For pulse laser materials processing often optical step index and gradient index multimode fibers with core diameters
ranging from 100 to 600 μm are used. The design of a high power fiber transmission system must take into account
limitations resulting from both surface and volume damage effects. Especially, breakdown at the fiber end faces and selffocusing
in the fiber volume critically influence the fiber performance. At least operation charts are desirable to select
the appropriate fiber type for given laser parameters.
In industry-relevant studies the influence of fiber core diameter and end face preparation on laser-induced (surface)
damage thresholds (LIDT) was investigated for frequently used all-silica fiber types (manufacturer LEONI).
Experiments on preform material (initial fiber material) and compact specimens (models of the cladding and coating
material) accompanied the tests performed in accordance with the relevant LIDT standards ISO 21254-1 and ISO
21254-2 for 1-on-1 and S-on-1 irradiation conditions, respectively. The relation beam diameter vs. LIDT was
investigated for fused silica fibers. Additionally, laser-induced (bulk) damage thresholds of fused silica preform material
F300 (manufacturer Heraeus) in dependence on external mechanical stress simulating fiber bending were measured. All
experiments were performed with 10-ns laser pulses at 1064 and 532 nm wavelength with a Gaussian beam profile.
The complete knowledge and description of light sources is a fundamental base of optical design. Partially coherent, paraxial light sources of homogeneous polarisation state are represented by the four-dimensional Wigner distribution function, which contains all information on amplitude, phase relations and spatial coherence. Hence, knowledge of the Wigner distribution enables the prediction of power density distributions after propagation through a wide range of optical systems by numerical simulation. A simple optical setup consisting of a spherical lens, a cylindrical lens, and a CCD camera can be used to experimentally retrieve the Wigner distribution function of a spatially confined light source by a tomographic reconstruction scheme. This paper briefly introduces the Wigner distribution and outline the reconstruction scheme. Measurements on partially coherent laser beams are presented including examples of successful predictions of power density distributions behind some optical systems.
Coherence properties of real laser beams can be crucial for many applications, e.g. in lithographic processes or for Bragg grating writing. Knowledge of the coherence distribution together with the amplitude and phase distribution allows for a complete beam characterization, which enables the numerical simulation of beam propagation through virtually any relevant optical system. Classical measurement methods of coherence properties are Young’s double hole interferometry and Shear interferometry. But due to their interferometric nature experimental realization of both methods is quite difficult and obtainable accuracies are usually not satisfying. The reconstruction of the Wigner distribution from a couple of measured intensity distributions in the waist region of a beam provides a simple measurement setup delivering fairly accurate results. This is demonstrated by an experimental comparison of these three methods.
The efficiency of high-average-power frequency doubling is always limited by thermal effects inside the nonlinear crystal. Usually it is assumed that the limitation is caused by a change of the index of refraction due to the temperature variation inside the crystal. This effect is characterized by the temperature acceptance of the nonlinear crystal. Additionally, the temperature distribution inside the crystal produces thermally induced mechanical stress that leads to phase mismatch due to the photoelastic effect. Depending on the sign, this effect will decrease or increase the conversion efficiency. In order to investigate the influence of stress- induced phase mismatch independently of phase mismatch caused by a temperature variation on the conversion efficiency, KTP (Type II) and LBO (Type I) crystals for second-harmonic generation (SHG) of 1064 nm radiation were temperature- stabilized and mechanical stress was applied along different crystal axes. The conversion efficiency of a weak probe beam was measured as a function of the stress.
Up to 0.75 Watt of blue laser light have been generated by second harmonic generation and additional sum frequency mixing of a 1.3 micrometer actively cw-modelocked Nd:YAG laser oscillator. Special attention has been paid to the design of the fundamental mode resonators, delivering output powers of up to 8 W at the fundamental wavelength with nearly diffraction limited beam quality. To overcome the thermally induced depolarizing birefringence a compensation scheme has been applied for further enhancement of the laser power.
Transversely cooled, cylindrical Nd:YAG laser rods exhibit a strong thermally induced birefringence when they are optically pumped. The birefringence leads to different refractive indices for radially and tangentially polarized light which affects the beam-quality and the degree of polarization achieved with conventional optical resonators. In spite of this drawbacks the traditional rod-geometry for Nd:YAG lasers is still of great interest because of its technological simplicity, comparatively low costs and acceptable efficiency. In the present study the limitations for the output power of single rod Nd:YAG lasers in TEM00- operation is investigated theoretically and experimentally. Furthermore the possibility of improving the brightness of Nd:YAG rod lasers beyond the traditional limits using dual rod systems with birefringence compensation is demonstrated. Birefringence compensation is established by image-relaying between two identical Nd:YAG laser rods and an additional 90 degree(s) polarization rotation. This compensation method requires specific dynamically stable optical resonators in order to achieve optimal compensation and high efficiencies. With a dual rod Nd:YAG-system up to 80 Watt TEM00 output power is achieved with a beam quality of M2 equals 2.8. The output power is approximately 8 times higher than using a single rod-system separately.
The absorption coefficients of nonlinear crystals for fundamental and second harmonic wave are of great importance for high average power second harmonic generation. A practical method to measure low absorption coefficients for high average power second harmonic generation. A practical method to measure low absorption coefficients is to use an interferometric laser calorimeter with high power lasers. Therefore Q-switched Nd:YAG laser systems with intracavity second harmonic generation are used. The measurements are made with optical powers up to 300 W and 45 W, respectively. Because of the high power, the resolution limit for the absorption coefficients is 0.001 percent/cm. The absorption coefficients of KTP and LBO crystals of different manufacturers are determined. The results are used for a numerical model which takes into account the decrease of conversion efficiency due to thermal effects caused by the absorption of laser power in the nonlinear crystal. This model describes saturation effects which appear in the range of 100 W in the green using a KTP crystal. A new idea for compensation of thermal effects will be presented.
The influence of thermally induced phase-mismatch in nonlinear crystals for frequency doubling caused by absorption of laser power is described. A numerical model is developed, which considers the spatial temperature distribution in the crystal and the corresponding wave-vector mismatch. For the temperature profile an approximate analytical expression is derived from the the heat-transfer equation in cylindrical symmetry. The conversion efficiency is calculated by solving the basic differential equations for frequency doubling with a spatial dependent wave-vector mismatch. Because the absorption coefficients are rather different at the fundamental and second harmonic wavelength, the heat-density in the crystal depends on the conversion efficiency and vice versa. Therefore an iteration method has to be used to calculate self-consistent solutions. Experimentally a Q- switched oscillator-amplifier-system with an average power of 200 W at 1064 nm is frequency doubled to an output of 103 W at 532 nm using a KTP crystal. The numerical calculations are in good agreement with the experimental results.
High average powers at 532 nm are of great interest for material processing applications because the absorption of metals and other materials increases with shorter wavelength. As the fundamental laser source we developed a Q-switched Nd:YAG oscillator-amplifier system with an average output power of 215 watts and a beam parameter product of less than 1 mm*mrad. The system operates with 100 Hz repetition rate and emits a burst of twenty Q-switch pulses during each flash-lamp pulse. The pulse duration of a single pulse increases from 100 to 200 nanoseconds with increasing pumping power. The high beam quality of the amplified beam is realized by a double-pass configuration of the amplifier with a phase conjugating SBS-cell. Additionally the thermally induced birefringence of the amplifiers is compensated for by a 90 degree-rotator between two identical amplifier rods. An output power of 164 W is frequency doubled in a KTP-crystal to 82 Watt average output power in the green which corresponds to 50% of doubling efficiency.
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