Output Pulse Profile Research Articles

Pulse Measurement from a Polluted Frequency Resolved Optical Gating Trace Based on Half-Trace Retrieval Algorithm

A half-trace retrieval algorithm based on an extended-ptychographical iterative engine algorithm is proposed to reconstruct the temporal structure of pulse from a polluted and recorded frequency-resolved optical gating (FROG) trace that was modulated by poor spatial profile of output pulses, stray light, or misalignment of the measurement setup. In the proposed algorithm, the probe pulse and the gated pulse were retrieved simultaneously from a recorded FROG trace with a half-delay range, and the measured pulse was obtained by combining the different edges of the probe pulse and the gated pulse. Numerical simulations were carried out to verify the feasibility of the proposed algorithm. A single-shot picoseconds (ps)−THG−FROG setup with a 100-μJ ps laser system and an online ps−SHG−FROG setup in PW laser system were built to test the proposed algorithm experimentally. The results show that the temporal structure of pulses retrieved by the half-trace retrieval algorithm is closer to the real temporal structure than that retrieved by the conventional ptychographical algorithm when the recorded FROG trace is badly polluted.

Boosting strong-coupling stimulated Brillouin amplification performance using an external magnetic field

This article presents a method for improving the amplification performance of strong-coupling stimulated Brillouin scattering by adding an external static magnetic field longitudinal to the laser propagation direction. Simulation results show that when the laser intensity is low, the output seed intensity can be increased by applying an external magnetic field and a right-hand circularly polarized laser pulse. When the seed intensity is higher, the amplification enters the regime of relativistic electron nonlinearity, the amplification oscillates, and the portion of the seed that is amplified moves from the first peak to the second and then the third. Under these circumstances, a reversed magnetic field helps to obtain a higher output intensity and a smoother output pulse profile. Adjusting the magnetic field dynamically, such as by using a double-pass scheme, combines these two advantages and maximizes the intensity enlarging and profile smoothing of the output pulse.

Studies of temporal characteristics of all-solid-state, gain-switched Cr:forsterite laser

A four-level rate-equation based model is presented to study the temporal dynamics of gain-switched Cr:forsterite laser. This model incorporates the effect of the excited-state absorption (ESA) present at the pump as well as the emitted wavelength from the forsterite laser. The value of the ESA cross-section at the pump wavelength (1064 nm) was experimentally obtained and utilized in the theoretical studies. The evolution of the upper laser level population and laser pulse inside the resonator cavity was numerically obtained by solving the rate-equations of the cavity photon density and population density. The Cr:forsterite laser pulse shows spiking behaviour as a consequence of the relaxation oscillation occurs during the buildup of the laser pulse inside the resonator cavity. The effect of several parameters related to pump pulse and cavity components on the output pulse parameters and the cavity buildup time were studied. The spiking behaviour in the output pulse profile of the Cr:forsterite laser was experimentally observed. The Cr:forsterite laser was pumped by the fundamental wavelength of Nd:YAG laser operated at 20 Hz pulse repletion rate in the confocal resonator cavity. The experimental results of pulse parameters were compared with the computed results and presented here.

Generalized temporal synthesis method for a birefringent laser pulse shaper.

We propose a new method for the synthesis of arbitrary temporal output pulse profiles by using a birefringent laser pulse shaper. This shaper contains a series of N birefringent crystals placed between input and output polarizers. The synthesis method starts with the linear time-invariant system theory to determine the optimal amplitudes of the replica pulses at the output polarizer and the time delay between them, and then uses the temporal Jones matrix formalism to obtain the angle of each crystal. The design procedure is demonstrated for an eight-stage birefringent laser pulse shaper. Several examples of predefined output pulse profiles are given to show the potential of the proposed birefringent laser pulse shaper.

