GHz Microwave Signal Research Articles

Monolithic optical resonator for ultrastable laser and photonic millimeter-wave synthesis

Optical resonators are indispensable tools in optical metrology that usually benefit from an evacuated and highly-isolated environment to achieve peak performance. Even in the more sophisticated design of Fabry-Perot (FP) cavities, the material choice limits the achievable quality factors. For this reason, monolithic resonators are emerging as promising alternative to traditional designs, but their design is still at preliminary stage and far from being optimized. Here, we demonstrate a monolithic FP resonator with 4.5 cm3 volume and 2 × 105 finesse. In the ambient environment, we achieve 18 Hz integrated laser linewidth and 7 × 10−14 frequency stability measured from 0.08 s to 0.3 s averaging time, the highest spectral purity and stability demonstrated to date in the context of monolithic reference resonators. By locking two separate lasers to distinct modes of the same resonator, a 96 GHz microwave signals is generated with phase noise -100 dBc/Hz at 10 kHz frequency offset, achieving orders of magnitude improvement in the approach of photonic heterodyne synthesis. The compact monolithic FP resonator is promising for applications in spectrally-pure, high-frequency microwave photonic references as well as optical clocks and other metrological devices. ©2024. All rights reserved.

Quantitative analysis of the tropospheric delay differences for 1–300 GHz microwave signals with ground-based microwave radiometric profiles

Tropospheric delay is a significant source of error in various microwave measurement technologies. As the tropospheric delay differences associated with the signal frequency can be neglected for signals near the L band, the Global Navigation Satellite System (GNSS) is frequently used to assist other microwave measurements on tropospheric delay correction for its high precision and all-weather capabilities. However, when the signal frequency differences are significant, further analysis of tropospheric delay related to signal frequency is required for higher accuracy in long-distance propagation. This study quantitatively analyzed the tropospheric delay differences from L-band to millimeter band (1–300 GHz) affected by dry air, water vapor, and liquid water, based on Microwave Radiometer (MWR) measurement profiles and the Millimeter-wave Propagation Model93 (MPM93) at Shanghai. Relative to the GNSS L-band signal of 1.2 GHz, the RMS of tropospheric delay differences over 1–300 GHz varies from 0.033 mm to 9.436 mm, with the maximum tropospheric delay differences up to 18.03 mm at 300 GHz. Tropospheric delay differences strongly correlate with surface temperature and surface pressure, except for some of the atmosphere absorption bands. These quantitative analyses are helpful for the comprehensive application of GNSS to assist other microwave measurement techniques.

A Symmetric Parity–Time Coupled Optoelectronic Oscillator Using a Polarization–Dependent Spatial Structure

We propose and experimentally investigate a symmetric parity-time (PT) coupled optoelectronic oscillator (COEO) based on a polarization-dependent spatial structure. In such a COEO system, the gain/loss and coupling coefficients of two orthogonal polarization optical waves can be controlled by adjusting the polarization controller (PC) and the bias voltage of a Mach-Zehnder modulator (MZM). The single-mode selection of a microwave signal can be implemented by the PT symmetry breaking of a special mode. The performance of the proposed COEO is experimentally examined, and a 10.0 GHz microwave signal with a phase noise of −109.1 dBc/Hz @ 10 kHz and a side mode suppression ratio of 51.4 dB is generated. Moreover, an optical frequency comb with a comb tooth spacing of 10.0 GHz and a bandwidth of 100 GHz within a 10 dB amplitude variation can be simultaneously generated.

Parity–Time-Symmetric Optoelectronic Oscillator Based on Stimulated Brillouin Scattering

Parity–time (PT) symmetry has been intensively exploited in optoelectronic oscillators (OEOs), enabling mode selection in mutually coupled feedback loops. However, two mutually coupled feedback loops are usually established to achieve a spatial PT-symmetric architecture, which increases system complexity and instability. Here, we propose and experimentally prove a PT-symmetric OEO based on stimulated Brillouin scattering (SBS). The PT symmetry is implemented in an overlapping spatial loop in which the gain and loss are realized based on SBS gain and SBS loss, respectively. By adjusting the pump power and the polarization states of the probe and pump lights, the gain and loss can be balanced, thus achieving stable single-mode oscillation. In experiment, a 9.66 GHz microwave signal with a side-mode suppression ratio of 41 dB is generated. At 10 kHz offset frequency, the single-sideband phase noise reaches as low as −120.1 dBc/Hz. This demonstration opens new avenues to enhance mode selectivity in an OEO and can potentially be integrated on a chip.

