A diagnostic system for measuring temperature, pressure, CO2 concentration and H2O concentration in a fluid stream is described. The system may include one or more probes that sample the fluid stream spatially, temporally and over ranges of pressure and temperature. Laser light sources are directed down pitch optical cables, through a lens and to a mirror, where the light sources are reflected back, through the lens to catch optical cables. The light travels through the catch optical cables to detectors, which provide electrical signals to a processer. The processer utilizes the signals to calculate CO2 concentration based on the temperatures derived from H2O vapor concentration. A probe for sampling CO2 and H2O vapor concentrations is also disclosed. Various mechanical features interact together to ensure the pitch and catch optical cables are properly aligned with the lens during assembly and use.