Temporal Intensity Modulation Research Articles

Frequency-mode-stable regenerative amplification at terahertz burst rates

Generation of high-fidelity amplified pulse bursts with a regular interpulse interval yields, in the spectral domain, an equidistant pattern of narrowband spectral modes, similar to frequency combs produced by cw mode-locked lasers but with greatly increased pulse energy. Despite their great potential for nonlinear spectroscopy, material processing, etc., such long frequency-stable bursts are difficult to generate and amplify because of prominent temporal intensity modulation even after strong dispersive pulse stretching. This study presents a burst generation method based on a master-oscillator regenerative-amplifier system that allows for chirped-pulse amplification (CPA) with high scalability in pulse number. A gradual smoothing of temporal intensity profiles at an increasing number of pulses is discovered, demonstrating an unexpected recovery of the CPA performance at terahertz intraburst repetition rates. In consequence, a self-referenced stable burst spectral peak structure with megahertz peak width is generated without risk of amplifier damage caused by interference of chirped pulses. This result eliminates limitations in burst amplification and paves the way for advancements in ultrashort-pulse burst technology, particularly for its use in nonlinear optical applications.

Analysis of pump-to-signal noise transfer in two-stage ultra-broadband optical parametric chirped-pulse amplification.

In optical parametric chirped-pulse amplification (OPCPA), pump temporal intensity modulation is transferred to the chirped-signal spectrum via instantaneous parametric gain and results in contrast degradation of the recompressed signal. We investigate, for the first time to our knowledge, the pump-to-signal noise transfer in a two-stage ultra-broadband OPCPA pumped by a single laser and show the dependence of pump-induced signal noise, characterized both before and after pulse compression, on the difference in pump-seed delay in the two stages. We demonstrate an up-to-15-dB reduction of the pump-induced contrast degradation via pump-seed delay optimization. Experiments and simulations show that, even when parametric amplifiers are operated in saturation, the pump-seed delay can be used to minimize the pump-induced contrast degradation that is attributed largely to the noises from the unsaturated edges of the pulse and that of the beam.

Development of the combined spatial and temporal pulse modulation for the laser impulse metal bonding process

The laser impulse metal bonding (LIMBO) process is a novel laser spot welding process to form a weld joint between a massive interconnector and thin metallization on a thermally sensitive substrate. As for the LIMBO process, the gap between joining partners in overlap configuration allows energetic separation of the melting and joining processes, whereas the underlying joining partner is only thermally stressed during the welding process. The LIMBO process shows, therefore, potential to substitute the ultrasonic wire bonding and the soldering process for joining interconnectors on metallized substrates such as direct bonded copper or PCB. Due to a separation between joining partners, the LIMBO process is divided into three stages that are achieved with temporal pulse modulation. The initial stage prepares a melt pool on an upper joining partner which penetrates through the material. After achieving a local melt pool through the upper joining partner, the temporal intensity modulation of the laser beam is initialized for the second stage. The enhanced laser beam intensity on the melt pool surface vaporizes the melt surface rapidly and induces a recoil metal plume. Hence, the recoil metal plume pressure accelerates the melt toward the underlying joining partner. After the contact, further energy input is required for the last stage to melt the underlying joining partner and subsequently to form a weld. Therefore, the laser beam intensity is once again temporally modulated to lower the intensity value. In this paper, the LIMBO process is extended with a further stage which combines spatial and temporal pulse modulation. From the outgoing position from the last stage of the LIMBO process, the laser beam is spatially modulated to perform a circular movement around the weld joint. Therefore, the spatial pulse modulation allows us to overcome the limitation of the spot welding process. In combination with temporal pulse modulation during the circular movement, the weld joint is expanded continuously. During the extended stage, the accessed heat is also considered, so the thermal threshold of the sensitive substrate is not exceeded. In addition, the overlap pulsed welding process is applied with the extended LIMBO process to achieve a linear weld joint area.

Coherent x rays with tunable time-dependent polarization

We describe a method for producing high power, coherent x-ray pulses from a free electron laser with femtosecond scale periodic temporal modulation of the polarization vector. This approach relies on the generation of a temporal intensity modulation after self seeding either by modulating the seed intensity or the beam current. After generating a coherent temporally modulated $s$-polarization pulse, the electron beam is delayed by half a modulation period and sent into a short orthogonally oriented undulator, serving as a $p$-polarization afterburner. We provide simulations of three configurations for realizing this polarization switching, namely, enhanced self seeding with an intensity modulation generated by 2 color self seeding, enhanced self seeding of a current modulated bunch, and regular self seeding of a current modulated bunch. Start to end simulations for the Linac Coherent Light Source-II are provided for the latter.