Theoretical and Experimental Investigations of Tunable Microwave Signal Generation Based on a 1-GHz All-Polarization-Maintaining Mode-Locked Fiber Laser

Photonics-based microwave generation brings the advantages of photonic oscillators, such as high stability, wide bandwidth, and low loss, to the microwave domain. In this paper, the generation of tunable microwave signals was investigated both theoretically and experimentally based on an all-polarization-maintaining 1-GHz mode-locked fiber laser. Based on beating between two highly chirped optical pulse trains with a relative time delay at the photodetector, tunable microwave signals could be obtained. The numerical simulations show that 40 GHz or higher microwave signals could be obtained by tuning the time delay and dispersion. To experimentally validate the theoretical model, the generation of tunable microwave signals from 2–4 GHz was demonstrated. Due to the utilization of polarization-maintaining devices, the optical output has a high degree of linear polarization of more than 99%, which verifies the enhanced system stability. These demonstrations are imperative for solidifying the advancements of recent years and could promote the utilization of photonics-based microwave generation in microwave photonics.

Biopsy marker localization with thermo-acoustic ultrasound for lumpectomy guidance.

Almost one in four lumpectomies fails to fully remove cancerous tissue from the breast, requiring reoperation. This high failure rate suggests that existing lumpectomy guidance methods are inadequate for allowing surgeons to consistently identify the proper volume of tissue for excision. Current guidance techniques either provide little information about the tumor position or require surgeons to frequently switch between making incisions and manually probing for a marker placed at the lesion site. This article explores the feasibility of thermo-acoustic ultrasound (TAUS) to enable hands-free localization of metallic biopsy markers throughout surgery, which would allow for continuous visualization of the lesion site in the breast without the interruption of surgery. In a TAUS-based localization system, microwave excitations would be transmitted into the breast, and the amplification in microwave absorption around the metallic markers would generate acoustic signals from the marker sites through the thermo-acoustic effect. Detection and ranging of these signals by multiple acoustic receivers on the breast could then enable marker localization through acoustic multilateration. Physics simulations were used to characterize the TAUS signals generated from different markers by microwave excitations. First, electromagnetic simulations determined the spatial pattern of the amplification in microwave absorption around the markers. Then, acoustic simulations characterized the acoustic fields generated from these markers at various acoustic frequencies. TAUS-based one-dimensional (1D) ranging of two metallic markers-including a biopsy marker that is FDA-approved for clinical use-immersed in saline was also performed using a bench-top setup. To perform TAUS acquisitions, a microwave applicator was driven by 2.66GHz microwave signals that were amplitude-modulated by chirps at the desired acoustic excitation frequencies, and the resulting TAUS signal from the markers was detected by an ultrasonic transducer. The simulation results show that the geometry of the marker strongly impacts the quantity and spatial pattern of both the microwave absorption around the marker and the resulting TAUS signal generated from the marker. The simulated TAUS signal maps and acoustic frequency responses also make clear that the marker geometry plays an important role in determining the overall system response. Using the bench-top setup, TAUS detection and 1D localization of the markers were successfully demonstrated for multiple different combinations of microwave applicator and metallic marker. These initial results indicate that TAUS-based localization of biopsy markers is feasible. Through microwave excitations and acoustic detection, TAUS can be used to localize metallic biopsy markers. With further development, TAUS opens new avenues to enable a more intuitive lumpectomy guidance system that could help to achieve better lumpectomy outcomes.

Reduction of Flicker Phase Noise in High-Speed Photodetectors Under Ultrashort Pulse Illumination

High-fidelity photodetection enables the transfer of the low noise inherent to optical oscillators to the microwave domain. However, when photodetecting optical signals of the highest timing stability, photodiode flicker (1/f) noise can dominate the resulting timing jitter at timescales longer than ~1 ms. With the goal of improving femtosecond-level timing fidelity when transferring from the optical to microwave domain, we vary the duty cycle of a train of optical pulses and show that the photodetector flicker phase noise on a photonically generated 1 GHz microwave signal can be reduced by ~10 dB under ultrashort pulse illumination, reaching as low as -140/f dBc/Hz. In addition, a strong correlation between amplitude and phase flicker noise is found, implying a single baseband noise source can modulate both quadratures of the microwave carrier. These findings expand the limits of the ultimate timing stability that can be transferred from optics to electronics.

