Multiframe observation of an intense femtosecond optical pulse propagating in air

4D spatio-temporal electric field characterization of ultrashort light pulses undergoing filamentation

We present an experimental method capable of capturing the complete spatio-temporal dynamics of filamenting ultrashort laser pulses. By employing spatially resolved Fourier transform spectrometry in combination with the capability to terminate the filament at any length, we can follow the nonlinear dynamics in four dimensions, i.e. the transverse domain, time and filament length. Our method thus not only enables the full characterization of the filamentation process throughout its evolution, but also allows to identify and select laser pulses with desired parameters.

Extending recordable time of light-in-flight recording by holography with double reference light pulses

Light-in-flight (LIF) recording by holography is a powerful technique for observing ultrashort light pulse propagation. However, the recordable time of the technique has been limited by the lateral length of the holographic plate. Then, to extend the recordable time of LIF recording by holography, we proposed a space-division multiplexing technique of holograms, which divides the holographic plate longitudinally and uses double reference light pulses. We experimentally demonstrated that the recordable time becomes twice as long as before for the first time, to the best of our knowledge, using the proposed technique. Specifically, we recorded the motion picture of the ultrashort light pulse propagation for 236 ps.

Short-pulse lasers for weather control

Filamentation of ultra-short TW-class lasers recently opened new perspectives in atmospheric research. Laser filaments are self-sustained light structures of 0.1-1 mm in diameter, spanning over hundreds of meters in length, and producing a low density plasma (10¹⁵-10¹⁷ cm^-3) along their path. They stem from the dynamic balance between Kerr self-focusing and defocusing by the self-generated plasma and/or non-linear polarization saturation. While non-linearly propagating in air, these filamentary structures produce a coherent supercontinuum (from 230 nm to 4 µm, for a 800 nm laser wavelength) by self-phase modulation (SPM), which can be used for remote 3D-monitoring of atmospheric components by Lidar (Light Detection and Ranging). However, due to their high intensity (10¹³-10¹⁴ W cm^-2), they also modify the chemical composition of the air via photo-ionization and photo-dissociation of the molecules and aerosols present in the laser path. These unique properties were recently exploited for investigating the capability of modulating some key atmospheric processes, like lightning from thunderclouds, water vapor condensation, fog formation and dissipation, and light scattering (albedo) from high altitude clouds for radiative forcing management. Here we review recent spectacular advances in this context, achieved both in the laboratory and in the field, reveal their underlying mechanisms, and discuss the applicability of using these new non-linear photonic catalysts for real scale weather control.

High-frame-rate observation of single femtosecond laser pulse propagation in fused silica using an echelon and optical polarigraphy technique

We have demonstrated high-frame-rate observations of a single femtosecond laser pulse propagating in transparent medium using the optical polarigraphy technique and an echelon. The echelon produced a spatially encoded time delay for the probe pulse to capture directly four successive images of an intense propagating pulse with picosecond time interval and femtosecond time resolution. Using this method, we observed the propagation process of a single femtosecond laser pulse in fused silica. The influence of pulse-energy fluctuation on the spatial and temporal distribution of the single laser pulse was visualized using the single-shot measurements.

Moving picture recording and observation of three-dimensional image of femtosecond light pulse propagation

We recorded and observed, for the first time, three-dimensional image of femtosecond light pulse propagation as continuous moving picture using light-in-flight recording by holography. We present the moving pictures of collimated and converging light pulses and some images extracted from them. We also discussed inherent feature appearing in the images. Such a discussion is essential to determine the actual shape of the propagating light pulse. This technique provides the means for observation of a temporally and spatially continuous moving picture of light itself and also enables the analysis of various kinds of ultrafast phenomena.

Femtosecond pulse propagation in nitrogen: numerical study of (3 + 1)-dimensional extended nonlinear Schrödinger equation with shock-term correction

We develop an accurate and efficient method for calculating evolution due to the extended nonlinear Schrödinger equation, which describes the propagation behavior of a femtosecond light pulse in a nonlinear medium. Applying Suzuki's exponential operator expansion to the evolution operator based on the finite-differential formulation, we realize the accurate and fast calculation that can be performed without large-scale computing systems even for (3 + 1)-dimensional problems. To study the correspondence between experiments and calculations, we calculate the propagation behavior of a femtosecond light pulse that is weakly focused in nitrogen gas of various pressures and compare the calculation results to the experimental ones. The calculation results reproduce the relative behavior of the spatial light pattern observed during the propagation. Additionally, the multiple-cone formation and interaction between two collimated pulses in nitrogen gas are also demonstrated as applications of the developed method.

Observation of propagating femtosecond light pulse train generated by an integrated array illuminator as a spatially and temporally continuous motion picture

We observed a propagating femtosecond light pulse train generated by an integrated array illuminator as a spatially and temporally continuous motion picture. To observe the light pulse train propagating in air, light-in-flight holography is applied. The integrated array illuminator is an optical device for generating an ultrashort light pulse train from a single ultrashort pulse. The experimentally obtained pulse width and pulse interval were 130 fs and 19.7 ps, respectively. A back-propagating femtosecond light pulse train, which is the -2 order diffracted light pulse from the array illuminator and which is difficult to observe using conventional methods, was observed.

Direct characterization of self-guided femtosecond laser filaments in air

High-power femtosecond laser pulses propagating in air form self-guided filaments that can persist for many meters. Characterizing these filaments has always been challenging owing to their high intensity. An apparently novel diagnostic is used to directly measure the fluence distribution of femtosecond laser pulses after they have formed self-guided optical filaments in air. The diagnostic is unique in that the information contained in the filaments is not lost owing to the interaction with the apparatus. This allows filament characteristics such as energy and size to be unambiguously determined for the first time.