Doublet Pulses Research Articles

Voluntary activation of maximal single and all finger power grip contractions.

When all 4 fingers are engaged together during a grip strength contraction, the force produced by an individual finger is less than the force produced when it acts in isolation. The purpose of this study was to evaluate if the reduced force output of a digit during an all-finger grip contraction is due to a decline in voluntary activation. Fifteen young adults (n=7 females) completed voluntary contractions of the index finger in isolation and all fingers together in a dynamometer capable of separately recording forces from each finger during voluntary and electrically evoked contractions. The median and ulnar nerves were electrically stimulated simultaneously at the elbow to record individual finger flexion forces from doublet (100 Hz) pulses. Doublet stimulations were applied during and immediately following contractions at 50, 65, 85, and 100% maximal voluntary contraction (MVC) forces. Two-way ANOVAs were used to compare the effects of sex and finger (single vs all) on flexion forces and voluntary activation. The index finger produced ∼25% more force when engaged in isolation compared to the all-finger contraction; however, there were no differences in voluntary activation between the single and all finger MVCs (p=0.344). The index finger force deficit was larger in females compared to males (34 vs 18%, p=0.030), but this was not explained by sex-related differences in voluntary activation. These data indicate that the additional force produced during single-finger contractions is not due to an alteration in voluntary activation, as all-finger contractions display near maximal activation of each digit.

Sub-GHz resolution line-by-line pulse shaper for driving superconducting circuits

We demonstrate a sub-GHz resolution, fully programmable Fourier-domain pulse shaper capable of generating arbitrary optical pulse patterns for superconducting circuit platforms. This high resolution allows line-by-line pulse shaping of a 1 GHz-spaced comb, and the pulse shaper can accommodate an optical bandwidth as large as 1 THz, which represents the highest resolution programmable line-by-line pulse shaping to our knowledge. Linear optical sampling with a dual-comb system confirms independent control of 1 GHz-spaced optical lines, and the low phase noise of the pulse shaper is characterized. We apply the pulse shaper as an optical drive for an array of Josephson junctions operating at a temperature of 4 K, where cryogenic photodetection of pulse doublets with user-defined separation characterizes the Josephson junction response. Furthermore, we demonstrate a pulse-density modulation pattern of 4 ps duration optical pulses that can serve as the high bandwidth drive of a quantum-based Josephson arbitrary waveform synthesizer. By leveraging the exquisite control, large bandwidth, and low noise of photonics, this represents an important advance toward the realization of high power and high spectral purity AC voltage standards at gigahertz frequencies without requiring 100 GHz bandwidth driving electronics.

Nonlinear frequency chirps from a stabilized injected phase-modulated fiber laser loop

A phase-modulated frequency-shifting loop is injected by a single-frequency laser at 1.5 μm. In so-called Talbot conditions, i.e., when the modulation frequency is an integer multiple of the inverse of the cavity round-trip time, the loop generates a frequency comb whose temporal trace consists in a train of pulse doublets whose positions in time depend on the frequency of the injection laser. When the modulation frequency is slightly detuned from the Talbot condition, nonlinear frequency chirps are predicted and observed in the output pulse train. We demonstrate that these nonlinear chirps are not restricted to sinusoidal shapes, and also that the loop can be stabilized by exploiting the intracavity phase modulation.

Generation of stable temporal doublet by a single-mode silicon core optical fiber

A highly nonlinear single mode anomalous dispersion silicon-core fiber is designed and optimized at the operating wavelength of 2.2 µm for the purpose of generating stable temporal pulse doublets. To designate the output pulse pair as a perfect Gaussian doublet, two new parameters, dissimilarity factor () and co-similarity index (μ cs) are introduced. Different input pulse parameters such as power, pulse width and chirp are optimized to obtain Gaussian temporal doublets at the shortest optimum length (∼few cm) which is sufficiently smaller in comparison to silica fibers reported earlier. The output pulse pairs remain as a doublet for quite a good stability length. In view of serving practical purposes, the possibilities of fluctuations of input power and pulse width are included to investigate the changes in stability length and effective repetition rate (ERR). The change in ERR along the fiber length produces a remarkable change in free carrier concentration in core, which has also been taken into account for the first time as per our knowledge to obtain the temporal pulse doublet in the so designed Si-core fiber.

