Broadband low-reflectivity coating for semiconductor power lasers by ion-beam and PECVD deposition

A new type of optical low-reflectivity coating based on a stepwise-graded-index multilayer has been designed and applied to GaAs/AlGaAs power laser mirrors. The coating design is based on a known principle of filter design theory (the Herpin principle) which enables an ideal graded-index film to be approximated by a much simpler combination of high and low index material layers. Only two materials are required (SiO2 and a-Si for instance) thus making it very easy to fabricate such coatings with standard deposition techniques such as ion beam sputtering or plasma enhanced chemical vapor deposition. The low-reflectivity region of this coating extends over a broad wavelength range, therefore making the overall reflectivity much less sensitive to thickness variations than is the case for single-layer coatings. The good optical qualities of such a coating and the ease of fabrication make it a very promising alternative to single-layer low-reflectivity coating. In particular, optical output power density in excess of 10 MW /cm2 on power laser mirrors was measured, which corresponds to one of the highest reported values for coated mirrors. © 1989.