Gaussian mode families, including Laguerre-Gaussian, Hermite-Gaussian and generalized Hermite-Laguerre-Gaussian beams, can be described via a geometric optics construction. Ray families crossing the focal plane are represented as one-parameter families of ellipses, parametrized by curves on an analog Poincaré sphere for rays. We derive the optical path length weighting the rays, and find it to be related to the Pancharatnam-Berry connection on the Poincaré sphere. Dressing the rays with Gaussian beams, the approximation returns the Laguerre-, Hermite-and generalized Hermite-Laguerre-Gaussian beams exactly. The approach strengthens the connection between structured light and Hamiltonian optics, opening the possibility to new structured Gaussian beams.
IntroductionGaussian beams are ubiquitous in contemporary optics as the simplest model of paraxially propagating, monochromatic light beams, as are the mode sets based on them, the Hermite-Gaussian (HG) and Laguerre-Gaussian (LG) modes [1-3]. The fundamental Gaussian and HG modes are familiar from the laser laboratory as modes of laser cavities, and LG modes epitomize structured light beams, carrying quantized orbital angular momentum and optical vortices on their axis. As such, HG and LG modes are the most familiar and studied examples of structured light beams.This familiarity desensitizes us to a remarkable property all of these beams share-up to a scaling, their intensity profile is the same in every plane, even to the far field. This is readily accounted for mathematically by the fact that Gaussian beam families are eigenfunctions of (isotropic fractional) Fourier transformations, or in the analogy between paraxial optics and 2+1-dimensional quantum mechanics, that two-dimensional harmonic oscillator eigenfunctions spread, under evolution by the free Schrödinger equation, in a formpreserving way [4][5][6]. The two-dimensional quantum harmonic oscillator itself is analogous, via the 'Schwinger oscillator model' [7,8], to the quantum theory of angular momentum: Gaussian beams become analogous to spin directions in an abstract space (LG vertical, HG horizontal) with mode order proportional to total spin [9]. The simplest example of mode order N=1 therefore resembles the Poincaré sphere for polarization [10], giving rise to the celebrated Poincaré sphere for modes [11][12][13][14][15][16]. Modes corresponding to an arbitrary abstract spin direction are the generalised Hermite-Laguerre-Gaussian (GG) beams [17,18].In [19] we proposed a new and broad definition of structured Gaussian beams, using the mathematics of semiclassical ray optics, based on the self-similarity of propagating Gaussian beams. This involved identifying each Gaussian beam with a two-parameter family of light rays, potentially overlapping, each of which is weighted by a complex amplitude: the sum of ray amplitudes at each point approximates the complex amplitude of the propagating wave field. This approximation can be improved by replacing each linear ray with a Gaussian beam whose axis correspon...