  In this work, an optic fiber based $\textit{in situ}$ illumination system
integrated into an aberration-corrected environmental transmission electron
microscope (ETEM) is designed, built, characterized and applied. With this
illumination system, the dynamic responses of photoactive materials to photons
can be directly observed at the atomic level, and other stimuli including
heating and various gases can also be applied simultaneously. Either a
broadband light source or a high power laser source aiming to expedite
photoreactions can be utilized, fitting different application needs. The optic
fiber enters the ETEM through the objective aperture port, with a carefully
designed curvature and a 30{\deg} cut at the tip to orient the emitted light
upwards onto the TEM specimen. The intensity distributions striking the sample
from the broadband and laser sources are both measured, and due to the
non-uniform distributions, an alignment procedure has been developed to align
the bright spot with the electron optical axis of the TEM. The imaging and
spectroscopy performances of the ETEM are proved to be maintained after
incorporating this illumination system. Furthermore, Langmuir evaporation is
observed when in situ laser light is applied to GaAs, demonstrating the
phenomenon of optical heating on suitable semiconductor materials.
