Thin-disk laser concept

7.1.1 Basic principle of the thin-disk concept

An important requirement for laser concepts is an effective heat removal. One possible realization is the optimization of the geometry of the laser media. Thin fibers and thin-disks exhibit an excellent relation between the volume and large surfaces. Hence, fiber as well as thin-disk lasers are well established for scientific and commercial usage in high brilliance lasers [T¨un10, Kel03, Gie07, Mar08, S¨ud09, Sch14].

The key component of a thin-disk laser is the laser crystal as a thin-disk with a dia-meter in the range of centidia-meters and a thickness in the range of tens to hundreds of micrometers. Usually the backside of the disk is mounted on a water cooled heat sink. This results in a nearby one-dimensional heat flow along the beam axis, so that the thin-disk shows nearby no thermal lens effects leading to an excellent laser beam quality. The disk concept is reviewed in [Gie07] highlighting the results in thin-disk lasers and the power scaling capability.

As it is illustrated in Fig. 7.1, the disk is pumped from the front and for the laser cavity it is used as a folding mirror. For this purpose the front of the disk is anti reflective coated whereas the back side is highly reflective coated, each for the pump and the laser wavelength.

Due to the small thickness of the disk it absorbs just a few percent of the pump light within a single pass. Therefore, multiple pump passes over the disk increase the absorbed pump power [Gie94]. With a few tens of pump passes over the disk an absorption of more than 90 % can be achieved [Gie07]. Fig. 7.1 shows how the pump beam from a multimode fiber is collimated and after the first pass the remaining pump beam is recycled and reimaged on the disk again by a special arrangement of prisms and a parabolic mirror.

Besides the outstanding heat removal of the thin-disk laser its power scaling capability is unique. The laser output power can be increased by increasing the pump power and the pump spot at the same time keeping the pump power density constant [Gie07].

However, in a fundamental mode laser the stability range of the resonator limits the pump spot diameter, since the laser is more sensitive for changes of the refractive power of the disk the larger the fundamental mode size is [Bau12a].

7.1.2 Properties of thin-disk laser materials

The demands on the thin-disk are complex wherefore a lot of research is in progress to find the optimal gain material with perfect thermo-elastic, mechanical, and opto-electronic properties. Challenges in manufacturing the thin-disk and mount it on the heat think are as demanding as special laser goals such as high output power, short pulse duration, and long-term stability.

For an efficient pump light absorption the doping level and the absorption cross section of the laser material should be high. Based on the absorption wavelength a suitable pump source should be available. Additionally, the material should exhibit a high

7.1 Thin-disk laser concept

Pump light recycling

Pump diode TDH Parabolic

mirror

Retro reflector Collimation

Pump mode

HR coating

Water cooled heat sink

Thin-disk AR coating

Thin-disk on heat sink Pump cavity with thin-disk head

Mirror configuartion for pump light

Laser mode

Figure 7.1: Schematic of the thin-disk laser concept. On the left the mirror ration for pump light recycling is illustrated. It is a three dimensional mirror configu-ration with the disk on the bottom level and the parabolic mirror for the pump light reflection above the disk. The panel in the middle shows the complete pump cavity with the pump diode. The pump mode is collimated and reflected on the thin-disk (TDH). The pump mode hits the disk under a slight angle whereas the laser mode is perpendicular to the disk. A zoom into the thin-disk head (TDH) is sketched on the right. It shows the disk with the anti reflective (AR) coating on the front and the highly reflective (HR) coating on the back side. The disk is mounted on a water cooled heat sink for a nearly one dimensional heat flow along the beam axis. The figure is adopted from [Bau12a].

thermal conductivity to keep the heat removal as high as possible. Another option to optimize the heat removal is a further reduction of the thickness of the disk that is challenging in terms of manufacturing processes. For the generation of short pulses a broad gain bandwidth is necessary.

A common thin-disk material that meets all requirements is Yb:YAG. It exhibits a small quantum defect of just 9 % [Koe06] and a high thermal conductivity leading to low crystal heating. When pumped at 940 nm the ratio of generated heat to absorbed energy is around 11 % [Koe06]. Standard InGaAs diodes can be used as pump sources for absorption wavelengths at 940 nm. Since the absorption bandwidth at 940 nm is quite broad, the laser tolerates small wavelengths shifts of the pump light that reduces the need for temperature controlled pump diodes [Koe06]. In contrast, for an absorp-tion wavelength of 969 nm wavelength-stabilized pump diodes are necessary [Wei12].

Furthermore, Yb:YAG crystals can be grown with nearby no parasitic defects that al-lows for high laser efficiency [Gie07].

However, since Yb:YAG is a quasi-three-level system the thermal population of the lower laser level is about 5 % at room temperature that requires high absorbed pump power to achieve transparency and reach inversion population [Koe06]. Another rea-son for the need of high pump intensity is a high saturation intensity of 9.7 kW/cm², respectively high saturation fluence of 9.2 J/cm², that results from a rather small cross section [Koe06]. According to Eq. (2.15) a high saturation fluence of the gain material tends to Q-switching. This can be seen in the power-up phase of the Yb:YAG thin-disk laser when due to small pump power Q-switching instabilities occur. Another disadvan-tageous property of Yb:YAG is the temperature dependence of the laser performance.

Based on a strongly temperature dependent thermal population the stimulated emis-sion cross section and thereby the optical efficiency of a laser can be increased by cooling the laser crystal [Koe06].

Despite these trade-offs Yb:YAG convinces with the above mentioned enormous small heat load and high crystal quality. Hence, Yb:YAG is a well established thin-disk ma-terial that allows for high average power systems with a good beam quality.

Consequently, milestone results regarding pulse energy and output power were achieved with Yb:YAG thin-disk lasers. Power scaling of Yb:YAG thin-disk lasers in the kW regime is possible: Just recently Schad et al. published output power of a single Yb:YAG thin-disk laser in multi-mode operation of more than 9 kW and in fundamen-tal mode as high as 4 kW [Sch14].

The highest average output power of a mode-locked Yb:YAG thin-disk laser without amplification at ambient air is 230 W [Bro14], the highest pulse energies are 41µJ [Bau12b]. In vacuum 275 W of average output power [Sar12a] and pulse energies of 80µJ [Sar14] were obtained. The shortest pulses of an Yb:YAG thin-disk laser have a pulse duration of 190 fs and were realized in a Kerr-lens mode-locked resonator [Pro12].

A lower limit for the pulse duration is given by the rather small bandwidth of Yb:YAG.

To push the limits in respect of pulse duration and average output power even fur-ther a variety of different laser crystals is investigated. For example Yb-doped oxides