Heart failure remains a leading cause of morbidity and mortality worldwide, highlighting the need for reliable tools to assess cardiac function. Myocardial oxygenation is one of the most direct indicators of tissue health, yet current methods lack compact, implantable solutions for continuous monitoring. This work presents an implantable optical sensor that exploits the ultraviolet-excited fluorescence of NADH as a marker of oxygenation. To overcome the limited penetration of ultraviolet light, near-infrared photons are externally delivered and converted into ultraviolet emission by lanthanide-based upconverting nanoparticles(UCNPs), enabling localized excitation without implanted power sources. A Fabry–Perot filter was incorporated to suppress blue emission that overlaps with NADH fluorescence while maintaining high ultraviolet transmittance. The filter design was optimized through multilayer simulations, and deposition conditions were tuned to improve film quality. Upconverting nanoparticles were drop-cast onto the filter surface, and material characterization confirmed the presence of significant nanoparticle coverage. An optical testing platform was further established using both a xenon-based source and a laser diode, which enabled validation of up-conversion performance and filter function. Collectively, these results demonstrate the feasibility of a compact, externally powered light emitter for implantable cardiac oxygen monitoring and establish a foundation for future development of minimally invasive biosensors.