Adaptive Pulse Shaping Research Articles

Adaptive dispersion compensation for remote fiber delivery of near-infrared femtosecond pulses

We report on remote delivery of 25-pJ broadband near-infrared femtosecond light pulses from a Ti:sapphire laser through 150 m of single-mode optical fiber. Pulse distortion caused by dispersion is overcome with precompensation by adaptive pulse shaping techniques, while nonlinearities are mitigated by use of an SF10 glass rod for the final stage of pulse compression. A near-transform-limited pulse duration of 130 fs was measured after the final compression.

Controlling nonlinear processes in microstructured fibers using shaped pulses

It is possible to modify pulse propagation and nonlinear interactions in microstructured fibers using phase-tailored ultrashort laser pulses. We experimentally investigate how pre-shaping of the input laser pulse can be used to alter its evolution and subsequent output characteristics. We also demonstrate how adaptive pulse shaping can be used to control the output properties of the pulse spectrum. Numerical simulations based on the nonlinear Schrodinger equation predict the output spectral profiles of the propagated pulse in good agreement with experimental results, and elucidate the relevant processes producing the optimal output.

Compact, robust, and flexible setup for femtosecond pulse shaping

We present an improved design and adjustment concept for femtosecond pulse shaping. The concept results in a compact and robust pulse shaping setup. A systematic adjustment procedure, high reproducibility and stability, as well as easy adaptability to different femtosecond laser sources are the key features of the presented design. The constructed prototype pulse shaper was tested in an open loop and feedback-controlled adaptive pulse shaping on two different femtosecond laser sources.

Control of the Coulomb explosion of I 2

We present calculated results for the optimization highly-charged fragment ion formation in the Coulomb explosion of I2 in an intense laser field. Calculations are performed using a simple genetic algorithm and a classical model for the Coulomb explosion process. We find that at low intensity the production of highly-charged fragment ions is optimized by a Fourier-limited pulse, whereas at higher intensity the Coulomb explosion is optimized by a sequence of pulses, with a time-separation determined by enhanced ionization at the critical internuclear distance. Our calculations provide insight into the sensitivity of adaptive pulse shaping experiments to the parameters and evolutionary approaches used.

Liquid-phase adaptive femtosecond quantum control: Removing intrinsic intensity dependencies

Femtosecond adaptive pulse shaping of 800-nm laser pulses is applied to control the multiphoton molecular excitation of the charge-transfer coordination complex [Ru(dpb)3](PF6)2 (where dpb=4,4′-diphenyl-2,2′-bipyridine) dissolved in methanol. A phase-only femtosecond pulse shaper provides a mechanism for multiparameter (128) variation of the incident field, and a closed-loop evolutionary algorithm optimizes pulse shapes within the vast search space. Molecular emission at 620 nm is used as experimental feedback which is proportional to the excited-state population in the long-lived MLCT3 (metal-to-ligand charge-transfer) state. The dominant intensity dependence of the multiphoton excitation process is removed by using second-harmonic generation (SHG) in a thin optical crystal as a general “reference” signal. Successful control of the emission/SHG ratio demands that the field adapt to the electronic structure or dynamic needs of the molecule in solution. This suggests that adaptive femtosecond pulse shaping can provide a general means of finding field shapes capable of selectively exciting molecules based on their unique optical properties.

Adaptive pulse shaping of phase and amplitude of an amplified femtosecond pulse laser by direct reference to frequency-resolved optical gating traces

By adaptive control of both spectral phase and amplitude of seed pulses with an acousto-optic programmable dispersion filter located before a pulse stretcher, accurate pulse shaping was achieved for ultrashort laser pulses delivered through a regenerative amplifier. Two-dimensional patterns obtained from frequency-resolved optical gating were used for calculating cost functions in an adaptive control algorithm without pulse shape reconstruction during the control of pulse shape.

Adaptive shaping of two-cycle visible pulses using a flexible mirror

We present a double-pass non-collinear optical parametric amplifier (NOPA) that delivers 4-fs visible–near-infrared pulses. The paper discusses geometrical and temporal properties of the NOPA configuration, which allow us to broaden the gain band of simultaneous parametric amplification in excess of 250 THz. The key elements of the bandwidth enhancement include (i) the use of a broadband second-harmonic pump, (ii) optimization of the incidence angle of all monochromatic spectral components of the pump in order to produce a favorable offset of the corresponding phase-matching curves, and (iii) selection of the chirp rate and time delay between the stretched pump and seed pulses. We next devise a two-stage compressor that incorporates a flexible mirror for adaptive pulse shaping and develop a simple and trustworthy feedback loop based on a one-dimensional spectral measurement. Our rapid numerical algorithm for adaptive control of the flexible membrane is found superior to more complex search routines that are less resistant to the fluctuations of the laser intensity. The automated optimization procedure results in the generation of two-cycle pulses with a carrier wavelength at ∼600 nm. The absence of deep modulation on the parametrically amplified spectrum in combination with the adaptive phase correction lead to a high quality of the temporal profile and allow concentrating 90% of the pulse energy within only a 7.5-fs time window.

