Microwave Reference Research Articles

High precision microwave measurement based on nitrogen-vacancy color center and application in velocity detection.

Wide-range high-precision velocity detection with nitrogen-vacancy (NV) color center has been realized. By treating the NV color center as a mixer, the high-precision microwave measurement is realized. Through optimization of acquisition time, the microwave frequency resolution is improved to the mHz level. Combined with the frequency-velocity conversion model, velocity detection is realized in the range of 0-100 cm/s, and the velocity resolution is up to 0.012 cm/s. The maximum deviation in repeated measurements does not exceed 1/1000. Finally, combined with the multiplexed microwave reference technique, the range of velocity can be extended to 7.4 × 105 m/s. All of the results provide reference for high-precision velocity detection and play a significant role in various domains of quantum precision measurement. This study provides a crucial technical foundation for the development of high-dynamic-range velocity detectors and novel quantum precision velocity measurement technologies.

Phase Locking of a THz QC-VECSEL to a Microwave Reference

High resolution frequency study and phase-locking have been performed on a terahertz quantum-cascade vertical-external-cavity surface-emitting-laser operating around 2.5 THz. A subharmonic diode mixer is used to down convert the THz signal to a 100 MHz intermediate frequency that is phase locked to a stable 100 MHz microwave reference. Between 90-95% of the QC-VECSEL signal is locked within 2 Hz of the multiplied RF reference, and amplitude fluctuations on the order of 1-10% are observed, depending on the bias point of the QC-VECSEL. The bandwidth of the locking loop is ∼1 MHz. Many noise peaks in the IF signal are observed, likely corresponding to mechanical resonances in the 10 Hz-10 kHz. These peaks are generally -30 to -60 dB below the main tone, and are below the phase noise level of the multiplied RF reference which ultimately limits the phase noise of the locked QC-VECSEL.

High-speed tunable microwave-rate soliton microcomb

Soliton microcombs are a promising new approach for photonic-based microwave signal synthesis. To date, however, the tuning rate has been limited in microcombs. Here, we demonstrate the first microwave-rate soliton microcomb whose repetition rate can be tuned at a high speed. By integrating an electro-optic modulation element into a lithium niobate comb microresonator, a modulation bandwidth up to 75 MHz and a continuous frequency modulation rate up to 5.0 × 1014 Hz/s are achieved, several orders-of-magnitude faster than existing microcomb technology. The device offers a significant bandwidth of up to tens of gigahertz for locking the repetition rate to an external microwave reference, enabling both direct injection locking and feedback locking to the comb resonator itself without involving external modulation. These features are especially useful for disciplining an optical voltage-controlled oscillator to a long-term reference and the demonstrated fast repetition rate control is expected to have a profound impact on all applications of frequency combs.

Frequency stabilization of a terahertz quantum-cascade laser to the Lamb dip of a molecular absorption line.

We demonstrate the frequency stabilization of a terahertz quantum-cascade laser (QCL) to the Lamb dip of the absorption line of a D2O rotational transition at 3.3809309 THz. To assess the quality of the frequency stabilization, a Schottky diode harmonic mixer is used to generate a downconverted QCL signal by mixing the laser emission with a multiplied microwave reference signal. This downconverted signal is directly measured by a spectrum analyzer showing a full width at half maximum of 350 kHz, which is eventually limited by high-frequency noise beyond the bandwidth of the stabilization loop.

Engineered zero-dispersion microcombs using CMOS-ready photonics

Normal group velocity dispersion (GVD) microcombs offer high comb line power and high pumping efficiency compared to bright pulse microcombs. The recent demonstration of normal GVD microcombs using CMOS foundry-produced microresonators is an important step toward scalable production. However, the chromatic dispersion of CMOS devices is large and impairs the generation of broadband microcombs. Here, we report the development of a microresonator in which GVD is reduced due to a coupled-ring resonator configuration. Operating in the turnkey self-injection locking mode, the resonator is integrated in a hybrid manner with a semiconductor laser pump to produce high-power efficiency combs spanning a bandwidth of 9.9 nm (1.22 THz) centered at 1560 nm, corresponding to 62 comb lines. Fast, linear optical sampling of the comb waveform is used to observe the rich set of near-zero GVD comb behaviors, including soliton molecules, switching waves (platicons), and their hybrids. Tuning of the 20 GHz repetition rate by electrical actuation enables servo locking to a microwave reference, which simultaneously stabilizes the comb repetition rate, offset frequency, and temporal waveform. This integrated hybrid system could be used in coherent communications or for ultrastable microwave signal generation by two-point optical frequency division.

