Rapid Frequency Sweep Research Articles

Stable soliton dual-microcomb generation via sideband thermal compensation for spectroscopy

Microcombs—generated by coherently pumping nonlinear microresonators—have emerged as a state-of-the-art scheme at the chip scale. Dual-comb spectroscopy (DCS) technology further takes advantage of the miniature system, and has been demonstrated as a powerful tool for real-time and broadband optical sampling of molecular spectra. Here, a novel soliton dual-microcomb generation method by rapid frequency sweep and sideband thermal compensation is put forward, and dual-microcomb optical spectra range beyond 200-nm has been successfully demonstrated in two microresonators with moderate quality factors. Compared to the dual-microcomb with a weak thermal compensation effect, the demonstrated dual-microcomb shows much lower-noise RF beat notes (<10 kHz) and smaller Allan deviations (1.0 × 10–4 at 1 ms) by increasing sideband power. Moreover, the dual-microcomb has been utilized in the gas absorption detection of H12CN for demonstration with high signal-to-noise ratios (SNRs) and fast acquisition rates. This work also lays a technical foundation for other dual-microcomb applications of ranging and microwave photonics.

Repetition rate locked single-soliton microcomb generation via rapid frequency sweep and sideband thermal compensation

Dissipative Kerr solitons (DKSs) with mode-locked pulse trains in high- Q optical microresonators possess low-noise and broadband parallelized comb lines, having already found plentiful cutting-edge applications. However, thermal bistability and thermal noise caused by the high microresonator power and large temperature exchange between microresonator and the environment would prevent soliton microcomb formation and deteriorate the phase and frequency noise. Here, a novel method that combines rapid frequency sweep with optical sideband thermal compensation is presented, providing a simple and reliable way to get into the single-soliton state. Meanwhile, it is shown that the phase and frequency noises of the generated soliton are greatly reduced. Moreover, by closing the locking loop, an in-loop repetition rate fractional instability of 5.5 × 10 − 15 at 1 s integration time and a triangular linear repetition rate sweep with 2.5 MHz could be realized. This demonstration provides a means for the generation, locking, and tuning of a soliton microcomb, paving the way for the application of single-soliton microcombs in low-phase-noise microwave generation and laser ranging.

Rapid Linear Frequency Swept Frequency-Modulated Continuous Wave Laser Source Using Iterative Pre-Distortion Algorithm

We present a simple iterative pre-distortion algorithm for achieving a rapid linear frequency sweep of semiconductor lasers. The algorithm achieves the desired frequency swept linearity with only four iterations. We derive a general formula for iterative pre-distortion by establishing the relationship between the laser output frequency and the drive current. The linear frequency-swept laser source obtained by this algorithm can be used in FMCW LiDAR systems. Experimentally, we investigated the algorithm using a 1550 nm distributed feedback (DFB) laser, achieving frequency swept excursion of 30.26 GHz, and frequency swept slope of 504 THz/s. We analyzed the linearity of the frequency swept results for the fourth iteration, achieving less than 5 MHz root mean square (RMS) value of frequency swept nonlinearity.

Harmonic ratcheting for fast acceleration

A major challenge in the design of rf cavities for the acceleration of medium-energy charged ions is the need to rapidly sweep the radio frequency over a large range. From low-power medical synchrotrons to high-power accelerator driven subcritical reactor systems, and from fixed focus alternating gradient accelerators to rapid cycling synchrotrons, there is a strong need for more efficient, and faster, acceleration of protons and light ions in the semirelativistic range of hundreds of MeV/u. A conventional way to achieve a large, rapid frequency sweep (perhaps over a range of a factor of 6) is to use custom-designed ferrite-loaded cavities. Ferrite rings enable the precise tuning of the resonant frequency of a cavity, through the control of the incremental permeability that is possible by introducing a pseudoconstant azimuthal magnetic field. However, rapid changes over large permeability ranges incur anomalous behavior such as the ``Q-loss'' and ``f-dot'' loss phenomena that limit performance while requiring high bias currents. Notwithstanding the incomplete understanding of these phenomena, they can be ameliorated by introducing a ``harmonic ratcheting'' acceleration scheme in which two or more rf cavities take turns accelerating the beam---one turns on when the other turns off, at different harmonics---so that the radio frequency can be constrained to remain in a smaller range. Harmonic ratcheting also has straightforward performance advantages, depending on the particular parameter set at hand. In some typical cases it is possible to halve the length of the cavities, or to double the effective gap voltage, or to double the repetition rate. This paper discusses and quantifies the advantages of harmonic ratcheting in general. Simulation results for the particular case of a rapid cycling medical synchrotron ratcheting from harmonic number 9 to 2 show that stability and performance criteria are met even when realistic engineering details are taken into consideration.

