Recently, a research team from the State Key Laboratory of Intense Field Laser Physics, Shanghai Institute of Optics and Precision Machinery (SIPM), Chinese Academy of Sciences (CAS), has proposed a method to significantly improve the pointing stability of high-frequency femtosecond filament and supercontinuum (SC) white light source, and effectively suppressed the intensity and beam pointing jitter in air filament-induced supercontinuum white light laser generation, in order to address the bottleneck of the high-frequency femtosecond laser gas filamentation. The results are published in the paper entitled "Stable, intense supercontinuum light generation at 1kHz by electric field assisted femtosecond laser filamentation in air". "The results were published in Light: Science & Applications.
Supercontinuum lasers covering the visible wavelength band are called "white light lasers" because of their ultra-wide spectral width and good coherence. It is widely used in biomedical imaging, molecular fingerprint spectroscopy, optical coherence tomography, tunable ultrafast pulse generation, femtosecond pulse compression, and other applications. White light lasers are usually generated from solid materials or photonic crystal fibers, hollow fibers, etc. Short pulsed laser gas filamentation provides the basis for the generation of the laser beam. Short-pulse laser gas filamentation provides a material-damage-free route to white-light laser generation, but pointing jitter caused by pulse accumulation limits applications at high re-frequencies. Limited by the damage threshold of solid materials or affected by the self jitter problem in the gas, it is difficult to obtain a stable transmission of high-energy, high-frequency white light laser sources, and it is also an urgent international problem in the field of supercontinuum light source generation and its applications.
Fig. 1 (a) (b) Schematic diagram of the experimental setup. (c) (d) True-color images of unloaded and loaded DC high-voltage electric field in the light filament and light filament-induced corona. (e) (f) Corresponding forward light spots.
In this study, the researchers utilized a high-frequency femtosecond laser to generate a large-energy supercontinuum white light source by forming a filament in air, and innovatively loaded the filament with an external high-voltage DC electric field, which solved the problem of thermally induced jitter of the filament at the root, based on the principle of inhibiting plasma compounding by means of the external electric field and prolonging its lifetime to reduce the heat deposition due to plasma compounding at the time of the next laser pulse, thus successfully reducing the thermal diffusion caused by the thermal diffusion of the filament, and thus successfully reducing the thermal diffusion caused by the thermal diffusion of the filament. In this way, the intensity of airflow disturbance caused by thermal diffusion is successfully reduced. In addition, the light filament induced corona discharge generated by the stable ion wind can also effectively overcome the light filament self-turbulent air flow, the experimental results show that under the action of the external electric field of high-frequency (1 kHz) light filament and forward super-continuum white light spatial pointing stability can be improved at least two times, and at the same time, the stability of the spectral intensity of the white laser light has been significantly improved, the researchers use 1 kHz / 6.54 mJ femtosecond laser pulse in the Using 1 kHz/6.54 mJ femtosecond laser pulses in air, the researchers successfully obtained a 3.55 mJ stabilized supercontinuum spectrum white light source. This work effectively solves the international problem of pointing and intensity jitter of high-frequency femtosecond laser gas filamentation, which not only opens the way for generating high-frequency, stable and large-energy super-continuum white light source in air, but also expands new opportunities for other applications such as secondary radiation source, imaging and material microprocessing based on high-frequency filamentation.
SIPM has made significant progress in generating stable and high-energy white light laser in air.
Fig. 2 (a) Standard deviation of the forward supercontinuum spectrum white light laser and filament pointing angle (SDEV) at 1 kHz as a function of applied high voltage. (b) Scattering angle SDEV of forward SC light (b) and filament (c) at different laser repetition rates.(d) Variation of effective electric field applied to the filament as a function of loaded high voltage.
Fig. 3 (a) Comparison of forward supercontinuum spectrum white light spectra with (FIL + 55 kV) and without (FIL) external DC high voltage, with the initial laser spectrum of the unfilamented (noFIL) as a reference. (b) Signal-to-noise ratio (1 kHz) of the intensity of the supercontinuum spectrum at different applied high voltages. (c) Supercontinuum spectrum white light laser energy as a function of pump laser energy (focusing lens focal length is 1 m).
