Lasers have been known for a long time and are used in a wide variety of fields such as industrial and material processing or measuring and control technology. A new application is being tested which aims to use continuous wave UV-lasers in metrology. For this application a nonlinear optical crystal is needed. Its processing is developed in a two-year project at the Institute for Precision Manufacturing and High-Frequency Technology of Deggendorf Institute of Technology. The crucial factor for the full optical performance in the UV range is the low roughness of the crystal surface, as it is installed between two prisms and the contactability between them should be ensured. In China, a nonlinear crystal that meets the requirements has already been designed and a production process for the raw crystal has been established. However, since the production of optically homogenous crystals has proven to be difficult, the availability of such is very limited. For this reason, a reference material with similar hardness and material behaviour is used in the process development in order not to be limited in the number of trials. It is important to be able to transfer the results from the reference material in an analogous way to the original crystal. One challenge of the project lies in the crystal thickness, since only a maximum thickness of three millimetres can be achieved for the purest form of the crystal required in the application. Therefore, it is important to handle the material sparingly during the process. In addition, the small dimensions of about ten to five millimetres and the brittleness of the material pose a problem. The goal of the project will be to develop a process that can circumvent all these problems so that small roughness of the crystal can be achieved by precision polishing.
A novel laser system with optical parameters that fill the gap between Q-switched and modelocked lasers has been developed. It consists of a high gain hybrid fiber-bulk amplifier seeded by a low power SESAM Q-switched oscillator. The mW level output power of the seed oscillator is preamplified by a single mode fiber which is limited by SRS effects. The final amplification stage is realized by a longitudinal pumped Nd:YVO4 crystal in a double pass setup. This MOPA configuration delivers sub-300ps pulses at repetition rates up to 1 MHz with an output power exceeding 60W. Nonlinear frequency conversion to 532nm and 355nm is achieved with efficiencies of >75% and >45%, respectively. Due to the high peak power, high repetition rate and high beam quality of this system, applications formerly only addressable at lower pulse repetition frequencies or with complex modelocked laser systems are now possible with high speed and lower cost of ownership. Application results that take benefit from these new laser parameters will be shown. Furthermore, the reduction of the pulse duration to sub-100ps and power scaling to output powers <100W by the use of the Innoslab concept are being presented.
The different concepts of combining fiber lasers for power-scaling are discussed. We report on three combined fibers with an output power of 100 W. Several proposals are made for further power scaling and the capacitance of a grating is tested in a simulation-experiment.
A Q-switched all-fiber laser application based on a novel micro-optical waveguide (MOW) on micro-actuating platform (MAP) light modulator is presented. A fused biconical taper (FBT) coupler acts as MOW, mounted on an electromechanical system, MAP, where an axial stress over the waist of FBT coupler is precisely controlled. The axial stress induced refractive index changes caused the coupling efficiency to result in modulation of optical power. The light modulator was implemented in a laser cavity as a Q-switching element. Q-switching of Yb3+-doped fiber laser was successfully achieved with the peak power of 192mW at 4.1W pump power and 699mW at 5.2W at the repetition rate of 18.6kHz. Further optimization of switching speed and modulation depth could improve the pulse extraction efficiency and the proposed structure can be readily applied in all-fiber Q-switching laser systems for marking applications.
In the last years a dramatic increase of the output power of rare-earth-doped fiber lasers and amplifiers with diffraction limited beam quality has been observed. These demonstrates impressively that fiber lasers and amplifiers are an attractive and power scalable solid-state laser concept. The main limiting factors for the laser output power are the damage of the fiber ends, heating of the fiber due to the quantum defect and nonlinear effects. To overcome these problems, an increasing of the core diameter and keeping the fiber single mode, by using solid core step-index large-mode-area fibers, allow the power scaling beyond 1 kW at diffraction limited beam quality. A further scaling is possible by using novel highly doped air-clad photonic crystal fibers with increased mode field diameters of the active core. This type of fibers has several new preferable features. In our contribution we will discuss the advantages of microstructured fibers to reduce nonlinear effects inside the fiber and the possibility to scale the output power of fiber lasers and amplifiers with excellent beam quality. We also show experiments with pulsed fiber amplifier systems using these microstructured large mode area fibers.
Experimental results based on rare-earth-doped fibers have impressively shown that fiber lasers and amplifiers are an attractive and power scalable solid-state laser concept. Based on ytterbium-doped large-mode-area double-clad fibers, in the continuous regime, output powers approaching the kW-range with diffraction limited beam quality have been shown. Average output powers in the order of 100 W have been demonstrated in the pulsed regime even for femtosecond fiber lasers. Further power scaling is limited by the end facets damage, thermo-optical problems or nonlinear effects. To overcome these restrictions microstructured fibers with several new preferable features can be used. In our contribution we will discuss power scaling of fiber lasers and amplifiers in the multi kW-range with excellent beam quality based on rare-earth-doped photonic crystal fibers.