Reliability and Reproducibility of the Cryogenic Sapphire Oscillator Technology

The cryogenic sapphire oscillator (CSO) is a highly specialized machine, which delivers a microwave reference signal exhibiting the lowest frequency fluctuations (Allan deviation σy(τ)) for the integration time τ between 1 s and 104 s, indeed 4 decades. In such interval, good units feature σy(τ) < 10-15, with < 10-14 drift in one day. The oscillator is based on a sapphire monocrystal resonating at 10 GHz in a whisperinggallery mode, cooled at ≈ 6 K for zero thermal coefficient and optimal quality factor Q.We report on the progress accomplished implementing eleven CSOs in about 10 years since the first sample delivered to the Malargüe station of the European Space Agency (ESA) in Argentina. Short-term stability is improved by a factor of 3.10, depending on τ, and the refrigerator’s electric power is reduced to 3 kW single-phase line. Frequency stability and overall performances are reproducible, with unattended operation between scheduled maintenance every two years. The CSO is now a semicommercial product suitable to scientific applications requiring extreme frequency stability with reliable unattended long-term operation, like the flywheel for primary frequency standards, the ground segment of Global Navigation Satellite System (GNSS), astrometry, Very Long Baseline Interferometry, and radio astronomy stations.

Satellite-Based Flood Mapping through Bayesian Inference from a Sentinel-1 SAR Datacube

Spaceborne Synthetic Aperture Radar (SAR) are well-established systems for flood mapping, thanks to their high sensitivity towards water surfaces and their independence from daylight and cloud cover. Particularly able is the 2014-launched Copernicus Sentinel-1 C-band SAR mission, with its systematic monitoring schedule featuring global land coverage in a short revisit time and a 20 m ground resolution. Yet, variable environment conditions, low-contrasting land cover, and complex terrain pose major challenges to fully automated flood monitoring. To overcome these issues, and aiming for a robust classification, we formulate a datacube-based flood mapping algorithm that exploits the Sentinel-1 orbit repetition and a priori generated probability parameters for flood and non-flood conditions. A globally applicable flood signature is obtained from manually collected wind- and frost-free images. Through harmonic analysis of each pixel’s full time series, we derive a local seasonal non-flood signal comprising the expected backscatter values for each day-of-year. From those predefined probability distributions, we classify incoming Sentinel-1 images by simple Bayes inference, which is computationally slim and hence suitable for near-real-time operations, and also yields uncertainty values. The datacube-based masking of no-sensitivity resulting from impeding land cover and ill-posed SAR configuration enhances the classification robustness. We employed the algorithm on a 6-year Sentinel-1 datacube over Greece, where a major flood hit the region of Thessaly in 2018. In-depth analysis of model parameters and sensitivity, and the evaluation against microwave and optical reference flood maps, suggest excellent flood mapping skill, and very satisfying classification metrics with about 96% overall accuracy and only few false positives. The presented algorithm is part of the ensemble flood mapping product of the Global Flood Monitoring (GFM) component of the Copernicus Emergency Management Service (CEMS).

Cryogenic Characterization of a Superconductor Quantum-Based Microwave Reference Source for Communications and Quantum Information

We are developing a new instrument, the RF Josephson arbitrary waveform synthesizer (RF-JAWS), for communications metrology and quantum information applications. An important aspect of the RF-JAWS design is the accurate and traceable characterization of its superconducting devices. In this article, we present a procedure for characterizing microwave superconducting devices in a cryogenic RF probe station via a vector network analyzer (VNA) calibrated with a custom cryogenic calibration kit colocated with the superconducting device under test (DUT) in the cryogenic environment. By de-embedding lossy and dispersive RF interconnects linking the superconducting DUT to the measurement apparatus at room temperature, we characterize the DUT exactly at the cryogenic on-wafer reference plane. More importantly, we operate our VNA with an external modulated source and our procedure features metrology-grade multiline thru-reflect-line calibration and absolute power and phase corrections, as opposed to the more common relative scattering-parameter correction. In addition, we apply an X -parameter model to account for impedance mismatch in cryogenic Josephson microwave sources. Our techniques are also suitable for cryogenic characterization of microwave superconducting devices for solid-state quantum computers and could help to optimize the quantum-classical interfaces in these systems.