Broadband microwave photonic frequency translator with a wide frequency shifting range.

A microwave photonic topology for shifting the frequency of an input microwave signal is presented. It operates based on a single sideband frequency mixing approach. The amount of microwave signal frequency shift is determined by a local oscillator frequency. The proposed frequency translator (FT) has a large bandwidth and a wide frequency shifting range. It can be designed to obtain a large spurious signal suppression ratio and a low frequency translation loss. Results are presented for the novel structure, which demonstrates the realization of a 2-18 GHz FT with a 10 kHz to 100 MHz frequency shifting range. The results also show the spurious signals are more than 31 dB below the frequencyshifted signal, and a low loss of only around 4 dB for frequency shifting a 10 GHz microwave signal.

Full-duplex WDM-RoF system based on OFC with dual frequency microwave signal generation and wavelength reuse

Abstract A novel full-duplex wavelength division multiplexing radio-over-fiber (WDM-RoF) system based on optical frequency comb (OFC), which can generate dual frequency microwave signal simultaneously and realize wavelength reuse for uplink transmission, is proposed. In this proposal, OFC, which serves as the optical carriers of the WDM-RoF network, is performed by cascading two electro-absorption modulators (EAMs) and a frequency modulator (FM). The optical carriers are modulated by the radio frequency (RF) signal in LiNbO3 Mach-Zehnder modulator (LN-MZM) to produce multiple optical single sidebands (OSSB). The multiple OSSB signals are divided into multiple individual channels by AWG, and each channel contains two optical carriers and two corresponding +1 order sidebands. The first optical carrier of each channel is reflected by fiber bragg grating (FBG) and reused to provide the optical source for the uplink transmission in the base station (BS), which simplify the configuration of base station. Dual-frequency microwave signals are generated in the downlink by beating the remaining second optical carrier and two +1 order sidebands. In the simulation, 6 WDM channels each carrying 2.5 Gb/s downlink data have been up-converted to 15 GHz and 25 GHz microwave signals simultaneously, and 6 WDM channels each carrying 2.5 Gb/s uplink baseband data are sent to the central station. Both downlink and uplink transmission with less than 1 dB power penalty on average after 20 km SMF transmission. This full-duplex WDM-RoF system shows high spectral-efficiency, large communication capacity, simple cost-efficient configuration and good performance over long distance transmission.

Piezoelectric Transduction of a Wavelength-Scale Mechanical Waveguide

We present a piezoelectric transducer in thin-film lithium niobate that converts a 1.7 GHz microwave signal to a mechanical wave in a single mode of a 1 micron-wide waveguide. We measure a -12 dB conversion efficiency that is limited by material loss. The design method we employ is widely applicable to the transduction of wavelength-scale structures used in emerging phononic circuits like those at the heart of many optomechanical microwave-to-optical quantum converters.

Ultra wideband measurement for arbitrary spectral response using double sideband modulation and low-frequency detection

An ultra-wideband measurement for arbitrary optical magnitude response is proposed and experimentally demonstrated by using optical carrier-suppressed double sideband modulation and fixed low-frequency detection. Compared with previous optical single sideband modulation-based methods, the proposed scheme doubles the measurement range. Moreover, the response can be detected at a fixed low frequency by incorporating microwave photonic frequency down-conversion. The proposed approach has the merit of measuring arbitrary response avoiding high-speed and wideband detectors. To verify the concept, simulation and experiment are carried out. The magnitude responses of two fiber Bragg gratings, which have bandwidths of 19.8 GHz and 67 GHz, are characterized with a resolution of 10 MHz by using a 43.5 GHz microwave signal source.

Microwave-to-optics conversion using a mechanical oscillator in its quantum groundstate.

