  We report a comprehensive micro-Raman study of a pressurized suspended
graphene membrane that hermetically seals a circular pit, etched in a
Si/SiO$_2$ substrate. Placing the sample under a uniform pressure load results
in bulging of the graphene membrane and subsequent softening of the main Raman
features, due to tensile strain. In such a microcavity, the intensity of the
Raman features depends very sensitively on the distance between the graphene
membrane and the Si substrate, which acts as the bottom mirror of the cavity.
Thus, a spatially resolved analysis of the intensity of the G- and 2D-mode
features as a function of the pressure load permits a direct reconstruction of
the blister profile. An average strain is then deduced at each pressure load,
and Gr\"{u}neisen parameters of $1.8\pm0.2$ and $2.4\pm0.2$ are determined for
the Raman G and 2D modes, respectively. In addition, the measured blister
height is proportional to the cubic root of the pressure load, as predicted
theoretically. The validation of this scaling provides a direct and accurate
determination the Young's modulus of graphene with a purely optical, hence
contactless and minimally invasive, approach. We find a Young's modulus of
$(1.05\pm 0.10) \rm TPa$ for monolayer graphene, in a perfect match with
previous nanoindentation measurements. This all-optical methodology opens
avenues for pressure sensing using graphene and could readily be adapted to
other emerging two-dimensional materials and nanoresonators.
