Repetition Rate Of Pulse Train Research Articles

Passive Q-switched mode locking of double-clading Yb:fiber laser with ion-implanted GaAs

We report the technique of the ion-implanted semi-insulating GaAs wafer used for passive Q-switched mode locking in double-cladding Yb:fiber laser. The wafer was implanted with 400-keV energy, 1016/cm2 dose As+ions, and was annealed at 600℃ for 20 min. At the pump power of 5W, we achieved output power of 200mW. The repetition rate of envelope of Q-switched mode locking is 50-kHz with a FWHM envelope of 4μs. The repetition rate of mode locked pulse train was found to be 15-MHz. This is the first report of such a kind of laser to the best of our knowledge.

The role of GABAergic inhibition in shaping directional selectivity of bat inferior collicular neurons determined with temporally patterned pulse trains.

This study examined the role of GABAergic inhibition in shaping directional selectivity of neurons in the inferior colliculus of the big brown bat, Eptesicus fuscus. When determined with temporally patterned pulse trains at different pulse repetition rates, 93 inferior colliculus neurons displayed three types of directional selectivity curves. A directionally selective curve always showed a maximum to a certain azimuthal angle (the best angle). A hemifield curve showed a maximum to a range of contralateral azimuthal angles. A non-directional curve did not show a maximum to any particular azimuthal angles. Directional selectivity curves of 42% neurons changed from hemifield or non-directional to directionally selective and the best angles of 16-21% neurons shifted toward the midline with increasing pulse repetition rate of pulse trains. Directional selectivity curves of most (74%) neurons that discharged impulses to each pulse of a pulse train also became sharper with increasing pulse repetition rate of pulse trains. Bicuculline application produced more pronounced broadening of directional selective curves of inferior colliculus neurons at higher than at lower pulse repetition rates. As a result, pulse repetition rate-dependent directional selectivity of inferior colliculus neurons was abolished. Possible mechanisms and biological significance of these findings are discussed.

Direct measurement of the group refractive index of air with interferometry between adjacent femtosecond pulses.

The group refractive index of air in laboratory conditions is measured directly between adjacent femtosecond laser pulses by a new interferometry technique. Measurement of the repetition rate of the mode-locked pulse train that gives the maximum amplitude of the interference-signal envelope enables us to determine the group refractive index of air within a standard deviation of 2 x 10(-7). This simple method without vacuum reference is attractive for measuring the group refractive index needed for precise distance measurements in open fields.

Technique for simultaneously multiplying the repetition rate of multiwavelength optical pulse trains

In this letter, we propose and numerically demonstrate a technique for synchronized multiplication of the repetition rate in multiwavelength optical pulse trains launched over a single fiber. The technique only requires linear reflection of the original signal in a single structure of superimposed chirped in-fiber Bragg gratings, which are specifically designed to provide a suitable group delay dispersion within each wavelength channel. The proposed multiplication process does not affect the individual pulse characteristics (shape and temporal width),and is highly efficient in terms of energy. Our proposal results in very compact structures capable of processing simultaneously a large number of wavelength channels.

Temporally patterned sound pulse trains affect intensity and frequency sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus.

This study examined the effect of temporally patterned pulse trains on intensity and frequency sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus. Intensity sensitivity of inferior collicular neurons was expressed by the dynamic range and slope of rate-intensity functions. Inferior collicular neurons with non-monotonic rate-intensity functions have smaller dynamic ranges and larger slopes than neurons with monotonic or saturated rate-intensity functions. Intensity sensitivity of all inferior collicular neurons improved by increasing the number of non-monotonic rate-intensity functions when the pulse repetition rate of pulse trains increased from 10 to 30 pulses per second. Intensity sensitivity of 43% inferior collicular neurons further improved when the pulse repetition rate of pulse trains increased still from 30 to 90 pulses per second. Frequency sensitivity of inferior collicular neurons was expressed by the Q10, Q20, and Q30 values of threshold frequency tuning curves and bandwidths of isointensity frequency tuning curves. Threshold frequency tuning curves of all inferior collicular neurons were V-shape and mirror-images of their counterpart isointensity frequency tuning curves. The Q10, Q20, and Q30 values of threshold frequency tuning curves of all inferior collicular neurons progressively increased and bandwidths of isointensity frequency tuning curves decreased with increasing pulse repetition rate in temporally patterned pulse trains. Biological relevance of these findings to bat echolocation is discussed.