Numerical investigation of the dependence of stimulated Brillouin scattering threshold on the pump intensity fluctuation

AbstractIn this paper, the dependence of stimulated Brillouin scattering (SBS) threshold on the sinusoidal modulated properties of pump pulse is studied. A 527-nm-wavelength, 5 ns square laser pulse with sinusoidal temporal modulation is used as the pump source, and a 600 mm liquid heavy fluorocarbon material FC-40 is used as the Brillouin medium. The numerically calculated results indicate that the SBS threshold increases with the increase of both temporal intensity modulation index and modulation frequency of the pump pulse. However, when the intensity distortion criterion is below 30% or the duration of modulation peaks below three times the phonon lifetime, the SBS threshold tends to remain stable. The numerical results provide assistance to judge the SBS threshold for unsmoothed pump pulses, especially for high power laser applications.

Backscatter spectra measurements of the two beams on the same cone on Shenguang-III laser facility.

In laser driven hohlraums, laser beams on the same incident cone may have different beam and plasma conditions, causing beam-to-beam backscatter difference and subsequent azimuthal variations in the x-ray drive on the capsule. To elucidate the large variation of backscatter proportion from beam to beam in some gas-filled hohlraum shots on Shenguang-III, two 28.5° beams have been measured with the Stimulated Raman Scattering (SRS) time-resolved spectra. A bifurcated fiber is used to sample two beams and then coupled to a spectrometer and streak camera combination to reduce the cost. The SRS spectra, characterized by a broad wavelength, were further corrected considering the temporal distortion and intensity modulation caused by components along the light path. This measurement will improve the understanding of the beam propagation inside the hohlraum and related laser plasma instabilities.

Hyperspectral imaging via labeled excitation light and background-free absorption spectroscopy

We introduce a new method of hyperspectral imaging which encodes a varying temporal intensity modulation onto the excitation (or illumination) power spectrum. Functionally, we have moved the spectrometer from the back end of an experiment to the front, where intensity modulations uniquely label wavelengths within the excitation power spectrum at different frequencies, thereby creating a temporal light label which we can identify after subsequent light–matter interactions. To demonstrate this method, we acquire two-dimensional micrographs of background-free absorption spectra by capturing the intensity modulations transferred from the excitation spectrum into the emitted fluorescent intensity. Both the temporal light labeling method and the demonstrated excitation-labeled fluorescence application are readily adaptable to hyperspectral acquisition rates far beyond the frame rates of high-speed cameras.

Low-adiabat rugby hohlraum experiments on the National Ignition Facility: Comparison with high-flux modeling and the potential for gas-wall interpenetration

Rugby-shaped gold hohlraums driven by a nominal low-adiabat laser pulse shape have been tested on the National Ignition Facility. The rugby affords a higher coupling efficiency than a comparably sized cylinder hohlraum or, alternatively, improved drive symmetry and laser beam clearances for a larger hohlraum with similar cylinder wall area and laser energy. A first (large rugby hohlraum) shot at low energy (0.75 MJ) to test laser backscatter resulted in a moderately oblate CH capsule implosion, followed by a high energy shot (1.3 MJ) that gave a highly oblate compressed core according to both time-integrated and –resolved x-ray images. These implosions used low wavelength separation (1.0 Å) between the outer and inner cones to provide an alternative platform free of significant cross-beam energy transfer for simplified hohlraum dynamics. Post-shot 2- and 3-D radiation-hydrodynamic simulations using the high-flux model [M. D. Rosen et al., High Energy Density Phys. 7, 180 (2011)], however, give nearly round implosions for both shots, in striking contrast with observations. An analytic assessment of Rayleigh-Taylor hydrodynamic instability growth on the gold–helium gas-fill interface shows the potential for significant linear growth, saturation and transition to a highly nonlinear state. Candidate seeds for instability growth include laser speckle during the early-time laser picket episode in the presence of only partial temporal beam smoothing (1-D smoothing by spectral dispersion and polarization smoothing) and intensity modulations from quad-to-quad and beam overlap. Radiation-hydrodynamic 2-D simulations adapted to include a dynamic fall-line mix model across the unstable Au-He interface show good agreement with the observed implosion symmetry for both shots using an interface-to-fall-line penetration fraction of 100%. Physically, the potential development of an instability layer in a rugby hohlraum is tantamount to an enhanced wall motion leading to hindered inner-beam propagation, due largely to the confluence of rugby shape and low ray angles relative to the hohlraum symmetry axis. A significant inward pointing shift of 500 μm in the outer cones for the third (full energy) shot of the series was used to improve the inner-beam propagation, resulting in a nearly symmetric x-ray self-emission image of the compressed core and reduced sensitivity to mix. Comparatively low time-dependent symmetry swings were also measured, and a significantly lower hot electron fraction was measured for potentially favorable fuel adiabat control. The outer cone stimulated Brillouin scatter levels jumped significantly, but remedial measures such as the use of a boron dopant in the Au wall are planned. A continuing trend of delayed implosion times is found in rugby hohlraums, suggesting levels of unaccounted hohlraum energy (∼150–200 kJ) similar to what is inferred in cylinder hohlraums. A mix-based physical scenario is described, based on suppressed channel heat flux to the dense gold wall from a temperature-gradient reversal induced by a multispecies plasma lapse rate [P. Amendt, C. Bellei, and S. C. Wilks, Phys. Rev. Lett. 109, 075002 (2012)].

