## 1 Introduction  
Ultrashort pulses are key to numerous fields of science and technology. The unique temporal and spectral properties of the pulses make them an extremely valuable tool in many fields, such as the investigation of ultrafast chemical reactions [1] and multi-photon microscopy [2]. Frequency combs are an important application of ultrashort pulses that received great attention since the Nobel Prize in Physics was awarded to [PERSON] and [PERSON] [3; 4; 5]. A comprehensive review regarding the applications of ultrashort laser systems can be found in [6; 7]. In the past five decades since the first demonstration of mode-locking in a He-Ne laser [8], the field of ultrafast optics \"exploded\", and the literature is enormous. The aim of this paper cannot be, therefore, to provide a complete survey of the bibliography in the field, but rather, to focus on the experimental and practical aspects of realizing a mode-locked solid-state oscillator. Specifically, we provide a detailed description of the theoretical and experimental physics of a Kerr lens mode-locked (KLM) Ti:sapphire oscillator, oriented mainly for those interested in setting up such an oscillator or in understanding its inner workings.  
The reason for choosing the Ti:sapphire oscillator as the default example in this review is trivial. The first realization of the KLM Ti:sapphire laser [9], followed by the theoretical explanation of the underlying physics [10; 11; 12; 13], are considered a transition point in ultrashort pulse generation. Although many gain media can produce ultrashort pulses, the Ti:sapphire crystal is by far the ultimate \"working horse\" of mode-locking, providing the shortest pulses with the highest peak powers. The excellent mechanical properties and cavity compactness of the Ti:sapphire oscillator renders it perfect for commercial production. The Ti:sapphire crystal emission spectrum in the near infra-red (NIR) regime, which spans nearly an octave in frequency, can produce ultrashort pulses down to the sub-two cycle regime [14; 15; 16; 17]. Consequently, the Ti:sapphire laser has become an extremely valuable research tool in many laboratories that focus on ultrafast phenomena and precision measurement. For these reasons, the description of the physical and experimental considerations involved in the realization of such an oscillator are beneficial to a wide audience of experimentalist (and theorists).  
In this review, we collect, under a single notation, the most important topics (in our view) for an essential, but detailed, description of the KLM Ti:sapphire laser. Further details will be correspondingly referenced. In addition, special focus is given to novel cavity designs that overcome some of the inherent limitations and disadvantages of the standard design of KLM Ti:sapphire lasers, such as shaping the oscillation spectrum, lowering the mode-locking threshold and eliminating sources of astigmatism. We suggest to the reader to employ educated reading when addressing this review. While we attempted to cover all relevant topics for experimental realization of a mode-locked Ti:sapphire oscillator, many readers are probably well familiar with some of the basic concepts, such as the principles of pulsed operation or dispersion control. We therefore advise the reader to first leaf through the review and then selectively read chapters of higher interest.