Mechanistic road upgrade structural design evaluation using rapid triaxial frequency sweep testing and linear elastic modeling

Saskatchewan Ministry of Highways and Infrastructure employ modified Shell nomographs for flexible pavement thickness design. These modified design nomographs for typical 1970s Saskatchewan field state conditions are based on the local calibration of Shell design curves. However, the modified Shell nomographs are not applicable for the design of recycled pavement systems, particularly full depth reclamation and granular base strengthened systems. This paper presents a mechanistic based thickness design methodology using linear elastic road modeling and a field validation study based on granular base stabilization and strengthening pilot project on Control Section (C.S.) 15-11 in Saskatchewan. The mechanistic methodology of this research included triaxial frequency sweep dynamic modulus and Poisson’s ratio characterization of various strengthened materials taken from the C.S. 15-11 test sections, pavement structural modeling using linear elastic theory, and calculating critical pavement design strain responses. As demonstrated in this research, strengthened pavement structures can be evaluated and designed based on mechanistic testing and linear elastic modeling results. The modeling results revealed rutting and shoving failure concurred with the field performance observed. Predicted primary deflection responses also concurred with those quantified in the field using a non-destructive falling weight deflectometer.

Flow-free transport of cells in microchannels by frequency-modulated ultrasound

We demonstrate flow-free transport of cells and particles by the use of frequency-modulated ultrasonic actuation of a microfluidic chip. Two different modulation schemes are combined: A rapid (1 kHz) linear frequency sweep around approximately 6.9 MHz is used for two-dimensional spatial stabilization of the force field over a 5 mm long inlet channel of constant cross section, and a slow (0.2-0.7 Hz) linear frequency sweep around approximately 2.6 MHz is used for flow-free ultrasonic transport and positioning of cells or particles. The method is used for controlling the motion and position of cells monitored with high-resolution optical microscopy, but can also be used more generally for improving the robustness and performance of ultrasonic manipulation micro-devices.

Rapid frequency sweep technique for MoM planar solvers

A new technique for accelerating the frequency sweep of MoM planar solvers is presented. It is referred to as the rapid frequency sweep (RFS) technique. The technique starts at the central frequency of the simulation band. At this frequency point, it calculates a number of basis integrals for each inner product term involved in the filling of the MoM impedance matrix. At each frequency point, a new set of weighting coefficients are evaluated. These coefficients are used to combine the basis integrals. The proposed RFS is applied to two microstrip circuits; namely, a lowpass filter and a branch line coupler. Results show that RFS offers high accuracy over a frequency band as wide as 150%. It is also very fast in comparison with the regular frequency sweep. For selected examples, a speed-up factor of about 12 is measured.

Delamination Detection in Composite Laminates from Variations of Their Modal Characteristics

As a first step in developing a health monitoring system, the effect of delamination on the modal frequencies of laminated composite beams has been investigated. A piezoceramic patch driven with a linear rapid frequency sweep was used to induce vibrations on the structure and its response registered via piezoelectric sensors. Modal frequencies were obtained using concepts of resonant ultrasound spectroscopy (RUS). Changes of the modal frequencies after delamination initiation, compared to those of a non-delaminated specimen, gave a good indication of the degree of damage, demonstrating the feasibility of using measured changes in the vibration characteristics to detect damage.

DELAMINATION DETECTION IN COMPOSITE LAMINATES FROM VARIATIONS OF THEIR MODAL CHARACTERISTICS

As a first step in developing a health monitoring system, the effect of delamination on the modal frequencies of laminated composite beams has been investigated. A piezoceramic patch driven with a linear rapid frequency sweep was used to induce vibrations on the structure and its response registered via piezoelectric film sensors. Modal frequencies were obtained by applying concepts of a novel method known as resonant ultrasound spectroscopy. Changes of the modal frequencies after delamination initiation, compared to those of a non-delaminated specimen, gave a good indication of the degree of damage, demonstrating the feasibility of using measured changes in the vibration characteristics to detect damage.

Application of the Rapid Frequency Sweep Technique for Delamination Detection in Composite Laminates

As a first step in developing a health monitoring system, the effect of delamination on the natural frequencies of laminated composite beams has been investigated. A piezoceramic patch driven with a linear rapid frequency sweep was used to induce vibrations on the structure and its response registered via piezoelectric film sensors. Modal frequencies were obtained through a method defined here as the maximum amplitude at resonance (MAR), which has proved to be a simple and effective procedure. Changes of the modal frequencies after delamination initiation, compared to those of an uncracked (non-delaminated) specimen, gave a good indication of the degree of damage, especially at high modal frequencies (9-12 kHz).

