Normalized Ensemble-Averaged OAM Spectrum in Disordered Statistical Q-Plates

Abstract: This work presents an exact, analytical derivation of the ensemble-averaged orbital angular momentum (OAM) power spectrum for a circularly polarized Gaussian beam traversing a statistical Q-plate with Gaussian spatial disorder. Utilizing the Gaussian moment theorem, a new closed-form expression for the averaged mutual coherence is obtained. This coherence function is then rigorously projected onto OAM modes, yielding an exactly normalized series representation whose absolute convergence is formally proven. The analysis meticulously resolves limiting disorder regimes: for coarse disorder, an ideal OAM spectrum, sharply peaked at its nominal OAM mode, is demonstrated. Conversely, for fine disorder, a specific fraction of total power exponentially decays with disorder variance into the nominal OAM mode, with remaining power distributed among nearby OAM modes, exhibiting an effective width inversely proportional to the dimensionless correlation length. Crucially, a new universal scaling framework, defined by a master control parameter and a universal coordinate, is introduced. This framework, rigorously derived from the exact solution, unifies the spectrum's description across all relevant disorder regimes and enables robust data collapse. Monte Carlo simulations, implemented with the same normalization and OAM projection, substantiate these claims by corroborating the predicted scaling and universal behavior, offering a foundational, device-level perspective on OAM universality in complex media.