Time-resolved information of fuel-concentration, equivalent ratio and temperature are very important for the development of modern engines. The presented fiber optic sensor is designed for acquisition of these parameters in the area close to the ignition spark, which is of particular interest. The measurement method is based on UV-laser induced fluorescence of tracer substances. Excitation and fluorescence light are separately guided through all-silica fibers. The measuring volume is defined by the optical design of the sensor head. Since the related components are completely integrated into a modified spark plug, the sensor can easily be used on close-to-production engines. We present the fundamental spectroscopic concept, the solution for the minimal invasive access through the spark plug and tracer spectra measured on a prototype.