  Semiconductor lasers are notoriously sensitive to optical feedback, and their
dynamics and coherence can be significantly modified through optical
reinjection. We concentrate on the dynamical properties of a very small (i.e.,
microscale) Vertical Cavity Surface Emitting Laser (VCSEL) operated in the low
coherence region between the emission of (partially) coherent pulses and ending
below the accepted macroscopic lasing threshold, with the double objective of:
1. studying the feedback influence in a regime of very low energy consumption;
2. using the micro-VCSEL as a surrogate for nanolasers, where measurements can
only be based on photon statistics. The experimental investigation is based on
time traces and radiofrequency spectra (common for macroscale devices) and
correlation functions (required at the nanoscale). Comparison of these results
confirms the ability of correlation functions to satisfactorily characterize
the action of feedback on the laser dynamics. Numerical predictions obtained
from a previously developed, fully stochastic modeling technique provide very
close agreement with the experimental observations, thus supporting the
possible extension of our observations to the nanoscale.
