Optical system for detecting the concentration of combustion products operating in situ and at high temperature based on measurement of the optical absorption of a gaseous mixture of combustion products through a photodetecting sensor based on gallium nitride (GaN), aluminium nitride (AlN), indium nitride (InN) and corresponding alloys. The operating temperature of the sensor preferably lies between 500° C. and 700° C., but the resistance of the active material permits use at even higher temperatures. This system can be used to …