Characterization of an optimized light source and comparison to pulsed dye laser for superficial and deep vessel clearance

An arc lamp-based device providing optimized spectrum and pulse shape was characterized and compared with two pulsed dye laser (PDL) systems using a vascular phantom. Safety and effectiveness for facial telangiectasia are presented in clinical case studies. An optimized pulsed light source's (OPL) spectral and power output were characterized and compared with two 595 nm PDL devices. Purpuric threshold fluences were determined for the OPL and PDLs on Fitzpatrick type II normal skin. A vascular phantom comprising blood-filled quartz capillaries beneath porcine skin was treated by the devices at their respective purpuric threshold fluences for 3 ms pulse widths, while vessel temperatures were monitored with an infrared (IR) camera. Patients with Fitzpatrick skin types II-III received a split-face treatment with the OPL and a 595 nm PDL. The OPL provided a dual-band output spectrum from 500 to 670 nm and 850-1,200 nm, pulse widths from 3 to 100 ms, and fluences to 80 J/cm(2). The smooth output power measured during all pulse widths provides unambiguous vessel size selectivity. Percent energy in the near infra-red increased with decreasing output power from 45% to 60% and contributed 15-26% to heating of deep vessels, respectively. At purpuric threshold fluences the ratio of OPL to PDL vessel temperature rise was 1.7-2.8. OPL treatments of facial telangiectasia were well-tolerated by patients demonstrating significant improvements comparable to PDL with no downtime. Intense pulsed light (IPL) and PDL output pulse and spectral profiles are important for selective treatment of vessels in vascular lesions. The OPL's margin between purpuric threshold fluence and treatment fluence for deeper, larger vessels was greater than the corresponding margin with PDLs. The results warrant further comparison studies with IPLs and other PDLs.

Conditional sign flip of two photons without pulse distortion

We theoretically demonstrate a deterministic conditional sign flip of a two-photon pulse that preserves the pulse profile. The nonlinear optical system used is a $V$-type three-level atom confined in a cavity, and the input photons are down-converted twin photons having temporal entanglement. The input and the output pulse profiles are nearly perfectly preserved when the reciprocal bandwidth of the input photons is tuned to the atomic linewidth.

Multimode Theory of Up-Conversion of Two Photons

The up-conversion dynamics of two photons in a three-level system is investigated theoretically. The analysis is based on a rigorous formalism that incorporates the multimode nature of the photon field, enabling quantitative handling of the spatial profiles of the photon pulses. We have derived an analytic formula that connects the wavefunction of two input photons to that of an up-converted output photon. The up-conversion efficiency and the output pulse profile are numerically calculated using this formula.

Sub and superluminal propagation through stratified media

Pulse propagation through 1-dimensional stratified media is studied. The general features of pulse propagation in relation to the delay or advancement of the pulse are discussed. It is shown that in a lossless system with spatial symmetry the reflected and the transmitted pulses have the same delay or advancement. The phase times and the explicit output pulse profiles for an input Gaussian pulse are calculated. While recalling recent results for a variety of systems (resonant absorbers or heterogeneous medium in a Fabry-Perot cavity, a slab of negative index medium, a left-handed/right-handed periodic structure, etc.), new results involving coupled waveguides and a high finesse cavity containing an electromagnetically induced transparency (EIT) medium are presented. Calculations clearly show that for pulses with a carrier tuned at the resonance of the stratified media, one generally has subluminal propagation. Alternatively, in stop gaps or away from the resonances one ends up with superluminal transit. In the case of the EIT cavity it is shown that the large dispersion offered by the intracavity medium can lead to substantial superluminality (ng≃−106) with peak transmission as high as 20%. This is in sharp contrast to large signal attenuation for usual superluminal transit. Note also that EIT, to the best of the authors knowledge, has so far been used to achieve only slow light.

Pulse Compression Using Direct Feedback of the Spectral Phase From Photonic Crystal Fiber Output Without the Need for the Taylor Expansion Method

Characterization and compensation of the complex spectral phase and the temporal profile of output pulses from a photonic crystal fiber (620-945-nm spectral broadening) were performed using a computer-controlled feedback system that combines a modified spectral-phase interferometry for direct electric-field reconstruction apparatus and only a 4-f chirp compensator having a spatial light modulator. These pulses were adaptively compressed from 12-fs input pulses to 6.8-fs. In addition, the compressed pulse profile showed excellent agreement with results measured independently with fringe-resolved autocorrelation.

