Beam dynamics measurement of pulsed infrared Nd:YAG Laser

Eduardo Cabrera et al., from the Universidad Complutense de Madrid reported from the first time dynamical transition observation in a class-B laser using a ICCD camera.

Experimental instantaneous intensity patterns

Experimental instantaneous intensity patterns

Figure 1: Experimental instantaneous intensity patterns with 3ns time resolution on a class-B Nd:YAG Laser. (3a), (3b) and (3c) with a time delay of 4µs from the beginning of the pulse. (3d) shows a calculated traverse pattern.
Figure 2: Experimental instantaneous intensity patterns with 9ns time resolution on a class-B Nd:YAG Laser. (2a) and (2 b) with a time delay of 4µs from the beginning of the pulse, (2c) 8µ s and (2d) with 40µs time delay. This figures are reprinted with permission from: E. Cabrera et al. Phys. Rev. A 73, 053820 (2006) by the American Physical Society

First time dynamical transition observation of a class-B laser.

Eduardo Cabrera et al., from the Universidad Complutense de Madrid examined the beam dynamics of a optical pumped Nd:YAG laser. They were able to measure the spatial beam profile of the pulsed Laser with a naonsecond time resolution. The spatial beam profile is a relevant information since this allows the determination of the spatial coherence and the brilliance of a measured Laser beam. Especially, as in former measurements with class B laser a large variety of spatio-temporal beam dynamics were observed. This beam dynamic can be measured with a instantaneous two-dimensional beam profile measurement. Class B laser are widely used in industrial applications, in medicine or in communications to cite just a few. Therefore, the study of its beam dynamics becomes to a worthwhile experiment. ICCD cameras providing a high speed shutter system and enable 2D beam profile measurement with nanosecond time resolution.

Beam dynamic measurement setup

The custom-made pumping geometry of the used Laser imposes an anisotropy that selects a "Cartesian symmetry" instead of a circular symmetry. The beam is reflected from a beam splitter to the intensified IR enhanced ICCD camera. The 4 Quik E high speed ICCD camera provides a minimum integration time of 1.2ns. The integration time and the delay time is remotly controlled. By scanning the delay time trough the pulse the beam dynamics of the IR can be detect. Applying the dual frame mode the ICCD camera is even capable of taking two frames per laser pulse, however, the described experiment was performed with one 2D beam profile at a Laser shot.

Highly ordered spatial profiles at the beginning

At the beginning of the Laser pulse (Fig 2) highly ordered structures apear although different kind of modes are present. In particular, lattice-like modes (Fig. 2a) and stripe-like modes (Fig. 2b and 2c) with different orientation are obtained. In Fig. 2d a more complex spatial profile was obtained with a combination of flower-like and stripe-like pattern. The presence of this great variety of profiles for the same experimental conditions seems to indicate a weak selection of the spatial symmetry.

Random beam dynamics with increasing time

As time increases the ordered structures disappear, leading to very disordered intensity profiles. Examples of this behavior can be seen in Figure 3 below. This two instantaneous beam pattern were taken with a time delay of 80µs from the beginning of the output pulse. The pattern on the left consists of a bunch of bright peaks randomly distributed over the transverse cross section, while superimposed traveling waves with different orientations can be seen on the right. These measurements show the dynamic transition of the beam profile from order to turbulence within one laser pulse. Hence, beam dynamic analysis of pulsed laser beams benefit form fastest shutter times available with ICCD cameras.

Title: Development of spatial turbulence from boundary-controlled patterns in class-B lasers
Author: Eduardo Cabrera, Oscar G. Calderón, Sonia Melle, and J. M. Guerra
Institute: Departamento de Optica, Universidad Complutense de Madrid, Spain

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