Conversion between signals in the microwave and optical domains is of great interest both for classical telecommunication, as well as for connecting future superconducting quantum computers into a global quantum network. For quantum applications, the conversion has to be both efficient, as well as operate in a regime of minimal added classical noise. While efficient conversion has been demonstrated using mechanical transducers, they have so far all operated with a substantial thermal noise background. Here, we overcome this limitation and demonstrate coherent conversion between GHz microwave signals and the optical telecom band with a thermal background of less than one phonon. We use an integrated, on-chip electro-opto-mechanical device that couples surface acoustic waves driven by a resonant microwave signal to an optomechanical crystal featuring a 2.7 GHz mechanical mode. We initialize the mechanical mode in its quantum groundstate, which allows us to perform the transduction process with minimal added thermal noise, while maintaining an optomechanical cooperativity >1, so that microwave photons mapped into the mechanical resonator are effectively upconverted to the optical domain. We further verify the preservation of the coherence of the microwave signal throughout the transduction process.

Photonic generation of frequency 16-tupling Millimeter wave signal using polarization property without an optical filter

In this paper, a novel filter free method of generating 16 tupled millimeter wave signal using polarization property is proposed. Phase modulators utilized in this scheme are independent of bias voltage deviation and have higher extinction ratio thus ensuring higher quality of generated mm wave signal. A large frequency tunable range is achievable, as no optical filter is used. A 160 GHz microwave signal is obtained at the polarizer output when a 10GHz RF local oscillator is employed, which is 16 folds the RF drive signal. The simulation results show that the generated signal offers an Optical Sideband Suppression Ratio (OSSR) of 63dB and Radio Frequency Spurious Suppression Ratio (RFSSR) of 51dB. The sideband suppression ratios obtained in the proposed scheme is higher than the previously suggested schemes on microwave signal generation using polarization property. Theoretical computations validate the simulation results. Various parameters affecting system performance are analyzed in later sections.

Wideband adaptive microwave frequency identification using an integrated silicon photonic scanning filter

Photonic-assisted microwave frequency identification with distinct features, including wide frequency coverage and fast tunability, has been conceived as a key technique for applications such as cognitive radio and dynamic spectrum access. The implementations based on compact integrated photonic chips have exhibited distinct advantages in footprint miniaturization, light weight, and low power consumption, in stark contrast with discrete optical-fiber-based realization. However, reported chip-based instantaneous frequency measurements can only operate at a single-tone input, which stringently limits their practical applications that require wideband identification capability in modern RF and microwave applications. In this article, we demonstrate, for the first time, a wideband, adaptive microwave frequency identification solution based on a silicon photonic integrated chip, enabling the identification of different types of microwave signals from 1 to 30 GHz, including single-frequency, multiple-frequency, chirped-frequency, and frequency-hopping microwave signals, and even their combinations. The key component is a high Q-factor scanning filter based on a silicon microring resonator, which is used to implement frequency-to-time mapping. This demonstration opens the door to a monolithic silicon platform that makes possible a wideband, adaptive, and high-speed signal identification subsystem with a high resolution and a low size, weight, and power (SWaP) for mobile and avionic applications.

Experimental Breast Phantom Imaging with Metamaterial-Inspired Nine-Antenna Sensor Array.

An experimental system for early screening of a breast tumor is presented in this article. The proposed microwave imaging (MI) system consists of a moveable array of nine improved negative-index metamaterial (MTM)-loaded ultrawideband (UWB) antenna sensor with incorporation of a corresponding SRR (split-ring resonator) and CLS (capacitively loaded strip) structure, in a circular array, the stepper motor-based array-mounting stand, the adjustable phantom hanging platform, an RF switching system to control the receivers, and a personal computer-based signal processing and image reconstruction unit using MATLAB. The improved antenna comprises of four-unit cells along one axis, where an individual unit cell integrates a balancing SRR and CLS pair, which makes the antenna radiation omnidirectional over the operating frequencies. The electrical dimensions of this proposed antenna are 0.28λ × 0.20λ × 0.016λ, measured at the lowest operating frequency of 2.97 GHz as the operating bandwidth of this is in between 2.97–15 GHz (134.82% bandwidth), with stable directional radiation pattern. SP8T 8 port switch is used to enable the eight receiver antennas to sequentially send a 3–8.0 GHz microwave signal to capture the backscattered signal by MATLAB software. A low-cost realistic homogeneous breast phantom with tumor material is developed and measured to test the capability of the imaging system to detect the breast tumors. A post-processing delay-multiply-and-sum (DMAS) algorithm is used to process the recorded backscatter signal to get an image of the breast phantom, and to accurately identify the existence and located area of multiple breast tumor tissues.