Optical frequency-to-time mapping using a phase-modulated frequency-shifting loop.

A real-time spectral analysis is demonstrated experimentally with a frequency-shifting loop that includes an electro-optic phase modulator. When a single-frequency laser seeds the loop, pulse doublets are emitted if the integer Talbot condition is satisfied. With a polychromatic seed, frequency-to-time mapping is demonstrated, namely the temporal output of the loop maps the spectral power of the seed, with a resolution of 400 kHz. Due to the phase modulation function, the mapping is shown to be nonlinear. The results are in agreement with the theoretical predictions of [H. Yang et al., J. Opt. Soc. Am. B37, 3162 (2020)JOBPDE0740-322410.1364/JOSAB.389801]. The extension to integrated systems for applications is discussed.

Sinusoidal Frequency-Modulated Waveforms Generated by a Phase-Modulated Frequency-Shifting Loop

We theoretically and experimentally investigate a phase-modulated frequency-shifting loop (PM-FSL) that includes an electro-optic phase modulator (EOPM) and a gain element, and is seeded by a single-frequency laser. By slightly detuning the modulation frequency of the EOPM off an integer multiple of the fundamental loop frequency, we generate an output waveform that exhibits a series of pulse doublets modulated at radio frequency (RF). We prove that the series consists of sinusoidal frequency-modulated (SFM) pulse doublets whose repetition rate and bandwidth are easily reconfigurable. We report the generation of the SFM waveforms with bandwidth above 7 GHz (limited by the detection bandwidth) by simply tuning the input RF tone over a span of a few kHz in the vicinity of 14.58 MHz (the round-trip frequency). The system is modeled using a time-delayed interference model that accounts for the modulation function of the EOPM, the loop delay time, and the detuning parameter. The model explains the formation of SFM pulse doublets and effectively reproduces all the experimental waveforms. This well-defined waveform may find applications in RF-optical signal processing and radar systems.

Optical pulse doublet resulting from the nonlinear splitting of a super-Gaussian pulse

We experimentally demonstrate the generation of a temporal pulse doublet from the propagation of an initial super-Gaussian waveform in a nonlinear focusing medium. The picosecond structures are characterized both in amplitude and phase and their close-to-Gaussian Fourier-transform limited shape is found in excellent agreement with numerical simulations. This nonlinear fiber-based single-stage reshaping scheme is energy efficient, can sustain GHz repetition rates and temporal compression factors around 10 are demonstrated.

Photonic generation of picosecond pulse doublets from the nonlinear splitting of super-Gaussian pulses

We experimentally demonstrate the generation of temporal pulse doublets from the propagation of initial super-Gaussian waveforms in an optical fiber with anomalous dispersion. The nonlinear focusing dynamics leads to picosecond structures that are characterized both in amplitude and phase. Their close-to-Gaussian Fourier-transform limited shape is found in excellent agreement with numerical simulations. This single-stage reshaping scheme is energy efficient, can sustain GHz repetition rates and temporal compression factors around 10 are demonstrated.

Helicopter stability and control derivatives identification in different flight conditions

Helicopter flight simulators require high-fidelity dynamic models based on a set of stability and control derivatives (SCDs). For different flight conditions, specific sets of SCDs must be identified and utilized in the flight simulator software. In this study, nine sets of SCDs of the AS 355 F2 helicopter have been estimated from experimental flight-testing data, considering different combinations of velocity, mass, and altitude. Linear dynamic models with eight states and four controls were determined for the different flight conditions, using the output-error method. The associated optimization problem was solved with the Gauss–Newton algorithm, and identification reliability was evaluated by the Cramer–Rao (CR) and relative correlation coefficient (RCC) indices. An instrumented AS 355 F2 Helicopter belonging to the Instituto de Pesquisas e Ensaios em Voo, Departamento de Ciencia e Tecnologia Aeroespacial, Sao Jose dos Campos, Brazil, was utilized for data collection in different flight conditions, totaling 16 test hours. The following natural dynamic modes have been excited: phugoid, short period, spiral and Dutch roll, by command inputs of the type: frequency sweep (sinusoidal), doublet pulse and 3-2-1-1. Considering CR and RCC criteria, the most appropriate maneuver for exciting each dynamic mode is suggested, and the sets of the nine flight conditions are SCDs reported.

