Semiconductor saturable absorber mirror (SESAM) devices have become a key component of ultrafast passive mode-locked laser sources. Here we describe in more detail how the key SESAM parameters such as saturation fluence, modulation depth, and nonsaturable losses are measured with a high accuracy. These parameters need to be known and controlled to obtain stable pulse generation for a given laser. A highprecision, wide dynamic range setup is required to measure this nonlinear reflectivity of saturable absorbers. The challenge to measure a low modulation depth and key measures necessary to obtain an accurate calibration are described in detail. The model function for the nonlinear reflectivity is based on a simple twolevel travelling wave system. We include spatial beam profiles, nonsaturable losses and higher-order absorption, such as twophoton absorption and other induced absorption. Guidelines to extract the key parameters from the measured data are given.PACS 07.60.Hv; 42.65.Re; 42.70.Nq

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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We design single-wall carbon nanotube (SWNT) thin-film saturable absorbers (SAs) integrated onto semiconductor distributed Bragg reflectors for mode-locking solid-state Er:Yb:glass lasers. We characterize the low nonsaturable loss, high-damage-threshold SWNT SAs and verify their operation up to a pulse fluence of 2 mJ/cm(2). We demonstrate passive fundamental continuous-wave mode locking with and without group-delay dispersion compensation. Without compensation the laser produces chirped 1.8 ps pulses with a spectral width of 3.8 nm. With compensation, we obtain 261 fs Fourier-transform-limited pulses with a spectral width of 9.6 nm.

We have investigated the effect of beryllium doping on the optical nonlinearity and on the carrier dynamics in low-temperature (LT) grown GaAs for various growth temperatures and doping levels. Pump–probe experiments with 20 fs pulses and quantitative measurements of the nonlinear absorption show that in undoped LT GaAs, ultrafast response times are only obtained at the expense of low absorption modulation. In contrast, in Be-doped LT GaAs, high absorption modulation is maintained for response times as short as 100 fs. These results are qualitatively explained accounting for the point-defect-related optical transitions in LT-GaAs.