On-chip high-frequency diagnostic of RSFQ logic cells

Hilbert Transform spectroscopy is described as a method to analyze the spectrum of pulse trains generated by RSFQ circuits. Simulations are carried out using parameters appropriate for NbN Josephson junctions for both generation and detection of SFQ pulse trains. It is shown that the pulse shape along with the pulse train repetition rate can be extracted through the use of a Josephson junction, used as a spectrometer, and located on-chip with the RSFQ circuit to test.

Temporal self-imaging effects: theory and application for multiplying pulse repetition rates

A time-domain equivalent of the spatial Talbot or self-imaging phenomenon appears when a periodic temporal signal propagates through a dispersive medium under first-order dispersion conditions. The effect is of great interest because it can be applied for multiplying the repetition rate of an arbitrary periodic pulse train without distorting the individual pulse features and essentially without loss of energy. In this way, pulse sequences in the terahertz regime can be generated from typical mode-locked pulse streams (with a few gigahertz repetition rates). The Talbot-based repetition-rate-multiplication technique can be implemented by using a linearly chirped fiber grating (LCFG) as the dispersive medium. As compared with other alternatives, an LCFG can be designed to provide the required bandwidth and dispersion characteristics in significantly more compact forms. Here, by using a signal-theory-based approach, we carry out a general theoretical analysis of the temporal self-imaging phenomenon and derive analytical expressions for all cases of interest (integer and fractional self-imaging effects). We also show how to design a LCFG for implementing the repetition-rate-multiplication technique and discuss the impact of nonidealities in the grating's response on the multiplication process. Results of our study are relevant from both a physical and a practical perspective.

Stabilization of the frequency, phase, and repetition rate of an ultra-short pulse train to a Fabry–Perot reference cavity

The carrier frequency, phase, and repetition rate of an ultra-short pulse train is controlled via simultaneous stabilization of more than 500 000 of its longitudinal modes to a Fabry–Perot reference cavity.

Generation of ultrahigh-speed tunable-rate optical pulses using strongly gain-coupled dual-wavelength DFB laser diodes

A novel and simple method to generate a variable-rate, ultrahigh-speed optical pulse train is demonstrated using a dual-wavelength, strongly gain-coupled distributed-feedback laser diode. The repetition rate of the optical pulse train is continuously tunable from 25 to 80 GHz and no high-speed electronics are required in this method.

The effect of the laser–atom interaction time on the influence of velocity-changing collisions in Doppler-free spectroscopy

Remarkable reduction of the influence of velocity-changing collisions in Doppler-free optogalvanic spectra has been achieved by using a square pulse train produced by the intensity modulation of a cw ring dye laser in conventional intermodulated optogalvanic spectroscopy. Decreasing the pulsewidth and the repetition rate of the square pulse train, the Gaussian collisional parameter, C, for the 1S 0 → 3P 1 transition of Yb atoms at low discharge current is reduced by two orders of magnitude when compared to that of a conventional intermodulated optogalvanic spectrum. The line narrowing of the Gaussian pedestals below the theoretical limit was also observed. Reduction of the collisional parameter as well as the line narrowing of Gaussian pedestals enhances the spectral resolution of this method and allows measurements of highly resolved Doppler-free spectra in hollow cathode discharge cells.

External fibre laser based pulse amplitude equalisationscheme forrational harmonic modelocking in a ring-type fibre laser

The authors have obtained, for the first time to their knowledge, pulse amplitude equalisation in a high repetition rate (up to 22.6 GHz) pulse train, which is generated from rational harmonic modelocking of an Er-doped fibre ring laser, by using an equalising fibre laser. The equalising fibre laser is based on a passively modelocked laser with an NALM (nonlinear amplifying loop mirror) and a Faraday rotating mirror.

Existence of electronic excitation enhanced crystallization in GeSb amorphous thin films upon ultrashort laser pulse irradiation.

