Graphene has garnered significant interest in optoelectronics due to its unique properties, including broad wavelength absorption and high mobility. However, its weak stability in ambient conditions requires encapsulation for practical applications. In this study, we investigate graphene CVD-grown field-effect transistors fabricated on Si/SiO2 wafers, encapsulated with aluminum oxide ( Al2O3) of different thicknesses. We measure and analyze their optoelectronic response across wavelengths from near-ultraviolet to near-infrared. We find that, while having a negligible role in the photogating process, the Al2O3 layer leads to stable and reproducible transferring curves operating in ambient conditions for over a month, with stable responsivities up to 1.5 A W-1 at the shortest wavelength. Moreover, the transferring curves are stable at elevated temperatures up to 107 degrees C . We also show that the sample performance can be tuned by changing the thickness of the SiO2 and Al2O3 layer which brings further perspectives in developing robust sample technologies, especially in the ultraviolet region where the responsivity increases. Aluminum oxide encapsulated graphene-based photodetectors can thus be interesting for applications in air and at elevated temperatures.