Wilfried Plass

High-resolution knife-edge laser beam profiling

Abstract A knife-edge method for profiling focused and unfocused laser beams with a high spatial resolution is presented. High resolution is achieved by dithering the knife-edge in the scan direction. The method is equivalent to the slit method but with a variable slit width also on a sub-μm scale. A signal-to-noise ratio of 10 6 : 1 has been demonstrated. The design is nearly as simple and of low-cost as that of the conventional knife-edge method. The inversion algorithm used for obtaining the beam profiles from knife-edge data is discussed. An excellent agreement is found between measured and calculated beam profiles. The accuracy of this method is demonstrated also with beam propagation measurements.

Simple high-precision method for measuring the specular reflectance of optical components.

We present a simple method to determine precisely the specular reflectance of optical components. The absence of transmissive elements in this method makes a wide spectral range available. High accuracy and precision are achieved with a fast, periodic change between the reference beam and the probe beam. Special efforts were made to eliminate inhomogeneities of beam intensity and detector sensitivity. With our experimental setup we obtain a precision of ±3 × 10(-4) at the wavelength of 10.6 µm and ±3 × 10(-5) at 1.06 µm for a single-bounce-measuring setup.

Calibrated CO2-laser power calorimeter

A continuously measuring carbon-dioxide laser power calorimeter designed for laser powers up to 25 kW is presented. The absorption of the detector is shown to be 99.98% at 10.6 micrometer and 99.1% at 1.06 micrometer. The flow-rate of the coolant is measured by determining the temperature increase due to an equivalent electrical heating of the coolant before the detector. An electrical calibration proves an accuracy of plus or minus 1% in the range from 40 W to 4 kW. A comparison with an instrument calibrated at the Physikalisch Technische Bundesanstalt (PTB) reveals a deviation of less than 0.5% at 850 W. The online-precision is limited by a relative and an absolute noise signal of 0.5% and 5 W, respectively. Methods to reduce the measurement errors occurring in the calorimetric design are presented.

Investigation of beam pointing stability, far-field divergence angle and power density distribution of a high-power CO2 laser

A high power carbon-dioxide laser beam, sampled by a diffraction grating, is monitored simultaneously by a Hartmann sensor to measure the wavefront and by a pyroelectric matrix camera to obtain the far field distribution. The maximum sampling frequency of the Hartmann sensor is 1 kHz with a resolution of 10 (mu) rad for the measurement of the beam direction. The data of the pyro- camera can be analyzed with a maximum frequency of 5 Hz and the beam direction in the far field can be measured with an angular resolution of 50 (mu) rad. Both methods reveal a periodic change of the beam direction with an amplitude of nearly 50 (mu) rad and a periodic change of the beam divergence with an amplitude of about 150 (mu) rad. Far field profiles measured with the matrix camera show that the angular stability is limited by fluctuations of the power density distribution. These changes of the transverse beam profile are closely connected to fluctuations of the laser gas pressure, which are due to the gas pressure regulation system.

Environmental stability of CO2 laser optics

Transmissive and reflective optics for high power CO2 lasers were exposed to define relative humidities and temperatures. Degradation effects were investigated by means of absorptance, reflectance, laser-induced damage threshold measurements and microscopic inspection. The LIDT measurements were performed in the short and long-pulse regime involving different test routines as 1 on 1, S on 1 and R on 1. For distinct coatings, microscopic investigation reveal a size-increase of the nodular defects. This increase results in a reduction of the defect induced LIDT, while the absorptance and reflectance remain unchanged. Alkali-halide components show an intensity-dependent laser-induced conditioning effect. This effect is analyzed with respect to different environmental parameters.

Round-robin test on optical absorption at 10.6 μm

The approved Draft International Standard 11551 on test methods for absorptance of optical laser components recently passed the international voting procedure. The utility of this standard practice document is the subject of the present round robin test. In order to cover a broad range of CO2-laser optical components, different types of metal mirrors and transmissive ZnSe-optics were included in the master sample set. After an initial inspection, this set passed through a series of optical laboratories in the United States, the United Kingdom, and Germany. The absorption of the samples was measured by calorimetric methods according to ISO DIS 11551, and the measurement results were compiled by the coordinating institute. The evaluation of the experiment was not started until all tests were completed. The results of the round robin test are discussed, compared and evaluated with respect to ageing mechanisms in optical coatings for CO2-laser systems. Although a great variety of different test facilities was employed by the round robin partners, a good agrement of the absorption values was observed for the wavelength of 10.6 micrometers . This demonstrates the versatility of the approved Draft International Standard 11551 for the calorimetric measurement of optical absorption in CO2-laser components.

