Generation of custom modes in a Nd:YAG laser with a semipassive bimorph adaptive mirror

Abstract

Custom modes at a wavelength of 1064 nm were generated with a deformable mirror. The required surface deformations of the adaptive mirror were calculated with the Collins integral written in a matrix formalism. The appropriate size and shape of the actuators as well as the needed stroke were determined to ensure that the surface of the controllable mirror matches the phase front of the custom modes. A semipassive bimorph adaptive mirror with five concentric ring-shaped actuators and one defocus actuator was manufactured and characterised. The surface deformation was modelled with the response functions of the adaptive mirror in terms of an expansion with Zernike polynomials. In the experiments the Nd:YAG laser crystal was quasi-CW pumped to avoid thermally induced distortions of the phase front. The adaptive mirror allows to switch between a super-Gaussian mode, a doughnut mode, a Hermite-Gaussian fundamental beam, multi-mode operation or no oscillation in real time during laser operation. For every application there is an ideal intensity distribution which yields optimum results. Such intensity distributions of the beam are commonly called custom modes because their spatial shape is optimised for a specific purpose. Either the special shape of the beam is advantageous for the generation of the laser radiation inside the cavity or for the laser induced process on a target. Due to their high potential in numerous applications we discuss the super-Gaussian mode (also known as top-hat or flat-top mode) and the doughnut mode as two examples of custom modes in the following.The super-Gaussian mode is characterised by a homogeneous intensity distribution and steep edges whereas the doughnut mode exhibits a ring shape with vanishing intensity in the centre of the beam. The intensity distributions of the super-Gaussian mode I SG (r) and the doughnut mode

Brightness enhancement in a solid-state laser by mode transformation

“…Intra-cavity approaches, however, have seen much interest over a number of years [10][11][12] and are highly attractive for improving laser brightness, as they potentially offer better pump-to-mode overlap, thus enabling higher energy extraction with an improved beam quality at the output. There are several design configurations available that consider either low or higher order mode selection [13][14][15][16][17][18][19], and particular to the selection of flattop beams (FTBs) for increased energy extraction and single-mode operation, there are several phase-only approaches that include the use of diffractive mirrors [20], graded-phase mirrors [21,22], diffractive elements [23], and intra-cavity deformable mirrors [24][25][26][27][28][29]. Other approaches include an intra-cavity amplitude filter [30], manipulation of the gain profile [31], an intra-cavity variable reflectivity mirror [32], and employing optical feedback in a microchip laser [33].…”

Structured Light from Lasers

“…External approaches using SLMs and DMDs have dynamic control built into them, reaching speeds of several kHz. It has long been possible to achieve dynamic control inside lasers using adaptive mirrors, first demonstrated for active transverse mode control inside a solid‐state laser, but the limited dynamic range and the lack of a reference wavefront has limited applications for the direct creation of structured light, with only a few studies on creating structured light fields (mostly super‐Gaussian beams) . Instead, intracavity adaptive optical solutions have primarily focused on dynamic phase control for beam quality and brightness optimization .…”

Systematic engineering of paraxial coherent fields with spherical optical systems

“…This deviation is most pronounced between φ = 10 • − 30 • . Intuitively beam-shaping (like e.g., in [8]) is expected to be done the best in this region, since the phase-front distortion is strongest. A physically more meaningful crucial parameter is the energy theoretically needed to superpose to the field to reach a phase conjugation…”