Manipulation of Gaussian derivative pulses and vector solitons in fiber lasers with analytic modeling by the coupled complex Ginzburg–Landau equations

Both positive- and negative-polarity Gaussian monocycle and doublet pulses, for which the pulse shapes are the first and second derivatives of Gaussian functions plus some continuous-wave backgrounds respectively, are generated in a ring-cavity erbium-doped fiber laser from polarization-locked vector solitons by using passive optical technology. The pulse states are switchable and are found to be the superposition of bright and dark solitons with different widths, amplitudes, time delays, polarizations, and wavelengths. Qualitative analysis of the properties of vector solitons are performed by solving coupled complex Ginzburg–Landau equations, and analytic modeling of the fiber laser is presented. By representing the envelopes of bright soliton by sech- and dark soliton by tanh-based functions, the incoherent superposition of these two soliton components have simulated the experimental observations, and the underlying mechanisms for the formation of monocycle and doublet pulses are attributed to the polarization locking of bright and dark solitons. The theoretical modeling is used to calculate the pulse parameters of the fiber laser, and the intracavity birefringence can be estimated. The results of tunable optical vector solitons are compared with atomic solitons in the system of Bose–Einstein condensation. Since the governing equations for soliton generation in fiber lasers and Bose–Einstein condensation have many properties in common, thus the simulation and propagation of pulsating waves may open a new route to explore the classical solitary dynamics in nonlinear optics and its quantum analogy in ultracold fields.

Pulse doublets generated by a frequency-shifting loop containing an electro-optic amplitude modulator.

We investigate theoretically and experimentally an all-fibered frequency-shifting loop which includes an electro-optic amplitude modulator (EOM) and an optical amplifier, and is seeded by a continuous-wave laser. At variance with frequency-shifted feedback lasers, or Talbot lasers, that contain an acousto-optic frequency shifter, the EOM creates at each round-trip two side-bands that recirculate inside the loop. Benefiting from the high modulation frequency of the EOM, a wide optical frequency comb up to 40 GHz is generated. We demonstrate an original double-pulse regime when the loop length is a multiple of the RF modulation wavelength applied to the modulator. The inter-pulse interval is governed by both the bias voltage and modulation depth of the EOM. Besides, some typical waveforms such as saw-tooth and rectangle are experimentally obtained by properly setting operating frequency, bias voltage and the RF power. The system is modeled by a linear interference model that takes the amplitude modulation function and loop delay into account. The model explains the formation of pulse doublets and reproduces well all the experimental waveforms. Furthermore, the un-seeded loop driven above threshold also generates mode-locked picosecond pulse doublets with a continuously adjustable delay up to the modulation period.

Investigation of different stimulation patterns with doublet pulses to reduce muscle fatigue.