Impulse excitation amplifier for Fourier transform mass spectrometry

In Fourier transform mass spectrometry (FTMS), static electric and magnetic fields are used to store gaseous ions in an analyzer cell. Typically, the ions can be stored for several minutes if the pressure in the analyzer cell is below 10−8 Torr. While they are stored, the ions move in circular cyclotron orbits having a radial frequency given by w=qB/m, where q/m is the charge-to-mass ratio of an ion, and B is the magnetic field strength. With most FTMS instruments, a mass spectrum of the stored ions is obtained by using a rapid radio frequency (rf) sweep, or ‘‘chirp,’’ to accelerate the ions. The chirp excitation signal produces an oscillatory electric field perpendicular to the magnetic field, and as the frequency of the oscillatory electric field approaches the cyclotron frequency of an ion, the ion is accelerated to a larger cyclotron orbit. In this article the theory of operation, design, and performance of an alternative excitation method called ‘‘impulse excitation’’ is described. Impulse excitation is different from chirp excitation because the ions are accelerated by a very short, high-voltage pulse rather than an oscillatory, radio frequency signal. One of the advantages of impulse excitation is that all the ions in the analyzer cell are accelerated simultaneously in less than a microsecond. This gives more stable FTMS signals and more reliable isotope ratios. An impulse excitation amplifier is inexpensive to build and far less complex than the hardware needed for chirp excitation.

Theory of impulse excitation for Fourier transform mass spectrometry

Most Fourier transform mass spectrometers utilize a rapid radio frequency sweep, or “chirp”, to accelerate and detect ions. Impulse excitation is different because the ions are accelerated by a very short, high voltage d.c. pulse. In this paper, an analytical theory of impulse excitation is presented. Ion trajectories are plotted to illustrate the effects of pulse width, pulse height and “risetime” and “falltime” of the pulse on the ion motion. To minimize mass discrimination, the risetime of the impulse signal must be very fast (50–100 ns) and the pulse width must be much shorter than the period of the cyclotron orbit of an ion.

Practical application of the rapid frequency sweep technique for structural frequency response measurement

SummaryThe rapid frequency sweep technique is the result of a considerable amount of research into the development of a transient method for the rapid measurement of structural frequency response, digital data analysis procedures being used to derive the required dynamic characteristics. Theoretical analyses, together with the results of experimental studies and experience gained during structural testing are presented here which illustrate practical considerations and limitations of the method; these range from discussion of exciter attachment, effects of transducers and choice of instrumentation to signal processing methods and related criteria. A simple guide to practical structural testing is given in the final section which outlines procedures to be followed during testing and signal acquisition.

The experimental characterization of wave propagation systems: II. Continuous systems and the effects of dispersion

This paper forms the second part of a study of the wave propagation approach and its application to the experimental characterization of structural/acoustic systems. It is shown that the measurement of time delays and attenuation factors by correlation techniques offers a useful complement to the measurement of frequency response in system characterization. Transient excitation is used and although the rapid frequency sweep is suitable for measurements in continuous systems, it is shown that the dispersion encountered in flexural wave propagation increases the difficulty in the interpretation of correlation data. To overcome this problem, another transient excitation signal is developed which has essentially single frequency content but which yields clear correlation functions, permitting the accurate measurement of group velocities at a single frequency.

The experimental characterization of wave propagation systems: I. non-dispersive waves in lumped systems

A study of non-dispersive wave propagation in lumped systems is described, with reference to the development of experimental techniques for the measurement of system parameters. The work is concerned in particular with the study of structural/acoustic systems. Transient, rapid frequency sweep excitation is used and both correlation and spectral analysis techniques are considered. Single and multi-path delay systems with and without resonant elements are discussed and it is shown that with a suitable choice of excitation parameters, the system characteristics can be successfully deduced.

Spectrally shaped transient forcing functions for frequency response testing

The transfer characteristics of dynamic force generating systems for frequency response measurements on mechanical systems often cause the generated force spectrum to differ in shape from the input voltage spectrum. By considering the spectral characteristics of the linear rapid frequency sweep, it is shown that good estimates of the modulus spectra of swept sinewaves with varying sweep rates may be calculated. It is shown how swept sinewave voltages of constant amplitude may be generated with an appropriately predetermined sweep rate to allow the transfer characteristics of the force generating system to be compensated for, thus producing a spectrally shaped transient forcing function at the output of the force generator. The application of the technique in vibration testing is demonstrated.

Variable resonance in constant frequency bats

Bats of the two closely related families Rhinolophidae and Hipposideridae, and also Chilonycteris rubiginosa of the family Phyllostomatidae, emit echo‐location pulses with a relatively long portion of constant frequency followed by a rapid frequency sweep. The greatest amplitude occurs either during the constant frequency or at some point in the sweep with the constant frequency at a lower amplitude. Two hypotheses are advanced to account for the changes in frequency of peak amplitude: a variable vocal tract resonance or “source generated” amplitude changes, and their relative merits are considered. The importance to the animals of high amplitude frequency sweeps is discussed, considering the conditions under which they are produced and whether the echoes from such pulses can be detected.

Evaluation of the dynamic characteristics of structures by transient testing

Transient methods of measuring the frequency response of structures may be used now that the digital computer is established as a means of analysing transient data. The spectral properties of the rapid frequency sweep are well defined and it is shown that this type of forcing function is extremely suitable for transient tests on structures. Methods of deriving the frequency response function from rapid frequency sweep tests are discussed and possible sources of error are investigated. Results from practical tests show how the method may be used to derive the frequency response characteristics of built-up structures.