  All-optical ultrasound detection bears unique advantages for photoacoustics,
including wider detection bandwidth, higher signal-to-noise per unit area and
lower susceptibility to electromagnetic noise. These benefits have established
optical ultrasound detection as a key method for photoacoustic applications in
biology and medicine. However, the use of free-space detectors renders this
approach sensitive to optical aberrations, which can degrade the pressure
sensitivity and result in deteriorated image quality. While spatial
mode-filtering through fibers has been proposed to alleviate these problems in
Fabry-P\'erot based pressure sensors, their real functional advantage has never
been properly investigated. In this paper we rigorously and quantitatively
compare the performance of free-space and fibre-coupled detectors in a custom
correlative setup. We demonstrate the superiority of the latter in terms of
both signal level and image quality in realistic all-optical photoacoustic
tomography settings.
