Observing mode propagation inside a laser cavity

Abstract

The mode inside a laser cavity may be understood as the interference of two counter-propagating waves, referred to as the forward and backward waves, respectively. We outline a simple experimental procedure, which does not require any additional components, to study the forward and backward propagating waves everywhere inside a laser cavity. We verify the previous theoretical-only prediction that the two fields may differ substantially in their amplitude profile, even for stable resonator systems, a result that has implications for how laser resonators are conceptualized and how the disparate traveling waves interact with nonlinear intra-cavity elements, for example, passive Qswitches and gain media.The eigenmodes, i.e. the spatial (transverse) structure of a field that reproduces after a round trip, of an empty resonator made up of two spherical mirrors depend upon the resonator symmetry. For rectangular (circular) symmetry they are the well-known Hermite-Gaussian (Laguerre-Gaussian) modes HG nm (LG pl ) [1]. The particularities of these eigenmodes are: (i) only the lowest-order modes, HG 00 and LG 00 , are characterized by a smooth intensity profile, and are described by the Gaussian function; (ii) the higher-order modes are made up of circular rings (spots) of light for the LG pl (HG mn ) family; and (iii) the higher-order modes spread

Fractal light from lasers

“…D). The output beam was captured using a CCD camera (Spiricon SPU260 BeamGage), placed in an image plane of S. Note that the field depends on the propagation direction (even in stable canonical resonators [28]), and the camera has been placed to record the image in the and M2 (radius of curvature R2, focal length F = R2/2), and an output coupler (OC) angled at 45 • with a 99.8% reflectivity. The geometrical length of the cavity was G, that of the Nd:YAG gain medium was g. S is the magnified self-conjugate plane.…”

“…D). The output beam was captured using a CCD camera (Spiricon SPU260 BeamGage), placed in an image plane of S. Note that the field depends on the propagation direction (even in stable canonical resonators [28]), and the camera has been placed to record the image in the magnified self-conjugate plane, S, not the de-magnified self-conjugate plane, s, which corresponds to the same plane but the opposite propagation direction.…”

A novel laser resonator for fractal modes

“…Both exhibiting the same pattern structure, but different magnifications: one version is built up from a successive magnification of the original pattern of the aperture while the second is built up from a de-magnification of the original aperture pattern. 12 The self-similarity pattern version depends mainly on the beam propagation direction inside the laser cavity. For example, if the beam inside the cavity propagates from plane S to M 1 , M 1 to M 2 ,and M 2 to plane S, the fractal pattern is magnified by a factor M (= R 2 /R 1 ).…”