Microwave envelope solitons in YIG film under oblique magnetisation

A detailed study of the output pulse profiles and envelope soliton characteristics are studied in a YIG film under oblique magnetisation for different input pulse widths, operating frequencies and ferrimagnetic resonance line widths. Dispersion characteristics and nonlinear characteristics of the YIG film are predicted by the first order multiple scale method. The analysis of the soliton in the YIG film is carried out using a split step Fourier method for arbitrary orientation of magnetisation. The conditions for the bright and dark soliton are established using the Lighthill criterion. A switchable bright to dark soliton is theoretically predicted by varying the angle of DC magnetisation. Pulse compression is also estimated for different magnetisation angles.

Record low-power all-optical semiconductor switch operation at ultrafast repetition rates above the carrier cutoff frequency

The switch repetition-rate limit of all-optical semiconductor switches, for which the dynamic extinction ratio decreases with increasing repetition rate, is examined and overcome. The extinction ratio is improved by optimization of the interferometer phase bias, which has previously been set to pi. The extinction-ratio increase accompanies a drastic change in the output pulse spectrum. Both the output pulse profiles and the spectra, before and after optimization, are successfully reproduced by simulation. As a result, switch repetition of 42 GHz (2.5 times higher than the semiconductor-carrier cutoff frequency) is achieved by use of 5-ps, 1548-nm pump pulses. What are believed to be record low input pump-pulse energies of 750 aJ (peak power, 140microW) at 10 GHz and 6 fJ (1.1 mW) at 42 GHz are used.

Investigation of distortions of the laser pulse profile in multipass amplifiers with a stimulated-Brillouin-scattering mirror

A simple qualitative model of beams focused stimulated Brillouin scattering (SBS), capable of providing a sufficiently accurate description of the Stokes pulse profile, was developed. The characteristics of deformation of a laser pulse during amplification in two-pass and four-pass amplifiers with an SBS mirror were investigated theoretically and experimentally for fully overlapping pulses. The influence of pumping and spontaneous emission was ignored. Approximate formulas were derived for determination of the duration of an output pulse and of its leading edge. The parameters required for the most effective control of the output pulse profile were determined.

Decay properties of microwave-magnetic-envelope solitons in yttrium iron garnet films

Microwave magnetic envelope (MME) wave-packet propagation in a nominal 7.2-\ensuremath{\mu}m-thick yttrium iron garnet film has been investigated to determine the decay properties of linear and nonlinear MME pulses. The data were obtained in the magnetostatic backward volume wave configuration with an in-plane static field of 1088 Oe and an operating frequency of 5 GHz. Output pulse profiles, peak powers, and integrated pulse energies were measured for 13 ns wide input pulses and propagation distances from 3 to 10 mm. The pulse energy decay rate \ensuremath{\mathrm{B}} is found to be 10.6\ifmmode\times\else\texttimes\fi{}${10}^{6}$nrad/s and independent of the input power level up to 400 mW, even though the nonlinear response begins at 80 mW. This \ensuremath{\mathrm{B}} value is twice the relaxation rate \ensuremath{\eta} from ferromagnetic resonance. In the linear regime below 80 mW, the amplitude decay rate \ensuremath{\alpha} of the dynamic microwave magnetization peak amplitude is nearly constant at a value ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{low}}$\ensuremath{\approx}7.8\ifmmode\times\else\texttimes\fi{}${10}^{6}$nrad/s, which is somewhat greater than \ensuremath{\mathrm{B}}/2 and significantly less than \ensuremath{\mathrm{B}}. This ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{low}}$ is greater than the decay rate due to damping, \ensuremath{\eta}=\ensuremath{\mathrm{B}}/2, because of dispersion. With the onset of the nonlinear soliton response above 80 mW, \ensuremath{\alpha} gradually increases and saturates for input powers greater than 200 mW at a value ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{high}}$ which is equal to the energy decay rate \ensuremath{\mathrm{B}}. This result indicates that the amplitude decay rate for MME solitons is very close to twice the relaxation rate. This result is predicted in the limit of a vanishingly small damping. Experimentally, it appears to be valid even when the relaxation is significant. The transition region from ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{low}}$ to ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{high}}$ has been quantitatively modeled through the nonlinear Schr\"odinger equation, and demonstrates an explicit change in the critical propagation length for soliton formation from 8 mm at the low power end of the transition to 3 mm at the high power end.

