Fiber optic attenuators are devices used to reduce or monitor the power level of a fiber optic signal. Basic types of fixed attenuation include single mode, dual window and multimode in D4/PC, FC, FC/UPC, MU, SC, SC/APC and UPC, ST and ST/UPC style connectors. The input of the attenuator has a 2. 2 mm wide key female connector, making it compatible with. Thorlabs' Fiber Optic Attenuator and Terminator Kits each contain six fixed attenuators, five terminators, and a selection of dust caps. for achieving a suitable signal level for a data receiver in a telecom system.