Ensuring Coating Environmental Stability
Ion beam sputtering (IBS) offers the ability to produce coated optics with superior stability with respect to changes in environmental conditions (e.g. humidity and temperature). This, in turn, improves the operating stability of the systems into which they are incorporated. This can be an issue with components manufactured using evaporative coating technology. In particular, these films are relatively porous, and can absorb moisture after they are produced. As a result, fluctuations in ambient humidity can change the optical thickness of the layers and shift coating response, in turn, affecting output. This is shown in the accompanying graph. In contrast, IBS produces coatings which are fully densified, and do not absorb moisture. Therefore, they are completely stable with respect to changes in humidity.
Environmental instability can even be an issue in optics used under “laboratory” conditions, or for systems, such as DPSS lasers, which are purged of moisture and hermetically sealed during fabrication. For example, when the laser is first powered on and heats ups, the coating may release moisture, causing a shift in its performance and a change in laser output. Thus, coated cavity optics which appeared to be within specification when accepted by the manufacturer can go out of specification when actually used.