There are a variety of optical coatings needed on laser bar facets to make them functional. There are also several different types of laser bar facet materials to be coated which complicates the problem a little bit. These coatings fall into three types; antireflection coatings, high reflectors and partial reflectors. A wide range of coating designs and materials to be used in the coatings has been studied.
The anti reflection coating (AR) typically used is either a single layer coating, obviously consisting of one material, or dual layer coatings consisting of two materials. There are a limited number of applications that may involve larger number of layers if more than one wavelength or a wavelength band of more than 40 nm needs to be covered. The single layer coatings usually do not provide very low reflectance. Dual layer coatings provide the ability to design coatings with very low reflectance. Manufacturing process limitatins allow for producing AR coatings with a residual reflection in the 0.1%-0.2% range. Although lower reflecting coatings can be designed, process control parameters and optical measurement problems limit the coating manufacturer to this range with R ≤ 0.2% being a standard specification. This paper will discuss the various coating designs for achieving low reflectance on InP and GaAs laser facets and the optical measurements problems.