Time-resolved organometallic photochemistry: Femtosecond fragmentation and adaptive control of CpFe(CO) 2X (X=Cl, Br, I)

We report a systematic experimental study of the influence of metalligand bonding properties on the ultrafast photofragmentation dynamics and on quantum control of organometallic molecules in the gas phase. In the first part, the femtosecond pump–probe technique is combined with reflectron time-of-flight mass spectrometry to record transient mass spectra of the photodissociation products of CpFe(CO) 2X (Cp=C 5H 5; X=Cl, Br, I). Sequential dissociation steps are observed for the X=Cl species while concerted and more complex mechanisms are seen in the molecules involving the larger halogens. The overall dissociation rate increases from the lighter toward the heavier halogen ligands in this series which can be explained qualitatively by the different metalhalogen bonding properties and their stabilizing effects on unsaturated intermediate species. In the second part, adaptive quantum control experiments are performed on the X=Cl and X=Br molecules. A femtosecond laser pulse shaper provides spectrally phase-modulated electric fields which are then iteratively optimized by a learning evolutionary algorithm. Direct experimental feedback from the photoproduct mass spectra is used in this search for specific optimization goals. A comparison between the two molecular species shows that laser fields optimized for one molecule also lead to an improvement in the species with the substituted halogen ligand, but not to the same degree as when the substituted molecule is directly optimized. Despite their chemical similarities, adaptive pulse shaping is sensitive enough to detect differences in the investigated metal complexes which are due to the electronic metalhalogen bonding properties. These results suggest an application in chemical analysis where specific signatures are desired for the individual compounds in mixtures of chemically similar molecules.

Control of bond-selective photochemistry in CH $\scriptstyle \mathsf {2}$ BrCl using adaptive femtosecond pulse shaping

Adaptive femtosecond laser pulse shaping is employed to achieve bond selective photodissociation/photoionization of CH2ClBr in the gas phase. The photoproduct signal measured in a reflectron time-of-flight mass spectrometer is used as feedback to improve iteratively the spectral phases of the laser pulse via an evolutionary algorithm. We observe an increase of the fission of the stronger versus the weaker carbon halogen bond by 100%. Single parameter control schemes proved unable to achieve bond-selectivity. The complexity of the control problem is addressed by mapping it onto the well-known problem of maximizing second-harmonic generation (SHG). Further spectroscopic results indicate that the control involves manipulation of wave-packet dynamics on the neutral surfaces.

Adaptive Pulse Shaping of Femtosecond Laser Pulses in Amplitude and Phase through a Single-Mode Fiber by Referring to Frequency-Resolved Optical Gating Patterns

We numerically demonstrate that adaptive pulse shaping can deliver ultrashort pulses arbitrarily shaped in both amplitude and phase through a single-mode optical fiber. For quick evaluation of the amplitude and phase of shaped pulses during iterative optimization, two-dimensional patterns obtained from frequency-resolved optical gating (FROG) measurement are used without pulse reconstruction. By controlling only the spectral phase of the input pulse before the fiber propagation to compensate the group delay dispersion and self-phase modulation, various desired pulse shapes can be generated at the fiber output. However, this scheme is limited to use with moderate peak powers.

Adaptive control methods for ultrafast pulse propagation in optical fibers

Adaptive control in combination with ultrafast pulse shaping provides a compelling approach to harness events that occur on the fastest timescale available. This paper illustrates the application of adaptive pulse shaping to femtosecond pulse propagation at /spl lambda/=1550 nm in single-mode optical fibers. The approach is illustrated first by through a numerical simulation of the technique. An experimental demonstration is described. The propagation of /spl sim/200-fs pulses is successfully achieved in the nonlinear regime by suitably preshaping the input pulse using an adaptive feedback loop.

Photoselective adaptive femtosecond quantum control in the liquid phase.

