Verrillon® VHM4000 product is a multimode step-index with a pure silica core for high resistance to hydrogen darkening. This product is available in both 50/125 and 62.5/125 versions. This design is available in a variety of coatings including Polyimide, high temperature acrylates, Silicone-PFA and hermetic Carbon. Typically, these fibers are used in down-hole distributed sensing and imaging applications where the temperature and hydrogen partial pressures are extreme. Due to its step index design, the VHM4000 product is suitable for short distance applications where the spatial resolution requirements are not extreme.