Microwave-envelope soliton threshold powers and soliton numbers.

The peak power of 5.8-GHz backward-volume-wave pulses propagated in a 7.2-\ensuremath{\mu}m-thick yttrium iron garnet film was measured as a function of input pulse width ${\mathit{T}}_{0}$ from 5.1 to 44 ns and input power from 0.05 to 3.5 W in a two-stripline microwave transducer structure. A threshold input power ${\mathit{P}}_{\mathrm{th}}$ for the onset of a nonlinear response was determined as a function of ${\mathit{T}}_{0}$. The dependence of ${\mathit{P}}_{\mathrm{th}}$ on ${\mathit{T}}_{0}$ followed a clear pattern consistent with theoretical considerations for soliton formation. Plots of ${\mathit{P}}_{\mathrm{th}}$ vs 1/${\mathit{T}}_{0}^{2}$ showed two regions where ${\mathit{P}}_{\mathrm{th}}$ scaled linearly with respect to 1/${\mathit{T}}_{0}^{2}$. The slopes of these two linear regions were in ratio 1:9, as expected from theory. A soliton-number parameter n calculated from the ${\mathit{P}}_{\mathrm{th}}$ data as a function of ${\mathit{T}}_{0}$ showed plateaus at n=1, 2, and 3. A one-to-one correlation was found between these calculated values of n and the number of peaks in output pulse profiles. These results demonstrate a simple connection between input pulse width, nonlinear power threshold, and soliton number.

Self-induced changes in the polarization of the output radiation in an elliptic single-mode fiber waveguide

An experimental investigation was made of a self-induced change of the polarization in an elliptic single-mode fiber waveguide due to a change in the refractive index in a strong field of a laser pulse. The experimental dependence of the energy of an output pulse on the energy coupled into the investigated waveguide was determined. The threshold power for a self-induced change in the polarization was obtained. The profile of the output pulse changed compared with that of the input pulse. A time asymmetry of the output pulse profile was detected.

Optical pulse shaping by filters based on spectral hole burning

A possibility of shaping the temporal profiles of picosecond light pulses by filtration through photochemically-burned spectral holes is demonstrated experimentally. The output pulse profiles calculated via the Fourier and Hilbert transformations from transparency spectra coincide with the ones measured directly.

Dynamics of population of the A3∑u+nitrogen metastable state in a self-sustained volume discharge of a pulsed CO2laser

Intracavity laser spectroscopy was used to study the dynamics of population of the ν = 2–8 vibrational levels of the A3∑u+ state in order to establish the possible influence of multistage ionization on the evolution of instability in a self-sustained volume discharge in CO2 laser active mixtures. The populations of the nitrogen vibrational levels Nν were calculated taking into account the real output pulse profile of a dye laser. It was found that multistage ionization can only influence the duration of stable operation of a self-sustained volume discharge by increasing the rate of growth of the spark channel in the discharge gap. This is why the addition of readily ionized substances to the gas that reduce the electron energy and therefore lower Nν can substantially improve the stability of the volume discharge and increase the active volume and output energy of a CO2 laser.

Spectral-temporal effects in resonant harmonic generation in gases

A theoretical investigation is made of unsteady quasiresonant optical third-harmonic generation in a gaseous nonlinear medium pumped by amplitude-modulated pulses of constant collimated radiation. It is shown that two effective unsteadiness mechanisms exist: wave unsteadiness, associated with the difference between the propagation velocities of the pump and harmonic waves, and local unsteadiness caused by the finite relaxation time of the appropriate nonlinear component of the polarization of the medium. In spectral terms these correspond to the resonant dispersion of the phase matching and the dispersion of the nonlinear polarizability of the medium, respectively. The output pulse profile is determined and the influence of the main parameters on its profile is analyzed. The stimulated emission spectrum is also studied for conditions when the pump field is a steady-state random Gaussian process.