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.
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