Space debris presents an increasing threat to the lifetime of commercial and military space assets. Laser-based space
debris removal systems could potentially mitigate this threat by targeting debris objects in the cm-range. In order to
reach this goal a minimum fluence of a few J/cm² on the debris object and a pulse repetition rate of several 10 Hz are
necessary. These requirements can be met by coupling 1000-2000 independent 10 J laser sources coherently and
employing a sending telescope with a diameter of 5 m. We analyze which parameters are critical to the effectiveness of
the transmission system and deduce design guidelines. In particular the effects of non-optimum filling factors, secondary
mirror size, emitter intensity distribution and phase jitter of the individual emitters are discussed and compared.
During the early 1990s, collaboration between the German Aerospace Center and the University of Stuttgart started to work on the Thin Disk concept. The core idea behind the thin disk design is the use of a thin, disk-shaped active medium that is cooled through one of the flat faces of the disk. This ensures a large surface-to-volume ratio and therefore provides very efficient thermal management. Today, the thin disk concept is used in various commercial lasers – ranging from compact, efficient low power systems to multi-kW lasers, including cw lasers and also pulsed (femtosecond to nanosecond) oscillators and amplifiers. The whole development of the Thin Disk laser was and will be accompanied by numerical modeling and optimization of the thermal and thermo-mechanic behavior of the disk and also the heat sink structure, mostly based on finite element models. For further increasing the energy and efficiency of pulsed Thin Disk lasers, the effects of amplified spontaneous emission (ASE) are a core issue. Actual efforts are oriented towards short pulse and ultra-short pulse amplifiers with (multi-)kW average power or Joule-class Thin Disk amplifiers, but also on new designs for cw thin disk MOPA designs.
A Thulium fiber laser pumped or InP diode laser stack pumped Cr:ZnSe thin disk cw multimode laser at 2.4 μm with an output power of 5 and 4 W, respectively, and with optical-tooptical efficiencies of 10% will be presented. An experimentally verified and numerically simulated thermal lensing induced and cyclic instability in the laser system will be shown. As a consequence, in order to prevent the lasing conditions in the resonator to be unstable, power scaling of a Cr:ZnSe thin disk laser is possible by enlarging the pump spot and reducing thereby the thermal lensing condition. Therefore, the instability is not initiated. As a conclusion, the investigated instability will show up in any laser active material which has a strong absorption of the pump beam, for instance in transition metal ion laser material systems in connection with any laser concept, like for instance in thin disk, bulk or slab designs.
In the past decade, the Thin Disk laser design was very successful as a high power laser design for cw lasers with good beam quality and high efficiency. Also pulsed lasers based on Thin Disk amplifiers achieved comparable high average power, but mostly with medium pulse energies. Latest results show that the thin disk still has not reached the scaling limits, neither in cw operation nor in pulsed operation.
The thin disk laser is a successful concept for high output power and/or high pulse energy, high efficiency and good beam quality in the 1 μm range. Holmium-doped materials are a promising approach to transform this success to the 2 μm range. Ho:YAG is especially interesting for high pulse energies due to the long fluorescence lifetime (~ 8 ms) which provides good energy storage capabilities. We have realized a Ho:YAG thin-disk laser with a cw output power of 15 W at 2.09 μm and a maximum optical-to-optical efficiency of 37%. The laser was pumped with a Tm-fiber laser. Numerical simulations of the Ho:YAG thin disk laser show the potential for further scaling. As broadly tunable alternative, also a Cr:ZnSe thin disk laser was investigated. A Tm-fiber laser and a fiber coupled diode stack were tested as pump sources. A laser power of 3.5 W was achieved with diode pumping.
The thin disk laser is a successful concept for high output power and/or high pulse energy, high efficiency and good
beam quality in the 1 μm range. Holmium-doped materials are a promising approach to transform this success to the
2 μm range. Ho:YAG is especially interesting for high pulse energies due to the long fluorescence lifetime (~ 8 ms)
which provides good energy storage capabilities. We have realized a Ho:YAG thin-disk laser with a cw output power of
15 W at 2.09 μm and a maximum optical-to-optical efficiency of 37%. The laser was pumped with a Tm-fiber laser.
Numerical simulations are used for further characterization.
A significant reduction of the influence of the thermal lens in thin-disk lasers in high power laser operation mode could
be achieved, using dynamically stable resonators. For designing the resonator, investigations of thermally induced phase
distortions of thin-disks as well as numerical simulations of the field distribution in the resonator were performed. This
characterization was combined with thermo-mechanical computations.
On the basis of these studies, about 500 W output power with an averaged <i>M<sup>2</sup></i> = 1.55 could be demonstrated, using one
disk. Almost 1 kW output power with good beam quality could be extracted, using two disks. For the purpose of further
power scaling in nearly fundamental mode operation, experiments using more than two disks are in preparation.
Experimental investigations concerning the operation characteristics of a coherent fiber laser MOPA array are presented.
The experimental set up consists of a single frequency fiber coupled 35 mW DBR diode laser at 1063 nm as master
oscillator and two polarization maintaining two stage 2 W Yb doped commercial fiber amplifiers. Phase control is
accomplished by fiber coupled acousto-optic frequency shifters prior to the amplifiers and an opto-electric phase locked
loop operating at 100 MHz. Phase measurement at the amplifier output is achieved by a combination of a free space and
fiberoptic interferometer in combination with RF photodiodes. A heterodyne signal of the amplifier output signal is
generated with respect to a reference signal derived from the master oscillator and works as input signal for the phase
control. Phase coupling of the array is demonstrated and the degree of coherence is determined from the contrast of the
far field diffraction pattern of the output beam as well as from analysis of the RF photodiode signals. The characteristics
of the phase control and phase stability are investigated and residual phase disturbances resulting from thermal and
acoustic effects as well as depolarization are identified. Achievable beam quality as a function of fill factor is compared
to theoretical computations. Finally, perspectives concerning a coherent 4x15 W MOPA array with three stage
amplifying systems are outlined.
In the past decade, the Thin Disk laser design was very successful as a high power laser design for cw lasers with good
beam quality and high efficiency. Several numerical models show that the actually demonstrated output powers are far
below the scaling limits due to amplified spontaneous emission (ASE). Also pulsed lasers based on Thin Disk amplifiers
achieved comparable high average power, but only with medium pulse energies (about 100 mJ). Numerical models show
that ASE effects limit the possible pulse energy to values of a few Joule, using the typical high power cw Thin Disk laser
To evaluate designs for higher pulse energies a time resolved numerical model of pump absorption and ASE was
developed and combined with a model of pulse amplification. The evaluation is focused on quasi-cw pumped pulse
amplifiers. It includes the analysis of temperature, stress, deformation and thermal lensing, using finite element methods.
This numerical model is used to develop thin disk designs for high output energies.
In principle, the thin-disk laser concept opens the possibility to demonstrate high power, high efficiency and good beam
quality, simultaneously. For this purpose, a very homogeneous pump power distribution on the disk is necessary as well
as very low phase distortions of the disk itself.
Spatial mode structure and thermal lens effects in an Yb:YAG thin-disk laser have been investigated as function of the
pump power in linear and folded resonators. Whereas thermal lens is shown to be very weak due to the thin disk
geometry, a strong correlation of the laser mode with respect to the power density distribution of the pump radiation is
exhibited. The experimental results are compared with numerical simulations of the field distribution within the
resonator as well as in the far field demonstrating the excellent homogeneity of the disk as laser active medium.