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"frequency"
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Monolithic piezoelectric control of soliton microcombs
High-speed actuation of laser frequency
1
is critical in applications using lasers and frequency combs
2
,
3
, and is a prerequisite for phase locking, frequency stabilization and stability transfer among optical carriers. For example, high-bandwidth feedback control of frequency combs is used in optical-frequency synthesis
4
, frequency division
5
and optical clocks
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. Soliton microcombs
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,
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have emerged as chip-scale frequency comb sources, and have been used in system-level demonstrations
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,
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. Yet integrated microcombs using thermal heaters have limited actuation bandwidths
11
,
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of up to 10 kilohertz. Consequently, megahertz-bandwidth actuation and locking of microcombs have only been achieved with off-chip bulk component modulators. Here we demonstrate high-speed soliton microcomb actuation using integrated piezoelectric components
13
. By monolithically integrating AlN actuators
14
on ultralow-loss Si
3
N
4
photonic circuits
15
, we demonstrate voltage-controlled soliton initiation, tuning and stabilization with megahertz bandwidth. The AlN actuators use 300 nanowatts of power and feature bidirectional tuning, high linearity and low hysteresis. They exhibit a flat actuation response up to 1 megahertz—substantially exceeding bulk piezo tuning bandwidth—that is extendable to higher frequencies by overcoming coupling to acoustic contour modes of the chip. Via synchronous tuning of the laser and the microresonator, we exploit this ability to frequency-shift the optical comb spectrum (that is, to change the comb’s carrier-envelope offset frequency) and make excursions beyond the soliton existence range. This enables a massively parallel frequency-modulated engine
16
,
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for lidar (light detection and ranging), with increased frequency excursion, lower power and elimination of channel distortions resulting from the soliton Raman self-frequency shift. Moreover, by modulating at a rate matching the frequency of high-overtone bulk acoustic resonances
18
, resonant build-up of bulk acoustic energy allows a 14-fold reduction of the required driving voltage, making it compatible with CMOS (complementary metal–oxide–semiconductor) electronics. Our approach endows soliton microcombs with integrated, ultralow-power and fast actuation, expanding the repertoire of technological applications of microcombs.
By monolithically integrating piezoelectric actuators on ultralow-loss photonic circuits, soliton microcombs—a spectrum of sharp lines over a range of optical frequencies—can be modulated at high speeds with megahertz bandwidths.
Journal Article
Optimal auxiliary frequency control of wind turbine generators and coordination with synchronous generators
Auxiliary frequency control of a wind turbine generator (WTG) has been widely used to enhance the frequency security of power systems with high penetration of renewable energy. Previous studies recommend two types of control schemes, including frequency droop control and emulated inertia control, which simulate the response characteristics of the synchronous generator (SG). This paper plans to further explore the optimal auxiliary frequency control of the wind turbine based on previous research. First, it is determined that the virtual inertia control has little effect on the maximum rate of change of frequency (Max-ROCOF) if the time delay of the control link of WTG is taken into consideration. Secondly, if a WTG operates in maximum power point tracking (MPPT) mode and uses the rotor deceleration for frequency modulation, its optimal auxiliary frequency control will contain only droop control. Furthermore, if the droop control is properly delayed, better system frequency response (SFR) will be obtained. The reason is that coordination between the WTG and SG is important for SFR when the frequency modulation capability of the WTG is limited. The frequency modulation capability of the WTG is required to be released more properly. Therefore, when designing optimal auxiliary frequency control for the WTG, a better control scheme is worth further study.
Journal Article
Flight trajectory prediction enabled by time-frequency wavelet transform
Accurate flight trajectory prediction is a crucial and challenging task in air traffic control, especially for maneuver operations. Modern data-driven methods are typically formulated as a time series forecasting task and fail to retain high accuracy. Meantime, as the primary modeling method for time series forecasting, frequency-domain analysis is underutilized in the flight trajectory prediction task. In this work, an innovative wavelet transform-based framework is proposed to perform time-frequency analysis of flight patterns to support trajectory forecasting. An encoder-decoder neural architecture is developed to estimate wavelet components, focusing on the effective modeling of global flight trends and local motion details. A real-world dataset is constructed to validate the proposed approach, and the experimental results demonstrate that the proposed framework exhibits higher accuracy than other comparative baselines, obtaining improved prediction performance in terms of four measurements, especially in the climb and descent phase with maneuver control. Most importantly, the time-frequency analysis is confirmed to be effective to achieve the flight trajectory prediction task.
Accurate flight trajectory prediction can be a challenging task in air traffic control, especially for maneuver operations. Here, authors develop a time-frequency analysis based on an encoder-decoder neural architecture to estimate wavelet components and model global flight trends and local motion details.
Journal Article
Assessing the influence of a rapid water drawdown on the seismic response characteristics of a reservoir rock slope using time–frequency analysis
To investigate the influence of a rapid water drawdown (RWD) on the seismic response characteristics of reservoir rock slopes, numerical dynamic analyses and shaking table tests are conducted on a rock slope containing discontinuities under a RWD using time–frequency analysis from the perspective of spectral and energy propagation characteristics. The results show that a RWD has a magnification effect on the seismic response of a surface slope, which is mainly manifested as the RWD causing the seismic energy of the surface slope to increase significantly. The RWD has a magnification effect on the Fourier spectrum amplitude of the low-order natural frequency band. A time–frequency domain analysis shows that the RWD has an influence on the characteristics of the seismic Hilbert energy spectrum (HES) in the low-frequency band of the surface slope and magnifies the amplitude of the marginal spectrum (MS) in the high-frequency band. In addition, the applicability of the Fourier spectrum, HES and MS in analysing the relationship between the RWD and the slope dynamic response is discussed. An analysis of the seismic HES shows that the RWD has a major impact on the overall dynamic response of the surface slope, while the RWD has a significant impact on the local dynamic response of the surface slope based on the Fourier spectrum and the MS. The influence mechanism of the RWD on the HES and MS of the slope is also discussed. Moreover, the influence of a RWD on the development process of seismic damage to the slope is clarified using an energy-based method.
Journal Article
Identification of time‐varying cable tension forces based on adaptive sparse time‐frequency analysis of cable vibrations
Summary For cable bridges, the cable tension force plays a crucial role in their construction, assessment and long‐term structural health monitoring. Cable tension forces vary in real time with the change of the moving vehicle loads and environmental effects, and this continual variation in tension force may cause fatigue damage of a cable. Traditional vibration‐based cable tension force estimation methods can only obtain the time‐averaged cable tension force and not the instantaneous force. This paper proposes a new approach to identify the time‐varying cable tension forces of bridges based on an adaptive sparse time‐frequency analysis method. This is a recently developed method to estimate the instantaneous frequency by looking for the sparsest time‐frequency representation of the signal within the largest possible time‐frequency dictionary (i.e. set of expansion functions). In the proposed approach, first, the time‐varying modal frequencies are identified from acceleration measurements on the cable, then, the time‐varying cable tension is obtained from the relation between this force and the identified frequencies. By considering the integer ratios of the different modal frequencies to the fundamental frequency of the cable, the proposed algorithm is further improved to increase its robustness to measurement noise. A cable experiment is implemented to illustrate the validity of the proposed method. For comparison, the Hilbert–Huang transform is also employed to identify the time‐varying frequencies, which are then used to calculate the time‐varying cable‐tension force. The results show that the adaptive sparse time‐frequency analysis method produces more accurate estimates of the time‐varying cable tension forces than the Hilbert–Huang transform method. Copyright © 2016 John Wiley & Sons, Ltd.
Journal Article