Modal decomposition for few mode fibers using the fractional Fourier system

Modal decomposition (MD) has become an indispensable diagnostic tool for optical fibers. A novel MD method using the fractional Fourier system is developed in this paper. Firstly, the existing phase retrieval (PR) algorithm based on the the fractional Fourier transform (FrFT) power spectra is extended to account for the effect of optical vortex. The extended capability can then be employed to reconstruct the phase of modal field to high fidelity in a non-iterative and non-interferometric manner. Combining the reconstructed phase with the measured near-field (NF) intensity, the modal field could be obtained and based upon which the complete MD (involving modal weights and phases) is performed. The validity and reliability of the method are demonstrated through several numerical examples including the noisy signals with different signal-to-noise ratio (SNR) levels.