Stress and defect damage of CO2 laser optics: time of damage analysis

Laser damage experiments in the long-pulse regime show for coated CO2-laser optics two distinct damage mechanisms. These damage mechanisms can be distinguished by their time of damage behavior. The time of damage is measured with a previously presented setup, which was refined to improve the accuracy of the measurements. The two mechanisms are interpreted as induced by either defects or stresses in the coating. For the defect induced case, damage occurs at or before the peak fluence of the laser pulse, while for the stress induced case, the damage is observed laser. For both mechanisms, analytical transient heat flow calculations are discussed. While for the defect induced damage, a good thermal contact of the small defects to the host explains the observed behavior, for the stress induced damage a 1D heat-flow model of a film-substrate system is used taking into account for actual temporal profile of the laser pulse. The experimental data of coated metal mirrors as well as ZnSe- and Germanium optics are very well described by this model even if bulk material parameters are used for the film. Some samples show both damage mechanisms. In this case the defect induced LIDT is lower than the stress induced one. The separation of these mechanisms is useful for an effective improvement of optical coatings.

S on 1 laser damage measurements on CO2-laser optics: defect- and stress-induced damage

Two different damage mechanisms are observed in laser damage experiments in the long-pulse regime for coated metal mirrors, partial reflectors and transmissive windows. These damage mechanisms can be distinguished by their time of damage behavior and are interpreted as induced by either defects or stresses in the coating. For both mechanisms, analytical calculations of the temperature evolution show a good agreement with the experimental data. Some samples show both damage mechanisms. The separation of these mechanisms by the time of damage analysis is used to study degradation effects due to an S on 1 irradiation. The experiments show that the defect induced damage threshold is in most cases independent of S, while the threshold of the stress induced damage decreases in a first approximation logarithmically with S. The integration of the Nomarski-microscope in the experimental setup allows for an effective evaluation of the experimental data according to the damage probability method of the ISO-standard ISO-DIS 11254-2. Microscopic inspection reveals noncumulative damage phenomena during S on 1 testing, which may prohibit the use of non-microscopic online detection techniques.

Evaluation of a round-robin test on optical absorption at 10.6 um

High quality optical components are of major importance for the development of new concepts and applications of carbon- dioxide lasers. Especially, as a consequence of the ever increasing output power of new laser systems, optical absorption has become a crucial topic for the production and commercialization of laser components. Therefore, activities for the standardization of optical absorption measurements were initiated in ISO/TC 172/SC 9/WG 6, which is dedicated to optics and optical instruments in lasers and laser related equipment. Recently, the corresponding standard document ISO/DIS 11551 on test methods for absorptance of optical laser components has passed the international voting procedure. For an evaluation of this standard practice, which is based on laser calorimetry, a round-robin test on optical absorption at 10.6 micrometer was started. In order to cover a broad range of carbon-dioxide laser optical components, different types of metal mirrors and transmissive ZnSe-optics were selected for the sample set, which was circulated between manufacturers and optical laboratories in the United States, the United Kingdom and Germany. This paper reports on the results of this experiment in respect to the new standard procedure described in ISO/DIS 11551. The contributions of the laboratories, which are based on a variety of different calorimetric measurement facilities, are compared and evaluated.

Temperature dependence of reflectance and transmittance of CO2 laser optics

The temperature dependence of reflectance and transmittance of commonly used CO2 laseroptics is investigated and presented. The knowledge of this dependence is not only important for a better understanding of damage mechanisms in the regime of long pulse and cw-operation, it may also be useful for engineering aspects of high power lasers. The measurements are performed for highly reflective and transmissive optics with an accuracy of +/- 2qq10-4 and a resolution better than 50ppm. The investigations are done in air to take into account the effects of the environment. Due to convection cooling, the maximum temperature is restricted to 200 degree(s) C. One observation is that oxidation of copper mirrors does not necessarily reduce the relfectance at 10.6 micrometers . Aluminum and silicon mirrors, ZnSe windows, and reflectors and germanium windows with different multilayer coatings are also investigated. The results for the uncoated metal mirrors are compared to the Drude theory.