Journal of Directed Energy, Volume 4, Number 1

This paper describes thin-disk laser history starting with the industrial laser environment in Germany in the 1970s. The background of the invention is discussed along with the German political and research environment. Thin-disk laser design and performance are then discussed in detail. Results for continuous-wave and pulsed operation as well as for amplification of short (nanosecond) and ultra-short (picosecond, femtosecond) pulses demonstrate the potential of thin-disk laser design. Advantages for using various laser materials are explained, as well as applicability of the thin-disk laser concept to optically pumped semiconductor structures. Finally, an overview is presented of German research and development practices and of patenting and licensing policies. The last section describes industrial applications of thin-disk laser technology.

This paper discusses the needs and requirements of modeling the thin disk laser with a focus on thermal modeling. Results concerning high power extraction (>10 kW with one disk), including thermal behavior, stress, and thermal lenses are presented. Challenges of modeling high energy storage in a thin disk amplifier are discussed. An approachfor modeling the influence of amplified spontaneous emission on the transient behavior of the inversion is described. In addition, an approach to find scaling limits due to amplified spontaneous emission is briefly described and results are presented.

With the continued advancement of high power, solid-state laser technology, a thorough understanding of optical fabrication techniques and how they impact spectral performance, thermal management, and damage threshold is required. Given their very unique form factor, thin disk lasers offer particular challenges in fabrication. This paper presents a review of fabrication technologies for thin-disk laser elements. In particular, we review current technologies for polishing, assembly, thin film coating, mounting, and metrology.

The operation of a 1030-nm, single thin-disk laser, which produced 6.5 kW of laser output power with 57% slope efficiency is reported. The Yb: YAG ceramic gain element is 200 μm thick, and bonded to a l-mm thick, undoped ceramic YAG cap. The gain element is pumped by diode lasers at 940 nm. The maximum incident pump intensity was 5.2 kW/cm2, yielding an output intensity of 2.6 kW/cm2 of multi mode laser radiation. Rigrod analysis suggested that the laser operates with inhomogeneous gain saturation. Enhanced spatial hole burning in the active-mirror gain element is responsible for this effective inhomogeneous saturation. Full modulation of the intracavity intensity within the gain at the high reflector leads to poor gain extraction close to the intensity null regions and reduced effective gain length. The independence of the pump threshold and output intensities on the pump spot size indicates that the axial gain is not clamped by the transverse amplified spontaneous emission for up to a pump spot diameter of 18 mm. Observed thermal lensing contributions include thermal expansion-induced disk flexure, pump edge-induced temperature profile, and strong thermal imprint of the cooling nozzle due to the direct jet impingement on the high-reflection coated side. Weak absorption of the 1030-nm intracavity intensity in the undoped cap led to excess heating that limited the extracted intensity.

Testing of a multidisk thin-disk laser has been conducted using a unique multipass resonator that provides aberration and mode controlas well as high gain. The disk modules used were derived from commercial thin disk lasers. The laser consistently produced a power level of 28 kW, beam quality of 2.7, and an optical-to-optical efficiency of 43%. No adaptive optics are required to achieve these results. Based on these results, a modified resonator should achieve single-mode operation with optical-to-optical efficiency of 55% and beam quality of less than 2.0 without adaptive optics. Disk scaling studies have shown that amplified spontaneous emission can be controlled to a disk size consistent with 100 kW or greater power levels.