Broad-band high damage threshold mid-IR coating for high power fluoride fiber laser

In this paper, we demonstrate a broadband anti-reflective (AR) coating over the mid-IR fluoride fiber endface (tip) for high power laser applications. The AR coating consists of multiple-pair Lithium fluoride (LiF) and Al2O3, and was deposited by electron beam physical vapor deposition with an iron assistant source at low temperature (<60°C). In addition, a super thin encapsulation layer (~20nm) of Al2O3 was applied over the AR coating by both traditional PVD and atomic layer deposition technology. The measurements show the coating has a reflectivity of < 1-1.5% in the range of 1.5-5.5μm. The laser induced damage threshold (LIDT) test using high power quantum cascade laser (QCL) shows the damage threshold is greater than 10 MW/cm2 with no sign of any damage on the coating. The durability and environmental tests of the AR coating with PVD coated encapsulation layer show good humidity resistance in open air and 95 RH% at 50ºC for 48 hours, no degradation of film quality and optical performance are observed. Further comparison study of ALD encapsulation layer vs pure PVD layers in more aggressive water vapor testing indicates ALD encapsulation layer has better moisture resistance.