Pulsed fiber lasers have become the primary laser source for the marking and engraving of various metals. The deleterious effects of back reflections from the part being marked or engraved are well-known. These effects include reduced beam quality, poor marking or engraving quality, and damage to the laser itself. As shown in Figure 1, back reflections can reach very high levels, near 90% in some cases. For this reason, optical isolators at the output of the pulsed fiber laser, particularly for average powers ≥20W have become integral parts of pulsed fiber lasers. Further, optical isolators can be exposed to a variety of operating conditions when used with pulsed fiber lasers for marking and engraving. This paper looks at some of those conditions and their impact on optical isolator performance and marking and engraving quality.