Coherent light sources can be used to manipulate the outcome of light-matter interactions by exploiting interference phenomena in the time and frequency domain. A powerful tool in this emerging field of 'quantum control' is the adaptive shaping of femtosecond laser pulses, resulting, for instance, in selective molecular excitation. The basis of this method is that the quantum system under investigation itself guides an automated search, via iteration loops, for coherent light fields best suited for achieving a control task designed by the experimenter. The method is therefore ideal for the control of complex experiments. To date, all demonstrations of this technique on molecular systems have focused on controlling the outcome of photo-induced reactions in identical molecules, and little attention has been paid to selectively controlling mixtures of different molecules. Here we report simultaneous but selective multi-photon excitation of two distinct electronically and structurally complex dye molecules in solution. Despite the failure of single parameter variations (wavelength, intensity, or linear chirp control), adaptive femtosecond pulse shaping can reveal complex laser fields to achieve chemically selective molecular excitation. Furthermore, our results prove that phase coherences of the solute molecule persist for more than 100 fs in the solvent environment.

Adaptive control of femtosecond pulse propagation in optical fibers

Nonlinear effects present fundamental obstacles to the propagation of femtosecond pulses of detectable energy in single-mode optical fibers, inducing severe distortion even after a very short (a few meters) propagation distance. We show here that adaptive pulse shaping can overcome these limitations by synthesizing pulses that are self-correcting for higher-order nonlinear effects when they are launched in the fiber. This approach would not only affect optical communications but also yield benefits in various disciplines requiring optimized fiber-based femtosecond pulse delivery, for example, nonlinear imaging techniques such as multiphoton microscopy, material processing, and medical diagnostics.

Optimization of adaptive feedback control for ultrafast semiconductor spectroscopy

We present an experimental study of the control of ultrafast semiconductor nonlinearities by adaptive feedback optical pulse shaping. In the feedback loop, an evolutionary algorithm directs the modulation of the spectral phase of 20-fs laser pulses. In this way, control is achieved over the broadband semiconductor continuum nonlinearity as measured in differential transmission experiments. Design guidelines are given for the implementation of the evolutionary algorithm. Our results demonstrate that a feedback loop with a carefully designed algorithm can serve as a new, sensitive tool in ultrafast semiconductor spectroscopy. Moreover, an optimized feedback loop allows for the substantial enhancement of ultrafast semiconductor nonlinearities.

Problem complexity in femtosecond quantum control

Adaptive femtosecond laser pulse shaping is employed to achieve active quantum control in selected gas phase photodissociation and photoionization processes of CpFe(CO) 2Cl. An evolutionary optimization algorithm uses experimental feedback from the photofragment mass spectra in a learning loop setup to improve the applied spectral laser pulse phases iteratively. The significance of the resulting complex temporal electric-field profiles is investigated by artificial reduction of the optimization parameter space. The degree of problem complexity is further studied by correlation measurements of the molecular control objective versus second-harmonic generation (SHG). In the case of direct ionization, the previously unknown molecular control problem can be mapped one-to-one onto the search space topology of SHG, whereas in the case of photoproduct ratio optimization, no clear correlation is observed. The control mechanism in the latter example is therefore much more complex and cannot be explained by a simple laser pulse intensity variation. Correlation techniques in a general sense are considered to be very helpful in solving the so-called problem of inversion, i.e., obtaining information about the underlying photophysical and photochemical processes.

Adaptive feedback control of ultrafast semiconductor nonlinearities

We experimentally demonstrate that adaptive feedback optical pulse shaping can be used to control ultrafast semiconductor nonlinearities. The control scheme is based on an evolutionary algorithm, which directs the modulation of the spectral phase of 20 fs laser pulses. The algorithm has optimized the broadband semiconductor continuum nonlinearity measured in differential transmission experiments. Our results show that insight into light–semiconductor interaction is obtained from the optimum laser pulse shape even if the interaction is too complex to predict this shape a priori. Moreover, we demonstrate that adaptive feedback control can substantially enhance ultrafast semiconductor nonlinearities by almost a factor 4.

Adaptive real-time femtosecond pulse shaping

Experimental results of a practical self-learning pulse-shaping system are presented. Real-time adaptive pulse shaping of uncharacterized pulses is achieved. A cross-correlation feedback measurement of the output pulses is used by a simulated-annealing algorithm to modify the pulses iteratively toward target shapes. This scheme can readily be used for coherent control of quantum dynamics.

Adaptive ultrashort pulse compression and shaping

A practical adaptive approach for ultrashort optical pulse compression and shaping is nummerically investigated. A simple single valued feedback measurement was suggested and proven useful for compression. The method is robust, and capable of handling the general case of pulse compression and shaping, where the input pulses are completely uncharacterized or partially characterized.