Holger Laabs

Twisting of three-dimensional Hermite—Gaussian beams

Abstract The mode converter first applied by Tamm and Weiss and generalized by Allen and co-workers can be used to transform any general astigmatic beam into a stigmatic beam with twist. For the special case of three dimensional Hermite—Gaussian beams the transformed beams are twisted Hermite—Gaussian beams with complex arguments. Only if a set of conditions is fulfilled are the transformed beams Laguerre—Gaussian beams. The influence of the input beam parameters on the output beam is discussed.

Measuring laser beam parameters: phase and spatial coherence using the Wigner function

Detailed laser beam characterization is essential for the proper choice of lasers source according to the respective application as well as for the optimization of optical systems. Since most lasers generate partially coherent beams, intensity and phase distributions are not enough to describe them. In addition the knowledge of the coherence distribution is necessary. So far different setups have been used to measure phase and coherence distribution with limited accuracy. Here we demonstrate a new measurement procedure which is based on the retrieval of the Wigner distribution, from which all relevant information can be derived. The setup is very simple and the results seems to be fairly accurate.

Symmetrization of astigmatic high power diode laser stacks

The astigmatic high power diode laser beam emitted from a two-dimensional stacked array is transformed into a beam with rotational symmetry. The symmetrization optic is a first order optical system consisting of a bifocal telescope and three rotated cylindrical lenses. The symmetrized beam has equal beam radii, far field divergence, and waist positions in x, y directions. Additionally a twist appears. The beam parameter product of the symmetrized beam is the average of the beam parameter products of the input astigmatic beam along x-, y-axes. The twist arises from the astigmatism of the original beam. The theoretical calculations are confirmed by the experiments.

Modal decomposition of partially coherent beams using the ambiguity function

Phase-space representations of optical beams such as the ambiguity function or the Wigner distribution function have recently gained considerable importance for the characterization of coherent and partially coherent beams. There is growing interest in beam properties such as the beam propagation factor and the coherence and phase information that can be extracted from these phase-space representations. A method is proposed to decompose a partially coherent beam into Hermite-Gaussian modes by using the ambiguity function. The modal weights and the possible phase relations of the Hermite-Gaussian modes are retrieved. The method can also be applied for the decomposition of the Wigner distribution function. Some examples are discussed in the scope of beam characterization.

Transformation of Hermite-Gaussian beams into complex Hermite-Gaussian and Laguerre-Gaussian beams

Hermite-Gaussian, Laguerre-Gaussian and complex Hermite- Gaussian modes are solutions of the paraxial wave equation. Using an astigmatic optical system each type of beams can be transformed into the others. This allows a generation of complex Hermite-Gaussian modes with twist whose propagation behavior is investigated in detail.

Gain-induced coupling of solid-state lasers

Abstract In this paper a theoretical model is presented which describes the coherent coupling of two laser modes inside the same crystal. The modes are pumped independently by two fibre-coupled laser diodes. The theoretical predictions of various locking effects are compared with first experimental results. Different phase-locking states and frequency locking are observed.

Diode-pumped multipath laser oscillators

Multipath resonators are well known as optical delay lines. By many zick-zack rays between two mirrors large optical path lengths can be obtained. This passive system can be converted into a laser oscillator by closed ray paths and a Nd:YAG crystal at one mirror. Optical pumping occurs by directly coupled diodes at the reflection points. Very efficient (more than 40% optical/optical) and compact systems up to 20 W, with the beam propagation factor m2 < 3 were generated. Q-switching and internal frequency doubling were also demonstrated. The fundamental physics of this system including the transverse mode-locking at the degeneration points of the resonator was investigated, experimentally and theoretically.

Beam transformation of a 500-W laser diode system

For fiber coupling and end-pumping of solid-state lasers, the astigmatic beam emitted from high power diode lasers has to be transformed into a beam with rotational symmetry. In this paper we present the result of beam transformation of a high power laser diode stack. The transformation optics consists of rotated cylindrical lenses, originally used in generating twisted beams from coherent and partially coherent fields. The astigmatic beam emitted from a 500 W 2D laser diode stack is transformed into a rotationally symmetric beam. The symmetrized beam has equal beam widths, equal beam divergences in x,y-directions and the same waist positions.

Surface-emitting laser diode beam characterization (Abstract Only)

We have conducted thorough analysis of a vertical-cavity surface-emitting laser (VCSEL) diode which produces TEM01* (donut) and higher-order modes. Our analysis includes the following quantities as a function of drive current: optical power, spectral content, relative intensity noise (RIN) up to 100 MHz, and beam characterization parameters. While this VCSEL produces higher-order modes which are not affected by optical feedback, its optical power (0.05 mW for TEM01*), long term stability, and sensitivity to collimating lens position make it a doubtful candidate for use in a beam characterization round robin. Also, we hope to present recently acquired data from the diode-pumped tunable transverse mode laser developed in Berlin and tested at NIST.