI platform.