Coherent terahertz-to-microwave link using electro-optic-modulated Turing rolls

Arising from modulation instability, Turing rolls in optical Kerr microresonators have been used in the generation of optical frequency combs and the synthesis of microwave and terahertz frequencies. In this work, by applying electro-optic modulation on terahertz-frequency Turing rolls, we implement electro-optic frequency division with a microcomb to synthesize variable low-noise microwave signals. We also actively stabilize the terahertz oscillations to a microwave reference via intracavity power modulation, obtaining fractional frequency instabilities that are better than those of the free-running situation by up to six orders of magnitude. This study not only highlights the extraordinary spectral purity of Turing roll oscillations but also opens the way for bidirectional terahertz-to-microwave links with hybrid optical frequency comb techniques.

High-Precision Time-Frequency Signal Simultaneous Transfer System via a WDM-Based Fiber Link

In this paper, we demonstrate a wavelength division multiplexing (WDM)-based system for simultaneously delivering ultra-stable optical frequency reference, 10 GHz microwave frequency reference, and a one pulse per second (1 PPS) time signal via a 50 km fiber network. For each signal, a unique noise cancellation technique is used to maintain their precision. After being compensated, the transfer frequency instability in terms of the overlapping Allan deviation (OADEV) for the optical frequency achieves 2 × 10−17/s and scales down to 2 × 10−20/10,000 s, which for the 10 GHz microwave reference, approaches 4 × 10−15/s and decreases to 1.4 × 10−17/10,000 s, and the time uncertainty of the 1 PPS time signal along the system is 2.08 ps. In this scheme, specific channels of WDM are, respectively, occupied for different signals to avoid the possible crosstalk interference effect between the transmitted reference signals. To estimate the performance of the above scheme, which is also demonstrated in this 50 km link independent of these signals, the results are similar to that in the case of simultaneous delivery. This work shows that the WDM-based system is a promising method for building a nationwide time and frequency fiber transfer system with a communication optical network.

Optically referenced 300 GHz millimetre-wave oscillator

Optical frequency division (OFD) via optical frequency combs (OFC) has enabled a leap in microwave metrology leading to noise performance never explored before. Extending the method to millimeter-wave (mmW) and terahertz (THz)-wave domain is of great interest. Dissipative Kerr solitons (DKSs) in integrated photonic chips offer a unique feature of delivering OFCs with ultrahigh repetition rates from 10 GHz to 1 THz making them relevant gears to perform OFD in the mmW and THz-wave domain. We experimentally demonstrate OFD of an optically-carried 3.6 THz reference down to 300 GHz through a DKS, photodetected with an ultrafast uni-traveling-carrier photodiode (UTC-PD). A new measurement system, based on the characterization of a microwave reference phase-locked to the 300 GHz signal under test, yields attosecond-level timing noise sensitivity, leveraging conventional technical limitations. This work places DKSs as a leading technology in the mmW and THz-wave field promising breakthroughs in fundamental and civilian applications.

Frequency microcomb stabilization via dual-microwave control

Optical frequency comb technology has been the cornerstone for scientific breakthroughs in precision metrology. In particular, the unique phase-coherent link between microwave and optical frequencies solves the long-standing puzzle of precision optical frequency synthesis. While the current bulk mode-locked laser frequency comb has had great success in extending the scientific frontier, its use in real-world applications beyond the laboratory setting remains an unsolved challenge due to the relatively large size, weight and power consumption. Recently microresonator-based frequency combs have emerged as a candidate solution with chip-scale implementation and scalability. The wider-system precision control and stabilization approaches for frequency microcombs, however, requires external nonlinear processes and multiple peripherals which constrain their application space. Here we demonstrate an internal phase-stabilized frequency microcomb that does not require nonlinear second-third harmonic generation nor optical external frequency references. We demonstrate that the optical frequency can be stabilized by control of two internally accessible parameters: an intrinsic comb offset ξ and the comb spacing frep. Both parameters are phase-locked to microwave references, with phase noise residuals of 55 and 20 mrad respectively, and the resulting comb-to-comb optical frequency uncertainty is 80 mHz or less. Out-of-loop measurements confirm good coherence and stability across the comb, with measured optical frequency instability of 2 × 10−11 at 20-second gate time. Our measurements are supported by analytical theory including the cavity-induced modulation instability. We further describe an application of our technique in the generation of low noise microwaves and demonstrate noise suppression of the repetition rate below the microwave stabilization limit achieved.