Introduction: Functional electrical stimulation is a common technique used in the rehabilitation of individuals with a spinal cord injury to produce functional movement of paralysed muscles. However, it is often associated with rapid muscle fatigue which limits its applications. Methods: The objective of this study is to investigate the effects on the onset of fatigue of different multi-electrode patterns of stimulation via multiple pairs of electrodes using doublet pulses: Synchronous stimulation is compared to asynchronous stimulation patterns which are activated sequentially (AsynS) or randomly (AsynR), mimicking voluntary muscle activation by targeting different motor units. We investigated these three different approaches by applying stimulation to the gastrocnemius muscle repeatedly for 10 min (300 ms stimulation followed by 700 ms of no-stimulation) with 40 Hz effective frequency for all protocols and doublet pulses with an inter-pulse-interval of 6 ms. Eleven able-bodied volunteers (28 ± 3 years old) participated in this study. Ultrasound videos were recorded during stimulation to allow evaluation of changes in muscle morphology. The main fatigue indicators we focused on were the normalised fatigue index, fatigue time interval and pre-post twitch–tetanus ratio. Results: The results demonstrate that asynchronous stimulation with doublet pulses gives a higher normalised fatigue index (0.80 ± 0.08 and 0.87 ± 0.08) for AsynS and AsynR, respectively, than synchronous stimulation (0.62 ± 0.06). Furthermore, a longer fatigue time interval for AsynS (302.2 ± 230.9 s) and AsynR (384.4 ± 279.0 s) compared to synchronous stimulation (68.0 ± 30.5 s) indicates that fatigue occurs later during asynchronous stimulation; however, this was only found to be statistically significant for one of two methods used to calculate the group mean. Although no significant difference was found in pre-post twitch–tetanus ratio, there was a trend towards these effects. Conclusion: In this study, we proposed an asynchronous stimulation pattern for the application of functional electrical stimulation and investigated its suitability for reducing muscle fatigue compared to previous methods. The results show that asynchronous multi-electrode stimulation patterns with doublet pulses may improve fatigue resistance in functional electrical stimulation applications in some conditions.

Development of a three-legged, high-speed, burst-mode laser system for simultaneous measurements of velocity and scalars in reacting flows.

We report the development of a three-legged, high-speed, high-energy, burst-mode laser system for the simultaneous measurement of velocity and key combustion species in turbulent reacting flows. The laser system is designed to simultaneously amplify a four-pulse sequence [including a doublet pulse for particle image velocimetry (PIV) measurements] with variable pulse separations at a repetition rate up to 500kHz and a burst duration of 1-10ms. With the three-legged, burst-mode laser system, we demonstrate simultaneous measurements of velocity using PIV and planar laser-induced fluorescence imaging of hydroxyl and formaldehyde in a turbulent jet flame.

Compact burst-mode Nd:YAG laser for kHz-MHz bandwidth velocity and species measurements.

A compact-footprint (0.18 m2) flash-lamp-pumped, burst-mode Nd:YAG-based master-oscillator pulsed-amplifier laser is reported with a fundamental 1064nm output of over 14J per burst. A directly modulated diode laser seed source is used to generate 10ms duration arbitrary sequences of 500kHz doublet or MHz singlet pulses for flow-field velocity or species measurements, respectively. Flexible pulse widths are used to balance the energy distribution of pulse doublets and achieve second-harmonic conversion efficiencies up to 42%. Burst-mode laser performance characteristics, measurement accuracies in turbulent flows, and prospects for kHz-MHz flow-field diagnostics are discussed.

A New Time-Domain Model for Multiple Scattering of UWB Signals Through Lossy Obstacles

In this paper, a new time-domain (TD) model is proposed for multiple scattering of ultra wideband (UWB) signals through lossy obstacles. Propagation through structures like wedge, single building, and double buildings is presented where buildings are supposed to have rectangular cross sections. Considering two-dimesnional (2-D) and realistic three-dimensional (3-D) scenarios, first an accurate path-tracing algorithm is proposed for multiple scattering of UWB signals through different 2-D and 3-D scenarios and then, TD solution is presented for realistic multiple scattering problems where a single ray-path can undergo diffraction, transmission, and reflection successively. Results are shown for both soft and hard polarizations. Considering Gaussian doublet pulse, the accuracy of the presented TD solution is confirmed by comparing the TD results with the numerical inverse fast Fourier transform (IFFT) of the corresponding frequency-domain (FD) results. It has been found that the field strength at the receiver (Rx) undergoes significant attenuation and distortion for different multiple scattering scenarios. For an in-depth analysis of pulse distortion at Rx, the UWB channel impulse response is analyzed for multiple scattering scenario. Power profile for multiple scattering scenario is also observed to signify the received power at the Rx. To characterize the multipath propagation, UWB multipath is analyzed in terms of time dispersion parameters like mean excess delay and root mean square delay spread. Further to confirm the generality of the proposed TD solution, the results are shown for a variety of other UWB pulses like monocycle and fourth-order Gaussian monocycle pulses. Finally, the computational efficiency of the TD and IFFT-FD methods is compared.

