In a covering outside its light-guiding portion, an optical fiber includes at least one crystalline intermetallic compound made up of at least two metals, the compound having a plateau pressure during hydride formation that is less than equal to 5102 atm as measured at 30 C. An example of the arrangement can include a source of pressurized coolant fluid connected to the coolant plate and adapted to cause the coolant fluid to be transferred between the coolant plate and the cooling device. The downhole pump includes two or more chambers for the accumulation of formation fluids and a valve assembly for filling and venting gas to and from the two or more chambers.