Optoelectronic reference X-band oscillator for radar systems

We present the design and results of studying the characteristics of an optoelectronic microwave oscillator in the free-running regime are presented, propose a method for synchronising it with a signal of a highly stable crystal oscillator using a phase-locked loop, and analyse the results of an experimental study on the frequency instability of an optoelectronic microwave reference oscillator. The optoelectronic microwave reference oscillator with optical gain and a 10 GHz oscillation frequency simultaneously provides ultra-low phase noise (less than – 142 dB Hz−1) at a 10 kHz frequency offset from the microwave carrier and a low level of spurs in the oscillation spectrum (no more than – 94 dBc). In this case, the temperature coefficient of the oscillation frequency is determined by the temperature instability of a highly stable crystal oscillator.

Multi-Frequency Phase-Coded Microwave Signal Generation With an Increased Number of Frequencies

A photonics-based approach to multi-frequency phase-coded microwave signal generation is proposed with an increased number of frequencies. Two microwave reference signals are sent to one arm of a dual-drive Mach-Zehnder modulator (DD-MZM), whereas the binary coding signal is sent to the other arm of the DD-MZM. The optical output of the DD-MZM is detected in a photodetector. Due to the nonlinearity in the electro-optic modulation, the generated photocurrent contains the nonlinear intermodulation products of the two reference signals, which are all modulated by the coding signal. An experiment is performed. Multi-frequency phase-coded microwave signals from 1 to 24 GHz with more than twelve different frequency components are generated. The performance of the generated signal including phase recovery accuracy and pulse compression capability is also verified.

Tunable Er-doped fiber optical frequency comb with a repetition rate adjustment larger than 1.6 MHz

Femtosecond optical frequency combs play important roles in various scientific and technical fields. For example, they can be used for frequency measurements, distance measurements, and in precision spectroscopy. The repetition rate of this type of comb can be tuned in a wide range by controlling the cavity length. This has here been done by using a combination of a motorized stage and a piezoelectric transducer (PZT). A very stable Er-doped fiber femtosecond laser, with a repetition rate of 200 MHz and a pulse duration of 68 fs, was then developed. In addition, a tunable repetition rate adjustment of 1.664 MHz was successively achieved. The fluctuation in the repetition rate was less than 100 Hz in 120 h in the free-running state. Moreover, the average output power was 105.05 mW, with a standard deviation of 0.09 mW for a duration of 110 h. The repetition rate and carrier-envelope offset frequency were continuously phase-locked to a microwave reference for more than 30 days with a standard deviation of 0.327 mHz and 0.569 mHz, respectively.

300 GHz wave generation based on a Kerr microresonator frequency comb stabilized to a low noise microwave reference.

In this Letter, we experimentally demonstrate low noise 300 GHz wave generation based on a Kerr microresonator frequency comb operating in the soliton regime. The spectral purity of a 10 GHz GPS-disciplined dielectric resonant oscillator is transferred to the 300 GHz repetition rate frequency of the soliton comb through an optoelectronic phase-locked loop. Two adjacent comb lines beat on a uni-traveling carrier photodiode emitting the 300 GHz millimeter-wave signal into a waveguide. In an out-of-loop measurement, we measure the 300 GHz power spectral density of phase noise to be -88dBc/Hz, -105dBc/Hz at 10 kHz, and 1 MHz Fourier frequency, respectively. Phase-locking error instability reaches 2×10-15 at 1 s averaging time. Such a system provides a promising path to the realization of compact, low power consumption millimeter-wave oscillators with low noise performance for out-of-the-laboratory applications.

Development of ultrastable fiber-optic time and frequency reference networks in Africa.

The unparalleled accuracy of modern-day atomic clocks has stimulated the development of time and frequency comparison techniques, with optical frequency transfer over fiber networks emerging as the preferred method. It has been demonstrated that frequency transfer over optical fibers has an order-of-magnitude better stability and accuracy than traditional satellite-based techniques. Precise time has become an essential service for most critical infrastructure and applications. New progress in LTE and 5G will demand more access to precise time with accuracies of under 10 ns. Although this technology exists in Africa, continuous improvements are required. With the arrival of the Square Kilometre Array (SKA) in Africa, the National Metrology Institute of South Africa (NMISA) upgraded its time and frequency infrastructure in order to support the stringent time accuracy requirements of the MeerKAT and SKA telescopes. Over the past five years, the Centre for Broadband Communication at the Nelson Mandela University has been conducting exciting and cutting-edge research looking at new and innovative ways for coherently disseminating high-speed data and clock over optical fiber networks. This paper reports on recent research progress made in developing synchronous optical networks in South Africa and across the African continent. This paper begins by presenting a pioneering all-optical approach for measuring the round-trip latency time along a spooled G.652 single-mode fiber. This has been realized by optically injecting a pulse-per-second (PPS) signal from a distributed feedback laser into the slave mode of a 1550 nm vertical cavity surface emitting laser (VCSEL) located at the receiver end. A round-trip fiber time delay of 113.2 µs was experimentally measured over 22 km. Furthermore, the jitter instability of an optically modulated PPS was measured as a function of temperature. A jitter of 434.82 ps was measured during the night-to-day temperature cycle (5°C-25°C). The impact of polarization fluctuations on jitter stability is presented. A maximum jitter of 417.88 ps was measured for the transmitted PPS along the aerial fiber. Lastly, a novel technique for distributing a stable microwave reference frequency, using an intensity modulated VCSEL, is presented. The novel frequency dissemination and synchronization system proposes the use of a VCSEL-based phase correction actuator together with the inherent chromatic dispersion properties of the fiber. Frequency instabilities of 4.18939×10-12 at 104s without active noise cancellation and 4.86×10-14 at 104s with active noise cancellation were successfully measured across the 26 km G.655 fiber link.