Scalable Ultra-Wideband Pulse Generation Based on Silicon Photonic Integrated Circuits

We demonstrate a scalable ultra-wideband (UWB) pulse generator using silicon photonic integrated circuits. A delay-and-superimpose principle for higher order UWB Guassian pulse generation is realized by monolithically integrating a pair of tunable microring resonators, a delayline waveguide, tap couplers, and an on-chip photodiode. The two microrings function as discrimination filters and generate the asymmetric monocycle pulses. For a proof-of-concept demonstration, the doublet pulses with a 10-dB fractional bandwidth of 100% and the triplet pulses with a 10-dB fractional bandwidth of 132% are generated by superposition of the two asymmetric pulses with a proper delay between them. The proposed scheme and the photonic chip can be scaled to generate more kinds of power-efficient pulses.

Soliton trapping and comb self-referencing in a single microresonator with χ(2) and χ(3) nonlinearities.

A shaped doublet pump pulse is proposed for a simultaneous octave-spanning soliton Kerr frequency comb generation and second-harmonic conversion in a single microresonator. The temporal soliton in the cavity is trapped atop a doublet-pulse pedestal, resulting in a greatly expanded soliton region compared to that with a general Gaussian pulse pump. The possibility of single-microresonator comb self-referencing in a single silicon nitride microring that can facilitate compact on-chip optical clocks is demonstrated via simulation.

Theoretical demonstration of all-optical switchable and tunable UWB doublet pulse train generator utilizing SOA wavelength conversion and tunable time delay

An all-optical ultrawide band (UWB) doublet pulse train signal generator is proposed and theoretically simulated by utilizing an inverse wavelength conversion base on the cross-gain modulation (XGM) effect in a semiconductor optical amplifier (SOA) and controllable time delay in two optical delay lines (ODLs). The proposed scheme is not only optically switchable in the polarity of pulse by switching the polarity of input pulse but also tunable in signal pulse width and radiofrequency (RF) spectrum by tuning the ODLs.

Monocycle and doublet pulses generation via photon echo in rare-earth-doped optical crystal

All-optical pulse generation opens up a field for ultrawideband (UWB) applications. However, controllable pulse width and pulse type are still challenging. Here, we present a theoretical model and stimulated results of monocycle and doublet waveforms generation using programmable optical photon echo progress. We synthesized instantaneously monocycle and doublet waveforms by adjustment of pulse width, pulse amplitude, pulse position, and time interval of subpulses. We verified the possible application of the proposed method to design U.S. Federal Communications Commission-compliant UWB waveforms, and therefore, it may provide an avenue for waveform generation.

Spatiotemporal analysis of turbulent jets enabled by 100-kHz, 100-ms burst-mode particle image velocimetry

100-kHz particle image velocimetry (PIV) is demonstrated using a double-pulsed, burst-mode laser with a burst duration up to 100 ms. This enables up to 10,000 time-sequential vector fields for capturing a temporal dynamic range spanning over three orders of magnitude in high-speed turbulent flows. Pulse doublets with inter-pulse spacing of 2 µs and repetition rate of 100 kHz are generated using a fiber-based oscillator and amplified through an all-diode-pumped, burst-mode amplifier. A physics-based model of pulse doublet amplification in the burst-mode amplifier is developed and used to accurately predict oscillator pulse width and pulse intensity inputs required to generate equal-energy pulse doublets at 532 nm for velocity measurements. The effect of PIV particle response and high-speed-detector limitations on the spatial and temporal resolution are estimated in subsonic turbulent jets. An effective spatial resolution of 266–275 µm and temporal resolution of 10 µs are estimated from the 8 × 8 pixel correlation window and inter-doublet time spacing, respectively. This spatiotemporal resolution is sufficient for quantitative assessment of integral time and length scales in highly turbulent jets with Reynolds numbers in the range 15,000–50,000. The temporal dynamic range of the burst-mode PIV measurement is 1200, limited by the 85-ms high-energy portion of the burst and 30-kHz high-frequency noise limit.