1. Introduction

Femtosecond lasers can be classified in terms of output parameters such as pulse duration, repetition rate, average power, peak power, carrier envelope phase stability (CEP), intensity noise and long-term stability. Depending on the application, only a few parameters out of that list can be interesting. The choice of a certain laser or oscillator technology is usually driven by applications and, ultimately, the users and the market will define which technology is beneficial for which application. However, before a certain technology is commercially pursued, researches invest many efforts in demonstrating its potential for diverse applications. This is currently happening to the thin-disk (TD) oscillators. Meanwhile, technologies like femtosecond slab

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

amplifiers [1] and fiber amplifiers [2] operating in the MHz repetition rate range have become commercially available and are already on the way to facilitating real applications. Compared to amplifiers, thin-disk oscillators are more compact and simple alternatives with some characteristics, which can be superior to amplifiers.

We consider femtosecond oscillators delivering average powers >100 W, pulse durations on the order of 200 fs and repetition rates of a few tens of MHz (see summary Table 1 on various oscillators). These parameters are obtained directly from oscillators without the involvement of any type of external amplification. From this perspective, thin-disk oscillators represent a separate class of lasers uniquely combining high peak and high average powers. Their main features are the amplification free nature, low noise and relative compactness. Further-on in the text, we focus only on this type of technology and omit any type of amplifiers or enhance-

The first mode-locked Yb:YAG thin-disk oscillator was demonstrated in the group of Prof. Keller [27] in 2000. That paper essentially merged two available technologies: the thin-disk and Semiconductor Saturable Absorber Mirrors (SESAM). The same group advanced this technology during the next decade and established many records in terms of peak power, pulse energy [28] and average power [29] directly obtained from femtosecond oscillators. Also other groups pushed these limits [30, 31] and investigated different gain materials and dispersion regimes [32]. However, the obvious merge of the Kerr-lens mode-locking (KLM) technique with the TD technology was not demonstrated till 2012 [33]. Although the basic idea of merging these two technologies was patented in 1999 [34], the experimental attempts to realize it were unsuccessful according to [35]. In 2012, the merging of KLM and TD technology was successfully demonstrated in our group [33]. This first encouraging experiment motivated us to proceed further in this direction. Over the last 5 years, numerous TD KLM oscillators were developed with other groups also joining this activity [36–38]. The oscillators developed in our group are summarized in Table 1 and are also shown as red dots in Figure 1. Additionally, the

(compressed)

40 3 13 300 9 Seed oscillator In use [3] 90 0.9 100 250 3.5 >50 20 MIR generation In use [4] 42 1.1 38 250 4.2 6 (10) 7.7 (10) MIR generation In use [5, 6] 270 14 19 330 37.8 Development itself Not in use [7]

10 (3.5) 0.7 (0.4) 100–200 70 (47) 0.6 Development itself In use [9] 100 4.1 24 190 19.3 65 30 Commercial system In use [10] All oscillators use Yb:YAG as a gain medium. Most of the oscillators are successfully operating in the laboratories with the

Table 1. Summary table of the KLM thin-disk oscillators developed at MPQ, LMU and UFI GmbH from 2012 till 2017.

155 10 15.5 140 63 130 30 XUV generation,

τp, fs (compressed) Application Status

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In use [8]

Raman spectroscopy

rapid evolution of TD KLM oscillators is shown in Figure 2b.

Pavg,W Ep, μJ frep, MHz τp, fs P, MW Pavg, W

parameter sets originally published.

ment cavities.

Originally, the authors' motivation was to pursue research applications in two far separated spectral ranges: extreme ultraviolet (XUV) and middle infrared radiation (MIR). Ideally, the laser sources described here would provide access to both broadband radiation in the XUV, the deep UV range 13–200 nm and broadband radiation in the infrared range 3–30 μm. Due to the intrinsic simplicity and compactness of thin-disk oscillators, these sources would serve as table-top alternatives to synchrotron beamlines [11, 12]. Moreover, this broadband radiation can be turned into frequency combs [13, 14] with some additional stabilization efforts [5] or even into a table-top attosecond pulse source [15, 16]. Consequently, direct frequency comb or dual frequency comb spectroscopy in the XUV and MIR regions would be advanced significantly with respect to the current state of the art. In addition, these sources can be transportable and would thus benefit more experiments and collaborations in the research community.