The energy density crystallization threshold of amorphous GeSb films has been studied for the first time as a function of the laser pulse duration in the range from 170 fs to 8 ns. The results obtained provide evidence of the existence of enhanced crystallization upon irradiation with pulses shorter than 800 fs, which is most likely related to electronic excitation effects. The study of the interaction of ultrashort laser pulses with solids has raised some important questions concerning the nature of the transformations occurring at the material surface in the presence of very strong levels of electronic excitation. The existence of electronic excitation induced phase transitions has been already demonstrated in several materials (Si [1], GaAs [2‐ 4], graphite [5]). In the case of crystalline semiconductors, excitation with pulses shorter than the longitudinal optical phonon emission time [6] may lead to a structural instability of the lattice, leading to the formation of a metastable transient phase with (semi)metallic character. It is not clear, however, whether the presence of such transient strong electronic excitation and/or transient phases may influence the final state or structure induced since, once the energy of the excited carriers has been transferred to the lattice, the structural transformation path is thermal in nature. Different nonthermal mechanisms which could lead to enhanced crystallization and to crystal damage recovery upon pulsed laser irradiation were proposed during the early 1980s [7,8]. More recently, both photonic effects and electron-hole plasma induced network softening have been invoked to explain structural changes induced in amorphous Si and Ge upon ns and ps laser pulse irradiation [9 ‐ 11]. High Sb-content GeSb amorphous and crystalline thin films have been shown to both crystallize and amorphize upon irradiation with subnanosecond laser pulses. Both processes show optical contrast [12] making this material a potential candidate for applications in ultrashort laser pulse driven phase change optical recording [13,14]. These characteristics also make this material interesting for the study of possible high electronic excitation induced and enhanced crystallization processes. The aim of this work is to elucidate whether the high electronic excitation, induced by irradiation with subnanosecond laser pulses, can induce or enhance the crystallization process of the amorphous phase. Such an enhancement process should lead to appreciable changes in the minimum energy density required for crystallization as the laser pulse duration changes, according to the differences in the excited carrier populations induced. We have performed what we believe to be the first experiment to determine the minimum energy density required for crystallization of an amorphous phase upon irradiation with laser pulses whose durations extend over 3 orders of magnitude, covering the range 170 fs to 8 ns. The samples used in the experiment were 50 nm thick, Ge 0.07Sb 0.93 films grown at room temperature in a multitarget magnetron dc sputtering system from Ge and Sb (99.999% purity) targets onto glass, carbon-coated mica, and Si(100) substrates. In order to improve the sensitivity of the material and to enhance the crystallization rate upon pulsed laser irradiation, two different kinds of samples were grown: (a) fully amorphous layers with the composition Ge 0.07Sb 0.93 and (2) bilayered films formed by an amorphous surface layer of similar composition and an underlayer slightly richer in Sb and therefore polycrystalline. Since the bilayered films showed smaller transformation times and improved sensitivity upon pulsed laser irradiation with 10 ps laser pulses at 583 nm, they were chosen for the present work. The samples on mica and Si were used, respectively, to prepare planar and cross-section transmission electron microscopy (TEM) specimens in order to characterize film structure and composition. The latter was determined by energy dispersive x-ray spectroscopy on planar TEM specimens. An ultrashort CPA laser pulse source (described in Ref. [15]) was used to irradiate the samples on glass. The experimental configuration is shown in Fig. 1. A commercial fs Ti:Al 2O 3 oscillator providing 150 fs duration pulses at a repetition rate of 82 MHz at 830 nm was used to seed an Ar 1 ion pumped Cr-LiSAF solid state regenerative amplifier. Before seeding the amplifier, the oscillator pulses were stretched to approximately 300 ps with a diffraction grating pulse stretcher. After amplification the pulses were recompressed with a grating pair compressor. The typical output of the laser system consisted of a 10 kHz repetition rate pulse train with energies per pulse in the order of 5 mJ. A second Pockels cell, synchronized to the one in the regenerative amplifier, was used

Programmable shaping of ultrabroad-bandwidth pulses from a Ti:sapphire laser

We have used a commercially available liquid-crystal spatial light modulator within a reflective optics pulse-shaping apparatus to shape ultrashort pulses with temporal resolution approaching 10 fs. Using the spatial light modulator as a phase modulator, we produce a variety of complex ultrafast waveforms, including odd pulses, high repetition rate (>23 THz) pulse trains, and asymmetric pulse trains. We also show that it is possible to compensate for large amounts of high-order phase dispersion (in excess of 60π) by appropriate cubic- and quartic-phase modulations of the pulse. Finally, we examine the limitations of shaping ultrabroad-bandwidth pulses. We find that, for specific classes of waveforms, Fourier-transform pulse-shaping techniques can be used for pulses with 5-fs durations, which exceed the current state of the art in ultrashort pulse generation. However, synthesis of general waveforms with 5-fs resolution will require compensating for nonlinear spatial dispersion of frequency in the masking plane.

