Tunable Ultrashort Laser Pulses Generated through Filamentation in Gases

Tunable and stable ultrashort laser pulses in the visible spectrum are generated with high efficiency by four-wave mixing process during the filamentation of near-infrared and infrared laser pulses in gases. It is shown that these tunable ultrashort pulses have a very low energy fluctuation and an excellent mode quality due to the processes of intensity clamping and self-filtering in the filament.

Figure 1

(a) 4WM Conversion efficiency in air (▴) and in argon (●) defined as the ratio in percentage of the 4WM pulse energy divided by the input IR seed energy. (b) Generated 4WM pulse spectra with the IR seed ($\lambda$) superposed in the filament zone.

Figure 2

Results for the 4WM generated pulse during the filamentation in air of a NIR laser beam and a 1.32 mm IR seed beam focused with an 82 cm focal length concave mirror (first row) and a 4 m focal length concave mirror (second row). (a),(d) Normalized 4WM pulse spectrum; (b),(e) the transform-limited pulse duration supported by the normalized spectrum; (c),(f) temporal intensity profile of compressed 4WM pulse as measured from the envelope of the interferometric autocorrelation. The solid line in (c) and (f) are the Gaussian fits of the experimental results.

Figure 3

Temporal evolution of energy per pulse normalized over the mean energy for (a) the IR seed, (b) the NIR pulse, (c) the generated 4WM pulse for NIR pump power ($P_{\mathrm{NIR}}$) 10 times below the critical power for self-focusing ($P_{\mathrm{cr}}$) in air, and (d) the generated 4WM pulse for pump power above the critical power for self-focusing in air ($P_{\mathrm{NIR}}=2.5P_{\mathrm{cr}}$). The rms energy fluctuations are indicated on the right-hand side.

Figure 4

Examples of NIR fluence distribution (left) used to generate the filament in air and the far-field fluence distribution of the generated 4WM pulse (right) at a pump power of $2.5P_{\mathrm{cr}}$ are shown in (a) for the undistorted NIR beam and (b) for the NIR beam passing through a square aperture. The ratio $\pi w_0\theta /\lambda$ of the 4WM beam was independent of the initial pump beam profile and below 1.01.