Tunable Optoelectronic Oscillator Using a Directly Modulated Microsquare Laser

A tunable optoelectronic oscillator (OEO) employing a directly modulated microsquare laser is proposed and experimentally demonstrated without using any external modulator and electrical bandpass filter. In our scheme, the OEO oscillates near the relaxation oscillation frequency at which the microsquare laser has the highest modulation efficiency. Microwave signals with the frequency tuned from 3.51 to 8.16 GHz are generated by tuning the bias current of the microsquare laser. Single sideband phase noise of -112.5 dBc/Hz is obtained at a frequency offset of 10 kHz for the generated 7.48 GHz microwave signal.

Silicon-Based Hybrid Integrated Photonic Chip for K ${}_{u}$ band Electromagnetic Wave Sensing

In this paper, we experimentally demonstrate a highly sensitive on-chip photonic electromagnetic wave sensor operating at 14.1 GHz with a 3 dB bandwidth of 4.84 GHz. The demonstrated electromagnetic sensor has several important advantages over conventional electrical electromagnetic sensors, including high sensitivity, compact in size, and strong immunity to electromagnetic interference. The sensor is comprised of a bowtie antenna and an asymmetric Mach-Zehnder interferometer (MZI). One arm of the MZI is an electro-optic polymer infiltrated one-dimensional slot photonic crystal waveguide (SPCW) and the other is a strip waveguide with teeth of subwavelength pitch. Bowtie antennas are designed and optimized to effectively collect 14.1 GHz microwave signal and applies it across the SPCW. The phase of the light guided in the SPCW therefore changes, and so does the amplitude of the output end of the MZI. The sensor is only 4.6 mm × 4.8 mm in size and has a low insertion loss of ~10 dB. Experimental results show the limit of detection at 14.1 GHz is 4.31 mW/m 2 , which is corresponding to a minimum detectable electric field of 1.8 V/m.

Simplified optical generation of microwave signal based on polarization multiplexing using an optical phase modulator

AbstractA new method to generate microwave signal with frequency doubled of the RF signal using an optical phase modulator by simply adjusting the polarization angle of the polarizer without any optical filters is experimentally proposed. The optical carrier is 19 dB lower than the 2 first‐order sidebands, which is better than the traditional optical carrier suppression double sideband modulation scheme. After detected by a high speed photodiode, a high spectrum purity 28 GHz microwave signal is generated easily. In addition, as pure optical carrier can also be recovered easily, the proposed scheme is cost‐effective and may be useful for the radio over fiber system with wavelength reuse.

Simultaneous Transfer of Stabilized Optical and Microwave Frequencies Over Fiber

We demonstrate simultaneous stabilization of optical and microwave signals on an optical-fiber link by independently stabilizing two optical signals that are separated by a nominated microwave frequency. This system can be implemented on conventional stabilized optical frequency transfer networks without degrading the stability of the optical signal, increasing the scope of applications of existing and future large-scale frequency transfer networks. For a 30 km optical-fiber link, we demonstrate a 193 THz optical carrier transfer absolute frequency stability of $4.9\times 10^{-5}$ Hz at $10^{3}$ s, and a 10.02 GHz microwave signal transfer absolute frequency stability of $1.3\times 10^{-4}$ Hz at $10^{3}$ s.

Triple Brillouin frequency spacing multiwavelength fiber laser with double Brillouin cavities and its application in microwave signal generation

We propose and experimentally investigate a multiwavelength Brillouin fiber laser (MBFL) with triple frequency spacing by employing a modular structure. In this scheme, we obtain nine channels optimized with frequency spacing of 0.259 nm. The single, double, and triple Brillouin frequency spacing for the MBFL can be easily realized by utilizing this modular structure. The impact of Brillouin pump (BP) power, BP wavelength, and erbium-ytterbium-doped fiber amplifier (EYDFA) output power on the performance of the MBFL is investigated, respectively. We also study the generation of beating frequency microwave signals based on single, double, and triple frequency spacing MBFL. 10.5 GHz, 21.48 GHz, and 31.77 GHz microwave signals with 3 dB linewidth of 16.4 MHz, 15.2 MHz, and 12.8 MHz are generated, respectively.