The point spread function (PSF) of an imaging system has a minimum size, a “diffraction limit,” determined by the size of the limiting aperture. Image features smaller than this PSF can be, in a conventional imaging system, resolved only if the intensity noise is low enough to permit deconvolution. Measuring image-plane intensity as a function of spatial mode rather than position has the potential to reduce the quantum noise and thus enable subdiffraction resolution at lower light levels or in shorter measurement times than can be tolerated with conventional imaging. Here we examine experimental measurements of intensity and intensity noise as a function of spatial mode. We characterize the impulse response of a spatial mode coupling measurement at the focal plane of an imaging system to the position of a far-field point source. Our measured intensity noise scales with power in a way that suggests photon shot noise is a significant contributor, and we find that the signal-to-noise ratio of our modal-basis measurement of point source position exceeds that of a conventional image-plane pixel array for subdiffraction objects imaged against dark backgrounds. The mode coupling is measured with a custom mode-separating fiber photonic lantern. Photonic lanterns and equivalent structures constructed from rigid waveguides are simple, passive devices that lend themselves to real-world implementations of this measurement scheme with minimal size, weight, power, and cost.
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