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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...

We demonstrate what is to our knowledge the first passively mode-locked thin-disk Yb:KY(WO(4))(2) laser. The laser produces pulses of 240-fs duration with an average power of 22 W at a center wavelength of 1028 nm. At a pulse repetition rate of 25 MHz, the pulse energy is 0.9microJ , and the peak power is as high as 3.3 MW. The beam quality is very close to the diffraction limit, with M(2)=1.1 .

The timing jitter, optical phase noise, and carrierenvelope offset (CEO) noise of passively mode-locked lasers are closely related. New key results concern analytical calculations of the quantum noise limits for optical phase noise and CEO noise. Earlier results for the optical phase noise of actively mode-locked lasers are generalized, particularly for application to passively mode-locked lasers. It is found, for example, that mode locking with slow absorbers can lead to optical linewidths far above the Schawlow-Townes limit. Furthermore, modelocked lasers can at the same time have nearly quantum-limited timing jitter and a strong optical excess phase noise. A feedback timing stabilization via cavity length control can, depending on the situation, reduce or greatly increase the optical phase noise, while not affecting the CEO noise. Besides presenting such findings, the paper also tries to clarify some basic aspects of phase noise in mode-locked lasers.PACS 42.50.Lc; 42.60.Fc IntroductionThe noise properties of mode-locked lasers, in particular the timing jitter [1, 2], the optical phase noise, and the carrier-envelope offset (CEO) noise [3,4], are important for many applications, e.g. in frequency metrology and data transmission. These types of noise can have different origins, the most important of which are usually mechanical vibrations of the laser cavity, thermal effects in gain medium and/or laser cavity, and quantum fluctuations. As is well known, the latter are related mainly to spontaneous emission in the gain medium and to vacuum noise entering the laser cavity through the output coupler mirror and other elements with optical losses. Depending on the circumstances, the noise performance can be close to quantum limited or many orders of magnitude above the quantum limit. One may expect that quantum-limited performance in terms of timing jitter and optical phase noise should usually come in combination, but in the following it will be demonstrated that e.g. mirror vibrations can lead to strongly enhanced optical phase noise,

We present a new technique for the efficient and simple production of amplitude-squeezed light with high coherent excitation, using a frequency doubler, in which only the fundamental wave is resonantly enhanced. Our theory predicts up to nearly 90% squeezing in the second-harmonic wave under ideal conditions. In our first experiment we measured up to 30% squeezing [having taken into account the limited quantum efficiency (63%) of our detection system] in a stable 50mW beam. Compared to doubly resonant &equency doubling our new approach has considerable technical advantages.