Rapid progress in the development of Yb-doped lasers was mainly supported by the availability of inexpensive pump sources, namely InGaAs diodes [17]. The price of these pump diodes has kept on decreasing over the last years due to increasing demand in industrial applications. Considering the current price of pump laser diodes, even moderate optical to optical efficiencies of femtosecond oscillators are not of a serious concern, especially for applications in research. The invention of Kerr-lens mode-locking [18] in combination with Ti:Sapphire (Ti: SA) [19] as gain-material had profound impact on applications both within and outside of research. When taking a closer look into the progress made in Ti:Sa and Yb:YAG, thin-disk oscillators one can conclude that the Yb-based TD oscillators resemble the technological evolution of the Ti:Sa oscillators. The main advantage of Yb-based TD technology is its peak and average power scalability with the access to relatively cheap pump diode sources. However, the Ti:Sa gain medium is uniquely broadband and delivers light in the visible range being hardly accessible to Yb-based gain media. Thus, Yb-based TD technology cannot completely substitute the Ti:Sa technology, especially in applications where no high average powers are required. Moreover, as soon as direct diode pumping of Ti:Sa oscillators will settle as the routine pumping scheme, a next competition round is to be expected between Yb-based and solid-state Ti:Sa oscillators, especially in the low average power regime. For those applications requiring XUV and MIR radiation, the intrinsic scalability of the thin-disk concept is of crucial importance. For instance, conversion efficiencies from a 1 μm driving source into the XUV hardly exceed 10<sup>5</sup> [20] and 10<sup>3</sup> into the MIR, when simple intra-pulse difference frequency generation is considered [21]. Thus, reasonable average powers in the mW-W range which are necessary for spectroscopy applications can be obtained by using very high average powers of the driving laser, on the order of 100–1000 W. It should be mentioned that femtosecond enhancement cavities [22–24] represent another type of technology well suitable for XUV generation experiments. So far, only by means of this relatively complex technology, direct XUV frequency comb spectroscopy was demonstrated [25] and highest average powers were achieved.

We consider femtosecond oscillators delivering average powers >100 W, pulse durations on the order of 200 fs and repetition rates of a few tens of MHz (see summary Table 1 on various oscillators). These parameters are obtained directly from oscillators without the involvement of any type of external amplification. From this perspective, thin-disk oscillators represent a separate class of lasers uniquely combining high peak and high average powers. Their main features are the amplification free nature, low noise and relative compactness. Further-on in the text, we focus only on this type of technology and omit any type of amplifiers or enhancement cavities.

amplifiers [1] and fiber amplifiers [2] operating in the MHz repetition rate range have become commercially available and are already on the way to facilitating real applications. Compared to amplifiers, thin-disk oscillators are more compact and simple alternatives with some characteris-

Originally, the authors' motivation was to pursue research applications in two far separated spectral ranges: extreme ultraviolet (XUV) and middle infrared radiation (MIR). Ideally, the laser sources described here would provide access to both broadband radiation in the XUV, the deep UV range 13–200 nm and broadband radiation in the infrared range 3–30 μm. Due to the intrinsic simplicity and compactness of thin-disk oscillators, these sources would serve as table-top alternatives to synchrotron beamlines [11, 12]. Moreover, this broadband radiation can be turned into frequency combs [13, 14] with some additional stabilization efforts [5] or even into a table-top attosecond pulse source [15, 16]. Consequently, direct frequency comb or dual frequency comb spectroscopy in the XUV and MIR regions would be advanced significantly with respect to the current state of the art. In addition, these sources can be transportable and would thus benefit more experiments and collaborations in the research community. Rapid progress in the development of Yb-doped lasers was mainly supported by the availability of inexpensive pump sources, namely InGaAs diodes [17]. The price of these pump diodes has kept on decreasing over the last years due to increasing demand in industrial applications. Considering the current price of pump laser diodes, even moderate optical to optical efficiencies of femtosecond oscillators are not of a serious concern, especially for applications in research. The invention of Kerr-lens mode-locking [18] in combination with Ti:Sapphire (Ti: SA) [19] as gain-material had profound impact on applications both within and outside of research. When taking a closer look into the progress made in Ti:Sa and Yb:YAG, thin-disk oscillators one can conclude that the Yb-based TD oscillators resemble the technological evolution of the Ti:Sa oscillators. The main advantage of Yb-based TD technology is its peak and average power scalability with the access to relatively cheap pump diode sources. However, the Ti:Sa gain medium is uniquely broadband and delivers light in the visible range being hardly accessible to Yb-based gain media. Thus, Yb-based TD technology cannot completely substitute the Ti:Sa technology, especially in applications where no high average powers are required. Moreover, as soon as direct diode pumping of Ti:Sa oscillators will settle as the routine pumping scheme, a next competition round is to be expected between Yb-based and solid-state Ti:Sa oscillators, especially in the low average power regime. For those applications requiring XUV and MIR radiation, the intrinsic scalability of the thin-disk concept is of crucial importance. For instance, conversion efficiencies from a 1 μm driving source into the XUV hardly exceed 10<sup>5</sup> [20] and 10<sup>3</sup> into the MIR, when simple intra-pulse difference frequency generation is considered [21]. Thus, reasonable average powers in the mW-W range which are necessary for spectroscopy applications can be obtained by using very high average powers of the driving laser, on the order of 100–1000 W. It should be mentioned that femtosecond enhancement cavities [22–24] represent another type of technology well suitable for XUV generation experiments. So far, only by means of this relatively complex technology, direct XUV frequency comb spectroscopy was demonstrated [25] and highest average powers were

tics, which can be superior to amplifiers.

