A unique in-situ film thickness monitor for infrared (IR) coatings has been designed and fabricated by using a visible laser (HeNe laser). The accuracy of the monitor has been verified by comparing the results of the optical monitoring using the HeNe laser with those obtained by the optical monitoring using an IR laser (CO2 Laser). The two lasers are aligned to couple the laser beams so that they are incident on the same spot of the sample bring coated and, after transmission through it, they are separated and directed to the respective detectors for the HeNe laser and CO2 laser. The coupling of the two beams helps to align the in-situ monitor, The percentage transmission of the CO2 laser beam is used for the monitoring of film thickness at 10.6 mu m Simultaneously, the transmission of the HeNe laser beam is also used for monitoring the film thickness as multiples of HeNe laser wavelength. The results have been compared using the in-situ film thickness monitor based on the IR laser specially designed for this purpose. The system has been incorporated in the Balzers Vacuum Coating Unit and has been used for the fabrication of an output laser mirror at 10.6 mu m Eight lambda/4 layers of the ThF4 and ZnSe have been coated on a ZnSe substrate by electron beam evaporation and the reflectivity R similar to 97% has been achieved.