H. Weber

Analysis of saturable absorbers, interacting with gaussian pulses

Abstract Mode-locked lasers, Q-switch systems and discrimination amplifiers make use of saturable absorbers. The interaction of those absorbers with light pulses is well known, either for very long or very short pulses, but not, if the pulse width is comparable with the absorber life time, which occurs very often in experiments. Moreover, the spatial structure of the pulse is mostly not considered. In this paper, we investigate in detail the interaction of gaussian pulses (spatial and temporal) with the two-level-absorber. Energy transmission, pulse shortening and pulse asymmetry are calculated as functions of pulse width and small signal transmission of the absorber.

Chaotic and periodic emission of high power solid state lasers

Abstract Nonlinear, dynamical systems even with a few degrees of freedom may show chaotic or periodic behaviour, depending on the parameters of the system. Recently it was demonstrated, both experimentally and theoretically, that the temporal emission of a laser can become chaotic, if several longitudinal modes oscillate [Brunner and Paul (1983), and Abraham et al. (1982)]. The chaotic emission is caused by the nonlinear interaction of the modes and the longitudinal gain structure [Komtomtseva et al. (1982)]. In this paper it is pointed out that the transverse mode structure and the radial gain profile produced by the transversal modes, may give rise to temporal instabilities of the laser emission. If the relevant parameters of the laser oscillator — Fresnel number, resonator losses, pump rate — exceed certain critical values, the output intensity becomes unstable. The damped relaxation oscillation changes into undamped periodic oscillation or, with increasing values of the above parameters, into chaotic emission. The theory, using the nonlinear Kirchhoff-Fresnel integral equation and the rate equation approach, is confirmed by experimental results.

Effective cross section of the Nd:YAG 1.0641-μm laser transition

The cross section of a laser transition is one of the most important parameters, because via the saturation intensity, Is=hν/(στ), the output power depends on it. For the Nd:YAG 1.0641‐μm laser transition, the published values vary between 4×10−19 and 8.8×10−19 cm2, as obtained by spectroscopic methods. This paper deals with a new method, which determines σ from the absolute value of the output power, the small‐signal gain, and the losses. Moreover, the extraction efficiency is evaluated, taking into account the mode structure.

Optimal pinhole radius for fundamental mode operation

In order to prevent higher-order transversal mode oscillation, a pinhole has to be inserted into the resonator. The pinhole radius must be adapted to the sizes of the fundamental gaussian beam. This paper deals with the optimal pinhole radius, which minimizes diffraction fringes and delivers the smallest far field divergence. Different laser systems were investigated experimentally. The range of the optimal value s = a/w, with a = pinhole radius and w = gaussian beam radius, is 1 < sopt < 1.5. It is shown, that the optimal value depends on the unsaturated gain G0 of the oscillator.

Focusing and imaging of laser beams

Abstract Imaging and focusing of laser beams has to be distinguished. It becomes important if non-gaussian beams (modes of unstable resonators, or superposition of several transversal modes) are considered. If the focus of a laser beam produced by a single lens is defined as the spot of minimum dimensions its position depends on the mode structure. By telescopic systems a focus position independent of the mode structure can be produced.

Misalignment Sensitivity of Unstable Resonators with Spherical Mirrors

Abstract The effects of misaligning the spherical mirrors of unstable resonators on the losses, and the near and far field have been investigated both theoretically and experimentally. The theoretical description is based on the Kirchhoff integral equation of the equivalent symmetric resonator with shifted integration limits, which was solved numerically for G-parameters smaller than 2·5 and equivalent Fresnel numbers in the range up to three. Experiments were performed with a pulsed Nd-YAG system. Intensity distributions in the near and far field were recorded with an image-processing system and the losses were measured absolutely by using a previously developed method for loss measurement. Furthermore, a comparison of the misalignment sensitivity of stable and unstable resonators indicates that the latter are promising candidates for resonators, even in solid-state lasers.

Losses and mode structure of unstable resonators with spherical mirrors

Losses and near and far field intensity distributions of unstable resonators with equivalent Fresnel numbers in the range up to three have been investigated both experimentally and theoretically. The losses were determined absolutely using a new method for loss measurement. Intensity distributions of lowest loss modes were recorded by means of an image‐processing system and compared to numerical solutions of Kirchhoff ’s integral equation of the empty cavity. The experiments were performed with a pulsed Nd‐YAG system.

Average refractive powers of an alexandrite laser rod

Abstract The average refractive powers (average inverse focal lengths) of the thermal produced by an alexandrite laser rod optically pumped at repetition rates between 0.4 and 10 Hz and with electrical flashlamp input pulse energies up to 500 J have been measured. The measuring setup is described and the measurement results are discussed.

Statistics of mode-locking threshold

Abstract Due to the statistics of spontaneous emission the second threshold of passively mode-locked lasers does not have a well-defined value, but it is probabilistic nature. A theoretical estimation was made byNew. In this paper, the probability of the second threshold and its dependence on the parameters of the laser system (absorber transmission, pump energy, resonator round-trip time, bandwidth, cross section) is investigated in detail.

Surface heat transfer coefficient, heat efficiency, and temperature of pulsed solid‐state lasers

The temperature of solid‐state lasers is a critical parameter. Efficiency and output power are strongly influenced by it. The two parameters which determine the temperature are the heat generation efficiency (HGE) and the surface heat transfer coefficient (SHTC) of the laser rod. These parameters allow the scaling of the rod temperature up to high pumping powers. Moreover, from the temperature inside the rod, the temperature gradients and the mechanical stress can be evaluated. Using transient temperature measurements, the SHTC and the HGE were determined for air‐ and water‐cooled Nd:YAG and alexandrite lasers. The SHTC can be confirmed by theoretical considerations.