92 High Power Laser Systems

achieved.

The first mode-locked Yb:YAG thin-disk oscillator was demonstrated in the group of Prof. Keller [27] in 2000. That paper essentially merged two available technologies: the thin-disk and Semiconductor Saturable Absorber Mirrors (SESAM). The same group advanced this technology during the next decade and established many records in terms of peak power, pulse energy [28] and average power [29] directly obtained from femtosecond oscillators. Also other groups pushed these limits [30, 31] and investigated different gain materials and dispersion regimes [32]. However, the obvious merge of the Kerr-lens mode-locking (KLM) technique with the TD technology was not demonstrated till 2012 [33]. Although the basic idea of merging these two technologies was patented in 1999 [34], the experimental attempts to realize it were unsuccessful according to [35]. In 2012, the merging of KLM and TD technology was successfully demonstrated in our group [33]. This first encouraging experiment motivated us to proceed further in this direction. Over the last 5 years, numerous TD KLM oscillators were developed with other groups also joining this activity [36–38]. The oscillators developed in our group are summarized in Table 1 and are also shown as red dots in Figure 1. Additionally, the rapid evolution of TD KLM oscillators is shown in Figure 2b.


All oscillators use Yb:YAG as a gain medium. Most of the oscillators are successfully operating in the laboratories with the parameter sets originally published.

Table 1. Summary table of the KLM thin-disk oscillators developed at MPQ, LMU and UFI GmbH from 2012 till 2017.

2. Kerr-lens mode-locking principle

commercialization.

artificial saturable absorber.

The refractive index n of a material depends on the incident electric field intensity. A Gaussian intensity distribution causes an increase of the refractive index in the central part of the beam relative to its outer regions therefore forming a nonlinear lens. The higher the light intensity, the stronger the action of such a lens. The lens becomes stronger for smaller beam radii ω and media with higher nonlinear refractive index n2. Self-focusing occurs for a highintensity, pulsed laser-beam (red, Figure 3) and reduces losses due to the hard aperture blocking the continuous wave (CW) beam of lower intensity. In a resonator-cavity, this mechanism initiates mode-locking and acts as an artificial saturable absorber. Catastrophic run away damage can happen when a critical power is reached and the length of the medium exceeds the self-focusing length. The first oscillator working on the KLM principle was discovered by Spence et al. [18] and referred to as self-mode-locking or magic modelocking. Piche [39] explained the mode-locking mechanism on the basis of self-focusing and only a few authors recognized the potential of the self-focusing effect for mode-locking before the invention of KLM [40, 41]. Since that time, KLM established itself as the method of choice for ultrashort-pulse generation and numerous studies were done on resonator design, theoretical numerical and analytical description of KLM and experiments on ultrashort pulse generation. Mostly, experiments were performed with the Ti:Sa gain medium, which has several outstanding features: an extremely broad gain bandwidth, short upper-state lifetime as well as high thermal conductivity [19]. Understanding that shortest possible pulses can only be obtained when nonlinearities and dispersion are balanced to form so-called soliton pulses [42, 43] preceded the invention of KLM. However, this technique constituted the decisive building-block to enable robust, usable solid-state femtosecond oscillators. With Kerr-lens mode-locked solitonic Ti:Sa oscillators up to several 100 mW average power and up to MW-level peak-power were realized with pulse durations approaching few optical cycles, all in a compact, reliable setup that was superior to the old dye-based technology. This ensured its worldwide adoption in many optical laboratories and nearly immediate

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Figure 3. Basic principle of KLM. Self-focusing occurs for a high-intensity beam (red) and reduces the losses due to a hard aperture (two black knifes) blocking the low-intensity (CW) beam. This mechanism initiates mode-locking and acts as an

Figure 1. Summary of diverse femtosecond thin-disk oscillators adopted from [26]. More details including the references can be found in [26].

Figure 2. (a) Four key elements: thin-disk technology, dispersive mirrors, Kerr-lens mode-locking and geometrical energy scaling concept form femtosecond thin-disk oscillator technology described and (b) graphical representation of the rapid thin-disk KLM oscillator development in our group.
