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Gain at the end configuration of a diode-pumped, Kerr-lens mode-locked Cr:LiSGaF laser


Optical Society of America -OSA-, Washington/D.C.; European Physical Society -EPS-, Geneva; IEEE Lasers and Electro-Optics Society:
CLEO Europe 1998. Technical digest
Piscataway, NJ: IEEE, 1998
ISBN: 0-7803-4233-X
Conference on Lasers and Electro-Optics Europe (CLEO Europe) <1998, Glasgow>
Fraunhofer ILT ()

Fulltext: Currently the widespread application of fs-lasers is limited by price and complexity. One approach for userfriendly systems are diode-pumped Kerr-lens mode-locked (KLM) fs-lasers with Cr:LiSGaF or Cr:LiSAF as the gain-medium. However bulky optical systems are necessary, to focus the laser diodes through the folding mirrors of a standard x- or z-fold cavity into the crystal. Efficient KLM and a small lasing threshold requires to focus the resonator mode tightly into the laser crystal. But in this case the poor beam quality of available red single emitter diode lasers prohibits an efficient mode matching To overcome these problems we designed the very asymmetric resonator in Fig. 1 and optimized it for high self amplitude modulation (SAM). With this setup and very compact optics for the beam shaping and polarization combination of the diodes we achieve efficient mode matching in the 5 mm long Cr:LiSGaF-crystal at a moderate lasing threshold. The 4 mm thick SF57 disk between a p air of folding mirrors with a ROC of 75 mm permits a very high SAM and self phase modulation (SPM). To compensate its group velocity dispersion (GVD) of 1920 fs2 first used SF10 prisms at 425 mm separation With a 2 per cent output-coupler and 660 mW power at the laser-crystal we achieved stable, nearly transform limited pulses centered at 837 nm of 68 fs duration, 56 mW average power and 100 MHz repetition rate. The laser was clearly soft aperture mode-locking, because we calculated, that only an aperture inside the laser-crystal or the SF57 plate provides the necessary nonlinear loss modulation. To avoid a strong sideband in the mode-locked spectrum, we replaced the SF 10 prisms by a pair of LAK31 prisms 865 mm apart. This reduced the calculated net third order dispersion (TOD) from -11000 fs3 to -2800 fs3, and with an 1 per cent output-coupler we achieved 34 fs pulses of 11 mW average power (Fig. 2). We measured a net GVD of -130 fs2 by the position of a sideband appearing at a lower amount of negative net GVD'. Despite the low intracavity power caused by resonator losses of 2 per cent, mode-locking is self-starting after running some 100 ms in continuos wave. The signal of a photodiode in Fig. 3 shows the change from continuos wave to mode-locking. Increasing SAM in relation to SPM by a more symmetric oscillator can reduce the tendency to generate sidebands' at a lower amount of net GVD and yield shorter pulses. However the beam diameter in the laser crystal and hence the lasing threshold rises with increasing symmetry. An improved surface finish of the SF57 disc and the prisms will decrease optical loss and reduce threshold. Pumping with a fiber-coupled high power diodebar will lead to larger tolerable values of the lasing threshold.