Incoherent-light temporal stretching of high-speed intensity waveforms.

We propose and demonstrate experimentally the first incoherent-light scheme for temporal imaging (magnification) of intensity waveforms. The scheme is based on a time-domain equivalent of a pinhole camera under incoherent illumination, involving two dispersive lines and temporal intensity modulation with a short gate. We report incoherent-light temporal stretching of radiofrequency waveforms by a magnification factor of 2.86, with a time-bandwidth product exceeding 160, i.e., a resolution of ~50 ps over a temporal aperture of ~8 ns, totally avoiding the use of chirp-controlled pulsed lasers. This work opens up new perspectives for realization of many critical high-speed signal-processing modules using practical incoherent light-wave schemes.

Control of temporal mapping and harmonic intensity modulation using two-color orthogonally polarized fields

We demonstrate theoretically control over the time-frequency mapping in two-color high-order harmonic generation. By changing the temporal delay between a driving field and its orthogonally polarized second harmonic we are able to preferentially select between the short- and long-orbit contributions to the harmonic emission along both polarization axes. This trajectory selection is interpreted through use of classical and quantum orbit models. We further observe, and explain, different modulations in intensity of odd and even order harmonic emission modeled via a full macroscopic propagation model coupled to a single-atom quantum orbit treatment.

Inner membrane dynamics in mitochondria

Combining the use of cells with sparse cristae marked with IMP-EGFP and short pulsed sub-saturating fluorescence excitation (non-saturation fluorescence microscopy/NSFM) revealed inhomogeneous fluorescence distribution along mitochondria in living cells. Also the matrix located TMRE was distributed non-uniformly and at least in part filling the gaps between the IMP-EGFP fluorescence: fluorescence intensities are modulated in space and time in part in an antidromic manner. The spatial modulations can be interpreted to represent cristae/matrix distributions. The temporal fluctuations of fluorescence vary within 0.3-3s. Because most peak positions of IMP fluorescence remain stationary up to at least several minutes, temporal intensity modulations may result from varying emissions related to the degree of excitation and/or represent wobbling of cristae, i.e. lateral movements, bending or size changes. Modulations by noise and non-saturated excitation have been reduced by 3 steps of deconvolution followed by averaging 4 images. This allowed a final temporal resolution of 150ms. Disappearance of cristae or formation of new ones takes place within a few seconds, but these are rare events. Thus position of cristae seems to be rather stable, but they regularly disassemble close to fission sites. Treatment with oligomycin strongly reduces "wobbling" activity.

Two-dimensional spatial-frequency-modulated imaging through parallel acquisition of line images

This Letter demonstrates a two-dimensional imaging technique that uses a line scan camera to resolve one spatial dimension and temporal modulation to resolve the perpendicular dimension. A temporal intensity modulation, which increases linearly in frequency along one direction is applied to an illumination beam. The modulated light distribution is imaged onto an object then onto a line scan camera oriented perpendicularly to the direction of the modulation sweep. A line diffuser is placed shortly before the line scan camera and diffuses light along the direction of modulation so that each pixel collects all modulation frequencies.

Flicker-induced eye movements and the behavioural temporal cut-off frequency in a nocturnal spider

We investigated changes in the eye muscle activity in the spider Cupiennius salei as a response to temporal intensity modulations. These spiders are known to enhance eye muscle activity in their principal eyes when moving stimuli are detected in the secondary eyes. We measured the activity of the dorsal eye muscle using a small telemetric unit attached to the spiders' prosoma and confronted the animals to flicker stimuli presented on a cathode ray tube monitor. We registered a significant increase in eye muscle activity as response to temporal light intensity modulations, which implies that no directed motion is required to trigger the spiders' response. This allowed the determination of the behavioural temporal cut-off frequency. None of the frequencies higher than 8.6 cycles s(-1) and all of the frequencies lower than 4.3 cycles s(-1) elicited a significant increase in eye muscle activity. A behavioural cut-off frequency of only a few cycles per second is well in line with the temporal properties of the photoreceptor cells determined using intracellular recordings. A relatively low temporal resolution and a relatively high spatial resolution suit well C. salei's lifestyle as a nocturnal sit-and-wait hunter.