Generation of 2 THz repetition rate pulse trains through induced modulational instability

Trains of 130 fs pulses at 2 THz repetition rates have been generated through induced modulational instability in a single-mode fibre from an anti-Stokes CW diode laser signal on a carrier frequency provided by a mode-locked Nd:YAG laser

Peripheral nerve stimulation suppression of C-fiber-evoked flexion reflex in rats. Part 2: Parameters of low-rate train stimulation of skin and muscle afferent nerves.

Transcutaneous electrical nerve stimulation (TENS) is an acknowledged clinical technique to alleviate chronic pain. Its effectiveness is still limited, however, and the stimulation parameters used are based on subjective reports from patients. In the present study, the systematic investigation of conditioning stimulation that was previously reported by the author has been extended to include short trains of stimuli at a low repetition rate (0.1 to 5 Hz) delivered to dissected skin and muscle nerves in the lightly anesthetized rat. The size of a C-fiber-evoked flexion reflex was utilized as a measure of transmission from nociceptive afferent nerve fibers in the spinal cord and was tested repeatedly after 30 minutes of conditioning TENS-like stimulation to adjacent nerves. At these low rates, stimulation of a muscle nerve was usually more effective in suppressing transmission from C-fibers to second-order neurons than was stimulation of a skin nerve. Furthermore, a stimulation strength recruiting both Group I-II and III muscle afferent fibers was more effective in depressing the C-fiber-evoked activity at all frequencies studied than was that activating Group I-II fibers only. A pulse-train repetition rate of around 1 Hz was most effective. These findings should be taken into account when carrying out clinical TENS treatment.

Responses to pure tone stimuli and model songs of ventral cord neurones of Corixa punctata Ill. (Heteroptera: Corixidae)

The hearing organs of Corixa punctata males were stimulated with pure tones and model songs. Carrier frequencies of the stimuli ranged from 1 to 3 kHz, thus covering the peak frequencies of the conspecific song spectra. The spike responses of ventral cord neurones were recorded extracellularly. Four different neurone types were found on the right (R) side of the prothoracic ganglion and were characterized as follows: (1) Threshold curves of the 1.2−2.4 1 R neurones showed a double peak of best frequency sensitivity (Fig. 1). According to stimulus carrier frequency their response saturated or reached a maximum between 10 and 25 dB above threshold (Fig. 2A). Neurones copied different stimulus durations in a phasic-tonic way (Fig. 3A). Time structure of a model song, simulating the conspecific sound emission, was faithfully copied in the frequency range between 1.6 and 2.8 kHz (Fig. 3E). Neurone's responses habituated at the repetition rate of pulse trains typical for the conspecific song; the degree of habituation was determined by the carrier frequency and sound intensity of the pulses (Fig. 6). (2) Threshold curves of the 1.6−2.4 2 R neurones showed two peaks of best sensitivity but were 7–14 dB less sensitive than the 1.6−2.4 1 R neurones (Fig. 1). They responded only with an “on”-burst of spikes irrespective of stimulus carrier frequency and stimulus duration; the response magnitude was mainly determined by stimulus intensity (Figs 2B, 3B, 4A and B). Because of fast habituation they responded only to the first pulse train in a sequence with natural repetition rate (Fig. 3F). (3) Neurones of the 2.4 1 R type responded best at frequencies between 2.0 and 2.8 kHz (Fig. 1). Their intensity/response curves were bell shaped (Fig. 2C). Prolonged responses were typical for stimuli shorter than 20 ms (Fig. 3C). Pure tone stimuli of longer duration or high pulse repetition rates, as they occur in the natural songs, inhibited responses in these neurones (Figs 3C, 4C and D, 5). A pulse locked response pattern was well expressed only to the first two pulses in a pulse train (Fig. 3G). Carrier frequency and sound intensity determined the degree of response habituation to the natural repetition rate of pulse trains (Fig. 6). (4) Neurones of the 1.0 1 R type did not exhibit special tuning to the song spectra of Corixa. They responded best to acoustic stimuli with carrier frequencies below 1.6 kHz (Fig. 1). At higher frequencies they were about 20 dB less sensitive than the three other neurones. Pure tone stimuli were copied in a phasic-tonic way (Fig. 3D).

