Dynamics Of Pulse Formation Research Articles

Flow-driven control of pulse width in excitable media.

Models of pulse formation in nerve conduction have provided manifold insight not only into neuronal dynamics but also the nonlinear dynamics of pulse formation in general. Recent observation of neuronal electrochemical pulses also driving mechanical deformation of the tubular neuronal wall, and thereby generating ensuing cytoplasmic flow, now question the impact of flow on the electrochemical dynamics of pulse formation. Here, we theoretically investigate the classical Fitzhugh-Nagumo model, now accounting for advective coupling between the pulse propagator typically describing membrane potential and triggering mechanical deformations, and thus governing flow magnitude, and the pulse controller, a chemical species advected with the ensuing fluid flow. Employing analytical calculations and numerical simulations, we find that advective coupling allows for a linear control of pulse width while leaving pulse velocity unchanged. We therefore uncover an independent control of pulse width by fluid flow coupling.

Research on collinear double pulse generation method of electro-optical Q-switched Nd:YAG laser

This letter introduces the collinear double-pulse generation method of the electro-optical Q-switched Nd:YAG laser and the rate equations describing its pulse formation dynamics. The two pulses share the same resonator and thus have a natural and highly coaxial characteristic. The effect of the voltage applied on the electro-optic crystal on the pulse energy and temporal waveform was studied. The time interval of the double-pulse laser can be tuned from 80 ns to 10 μs, thus satisfying the needs of a general double-pulse laser. This method is ideal for obtaining a collinear double-pulse laser and can be extended to other lasers.

Switchable property of dissipative soliton and dispersion-management soliton in fiber laser

We numerically and experimentally demonstrated that the central wavelength of the output pulses in an Er-doped mode-locked fiber laser can be switched from ∼1561 to ∼1531 nm, while these two pulses exhibit dispersion-management soliton and dissipative soliton, respectively. The switch capability of our fiber laser with different pulse waveforms can be attributed to combining the double-hump gain spectrum of Er-doped fiber and third-order dispersion (TOD) effect. By considering the TOD effect, the two wavelengths can be operated in different dispersion regime. The transient switch dynamics of these different type of solitons are also observed by time stretched technique. These findings provide a good foundation for comprehensively studying pulse formation dynamics in laser cavities, and benefit for design dual-wavelength fiber lasers.

Chirped-pulse generation from optical parametric oscillators with an aperiodic quasi-phase-matching crystal.

We describe chirped-pulse generation from an optical parametric oscillator using an aperiodic quasi-phase-matching crystal as the gain medium. We examine the pulse formation dynamics, and reveal that the excitation of chirped-pulse oscillation is assisted by the position-dependent conversion-wavelength and the pump-to-signal temporal walk-off along the gain medium. Stable oscillation is confirmed by numerical simulations and experimental demonstrations. In a preliminary experiment, we obtained a signal wave covering 1293-1734 nm and an idler wave covering 2.9-5.0 µm from an oscillator based on a 2-mm-long aperiodically poled ${{\rm LiNbO}_3}$LiNbO3 crystal. These results represent a new, to the best of our knowledge, pulse formation mechanism inside a nonlinear oscillator, which is promising for generating light pulses with very wide instantaneous bandwidth and at arbitrary wavelengths.

Simultaneous emission of Gaussian-like and parabolic-like pulse waveforms in an erbium-doped dual-wavelength fiber laser

We report on the observation of different pulse formation dynamics in a nonlinear polarization evolution (NPE)-based broadband erbium-doped fiber laser when the net cavity group-velocity dispersion (GVD) is managed to be close to zero. The fiber laser can generate pulses with a single wavelength or dual wavelengths by adjusting the waveplates. When the laser operates in dual-wavelength emission, the output pulses corresponding to the two wavelengths exhibit Gaussian- and parabolic-like waveforms, respectively, indicating that the laser can simultaneously operate in stretched-pulse and self-similar regimes. The generation of dual-wavelength emission with different pulse waveforms can be attributed to an overdriven NPE switch acting on a chirped broadband pulse and different dispersion mechanisms. These findings provide a good foundation for comprehensively studying pulse formation dynamics in laser cavities.

RF pulse formation dynamics in gyromagnetic nonlinear transmission lines

We present the results of rf oscillations growth dynamics as the high voltage pulse propagates through the gyromagnetic nonlinear transmission line (NLTL). Several equidistant electrical probes were placed inside the NLTL in order to examine the oscillatory wave formation and the role of higher order modes in this process. RF pulses are generated at 2 GHz frequency with nonaveraged peak power of 100 MW. Azimuthal asymmetry of the wave propagating through NLTL is detected.

Short pulse generation and mode control of broadband terahertz quantum cascade lasers

We report on a waveguide engineering technique that enables the generation of a bandwidth up to 1 THz and record ultra-short pulse length of 2.5 ps in injection seeded terahertz quantum cascade lasers. The reported technique is able to control and fully suppress higher order lateral modes in broadband terahertz quantum cascade lasers by introducing side-absorbers to metal-metal waveguides. The side-absorbers consist of a top metalization set-back with respect to the laser ridge and an additional lossy metal layer. In continuous wave operation the side-absorbers lead to octave spanning laser emission, ranging from 1.63 to 3.37 THz, exhibiting a 725 GHz wide at top within a 10 dB intensity range as well as frequency comb operation with a bandwidth of 442 GHz. Numerical and experimental studies have been performed to optimize the impact of the side-absorbers on the emission properties and to determine the required increase of waveguide losses. Furthermore, these studies have led to a better understanding of the pulse formation dynamics of injection-seeded quantum cascade lasers.

