  Coherent phonon generation by optical pump-probe experiments has enabled the
study of acoustic properties at the nanoscale in planar heterostructures,
plasmonic resonators, micropillars and nanowires. Focalizing both pump and
probe on the same spot of the sample is a critical part of pump-probe
experiments. This is particularly relevant in the case of small objects. The
main practical challenges for the actual implementation of this technique are:
stability of the spatio-temporal overlap, reproducibility of the focalization
and optical mode matching conditions. In this work, we solve these three
challenges for the case of planar and micropillar optophononic cavities. We
integrate the studied samples to single mode fibers lifting the need for
focusing optics to excite and detect coherent acoustic phonons. The resulting
excellent reflectivity contrast of at least 66% achieved in our samples allows
us to observe stable coherent phonon signals over at least a full day and
signals at extremely low excitation powers of 1uW. The monolithic sample
structure is transportable and could provide a means to perform reproducible
plug-and-play experiments.
