Appl. Phys. B 80, 151-158 (2005) Lasers and Optics Applied Physics B ABSTRACT The reflectivity of a semiconductor saturable absorber mirror (SESAM) is generally expected to increase with increasing pulse energy. However, for higher pulse energies the reflectivity can decrease again; we call this a 'roll-over' of the nonlinear reflectivity curve caused by inverse saturable absorption. We show for several SESAMs that the measured roll-over is consistent with two-photon absorption only for short (femtosecond) pulses, while a stronger (yet unidentified) kind of nonlinear absorption is dominant for longer (picosecond) pulses. These inverse saturable absorption effects have important technological consequences, e.g. for the Q-switching dynamics of passively mode-locked lasers. A simple equation using only measurable SESAM parameters and including inverse saturable absorption is derived for the Q-switched modelocking threshold. We present various data and discuss the sometimes detrimental effects of this roll-over for femtosecond high repetition rate lasers, as well as the potentially very useful consequences for passively mode-locked multi-GHz lasers. We also discuss strategies to enhance or reduce this induced absorption by using different SESAM designs or semiconductor materials.PACS 42.60.Fc; 42.70.Nq; 78.20.Ci IntroductionSince 1992, semiconductor saturable absorber mirrors (SESAMs) have been used with great success for selfstarting passive continuous-wave mode locking of various types of solid-state lasers [1][2][3][4]. A technological key point is to avoid Q-switching instabilities [5], which can be provoked as an unwanted side effect of using a saturable absorber in a laser cavity. Especially for lasers with high pulse repetition rates [6,7] or high average output powers [8], the tendency for Q-switched mode locking (QML), which is typically observed below a certain threshold for the intracavity power, is a crucial limiting factor. Often it is difficult to achieve a low enough QML threshold.Theoretical results for the QML threshold [5] have generally been found to be in good agreement with experimental u Fax: +41-1-633-1059, E-mail: grange@phys.ethz.ch values. However, particularly for some recent high repetition rate lasers, the QML threshold was found to be significantly lower than expected. Recently, it was shown that for some Er:Yb:glass lasers this could be explained with modified saturation characteristics of the SESAMs used, namely with a rollover of the nonlinear reflectivity for higher pulse fluences [9]. Two-photon absorption (TPA) has been widely used for optical power limiter [10]. And it has long been known that TPA causes a roll-over in the nonlinear reflectivity which lowers the Q-switching threshold [11,12]. However, for picosecond pulse durations as in the mentioned Er:Yb:glass lasers this effect would be too weak to be significant for practical values of the pulse fluence. Therefore, it was surprising that a significant roll-over was observed even in this regime [9], while many earlier exp...

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