Theory of passive mode-locking of semiconductor disk lasers in the blue spectral range by metal nanocomposites

We theoretically study femtosecond pulse generation by passive mode-locking of semiconductor disk lasers operating in the blue spectral range using metal nanocomposites as slow saturable absorbers. By using the relation for the nonlinear dielectric response of a layer of silica glass doped with spherical silver nanoparticles and the master equation for mode-locking, we investigate the dynamics of pulse formation and the achievable pulse parameters and predict the generation of pulses as short as 50 fs at 420 nm in such lasers.

Dynamics of pulse formation in mode-locked semiconductor disk lasers

We report on optically pumped semiconductor disk lasers passively mode locked with a semiconductor saturable absorber mirror. The effect of second- and third-order dispersion and dynamic gain saturation on the pulse formation is studied both experimentally and numerically. Improvement of the pulse quality and duration in the disk laser with resonant periodic gain structure requires accurate control of dispersion by wavelength detuning.

Calculation of the Spatio-Temporal Dynamics of Q-switchedYb3+:YAG Laser with an Unstable Cavity and a Super-Gaussian Mirror

A numerical simulation used to compute the spatio-temporal dynamics ofpulse formation of diode-pumped Q-switched Yb:YAG laser is carried out.The model takes the laser amplification and gain saturation, theproperties of the laser cavity, and the diffractive effects of the laserdisc into account. The numerical calculation is performed for a confocalpositive-branch unstable resonator with a super Gaussian couplingmirror. The simulation results show that the laser pulse starts from aGaussian intensity distribution and becomes rapidly non-Gaussian. Thecorresponding beam quality M2 factor is seen to vary approximatelyfrom 1.5 at the beginning of the formation of pulse to more than 10 inthe tail of the pulse, with a value of 11.6 at the peak of the pulse.

Sequence effect of optical elements in a femtosecond Ti:sapphire laser oscillator

We have numerically and experimentally verified that the sequence of optical elements in a femtosecond Ti:sapphire laser affects the laser’s characteristics. Even with the same net dispersion in a cavity, the output spectrum showed a significant change in width and shape when the sequence of dispersive elements was changed in a Kerr-lens mode-locked Ti:sapphire laser with fused-silica prisms for dispersion compensation. It was also shown that the output coupler’s position can affect the dynamics of pulse formation by the combined action of output coupling and self-phase modulation. We explain such phenomena based on pulse formation in a laser cavity with discrete distribution of dispersion and self-phase modulation.

Pulse Dynamics in an Actively Mode-Locked Laser

We consider pulse formation dynamics in an actively mode-locked laser. We show that an amplitude-modulated laser is subject to large transient growth and we demonstrate that at threshold the transient growth is precisely the Petermann excess noise factor for a laser governed by a nonnormal operator. We also demonstrate an exact reduction from the governing PDEs to a low-dimensional system of ODEs for the parameters of an evolving pulse. A linearized version of these equations allows us to find analytical expressions for the transient growth below threshold. We also show that the nonlinear system collapses onto an appropriate fixed point, and thus in the absence of noise the ground-mode laser pulse is stable. We demonstrate numerically that, in the presence of a continuous noise source, however, the laser destabilizes and pulses are repeatedly created and annihilated.

Double resonance at inhomogeneously broadened transitions in common-upper-level scheme during the formation of self-induced transparency pulses in the pump channel

The transient double resonance excited in a common-upper-level scheme is studied theoretically in a medium whose spectral inhomogeneity is caused by the Doppler effect. The formation of the 0π, 2π- and 4π-pulses in the pump channel is considered. Studies of the pulse formation dynamics have revealed, in particular, that the exponential gain of a signal pulse is possible only if the pump channel contains at least one 2π-pulse. If there are more than one 2π-pulses in the pump channel, each pump pulse generates its own signal pulse whose characteristics are determined by the parameters of the pump pulse.

Stabilization of terahertz beats from a pair of picosecond dye lasers by coherent photon seeding

Well-defined beats of terahertz frequencies have been generated by two synchronously-pumped mode-locked dye lasers stabilized by coherent photon seeding. External cavities ended by phase-conjugate mirrors were used to provide the necessary feedback. The two lasers were operated at different tunable wavelengths, and the beat frequency was scanned from 0.5 to 18 THz. In the absence of coherent photon seeding, the dynamics of pulse formation in each dye laser was subject to spontaneous emission noise, leading to a severe degradation of the beat signal.

Emission buildup in Q-switched fiber lasers

Amplified spontaneous emission is experimentally found to play an important role in the dynamics of pulse formation in Q-switched fiber lasers and leads to stepwise (or quantized) behavior of the emission buildup. This effect could be included in detailed modeling of pulsed fiber lasers and might be interesting in some applications.

Additive pulse mode-locked Nd: YAG laser

Additive pulse mode locking applied to lamppumped Nd: YAG lasers results in an attractive source of picosecond pulses at 1.06 μm or 1.32 μm with average powers at the Watt level. We provide detailed information on construction and operation and give data on performance. A modified active stabilization scheme allows not only improved stability of operation but also insight into the dynamics of pulse formation.

Pulse-forming dynamics of a cw passively mode-locked Ti:sapphire/DDI laser

The dynamics of pulse formation in a cw passively mode-locked Ti:sapphire/DDI laser has been investigated. It is found that the time required for the circulating energy in the cavity to build up and bleach the dye absorber is approximately 140 μs. A further 230 μs is required for the buildup of steady-state picosecond pulses. Concurrently the laser output spectrum initially narrows in the cw regime, begins to broaden as the laser becomes modulated, and finally establishes the steady-state spectral distribution.