Multiterawatt femtosecond Cr:LiSAF laser


Cr:LiSAF has become an attractive alternative gain medium for the high intensity amplification of femtosecond optical pulses. It uniquely combines the advantages of Ti:sapphire and Nd:glass, materials commonly used as gain media in laser systems based on chirped pulse amplification. Cr:LiSAF has a broad spectral emission bandwidth similar to that of Ti:sapphire allowing the amplification of femtosecond optical pulses. In addition it has an upperstate lifetime of 67 μs long enough for efficient flashlamp-pumping leading to compact laboratory sized low-cost amplifiers using a mature technology developed over many years. Based on our investigations of the optical gain, nonlinear and optical damage properties of Cr:LiSAF we have developed a femtosecond Cr:LiSAF laser system with a final amplifier aperture of 25 mm using chirped pulse amplification. This system currently produces 90-fs output pulses with a peak power of 8 TW. Its configuration is shown in Fig. 1. The laser system uses a Kerr-lens mode-locked Ti:sapphire laser, a pulse stretcher, a regenerative amplifier followed by three additional double pass amplifiers with increasing aperture up to 25 mm and a pulse compressor. Details of this laser system are described elsewhere. The energy of the single pulse selected by the pulse slicers at the output of the regenerative amplifier is 4.5 mJ with a stability of better than ±5%. When directly compressed it reliably produces 2.5 mJ (sech2) pulses of 95 fs duration in a diffraction limited Gaussian beam. The spectral width of the amplified pulse is 8.5 nm resulting in a time-bandwidth product of 0.32 which is close to the Fourier-transform limit of 0.32. The 7 mm preamplifier then produces pulse energies of 75 mJ at a repetition rate of 1 Hz which are then further amplified by the 10 mm amplifier to 280 mJ at the same repetition rate. Two additional passes through the 25 mm final amplifier results in 1.45 J pulse energy in a single shot mode (1 shot/10 min) in a approx.2x diffraction limited beam. After recompression the pulse energy is 750 mJ. A single shot autocorrelation of these pulses is shown in Fig. 2. The measure (FWHM) width is 140 fs, which corresponds to a pulse duration of 90 fs assuming a sech pulse shape. We are now making further improvements to the output performance of this system. We estimate that the final amplifier with 25 mm aperture can support pulse energies of several joules. Replacement of some of the amplifier crystals with low-loss Cr:LiSAF will lead to a 3x increase in output power and an improvement in focus-ability. In addition, a further increase of a factor of 2 can be gained from anti-reflection coatings to all our amplifier rods. We also envisage to redesign the system for minimum third and higher order dispersion to amplify pulses much shorter than 100 fs. We believe that with some of these improvements our laser system has the potential to reliably provide focused intensities well in excess of 1020 W/cm2 in the near future.

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Publication Title

Proceedings of the International Quantum Electronics Conference (IQEC'94)

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Article; Proceedings Paper



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0028561733 (Scopus)

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