Underwater stridulation by corixids: Stridulatory signals and sound producing mechanism in Corixa dentipes and Corixa punctata

1. (1) The species-specific songs of Corixa dentipes and Corixa punctata males are composed of pulse trains, each of which contains about eleven sound pulses. C. dentipes has a repertoire of four different songs (Song A-D). These songs differ in overall duration, pulse-train repetition rate, and amount of variability in pulse-train rate, pulse rate and sound intensity during the song or individual verces. C. punctata produces only one song. Its sound pattern is considerably more variable, even for an individual animal, than the very precisely performed C. dentipes songs. 2. (2) In the case of C. dentipes, the highest measured peak sound pressure level (SPL) of single pulses was 133 dB re 1 μPa at a distance of 0.1 m from the animal, and the mean of the largest pulses per pulse train over the entire Song A was 115 dB. The song of C. punctata is less loud, with a SPL ca. 10–20 dB lower than that of the C. dentipes songs. The measurements permit estimation of the alternating pressures at the thoracic tympanal organs during acoustic communication. 3. (3) The stridulatory apparatus consists of two specialized structures. The file (pars stridens) is a field of ca. 20 parallel rows of thickened bristles (pegs) on the femur of each foreleg. There are distinct species- and sex-specific differences in the spacing of the rows and the thickness of the pegs. The counterpart of the file, the maxillary plate or scraper (plectrum), is a specially thickened cuticular projection on the side of the head. 4. (4) A single downstroke of the file over the scraper produces a train of acoustic pulses. Each pulse is associated with the contact between one row of pegs in the file and the scraper. Structure and movement of the stridulatory apparatus indicate that at most three adjacent pegs in a row contact the scraper to generate a single pulse. 5. (5) The foreleg movements during various songs can be unilateral, alternating or synchronous. Results indicate that even when the two forelegs move in synchrony the sounds are produced by only one leg, so that in general superposition of stridulatory pulses is avoided. The speed of the downstroke, the repetition rate of consecutive leg movements and the duration of stridulation are temperature-dependent. The Q 10 for the pulse-train rate is between 2.7 and 2.0 for water temperatures between 4°C and 25°C.

Evaluation of electrical parameters for diaphragm pacing: An experimental study

A study of electrical parameters for pacing of the diaphragm in dogs was undertaken to determine which level of current, stimulation waveform, electrode design, impulse frequency, and pulse train repetition rate caused least fatigue of the diaphragm muscle. Stimulation of the phrenic nerve at a current level causing maximal tidal volume resulted in greater fatigue than stimulation at a somewhat lesser current level. A bidirectional waveform was less fatiguing than a unidirectional waveform, but the difference was not striking. Whether the electrode was monopolar or bipolar was not a factor, although recovery from fatigue was slightly faster and more complete after cessation of stimulation with the former. A marked differences in the development of fatigue occurred with the use of different stimulation frequencies. A high-frequency, narrow pulse interval (33 Hz; 30 msec PI) caused rapid and marked fatigue, a higher-frequency, wider pulse interval (25 Hz; 40 msec PI) somewhat less fatigue, while a stimulation frequency similar to the discharge of motor neurons to slow-contracting muscle (11 Hz; 90 msec PI) no fatigue, indeed, with continued stimulation, an improvement in tidal volume. There also appeared to be less fatigue with a pulse train repetition rate (rate of inspiration) of 10 per minute rather than 20. The results of these experiments are of importance in planning an effective pacing program for clinical use. It may be particularly helpful in the designing of stimulators for patients who require artificial ventilatory support but who are unable to be maintained on the available electrical apparatus for clinical application.

A technique for very-high-speed pulse generation with variable repetition rate

A simple technique is proposed for the generation of high-speed variable repetition rate pulse-trains by summing the output pulses from a series at variable delay step recovery diode circuits. The diode switching times determine the maximum repetition rate, which is several gigahertz.

Optical interaction with high-field domain nucleation in GaAs

Control of nucleation of high-field domains in GaAs has been reported for essentially one-dimensional samples. The present work reports an optically triggered bimodal form of domain launch in a 3- dimensional sample. The preliminary experiments describing this effect are given, indicating an optically induced time shift in domain launch without a corresponding change in the repetition rate of a domain-generated pulse train. A useful application of this effect to a circulating digital optical memory is described.