Photonic temporal integration of broadband intensity waveforms over long operation time windows

We propose and experimentally demonstrate a novel design for temporal integration of microwave and optical intensity waveforms with combined high processing speed and a long operation time window. It is based on concatenating in series a discrete-time (low-speed) photonic integrator and a high-speed analog time-limited intensity integrator. This scheme is demonstrated here using a cascaded fiber-based interferometers' system (as a passive eight-point discrete-time integrator) and an analog time-limited intensity integrator. The latter is based on temporal intensity modulation of the input waveform with a rectangular-like incoherent energy spectrum followed by linear dispersion. Using this setup, we experimentally achieve accurate time integration of intensity signals with ~36 GHz bandwidths over an operation time window of ~4 ns, corresponding to a processing time-bandwidth product of >144.

Turn-key module for neutron scattering with sub-micro-eV resolution

We report the development of a compact turn-key module that boosts the resolution in quasielastic neutron scattering by several orders of magnitude down to the low sub-micro-eV range. It is based on a pair of neutron resonance spin flippers that generate a well defined temporal intensity modulation, also known as Modulation of IntEnsity by Zero Effort (MIEZE). The module may be used under versatile conditions, in particular, in applied magnetic fields and for depolarizing and incoherently scattering samples. We demonstrate the power of MIEZE in studies of the helimagnetic order in MnSi under applied magnetic fields.

Amplitude modulation filtering of FM-to-AM conversion due to the focusing grating of LMJ

FM-to-AM conversion is an important issue for high power lasers such as the Laser MegaJoule. Indeed temporal intensity modulation may prevent a good ignition of the target. FM is necessary to achieve optical smoothing: phase modulation broadens the spectrum and the different generated wavelengths will create different speckle patterns thanks to dispersion systems (gratings) that spatially average the focal spot on the target. We show in this paper that dispersion and focusing systems linearly filter intensity modulations by averaging due to time delay introduced by the gratings. For the LMJ configuration this filter is equivalent to a Gaussian low-pass filter with a 7GHz 3dB-bandwidth.

Measurement of nonlinear temporal phase shifts using spectral Foucault technique

Temporal phase shifts induced by cross-phase modulation are directly characterised with a spectral equivalent of the Foucault technique used to spatially resolve wavefronts. The temporal phase induced by a high power pulsed pump on a monochromatic probe is converted to a temporal intensity modulation via spectral filtering. A measurement of the modulated instantaneous power of the filtered signal allows direct determination of the time-resolved nonlinear phase shift. An experimental demonstration is performed on a highly nonlinear fibre.

Measurement of nonlinear temporal phase shifts using spectral Foucault technique

Temporal phase shifts induced by cross-phase modulation are directly characterised with a spectral equivalent of the Foucault technique used to spatially resolve wavefronts. The temporal phase induced by a high power pulsed pump on a monochromatic probe is converted to a temporal intensity modulation via spectral filtering. A measurement of the modulated instantaneous power of the filtered signal allows direct determination of the time-resolved nonlinear phase shift. An experimental demonstration is performed on a highly nonlinear fibre.

Crosstalk of Signals in Pump Probe Structure with Self-phase Modulation and Cross-phase Modulation

The transmission of optical signals under the effects of cross-phase modulation (XPM) and self-phase modulation (SPM) together in pump-probe wave structure is investigated in intensity modulation-direct detection wavelength division multiplexing (IM-DD WDM) systems. A simple expression for frequency domain IM of probe channel is derived. The analytical method to determine the temporal shape of probe wave when arbitrary wave is inputted in pump channel is confirmed. The impact of channel separation and transmission rate to XPM is assessed. We find that the SPM in pump channel increase temporal intensity modulation index reflected XPM to a certain extent. The analytical result agrees well with numerical simulation.

Frequency-chirped readout of spatial-spectral absorption features

This paper examines the physical mechanisms of reading out spatial-spectral absorption features in an inhomogeneously broadened medium using linear frequency-chirped electric fields. A Maxwell-Bloch model using numerical calculation for angled beams with arbitrary phase modulation is used to simulate the chirped field readout process. The simulation results indicate that any spatial-spectral absorption feature can be read out with a chirped field with the appropriate bandwidth, duration, and intensity. Mapping spectral absorption features into temporal intensity modulations depends on the chirp rate of the field. However, when probing a spatial-spectral grating with a chirped field, a beat signal representing the grating period can be created by interfering the emitted photon echo chirped field with a reference chirped field, regardless of the chirp rate. Comparisons are made between collinear and angled readout configurations. Readout signal strength and spurious signal distortions are investigated as functions of the grating strength and the Rabi frequency of the readout pulse. Using a collinear readout geometry, distortions from optical nutation on the transmitted field and higher-order harmonics are observed, both of which are avoided in an angled beam geometry.