Klaus Wittig

Scalable concept for diode-pumped high-power solid-state lasers

A new, scalable concept for diode-pumped high-power solid-state lasers is presented. The basic idea of our approach is a very thin laser crystal disc with one face mounted on a heat sink. This allows very high pump power densities without high temperature rises within the crystal. Together with a flat-top pump-beam profile this geometry leads to an almost homogeneous and one-dimensional heat flux perpendicular to the surface. This design dramatically reduces thermal distortions compared to conventional cooling schemes and is particularly suited for quasi-three-level systems which need high pump power densities. Starting from the results obtained with a Ti:Sapphire-pumped Yb:YAG laser at various temperatures, the design was proved by operating a diode-pumped Yb:YAG laser with an output power of 4.4 W and a maximum slope efficiency of 68%. From these first results we predict an exctracted cw power of 100 W at 300 K (140 W at 200 K) with high beam quality from a single longitudinally pumped Yb: YAG crystal with an active volume of 2 mm3. Compact diode-pumped solid-state lasers in the kilowatt range seem to be possible by increasing the pump-beam diameter and/or by using several crystal discs.

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.

Efficient high-power-diode-pumped thin-disk Yb:YAG-laser

Our new, scalable disk laser concept allows efficient diode-pumped high-power operation of quasi-three-level lasers with high beam quality. The use of a very thin crystal disk with one face mounted on a heat sink together with the low heat generation of Yb:YAG (< 11%) enables high pump power densities (> 5 kW/cm2) required for efficient operation without strong thermal degradation or optical distortion. Compact diode-pumped solid-state lasers in the kilowatt range seem to be achievable by increasing the pump beam diameter and/or by using several crystal disks. We present our latest results, obtained with a (phi) 2 mm X 0.4 mm Yb:YAG crystal operated at temperatures between 200 K and 300 K with an output power of up to 13.5 W and more than 50% optical-to-optical efficiency at 9.5 W. Simulations of thermal deformation and stress as well as of efficiency are also presented. Additionally, recent measurements of the tuning range of the diode-pumped Yb:YAG-laser, indicting the potential of femtosecond pulse generation, are shown.

Coherent fiber coupling of laser diodes

Laser diodes with diffraction-limited beam quality offer high power densities of the order of 107 - 108 W/cm2, but are limited in output power to some watts. Scaling to higher powers has to be realized by superposition of a number of laser diodes. Coherent superposition allows us to further increase the power density in the far field. This is realized by injection locking of three slave laser diodes (Toshiba TOLD) 9140, 20 mW, 690 nm) by one master laser diode (TOLD 9140) and superpositioning of the three slaves lasers by a lens array. The feedback of the slaves into the master is suppressed by two Faraday isolators. For superpositioning the light of the slaves while maintaining the high beam quality, the light of each diode is coupled into an optical single-mode fiber. Phase shifts due to mechanical or thermal disturbances of the single-mode fibers for frequencies up to 1 kHz are compensated by a single-mode optical fiber piezoceramic phase modulator and an electronic control circuit. A phase stability with a maximum phase error smaller than 6 degrees is kept over an hour. The power-density distribution in the focal plane of the focusing lens shows a peak power 2.6 times that of the incoherent superposition and a modulation corresponding to the Fourier transform of the nearfield distribution of the lens array.

Algebraic approach to characterizing paraxial optical systems

The paraxial propagation formalism for ABCD systems is reviewed and written in terms of quantum mechanics. This formalism shows that the propagation based on the Collins integral can be generalized so that, in addition, the problem of beam quality degradation that is due to aberrations can be treated in a natural way. Moreover, because this formalism is well elaborated and reduces the problem of propagation to simple algebraic calculations, it seems to be less complicated than other approaches. This can be demonstrated with an easy and unitary derivation of several results, which were obtained with different approaches, in each case matched to the specific problem. It is first shown how the canonical decomposition of arbitrary (also complex) ABCD matrices introduced by Siegman [Lasers, 2nd ed. (Oxford U. Press, London, 1986)] can be used to establish the group structure of geometric optics on the space of optical wave functions. This result is then used to derive the propagation law for arbitrary moments in eneral ABCD systems. Finally a proper generalization to nonparaxial propagation operators that allows us to treat arbitrary aberration effects with respect to their influence on beam quality degradation is presented.

Techniques for measuring the moments of laser beam irradiance distributions

Methods for determining the position and diameter of a laser beam are discussed. Of particular interest is a method that makes use of optical filters having transmittance that varies with one or more spatial coordinates. With the appropriate distribution of transmittance, a simple measurement of the fractional power transmitted by a particular filter can, after proper scaling, directly give beam position and beam diameter.

Fiber coupling of single-mode laser diodes with power-density conservation

Laser diodes with diffraction limited beam quality offer high power densities of the order of iO - 108 W/cm2, but are limited in output power to some Watts. Scaling to higher powers without using a solidstate laser converter has to be realized by incoherent superposition of the outputs of a number of laser diodes. For that the radiation of single-mode laser diodes is coupled into single-mode fibers which at the other end are shaped into a bundle of hexagonal symmetry. The radiation leaving the fiber bundle is collimated with an array of achromats and focused with an additional lens onto the target. With 19 fibercoupled 690-nm diodes (TOLD 915 1, 20 mW at the fiber end) a total cw power of 338 mW in a spot of 19.4 jim diameter (at I/Is = l/e2of a nearly Gaussian cross section) was achieved. The peak power density was 263 kW/cm2, which is approximately 1 .7 times that of a single fiber. Optimizing the filling factor should further increase the power density. Keywords: single-mode lasers diodes, single-mode fibers, scalable system. incoherent coupling

Die algebraische Beschreibung paraxialer optischer Systeme

Um die Hamiltonsche Formulierung der geometrischen Optik zum formalen Ausgangspunkt einer algebraischen bzw. quantenmechanischen Beschreibung der paraxialen Wellenoptik zu nehmen, bedarf es zunachst der Erlauterung des Zusammenhangs zwischen der symplektischen Symmetrie und den als Poisson-Klammern bezeichneten Objekten der klassischen Mechanik, die bei der Quantisierung in Kommutatoren zwischen Operatoren ubergehen.

Experimentelle Verfahren zur Bestimmung statistischer Momente

Nachdem in den letzten Kapiteln die theoretischen Grundlagen zur Charakterisierung von Laserstrahlung mittels statistischer Momente gelegt wurden, sollen in diesem Kapitel experimentelle Verfahren zur direkten Messung solcher Momente diskutiert werden.

Numerische Umsetzung des algebraischen Formalismus der wellenoptischen Propagation

In diesem abschliesenden Kapitel wird die im ersten Teil der Arbeit entwickelte quantenmechanische Interpretation der paraxialen Propagationstheorie konkret numerisch umgesetzt, um fur praktische Anwendungszwecke Berechnungsmethoden zur Verfugung zu haben. Es wird sich zeigen, das die numerischen Verfahren sehr schnell sind, was im Hinblick auf eine Rechenzeitersparnis bedeutsam ist. Des weiteren wird sich die Berechnung beliebiger Mischmomente, wie sie in Kapitel 4.1 Gl.(176) eingefuhrt wurden, fast trivial gestalten, da sich samtliche Integrationen durch Skalarproduktbildung von Vektoren beschreiben lassen. Eine numerisch bequeme Handhabung statistischer Momente ist, wie in Kapitel 5 und 6 deutlich wurde, ein wichtiges Werkzeug bei der Laserstrahldiagnostik.