Development of a photoelectric phase-locked loop model to better synchronize frequency combs and microwaves.

The high phase coherence between ultralow-noise microwaves and ultrahigh-stable optical frequency combs (OFCs) is of both scientific and technological relevance for telecommunication, timekeeping, astronomy, and metrology. Here, a photoelectric phase-locked loop (PLL) model with ultralow phase noise based on the optical-microwave phase detector technique has been proposed and experimentally demonstrated. A detailed mathematical model for tight, real-time phase synchronization of OFCs and microwaves is developed to investigate the feasibility and analyze the characteristics of the phase-coherent system. We fabricate a compact PLL circuit with a proportional-integral-derivative regulator for the synchronization of an OFC to a microwave reference. Once synchronized, the long-term stability of the OFC agrees to 2.4×10-14 at a 1000 s averaging time, which is enhanced by more than 4 orders of magnitude. Besides, the OFC almost acquires the same frequency stability as the microwave source. The ability to better phase synchronize OFCs and microwaves enables a wide range of applications beyond the laboratory.

Silicon Photonics Optical Frequency Synthesizer

AbstractThe knowledge of the exact frequency of an optical source has always been one of the ultimate goals in optics. Since the discovery of the laser, complex systems have been developed to address this challenge. That effort reached a significant milestone with the advent of the femtosecond laser frequency comb that reduced the system size from an entire lab down to the bench‐top. That spurred interest in the development of integrated optical frequency synthesizers that can generate precisely different optical frequencies on demand and can be deployed widely. In this work, such an optical frequency synthesizer using supercontinuum waveguide and second harmonic generator on silicon photonics platform is demonstrated. Integrated silicon photonics based tunable continuous wave laser is phase‐locked to a microwave reference, to synthesize absolute optical frequencies in the telecom band. A relative frequency instability of 1 × 10−12 at 1 s level is achieved by utilizing an integrated self‐referencing scheme that exploits the strong 3rd order and electric‐field‐induced 2nd order nonlinearities of silicon waveguides. With this work, an all on‐chip silicon photonics based frequency synthesizer seems promising for mass production of next generation broad‐band coherent optical communication systems, spectroscopic, detection, and ranging systems and future integrated quantum systems with Hz‐level precision.

Photonics-based multi-band linearly frequency modulated signal generation and anti-chromatic dispersion transmission.

A photonics-based anti-chromatic dispersion transmission scheme for multi-band linearly frequency modulated (LFM) signals is proposed and experimentally demonstrated. In the central station (CS), the key component is an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator, of which the up-arm and down-arm are driven by a microwave reference signal and an intermediate-frequency (IF) LFM signal respectively. By properly adjusting the DP-QPSK modulator, optical frequency comb (OFC) and frequency shift lightwave are generated. After polarization coupling and remote transmission, the orthogonal-polarization optical signals are introduced into balanced photodetector for heterodyne detection. Thence, multi-band LFM signals are generated and transmitted to remote base stations (BS) with the largest power for the anti-chromatic dispersion ability. Experiments are conducted to verify the analysis. Multi-band LFM signals at L (1.5 GHz), C (7 GHz), X (10 GHz), Ku (15.5 GHz) and K (18.5 GHz) bands with flatness of 1.9 dB are simultaneously obtained in the CS after 50 km fiber transmission, while the normally double-sideband modulation approach experiences a significant power fading for the fiber dispersion. Tunability of the system is evaluated, and detection performances of the generated signals are also analyzed.