A geometrical optics approach to encircled-flux compliant source modeling for multi-mode fiber illumination and connection attenuation

•Geometrical-optics based source modeling for multimode fiber connection attenuation.•Spatially stable propagation of LP-mode conforming ray-invariant ray distributions.•Agreement between model and measurements of attenuation and far-field pattern.•Effect of coherent speckle pattern on connection attenuation variation. We construct ray-based encircled-flux compliant sources for multi-mode optical fiber launches using classic geometrical optics. In particular, we make an LP-mode conforming launch that excites only guided rays by distributing the β¯ ray invariant according to a (scaled) counterpart of the modal power distribution of LP-modes. Propagation of the ray distribution is spatially stable and simulations of attenuation due to fiber connections and 1 mm far-field behavior using this source agree well with measurements. We caution against models that configure ray positions and ray directions independently and show that they are prone to produce unstable ray distributions. Lastly, we show that modal noise due to laterally misaligned fibers can be replicated using geometrical optics by configuring our LP-mode conforming sources to match the radial power distribution of 3000 vectorial full-wave spatio-temporal coherent speckle patterns.